Improved Spark Stand for Optical Emission Spectrometry

Abstract

A spark stand for an optical emission spectrometer, comprising: a spark chamber; a gas inlet for flowing gas into the spark chamber; a gas outlet for carrying gas from the spark chamber; wherein one or more internal surfaces of the spark chamber, gas inlet and/or gas outlet comprise an anti-adhesion material. The anti-adhesion material can enable reduced adhesion of ablated material, such as metallic dusts for example, onto the surfaces within the spark stand.

Claims

1. A spark stand for an optical emission spectrometer, comprising: a table having an aperture for receiving a solid sample to be analysed such that the solid sample covers the aperture; a spark chamber having an electrode therein; wherein the aperture is positioned over the spark chamber; a gas inlet for flowing gas into the spark chamber; a gas outlet for carrying gas from the spark chamber; wherein one or more internal surfaces of the spark chamber, the gas inlet, or the gas outlet comprise an anti-adhesion material.

2. A spark stand according to claim 1, wherein the anti-adhesion material has dynamic and static coefficients of friction of 0.5 or less, or 0.4 or less, or 0.3 or less.

3. A spark stand according to claim 1, wherein the anti-adhesion material comprises a polymeric material.

4. A spark stand according to claim 1, wherein the polymeric material comprises a fluorinated polymeric material.

5. A spark stand according to claim 4, wherein the fluorinated polymeric material comprises at least one of: a fluorinated polyalkylene, a fluorinated functional alkane polymer, or a fluorinated parylene polymer.

6. A spark stand according to claim 5, wherein the fluorinated polymeric material comprises at least one of: polytetrafluoroethylene (PTFE), fluorinated propylene ethylene (FPE), perfluoroalkoxy alkane (PFA), parylene F-AF4, or parylene F-VT4.

7. A spark stand according to claim 4, wherein the fluorinated polymeric material comprises a perfluorinated polymeric material.

8. A spark stand according to claim 3, wherein the anti-adhesion material comprises a mixture of two or more different polymeric materials.

9. A spark stand according to claim 1, wherein the anti-adhesion material comprises a ceramic material.

10. A spark stand according to claim 1, wherein the anti-adhesion material is provided as a coating.

11. A spark stand according to claim 1, wherein the anti-adhesion material is provided as a block of material.

12. A spark stand according to claim 1, wherein the anti-adhesion material forms a substrate material of the spark stand.

13. An optical emission spectrometer comprising: a spark stand having: a table having an aperture for receiving a solid sample to be analyzed such that the solid sample covers the aperture; a spark chamber having an electrode therein, wherein the aperture is positioned over the spark chamber; a gas inlet for flowing gas into the spark chamber; and a gas outlet for carrying gas from the spark chamber, wherein one or more internal surfaces of the spark chamber, the gas inlet, or the gas outlet comprise an anti-adhesion material.

14. A method of optical emission spectrometry, comprising: providing a spark stand having a spark chamber, a gas inlet for flowing gas into the spark chamber and a gas outlet for carrying gas from the spark chamber; and providing an anti-adhesion material at one or more internal surfaces of the spark chamber and/or gas inlet and/or gas outlet.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0026] FIG. 1 shows schematically a cross-sectional view of a spark stand.

[0027] FIG. 2 shows the chemical structures of fluorinated polymers used as surface coatings in the Examples.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0028] FIG. 1 shows a schematic cross-sectional view of a spark stand 1 that forms part of an optical emission spectrometer. The spark stand comprises a spark chamber 11 of generally cylindrical geometry, i.e. having a cylindrical chamber wall. The spark chamber houses a cylindrical electrode 7 having a pin-shaped end, which is surrounded by an insulator 4 to prevent discharges to the chamber wall. Insulator 4 is rotationally symmetric about the electrode 7. The view of the spark stand 1 has been cut away from other parts of the optical emission spectrometer. For example, a spectrograph is not shown that receives light emission from the spark chamber through an optical conduit 5.

[0029] The spark stand comprises an upper table 1A having an aperture 3 positioned over the spark chamber 11. In use, a metallic sample (not shown) is mounted onto the table so that a face of the sample covers aperture 3. A spark is ignited between the electrode 7 and the surface of the sample that faces the electrode. This generates a plasma which ablates and vaporises matter from the sample, followed by atomisation, excitation and light emission. The light is analysed by the spectrograph (not shown) to determine information about the composition of the sample.

