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EP-4736217-A1 - APPARATUS FOR INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

EP4736217A1EP 4736217 A1EP4736217 A1EP 4736217A1EP-4736217-A1

Abstract

A sampling interface enables the sampling of ions in a mass spectrometer for subsequent spectroscopic analysis. The sampling interface comprises: an inlet receiving a quantity of particles from an ion source, the quantity of particles including ions for spectroscopic analysis; a skimmer downstream of the inlet and comprising an aperture through which particles from the inlet pass; an extraction lens downstream of the skimmer and configured for extracting ions from the particles passed through the aperture at the skimmer; an adjustable voltage source applying a bias voltage to the extraction lens so as to generate an electric field in at least a region between the extraction lens and the skimmer; and a controller controlling the adjustable voltage source to apply a bias voltage to the extraction lens, in order to control the reduced electric field in the region immediately downstream of the skimmer.

Inventors

  • BELOV, MIKHAIL

Assignees

  • Thermo Fisher Scientific (Bremen) GmbH

Dates

Publication Date
20260506
Application Date
20240628

Claims (20)

  1. 1 . A sampling interface for use in mass spectroscopy apparatus, the sampling interface being arranged so as to enable the sampling of ions in a mass spectrometer for subsequent spectroscopic analysis, the sampling interface comprising: an inlet for receiving a quantity of particles from an ion source, the quantity of particles including ions for spectroscopic analysis; a skimmer arranged downstream of the inlet, the skimmer comprising an aperture through which particles from the inlet pass; an extraction lens arranged downstream of the skimmer, the extraction lens being configured for extracting ions from the particles passed through the aperture at the skimmer; an adjustable voltage source for applying a bias voltage to the extraction lens so as to generate an electric field in at least a region between the extraction lens and the skimmer; and a controller configured to control the adjustable voltage source to apply a bias voltage to the extraction lens, in order to control the reduced electric field in the region immediately downstream of the skimmer.
  2. 2. The sampling interface of claim 1 , wherein the reduced electric field is a ratio of the electric field in at least the region between the extraction lens and the skimmer to the number density in the region immediately downstream of the skimmer.
  3. 3. The sampling interface of claim 1 or claim 2, wherein the controller is configured to receive a measured value of a system parameter representative of the number density in a region immediately downstream of the skimmer, and determine a magnitude of the bias voltage to be applied by the adjustable voltage source based on the measured value of the system parameter.
  4. 4. The sampling interface of claim 3, wherein the controller is configured to determine the magnitude of the bias voltage to be applied by the adjustable voltage source based on the measured value to maintain the reduced electric field at an approximately constant value.
  5. 5. The sampling interface of claim 4, wherein an approximately constant value is constant to within ±10%, or more preferably constant to within ±5%.
  6. 6. The sampling interface of any preceding claim, wherein the system parameter is pressure in a region immediately downstream of the skimmer.
  7. 7. The sampling interface of any preceding claim, wherein the measured value of the system parameter is measured at a sensor arranged in a region immediately downstream of the skimmer.
  8. 8. The sampling interface of claim 7, when dependent on claim 6, wherein the sensor is a pressure sensor.
  9. 9. The sample interface of any preceding claim, wherein the inlet is configured for receiving a quantity of particles from an inductively-coupled plasma ion source.
  10. 10. A torch for generating an inductively coupled plasma, the torch comprising: a torch tube comprising a support end and an open end, wherein a portion of the open end defines a cavity for at least partially confining the inductively coupled plasma; an injector tube comprising a bore extending through the injector tube between an injector inlet end for receiving a sample flow into the bore and an injector outlet end for conveying the sample flow out of the bore, wherein the injector tube is positioned to extend at least partially through the torch tube; and wherein the diameter of the bore decreases between a first location and a second location, and the diameter of the bore is constant or decreases between the second location and a third location, the first location being closer to the injector inlet end than the third location, and the second location being between the first and third location in the direction of sample flow through the bore of the injector tube.
  11. 1 1 . The torch of claim 10, wherein the diameter of the bore at the second location is no greater than 1 10% the diameter of the bore at the third location.
  12. 12. The torch of claim 10 or claim 11 , wherein the diameter of the bore at the second location is equal to the diameter of the bore at the third location.
  13. 13. The torch of any one of claims 10 to 12, wherein the diameter of the bore at the first location is more than twice the diameter of the bore at the second location, and preferably is more than three times the diameter of the bore at the second location.
  14. 14. The torch of any one of claims 10 to 13, wherein the third location is at the injector outlet end of the injector tube.
  15. 15. The torch of any one of claims 10 to 14, wherein the diameter of the bore comprises a conical section extending between the first location and the second location.
  16. 16. The torch of any one of claims 10 to 15, wherein the injector tube is positioned to extend at least partially through the torch tube so as to define a channel between an outer surface of the injector tube and an inner wall of the torch tube for passage of a flow of a confining gas.
  17. 17. The torch of claim 16, wherein a diameter of the outer surface of the injector tube is greater at the injector outlet end than at the injector inlet end.
  18. 18. The torch of claim 16 or claim 17, wherein the diameter of the outer surface of the injector tube has a tapering portion terminating at the injector outlet end, such that the diameter of the outer surface of the injector tube narrows in a direction extending from the injector outlet end towards the injector inlet end.
  19. 19. The torch of claim 18, wherein the radial spacing between the outer surface of the injector tube and the inner wall of the torch tube is approximately constant in a region aligned with the tapering portion of the outer surface of the injector tube.
  20. 20 The torch of any one of claims 10 to 19, wherein a bore extending through the torch tube between the support end and the open end and having the injector tube extending therethrough comprises a tapering section, wherein the diameter of the bore extending through the torch tube increases between an entry to the tapering section and an exit to the tapering section, wherein the entry to the tapering section is closer to the support end than the exit to the tapering section.

