Autofocus LIBS system

Abstract

The LIBS analyzer (2) comprises: a focusing lens device (6); a laser (8) for propagating a laser beam (L) through the focusing lens device (6) to focus at its focal plane (F); a detector (10) for generating light intensity data; a translation mechanism (22) for changing the positioning point of the focal plane (F); and a controller (14) for automatically controlling the translation mechanism (22) to achieve an optimal position in which the focal plane (F) and the upper surface (18) of the sample (20) coincide, calculated by the controller (14) based on applying a mathematical transformation relating light intensity to the optimal position to the light intensity data of the plasma generated by the laser beam (L).

Description

Technical Field

[0001] The present application relates to an auto-focusing laser induced breakdown spectroscopy ('LIBS') analysis system and a method of auto-focusing in a LIBS analysis system. Background Art

[0002] LIBS analysis systems are known and used to detect the presence and / or concentration of lower atomic number elements in a sample with a certain degree of accuracy. These devices typically include a high-powered laser that heats a portion of the sample sufficiently to create a plasma. As the plasma cools, eventually the electrons return to their ground state. During this process, photons are emitted at wavelengths that are unique to the specific elements that make up the sample. The detection and analysis of these photons in a LIBS analysis system enables quantitative and / or qualitative determination of the elemental composition of the sample.

[0003] In order to achieve a high enough power density to generate a plasma, the laser must be focused to a small spot size, typically 100 picometers or less. The very small beam spot size on the sample requires that the laser must be precisely focused on the surface of the sample being analyzed to obtain consistent analytical results. Since the sample being analyzed is usually non-uniform, at least relative to the size of the laser beam sampling area, LIBS analysis systems usually provide relative motion between the incident laser beam and the sample surface so that the sample can be analyzed at multiple different surface areas to obtain a more representative determination of its elemental composition. However, the sample being analyzed often has a relatively uneven surface, so the laser must be repeatedly refocused as the sample is moved to expose different areas of its surface for LIBS analysis.

[0004] From US 9,267,842, it is known that a handheld autofocus LIBS analysis system is provided, comprising a focusing lens configured to move along a first axis, an ablation laser having an output directed along the first axis and passing through the focusing lens, a spectrometer for outputting intensity data from a sample mounted for relative movement in a plane perpendicular to the first axis, and a controller system, the controller system being responsive to the output of the spectrometer and configured to effect relative movement of the sample to expose a plurality of different regions for analysis, and repeatedly stimulating the laser at each different region, processing the output of the spectrometer, and adjusting the position of the focusing lens relative to the sample until the output of the spectrometer indicates a maximum or near-maximum intensity of the plasma generated by the ablation laser output focused on a point in the region of the sample. Unfortunately, this autofocus scheme requires multiple measurements to be taken at each sampling region, each time at a different position of the lens along the first axis, in order to establish an optimal focus of the laser output for LIBS analysis of the region. This results in increased analysis time for the sample, and may result in errors in the analysis results due to a large number of laser ablation instances of the same region. Summary of the invention

[0005] According to a first aspect of the present invention, a LIBS analysis system is provided, comprising: a focusing lens having a focal plane; a laser configured to propagate a laser beam through the focusing lens in a direction along an optical path to focus on the focal plane; a detector having an output proportional to the intensity of incident electromagnetic radiation; a sample holder for holding a sample, whose upper surface intersects the optical path; a translation mechanism operable to achieve relative movement of the sample holder and the focusing lens to change the location of the focal plane relative to the sample holder along the optical path; and a controller configured to automatically control the operation of the translation mechanism to achieve relative movement to achieve an optimal position in which the focal plane and an analysis area of ​​the upper surface intersecting the optical path are in coincidence or near coincidence, wherein the controller is further configured to calculate the optimal position based on a mathematical transformation stored in a memory accessible to the controller, the transformation associating measurement data of an area of ​​the upper surface intersecting the optical path with the optimal position, and generating output using electromagnetic radiation obtained from a plasma generated by the laser beam striking one or more other areas of the upper surface of the sample. By using a transformation to calculate rather than measure the optimal position of the laser at each sampling area, the number of non-analytical measurements is reduced.

