WO2023139532A1

WO2023139532A1 - Ion source

Info

Publication number: WO2023139532A1
Application number: PCT/IB2023/050500
Authority: WO
Inventors: Marco CISTERNI
Original assignee: Cisterni Marco
Priority date: 2022-01-21
Filing date: 2023-01-20
Publication date: 2023-07-27

Abstract

The invention relates to an ion source comprising: - an inlet for a sample to be analysed; - a plurality of components forming a path for an ion flow, wherein at least one of the components forming the path for the ion flow comprises graphite or consists essentially or entirely of graphite, and the at least one component is at least partially coated, preferably entirely coated, with silicon carbide (SiC). This ion source has demonstrated to be particularly effective for analysing one or more of the following substances: - high-boiling organic substances containing medium to long chain hydrocarbons, preferably esters or ethers of long chain hydrocarbons; - volatile compounds, for example perfumery compounds; - nitro compounds, preferably aromatic nitro compounds, more preferably nitrobenzene compounds; - halogenated composites, for example containing at least two halogen substituents, preferably chlorine, bromine, iodine.

Description

“Ion source”

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DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to an ion source in which at least one of the components comprises graphite and is at least partially coated with silicon carbide (SiC).

BACKGROUND OF THE INVENTION

Mass spectrometry is a technique widely used for qualitative and quantitative analysis of substances and mixtures of substances, also complex ones. Mass spectrometers are often used as detectors coupled to a gas chromatograph (GC-MS system). In a mass spectrometer, the ion source has the function of receiving the substances to be analysed, ionising them, and sending them, after they have been ionised and fragmented, to the other components of the spectrometer, which selects the ions based on their mass-to- charge ratio.

An ion source comprises an ionisation chamber, inside which a beam of electrons emitted by a heated filament flows through a sample to be ionised, and is then discharged onto a collector, and a system for accelerating the ions produced in the chamber. The most widely used ion sources are electron ionisation (El) or chemical ionisation (Cl) sources. In an electron ionisation (El) ion source, a beam of electrons (emitted by an incandescent metal filament) passes through the chamber and collides with the substance to be analysed contained in the chamber itself, bringing about the ionisation and fragmentation thereof. In a chemical ionisation ion source, by contrast, the ionisation of the substance to be analysed is mediated by the presence of a carrier gas.

The most commonly used carrier gases are helium, nitrogen, noble gases and hydrogen. The walls of the ionisation chambers of ion sources, both electron ionisation and chemical ionisation chambers, are generally made with electrically and thermally conductive materials, such as, for example, stainless steel or other metals or metal alloys. These materials meet certain requirements, including: stability at the operating temperatures, chemical inertia, and electrical and thermal conductivity.

Furthermore, such materials are particularly useful when the carrier gas is helium, whereas they do not allow reliable analyses to be performed when the carrier gas is hydrogen or when the hydrogen is used as a cleaning agent for the ion chamber in the presence of helium as the carrier gas, due to the undesirable interactions between the analytes (and their ions), the hydrogen and the materials of the ion source, whether based on stainless steel or other metals.

Because of these interactions, changes occur in the mass spectrum and there is distortion (tailing) of the chromatographic peaks. These problems are particularly serious with analytes that have functional groups or chemical bonds that can be reduced in the presence of hydrogen. In some cases, the main ions of the analyte show undistorted peaks, but the total ion chromatogram (TIC) will have substantial distortion deriving from the products of degradation formed by the interaction between the hydrogen and the compound to be analysed.

The problem of the distortion of the mass spectrum can be a serious problem. For some compounds, the spectrum can be so different from the spectra of the reference library (for example, National Institute of Standards and Technology (NIST) Mass Spectral Database) used for identification as to be mistakenly identified as a different compound. In the art there are known ion sources which attempt to overcome the problem of the interference between analytes and hydrogen.

WO201 6/083964 describes an ion source in which the inner walls of the ionisation chamber are covered with a graphite layer. This ion source makes it possible to minimise the spectral distortions due to the interaction between the ionisation chamber walls and the samples of substances to be analysed when the mass spectrometer is used as the detector in a GC-MS system that makes use of hydrogen as the carrier gas; furthermore, it enables the response of the GC-MS instrument to some substances to be improved, thus improving the shape and height of the peaks associated with them.

This solution is effective for attenuating the spectral alterations of a large number of organic substances; however, some classes of particularly reactive substances, such as nitro compounds, can also show an alteration of the mass spectrum even when the ionisation chambers described in WO2016/083964 are used; moreover, even when the ion source described in WO2016/083964 is used, there can be a reduction in the response and sensitivity of the GC-MS system vis-a-vis high-boiling organic substances that contain medium to long chain hydrocarbons in the molecular structure. EP3454359 aims to overcome the problems encountered by WO2016/083964 by providing ion sources made of pyrolytic graphite having a certain density or ion sources made of graphite with a pyrolytic graphite coating. This solution has demonstrated to be particularly effective for minimising the alterations of the spectrum and response of specific organic substances, such as nitro compounds and high-boiling organic substances containing hydrocarbons, in particular medium to long chain hydrocarbons. Other ion sources known in the art are described in W02010/131008 and WO2021/194617.

W02010/131008 relates to an ion source made of stainless steel or alloys thereof with a coating that can be, among other things, silicon carbide (SiC). The examples, however, all refer to a titanium carbide (TiC) coating and show an increase in the signal intensity compared to an uncoated ion source only in relation to some analytes. However, this document does not provide solutions for solving the technical problem of the peak distortion and spectral alterations that occur in the presence of hydrogen; it only describes effects on the sensitivity of analysis. In fact, hydrogen is not even described as a possible carrier gas.

