Membrane-based purge gas and sample transport for sample processing by laser ablation.

Abstract

A system and method are described for transferring gas from an ablation cell to an inductively coupled plasma analyzer through a gas exchange membrane transfer line to replace gas introduced into the ablation cell with a sweep gas. An embodiment of the system includes, but is not limited to, a laser ablation cell configured to generate a sample transfer stream by laser ablation of a sample and to introduce a carrier gas that flows the ablated sample from the laser ablation cell, an inductively coupled plasma analyzer configured to measure one or more analytes in the sample transfer stream, and a gas exchange membrane transfer line fluidly coupled between the laser ablation cell and the inductively coupled plasma analyzer, the gas exchange membrane transfer line configured to replace gas in the sample transfer stream with the sweep gas by gas exchange through a membrane of the gas exchange membrane transfer line.

Description

[Technical field]

[0001] (Related Applications) This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 63 / 319,846, entitled “MEMBRANE-BASED PURGE GAS AND SAMPLE TRANSFER FOR LASER ABLATION SAMPLE PROCESSING,” filed March 15, 2022. U.S. Provisional Application No. 63 / 319,846 is incorporated herein by reference in its entirety. [Background technology]

[0002] Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) and Laser Ablation Inductively Coupled Plasma Optical Emission Spectroscopy (LA-ICP-OES) techniques can be used to analyze the composition of a target, such as a solid or liquid target material. Often, a sample of the target is provided to the analytical system in the form of an aerosol (i.e., a suspension of solid and sometimes liquid particles and / or vapor in a carrier gas, such as helium gas). The sample is typically produced by placing the target in a laser ablation chamber, introducing a flow of carrier gas into the chamber, and ablating a portion of the target with one or more laser pulses to produce a plume containing particles and / or vapors emitted from the target or otherwise generated and suspended in the carrier gas. The target material entrained in the flowing carrier gas is transported through a transport conduit to the analytical system and ionized with an inductively coupled plasma (ICP) torch.

[0003] The plasma containing the ionized particles and / or vapor is then analyzed by analytical systems such as mass spectrometry (MS), optical emission spectrometry (OES), isotope ratio mass spectrometry (IRMS), or electrospray ionization (ESI) systems. For example, ICP spectrometry is a commonly used analytical technique for measuring trace element concentrations and isotope ratios in liquid samples. ICP spectrometry employs an electromagnetically generated partially ionized argon plasma that reaches a temperature of approximately 7,000 K. When a sample is introduced into the plasma, the high temperature causes the atoms of the sample to ionize or emit light. Each chemical element produces a characteristic mass and emission spectrum, so by measuring the emitted mass or light spectrum, the elemental composition of the original sample can be determined. Summary of the Invention

[0004] A system and method are described for transferring a sample gas from an ablation cell to an inductively coupled plasma analysis system through a gas exchange membrane transfer line to replace the gas introduced into the ablation cell with a sweep gas without extinguishing a plasma used by the analysis system to measure analytes in the sample gas. An embodiment of the system includes, but is not limited to, a laser ablation cell configured to generate a sample transfer stream by laser ablation of the sample and introduce a carrier gas that flows the ablated sample from the laser ablation cell, an inductively coupled plasma analysis device configured to measure one or more analytes in the sample transfer stream, and a gas exchange membrane transfer line fluidly coupled between the laser ablation cell and the inductively coupled plasma analysis device, the gas exchange membrane transfer line configured to replace the gas in the sample transfer stream with the sweep gas by gas exchange through a membrane of the gas exchange membrane transfer line.

[0005] Method embodiments include, but are not limited to, transferring a sample transport stream comprising a plurality of sample particles and at least one gas from a laser ablation system to a gas exchange membrane transport line, introducing a sweep gas into the gas exchange membrane transport line, separating at least a portion of the at least one gas from the plurality of sample particles via exchange with at least a portion of the sweep gas, and transferring the sample transport stream through the gas exchange membrane transport line to an inductively coupled plasma analyzer.

