Optical emission spectrometer and method of optical emission spectrometry

Abstract

Optical assembly 20 directs light, perhaps axial light, in a first direction from plasma source 10 along a first optical path to entrance slit 60 of an optical emission spectrometer. First mirror 40 selectively switches reference light from reference light source 30 onto the first optical path and to the entrance slit for calibrating the spectrometer. Radial light from the plasma source may be directed along a second optical path to the entrance slit (Fig. 4) when selectively switched. Second mirror 50 selectively switches reference light onto the second optical path for calibrating the spectrometer. The first and second mirrors may each comprise a reflective portion and a transmissive portion. The portions may define complementary shapes on the two mirrors. The spectrometer may comprise an echelle grating to direct light onto a detector to image an echelle spectrum comprising plural orders, or may comprise a concave grating to image a linear spectrum on the detector.

Description

Field of the disclosure The present disclosure relates to optical spectrometry. In particular, the present disclosure relates to optical emission spectrometry. Background Optical emission spectrometry involves the analysis of light emitted from sample molecules as electrons transition from a higher energy level to a lower energy. Typically, electrons of a sample molecule are excited to a higher energy level by means of an energy source, for example a plasma source. An optical emission spectrometer aims to capture the light emitted and to analyse the wavelength of the light in order to determine information about the composition of the sample. In order to accurately characterise the sample molecules, the optical emission spectrometer typically includes an optical system which directs light from the plasma source to a detector of the optical spectrometer. It is desirable for the optical emission spectrometer to accurately determine the wavelength of the light emitted from the plasma source. US -B-7,319,519 discloses a method for the calibration of echelle spectra, in which the wavelength are distributed across a number of orders. The method comprises recording a line-rich reference spectrum with known wavelengths for a number of the lines and determining the position of a number of peaks for the reference spectrum in the recorded spectrum. Against this background, it is desirable to provide an improved, or at least commercially relevant alternative, optical emission spectrometer and method of optical emission spectrometry. Summary According to a first aspect of the disclosure, an optical emission spectrometer is provided. The optical emission spectrometer comprises a plasma source, an optical assembly, a reference light source, and a first mirror. The plasma source is configured to emit light. The optical assembly comprises a plurality of optical components, the optical assembly configured to direct light travelling in a first direction from the plasma source to an entrance slit of the optical emission spectrometer along a first optical path. The reference light source is for calibrating the spectrometer, wherein the reference light source is configured to output reference light along a reference light path. The first mirror is configured to selectively switch the refence light on to the first optical path such that the reference light is directed to the entrance slit for calibration of the optical emission spectrometer. As such, the optical emission spectrometer of the first aspect is configured to measure light which is emitted from a plasma source travelling along the first direction. The optical assembly of the optical emission spectrometer provides a first optical path to direct the light from the plasma source to the entrance slit of the optical emission spectrometer. In order to improve the accuracy of the optical emission spectrometer, it is advantageous to calibrate the optical emission spectrometer. The optical emission spectrometer of the first aspect may be calibrated using a reference light source which is integrated with the optical emission spectrometer. Thus, the first optical path of the optical emission spectrometer may be calibrated in a reliable manner without requiring any external light sources or calibration samples. In particular, the reference light source is straightforward to switch on to the first optical path using the first mirror, such that a calibration may be performed in real-time. That is to say, the optical emission spectrometer does not require any external adjustment (e.g. use of a calibration sample, or an internal standard) in order to calibrate the optical emission spectrometer. Accordingly, the first optical path may be calibrated repeatedly, or in real-time during a series of measurements performed using the optical emission spectrometer. In order to perform the calibration(s), the optical emission spectrometer of the first aspect comprises a first mirror which is configured to selectively switch the reference light on to the first optical path such that the reference light is imaged on the entrance slit of the optical emission spectrometer. As such, the reference light can be imaged on the first optical path without adjusting any component of the optical assembly defining the first optical path. Thus, calibration measurements may be performed without requiring any adjustment of the optical assembly, thereby further improving the accuracy of the optical spectrometer. In some embodiments, the first mirror is configured to be moveable between: a first position away from the first optical path, and a second position in which the first mirror is configured to reflect reference light from the reference light path on to the first optical path. As such, the first mirror may be moved between the first and second positions in order to selectively switch the reference light on to the first optical path. Accordingly, calibration of the optical emission spectrometer may be triggered by movement of the first mirror, rather than any adjustment of the optical assembly providing the first optical path. In some embodiments, the reference light path may intersect the first optical path. Further, in some embodiments, the first mirror may be positioned along the reference light path, for example at the intersection of the reference light path and the first optical path, in order to selectively switch the reference light. In some embodiments, the plasma source may be configured to produce axial light in an axial direction and radial light in a radial direction. In some embodiments, the first direction in which light is emitted by the plasma source may be an axial direction, or a radial direction of the plasma source. According to this disclosure, an axial direction of a plasma source is understood to be a direction which is aligned with a principal axis of the plasma source. The radial direction is understood to be a direction which is transverse to the axial direction. As such, an axial direction and a radial direction of a plasma source may be two directions in which light is emitted by a plasma source which are transverse to each other. In some embodiments, the optical assembly comprises a plurality of optical components which are configured to direct radial light in the radial direction from the plasma source to the entrance slit of the optical emission spectrometer along a second optical path, and the optical emission spectrometer comprises a second mirror configured to selectively switch the refence light onto the second optical path for calibration of the optical emission spectrometer. As such, in some embodiments, the optical emission spectrometer may be configured to perform measurements using light emitted from the plasma source in a radial direction or light emitted in an axial direction. By providing first and second mirrors, the reference light source may be used to calibrate both the first and second optical paths without requiring any further adjustment of the optical assembly. Furthermore, the first and second optical paths may be calibrated using the same reference light source. Thus, the accuracy of the calibration of the optical emission spectrometer may be further improved. In some embodiments, the second mirror may be configured to be moveable between a first position away from the second optical path, and a second position in which the second mirror is configured to reflect reference light from the reference light path on to the second optical path such that reference light is directed to the entrance slit of the optical emission spectrometer. As such, the second mirror may, in some embodiments, be moved between two positions in order to switch reference light on to the second optical path. In some embodiments, the first mirror may comprise a reflective portion and a transmissive portion, wherein when the first mirror is in the second position the reflective portion may be configured to reflect reference light incident on the reflective portion on to the first optical path, and the transmissive portion may be configured to transmit reference light incident on the transmissive portion such that it intersects the second optical path. As such, the first mirror may be positioned in the second position such that some reference light is reflected on to the first optical path by the reflective portion of the first mirror. Reference light which is incident on the transmissive portion may be transmitted on to the second mirror / second optical path. Thus, in some embodiments, the first and second mirrors may both be positioned in the second position to allow the first and second optical paths to be calibrated without requiring any further adjustments to the position of the mirrors. In some embodiments, the second mirror may comprise a complementary reflective portion, such that when the second mirror is in the second position, the complementary reflective portion is configured to reflect reference light which is transmitted by the transmissive portion of the first mirror. As such, the first and second mirrors