[0006] In an embodiment, the controller is configured to control the LIBS analysis system to perform a profile generation cycle, during which the controller is configured to operate the translation mechanism to achieve relative motion to achieve a plurality of different positions of the focal plane at a first other region of the upper surface along the optical path relative to the sample holder; operate the laser to generate plasma at each of the plurality of different positions, and retrieve a representation of the detector output obtained from the electromagnetic radiation from the plasma generated at each of the plurality of different positions into a memory as intensity data indexed with the position for generating a mathematical transformation. In some embodiments, the controller is configured to process the intensity data to generate a mathematical expression relating the detector output to the position of the focal plane and store the mathematical expression in the memory as a mathematical transformation. Thus, the number of plasma generation events performed in the sampling area for the purpose of autofocusing is reduced, preferably to one event.

[0007] In one embodiment, the controller is configured to operate the translation mechanism to move the sample stage in a plane perpendicular to the optical path to continuously intersect multiple different other areas of the upper surface with the optical axis, each different other area having a different known positioning point in the plane; operate the laser beam to generate plasma at each of the multiple different other areas, and generate a mathematical transformation based on the comparison of the detector output at each of the different known positioning points in the plane with the intensity data. The mathematical transformation links the information identifying the optimal position with the positioning point of the area in the plane perpendicular to the optical path, and the controller is configured to operate the LIBS system to collect the positioning point of the analysis area as measurement data. This has the following advantages, that is, the optimal positioning point of the area on the sample surface can be calculated based on the knowledge of the positioning point of the area, without having to ablate the area first. Therefore, LIBS analysis measurements can be performed on unablated areas of the sample surface.

[0008] According to a second aspect of the present invention, a method for operating a LIBS analysis system according to the first aspect is provided, comprising automatically adjusting the focus positioning point of the laser beam output by the laser of the LIBS analysis system to an optimal positioning point by automatically controlling the operation of a translation mechanism to achieve relative movement so as to achieve an optimal positioning point where the focal plane and the area of ​​the upper surface intersecting with the optical path are in coincidence or close to coincidence, wherein the method further comprises using a controller to generate a mathematical transformation, which mathematical transformation associates the measurement data of the upper surface area intersecting with the optical path with the optimal mathematical transformation, and generates using an output obtained from electromagnetic radiation of a plasma generated by irradiating the laser beam to one or more other areas of the upper surface of the sample; obtaining measurement data of the upper surface area and applying the mathematical transformation to the measurement data to determine the optimal positioning point. Therefore, the advantages corresponding to the first aspect can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] These and other advantages of the present invention will be better understood by considering the following description of one or more non-limiting embodiments with reference to the accompanying drawings, in which:

[0010] Figure 1 shows a schematic functional block diagram of a LIBS system according to the present invention;

[0011] Figure 2 Shows Figure 1 Plan view of the sample and sample stage of the LIBS system represented;

[0012] Figure 3 A flow chart showing the main steps associated with the operation of a LIBS system according to the present invention;

[0013] Figure 4 shows a plot of the intensity of two carbon-related plasma emissions versus position along the z-axis;

[0014] Figure 5 A flow chart showing the main steps associated with a LIBS system analysis cycle according to the present invention;

[0015] Figure 6 A flow chart showing the main steps associated with the generation of a mathematical transformation according to an embodiment of the method of the present invention;

[0016] Figure 7 It shows that the Figure 6 A flow chart of the main steps associated with the analysis cycle of the LIBS system according to the present invention for generating mathematical transformations; and

[0017] Figure 8 A grid plot showing the motion data of the sample surface. DETAILED DESCRIPTION

[0018] According to an embodiment of the LIBS analysis system 2 of the present invention, Figure 1 It is schematically shown as comprising a sample stage 4; a focusing lens device 6; a laser 8; a detector 10; an element composition determination unit 12 and a controller 14.

[0019] The laser 8 is oriented so that when it is excited, in this embodiment, under the control of the controller 14, a collimated laser beam L is generated, which is guided along an optical path O through a focusing lens device 6 and toward the sample stage 4 to impact an analysis area 40 of the upper surface 18 of the sample 20 located on the sample stage 4. Typically, the sample 20 has a known size and an approximate geometry, and in this embodiment is a cylindrical sample with a circular cross-section. In the following, it will be assumed that the sample 20 has a generally convex upper surface 18, with its highest point located at the center. This is typically the shape presented by a LIBS sample formed of pressed particles or granular materials. However, as will be explained below, such knowledge of the sample geometry and topology is not necessary for the present invention.