[0006] An embodiment of the system includes, but is not limited to, a laser ablation system configured to generate a sample transport stream by laser ablation of a sample and introduce a carrier gas that flows the ablated sample from the laser ablation system, an inductively coupled plasma analyzer configured to measure one or more analytes in the sample transport stream, and a gas exchange membrane transport line fluidly coupled between the laser ablation system and the inductively coupled plasma analyzer, the gas exchange membrane transport line configured to replace gas in the sample transport stream with a sweep gas by gas exchange through a membrane of the gas exchange membrane transport line.

[0007] This Summary is provided to introduce in a simplified form some of the concepts described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0008] The detailed description will now be made with reference to the accompanying drawings. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 illustrates a system for controlling the flow of purge gas introduced into an ablation cell between samples, according to an embodiment of the present disclosure. [Diagram 2]FIG. 1 is a schematic diagram of a gas exchange membrane coupled between a laser ablation system and an analytical system to replace a sample gas or a purge gas with a sweep gas, according to an embodiment of the present disclosure. [Diagram 3] Diagram showing the gas exchange membrane of FIG. 2 during the gas exchange procedure. [Figure 4] Plot of nitrogen intensity versus amount of nitrogen added after the gas exchange membrane to provide a nitrogen calibration curve. [Diagram 5] A graph showing the percentage of nitrogen detected by the analysis system after gas exchange through the gas exchange membrane and the percentage of nitrogen added prior to the gas exchange membrane. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Laser ablation is a sampling method that uses a focused beam of pulsed laser light in a laser ablation cell to convert a portion of a solid or liquid sample from a target into the aerosol phase, which is then transported in a gas stream to an analytical instrument such as an inductively coupled plasma mass spectrometer (ICPMS). Multiple laser pulses may be used to ablate different areas of the sample to generate an adequate signal and ensure a representative sample is collected for analysis.

[0011] Laser ablation systems can facilitate automated handling of multiple samples to one laser ablation cell by utilizing automated sample loading techniques. For example, a laser ablation system can include a sample loading system (e.g., a sample carousel) for housing multiple samples and sequentially introducing the samples to the laser ablation cell to generate individual ablated samples. For example, the sample carousel can rotate to bring a particular sample closer to underneath the laser ablation cell. A piston located below the carousel can lift the sample into or vertically to the laser ablation cell and hold the sample in place during laser ablation sampling. After transfer of the ablated target material from the laser ablation cell to an analyzer, the piston can lower the sample and the carousel brings the next sample closer to underneath the laser ablation cell. When switching between samples, atmospheric gases can enter the exposed laser ablation cell, such that the lowering of the piston exposes the bottom of the laser ablation cell to the atmosphere or atmospheric gases entrained within the laser ablation system. If atmospheric gases are sent to the analyzer, the ICP torch plasma may be extinguished, resulting in downtime of the ICP analyzer while the ICP torch is reignited and recalibration or stabilization techniques are performed, resulting in reduced sample throughput and increased operating costs for the ICP analyzer (e.g., due to increased nebulizer gas usage, power requirements, etc.).

[0012] One option to prevent atmospheric gases from entering the ICP torch is to utilize a pinch valve to selectively choose the path that gases from the laser ablation cell take. For example, the transfer line from the laser ablation cell can be split into two gas lines made of flexible / flexible material and enter the pinch valve. The first gas line connects the laser ablation cell to the ICP torch, and the second gas line connects the laser ablation cell to a purge location (vent, waste, etc.). When switching between samples, the pinch valve compresses the flexible tubing to close the first gas line, preventing atmospheric gases from reaching the ICP torch, while the second gas line remains open to purge atmospheric gases. During sampling, the pinch valve opens the first gas line and compresses the flexible tubing to close the second gas line, preventing the sample gases from being purged, while allowing the sample to pass through the first gas line to the ICP torch.