WO2021/194617 describes an ion source made of stainless steel or similar inert or high efficiency materials, coated at least partially with silicon, silicon hydride or a combination thereof. The results of the analyses conducted with this source show improvements in the analysis of some types of analytes where hydrogen is used as the carrier gas compared to classic sources and compared to graphite sources.

Despite the solutions already available in the literature, there remains a need for ion sources that show spectral fidelity when used with hydrogen gas.

SUMMARY OF THE INVENTION

In a first aspect thereof, the present invention relates to an ion source comprising:

- an inlet for a sample to be analysed;

- a plurality of components forming a path for an ion flow, in which at least one of the components forming the path for an ion flow comprises graphite or consists essentially or entirely of graphite, and the at least one component is at least partially coated with silicon carbide, preferably entirely coated with silicon carbide (SiC).

In a preferred embodiment, the inner surface of at least one of the components of the ion source comprising graphite, which forms the path for the ion flow, is at least partially coated with SiC; preferably, the entire inner surface is coated with SiC.

The carrier gas is selected from hydrogen, nitrogen, at least one noble gas and mixtures thereof. In one embodiment, the carrier gas is hydrogen.

In a further aspect, the present invention relates to an ion source comprising:

(i) an ionisation chamber;

(ii) at least one repeller electrode; and

(iii) an exit wall, wherein at least one of the elements (i) to (iii) comprises graphite or consists essentially or entirely of graphite, and is at least partially coated, preferably totally coated, by a layer of silicon carbide (SiC).

In one embodiment, the inner surface of at least one of the elements (i) to (iii) comprising graphite or consisting essentially or entirely of graphite, is coated at least partially with silicon carbide; it is preferably totally coated with silicon carbide.

In another aspect, the invention relates to a method of analysing a sample with a mass spectrometer which includes the use of the ion source described herein. According to this method, a carrier gas comprising the sample is made to flow through the inlet of the ion source; the sample is ionised by at least one of the components forming the path for the ion flow, providing ions that are analysed according to their mass-to-charge ratio, wherein at least one of the components is made according to the invention.

In other words, the invention relates to a mass spectrometry method comprising:

(a) ionising a sample in the ion source as described above, preferably a gaseous sample comprising at least one organic substance, thereby producing ions; and

(b) analysing the ions produced in the ion source according to their mass-to-charge ratio. In another aspect, the invention relates to a method of analysing a sample with a gas chromatograph-mass spectrometer which includes the use of the ion source described herein. According to this method, a carrier gas comprising the sample is first made to flow through a separation column and subsequently ionised by at least one of the components forming the path for the ion flow, thereby providing ions that are analysed according to their mass-to-charge ratio, wherein at least one of the components is made according to the invention.

In other words, the present invention relates to a gas chromatography-mass spectrometry method comprising:

(A) providing a first gaseous sample comprising at least one analyte and a carrier gas selected from hydrogen, nitrogen, at least one noble gas and mixtures thereof;

(B) providing a separation column comprising at least one stationary phase that is able to selectively adsorb said at least one analyte;

(C) introducing the first sample at one end of the separation column and allowing the sample to flow through the column, thereby obtaining at least one second gaseous sample;

(D) ionising said at least one second gaseous sample in one mass spectrometer comprising an ion source as described above, thereby producing ions; and

(E) analysing the ions produced according to their mass-to-charge ratio.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 shows an example of an ion source made according to the invention;

Fig. 2 shows chromatograms of isobornyl acetate (CAS 125-12-2): A) reference spectrum of the National Institute of Standards and Technology (NIST); B) metal ion source; C) pyrolytic graphite ion source; D) ion source according to the invention.

Fig. 3 shows chromatograms of the substance styrallyl acetate/benzenemethanol, methyl acetate (CAS 93-92-5): A) reference spectrum of the National Institute of Standards and Technology (NIST); B) metal ion source; C) pyrolytic graphite ion source; D) ion source according to the invention.

Fig. 4 shows chromatograms of the substance “Rosafix’Vbenzenemethanol, trichloromethyl acetate (CAS 90-17-5): A) reference spectrum of the National Institute of Standards and Technology (NIST); B) metal ion source; C) graphite + pyrolytic graphite ion source; D) ion source according to the invention (graphite +SiC).

Fig. 5 shows a detail of the chromatogram obtained with the metal source of “Rosafix’Vbenzenemethanol, trichloromethyl acetate in Fig. 4B, with reference to ions 104, 107 and 172. Fig. 6 shows the chromatograms of the substance Heptachlor (CAS 76-44-8): A) reference spectrum of the National Institute of Standards and Technology (NIST): on the left, chromatogram of heptachlor; on the right, spectrum of 4-7-methano-1 H- indene,4,5,6,7,8,8-hexachloro-3a,4,7,7,a-tetrahydro (i.e. heptachlor with a Cl substituted by H); B) metal ion source; C) graphite + pyrolytic graphite ion source; D) ion source according to the invention (graphite + SiC).

Fig. 7 shows the chromatograms of the substance Lindane (CAS 58-89-9): A) reference spectrum of the National Institute of Standards and Technology (NIST); B) metal ion source; C) graphite + pyrolytic graphite ion source; D) ion source according to the invention (graphite + SiC).

Fig. 8 shows the chromatograms of the substance Fenitrothion (CAS 122-14-5): A) reference spectrum of the National Institute of Standards and Technology (NIST); B) metal ion source; C) graphite + pyrolytic graphite ion source; D) ion source according to the invention (graphite + SiC).