may be provided in a complementary manner. In some embodiments, the transmissive portion (of the first mirror) may define a first shape on the first mirror. In some embodiments, the complementary reflective portion (of the second mirror) may define a complementary first shape on the second mirror, wherein the first shape and the complementary first shape may be aligned along the reference light path. As such, the light which is transmitted through the first mirror may be reflected on to the second optical path via the first mirror. Thus, when the first and second mirrors are each in respective second positions, reference light may be present on both the first and optical paths. In some embodiments, the first shape and the complementary first shape may each define one or more of: a circle, a regular polygon, an irregular polygon, and an ellipse. In some embodiments, the transmissive portion of the first mirror may comprise a plurality of shapes. For example, the transmissive portion may comprise a first shape and a second shape. The first and second shapes may be the same shapes or different shapes. As such, it will be appreciated that the transmissive portion of the first mirror may be provided with any two-dimensional pattern, with the complementary reflective portion of the second mirror having a complementary two-dimensional pattern. In some embodiments, the first mirror may comprise an optically transparent plate having a reflective coating partially covering a major surface of the optically transparent plate, wherein the reflective coating provides the reflective portion of the first mirror. In some embodiments, the second mirror may comprise an optically transparent plate having a reflective coating partially covering a major surface of the optically transparent plate, wherein the reflective coating provides the complementary reflective portion of the second mirror. In some embodiments, when in the first position, the first mirror may intersect the reference light path. As such, when in the first position, the transmissive portion of the first mirror may be configured to transmit reference light to the second mirror. In some embodiments, the optical emission spectrometer may further comprise an optical fibre. The optical fibre may be configured to direct reference light from the reference light source towards the first and second mirrors. As such, the optical emission spectrometer may be configured to accommodate the reference light source and the reference light path in a space efficient manner. For example, the optical fibre may be configured to receive reference light from the reference light source along a first reference direction and to output reference light along the reference light path to the first and second mirrors, wherein the direction of the reference light path is different to reference light direction. For example, in some embodiments, the direction of the reference light path may not intersect with the reference light direction. In some embodiments, the optical assembly may comprise an adjustable mirror. The adjustable mirror may be configured to be adjustable between a first position in which the adjustable mirror reflects light from the first optical path to the entrance slit of the spectrometer and a second position in which the adjustable mirror reflect light from the second optical path to the entrance slit of the spectrometer. In some embodiments, the optical assembly may comprise a first focusing mirror provided on the first optical path. The first focusing mirror may be configured to receive axial light from the plasma source and reference light from the first mirror, and to focus the axial light and the reference light to the adjustable mirror of the optical assembly. In some embodiments, the optical assembly may comprise a second focusing mirror provided on the second optical path. The second focusing mirror may be configured to receive radial light from the plasma source and the reference light from the second mirror, and to focus the radial light the reference light to the adjustable mirror of the optical assembly. Thus, the optical assembly may be provided in a space efficient configuration to focus light from the plasma source and the reference light source on to the entrance slit of the optical emission spectrometer. In some embodiments, the reference light source may comprise one or more of: a hollow cathode lamp, a glow discharge lamp, a spectral lamp, a laser, a LED (preferably a narrowband LED) and the like. As such, it will be appreciated that the reference light source may be provided by any light source which outputs on or more well-defined spectral lines (i.e. a bandwidth of less than 1 nm) of a known wavelength. Preferably, the reference light source may emit a plurality of spectral lines across a range of wavelengths, such that the optical emission spectrometer may be calibrated across a desired range of wavelengths, or across a desired range of detector locations. In some embodiments, the optical emission spectrometer may comprise one or more of: a reference light shutter configurable to occlude the reference light source from the reference light path, an axial light shutter configurable to occlude the plasma source from the first optical path, and a radial light shutter configurable to occlude the plasma source from the second optical path. As such, the optical emission spectrometer may be provided with a reference shutter which may allow the reference light source to be operated continuously while the optical emission spectrometer is in use. For example, in some embodiments, it may not be desirable to be repeatedly turning the reference light source on and off each time a calibration measurement is performed. Thus, a reference light shutter may be used to allow the reference light source to be continuously on and not interfere with other measurements of the optical emission spectrometer. Similarly, an axial light shutter and / or a radial light shutter may be provided to allow the plasma source to be continuously operated whilst performing a calibration measurement. In some embodiments, the optical emission spectrometer may comprise a detector. In some embodiments, the optical emission spectrometer may comprise an echelle grating configured to receive light from the entrance slit and direct the light to the detector, wherein an echelle spectrum comprising a plurality of orders is imaged on the detector. In some embodiments, the optical emission spectrometer may comprise a concave grating configured to receive light from the entrance slit and to direct the light to the detector, wherein a linear spectrum is imaged on the detector. According to a second aspect of the disclosure, a method of optical emission spectrometry using the optical emission spectrometer of the first aspect is provided. The method comprises performing a calibration measurement in which the first mirror is positioned such that reference light is directed from the reference light source to the entrance slit of the spectrometer along the first optical path, wherein the optical emission spectrometer detects a known wavelength of the reference light source. Thus, the method of the second aspect performs a calibration measurement for the optical emission spectrometer using an integrated reference light source. The optical emission spectrometer may determine a calibration for the optical emission spectrometer based on an association between the known wavelength and a position of the spectral peak of known wavelength incident on a detector of the optical emission spectrometer. Thus, the optical emission spectrometer of the first aspect may be calibrated in a quick, accurate, and reliable manner. In some embodiments, the method may further comprise performing a sample measurement in which the first mirror is positioned such that light is directed from the plasma source to the entrance slit of the optical emission spectrometer along the first optical path. As such, the method may comprise performing a calibration measurement and a measurement of a sample. In some embodiments, calibration measurements may be repeated, or interleaved throughout a plurality of sample measurements performed using the first optical path. In some embodiments, the method may further comprise: performing an axial sample measurement in which the first mirror is positioned such that axial light is directed from the plasma source to the entrance slit of the optical emission spectrometer; and / or performing a radial sample measurement in which the second mirror is positioned such that radial light is directed from the plasma source to the entrance slit of the optical emission spectrometer. As such, the method of the first aspect may allow measurements of a sample present in the plasma source to be performed by changing the position of the first and / or second mirrors. Thus, the optical emission spectrometer may efficiently transition between performing sample measurement and calibration measurements. In some embodiments, a method of optical emission spectrometry may be provided in which a plurality of axial sample measurements and / or a plurality of radial sample measurements are performed. As part of the method, at least one axial calibration measurement and / or at least one radial calibration measurement may be performed during the plurality of sample measurements. For example, the axial calibration measurement and / or the radial calibration measurement may be interleaved with the plurality of sample measurements. As such, the calibration of the optical emission spectrometer may be checked (and re-calibrated) in real-time. Thus, according to the method of the second aspect, small real-time fluctuations in the operating conditions of the optical emission spectrometer may be accounted for by performed repeat calibration of the optical emission spectrometer in real time. Brief description of the figures Embodiments of the disclosure will now be described with reference to the following nonlimiting figures in which: Fig. 1 is a block diagram of an optical emission spectrometer according to this disclosure; Fig. 