A method for the quantification of chlorinated paraffins

Abstract

A mass spectrometer for the quantification of chlorinated paraffins, the mass spectrometer comprising: (a) an inlet system comprising a gas chromatography column 205; (b) an ion source comprising an i

Description

The present invention relates to a method for the quantification of chlorinated paraffins in an analytical sample. More particularly, the method involves quantitative analysis by mass spectrometry using a mass spectrometer. The present invention also relates to a mass spectrometer for use in the quantification of chlorinated paraffins. Chlorinated paraffins (CPs) have been commercially synthesised since the 1930s, typically produced by reaction of n-alkanes (i.e. straight chain paraffins) with gaseous chlorine at elevated temperatures of about 80°C to 120°C. The radical substitution reaction may even be accelerated by irradiation with ultra-violet light. Chlorinated paraffins have many industrial uses and are still used today in a range of applications, commonly as additives in lubricants or coolants used in metal processing, as well as being used as flame retardants or plasticisers in plastics and other materials such as synthetic rubbers, paints and adhesives. They have also been used in the leather industry as fatliquoring agents. Chlorinated paraffins have the general molecular formula CxH2x+2-yCly, and generally have a chlorine concentration varying from 20 wt% to 70 wt%, typically 30 wt% to 70 wt%. They are generally divided into three groups: short-chain CPs (SCCPs, C10-13 and CI1-13), medium-chain CPs (MCCPs, C14-17 and Ch-17), and long-chain CPs (LCCPs, C18-30 and Ch-30). These may also be known by their CAS numbers: 85535-84-8, 85535-85-9, and 63449-39-8, respectively. As noted in, for example, Toxics 2022, 10, 778 “Bioaccumulation and Biotransformation of Chlorinated Paraffins”, chlorinated paraffins are a class of persistent, toxic, and bioaccumulative compounds which have received increasing attention worldwide in recent decades for their environmental occurrence and ecological and human health risks. They do not break down easily, leading to long-term environmental presence. Chlorinated paraffins can accumulate in the tissues of living organisms, and this bioaccumulation can magnify through the food chain, affecting top predators and potentially humans. Exposure has been linked to various health issues, including liver, thyroid, and kidney tumours. They are particularly toxic to aquatic organisms, even at low concentrations. Indeed, in 2017, it was agreed to list SCCPs in the Stockholm Convention on Persistent Organic Pollutants such is the growing concern over these chemicals to human health and the environment. Therefore, monitoring, and effective analysis, is critical for controlling global use of SCCPs (though decision SC-8 / 11 of COP8 provides exemptions for the above described uses). As concerns over their environmental and health impacts grow, it can be anticipated that in future there will be ever more stringent regulations in place increasing the need for a reliable and accurate analytical method for quantitative analysis. It has long been known in the art that analysis of this class of compounds is very difficult to perform, at least in part due to the nature of chlorinated paraffins as an inherently complex mixture of thousands of individual congeners. Chemosphere 2022, 3, 131878 “Determination of chlorinated paraffins (CPs): Analytical conundrums and the pressing need for reliable and relevant standards”, for example, identifies there has been a long-felt need for many decades for an analytical method for the quantitative analysis of chlorinated paraffins, particularly short-chain chlorinated paraffins (SCCPs) and medium-chain chlorinated paraffins (MCCPs), though especially SCCPs. To date, there is no established effective method for quantitative analysis of CPs. It is also known in the art that the lack of certified reference materials poses a problem for the assessment of the quality assurance / quality control of any analytical procedure. The Water Framework Directive (WFD) Guidance published a Technical Report - 2009 - 025, Guidance Document No. 19 “Guidance on Surface Water Chemical Monitoring under the Water Framework Directive” in which the Substance Guidance Sheets of Annex II, pages 11-13, relate to SCCPs and it is identified that “[A]lthough some work has been conducted on development of selective and sensitive methods for SCCP analysis in recent years, for the time being, no fully validated procedure is available that could be recommended for routine monitoring of SCCPs in environmental samples. SCCP concentrations in environmental samples analysed by GC-ECNI-MS can vary widely (by a factor of ten) depending on chlorine content of the standard used for quantification". As published in the EFSA Journal in 