[0020] The focusing lens device 6 is configured to focus the laser beam L on a focal plane F located at a given position along the optical path O. The relative movement of the focusing lens device 6 and the sample stage 4 can be performed under the control of the controller 14 so that the focal plane F coincides (or nearly coincides) with the upper surface 18 of the sample 20. When such coincidence (or nearly coincidence) is achieved, the plasma is generated by the laser beam L hitting the analysis area 40 of the upper surface 18, and it has a maximum (or nearly maximum) intensity.

[0021] The controller 14 may include one or more microprocessors, data processors, memory devices, digital signal processors, analog and / or digital circuits or similar components, and / or application specific integrated circuit devices and may be distributed, for example, one data processor may be associated with the elemental composition determination unit 12, while another data processor may be configured to determine intensity distribution data (as described below), and a microcontroller may be associated with the laser 8, and another microcontroller may be associated with the translation mechanism 22. The same is true for the algorithms, software, firmware, etc. used to configure the controller 14 to operate as described below.

[0022] A translation mechanism 22 is provided to achieve relative motion and may include a stepper motor, a worm drive, an electromagnetic coil, or other translation device. In the present embodiment, the translation mechanism is configured to control the movement of the sample stage 4 at least in a direction parallel to the optical path O (the "z" direction) in response to instructions from the controller 14. In the present embodiment, the translation mechanism 22 is also configured to achieve the movement of the sample stage 4 in a plane perpendicular to the z direction ( Figure 1 and Figure 2 The translation mechanism 22 is configured to control the movement of the sample stage 4 independently in three orthogonal directions (x-direction, y-direction, and z-direction). In some embodiments, the translation mechanism 22 can be distributed, including two or more separate units, and can additionally or alternatively control the movement of the focusing lens device 6 to achieve one or both of the z-direction and the relative movement in the xy plane. In some embodiments, the translation mechanism 22 can be configured to achieve rotation of the sample stage 4 (and therefore the sample 20) in the xy plane around an axis parallel to the z-direction.

[0023] The detector 10 is configured to generate an output proportional to the intensity of the electromagnetic radiation incident thereon. In the present embodiment, the detector 10 comprises a spectrometer configured to generate an electrical signal output indicative of the amplitude of the received electromagnetic radiation as a function of wavelength for processing by one or both of the controller 14 and the elemental composition determination unit 12. The detector 10 is positioned to receive electromagnetic radiation from a plasma generated by a laser beam L generated by a laser 8 striking an upper surface 18 of a sample 20. The electromagnetic radiation from the plasma travels along a detection path, which in the present embodiment comprises a focusing lens arrangement 6; an optical element 24, and is connected to the detector 10.

[0024] The optical element 24 is located along the optical path O between the laser 8 and the focusing lens arrangement 6 and is formed to pass the laser beam L and reflect lower energy electromagnetic radiation from the plasma in a direction towards the detector 10 .

[0025] The elemental composition determination unit 12 is configured to receive the electrical signal output from the detector 10 and determine the composition of the analysis area 40 of the sample 20 impacted by the focused laser beam L in a manner known in the art of LIBS analysis. In some embodiments, the operation of the elemental composition determination unit 12 is controlled by the controller 14 so that the composition determination is performed only when the focal plane F coincides (or nearly coincides) with the upper surface 18 of the sample 20.

[0026] The controller 14 is configured by suitable programming to operate the LIBS system 2 substantially as follows Figure 3 4, so as to perform a profile generation cycle for each sample 20 placed on the sample stage 4. During the profile generation cycle, intensity data is collected for a first other area 16 of the upper surface 18 that is different from the analysis area 40. The intensity data represents intensity values ​​measured at a plurality of different positions along the optical path O relative to the position index along the focal plane F for the focal plane F. In the present embodiment, the controller 14 actuates the translation mechanism 22 to position the sample stage 4 at an initial position along the optical path O relative to the focal plane F ( Figure 3 2), and preferably, but not essentially, aligning the center of the upper surface 18 with the optical path O so that the intensity data subsequently generated will be for the first other area 16, here at the highest part of the sample surface 18. Then, in this embodiment, the controller 14 activates the laser 8 to generate a laser beam L, which strikes the upper surface 18 of the sample at the first other area 16. The electrical signal output received from the detector 10 is stored in the memory 26 indexed relative to the representation of the initial position ( Figure 3 The controller 14 then operates to actuate the translation mechanism 22 to move the sample stage 4 to a new position ( Figure 3 Step 6); causing the laser 8 to emit the light beam L again to hit the first other area 16 and store the electrical signal output received from the detector 10 in the memory 26 with respect to the index of the new position ( Figure 38) as an additional element of the intensity data. These steps (steps 6 and 8) are repeated until it is determined in the controller (step 10) that the final position along the optical path O has been reached. The initial position and the final position of the sample stage 4 along the optical path O relative to the focal plane F correspond to a selected predetermined position range, which is selected to ensure that the position where the focal plane F coincides (or nearly coincides) with the upper surface 18 of the sample 20 on the sample stage 4 is surrounded. This position represents the optimal position where the intensity of the electromagnetic radiation incident on the detector 10 is maximum (or close to maximum). In one embodiment, the change in position is performed iteratively so that the sample stage 4 adopts discrete positions between the initial position and the final position. In another embodiment, the change in position is performed substantially continuously between the initial position and the final position. In some embodiments, the laser 8 can be excited by the controller 14 only during one direction of movement of the sample stage 4, while in other embodiments, the laser 8 is excited during bidirectional movement of the sample stage 4.