[0030] The spark ignition takes place under an argon (Ar) atmosphere, which is provided by a flow of argon into the spark chamber 11 through gas inlet orifice 20 which is connected to gas inlet conduit 22. The gas inlet conduit 22 is fed with argon gas from an argon source upstream. The gas flows in the direction indicated by arrows 2. For example, argon gas of purity better than 99.997% may be fed into the spark chamber via the gas inlet at a rate of 5 slpm (standard litres per minute) during sample analysis. Ablated matter is carried from the spark chamber by the gas flow through a gas outlet orifice 30 and gas outlet conduit 32 to an exhaust pipe 6. The gas inlet and outlet orifices 20 and 30 lie on opposing sides of the spark chamber 11. The gas inlet and outlet conduits 22 and 32 are provided by channels formed in one or both of two modular metallic components that are stacked together: a lower table 1B and the upper table 1A. The lower table 1B is generally fixed in place and the upper table 1A is removable. The upper table 1A can thus be removed to allow cleaning of the spark chamber and gas inlet and outlet conduits. The lower, fixed table 1B is fixed in place by screws (not shown), which can be removed, if required, to allow removal of the table 1B and cleaning of the exhaust pipe. Over multiple cycles of analysis, i.e. many sparks, part of the ablated matter adheres on the insulator 4 surrounding the electrode 7 and accumulates on the surfaces of the spark chamber wall and conduit 32, causing performance degradation and requiring regular maintenance by cleaning of the insulator 4, spark chamber 11 and the fixed (1B) and movable (1A) tables.

[0031] In accordance with the invention, one or more of the internal surfaces of the spark chamber 11 and/or gas inlet 22 and/or gas outlet 32 comprise an anti-adhesion material. In an embodiment, one or more of the internal surfaces of at least the spark chamber 11 and/or gas outlet conduit 32 comprise the anti-adhesion material. Preferably, at least internal surfaces of the spark chamber 11 and gas outlet conduit 32 comprise the anti-adhesion material. The internal surface of the gas inlet is less prone to accumulation of ablated material as the direction of gas flow sweeps ablated material away from the inlet towards the outlet. However, in some embodiments, an internal surface of the gas inlet 22 can also comprise the anti-adhesion material, such as a portion of the conduit adjoining the inlet 20.

[0032] In some embodiments, the anti-adhesion material is provided as a coating, i.e. a coating having the function of reducing the adhesion of particles compared to the uncoated surface. The coating can be provided as a polymer coating, especially a fluorinated polymer coating. The coating can be provided as a powder coating (high performance powder coating) and/or a dry film coating.

[0033] Preferred properties of the anti-adhesion material include any one or more of: i) low static and dynamic friction coefficients, preferably each 0.5 or less, allowing for minimum ablated dust accumulation by mechanical forces; ii) strong abrasion resistance, for example having a wear rate of <0.001 mm3/(N*m); iii) high vacuum ultraviolet (VUV) resistance, which is important in the vicinity of the spark/arc light source, for example a) high chemical bond dissociation energy (Carbon-Fluorine is ˜546 kJ/mol) and b) low photon penetration depth (<200 nm); iv) high dielectric strength, for example at least 50 MV/m, which is important to maintain spark/arc stability in the vicinity of the electrode; and v) high resistance to chemical solvents, which are used for cleaning during maintenance procedures. The surface roughness of the anti-adhesion material may depend on the surface of the substrate. The surface roughness of the anti-adhesion material can be 10 μm or less. Another desirable property of the anti-adhesion material is a Rockwell hardness of at least R50, e.g. >R54 (PTFE).

[0034] The coating can be applied by methods such as chemical vapour deposition (CVD), which selectively and controllably grows a thin, conformal film onto the selected surfaces. Preferred anti-adhesion materials include polymers such as fluorinated polymers and perfluorinated polymers. Examples include: fluorinated polyalkylenes, e.g. polytetrafluoroethylene (PTFE) and fluorinated propylene ethylene (FPE) co-polymer; fluorinated functional alkane polymers, e.g. fluorinated alkoxyalkanes polymers, such as perfluoroalkoxy alkane (PFA) polymer, which are copolymers of tetrafluoroethylene (C.sub.2F.sub.4) and perfluoroethers; and fluorinated arylene polymers, e.g. parylenes, such as parylene F-AF4 and parylene F-VT4. The anti-adhesion material can be a mixture of any two or more types of polymers.

[0035] In other embodiments, the composition of the fixed (1B) and movable (1A) tables, or a part of them, can be modified or substituted to be inherently anti-adherent to the ablated metallic dust. Thus, the spark table substrates present internal surfaces in the spark chamber and gas inlet and outlet that are anti-adherent to the dust. As the regions surrounding the plasma operate at very high temperatures (thousands of Kelvin), it may be possible to replace a part of the conventional metallic substrate of the tables 1A and 1B surrounding the chamber, inlet and/or outlet with, for example, the anti-adherent material such as fluorinated polymers. The anti-adherent material, such as fluorinated polymers, may be provided as blocks. This may be achieved, for example, by mechanically substituting part of the metal substrate of the tables surrounding the chamber, inlet and/or outlet with blocks of the anti-adherent material.

[0036] In other embodiments, the whole composition of the fixed table (1B) and/or movable table (1A) of the spark stand could be made of the anti-adhesion material, i.e. instead of the conventional metal material that is used, the table could be made of, for example, an anti-adhesion polymer or ceramic material. This may be achieved, for example, by engineering the tables with polymers or ceramic materials, such as zirconium oxide (ZrO.sub.2) or boron aluminium magnesium alloys (BAM).