Fig. 9 shows the chromatograms of a sample containing polychlorinated biphenyls (PCBs): at the top, ion source according to the invention (graphite + SiC); at the bottom, metal ion source.

DETAILED DESCRIPTION OF THE INVENTION

In the context of the present invention, the term “graphite” refers to a material consisting essentially of graphitic carbon (apart from the inevitably present impurities), i.e. a material in which carbon presents itself in the allotropic form of graphite, irrespective of any structural defects that may be present. The graphitic material that is useful for the implementation of the present invention presents itself as a material characterised by a long-range crystalline order, preferably in three dimensions, determinable by diffractometric analysis.

In the present description and in the appended claims, the term “inner surface” refers to the surface of an element, in particular of at least one of the elements (i) to (iii), which is in contact with the sample subjected to ionisation in the ion source.

In the context of the present invention, the term “high-boiling organic substance” refers to an organic substance with a boiling point greater than or equal to 250°C, preferably greater than or equal to 300°C, measured at a pressure of about 0.1 MPa. In the context of the present invention, the term “medium to long chain hydrocarbon” refers to a saturated or unsaturated, cyclic or acyclic, linear or branched hydrocarbon containing at least 6, preferably at least 8, carbon atoms.

In the context of the present invention, the term “nitro compound” refers to an organic compound comprising at least one -NO2 group bound to a carbon atom.

The term "ionisation chamber" is used here to refer to a solid structure that encloses a volume in which the sample, typically a gas, is ionised.

The term “repeller electrode” means an electrode, set at an appropriate electrical potential, which generates a field intended to direct the ions produced in the ionisation chamber towards the subsequent components of the mass spectrometer.

The term “exit wall” means a wall provided with a suitable opening that allows the passaggio of the ions produced in the ionisation chamber towards the subsequent components of the mass spectrometer.

The “optional further components”, intended to increase the ionisation of the substances to be analysed as much as possible and optimise the transmission of ions to the subsequent components of the mass spectrometer, can also include further electrodes which, by suitably modifying the overall electric field, can better direct the ions in their path inside the instrument.

The term “component substantially consisting of graphite” means that the component comprises from 80% to 99.5% graphite, preferably from 90% to 99%.

The term “component entirely consisting of graphite” means that the component comprises 100% graphite, apart from the impurities necessarily present.

The expressions “layer of silicon carbide” and “silicon carbide coating” mean that the layer/coating comprises silicon carbide or consists essentially of silicon carbide or consists entirely of silicon carbide.

The term “layer/coating substantially consisting of silicon carbide” means that the layer/coating comprises from 80% to 99.5% silicon carbide, preferably from 90% to 99%. The term “layer/coating entirely consisting of silicon carbide” means that the layer/coating comprises 100% silicon carbide, apart from the impurities necessarily present.

The term “volatile substance” means a substance having a boiling point below 250°C at a standard pressure of 101.3 kPa, as in the case for example of most perfumery substances. The ion source according to the invention can be an electron ionisation (El) or a chemical ionisation (Cl) source; it is preferably an electron ionisation ion.

The ion source comprises an inlet for the sample and a plurality of components having a surface and forming a path for an ion flow, wherein at least one of the components comprises graphite or consists substantially or entirely of graphite, and the surface of at least one of the components comprising graphite or consisting substantially or entirely of graphite is at least partially coated with silicon carbide.

When the ion source is part of a device, for example a mass spectrometer, one or more of the components of the ion source that are between the sample inlet and the ion flow outlet, comprises graphite or consists substantially or entirely of graphite, and is at least partially coated with silicon carbide. In particular, the inner surface of the at least one component is at least partially coated with silicon carbide.

The components of the mass spectrometer downstream of the ion flow outlet are not coated with silicon carbide.

When the surface of at least one of the components forming the path for an ion flow is at least partially coated with silicon carbide, the portion of the surface of the component coated with silicon carbide can be equal to or less than a about 50% or can be equal to or greater than 50%. In particular, the portion of the surface coated with silicon carbide can be between 55% and 99.9%, for example equal to 55%, 60%, 65%, 70%, 75%, 85%, 90%, 95%, 98%, 99%, 99.5%, or 99.9% of the surface. The portion of the surface of every given component that is coated with silicon carbide can be identical for all the coated components, or it can be different for every particular component.

The surface of the at least one component comprising graphite or consisting substantially or entirely of graphite, can be entirely coated with silicon carbide. In this case, the portion of coated surface is equal to 100%.

The plurality of components can include one, two, three, four, five or six components partially or entirely coated with silicon carbide, wherein the component or components that are coated with silicon carbide comprise graphite or consist substantially or entirely of graphite.

In one embodiment, the component comprising graphite or consisting substantially or totally of graphite, is selected from: an ionisation chamber, at least one repeller electrode and an exit wall; it is preferably an ionisation chamber. In one embodiment the ionisation chamber, the at least one repeller electrode and/or the exit wall are all made as described in the present invention.

The silicon carbide coating of at least one of the components of the ion source is at least one layer of silicon carbide, preferably having a thickness of between about 30 and about 120 microns. For example, the layer of silicon carbide can have a thickness of about 30, 40, 50, 60, 70, 80, 90, 100, 110, or about 120 microns.

The components described here as coated with a layer of silicon carbide can have at least one layer of silicon carbide with a thickness that is substantially equal or similar, or every component coated with at least one layer of silicon carbide can have a layer of silicon carbide with a thickness differing from that of the other components. The layer of silicon carbide can be deposited on the component using known techniques, including CVD (chemical vapour deposition), ALD (atomic layer deposition), PVD (physical vapour deposition), sputtering, evaporation, plating techniques and similar techniques known in the art.