[0027] In some embodiments, the controller 14 can energize the translation mechanism 22 during the intensity data generation cycle to move the sample stage 4, for example, to move a circle (such as Figure 2 The upper surface 18 is exposed in a first other region 16, whereby a new portion of the upper surface 18 is exposed for each new position of the sample stage 4 along the optical path O. Each new portion of the first other region 16 should be sufficiently close to the previous new portion of the region 16 so that the topography of the upper surface 18 does not change significantly (i.e., there is no measurable effect on the plasma due to height changes), and the composition remains constant within the limits that changes in composition do not significantly affect the plasma intensity.

[0028] In some embodiments, the intensity of the plasma is measured at one or more specific wavelengths at each position between the initial position and the final position. In some embodiments, two specific wavelengths may be selected, such as Figure 4 C 193 (dark line) and C 230 (bright line) of carbon plasma emission lines at 193 nanometers (nm) and 230 nm. 193 The emission lines are due to neutral carbon, while C 230 is due to ionized carbon and occurs at a rate greater than C 193 Due to the characteristics of the generated plasma, a higher temperature occurs in the plasma at a certain distance above the sample upper surface 18. This in turn means that the position of the sample stage 4 along the z direction where the maximum plasma intensity is reached is the same for each emission line (C 193 and C 230) will be different. This difference can be used to determine the direction to move the sample stage 4 in the autofocus procedure described below. It should be understood that the elemental emission line can be selected based on the expected sample composition, for example, the emission line due to the element constituting the basic sample matrix should be selected rather than the emission line due to potential sample impurities (for example, carbon, hydrogen or nitrogen emission lines in organic samples). Another emission line can be selected to represent the continuous spectrum plasma emission, because it is known that the intensity of the continuous spectrum emission increases with the increase of plasma temperature.

[0029] Once the controller 14 has been operated to achieve all desired positions between the initial position and the final position, the controller 14 is configured to access the memory 26 ( Figure 3 Step 12) and processing the stored intensity data to identify the optimal position (z) of the sample stage 4 along the z direction corresponding to the maximum (or near-maximum) intensity of the stored intensity data. OPT )( Figure 3 Step 12a), and activating the translation mechanism 22 to move the sample stage 4 to the optimal position thus determined ( Figure 3 Step 12b). For example, from Figure 4 It can be seen that in this embodiment, z OPT About 0.6 millimeters (mm).

[0030] The controller 14 is also configured to control the LIBS system 2 to perform a calibration generation cycle ( Figure 3 Step 14), during which cycle the controller 14 uses the intensity data stored in the memory 26 to generate a mathematical transformation that relates the optimal position along the z direction to a measurable characteristic of an area of ​​the sample upper surface 18 and stores the mathematical transformation in the memory 26 for use in the autofocus routine described below.

[0031] In some embodiments, the calibration generation cycle (step 14) includes generating a mathematical transformation that relates the position of the sample stage 4 along the z direction to the intensity using the intensity data stored in the memory 26, the intensity data being obtained from the first other region 16 of the upper surface 18 of the sample 20 during the profile generation cycle. In these embodiments, the analysis cycle (see Figure 5 ), the controller 14 operates the translation mechanism 22 to move the sample 20 in the xy plane to present an analysis area 40 of the upper surface 18, which is not used in the mathematical transformation generation in step 14. In step 18, the controller 14 operates the laser 8 to generate plasma from the analysis area 40, and the intensity of the generated plasma is obtained as the measurement data of the area of ​​the sample upper surface 18, and the mathematical transformation converts the optimal position z to OPTThe controller 14 then initiates an autofocus routine at step 20 during which a mathematical transformation is applied to the intensity obtained and the position along the z-axis corresponding to the intensity is determined. This position is compared to the optimal position z OPT Compare them and calculate the differences.