2020 following a request from the European Commission, “Risk assessment of chlorinated paraffins in feed and food” identifies therein that “[T]here is no ideal technique that can be used for analysis of CPs; all available methods have limitations and are a compromise in some aspect, and thus can generate different results. The most commonly used technique is gas chromatography (GC) combined with LRMS in the electron capture negative ionisation (ECNI) mode or high-resolution MS (HRMS). The ECNI technique is a sensitive technique for the detection of CP congeners with >Cis- Similarly, as described in paragraph 66 of the very recent publication “Guidance on best available techniques and best environmental practices relevant to the Short Chain Chlorinated Paraffins (SCCPs) listed under the Stockholm Convention on Persistent Organic Pollutants” issued by The Stockholm Convention Secretariat and United Nations Environment Programme (UNEP), published July 2024: “Currently, the determination of SCCPs is mostly performed by mass spectrometry (MS) in the Electron Capture Negative Ionisation (ECNI) mode. This approach is prone to interferences from other chlorinated compounds and medium-chain chlorinated paraffins, leading to errors in the quantification of SCCPs", highlighting that there has been very little technical development over at least the last 15 years to address this highly important problem. There have nevertheless been various efforts in the prior art in light of this problem to analyse SCCPs. For example, an Application Note published in 2019 by Agilent Technologies, Inc. “Short Chain Chlorinated Paraffins (SCCP) Analysis Using Negative Chemical Ionization (Cl) and Low Energy El by High-Resolution 7250 GC / Q-TOF” describes the analysis of SCCPs using an Agilent 7250 GC / Q-TOF system equipped with a low energy-capable electron ionisation (El) source as well as an interchangeable Cl source in both negative Cl and low energy El modes to ensure high selectivity and sensitivity across SCCP congeners with various degrees of chlorination. Organohalogen Compounds 2021, 82, 13-15 “The Potential of Soft Ionizations for Analyzing Chlorinated Paraffins” is a conference abstract published in respect of the International Symposium on Halogenated Persistent Organic Pollutants by Dioxin20XX which relates to the analysis of an SCCP standard mixture with a chlorine content of 55%. The analyses were conducted on a GC (7890B, Agilent Technologies, Inc.) equipped with Zoex thermal modulator (Zoex Corporation). A time-of-flight mass spectrometer (TOFMS) with a photoionisation-electron ionisation (Pl-El) combined ion source (JMS-T200GC “AccuTOF GCx-plus”, JEOL Ltd.) was coupled to GCxGC. Different dilutions of SCCP standard mixture were measured under the same GCxGC condition with PI, standard (70 eV) El, and low energy (15 eV, 20 eV, 25 eV) El, respectively. Chemosphere 2024, 352, 14100 “Quantification of chlorinated paraffins by chromatography coupled to high-resolution mass spectrometry - Part A: Influence of gas chromatography and ionisation source parameters” compared GC-ECNI-HRMS parameters for the analysis of chlorinated paraffins (polychlorinated n-alkanes - PCAs). The analyses were carried out using a Trace GC 1310 device equipped with a Tri Plus RSH autosampler and coupled to a Q Exactive mass spectrometer (Thermo Fisher Scientific™) with the ionisation of the PCAs carried out in ECNI. Accordingly, despite the efforts made, there still remains a need in the art for a simple and effective method for an accurate quantitative analysis of chlorinated paraffins, especially short-chain chlorinated paraffins with a low degree of chlorination (such as less than Cis). The present invention aims to overcome, or at least reduce, the aforementioned problems in the prior art, or to at least provide a commercially viable alternative thereto. Thus, a first aspect of the present invention provides a method for the quantification of chlorinated paraffins, the method comprising: (i) providing a sample comprising chlorinated paraffins for analysis; (ii) ionising the sample by field ionisation; and (iii) quantitatively analysing the amount of each chlorinated paraffin in the sample with a mass spectrometer in selected ion monitoring mode. The present disclosure will now be described further. In the following passages, different aspects / embodiments of the disclosure are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. The method of the present invention provides an efficient and importantly accurate quantitative analysis of the concentration of the chlorinated paraffins in an analytical sample. As such, the method permits an accurate determination of the concentration of CPs in the bulk sample from which the analytical sample is obtained. Any conventional bulk sample may be used, and the method may therefore be used to determine the CP