2 shows a schematic diagram of the optical emission spectrometer performing a measurement of axial light according to an embodiment of the disclosure; Fig. 3 shows a schematic diagram of the optical emission spectrometer performing a calibration measurement of the first optical path according to an embodiment of the disclosure; Fig. 4 shows a schematic diagram of the optical emission spectrometer performing a measurement of radial light according to an embodiment of the disclosure; Fig. 5 shows a schematic diagram of the optical emission spectrometer performing a calibration measurement of the second optical path according to an embodiment of the disclosure; Fig. 6 shows a schematic diagram of a first mirror; Fig. 7 shows a schematic diagram of a second mirror; Figs. 8a, 8b, 8c, 8d, 8e and 8f show schematic diagrams of the first and second mirrors according to further embodiments of the disclosure; Fig. 9 shows a schematic diagram of an optical emission spectrometer including a plurality of shutters; Fig. 10 shows a block diagram of a method of optical emission spectrometry according to this disclosure; and Fig. 11 shows a block diagram of a further method of optical emission spectrometry according to this disclosure. Detailed description According to an embodiment of the disclosure an optical emission spectrometer is provided. Fig. 1 is a block diagram of an optical emission spectrometer 1 according to this disclosure. As shown in Fig. 1, the optical emission spectrometer 1 comprises: a plasma source 10, an optical assembly 20, a reference light source 30, a first mirror 40, a second mirror 50, an entrance slit 60, a diffractive optics assembly 70 and a detector 80. A further diagram of the plasma source 10, optical assembly 20, reference light source 30, first mirror 40, second mirror 50, and entrance slit 60 is shown in Fig. 2. The plasma source 10 is configured to emit light. The plasma source 10 may be configured to produce axial light in an axial direction and radial light in a radial direction. The plasma source 10 may be an inductively coupled plasma (ICP) source. In such embodiments, the optical emission spectrometer 1 may be an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). As shown in Fig. 2, the optical assembly 20 is configured to receive the axial light and the radial light and to direct the axial light and the radial light to the entrance slit 60. The optical assembly 20 comprises a plurality of optical components 21, 22, 23, 24. One example of an optical assembly 20 is shown in Fig. 2. The optical assembly 20 of Fig. 2 comprises a (flat) radial mirror 21, an axial focusing mirror 22, a radial focusing mirror 23 and a slit focusing mirror 24. Of course, it will be appreciated that in other embodiments, other combinations of flat mirrors, focusing mirrors (e.g. a convex mirror), lenses and / or any other optical components may be used to direct light from the plasma source 10 to the entrance slit 60. The optical assembly 20 is configured to direct light travelling in an axial direction (i.e. a first direction) from the plasma source 10 to an entrance slit 60 of the optical emission spectrometer 1 along a first optical path. As shown in Fig. 2, the first optical path extends from the plasma source 10 in the axial direction to the axial focusing mirror 22. The axial focusing mirror 22 is configured to receive the axial light and to reflect the axial light on to the slit focusing mirror 24. The slit focusing mirror 24 is configured to reflect the axial light from the axial focusing mirror towards the entrance slit 60. The slit focusing mirror 24 is configured to focus the axial light on the entrance slit 60 of the optical emission spectrometer 1. Accordingly, the axial light is directed along a first optical path defined by the optical components 22, 24 of the optical assembly 20. The reference light source 30 is a light source which is suitable for calibrating the spectrometer 1. As such, the reference light source 30 may be configured to output light having a wavelength, or a range of wavelengths which the spectrometer 1 is capable of measuring. As shown in Fig. 2. the reference light source 30 is configured to output reference light along a reference light path. In some embodiments, the reference light output on the reference light path may be collimated, for example by providing a collimating lens (not shown) as part of the reference light source 30. As shown in Fig. 2, the refence light path intersects the first light path and the second light path of the optical assembly 20. In some embodiments, the refence light source 30 may be one or more of: a hollow cathode lamp, a narrowband LED, a laser, a glow discharge lamp and a spectral lamp. As such, it will be appreciated that the reference light source 30 may be used to calibrate the optical emission spectrometer 1 across a range of wavelengths. According to this disclosure, the reference light source 30 is configured to output light for calibrating the optical emission spectrometer 1. In particular, the reference light source 30 outputs light for calibrating a position of a spectral peak on the detector 80 of the optical emission spectrometer. By determining the position of a spectral peak (or a plurality of peaks) of the reference light source 30 (having a known wavelength(s)), the optical emission spectrometer 1 may determine a position calibration for the optical emission spectrometer 1. In some embodiments, the reference light source 30 may be configured to directly output reference light along the reference light path (e.g. as shown in Fig. 2). In some embodiments, one or more reference optical components (not shown in Fig. 2) may be provided to direct light output from the reference light source to the reference light path. For example, in some embodiments an optical fibre (not shown) may be provided which is configured to direct reference light from the reference light source towards the first and second mirrors 40, 50. One or more focusing lenses or mirrors may be provided to focus light from the reference light source 30 into the optical fibre and to focus light output from the optical fibre to on to the first and second mirrors. By utilising an optical fibre to direct reference light, the optical assembly 20 and reference light source 30 may be arranged in a more space-efficient manner. The diffractive optics assembly 70 (e.g. as shown in Fig. 1) may be configured to direct light from the entrance slit 60 to a detector 80 (e.g. as shown in Fig. 1) of the optical emission spectrometer 1. The diffractive optics assembly 70 may comprise one or more diffractive component which may be configured to diffract light such that light of different wavelengths is distributed in space. For example, the diffractive optics assembly 70 may comprise an echelle grating (not shown in Fig. 1). The echelle grating may be configured to receive light from the entrance slit 60 and to direct the light to the detector 80, wherein an echelle spectrum comprising a plurality of orders is imaged on the detector 80. Alternatively, the diffractive component may be a concave grating (not shown in Fig. 1). The concave grating may be configured to receive light from the entrance slit 60 and to direct the light to the detector 80, wherein a linear spectrum is imaged on the detector 80. As such, it will be appreciated that light from the plasma source 10 or the reference source 30 may be imaged on the detector 80 as a spatially distributed spectrum. The detector 80 may be configured to receive light from the diffractive optics and generate a signal which is indicative of the spectrum incident on the detector 80. For example, in some embodiments, the optical spectrum incident on the detector 80 may be a two-dimensional spectrum. In such embodiments, the detector 80 may be a CCD (charged coupled device) array. A typical CCD array may have at least approximately 1024 x 1024 pixels (i.e. 1 Megapixel). The CCD array may be arranged for producing spectrum intensity values corresponding with the measured amount of light of the spectrum, and for transferring the spectrum values to a processor (not shown) for further analysis. Where the diffractive optics assembly 70 comprises an echelle grating, the orders of the echelle spectrum may be distributed across the CCD array. As such, the detector 80 may be a multichannel detector that is configured to detect a plurality of different wavelengths. The detector 80 (such as in the embodiment of Fig. 1) may be configured to detect a two-dimensional spectrum. In other embodiments, the detector 80 may be a Complementary Metal Oxide Semiconductor (CMOS) detector or a Charge Injection Device (CID) detector. The optical emission spectrometer 1 may determine a calibration for the optical emission spectrometer by associating a spectral peak of a known wavelength of the reference light source 30 and a position (location) of the detector 80 on which the spectral peak is incident. In the embodiment of Figs. 2 and 3, the reference light path extends in a direction which is transverse to the axial direction of the axial light. As such, the reference light path may intersect the axial light path orthogonally. The first mirror 40 is configured to be moveable between a first position and a second position. As shown in Fig. 2, when the first mirror 40 is in the first position, the first mirror 40 is positioned away from the first optical path. That is to say, in the first position, the first mirror 40 does not occlude the first optical path. Accordingly, when the first mirror 40 is in the first position, substantially all of the available axial light is directed to reach the entrance slit 60 via the optical assembly 20. As shown in Fig. 3, the first mirror 40 may be moved to be in a second position in which the first mirror 40 is configured to reflect reference light from the reference light path on to the first optical path. As such, when the first mirror 40 is in the second position, the reference light may be reflected by the first mirror 40 such that it is aligned with the first optical path. Accordingly, the first mirror 40 is