SPECIFIC EXAMPLES

[0037] Tests were performed in which coatings of various compositions were applied onto internal surfaces of a spark stand of the type shown in FIG. 1, where metallic dusts ablated from the sample are known to be deposited. The spark stand was part of an ARL™ iSpark™ optical emission spectrometer from Thermo Fisher Scientific™. Each of the coatings was applied to the outlet conduit 32 between the spark chamber 11 and the exhaust pipe 6 (shaded region shown in FIG. 1). Fluorinated polymers were tested due to their low friction coefficient (excellent non-stick properties) and their high resistance to a) VUV light, b) abrasion and c) chemical solvents.

[0038] Four types of fluorinated polymers were tested:

1) Polytetrafluoroethylene (PTFE),

2) Parylene F-AF4,

3) Parylene F-VT4,

4) Fluorinated Propylethylene (FPE).

[0039] The chemical structures are shown in FIG. 2: (a) PTFE; (b) Parylene F-AF4; (c) Parylene F-VT4; and (d) FPE.

[0040] The tests were conducted by analysing aluminium (Al) samples, except where mentioned below where iron (Fe) and copper (Cu) were also tested. Each test comprised 2500 runs, where a run corresponded to an attack onto a single sample location. Each run comprised several thousand sparks. The spark frequency was 300 Hz and each run had a duration of 28.2 seconds.

[0041] The effectiveness of the coatings was evaluated by 1) visually inspecting the spark stand conditions; 2) measuring the weight of the exhaust dusts that adhered on the surfaces; and 3) evaluating the difficulty and time required to clean the coated areas.

[0042] Each test comprised a control test performed without coated surfaces as a reference, followed by a test with the coated surfaces.

[0043] Results

(a) Polytetrafluoroethylene (PTFE)

[0044] An adhesive of Polytetrafluoroethylene (PTFE) was applied to the outlet conduit 32 (shaded region) as shown in FIG. 1. The layer consisted of a coated adhesive and had a thickness of 1 mm. After testing, the visual inspection of the coated spark tables (1A and 1B) showed a marked reduction of both the thickness and the area covered by exhaust dusts. The weight of the exhaust dusts was reduced by 84% compared to the test made without a coating. A few seconds were required to clean the coating with isopropyl alcohol and paper, leaving essentially no dust remnants on the surface.

(b) Parylene F-AF4

[0045] A coating of Parylene F-AF4 with a thickness of 10 μm was applied to the surface by a chemical vapour deposition (CVD) process. After testing, the visual inspection of the coated spark tables showed adherence of exhaust dusts but the adhered dust weight was reduced by 30%. A few seconds were required to clean the coating with isopropyl alcohol and paper, leaving essentially no dust remnants on the surface.

(c) Parylene F-VT4

[0046] A coating of Parylene F-VT4 with a thickness of 50 μm was applied by chemical vapour deposition (CVD) process. Parylene After testing, the visual inspection of the coated tables showed a clear reduction of both the thickness and area covered by exhaust dusts. The adhered dust weight was reduced by 77%. A few seconds were required to clean the coating with isopropyl alcohol and paper, leaving essentially no dust remnants on the surface.

[0047] A different coating of Parylene F-VT4 with a thickness of 20-25 μm showed a reduction of the adhered dust weight by 64%. Parylene F-VT4 is chemically inert, has a coefficient of friction of 0.39/0.35 (static/dynamic) and a Rockwell hardness of R80.

(d) Fluorinated Propyl Ethylene (FPE)

[0048] A coating of FPE with a thickness of 50 μm was applied by chemical vapour deposition (CVD) process. However, the adhered dust weight was reduced by 53%. A few seconds were required to clean the coating with isopropyl alcohol and paper, leaving essentially no dust remnants on the surface.

[0049] A different coating of FPE with a thickness of 80-90 μm showed a reduction of the adhered dust weight by 56%. The same coating of FPE with a thickness of 80-90 μm was also tested with iron (Fe) and copper (Cu) samples. The test with Fe showed a reduction of the adhered dust weight by 64% and the test with Cu showed a reduction of the adhered dust weight by 30%. FPE is chemically inert, has a coefficient of friction of 0.25/0.20 (static/dynamic) and a Rockwell hardness of R54.

[0050] The results demonstrate that the coatings of anti-adhesion materials enable longer time intervals between cleaning operations (thus improving maintainability of the instrument), fast cleaning times and improved plasma conditions as the analytical conditions are less affected by adhered exhaust dusts.

[0051] It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention.

[0052] The use of any and all examples, or exemplary language (“for instance”, “such as”, “for example” and like language) provided herein, is intended merely to better illustrate the invention and does not indicate a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0053] As used herein, including in the claims, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as “a” or “an” means “one or more”.

[0054] Throughout the description and claims of this specification, the words “comprise”, “including”, “having” and “contain” and variations of the words, for example “comprising” and “comprises” etc, mean “including but not limited to”, and are not intended to (and do not) exclude other components.