concentrations in, by way of example only, water samples, air samples, soil samples, plastic / rubber samples, food samples and / or other biological material (biota). Suitable sample preparation will be well-known to those skilled in the art to prepare an analytical sample. The sample preparation of CPs generally involves (or consists of) extraction, clean-up and optionally fractionation steps before the final analysis (i.e. steps (ii) and (iii) in the present method by mass spectrometry). Extraction can typically be performed with a Soxhlet or accelerated organic solvent extraction for solid samples. Solid phase extraction (SPE) or liquid-liquid extraction (LLE) may be used for water samples. A clean-up step may be used to remove matrix materials. For example, various absorbents and filters may be used to remove such components, such as by gel permeation chromatography or a column of activated silica gel. As such, step (i) of the present method may comprise preparing the sample for analysis. A number of standards have been set for CP analysis in different matrices, some of which include ISO 18219-1:2021 (Leather), ISO 22818:2021 (Textiles), and ISO 12010:2019 (Water quality). Each standard provides procedures for sample preparation. The method further comprises ionising the sample by field ionisation. That is, the analytical sample may be injected into the mass spectrometer and vapourised to provide a gaseous sample, as is well-known for mass spectrometers, and into the ion source. The ion source comprises a field ionisation electrode for the field ionisation step, whereby the gaseous sample is passed over the field ionisation electrode. Field ionisation (Fl) is the ionisation of a gaseous molecule by an intense electric field which is typically provided by a sharp electrode at a high potential. Such a sharp electrode may also be called an emitter, and may be a single tip, a filament wire (e.g. with a diameter of less than 5 pm) or razor blade, or more preferably a filament wire with protruding “whiskers” or “dendrites”. Filament wires are typically formed from platinum or tungsten, and may have carbon whiskers. Various field ionisation electrodes are commercially available. A high electric field gradient (e.g. greater than 108 V / cm) at the sharp point (e.g. the tips of the whiskers) results in the ionisation by quantum mechanical tunnelling of electrons producing molecular radical cations M+'. Field ionisation is particularly advantageous since, unlike the range of ionisation techniques used in the prior art, including Cl and low energy El, the inventor has found that there is minimal, or essentially no, fragmentation of the chlorinated paraffins by this method. As such, the molecular mass / ion peaks of each chlorinated paraffin of a specific chlorination degree can be achieved. However, due to the inherently low concentration of CPs in bulk samples and the subsequent analytical sample, sensitivity is a particular concern. Whilst field ionisation is a known soft ionisation technique, this is generally avoided in the art due to its low sensitivity resulting from inherently very low ion currents. It is believed that there is a general prejudice in the art for the use of Fl due to its low sensitivity, such that it is not the obvious ionisation technique to use for CP analysis, particularly for quantitative analysis. However, the present method further comprises quantitatively analysing the amount of each chlorinated paraffin in the sample specifically with a mass spectrometer in selected ion monitoring (SIM) mode. It has been found that the unique combination of Fl with SIM addresses the problems in the prior art because the targeted approach of using SIM, rather than a full scan approach, improves the sensitivity (i.e. the signal to noise ratio, also referred to as S / N). In this analysis mode, the instrument allocates more analysis time to detecting a greater number of target ions by using longer dwell times. Dwell time refers to the specific measuring time assigned to each target ion as per the SIM / MID setup. By extending the dwell times for critical target analyte masses, better ion statistics are achieved, enhancing sensitivity. Since Fl provides a molecular ion peak with minimal or no fragmentation, the amount of each paraffin of a given chlorination degree can in turn be accurately quantified by the molecular ion peak. By no fragmentation it is meant that the carbon backbone of the chlorinated paraffin remains intact, though minimal fragmentation may occur through loss of HCI (e.g. providing M+' and / or M-HCI+' ion peaks). Selected ion monitoring mode is also, less preferably, referred to in the art as multiple ion detection (MID). SIM is defined in the IUPAC gold book (https: / / goldbook.iupac.org / terms / view / S05547) as the “operation of the mass spectrometer in which the intensities of several specific ion beams are recorded rather than the entire mass spectrum!’