configured to selectively switch the refence light on to the first optical path such that the reference light is directed to the entrance slit 60 for calibration of the optical emission spectrometer 1. That is to say, when the first mirror 40 is in the first position, the plasma source 10 may be operated in order to perform a sample measurement using the optical emission spectrometer 1. When the first mirror 40 is in the second position, the reference light source 30 may be operated in order to perform a calibration measurement using the optical emission spectrometer 1. As shown in Figs. 2 and 3, the first mirror 40 may be switched between the first and second positions by rotating the first mirror 40 about one end of the first mirror 40. In other embodiments, the first mirror 40 may move between the first and second positions by a translational movement (e.g. sliding the mirror) or a combination of a rotational and translational movement. The first mirror 40 shown in Figs. 2 and 3 is a flat mirror. It will be appreciated that the first mirror 40 may be any mirror suitable for directing the reference light onto the axial light path when the first mirror 40 is in the second position. For example, in some embodiments the first mirror 40 may be a curved mirror. When the first mirror 40 is in the second position, the reference light may be received by the axial focusing mirror 22 and reflected to the slit focusing mirror 24. As such, when performing a calibration measurement, the reference light may be directed to the entrance slit 60 by the same optical components 22, 24 of the optical assembly 20 which are used to direct axial light of the plasma source 10 when performing a sample measurement using axial light. While the embodiment of Figs. 2 and 3 includes first and second optical paths, it will be appreciated that in some embodiments, the optical emission spectrometer 1 may be provided with a single mirror 40 and the first optical path only (i.e. the second optical path and second mirror 50 are not provided). The first mirror 40 may be adjustable between two positions to allow the optical emission spectrometer 1 to perform sample measurements using the plasma source 10 and reference measurements using the reference light source 30. It will be appreciated that the single optical path may receive either radial light and / or axial light from the plasma source 10. The optical emission spectrometer 1 of Figs. 2 and 3 may also be configured to perform sample measurements and calibration measurements using radial light from the plasma source 10. As such, the optical emission spectrometer 1 of Figs. 2-5 comprises a first optical path and a second optical path. Figs. 4 and 5 show diagrams of the optical emission spectrometer 1 performing sample and calibration measurements respectively using the radial light path of the optical assembly 20. As shown in Figs. 4 and 5, the flat mirror 21, radial focusing mirror 23, and slit focusing mirror 24 of the optical assembly 20 are arranged to direct radial light from the plasma source 10 to the entrance slit 60. The radial light is directed along a second optical path defined by the optical components 21, 23, 24 of the optical assembly 20. The second mirror 50 is configured to be moveable between a first position and a second position. As shown in Fig. 4, when the second mirror 50 is in the first position, the second mirror 50 is positioned away from the second optical path. That is to say, in the first position, the second mirror 50 does not occlude the second optical path. Accordingly, when the second mirror 40 is in the first position, substantially all of the radial light reflected by the flat mirror 21 may be directed to reach the entrance 60 slit via the optical assembly 20. As shown in Fig. 5, the second mirror 50 may be moved to be in a second position in which the second mirror 50 is configured to reflect reference light from the reference light path on to the second optical path. As such, when the second mirror 50 is in the second position, the reference light may be reflected by the second mirror 50 such that it is aligned with the second optical path. Accordingly, the second mirror 50 is configured to selectively switch the refence light on to the second optical path such that the reference light is directed to the entrance slit 60 for calibration of the optical emission spectrometer 1. That is to say, when the second mirror 50 is in the first position, the plasma source 10 may be operated in order to perform a sample measurement using the optical emission spectrometer 1. When the second mirror 50 is in the second position, the reference light source 30 may be operated in order to perform a calibration measurement using the optical emission spectrometer 1. As shown in Figs. 4 and 5, the second mirror 50 may be switched between the first and second positions by rotating the second mirror 50 about one end of the second mirror 50. In other embodiments, the first mirror 50 may move between the first and second positions by a translational movement (e.g. sliding the mirror) or a combination of a rotational and translational movement. The second mirror 50 shown in Figs. 4 and 5 is a flat mirror. It will be appreciated that the second mirror 50 may be any mirror suitable for directing the reference light onto the second optical path when the second mirror 50 is in the second position. For example, in some embodiments, the second mirror 50 may be a curved mirror. When the second mirror 50 is in the second position, the reference light may be received by the radial focusing mirror 23 and reflected to the slit focusing mirror 24. As such, when performing a calibration measurement, the reference light may be directed to the entrance slit 60 by the optical components 23, 24 of the optical assembly 20 which are also used to direct radial light of the plasma source 10 when performing a sample measurement using axial light. In some embodiments, the first and second mirrors 40, 50 may be arranged in order to allow calibration measurements to be performed which have improved accuracy. As shown in Fig. 3, the first mirror 40 may comprise a transmissive portion 42 and a reflective portion 44. A plan view of one possible design for the first mirror 40 is shown in Fig. 6. The transmissive portion 42 of the first mirror 40 is configured to transmit light which is incident on the first mirror 40 though the first mirror 40 (i.e. through the thickness of the first mirror 40). By transmit, it will be understood that the transmissive portion 42 is configured to substantially transmit at least some wavelengths of visible light through a thickness of the first mirror 40. As such, when the first mirror 40 is in the second position, the transmissive portion may be configured to transmit at least some of the reference light from the reference light source through the first mirror 40 and towards the second mirror 50. The transmissive portion 42 of the first mirror may also be configured to transmit at least some of the axial light on the first optical path through the first mirror 40 and towards the axial focusing mirror 22. The reflective portion 44 may be configured to reflect reference light incident on the reflective portion 44 on to the first optical path. In some embodiments, the first mirror 40 comprises an optically transparent plate having a reflective coating partially covering a major surface of the optically transparent plate, wherein the reflective coating provides the reflective portion 44 of the first mirror 40. In the embodiment of Fig. 6, the reflective portion 44 of the first mirror 40 may be provided by a metallic film (e.g. aluminium, silver etc.) provided on a surface of a glass substrate. The transmissive portion 42 may define a region of the surface of the glass substrate on which the reflective portion 44 is not provided. In the embodiment of Fig. 6, the first mirror 40 comprises a glass substrate having a generally rectangular shape. The transmissive portion 42 is a circular or elliptical region located generally in the centre of the surface of the first mirror 40. The reflective portion 44 covers the remainder of the surface of the first mirror 40 surrounding the transmissive portion 42. As shown in Fig. 2, the first mirror 40 may be aligned relative to the reference light path such that the reference light path extends through the transmissive portion 42 of the first mirror 40 when the first mirror 40 is in the first position. As shown in Fig. 3, the first mirror 40 may also (or alternatively) be aligned relative to the reference light path such that the reference light path extends through the transmissive portion 42 of the first mirror 40 when the first mirror 40 is in the second position. As such, in the embodiment of Figs. 2 and 3, the reference light source may illuminate at least a portion of the second mirror 50 when the first mirror 40 is in the first position or the second position. In some embodiments, for example as shown in Figs. 4 and 5, the second mirror 50 may comprise a complementary reflective portion 52, such that when the second mirror 50 is in the second position, the complementary reflective portion 52 is configured to reflect reference light which is transmitted by the transmissive portion 42 of the first mirror 40. A plan view of a design for the second mirror 50 is shown in Fig. 7. In embodiments where the reference light on the reference light path is collimated, the complementary reflective portion 52 may have a design which is substantially the same shape and size as the transmissive portion 42. As such, the design of the second mirror 50 in Fig. 7 complements the design of the first mirror 40 shown in Fig. 6. As shown in Fig. 4, when the second mirror 50 is in the first position, the second mirror 50 does not occlude the second light path. The first position may ensure that any reference light incident on the second mirror 50 (i.e. reference light transmitted via the transmissive portion 42 of the first mirror 40) is not reflected on to the first or second light path. As shown in Fig. 5, when the second mirror 50 is in the second position, the