The coating/layer of silicon carbide described here comprises silicon carbide or consists substantially or entirely of silicon carbide, apart from the impurities necessarily present. The graphite comprised in the at least one component of the ion source or of which the at least one component consists essentially or entirely, does not comprise amorphous carbon; in particular, it does not comprise DLC (diamond-like carbon).

Preferably, the graphite can have density of between 1 .80 and 2.26 g/cm³, preferably between 1.80 and 2.25 g/cm³.

The graphite is selected from polycrystalline graphite, preferably isotropic polycrystalline graphite, and pyrolytic graphite, preferably highly oriented pyrolytic graphite.

Polycrystalline graphite is a graphitic material with coherent crystallographic domains (crystalline grains) of limited size. Polycrystalline graphite can be obtained by high- temperature graphitisation of a carbon material. Preferably, the polycrystalline graphite can be isotropic, i.e. it may not have a preferential crystallographic orientation of the microstructure and thus show the same physicochemical properties in all directions. Isotropic polycrystalline graphite can be obtained by uniformly pressing (isostatic pressing) the carbon material during the graphitisation process. Polycrystalline graphite and isotropic polycrystalline graphite are commercially available graphitic materials. Pyrolytic graphite is a graphitic material with a high degree of crystallographic orientation along the axis perpendicular to the surface of the material. Pyrolytic graphite can be obtained by graphitisation of pyrolytic carbon or by chemical vapour deposition (CVD) at temperatures higher than about 2,500°K (about 2,227°C). Highly oriented pyrolytic graphite can be obtained by chemical vapour deposition at temperatures of about 3,300°K (about 3,027°C) with suitable hot-pressing processes in order to develop the orientation of the planes. CVD processes for obtaining pyrolytic graphite and highly oriented pyrolytic graphite are in themselves known in the art.

Pyrolytic graphite, optionally highly oriented, generally has a higher density than polycrystalline graphite. Furthermore, it is characterised by a very low surface porosity.

In one embodiment, the graphite can be polycrystalline graphite having a density of about 1 .65-2.00 g/cm³, preferably about 1 .80-2.00 g/cm³

In one embodiment, the graphite can be pyrolytic graphite having a density of about 2.00 - 2.26 g/cm³, preferably about 2.00 - 2.25 g/cm³, preferably about 2.15 - 2.23 g/cm³.

In one embodiment, the graphite is selected from polycrystalline graphite, preferably isotropic polycrystalline graphite, and pyrolytic graphite, preferably highly oriented pyrolytic graphite, whose inner surface is coated at least partially or entirely with silicon carbide.

The Applicant has surprisingly found that when at least one of the components of the ion source comprises or consists essentially or completely of graphite and is at least partially coated with silicon carbide, the ion source according to the invention can be used for mass spectrometry of organic substances that are reactive towards the materials making up conventional ion sources.

In particular, the ion source as described above can be advantageously used for ionisation of high-boiling organic substances containing medium to long chain hydrocarbons, nitro compounds and mixtures thereof, as it reduces to a minimum the alterations of the mass spectrum and/or the loss of instrument sensitivity for the substances analysed, caused by the interaction of the substances themselves with the inner surfaces of the ion source.

These advantages are manifested in particular when the carrier gas is hydrogen, as the ion source according to the invention allows the interference between the compounds to be analysed and the hydrogen to be minimised. The ion source according to the invention comprises:

(i) an ionisation chamber; and/or

(ii) at least one repeller electrode; and/or

(iii) an exit wall wherein

- at least one of the elements (i) to (iii) comprises or consists substantially or completely of graphite; and

- at least one of the elements (i) to (iii), preferably the inner surface of at least one of the elements (i) to (iii), is at least partially coated with silicon carbide, preferably with at least one layer comprising silicon carbide or consisting essentially or entirely of silicon carbide. At least one of the elements (i) to (iii) comprises or consists essentially or completely of graphite selected from: polycrystalline graphite, preferably isotropic polycrystalline graphite, and pyrolytic graphite, preferably highly oriented pyrolytic graphite, whose inner surface is at least partially, or else entirely, coated with silicon carbide. The silicon carbide coating forms at least one layer on the graphite component which coats the inner surface at least partially or entirely.

Preferably, the inner surface of the ionisation chamber (i) is at least partially coated with at least one layer of silicon carbide; more preferably, the inner surface of the ionisation chamber (i) and of the at least one repeller electrode (ii) is at least partially coated with at least one layer of silicon carbide; even more preferably the inner surface of all the elements (i) to (iii) is at least partially coated with at least one layer of silicon carbide.

Preferably, the inner surface of the ionisation chamber (i) is entirely coated with at least one layer of silicon carbide; more preferably, the inner surface of the ionisation chamber (i) and of the at least one repeller electrode (ii) is entirely coated with at least one layer of silicon carbide; even more preferably, the inner surface of all the elements (i) to (iii) is entirely coated with at least one layer of silicon carbide.

According to this embodiment, the elements (i) to (iii) of the ion source can be made in a shape and size that are in themselves known in the art, optionally configured to replace analogous elements of ion sources present on the market.

The ion source can comprise “optional further components” intended to increase the ionisation of the substances to be analysed as much as possible. In this case, also at least one of the other optional additional components comprises or consists substantially or completely of graphite and is at least partially coated with silicon carbide, preferably with at least one layer comprising silicon carbide or consisting essentially or entirely of silicon carbide.