. Such an operating mode of a mass spectrometer compensates for the loss in sensitivity providing an unexpectedly simple solution to a previously longstanding complex and difficult problem. The present method of quantitative analysis is particularly suitable for the analysis of short-chain chlorinated paraffins, and even more advantageously, paraffins of a low degree of chlorination, e.g. less than or equal to Cis. That is, based on the formula CxH2x+2-yCly, the degree of chlorination “y” is from 1 to 5, even more preferably from 1 to 4. As such, it is preferred that the degree of chlorination of at least 10 wt% of the chlorinated paraffins (especially short-chain chlorinated paraffins) in the sample is 5 or less, preferably 4 or less, and this may even be at least 20 wt% with no specific upper limit (though typically this is less than 60 wt%). Methods in the prior art are not capable of quantitative analysis of these low degrees of chlorination and this has remained a stumbling block in the art. It is generally preferred that the mass spectrometer is a high resolution mass spectrometer, preferably having a resolving power of greater than 30,000 (e.g. FWHM @ m / z 200), preferably greater than 60,000. The components of a mass spectrometer are well-known, and generally include an inlet system coupled to an ion source for ionising the sample. The ionised sample is then passed to a mass analyser whose function is to separate the ions based on their mass-to-charge (m / z) ratio which may then be directed to an ion detector. The ion detector is connected to a signal processor or computer for the subsequent data processing converting the signal current to digital information for display. In one particularly preferred embodiment, the inlet system comprises a gas chromatography column, and the sample inlet to the ion source may then be regarded as the outlet of the column. As such, immediately before step (ii) of ionising the sample, the sample is separated in the gas chromatography column. In one preferred embodiment, the mass spectrometer comprises an ion trap mass analyser, preferably an Orbitrap™ mass analyser. Such an ion trap mass analyser simultaneously functions as the mass analyser and ion detector since, as the ions oscillate back and forth along the axis of the central electrode as different frequencies, an image current from the trapped ions can be detected on the outer electrode and converted to a mass spectrum. In other preferred embodiments, the mass spectrometer comprises a sector field mass analyser, and preferably is a double focussing sector field mass spectrometer. One example of a suitable instrument is a DFS™ Magnetic Sector GC-HRMS system, available from Thermo Scientific™. Such an instrument is known for use in performing Dioxin and Persistent Organic Pollutants (POPs) analysis. Another example of a suitable instrument is a Neoma™, available from Thermo Scientific™. In embodiments of the present method, particularly those in which the mass spectrometer is a sector field mass spectrometer, step (ii) of ionising the sample preferably comprises ionising the sample together with one or more reference compounds. Reference compounds can be used to adjust the calibration scale as well as the relative intensities of the ion peaks, i.e. the reference allows the use of mass calibration in SIM mode. Intensity consistence of the mass traces of such reference gas is used as quality criterion for the quality of the sample - this is regulated in official methods such as EPA 1613 for dioxins and furans. Typically used references for the low level analysis of POPs, which include perfluorotributylamine (PFTBA- also referred to as FC43) or perfluorokerosene (PFK), are not the most suitable for field ionisation since it is believed that they do not ionise well. It is preferred that the one or more reference compounds are non-halogenated, and advantageously may simply be hydrocarbons which ionise well and exhibit essentially no fragmentation under field ionisation. Preferably, the one or more reference compounds are each a Cs to C30 hydrocarbon (such as a linear or branched alkane), e.g. from Cwto C30. Preferably, the one or more reference compounds are vapourable. In some preferred embodiments, at least two reference compounds are introduced into the mass spectrometer at the same time as the sample, and may be introduced through a separate port to the sample into the mass spectrometer. The at least two reference compounds may then be used to quantitatively analyse the amount of short-chain chlorinated paraffins in the sample using a lock-plus-cali mass technique. By such a method, the masses of the two reference compounds are selected so as to “book-end” the expected masses of the chlorinated paraffins that are being detected, i.e. the SIM / MID window for the