[0032] In some embodiments, the emission intensities at two different wavelengths can be determined corresponding to the different temperature emissions. Figure 4 As shown, for positions along the z direction, at these positions, the high temperature emission (here C 230 emission) is lower than the other emission (here C 193 emission), which would correspond to being below the optimal positioning point z OPT of the sample surface, and when the measured intensity is higher than another, this will correspond to the optimal positioning point z OPT This information can be used in the controller 14 to determine the desired direction of movement of the sample stage 4 along the z direction.

[0033] At step 22, the controller 14 then operates the translation mechanism 22 to move the sample stage 4 an amount corresponding to the calculated difference (and optionally in a direction determined from a comparison of the emission intensities at the two wavelengths as discussed above). The focal plane F is thereby made coincident (or nearly coincident) with the upper surface 18 of the sample 20 on the sample stage 4 at the analysis region 40. At step 24, the controller 14 then operates the laser 8 to again generate plasma from the analysis region 40. At step 26, the controller 14 operates the elemental composition determination unit 12 to analyze the electrical signal output indicating the amplitude of the electromagnetic radiation as a function of the wavelength it receives from the detector 10 and incident on the detector 10. The composition of the analysis region 40 of the sample 20 is then determined by the elemental composition determination unit 12 from the wavelength-dependent intensity data in a manner known in the art of LIBS analysis.

[0034] In some embodiments, the calibration generation cycle (step 14) includes executing Figure 614a, the controller 14 operates the translation mechanism 22 to move the sample stage in the xy plane by a predetermined amount so as to expose a further other area of ​​the upper surface 18 of the sample 12 to the laser beam L. In step 14b, the controller 14 excites the laser 8 to generate a plasma in the further other area and the detector 10 generates an output corresponding to the intensity of electromagnetic radiation from the plasma. In step 14c, the controller 14 operates to determine a position along the z direction corresponding to the intensity. The position is compared with the optimal position, and in some embodiments, the difference Δz between the two is calculated, which is indexed in step 14d relative to the position of the further area (e.g., the absolute x, y position of the sample stage 4 or the position Δx, Δy measured relative to the x, y position of the first other area 16 or other origin of the coordinate system) in the memory 26. In some embodiments, the difference Δz can be determined in a later step, and the determined position along the z direction is stored in the memory 26 indexed relative to an indication of the location point of the associated further other area.

[0035] These steps are repeated until a sufficient number of additional regions of known positioning points in the xy plane have been exposed to the laser beam L, so that a mathematical transformation can be generated. In some embodiments, the sample stage 4 is set at the optimal positioning point z OPT In the case of Figure 2 Three circles 34, 36, 38 are shown in FIG.

[0036] At step 14f, a mathematical transformation is generated that will achieve the optimal position z OPT The required movement in the z direction (Δz) is mapped to the region's location point in the xy plane. This can be accomplished by spline or polynomial surface fitting, for example using software such as MathWorks from Natick, MA 01760-2098, USA. Corresponding algorithms are provided in commercially available software such as , which produce a calibration that interpolates between the measurement points.

[0037] In these embodiments, the analysis cycle (see Figure 7 ), using the surface mapping as a mathematical transformation at step 28, controller 14 operates translation mechanism 22 to move sample 20 by a known amount in the xy plane (Δx, Δy) to present an analysis region 40 of upper surface 18 that was not used in the generation of the mathematical transformation at step 14. An absolute (or, alternatively, relative) location point of analysis region 40 in the xy plane, typically an indication of x and y coordinates, is used as measurement data for an area of ​​upper surface 18 of the sample, and the mathematical transformation converts the optimal position z to the optimal position z.OPT The controller 14 then initiates an autofocus routine during which, at step 30, a mathematical transformation is applied to the acquired location point information and the optimal position z is calculated. OPT The required amount of movement Δz along the z-axis is for the analysis region 40. At step 32, the controller 14 then operates the translation mechanism 22 to move the sample stage 4 an amount corresponding to the calculated amount of movement Δz, so that the focal plane F coincides (or nearly coincides) with the upper surface 18 of the sample 20 on the sample stage 4 at the analysis region 40. At step 34, the controller 14 operates the laser 8 to generate plasma from the analysis region 40, and at step 36, operates the elemental composition determination unit 12 to analyze the electrical signal output, which is representative of the magnitude of electromagnetic radiation from the generated plasma as a function of the wavelength it receives from the detector 10 and incident on the detector 10, and determines the composition of the analysis region 40 of the sample 20 from the wavelength-dependent intensity data in a manner known in the art of LIBS analysis.