complementary reflective portion 52 of the second mirror 50 is configured to reflect reference light which is incident on complementary reflective portion 52 on to the second optical path. So, as will be appreciated from at least Figs. 6 and 7, the transmissive portion 42 of the first mirror may define a first shape on the first mirror 40. The complementary reflective portion 52 of the second mirror may then define a complementary first shape on the second mirror 50 wherein the first shape and the complementary first shape are aligned along the reference light path. In some embodiments, such as the embodiment of Fig. 7, the second mirror 50 may also comprise a complementary transmissive portion 54. The complementary transmissive portion 54 may be configured to transmit light through the thickness of the second mirror 50. Thus, as shown in at least Fig. 5, when the second mirror 50 is in the second position, the second mirror 50 may allow at least some of the radial light to reach the entrance slit via the complementary transmissive portion 54 of the second mirror 50. In some embodiments, the second mirror 50 comprises an optically transparent plate having a reflective coating partially covering a major surface of the optically transparent plate, wherein the reflective coating provides the complementary reflective portion 52 of the second mirror. For example, the optically transparent plate may be a glass substrate which is similar to the glass substrate of the first mirror 40 in Fig. 6. The reflective coating may be provided by a metallic film, similar to the reflective portion 44 of the first mirror 40 of Fig. 6. While the embodiment of Figs. 2-7 shows first and second mirrors 40, 50 having a first shape and a complementary first shape which are generally circular, it will be appreciated that other patterns may be used for the first and second mirrors 40, 50. In some embodiments, the first mirror 40 may comprise a plurality of shapes, with the second mirror 50 comprising a plurality of complementary shapes. Figs. 8a and 8b show examples of a first and second mirror 40a, 50a having a plurality of shapes. As such the first mirror 40a of Fig. 8a comprises a plurality of transmissive portions 42a and the second mirror 50a of Fig. 8b comprises a plurality of complementary reflective portions 52a. While the first mirror 40a of Fig. 8a includes a single contiguous reflective portion 44a, in other embodiments a plurality of reflective portions 44a may be provided. The second mirror 50a of Fig. 8b includes a complementary transmissive portion 54a which corresponds to the shape of the reflective portion 44a. In some embodiments, the first shape and the complementary first shape may each define one or more of: a circle, a regular polygon, an irregular polygon, and an ellipse. For example, the first mirror 40b of Fig. 8c comprises a transmissive portion 42b having a first shape which is generally rectangular, and the second mirror 50b of Fig. 8d comprises a complementary reflective portion 52b which has a rectangular complementary first shape. The first mirror 40b includes a reflective portion 44b which surrounds the rectangular transmissive portion 42b. The second mirror 50b of Fig. 8d includes a complementary transmissive portion 54b which corresponds to the shape of the reflective portion 44b. The first mirror 40c of Fig. 8e comprises a transmissive portion 42c having a first shape which is generally elliptical, and the second mirror 50c of Fig. 8f comprises a complementary reflective portion 52c which has an elliptical complementary first shape. The first mirror 40c includes a reflective portion 44c which surrounds the elliptical transmissive portion 42c. The second mirror 50c of Fig. 8f includes a complementary transmissive portion 54c which corresponds to the shape of the reflective portion 44c. As will be appreciated from at least Figs. 2-5, the entrance slit 60 may receive light from the first optical path and the second optical path of the optical assembly 20. In order to select whether light from the first optical path or the second optical path is focused on to the entrance slit, the slit focusing mirror 24 may be an adjustable mirror. As such, in some embodiments, the slit focusing mirror 24 may be configured to be adjustable between a first position in which the slit focusing mirror 24 reflects light from the first optical path to the entrance slit 60 and a second position in which the slit focusing mirror 24 reflects light from the second optical path to the entrance slit 60. As such, in some embodiments, the slit focusing mirror 24 may be used to select the which light is incident on the entrance slit 60. In the embodiment of Figs. 2-5, the slit focusing mirror 24 is rotatable about an axis which extends normal to the plane defined by the radial and axial directions of light. In the first position, the slit focusing mirror 24 is configured to reflect light from the first optical path to the entrance slit 60. In the embodiments of Figs. 2-5, the slit focusing mirror 24 is configured to rotate clockwise from the first position to the second position where the slit focusing mirror is configured to reflect light from the second optical path to the entrance slit 60. It will be appreciated that the slit focusing mirror 24 is one example of an optical component which can be used to focus light from the first and second optical paths on to the entrance slit 60. In other embodiments, other combinations of optical components may be provided as part of an optical assembly 20 in order to provide similar functionality. In some embodiments, selection of the light incident on the entrance slit 60 may also be controlled by way of one or more shutters. For example, as shown in Fig. 9, the optical emission spectrometer 1 may comprise a reference light shutter 34. The reference light shutter 34 may be configurable to occlude the reference light source 30 from the reference light path. For example, the reference light shutter 34 may be located in front of the reference light source 30. The reference light shutter 34 may, in a first position (shown in Fig. 9), occlude the reference light path such that refence light is not incident on the first and / or second mirrors 40, 50. Ina second position (indicated schematically in Fig. 9 with dashed lines), the reference light shutter 34 may allow refence light to be incident on the first mirror 40 and / or the second mirror 50. As such, the reference shutter 34 may be used to control the transmission of reference light into the optical assembly 20 whilst maintaining the operation of the reference light source 30. That is to say, the use of a reference light shutter 34 may avoid repeatedly turning the reference light source 30 on and then off again each time a calibration measurement is to be performed. By using a reference light shutter 34 to reduce the number of times the reference light source is switched on and / or off, the lifetime of the reference light source 30 may be extended. Furthermore, some reference light source 30 may have a warm-up period when the reference light source 30 is turned on. During the warm-up period, the spectral characteristics of the reference light source 30 may change. Thus, by reducing the occasions where the reference light source 30 is turned on and / or off, the accuracy of the calibration measurements may be improved. In some embodiments, a plasma light shutter may be provided. The plasma light shutter may be configurable to occlude the plasma source 10 from the optical assembly 20. In some embodiments, a single plasma light shutter may be provided which may occlude the plasma source 10 from both the first and second optical paths. In the embodiment of Fig. 9, the plasma source 10 is provided with an axial light shutter 26 and a radial light shutter 28. The axial light shutter 26 may, in a first position (shown in Fig. 9), occlude the first light path such that axial light is not incident on the first mirror 40 and / or the axial focusing mirror 22. In a second position (indicated schematically in Fig. 9 with dashed lines), the axial shutter 26 may allow axial light to be incident on the first mirror 40 and / or the axial focusing mirror 22. The reference light shutter 28 may, in a first position (shown in Fig. 9), occlude the second light path such that axial light is not incident on the second mirror 50 and / or the radial focusing mirror 23. In a second position (indicated schematically in Fig. 9 with dashed lines), the radial shutter 28 may allow radial light to be incident on the second mirror 50 and / or the radial focusing mirror 23. Next a method 100 of optical emission spectrometry using the optical emission spectrometer 1 will be described. A block diagram of method 100 is shown in Fig. 10. In step 101, the method comprises performing a calibration measurement using the optical emission spectrometer 1. The calibration measurement performed may be a radial calibration measurement or an axial calibration measurement. To perform an axial calibration measurement, the first mirror 40 may be positioned in the second position such that reference light is directed from the reference light source 30 to the entrance slit 60 along the first optical path, wherein the optical emission spectrometer 1 detects a known wavelength of the reference light source. In the embodiment of Figs. 2-7, the slit focusing mirror 24 may also be positioned in the first position in order to direct light along the first optical path to the entrance slit 60. To perform a radial calibration measurement, the first mirror 40 may be positioned in the first or second position such that reference light is directed from the reference light source 30 to the second mirror 50. The second mirror is positioned in the second position such that reference light is directed to the entrance slit 60 along the second optical path, wherein the optical emission spectrometer 1 detects a known wavelength of the reference light source. In the embodiment of Figs. 2-7, the slit focusing mirror 24 may also be positioned in the second position in order to direct light along the