analysis. Although both reference masses are used for the inherent scan calibration it is common practice to name the lower reference mass the “lock mass” and the upper reference mass the “calibration mass” or “cali mass”. The analysis using a lock-plus-cali mass technique is described in, for example, Application Note: 30116 “High Resolution Multiple Ion Detection (MID)” published by Thermo Fisher Scientific™. The analysis is divided into multiple time sections each containing different specific target masses. In a section dialog of the software, all specific target masses for each SIM / MID time section are defined plus two reference gas masses, the lock and cali mass used for the constant automatic mass recalibration of the mass spectrometer. This approach ensures maximum mass accuracy and stability for the high-resolution analysis of low-level contaminants. Within each SIM / MID analysis time section, the traces of target masses and reference masses are acquired. During the acquisition, a permanent automatic mass re-calibration is carried out. This automatic mass calibration process includes the following steps: first step is the first so called locking of the reference mass signal specified as lock mass, secondly, an electric calibration using a second reference mass peak, the so-called cali mass, is carried out, and in the third and final step the successive monitoring of the target masses takes place. While step two and three describe a so-called SIM / MID mass scanning cycle which is constantly repeated, the locking process is executed only once at the start of each of the SIM / MID analysis time sections. During the lock process at the start of each SIM / MID time section the system identifies the reference mass, labelled as lock mass using a small mass scan over a defined narrow mass range, called lock window. It corrects its position to the exact theoretical value which is the basis for the following calibration. A second aspect of the present invention provides a mass spectrometer for the quantification of chlorinated paraffins, the mass spectrometer comprising: (a) an inlet system comprising a gas chromatography column; (b) an ion source comprising an ion volume, the ion volume housing a field ionisation electrode affixed therein; and (c) a mass analyser; wherein the ion volume is configured to receive the gas chromatography column so as to permit varying of the distance and / or the alignment between the field ionisation electrode and an outlet of the gas chromatography column within the ion volume. The inlet system is for introducing the sample in the gas phase to the ion source of the mass spectrometer. The sample is introduced gas chromatography and in to the ion source via a gas chromatography column. As described herein, it is preferred that the mass analyser comprises a sector field mass analyser (preferably double focussing), or that the mass analyser comprises an ion trap mass analyser such as an Orbitrap™ mass analyser. As will be appreciated, the spectrometer will be configurable to operate in selected ion monitoring mode when in use in analysing a sample comprising chlorinated paraffins. The mass spectrometer of the second aspect further comprises an ion source comprising an ion volume. The ion volume is a component of the ion source within which the ions are generated, and in the present mass spectrometer, the ion volume houses the field ionisation electrode. More specifically, the field ionisation electrode is uniquely integrated into the ion volume (i.e. mounted to the wall(s) of the ion volume) with the electrode arranged within the space. In known field ionisation apparatus, the electrode is not affixed to the ion volume or the ion source which has been found to make accurate alignment with the sample inlet (i.e. the GC column outlet) and setting of a distance therebetween problematic. As such, it is conventional in mass spectrometers which comprise a field ionisation electrode that the distance is more than 5.0 mm (such as from 5.0 mm to 8.0 mm). However, the inventor has found that it is particularly beneficial for the quantitative analysis described herein for the distance between the sample inlet and the field ionisation electrode to be less than 4.0 mm (i.e. the sample pathway). As will be appreciated, this is generally the shortest distance between the point-like outlet and the electrode which can take the form of a wire or edge. This narrow sample pathway is in view of the sample density since the volume of the sample cannot be too small as it passes the electrode. The present mass spectrometer therefore allows for the appropriate alignment of the inlet and the electrode to ensure effective ionisation for a sample with a low concentration of analyte (i.e. CPs) particularly so as to set the distance of the sample pathway to less than 4.0 mm. It is more preferred that the sample pathway