[0013] However, pinch valves introduce many drawbacks to the operation of laser ablation systems because they frequently change the inner diameter of the tubing that passes between the laser ablation cell and the ICP analyzer. For example, the opening and closing action of the pinch valve can become unstable due to the flexible tubing sticking during the opening and closing action (e.g., the inner walls of the tubing stick together) or the flexible tubing deforming over time (e.g., the flexible tubing material becomes hard). Tube sticking and other tube deformations can increase over time due to mechanical failure or aging of the tube. In addition, the tubes are prone to clogging and sample or airborne particulates can accumulate at the pinch site, affecting the consistency of sample analysis by the ICP analyzer and requiring frequent tube maintenance or replacement, which is often only noticed after a sample procedure is recognized as having failed, leading to reprocessing of samples after tube maintenance and the associated loss of time. Mechanical problems with such pinch valves can result in problems such as, among other things, fluctuations in cell pressure in the laser ablation cell during sample processing and fluctuations in the sensitivity of the ICP analyzer for measuring analytes in the sample gas transferred from the laser ablation cell.

[0014] Thus, in one embodiment, the present disclosure is directed to a system and method for transferring gas from a laser ablation system to an inductively coupled plasma analysis system through a gas exchange membrane transfer line to exchange the gas introduced into the laser ablation system with a sweep gas without extinguishing the plasma used by the analysis system to measure analytes in the sample gas. The gas exchange membrane transfer line is fluidly coupled between the laser ablation system (or a portion thereof, such as when utilizing a laser ablation cell) and the analysis system, and includes a tube formed from a gas exchange membrane (e.g., a polytetrafluoroethylene (PTFE) membrane) housed within an outer tube. A sweep gas (e.g., Ar gas) is introduced into one end of the transfer line and into the annular space between the tubes. A sample transport stream is introduced into the interior of the gas exchange membrane tube at the end of the transfer line opposite the introduction of the sweep gas, such that the sweep gas and the sample transport stream flow in opposite directions. In an embodiment, the flow rate of the sweep gas can be controlled by a mass flow controller (MFC).

[0015] Gas present in the sample transport stream passes through the membrane (e.g., by counterbalance forces) into the annular space between the tubes and is eventually exhausted out the transport line and then discarded through the exhaust. The sweep gas (e.g., provided via the MFC) passes from the annular space through the membrane and into the sample transport stream. Thus, the sample transport stream has had all or substantially all of the preceding gas (e.g., nitrogen gas, purge gas, atmospheric gas) replaced by the sweep gas (e.g., Ar) before being transferred to the analytical system. This allows the addition of purge gas to the ablation cell and purging the entire flow path without adding any physical clamps or restrictions to the system.

[0016] (Example) 1-5, a system 100 for transferring gas from a laser ablation system to an inductively coupled plasma analytical system via a gas exchange membrane transfer line to replace the gas introduced into the laser ablation system with a sweep gas according to an exemplary embodiment of the present disclosure is shown. The system 100 generally includes a laser ablation system 102 in fluid communication with each of a gas exchange membrane transfer line ("transfer line 104") and an analytical system 106. The laser ablation system 102 may include a laser ablation cell or other chamber that holds the sample while a focused beam of pulsed laser light converts a portion of the sample into an aerosol phase that is transported in a gas flow to the transfer line 104. The transfer line 104 introduces the sweep gas to replace the gas used by the laser ablation system 102 to transfer the sample from the laser ablation system or to purge the atmosphere introduced during the exchange of the sample. The flow of material through the transfer line 104 from the laser ablation system 102 to the analysis system 106 is referred to herein as the "sample transport stream," and the composition of the sample transport stream may vary over the length of the transfer line 104 as the sweep gas exchanges with gases present in the laser ablation system (e.g., carrier gas, atmospheric gas, purge gas, etc.). Examples of gases present in the laser ablation system during a sample transfer or purge cycle include, but are not limited to, helium, nitrogen, carbon dioxide, oxygen, etc. Examples of sweep gases include, but are not limited to, argon. In this manner, the sample transport stream may contain varying compositions of ablated sample, carrier gas, atmospheric gas, purge gas, sweep gas as the sample is transferred to the analysis system 106, with more sweep gas replacing gas transferred from the laser ablation system 102 as the sample transport stream passes through the transfer line 104. The transfer line 104 also fluidly couples the laser ablation system 102 with the analysis system 106.The analytical system 106, in one embodiment, is an inductively coupled plasma analytical system, such as an ICP-MS system, an ICP-OES, or the like, used to measure one or more analytes in the sample transport stream.