second optical path to the entrance slit 60. Data generated by the detector 80 (as shown in Fig. 1) during the calibration measurement may be used to calibrate the optical emission spectrometer 1. For example, where the detector 80 is an array detector, the plurality of spectral peaks of the reference light may be imaged on the detector 80 during the calibration measurement. One or more of the spectral peaks of the reference light source may have a known wavelength. Thus, the optical emission spectrometer 1 may associate one or more detector locations with a respective wavelength based on the locations of spectral peaks of the reference light imaged on the detector. When performing calibration measurements using an optical emission spectrometer 1 comprising one or more shutters, the shutters may be operated to occlude the light source(s) not in use during the calibration measurement. For example in the embodiment of Fig. 9, the axial shutter 26 and the radial shutter 28 may be moved into respective first positions when performing a calibration measurement. When performing a calibration measurement, the reference light shutter 26 may be in the second position. For embodiments where one or more shutters for the plasma source 10 are not provided, the plasma source may, in some embodiments, be switched off briefly while the calibration measurement(s) are performed. Accordingly, the optical emission spectrometer 1 may be calibrated (or re-calibrated) using a reference light source 30 which is integrated into the optical emission spectrometer 1. As such, the calibration process may be performed automatically by the optical emission spectrometer 1 without any intervention or input from a user. Thus, the optical emission spectrometer 1 may switch between performing a sample measurement and a calibration measurement in a relatively quick manner. For example, the optical emission spectrometer 1 may repeatedly perform calibration measurements over a period of time in which sample measurements are being performed. Performing such calibration measurements (in a relatively short time duration) may allow the optical emission spectrometer 1 to perform sample measurements with improved accuracy without requiring a user to manually recalibrate the optical emission spectrometer 1 repeatedly. In step 102, the method may comprise performing a sample measurement using the optical emission spectrometer 1. The sample measurement performed may be an axial sample measurement or a radial sample measurement. To perform an axial sample measurement, the first mirror 40 may be positioned in the first position and the slit focusing mirror 24 may be positioned in the first position such that light is directed from the plasma source 10 to the entrance slit 60 of the optical emission spectrometer 1 along the first optical path. The light incident on the entrance slit is then imaged by the detector 80. To perform a radial sample measurement, the second mirror 50 may be positioned in the first position such that light is directed from the plasma source 10 to the entrance slit 60 of the optical emission spectrometer 1 along the second optical path. In the embodiment of Figs. 2-7, the slit focusing mirror 24 may also be positioned in the second position in order to direct light along the second optical path to the entrance slit 60. The light incident on the entrance slit is then imaged by the detector 80. When performing a sample measurement (e.g. a radial sample measurement or an axial sample measurement), the optical emission spectrometer 1 may utilise data from a calibration measurement (e.g. a radial calibration measurement and / or an axial calibration measurement) when assigning wavelengths to the data generated by the detector 80. For example, the optical emission spectrometer 1 may utilise radial calibration information from a radial calibration measurement to calibrate a radial sample measurement. The optical emission spectrometer 1 may also utilise axial calibration information from an axial calibration measurement to calibrate an axial sample measurement. When performing sample measurements using an optical emission spectrometer 1 comprising one or more shutters, the shutters may be operated to occlude the light source(s) not in use during the sample measurement. For example in the embodiment of Fig. 9, the reference light shutter 26 may be moved into the first position when performing a sample measurement. For sample measurements, the slit focusing mirror 24 may be used to select whether light from the first or second optical path is imaged on the slit. As such, the axial and radial shutter 26, 28 may both be in the second position when performing sample measurements. By utilising a reference light source 30 to calibrate the optical emission spectrometer 1, the optical emission spectrometer 1 may switch between performing a calibration measurement and a sample measurement in a straightforward manner (e.g. by movement of the first mirror 40). As such, the optical emission spectrometer 1 may not require any changes to the sample being measured (e.g. measurement of a calibration mixture or an internal standard) in order to calibrate the spectrometer. As such, the optical emission spectrometer 1 may be calibrated in a reliable and robust manner. Furthermore, the calibration measurements may be performed in real-time in order to correct for drift of the spectrometer throughout a series of sample measurements. In some embodiments, one or more calibration measurements of the optical emission spectrometer 1 may be performed in advance of a performing a sample measurement (e.g. as shown in method 100 of Fig. 10. In some embodiments, where a plurality of sample measurements are to be performed, calibration measurements may be interleaved throughout the sample measurements. As such, each sample measurement performed may utilise calibration information for the optical emission spectrometer 1 which has been obtained in real-time. One example of a method 200 of optical emission spectrometry is shown in Fig. 11. In Fig. 11, an axial calibration measurement 201 and a radial calibration measurement 202 are performed initially. A plurality of axial sample measurements 203 and a plurality of radial sample measurements 204 are then performed. In the embodiment of Fig. 11, the sample measurements alternate between radial and axial sample measurements 203, 204 using the optical emission spectrometer 1. After performing a number of radial and axial sample measurements 203, 204 (e.g. at least 3, 5, 7 or 10, 15 or 20), the optical emission spectrometer 1 may repeat the axial calibration measurement 201a and the radial calibration measurement 202a, before performing further radial and axial sample measurements 203a, 204a. The further radial and axial sample measurements 203a, 204a may utilise calibration information from the most recent calibration measurements 201a, 202a performed. By interleaving calibration measurements in real-time, the optical emission spectrometer 1 may perform a plurality of sample measurements with improved accuracy. In particular, by repeating the calibration throughout a set of sample measurements, the optical emission spectrometer may account for any measurement drift which may occur over the course of the experiment. By way of example, in some embodiments, the optical emission spectrometer 1 may perform a plurality of sample measurements, 203, 204 each sample measurement having a duration of no greater than: 2000 ms, 1000 ms, 500 ms, or 250 ms. In order to switch from performing a sample measurement to performing a further calibration measurement, a switching time of no greater than: 2000 ms, 1000 ms, or 500 ms may be provided. The optical emission spectrometer may then perform one or calibration measurement, each calibration measurement having a duration of no greater than: 2000 ms, 1000 ms, 500 ms, or 250 ms. In order to switch from performing a calibration measurement to performing further sample measurements, a switching time of no greater than: 2000 ms, 1000 ms, or 500 ms may be provided. As such, when performing a series of sample measurements, interleaving a calibration measurement between successive sample measurements may introduce a delay of no greater than: 6000 ms, 4000 ms, 2000 ms, or 1000 ms. As such, it will be appreciated that the optical emission spectrometer may switch between calibration and sample measurements without an excessive delay between sample measurements performed before and after the calibration measurements. As such, in some embodiments, the optical emission spectrometer 1 may perform a plurality of sample measurements. For example, the plurality of sample measurements may be performed at a frequency of at least: 0.5 Hz, 1 Hz, 2 Hz, or 4 Hz. While performing a plurality of sample measurements, the optical emission spectrometer 1 may interleave calibration measurements. The calibration measurements may be repeated during the sample measurements in order to correct for any drift of the optical emission spectrometer 1. For example, the optical emission spectrometer 1 may perform a calibration measurement at a frequency of at least: 0.01 Hz, 0.02 Hz, 0.05 Hz, 0.1 Hz, or 0.2 Hz. In some embodiments, the optical emission spectrometer 1 may perform a calibration 5 measurement after performing no more than: 100 sample measurements, 50 sample measurements, 30 sample measurements, 20 sample measurements, or 10 sample measurements. As such, the optical emission spectrometer 1 may automatically perform repeat calibration measurements over the duration of a plurality of sample measurements in order to maintain a relatively high degree of measurement accuracy / confidence over the 10 duration of the sample measurements. Thus, in accordance with embodiments of the disclosure, an optical emission spectrometer 1 and a method 100, 200 of optical emission spectrometry is provided. It will be appreciated that the above disclosure is not limited to the embodiments discussed above 15 and that various modifications will be apparent to the skilled person. The scope of the invention is defined by the following claims.