is less than 1.0 mm, and even more preferably less than 0.5 mm. As will be appreciated, the inlet will desirably not touch the Fl electrode due to the delicate nature of the electrode (especially those which include whiskers) such that the distance / sample pathway is greater than 0 mm, such as greater than 0.1 mm. In some preferred embodiments, the ion source is removable. That is, the ion source may be an exchangeable component of the mass spectrometer which can allow for exchange of the ion source (such as to replace a dirty ion source with a cleaned ion source) without breaking the vacuum that is present within the mass analyser. An ion volume comprises an outlet through which ions which are formed by the field ionisation electrode a drawn and passed to the mass analyser. Such an outlet may be referred to as a “drawlens”. For field ionisation, the high voltage applied to the field ionisation electrode may be multiple kV, such as at least 3 kV or at least 5 kV. The drawlens may also have a high voltage of opposite polarity applied, such as at least -3 kV, or at least -5 kV to draws the ions to the mass analyser. The field ionisation electrode may be arranged within the ion volume at least 5 mm from the drawlens to prevent arcing, or even at least 10 mm. The removable ion source component may therefore have outer electrical connections which are configured to connect with electrical connections provided in a body of the mass spectrometer to provide the ion source with such electrical power when installed. A user may then insert the end of the gas chromatography column into the ion source to align with the field ionisation electrode affixed within the ion volume. It is also preferred that the ion volume is configured to receive the gas chromatography column in a direction perpendicular to an outlet of the ion volume. That is, in use the flow from the gas chromatography column at the outlet thereof is provided in a direction approximately perpendicular to the subsequent flow of the ions through the drawlens to the mass analyser. This direction is preferred since it avoids the filament blocking the path of the ions. In some embodiments, the inlet system may comprise two gas chromatography columns, and each of which may be arranged perpendicularly on opposite sides of the field ionisation electrode. In view of the foregoing, one particular specific embodiment of the method for the quantification of short-chain chlorinated paraffins comprises: (i) providing a sample comprising short-chain chlorinated paraffins for analysis, particularly short-chain chlorinated paraffins having a degree of chlorination of 5 or less, preferably from 1 to 4, and separating the sample in a gas chromatography column; (ii) ionising the sample received from the outlet of the gas chromatography column by field ionisation, wherein a sample pathway between the outlet and an electrode for the field ionisation is less than 4.0 mm; and (iii) quantitatively analysing the amount of each short-chain chlorinated paraffin in the sample with a mass spectrometer in selected ion monitoring mode; wherein the mass spectrometer comprises a sector field mass analyser, and the method comprises ionising the sample together with at least two reference compounds (preferably each being a hydrocarbon) which are used to quantitatively analyse the amount of chlorinated paraffins in the sample using a lock-plus-cali mass technique. Figures The present invention will now be described further with reference to the following exemplary non-limiting Figures, in which: Figure 1 illustrates the set-up of sample vials used in the Example. Figure 2 is a mass spectrum of 1,4-dichlorobutane obtained using El on a DFS™ mass spectrometer. Figure 3 is a mass spectrum of 1,4-dichlorobutane obtained using Fl on a DFS™ mass spectrometer. Figure 4 is a close-up view of the molecular ion peak in the mass spectrum of Figure 3. Figure 5 illustrates the mass traces of five isotopes of 1,4-dichlorobutane as well as the calibration mass used in the Example. Figure 6 is a perspective view of a schematic of an ion volume. Figure 7 is a rear view of the ion volume of Figure 6. Figure 8 is a perspective view of a schematic of a further ion volume. Figure 1 illustrates the set-up of sample vials 100, 120 used in the Example described hereinbelow. A first 2 mL vial 100 comprises a liquid hydrocarbon mixture 105 and a headspace 110, suitable for GC / MS. Headspace sampling is a well-known technique in which the headspace 110 (i.e. the gas layer above the sample) is analysed. The end of a fused silica capillary 115, coupled at its other end to a mass spectrometer (not shown), is inserted into the headspace 110 of the first vial 100, whose composition is in equilibrium with the liquid 105. A second vial 120 comprises a liquid hydrocarbon mixture 125 which is identical to the liquid hydrocarbon mixture 