[0038] Now refer to Figure 8 Description Reference Figure 1 The embodiment of the LIBS analysis system 2 described is configured according to Figure 2 , Figure 3 , Figure 4 , Figure 6 and Figure 7 The method operates as described.

[0039] In the intensity data generation cycle for the sample 20 located on the sample stage 4 (see Figure 3 ) period, the controller 14 accesses and runs the program code stored in the memory 26 to cause C 193 and C 230 The intensity data of the emission line is collected and stored in the memory 26. Specifically, in the present embodiment, the controller 14 activates the translation mechanism 22 to move the sample stage 14 in a circle and sequentially expose multiple new portions of the first other area 16 of the upper surface 18 to the laser beam L. In this embodiment, the translation mechanism 22 is activated to move the sample 20 so that the multiple portions of the first other area will be located on the circle 16 close to the center of the cylindrical sample 20. In each of the multiple new portions of the first other area 16, the controller 14 controls the translation mechanism 22 to move the sample stage 4 to a new position in the z direction, selecting each position so that all positions together span a predetermined position range, including the optimal position z. OPT and below and above this optimal position Z OPT The intensity data stored in the memory 26 represent the intensity values ​​of the electromagnetic radiation generated by the plasma, here 193 nm and 230 nm, indexed relative to the position along the z direction, such as Figure 4As shown above. Figure 4 As discussed, curve 30 represents the intensity change at position z in millimeters at 193 nm, and curve 32 represents the intensity change at the same position at 230 nm. By comparing the two intensities at the same position z, the direction of movement of the sample surface in the z direction can be determined to obtain the optimal position Z OPT .

[0040] The intensity data associated with the emission at 193 nm (generated closest to the sample surface 18) stored in the memory 26 is processed by the controller 14 to determine the position in the z direction at which the measured intensity is at a maximum. This determined position represents the optimum position Z OPT At this positioning point, the optimal position ZOPT, where the focal plane F of the laser beam L coincides (or almost coincides) with the upper surface 18 of the sample 20, is determined to be 0.6 mm.

[0041] Once the controller 14 determines the optimal position z OPT , the LIBS analysis system 2 executes a calibration generation cycle under the control of the controller 14 (see Figure 6 During the calibration generation cycle, the system 2 is controlled to obtain a plurality of known positioning points on the upper surface 18 of the sample 20 and the sample holder 4 in the z direction relative to the optimal positioning point z OPT (Optimal position z OPT ) of the fixed same positioning point. Specifically in this embodiment, the position of the sample stage 4 is relative to the optimal position z OPT (Here we choose the fixed position as z OPT ) in a fixed position along the z direction, the controller 14 activates the translation mechanism 22 to move the sample stage 14 to sequentially expose a plurality of other regions describing circles 34, 36, 38 of the upper surface 18 of the sample 20 to the laser beam L, wherein each of the plurality of other regions has a known location point in the xy plane on the sample surface 18 that is different from all other regions previously exposed to the laser beam L. In this embodiment, the translation mechanism 22 is activated by the controller 14 to move the sample stage 4 (and therefore the sample 20) so that the plurality of other regions will be located on one or more (here three) circles 34, 36, 38 on the upper surface 18 of the sample 20.