Figure 1 shows a diagram of a preferred embodiment of the invention, in which the ion source is denoted in its entirety by 1. Said ion source 1 comprises: a housing 2 configured to form the electron ionisation chamber 3; a repeller electrode 4; and an exit wall 5 configured to permit the passage of the ions 6 generated in the ionisation chamber 3 by colliding the electrons 7 with the sample to be analysed (not represented). The ion source 1 is configured to generate the ions 6 by colliding the sample to be analysed with the electrons 7, wherein the electrons 7 are emitted by a filament 8 and collected by a collector 9. The ions 6 generated by the collision are accelerated by the repeller electrode 4 towards the exit wall 5 and come out of the ion source. Once outside the ion source, the ions 6 can be separated by an analyser, in itself known in the art, on the basis of their mass-to-charge ratio.

According to this embodiment, the ionisation chamber comprises graphite, preferably polycrystalline or pyrolytic graphite, or consists essentially or entirely of graphite and the inner surface of the ionisation chamber 3 is partially or entirely coated with at least one layer of silicon carbide. The exit wall 5 can optionally comprise or consist essentially or entirely of graphite. In this case, the inner wall thereof is coated at least partially or entirely with at least one layer of silicon carbide.

The polycrystalline graphite is preferably isotropic; the pyrolytic graphite is preferably highly oriented.

The repeller electrode 4 can also optionally comprise or consist essentially or entirely of graphite and be partially or totally coated, on the inner surface, with at least one layer of silicon carbide.

The ion source according to the invention can be used in a mass spectrometer so as to reduce or eliminate alterations in mass spectra and chromatograms of organic substances. This advantage is more greatly evident for some chemical species, such as, for example, high-boiling organic substances containing hydrocarbons, preferably medium to long chain saturated aliphatic hydrocarbons, volatile substances having a boiling point lower than 250°C at a standard pressure of 101.3 kPa, as in the case, for example, of the majority of perfumery substances, nitro compounds and composites containing many chlorine groups.

In particular, the Applicant has observed that using the ion source according to the present invention, the alterations of the spectra of nitro compounds and highly chlorinated compounds due to the interaction of the nitro groups or chlorine groups with the inner surfaces of the ion source and with the carrier gas can be significantly reduced or eliminated.

Furthermore, by using the ion source according to the present invention the instrument’s response (sensitivity) to high-boiling organic substances containing hydrocarbons, preferably medium to long chain hydrocarbons and volatile substances, such as perfumery substances, can be advantageously improved.

In a further aspect thereof, the present invention relates to the use of an ion source as described above to minimise the alterations of the mass spectrum and/or increase the analytic sensitivity of a mass spectrometer in analysing at least one organic compound selected from nitro compounds, highly chlorinated composites, high-boiling organic compounds containing at least one medium to long chain hydrocarbon and volatile compounds.

Preferably, the nitro compounds can be aromatic nitro compounds.

Preferably, the high-boiling organic substances containing a medium to long chain hydrocarbon, can be esters or ethers containing medium to long chain hydrocarbons.

In a further aspect thereof, the present invention relates to a mass spectrometer comprising the ion source as described above.

This mass spectrometer has the advantage of having greater precision, accuracy and sensitivity as a detector when used for the analysis of the substances specified, preferably when used in a gas chromatography-mass spectrometry (GC-MS) system.

Preferably, the mass spectrometer is operated with hydrogen as the carrier gas.

In a further aspect thereof, the present invention refers to a mass spectrometry method comprising:

(a) ionising, in the ion source as described above, a sample, preferably a gaseous sample, comprising at least one analyte, thereby producing ions; and

(b) analysing the ions produced in the ion source according to their mass-to-charge ratio. The method can be implemented with a mass spectrometer comprising the ion source according to the invention.

The mass spectrometry method according to the invention has the advantage of greater accuracy, precision and/or sensitivity in detecting the mass spectra of some organic substances, described below.

Preferably, the sample can further comprise at least one gas selected from hydrogen, nitrogen, a noble gas and mixtures thereof. Preferably, the gas is hydrogen.

Some chemical substances, such as, for example, nitro compounds and highly chlorinated compounds, are particularly reactive both towards the materials making up the inner surfaces of ion sources, and towards some carrier gases, such as, for example, hydrogen, used in GC-MS analysis. The spectra of such substances can thus undergo alterations upon interacting both with the carrier gas and with the inner surfaces of the ion source.

High-boiling organic substances containing hydrocarbons, preferably medium to long chain hydrocarbons, such as, for example, esters containing medium to long chain hydrocarbons, can be adsorbed onto the inner surface of the components of the ion sources, resulting in a lowering of the height of the peaks and the appearance of tails, which compromise the quantitative and qualitative analysis of these compounds.

Such problems can be solved or minimised using a mass spectrometer comprising an ion source according to the invention.

Mass spectrometers can be used as detectors for the identification of substances in combination with various chromatographic techniques, such as, for example, gas chromatography and high-performance liquid chromatography (HPLC). The mass spectrometer comprising the ionisation chamber according to the invention is particularly suitable for being used as a detector in a GC-MS system.

Therefore, in a further aspect thereof, the present invention relates to a gas chromatography-mass spectrometry method (GC-MS method) comprising:

(B) providing a separation column comprising at least one stationary phase that is able to selectively adsorb said at least one analyte; (C) introducing the first sample at one end of the separation column and allowing the sample to flow through the column, thereby obtaining at least one second gaseous sample;

(D) ionising said at least one second gaseous sample in a mass spectrometer comprising an ion source as described above, thereby producing ions; and

(E) analysing the ions produced according to their mass-to-charge ratio.

In one embodiment, the noble gas is selected from helium, argon and mixtures thereof.

In one embodiment, the first gaseous sample comprises a plurality of analytes and step (C) is carried out by separating the plurality of analytes according to their affinity for the stationary phase.