105, but further comprises a chloroalkane for analysis. The capillary 115 can be moved to be inserted into the headspace 130 of the second vial 120. Figure 2 is a mass spectrum (m / z vs relative abundance) of 1,4-dichlorobutane obtained using El on a DFS™ mass spectrometer as a model for an SCOP. In accordance with the mass spectrum available from NIST, no molecular ion peak is observed or reported in the mass spectrum at about m / z 126. Instead the largest peak observed is a fragment at about m / z 55 with many other fragments also observed at other m / z ratios. Figure 3 is a mass spectrum of 1,4-dichlorobutane obtained using Fl on a DFS™ mass spectrometer which shows only the isotope pattern of the molecular ion peak at about m / z 126 and one fragment with an intact carbon backbone at about m / z 90, but no further fragments in the low mass range. Figure 4 is a close-up view of the molecular ion peak in the mass spectrum of Figure 3 at about m / z 126. The isotope pattern shows an excellent fit with theoretical calculated values. Specifically, peaks are observed at m / z 125.9996 (C4H8CI2; calc, m / z 125.9998; -0.1082 mmu); m / z 127.0033 (C313CH8CI2; calc, m / z 127.0031; 0.1686 mmu); m / z 127.9979 (C4H8CI37CI; calc, m / z 127.9968; 1.0871 mmu); m / z 129.0094 (C313CCI37CI; calc. m / z 129.0002; 9.2833 mmu); m / z 129.9949 (C4H837CI2; calc, m / z 129.9939; 1.0618 mmu); m / z 131.0151 (C313CH837CI2; calc. m / z 130.9972; 17.8486 mmu). Figure 5 illustrates the mass traces of five isotopes of 1,4-dichlorobutane as well as the molecular ion of the calibration mass used in the Example. From top to bottom, the mass traces shown are those for the m / z windows of 1,4-dichlorobutane (i) 125.9972-126.0022, (ii) 127.0006-127.0056, (iii) 127.9942-127.9994, and (iv) 128.9976-129.0028, with the bottom two traces for the calibration mass at m / z 128.0928-128.2198. Figure 6 is a perspective view of a schematic of an ion volume 200 into which a gas chromatography column 205 has been arranged and aligned such that the flow of sample is directed directly at the field ionisation electrode 210. The outlet 205a of the gas chromatography column 205 is arranged very close to the field ionisation electrode, for example, providing a sample pathway of about 0.5 mm. This close coupling of the outlet 205a and the electrode 210 may also be seen in the rear view of the ion volume 200 in Figure 7. The ion volume 200 has an outlet 215a provided by a drawlens 215. Following ionisation by the electrode 210, which may have an applied voltage of about 5 kV, as shown by arrow 220 the ions are passed to a mass analyser through an outlet 215a provided by the drawlens 215, which may have an applied voltage of about -5 kV. The gas chromatography column 205 is arranged to provide a flow of sample in a direction perpendicular to arrow 220 (and perpendicular to the length of the wire electrode 210). Figure 8 is a perspective view of a schematic of a further ion volume. This may be equivalent to ion volume 200, though with an additional gas chromatography column arranged perpendicularly, but on the opposite side of the field ionisation electrode to the first gas chromatography column. Example To prove the concept of targeted analysis of SSCPs using an MID method with field ionisation, the following experiment was carried out. The headspace of a hydrocarbon mixture (iso-octane and n-nonane) in a 2 mL vial was introduced via a fused silica capillary into the ion source of an MS equipped with an Fl filament by the vacuum of the MS (as shown in Figure 1). An MID experiment was started, and a raw data file was acquired using the molecular peak of the iso-octane as lock mass and the n-nonane molecular peak as calibration mass. To mimic a chromatographical peak, the capillary was changed for a short period of time to the headspace of a second vial containing the same hydrocarbon mixture plus a small amount of 1,4-dichlorobutane and back to the headspace of the first vail containing only the hydrocarbon mixture. The mass traces of five isotopes of 1,4-dichlorobutane were also monitored (see Figure 5). During the experiment the vials were kept at room temperature. All mass traces of the selected 1,4-dichlorobutane isotopes show an intensity increase during the period when the capillary was moved to the headspace of the vial containing the 1,4-dichlorobutane (see Figure 5). The peak intensity of each mass trace was integrated, and the area of the peak was compared to the theoretical relative occurrence of each of the selected isotope of 1,4-dichlorobutane. The peak area data is illustrated in the Table below: Isotope m / z Relative Abundance Realtive Abundance [%] Theory [%] C4H835CI2 125.9998 332457 100.00 100 C313CH835CI2 127.0031 16716 5.03 4.33 C4H835CI37CI 127.9968 196278 59.04 63.92 C313CH835CI37CI 129.0002 9518 2.86 2.77 C4Hs37CI2 129.9939 30881 9.29 10.22 The