[0042] At each of a plurality of other regions on the circle 34, 36, 38, the controller 14 operates the laser 8 to generate a laser beam L to strike the upper surface 18 at the other region and generate a plasma. The intensity values ​​of the electromagnetic radiation from the plasma incident on the detector 10 at two wavelengths of 193 nm and 230 nm are stored in the memory 26 together with representations of localization points of the other regions, for example Cartesian coordinates (x, y) with the origin at the center of the sample 20. In the controller 14, each such intensity value of the emission at 193 nm is compared with the intensity values ​​constituting the intensity data obtained at a plurality of different positions along the z direction of the sample stage 4 and at positions along the z direction determined from these positions where the intensity values ​​of the 193 nm emission coincide (or nearly coincide) with the intensity values ​​of the intensity data. Thus, the value Δz can be calculated as the difference between the determined position along the z direction and the optimal position z OPT Thus, the value Δz represents the movement in the z direction required to bring the surface of the region into coincidence (or near coincidence) with the focal plane F of the laser beam L at that region. In this embodiment, the controller 14 accesses a mathematical transformation stored in the memory 26 by which the intensity values ​​are associated with positions along the z direction and applies the intensity value of the 193nm emission at the region to the transformation to calculate the position where coincidence is achieved. As described above, the 193nm and 230nm intensities can be compared at that region to determine the optimal position z to achieve. OPT The desired direction of movement along the z-axis. One or both of the determined position or the calculated value Δz of the desired movement (preferably the latter) are stored in memory 26 as movement data indexed relative to a location point of another region in the xy plane.

[0043] When collecting motion data for all of the various other regions associated with circles 34, 36, 38, controller 14 processes the collected indexed motion data to generate a mathematical transformation that relates the motion data to an anchor point for one region in the xy plane and stores the transformation in memory 26 (by Figure 8 The mathematical transformation can be performed using a mathematical fitting algorithm known in the art, such as the MathWorks ® from Natick, MA 01760-2098, USA. From the data point ( Figure 8 It is generated by cubic spline fitting of P1 in the example.

[0044] The controller 14 then operates to control the LIBS analysis system 2 to perform an analysis cycle (see Figure 7), during which the controller 14 activates the translation mechanism 22 to move the sample stage 4 at least once by a known amount and expose an analysis region 40 of the upper surface 18 of the sample 8 to the laser beam L that was not previously exposed to the laser beam L. Since the sample stage 4 has moved by a known amount, the position of the analysis region 40 is known. The controller 14 operates to access a mathematical transformation and apply it to the known position of the analysis region 40 (here, the x, y coordinates of the region 40) to determine the position z of the sample stage 4 in the z direction OPT At this position, the focal plane F coincides (or nearly coincides) with the upper surface 18 at the analysis area 40, and then the translation mechanism 22 is controlled to move the sample stage in the z direction by an amount Δz to achieve the optimal position z OPT . The controller 14 then energizes the laser 8. The laser beam L generated by the energized laser 8 strikes the exposed analysis region 40 and generates plasma. The output of the detector 10, representing the intensity of the electromagnetic radiation from the generated plasma striking it, is transmitted to the elemental composition determination unit 12 to be analyzed therein in a manner known in the art to quantitatively or qualitatively determine the concentration or presence of an element of interest of the analysis region 40 of the upper surface 18 of the sample 20 from which the region was generated.

Claims

1. LIBS analysis system (2), comprising: A focusing lens arrangement (6) having a focal plane (F); a laser (8) configured to propagate a laser beam (L) through the focusing lens arrangement (6) in a direction along an optical path (O) to be focused at the focal plane (F); a detector (10) having an output proportional to the intensity of incident electromagnetic radiation; a sample holder (4) for holding a sample (20), the upper surface (18) of the sample (20) intersecting the optical path (O); a translation mechanism (22) operable to effect relative movement of the sample holder (4) and the focusing lens arrangement (6) to change the position of the focal plane (F) along the optical path (O) relative to the sample holder (4); and a controller (14) configured to automatically control the operation of the translation mechanism (22) to achieve relative movement to achieve an optimal position z where the focal plane (F) and an analysis area (40) of the upper surface (18) intersecting the optical path (O) coincide with or nearly coincide with each other. OPT , wherein the system (2) further comprises a memory (26) accessible to the controller (14) and having stored therein a mathematical transformation generated using an output of the detector (10) obtained from electromagnetic radiation from a plasma generated by the laser beam (L) impinging on an upper surface (18) of the sample (20) in an area other than the analysis area (40) or in a plurality of different areas, the mathematical transformation relating measurement data of the analysis area (40) to an optimal position, and wherein the controller (14) is further configured to calculate the optimal position z based on the mathematical transformation stored in the memory (26) OPT and controlling the operation of the translation mechanism (22) to achieve the optimal position z OPT .