[0017] The transfer line 104 shown in Figures 2 and 3 includes an outer tube 200 formed of a gas impermeable material surrounding an inner tube 202 constructed of a gas exchange membrane material. In an embodiment, the gas exchange membrane material is a polytetrafluoroethylene (PTFE) membrane, but the transfer line 104 is not limited to such materials and may include alternative or additional materials suitable for the exchange of sweep gas while being substantially impermeable to sample particulates. A sample transport stream is introduced from the laser ablation system 102 into the interior of the inner tube 202 at a first end 204 of the transfer line 104 and transported through the transfer line 104 to a second end 206 of the transfer line 104 that is fluidly coupled to the analysis system 106. A sweep gas is introduced into an annular region 208 between the outer tube 200 and the inner tube 202 and transported through the transfer line 104 toward the first end 204. In an embodiment, the sweep gas is introduced into the transfer line 104 via a sweep gas inlet 210 adjacent the second end 206 of the transfer line 104 and removed from the transfer line 104 via a sweep gas outlet 212 adjacent the first end 204 of the transfer line 104. However, for longer transfer lines 104 (e.g., transfer lines greater than 6 feet), the sweep gas inlet 210 may be located closer to the middle of the transfer line 104, away from the second end 206, so that the length of the transfer line 104 provides sufficient residence time for the sweep gas to exchange with the sample transport stream. In an embodiment, the flow rate of the sweep gas is managed by a mass flow controller (MFC) for introduction into the sweep gas inlet 210 at a precisely controlled flow rate.

[0018] Although the transfer line 104 is described herein as including a substantially concentric arrangement of the inner tube 202 and the outer tube 200 to provide an annular region 208 therebetween, in this disclosure the transfer line 104 is not limited to such a configuration. For example, the inner tube 202 and the outer tube 200 may be offset such that the region therebetween is irregularly shaped (i.e., not annular), one or more of the inner tube 202 or outer tube 200 may have a non-circular cross-section, etc., without departing from the scope of the present disclosure.

[0019] The sweep gas and the sample transport stream flow in substantially opposite directions through the transport line 104, with the inner tube 202 initially separating the flows. For example, the inner tube 202 separates an annular region 208 that initially holds the sweep gas from the interior of the inner tube 202 into which the sample transport stream enters. The PTFE membrane of the inner tube 202 is substantially permeable to gases (e.g., sweep gas (e.g., Ar), sample transport gas (e.g., He), purge gas (e.g., N2), etc.) and substantially impermeable to sample particulates generated from the laser ablation system 102. Thus, a portion of the sweep gas flows from the annular region 208 across the membrane to the interior of the inner tube 202 and mixes with the sample transport stream for passage to the analysis system 106, while a portion of the gas initially present in the sample transport stream flows from the inner tube 202 across the membrane to the annular region 208 for exit from the sweep gas outlet 212. For example, Figure 3 illustrates an embodiment in which helium gas initially present in the sample transport stream is transported from the inner tube 202 across the membrane to the annular region 208, and an argon sweep gas is transported from the annular region 208 across the membrane to the inner tube 202 and mixed with the sample transport stream for passage to the analytical system 106. As shown, substantially all of the helium initially present in the sample transport stream is removed from the sample transport stream and exits the transport line 104 via the sweep gas outlet 212. For example, the concentration of the sweep gas in the annular region 208 can facilitate the exchange of sample transport stream gas such that helium gas is exchanged with the argon sweep gas over the length of the transport line 104. In an embodiment, the removal of the initial sample transport stream gas through the transport line 104 is greater than 99%.

[0020] The length of the transfer line 104 can affect the amount or degree of gas exchange between the gas flowing through the inner tube 202 and the annular region 208. In an embodiment, a transfer line 104 having a length of about 6 feet has been experimentally shown to replace substantially all of the initial gas in the sample transport stream with the sweep gas (e.g., 99% or greater replacement). However, the system 100 is not limited to a transfer line 104 of about 6 feet, where the system 100 may include a transfer line 104 of more than 6 feet or less than 6 feet, where such length may depend, for example, on the amount of gas exchange desired, the flow rate of the gas stream, the type of gas exchange membrane, or other factors.