Claims

1. An optical emission spectrometer comprising:a plasma source configured to emit light;an optical assembly comprising a plurality of optical components, the optical assembly configured to direct light travelling in a first direction from the plasma source to an entrance slit of the optical emission spectrometer along a first optical path;a reference light source for calibrating the spectrometer, the reference light source configured to output reference light along a reference light path;a first mirror configured to selectively switch the refence light on to the first optical path such that the reference light is directed to the entrance slit for calibration of the optical emission spectrometer.

2. An optical emission spectrometer according to claim 1, whereinthe first mirror is configured to be moveable between:a first position away from the first optical path, anda second position in which the first mirror is configured to reflect reference light from the reference light path on to the first optical path.

3. An optical emission spectrometer according to claim 1 or claim 2, wherein the reference light path intersects the first optical path.

4. An optical emission spectrometer according to any of claims 1 to 3, whereinthe plasma source is configured to produce axial light in an axial direction and radial light in a radial direction, wherein the axial direction is the first direction.

5. An optical emission spectrometer according to any of claims 1 to 3, whereinthe plasma source is configured to produce axial light in an axial direction and radial light in a radial direction, wherein the radial direction is the first direction.

6. An optical emission spectrometer according to claim 4, whereinthe optical assembly comprises a plurality of optical components which are configured to direct radial light in the radial direction from the plasma source to the entrance slit of the optical emission spectrometer along a second optical path; andthe optical emission spectrometer comprises:a second mirror configured to selectively switch the refence light onto the second optical path for calibration of the optical emission spectrometer.

7. An optical emission spectrometer according to claim 6, whereinthe second mirror is configured to be moveable between:a first position away from the second optical path, anda second position in which the second mirror is configured to reflect reference light from the reference light path on to the second optical path such that reference light is directed to the entrance slit of the optical emission spectrometer.

8. An optical emission spectrometer according to claim 7, whereinthe first mirror comprises a reflective portion and a transmissive portion, wherein when the first mirror is in the second position the reflective portion is configured to reflect reference light incident on the reflective portion on to the first optical path, and the transmissive portion is configured to transmit reference light incident on the transmissive portion towards the second mirror.

9. An optical emission spectrometer according to claim 8, whereinthe second mirror comprises a complementary reflective portion, such that when the second mirror is in the second position, the complementary reflective portion is configured to reflect reference light which is transmitted by the transmissive portion of the first mirror.

10. An optical emission spectrometer according to claim 9, whereinthe transmissive portion defines a first shape on the first mirror, and the complementary reflective portion defines a complementary first shape on the second mirror, wherein the first shape and the complementary first shape are aligned along the reference light path.

11. An optical emission spectrometer according to claim 10, whereinthe first shape and the complementary first shape may each define one or more of: a circle, a regular polygon, an irregular polygon, and an ellipse.

12. An optical emission spectrometer according to any of claims 8 to 11, whereinthe first mirror comprises an optically transparent plate having a reflective coating partially covering a major surface of the optically transparent plate, wherein the reflective coating provides the reflective portion of the first mirror.

13. An optical emission spectrometer according to any of claims 8 to 12, wherein the second mirror comprises an optically transparent plate having a reflective coating partially covering a major surface of the optically transparent plate, wherein the reflective coating provides the complementary reflective portion of the second mirror14. An optical emission spectrometer according to any of claims 2 to 13, wherein when in the first position, the first mirror intersects the reference light path.

15. An optical emission spectrometer according to any of claims 1 to 14, further comprising:an optical fibre which is configured to direct reference light from the reference light source towards the first and second mirrors.

16. An optical emission spectrometer according to any of claims 6 to 15, whereinthe optical assembly comprises:an adjustable mirror;a first focusing mirror provided on the first optical path, the first focusing mirror configured to:receive axial light from the plasma source and reference light from the first mirror, andto focus the axial light and the reference light to the adjustable mirror of the optical assembly; anda second focusing mirror provided on the second optical path, the second focusing mirror configured to:receive radial light from the plasma source and the reference light from the second mirror, andto focus the radial light the reference light to the adjustable mirror of the optical assembly,wherein the adjustable mirror is configured to be adjustable between a first position in which the adjustable mirror reflects light from the first optical path to the entrance slit ofthe spectrometer and a second position in which the adjustable mirror reflect light from the second optical path to the entrance slit of the spectrometer.

17. An optical emission spectrometer according to any of claims 1 to 16, wherein the reference light source comprises one or more of: a hollow cathode lamp, a narrowband LED, a laser, a glow discharge lamp and a spectral lamp.

18. An optical emission spectrometer according to any of claims 1 to 17, further comprising one or more of:a reference light shutter configurable to occlude the reference light source from the reference light path;a plasma light shutter configurable to occlude the plasma source from the optical assembly.

19. An optical emission spectrometer according to any of claims 1 to 18, further comprising:a detector; andan echelle grating configured to receive light from the entrance slit and to direct the light to the detector, wherein an echelle spectrum comprising a plurality of orders is imaged on the detector.

20. An optical emission spectrometer according to any of claims 1 to 18, further comprising:a detector; anda concave grating configured to receive light from the entrance slit and to direct the light to the detector, wherein a linear spectrum is imaged on the detector.

21. A method of optical emission spectrometry using the optical emission spectrometer of any of claims 1 to 20, the method comprising:performing a calibration measurement in which the first mirror is positioned such that reference light is directed from the reference light source to the entrance slit of the spectrometer along the first optical path, wherein the optical emission spectrometer detects a known wavelength of the reference light source.

22. A method of optical emission spectrometry according to claim 20, the method further comprising:performing a sample measurement in which the first mirror is positioned such that light is directed from the plasma source to the entrance slit of the optical emission spectrometer along the first optical path.

23. A method of optical emission spectrometry using the optical emission spectrometer of any of claims 6 to 20, the method comprising:performing an axial calibration measurement in which the first mirror is positioned such that reference light is directed from the reference light source to the entrance slit of the spectrometer along the first optical path, wherein the optical emission spectrometer detects a known wavelength of the reference light source; andperforming a radial calibration in which the second mirror is positioned such that reference light is directed from the reference light source to the entrance slit of the spectrometer along the second optical path, wherein the optical emission spectrometer detects a known wavelength of the reference light source.