Description

APPARATUS FOR INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY Field of the disclosure The present disclosure concerns improvements in or relating to mass spectrometry. More particularly, the invention relates to a sampling interface and a torch for generating a plasma, both for use within mass spectrometry apparatus. In a particular, examples of the present invention relate to apparatus and a method for inductively coupled plasma-mass spectrometry (ICP-MS). Background to the disclosure Mass spectrometers are specialist scientific instruments used for spectroscopic analysis of ionised or charged particles, in order to determine the elemental composition of a sample. Typically, ionised particles of a sample are passed through a mass analyser to separate the various charged particles into their mass-to-charge ratio, before being received at one or more detectors. Prior to entry into the mass analyser, a sample must be converted into charged particles or ions and these charged particles or ions are then focused and directed through the mass spectrometer towards the mass analyser. One type of mass spectrometry uses an inductively coupled plasma to transform a sample into a plurality of particles, including charged particles or ions. The plasma may be generated, and introduction of the sample to the plasma may take place at, a plasma source (also known as a plasma torch). A stream of particles from the plasma is then passed through the mass spectrometer, in particular via a sampling interface. The sampling interface is a portion of a mass spectrometer arranged to enable the sampling of ions from a generated plasma and then subsequently to direct and focus those sampled ions towards a mass analyser for spectroscopic analysis. Common elements of a sampling apparatus include a sampling cone, a skimmer, and an extraction lens, each arranged in the direction of flow of the sample particles. In a mass spectrometer incorporating a typical sampling interface and known conical torch, it is observed that degradation of instrument performance can occur over longer analysis cycles and especially for high-matrix samples (being solutions containing highly concentrated salts, acids, bases or other chemicals). In particular, degradation can be observed in the form of loss of analytical sensitivity (such as orders of magnitude of signal decay over an hour-long experiment) and reduced signal reproducibility (manifested in suppression of sample signals in the presence of ‘high-matrix’ constituents, compared to distinguishing ‘clean’ sample signals). Such degradation in performance has been attributed to contamination downstream of the conical torch, including build-up of high-concentration matrix components at the sampling cone and skimmer. In particular, build-up has been observed around apertures (through which charged particles pass) of both the sampling cone and the skimmer, thereby reducing the diameter of the apertures. FIGURE 1 shows an example of such a build up at a skimmer 10 with a nominal aperture diameter of 600 pm that has been contaminated by high concentration salts after a four-hour experiment. It can be seen that the skimmer aperture 20 now has a diameter reduced to around 420-450 pm by the build-up. This decrease in the skimmer aperture diameter in turn results in a pressure drop immediately downstream of the skimmer assembly, causing the type of overall performance degradation discussed above. One known approach to mitigate a loss of analytical sensitivity is by de-tuning certain instrument parameters (for example, the pressure in the region between the sampling cone and the skimmer). Another option is to reduce the gas flow through the skimmer orifice, to decrease the amount of build-up. However, these techniques only partly solve a loss of sensitivity, but in addition can further diminish signal reproducibility. US Patent No. 9,202,679 considers a sampling interface in which a bias voltage potential may be applied to a skimmer in order to control the kinetic energy of ions entering a collision region immediately behind the skimmer. However, applying a potential to the skimmer aperture fails to prevent the build-up of salt crystals on the skimmer surface. Moreover, only small voltages (for instance, of just a few volts) can be applied without disrupting the beam of particulates generated by the plasma. As such, further options to reduce the signal degradation would be desirable. Modifications have also been made to the plasma torch. For instance, US Patent No. 10,212,798 describes a torch for generating an inductively coupled plasma comprising a torch tube concentric around a central, injector tube. An annular channel is defined between the torch tube and the injector tube, and the torch tube comprises tapered portions which open out towards each end such that the annular channel has a diameter which narrows in a central section. Gas can be passed through the annular channel in a helical formation for cooling of