[0021] Because the gas in the sample transport stream can be replaced with a sweep gas, system 100 can introduce a purge gas into the laser ablation system to purge atmospheric gases present in the ablation cell upon sample exchange without extinguishing the plasma of the ICP torch of analysis system 106. Additionally, system 100 can control the composition of gas in the sample transport stream without using physical clamps or restrictions on the flow paths within system 100, such as via pinch valves, thus keeping the transport lines between laser ablation system 102 and analysis system 106 open.

[0022] (Experimental Example - Nitrogen Content of Sample Transport Stream) In one embodiment, system 100 was utilized to measure the amount of nitrogen gas removed from a sample transport stream via a transfer line 104 having a length of approximately 6 feet leading to an analytical system 106. In the first part of the experiment, multiple nitrogen samples were introduced into the ICPMS system without an intervening gas exchange membrane (e.g., downstream of the gas exchange membrane) at different introduction rates controlled via a mass flow controller (MFC) to quantitatively measure the amount of nitrogen received for various nitrogen flow rates. Example nitrogen flow rates ranged from about 0.1 mL / min to about 1 mL / min.

[0023] By introducing nitrogen into the ICPMS system without passing through the gas exchange membrane, all of the nitrogen introduced through the MFC was received by the ICPMS system without being affected by gas exchange through the gas exchange membrane in transfer line 104. The ICPMS system reported the measured intensity of the nitrogen. Based on these measurements, a calibration curve of intensity versus flow rate was created, an example of which is shown in FIG. 4.

[0024] In the next portion of the experiment, multiple nitrogen samples were introduced into the ICPMS through transfer line 104 at different flow rates controlled via the MFC. A gas exchange membrane was interposed in the transfer line 104 between the nitrogen sample introduction site and the ICPMS. Exemplary nitrogen flow rates were from about 600 mL / min to about 1500 mL / min. An argon sweep gas was introduced into the transfer line 104 to exchange the nitrogen introduced via the MFC for argon before it reached the ICPMS. The ICPMS system reported the measured intensity of nitrogen, which represents the nitrogen that was not removed or otherwise exchanged by the transfer line 104 during transport to the ICPMS system.

[0025] A chart of the nitrogen flow rate measured by the ICPMS after passing through the transfer line 104 and the nitrogen flow rate introduced upstream of the transfer line 104 via the MFC was created based on these measurements, an example of which is shown in FIG. 5. The calibration curve shown in FIG. 4 was used to obtain the measurement data on the Y-axis, and the nitrogen intensity measured after the gas exchange in the transfer line 104 was plotted on the calibration curve to determine the flow rate of nitrogen remaining after the gas exchange. As shown in FIG. 5, the transfer line 104 provided greater than 99% removal of the nitrogen introduced into the transfer line 104 for a wide range of gas flow rates. For example, for a nitrogen flow rate of 800 mL / min (shown as a vertical line on the chart), which is in the range of gas flow rates for typical laser ablation samples, the ICPMS system measured less than 0.5 mL / min, resulting in greater than 99.93% nitrogen gas removal.

[0026] (Conclusion) Although the present subject matter has been described in terms of structural features and / or process operations, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims

1. 1. A system for controlling a flow of gas from a laser ablation cell, comprising: a laser ablation cell configured to generate a sample transport stream by laser ablation of the sample and to introduce a carrier gas that flows the ablated sample from the laser ablation cell; an inductively coupled plasma analyzer configured to measure one or more analytes in the sample transport stream; a gas exchange membrane transfer line fluidly coupled between the laser ablation cell and the inductively coupled plasma analyzer; The system, wherein the gas exchange membrane transfer line is configured to replace gas in the sample transfer stream with a sweep gas by gas exchange across the membrane of the gas exchange membrane transfer line.

2. a flow controller fluidly coupled to the source of sweep gas; The system of claim 1 , wherein the flow controller is configured to introduce a sweep gas at a controlled flow rate into the inlet of the gas exchange membrane transfer line.

3. The gas exchange membrane transfer line includes an inner tube surrounded by an outer tube; The system of claim 1 , wherein at least a portion of the inner tube includes the gas exchange membrane.