comparison passes the requirement of an acceptable variation of ±15% for the ratio between the most abundant isotope (Quantification mass) and second most abundant isotope (Ratio mass) as required in official methods such as EPA 1613 for Dioxins and Furans or EPA 1668 for PCBs. As used herein, the singular form of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features and is also intended to include the option of the features necessarily being limited to those described. In other words, the term also includes the limitations of “consisting essentially of” (intended to mean that specific further components can be present provided they do not materially affect the essential characteristic of the described feature) and “consisting of” (intended to mean that no other feature may be included such that if the components were expressed as percentages by their proportions, these would add up to 100%, whilst accounting for any unavoidable impurities), unless the context clearly dictates otherwise. Numerical lower and upper limits of features described herein may preferably be combined to provide a closed range. The foregoing detailed description has been provided byway of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents. For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference. The present invention with now be described further with reference to the following numbered embodiments: 1. A method for the quantification of chlorinated paraffins, the method comprising: (i) providing a sample comprising chlorinated paraffins for analysis; (ii) ionising the sample by field ionisation; and (iii) quantitatively analysing the amount of each chlorinated paraffin in the sample with a mass spectrometer in selected ion monitoring mode. 2. The method according to embodiment 1, wherein the chlorinated paraffins are short-chain chlorinated paraffins. 3. The method according to embodiment 2, wherein the degree of chlorination of at least 10 wt% of the short-chain chlorinated paraffins in the sample is 5 or less. 4. The method according to any one of embodiments 1 to 3, wherein the mass spectrometer comprises an ion trap mass analyser, preferably an Orbitrap™ mass analyser. 5. The method according to any one of embodiments 1 to 3, wherein the mass spectrometer comprises a sector field mass analyser, and the method comprises ionising the sample together with one or more reference compounds. 6. The method according to embodiment 5, wherein the one or more reference compounds are non-halogenated, preferably hydrocarbons. 7. The method according to embodiment 5 or embodiment 6, wherein the one or more reference compounds are each a Cs to C30 hydrocarbon. 8. The method according to any one of embodiments 5 to 7, wherein at least two reference compounds are introduced with the sample into the mass spectrometer. 9. The method according to embodiment 8, wherein the at least two reference compounds are used to quantitatively analyse the amount of chlorinated paraffins in the sample using a lock-plus-cali mass technique. 10. The method according to any preceding embodiment, wherein the mass spectrometer is a high resolution mass spectrometer. 11. The method according to embodiment 10, wherein the mass spectrometer is a double focussing high resolution mass spectrometer. 12. The method according to any preceding embodiment, wherein immediately before step (ii), the sample is separated in a gas chromatography column. 13. The method according to embodiment 12, wherein a sample pathway between an outlet of the gas chromatography column and an electrode for the field ionisation is less than 4.0 mm. 14. The method according to embodiment 13, wherein the sample pathway is less than 1.0 mm, preferably less than 0.5 mm.

Claims

1. A mass spectrometer for the quantification of chlorinated paraffins, the mass spectrometer comprising:(a) an inlet system comprising a gas chromatography column;(b) an ion source comprising an ion volume, the ion volume housing a field ionisation electrode affixed therein; and(c) a mass analyser;wherein the ion volume is configured to receive the gas chromatography column so as to permit varying of the distance and / or the alignment between the field ionisation electrode and an outlet of the gas chromatography column within the ion volume.

2. The apparatus according to claim 1, wherein the mass analyser comprises a sector field mass analyser.

3. The apparatus according to claim 1, wherein the mass analyser comprises an ion trap mass analyser, preferably an Orbitrap™ mass analyser.

4. The apparatus according to any one of claims 1 to 3, wherein the ion source is removable.

5. The apparatus according to any one of claims 1 to 4, wherein ion volume is configured toreceive the gas chromatography column in a direction perpendicular to an outlet of the ion volume.A

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