24. A method according to claim 23, further comprisingperforming an axial sample measurement in which the first mirror is positioned such that axial light is directed from the plasma source to the entrance slit of the optical emission spectrometer; and / orperforming a radial sample measurement in which the second mirror is positioned such that radial light is directed from the plasma source to the entrance slit of the optical emission spectrometer.

25. A method according to claim 24, whereina plurality of axial sample measurements and / or a plurality of radial sample measurements are performed, andat least one axial calibration measurement and / or at least one radial calibration measurement is performed during the plurality of sample measurements.“Amendments to the claims have been filed as followsCLAIMS:

1. An optical emission spectrometer comprising:a plasma source configured to emit light;5 an optical assembly comprising a plurality of optical components, the opticalassembly configured to direct light travelling in a first direction from the plasma source to an entrance slit of the optical emission spectrometer along a first optical path;a reference light source for calibrating the spectrometer, the reference light source configured to output reference light along a reference light path;10 a first mirror configured to selectively switch the refence light on to the first opticalpath such that the reference light is directed to the entrance slit for calibration of the optical emission spectrometer, whereinthe first mirror is configured to be moveable between:a first position away from the first optical path, and15 a second position in which the first mirror is configured to reflect referencelight from the reference light path on to the first optical path.

2. An optical emission spectrometer according to claim 1, wherein the reference light path intersects the first optical path.

203. An optical emission spectrometer according to any of claims 1 to 2, wherein the plasma source is configured to produce axial light in an axial direction and radiallight in a radial direction, wherein the axial direction is the first direction.25 4. An optical emission spectrometer according to any of claims 1 to 2, whereinthe plasma source is configured to produce axial light in an axial direction and radial light in a radial direction, wherein the radial direction is the first direction.

5. An optical emission spectrometer according to claim 3, wherein30 the optical assembly comprises a plurality of optical components which areconfigured to direct radial light in the radial direction from the plasma source to the entrance slit of the optical emission spectrometer along a second optical path; and the optical emission spectrometer comprises:a second mirror configured to selectively switch the refence light onto the second35 optical path for calibration of the optical emission spectrometer.

6. An optical emission spectrometer according to claim 5, wherein the second mirror is configured to be moveable between: a first position away from the second optical path, and5 a second position in which the second mirror is configured to reflect reference lightfrom the reference light path on to the second optical path such that reference light is directed to the entrance slit of the optical emission spectrometer.

7. An optical emission spectrometer according to claim 8, wherein10 the first mirror comprises a reflective portion and a transmissive portion, whereinwhen the first mirror is in the second position the reflective portion is configured to reflect reference light incident on the reflective portion on to the first optical path, and the transmissive portion is configured to transmit reference light incident on the transmissive portion towards the second mirror.I PlU /  15CM8. An optical emission spectrometer according to claim 7, wherein'l““ the second mirror comprises a complementary reflective portion, such that when thesecond mirror is in the second position, the complementary reflective portion is configured “ to reflect reference light which is transmitted by the transmissive portion of the first mirror.CO 209. An optical emission spectrometer according to claim 8, wherein the transmissive portion defines a first shape on the first mirror, and the complementary reflective portion defines a complementary first shape on the second mirror, wherein the first shape and the complementary first shape are aligned along the25 reference light path.

10. An optical emission spectrometer according to claim 9, wherein the first shape and the complementary first shape may each define one or more of:a circle, a regular polygon, an irregular polygon, and an ellipse.3011. An optical emission spectrometer according to any of claims 7 to 10, wherein the first mirror comprises an optically transparent plate having a reflective coatingpartially covering a major surface of the optically transparent plate, wherein the reflective coating provides the reflective portion of the first mirror.3512. An optical emission spectrometer according to any of claims 7 to 11, wherein the second mirror comprises an optically transparent plate having a reflective coating partially covering a major surface of the optically transparent plate, wherein the reflective coating provides the complementary reflective portion of the second mirror513. An optical emission spectrometer according to any of claims 1 to 12, wherein when in the first position, the first mirror intersects the reference light path.

14. An optical emission spectrometer according to any of claims 1 to 13, further10 comprising:an optical fibre which is configured to direct reference light from the reference light source towards the first and second mirrors.15253015. An optical emission spectrometer according to any of claims 5 to 14, whereinthe optical assembly comprises:an adjustable mirror;a first focusing mirror provided on the first optical path, the first focusing mirror configured to:receive axial light from the plasma source and reference light from the first mirror, andto focus the axial light and the reference light to the adjustable mirror of the optical assembly; anda second focusing mirror provided on the second optical path, the second focusing mirror configured to:receive radial light from the plasma source and the reference light from the second mirror, andto focus the radial light the reference light to the adjustable mirror of the optical assembly,wherein the adjustable mirror is configured to be adjustable between a first position in which the adjustable mirror reflects light from the first optical path to the entrance slit of the spectrometer and a second position in which the adjustable mirror reflect light from the second optical path to the entrance slit of the spectrometer.

16. An optical emission spectrometer according to any of claims 1 to 15, whereinthe reference light source comprises one or more of: a hollow cathode lamp, a narrowband LED, a laser, a glow discharge lamp and a spectral lamp.101517. An optical emission spectrometer according to any of claims 1 to 16, further comprising one or more of:a reference light shutter configurable to occlude the reference light source from the reference light path;a plasma light shutter configurable to occlude the plasma source from the optical assembly.

18. An optical emission spectrometer according to any of claims 1 to 17, further comprising:a detector; andan echelle grating configured to receive light from the entrance slit and to direct the light to the detector, wherein an echelle spectrum comprising a plurality of orders is imaged on the detector.

19. An optical emission spectrometer according to any of claims 1 to 17, further comprising:a detector; anda concave grating configured to receive light from the entrance slit and to direct the light to the detector, wherein a linear spectrum is imaged on the detector.

20. A method of optical emission spectrometry using the optical emission spectrometer25 of any of claims 1 to 19, the method comprising:performing a calibration measurement in which the first mirror is positioned such that reference light is directed from the reference light source to the entrance slit of the spectrometer along the first optical path, wherein the optical emission spectrometer detects a known wavelength of the reference light source.3021. A method of optical emission spectrometry according to claim 20, the method further comprising:performing a sample measurement in which the first mirror is positioned such that light is directed from the plasma source to the entrance slit of the optical emission35 spectrometer along the first optical path.

22. A method of optical emission spectrometry using the optical emission spectrometer of any of claims 5 to 19, the method comprising:performing an axial calibration measurement in which the first mirror is positioned5 such that reference light is directed from the reference light source to the entrance slit of the spectrometer along the first optical path, wherein the optical emission spectrometer detects a known wavelength of the reference light source; andperforming a radial calibration in which the second mirror is positioned such that reference light is directed from the reference light source to the entrance slit of the10 spectrometer along the second optical path, wherein the optical emission spectrometer detects a known wavelength of the reference light source.LD1523. A method according to claim 22, further comprisingperforming an axial sample measurement in which the first mirror is positioned such that axial light is directed from the plasma source to the entrance slit of the optical emission spectrometer; and / orperforming a radial sample measurement in which the second mirror is positioned such that radial light is directed from the plasma source to the entrance slit of the opticalemission spectrometer.2024. A method according to claim 23, whereina plurality of axial sample measurements and / or a plurality of radial sample measurements are performed, andat least one axial calibration measurement and / or at least one radial calibration25 measurement is performed during the plurality of sample measurements.

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