4. The system of claim 3 , wherein the inner tube defines an inlet configured to receive a sweep gas into a region defined between the inner tube and the outer tube.

5. The system of claim 4 , wherein the region defined between the inner tube and the outer tube is an annular region.

6. 5. The system of claim 4, wherein the inlet is positioned adjacent to an end of the gas exchange membrane transfer line coupled to the inductively coupled plasma analyzer such that a sweep gas and a sample transport stream flow at least partially through the gas exchange membrane transfer line in a reverse flow path arrangement.

7. 13. The system of claim 1, further comprising a sample carousel configured to hold a plurality of samples for sequential introduction into the laser ablation cell to generate individual sample transport streams from each sample.

8. 1. A method for controlling a flow of gas from a laser ablation cell, comprising: transferring a sample transport stream comprising a plurality of sample particles and at least one gas from the laser ablation system to a gas exchange membrane transport line; introducing a sweep gas into the gas exchange membrane transfer line; separating at least a portion of the at least one gas from the plurality of sample particles via exchange with at least a portion of the sweep gas; and transferring the sample transfer stream through said gas exchange membrane transfer line to an inductively coupled plasma analyzer.

9. 9. The method of claim 8, wherein introducing a sweep gas into the gas exchange membrane transfer line comprises introducing the sweep gas into the gas exchange membrane transfer line at a controlled flow rate via a flow controller.

10. The gas exchange membrane transfer line includes an inner tube surrounded by an outer tube; at least a portion of the inner tube includes a membrane permeable by a sweep gas and at least one gas; The method of claim 8 , wherein the membrane is substantially impermeable to a plurality of sample particles.

11. introducing a sweep gas into the gas exchange membrane transfer line comprises introducing a sweep gas into a region between the inner tube and the outer tube; 11. The method of claim 10, wherein transferring a sample transport stream comprising a plurality of sample particles and at least one gas from a laser ablation system to a gas exchange membrane transport line comprises transferring the sample transport stream through the inner tube.

12. The method of claim 11 , wherein at least a portion of the flow of the sweep gas and the sample transport stream flows in counter-current directions through the gas exchange membrane transport line.

13. 9. The method of claim 8, wherein separating at least a portion of the at least one gas from the plurality of sample particles via exchange with at least a portion of the sweep gas comprises separating at least 99% of the at least one gas from the plurality of sample particles via exchange with at least a portion of the sweep gas.

14. 1. A system for controlling a flow of gas from a laser ablation system, comprising: a laser ablation system configured to generate a sample transport stream by laser ablation of a sample and to introduce a carrier gas that flows the ablated sample from the laser ablation system; an inductively coupled plasma analyzer configured to measure one or more analytes in the sample transport stream; a gas exchange membrane transfer line fluidly coupled between the laser ablation system and the inductively coupled plasma analyzer; The gas exchange membrane transfer line is configured to replace gas in the sample transfer stream with a sweep gas by gas exchange across the membrane of the gas exchange membrane transfer line.

15. a flow controller fluidly coupled to the source of sweep gas; 15. The system of claim 14, wherein the flow controller is configured to introduce a sweep gas at a controlled flow rate into the inlet of the gas exchange membrane transfer line.

16. The gas exchange membrane transfer line includes an inner tube surrounded by an outer tube; The system of claim 14 , wherein at least a portion of the inner tube includes the gas exchange membrane.

17. The system of claim 16 , wherein the inner tube defines an inlet configured to receive a sweep gas into a region defined between the inner tube and the outer tube.

18. The system of claim 17 , wherein the region defined between the inner tube and the outer tube is an annular region.

19. 20. The system of claim 18, wherein the inlet is positioned adjacent to an end of the gas exchange membrane transfer line coupled to the inductively coupled plasma analyzer such that a sweep gas and a sample transport stream flow at least partially through the gas exchange membrane transfer line in a reverse flow path arrangement.

20. 15. The system of claim 14, further comprising a sample carousel configured to hold a plurality of samples for sequential introduction into the laser ablation cell to generate individual sample transport streams from each sample.

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