The mass spectrometry or gas chromatography-mass spectrometry methods described above can be particularly suitable for analysing samples comprising at least one analyte selected from:

- high-boiling organic substances containing medium to long chain hydrocarbons, preferably esters or ethers of long chain hydrocarbons;

- volatile compounds, for example perfumery compounds;

- nitro compounds, preferably aromatic nitro compounds, more preferably nitrobenzene compounds;

- halogenated composites, for example containing at least two halogen substituents, preferably chlorine, bromine, iodine.

The Applicant has surprisingly found that MS or GC-MS analysis of the substances specified above improves considerably with the use of the ion source according to the invention comprising graphite coated at least partially with silicon carbide. This has been verified, in particular, for substances containing nitro groups or chlorinated substances, which are the ones that interact most with hydrogen in the case of metal ion sources.

If the ion source is made of graphite, pyrolytic graphite in particular, one observes a general decrease in the interaction between the substances to be analysed and the hydrogen, with a consequent improvement in the chromatogram. However, graphite poses problems tied to mechanical strength, which is lower than that of metal. The SiC coating envisaged by the invention allows the mechanical strength of the chamber to be decidedly increased, while at the same time increasing the inertia towards the substances to be analysed and hydrogen.

The combination between graphite and SiC makes it possible to maximise the properties specified above and also to exploit graphite’s properties of thermal conductivity, which are superior to those of steel and the metal alloys commonly used for ion sources. This ensures that the distribution of heat among the various parts of the ion source is optimal with graphite as compared with metal. This is very important in order to have all the inner surfaces at the same temperature. In fact, the source is usually heated at one end, but if the material does not distribute heat well, colder areas are created in which substances tend to condense and accumulate, creating problems of sensitivity and an increase in the noise of the analysis.

With the use of graphite, the substances to be analysed remain in a gaseous state without creating areas of accumulation or condensation. In addition, the SiC coating increases the inertia of the ion chamber and its mechanical strength.

Compared to metal ion sources, the ion source of the invention provides a better reproducibility of analysis, and requires no pre-conditioning before use, that is, no “preliminary” analyses need to be performed before proceeding to the actual analysis. Compared with the use of graphite ion sources, optionally coated with pyrolytic graphite, the ion sources of the invention have an advantage of robustness, imparted by the coating made of SiC, which is a very hard material.

This enables the construction of large-sized parts, which may also be fastened with screws to the other parts of the instrument, and not necessarily situated inside other metal parts. With graphite/pyrolytic graphite it is preferable to make smaller parts, to be inserted inside the “standard” metal parts of the instrument; however, the geometry of the ion source does not always allow this to be done, especially if the parts become too small to be produced.

The hard surface of the SiC coating makes it possible to have better resistance to wear and to carry out an abrasive and thus very effective cleaning. This has a positive impact on the lifespan and performance of the ion source.

The Applicant’s comparative experiments demonstrated that, with the ion source of the invention, one obtains better spectra for less volatile substances, substances containing many chlorine substituents, and nitro groups, for which graphite has inferior performance, unless the temperature of the ion source is greatly increased.

As regards the advantages of the present invention compared to the known solution of ion metal sources coated with silicon, the SiC coating shows a greater chemical inertia than silicon, above all with reference to samples containing alkyl halides or samples containing highly chlorinated substances.

The comparison performed by the Applicant between chromatograms produced with a metal ion source, a graphite ion source and an ion source made according to the invention, shows a decided improvement in the latter case, compared both to the metal ion source and the graphite one.

Below is a description of the experiments carried out by the Applicant which demonstrate the advantages connected to the present invention.

EXAMPLES

Perfumery substances, as an example of volatile substances, and pesticides, as an example of highly chlorinated substances or ones containing nitro groups, were analysed. The quality of the spectra acquired with the various ion sources was verified by comparing it with the reference spectra for the various substances present in the library of the National Institute of Standards and Technology (NIST).

The reference spectra were obtained using helium as the carrier gas; helium is an inert gas and does not give rise to undesirable reactions (which distort the spectra).

The aim of the invention is to obtain spectra that are as close as possible to the reference spectra.

All spectra are considered in reference to the top of the peak of the substance.

Operating conditions under which the tests were performed

GC oven

Initially 80°C, then 6 °C/min up to 300°C, then static for 3 min.

GC injector

300 °C, split injection of 5:1 or 10:1 (pesticides) or 80:1 (perfumery substances). Column

HP-5ms 30 m x 250 m x 0.25 pm column.

Carrier gas

Hydrogen, flow at 1 ml/min.

Detector

MSD, SCAN mode 30-400 amu.

Ion sources tested:

(i) metal;

(ii) graphite + pyrolytic graphite coating;

(iii) graphite + SiC coating.

Source temperature: 230°C (perfumery substances) or 290°C (pesticides)

Example 1 - Perfumery substances

Isobornyl acetate was analysed with the ion sources specified. The results are shown in Figure 2B-D (A: metal source, B: graphitic source, and C: graphite + SiC source), with reference to the NIST spectrum (figure 2A).

Styrallyl acetate/benzenemethanol, methyl acetate (CAS 93-92-5) was analysed with the ion sources specified. The results are shown in Figure 3B-D (A: metal source, B: graphitic source, and C: graphite + SiC source), with reference to the NIST spectrum (figure 3A).

With reference to Figures 2 and 3, it is evident that various problems occurred in the spectra with the metal source.

In the case of isobornyl acetate and styrallyl acetate, the spectra acquired with the metal source show numerous peaks that are absent in the reference spectra coming from the NIST library.