2. The LIBS analysis system (2) according to claim 1, wherein: The controller (14) is configured to operate the translation mechanism (22) to achieve the relative motion to achieve a plurality of different positions of the focal plane (F) relative to the sample holder (4) along the optical path (O) at one other region of the upper surface (18); operate the laser (8) to generate plasma at each of the plurality of different positions, and retrieve into the memory (26) a representation of the detector output obtained from the electromagnetic radiation from the plasma generated at each of the plurality of different positions indexed relative to the position as intensity data for generating the mathematical transformation.

3. The LIBS analysis system (2) according to claim 2, wherein: The controller (14) is configured to process the intensity data to generate a mathematical expression relating detector output to the position of the focal plane (F) and to store the mathematical expression in the memory as the mathematical transform.

4. The LIBS analysis system (2) according to claim 3, wherein: The controller (14) is configured to operate the laser (8) to generate plasma at the analysis region (40); acquire the output of the detector (10) obtained based on the electromagnetic radiation from the generated plasma into the memory (26) as the measurement data; and calculate the optimal position of the analysis region (40) by applying the mathematical transformation to the measurement data.

5. The LIBS analysis system (2) according to claim 2, wherein: The controller (14) is also configured to operate the translation mechanism (22) to move the sample holder (4) in a plane perpendicular to the optical path (O) so that multiple different other areas of the upper surface (18) continuously intersect the optical path (O), each different other area having a different known positioning point in the plane; operate the laser (8) to generate plasma at each of the multiple different other areas, and generate the mathematical transformation based on a comparison of the output of the detector (10) at each of the different known positioning points in the plane with the intensity data.

6. The LIBS analysis system (2) according to claim 5, wherein: The mathematical transformation associates information identifying the optimal position with a location point in a region perpendicular to the optical path in a plane, and the controller (14) is configured to operate the LIBS analysis system (2) to collect the location point of the analysis region (40) as the measurement data.

7. A method for operating the LIBS analysis system (2) according to claim 1, comprising automatically adjusting the focus positioning point of the laser beam (L) output by the laser (8) to an optimal position z of the optimal positioning point where the focal plane (F) and the analysis area (40) of the upper surface (18) intersecting the optical path (O) are in coincidence or nearly coincidence by automatically controlling the operation of the translation mechanism (22) to achieve relative movement. OPT , to achieve the optimal position z OPT , wherein the method further comprises generating, using the controller (14), a mathematical transformation that relates the measured data of the analysis area (40) to the optimal position z OPT The method comprises the steps of: correlating and using the output of the detector (10) obtained from the electromagnetic radiation from the plasma generated by the laser beam (L) impinging on an area other than the analysis area (40) of the upper surface (18) of the sample (20); obtaining measurement data of the analysis area (40) of the upper surface (18) and applying the mathematical transformation to the measurement data to determine the optimal position z at the analysis area (40) OPT .

8. The method of claim 7, wherein: The method further comprises the steps of automatically controlling the translation mechanism (22) to effect relative motion to achieve a plurality of different positions of the focal plane (F) relative to the sample holder (4) along the optical path (O) at another region of the upper surface (18); operating the laser (8) to generate plasma at each of the plurality of different positions, and retrieving a representation of the detector output obtained from the electromagnetic radiation from the plasma generated at each of the plurality of different positions into the memory (26) as intensity data indexed relative to the position for use in generating a mathematical transformation.

9. The method of claim 8, wherein: Generating the mathematical transform includes processing intensity data in the controller (14) to produce a mathematical expression relating detector output to the position of a focal plane (F) and storing the mathematical expression as the mathematical transform in the memory.

10. The method of claim 9, wherein: Obtaining the measurement data comprises operating the laser to generate plasma in the region of the upper surface; adjusting the focal position of the laser beam (L) output by the laser (8) to an optimal position z OPT comprising calculating the optimum position z of the region by applying said mathematical transformation to the detector output obtained from the electromagnetic radiation from the generated plasma OPT and operating the translation mechanism (22) to move the sample holder (4) to an optimal position z OPT .

11. The method of claim 8, wherein: Generating the mathematical transformation includes operating the controller (14) to energize the translation mechanism (22) and move the sample holder (4) in a plane perpendicular to the optical path to successively intersect multiple different other regions of the upper surface with the optical path; operating the laser (8) to generate plasma, and storing detector outputs obtained from electromagnetic radiation from the plasma generated at each of the multiple different other regions in the memory indexed relative to the location point of the corresponding region in the plane, for generating the mathematical transformation based on a comparison of the detector output at each location point of the corresponding region in the plane with the intensity data.

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