These distortions in the spectra make it very difficult to identify the substance by searching in the library (because the spectrum becomes very different).

It is easy to verify that these “distorted” spectra derive from the overlap of the “original” spectrum of the substances with spectra deriving from products of the reactions between those substances and hydrogen (in general reactions whereby the substances are reduced to hydrocarbons). For example, in the case of styrallyl acetate, the spectrum of styrene appears and becomes prevalent compared to the “original” reference spectrum of styrallyl acetate.

In the case of isobornyl acetate, a spectrum correlated to the hydrocarbon part of the substance appears and can be interpreted as camphene.

In the case of isobornyl acetate, in particular, attention should be paid to ion 93, which has a count equal to that of ion 95 with the metal source, whereas it should be about 50% compared to ion 95 according to NIST.

In the case of the styrallyl acetate, on the other hand, attention needs to be paid to the ratio between ions 105 and 104, and also between ions 122 and 104; in addition to the anomalous presence of ion 91 in the case of the metal source.

For both substances, the spectra acquired with the “graphite + SiC” source show no anomalous peaks and are much more similar to the reference spectra of the substances.

“Rosafix” / benzenemethanol, trichloromethyl acetate was analysed with the ion sources specified. The results are shown in Figure 4B-D (A: metal source, B: graphitic source, and C: graphite + SIC source), with reference to the NIST spectrum (figure 4A).

With reference to Figure 4, the spectrum acquired with the metal source is clearly the one that shows the largest number of peaks of distortion with the highest relative intensity. Furthermore, ions with an m/z greater than 210 are completely lacking.

The spectra acquired with the source coated with SiC and the source coated with pyrolytic graphite are much closer to the reference spectrum; the spectrum acquired with the source coated with pyrolytic graphite is worse due to the higher ratio between ion 172 and the base ion 107.

The 172/107 ratio should be about 10% according to the reference spectrum; the spectrum acquired with SiC is the closest, whereas with the metal source it exceeds 50%. The same may be said for the ratio between 104 and 107, which should be less than 10%, whereas it is over 50% with the metal source, as well as showing serious tailing, as can be seen in figure 5, which represents the extraction of ions 104, 107, 172 along the peak of the spectrum obtained with the metal source in figure 4B.

Example 2 - Pesticides Heptachlor was analysed with the ion sources specified. The results are shown in Figure 6B-D (A: metal source, B: graphitic source, and C: graphite + SiC source), with reference to the NIST spectrum (figure 6A).

This is a highly chlorinated substance. In the spectrum acquired with the metal source, one notes a strong distortion due mainly to the presence of the ion at 66 m/z.

This ion is the base peak of the spectrum of the substance 4-7-methano-1 H- indene,4,5,6,7,8,8-hexachloro-3a,4,7,7,a-tetrahydro, which can be considered ideally as derived from heptachlor, with the substitution of one of the Cl atoms with an H atom, which is the reaction likely to occur in the ion source.

The fact that the situation is different with the other sources shows that the reaction occurs on the surface of the ion source (with probable catalytic activity on the surface) and not in the gaseous phase.

The peak 66 derives from the fragment comprising the cyclopentene ring (visible in the structure of 4-7-methano-1 H-indene,4,5,6,7,8,8-hexachloro-3a,4,7,7,a-tetrahydro); it is a fragment devoid of Cl atoms; in fact, the pair of ions 2 amu apart typical of fragments comprising a Cl atom is not present.

In the reference spectrum of the substance (NIST; figure 6A) one notes the pair of ions at m/z 100 and 102, wherein 100 is the second highest ion in the entire spectrum; this pair derives from the fragment originating from the chlorocyclopentene present in the structure of heptachlor.

In the case of 4-7-methano-1 H-indene,4,5,6,7,8,8-hexachloro-3a,4,7,7,a-tetrahydro, this chlorocyclopentene ring has been reduced to cyclopentene, which in turn leads to the peak at 66 m/z in the spectrum.

The fact that, in the case of the spectrum acquired with the metal source, the relative count of the ion at 66 m/z ends up greatly exceeding not only ion 100, but even 272 (which is the base peak of the spectrum of heptachlor) would suggest that the “catalytic reduction” leading to the loss of Cl and the substitution with H already occurs in the molecule of heptachlor, rather than in the single fragment of chlorocyclopentene. If this were the case, one would observe a decrease in ion 100 (and 102) in favour of 66, whereas the rest of the spectrum would remain more or less unchanged.

However, what is observed is a distortion of the entire spectrum, with the spectrum of the substance “derived from the reaction with H”, 4-7-methano-1 H-indene,4,5,6,7,8,8- hexachloro-3a,4,7,7,a-tetrahydro, overlapping with the normal spectrum of heptachlor until becoming distinctly prevalent.

The result is, on the one hand, a net loss of sensitivity with respect to the ions characteristic of the substance, for example the ones at 272 and 100 m/z, which would normally be used to search for the substance; and on the other hand, a considerable worsening of the match between the acquired spectrum and the one in the library for the substance, eventually leading to a mistaken recognition of the substance itself.

The ratio between ions 66 and 100 is much lower, and thus more favourable, in the case of the spectrum acquired with the pyrolytic graphite source; and even more so in the case of the SiC-coated graphite source.

Lindane was analysed with the ion sources specified. The results are shown in Figure 7B-D (A: metal source, B: graphitic source, and C: graphite + SiC source), with reference to the NIST spectrum (figure 7A).

With reference to Figure 7, it may be noted that the lindane spectrum acquired with the metal source is completely wrong.

In this case one can clearly note, on the metal surface, the ion at 78 m/z (as well as the ions 50, 51 , 52) deriving from the catalytic reduction of lindane (hexachlorocyclohexane) to benzene.

What is observed, on the metal source, is in fact the spectrum of benzene instead of that of lindane (the catalytic reduction of lindane with hydrogen on a metal surface leads to benzene and not to cyclohexane; see for example:

This demonstrates that, on a metal surface, lindane is almost entirely reduced to benzene. In contrast, both in the source with pyrolytic graphite, and even more so with the SiC- coated source, one observes that the relative height of the peak at 78 m/z compared to the base ion is much lower and almost corresponds to the one observed in the reference spectrum of lindane.

In conclusion, the source coated with SiC is the one that shows the best behaviour, together with the pyrolytic graphite source, whereas the metal source is not suitable for the analysis of this substance. Fenitrothion was analysed with the ion sources specified. The results are shown in Figure 8B-D (A: metal source, B: graphitic source, and C: graphite + SiC source), with reference to the NIST spectrum (figure 8A).

In this case, the “worst” spectrum is the one acquired on graphite plus pyrolytic graphite. It is interesting to verify ions 247, 138, and 93.

247 is easily identifiable as deriving from the reaction NO2 — NH2 (in the presence of H2); in fact, it derives from 277 (molecular ion) with the subtraction of 30 amu (difference between the weight of NO2 and NH2).

138 likewise derives from reactions with hydrogen, and like 93 is rather high in the spectrum acquired with the source made of graphite plus pyrolytic graphite.

The spectrum acquired with the source made of graphite plus SiC is the one that best matches the reference.

Example 3 - High-boiling substances (series of PCBs, polychlorinated biphenyls)

Figure 9 shows the difference between the chromatogram acquired with the SiC source (top) and metal source (bottom), of a sample containing PCBs (polychlorinated biphenyls).

The peaks present are, in order: PCBs Nos. 28, 52, 101 , 138, 153, 180.

The ratio between the area of the peak of PCB 180 (heptachlorobiphenyl) and the area of the peak of PCB 28 (trichlorobiphenyls), is equal to 88% in the case of SiC, whereas it is equal to 40% in the case of the metal source.

This shows, as can also be seen for the other peaks, that in the metal source the instrument’s response decreases rapidly as the molecular weight (and boiling point) of the substances increases.

Furthermore, one notes a tailing of the peaks in the chromatogram acquired with the metal source.

The signal-to-noise ratio with the SiC source is about 250 for PCB 28, and 200 for PCB 180, thus showing a constant sensitivity and response over the whole series.

With the metal source, by contrast, one finds a signal-to-noise ratio of about 200 for PCB 28, and only 70 for PCB 180, thus showing a large reduction in response and sensitivity (- 65% between one substance and the other). The boiling point of trichlorobiphenyls, including PCB 28, is about 330°C, whilst the boiling point of heptachlorobiphenyls, including PCB 180, is about 420 °C.

This is an indicator of a reduction in sensitivity for high-boiling substances, in the case of the metal source, which does not occur with SiC. This can be caused not only by combined effects of the presence of hydrogen and the metal surface, but also by a non-optimal distribution of heat, with some points of the ion source at a lower temperature than expected.

Finally, the spectra of many of these PCBs show an alteration in the case of the metal source, which is absent in the case of the SiC-coated source.

Claims

1. Ion source comprising:

- an inlet for a sample to be analysed;

- a plurality of components forming a path for an ion flow, in which at least one of the components forming the path for the ion flow comprises graphite or consists essentially or entirely of graphite, and the at least one component is at least partially coated, preferably entirely coated, with silicon carbide (SiC).

2. Ion source according to claim 1 , wherein the inner surface of at least one of the components forming the path for the ion flow is at least partially coated with silicon carbide.

3. Ion source according to claim 1 or 2, wherein the at least one component is totally coated with silicon carbide.

4. Ion source according to any one of claims 1 to 3, wherein the at least one component, preferably the inner surface of the at least one component, is coated with at least one silicon carbide layer.

5. Ion source according to any one of claims 1 to 4, wherein the ion source is a chemical ionisation ion source or an electron ionisation ion source.

6. Ion source according to any one of claims 1 to 5, comprising:

(i) an ionisation chamber;

(ii) at least one repeller electrode; and

(iii) an exit wall, in which at least one of the elements (i) to (iii) comprises graphite or consists essentially or entirely of graphite, and is at least partially coated, preferably entirely coated, with a layer of silicon carbide (SiC).

7. A mass spectrometer or gas chromatograph-mass spectrometer comprising an ion source according to any one of claims 1 to 6.

8. A method of analysing a sample with a mass spectrometer comprising the ion source according to any one of claims 1 to 6, comprising:

- flowing a carrier gas, preferably selected from hydrogen, nitrogen, at least one noble gas and mixtures thereof, including a sample for analysis, through the inlet of the ion source;

- ionising the sample with at least one of the components forming the path for the ion flow, providing ions;

- analysing ions according to their mass-to-charge ratio.

9. A method of analysing a sample with a gas chromatograph-mass spectrometer comprising the ion source according to any one of claims 1 to 6, comprising:

- flowing a carrier gas, preferably selected from: hydrogen, nitrogen, at least one noble gas and their mixtures, including a sample to be analysed, through a separation column;

- analysing ions according to their mass-to-charge ratio.

10. Method according to claim 8 or 9, wherein the sample to be analysed comprises one or more of the following substances:

- volatile compounds, preferably perfumery compounds;

- nitro compounds, preferably aromatic nitro compounds, more preferably benzene nitro compounds;

- halogenated composites, preferably containing at least two halogen substituents, preferably chlorine, bromine, iodine.