SYSTEM FOR ANALYZING A SAMPLE

A method for analyzing a sample using a system, wherein the sample is a solid sample comprising a first chemical element and a second chemical element, wherein the system comprises a plasma spectrometer and an analytical device, the method including: determining a concentration of the first chemical element using the analytical device; determining a sensitivity of the plasma spectrometer to the first chemical element and to the second chemical element; measuring a signal intensity of the first chemical element using the plasma spectrometer; measuring a signal intensity of the second chemical element using the plasma spectrometer; and calculating a concentration of the second chemical element using the determined concentration of the first chemical element, the sensitivities to the first chemical element and to the second chemical element, and the signal intensities of the first chemical element and the second chemical element.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to and claims the priority benefit of German Patent Application No. 10 2022 126 023.2, filed Oct. 7, 2022, and German Patent Application No. 10 2022 126 192.1, filed Oct. 10, 2022, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to systems and methods for analyzing a sample comprising a plasma spectrometer and an analytical device and to methods to calibrate such systems.

BACKGROUND

Plasma spectrometers utilize a plasma for ionizing a sample and generating ions and/or photons from the sample. The ions are typically analyzed by means of a mass spectrometer, whereas the photons are usually analyzed by means of optical spectrometers.

In a mass spectrometer, the molecules or atoms of a sample are first transferred into the gas phase and ionized. For ionization, various methods known from the state of the art are available, such as inductively coupled plasma ionization (ICP) which ionizes the sample by means of a plasma. Up to now, several different types of inductively coupled plasma mass spectrometers (ICP-MS) are available, as e.g., the quadrupole ICP-MS or time-of-flight ICP-MS.

After ionization, the ions pass through a vacuum interface to a mass analyzer, in which they are separated according to their mass-to-charge ratio (m/z). Different types of interfaces and modes of operation are based, for example, on the application of static or dynamic electric and/or magnetic fields or on different times of flight of different ions. In particular, different types of interfaces include single, multiple or hybrid arrangements of analyzers, such as quadrupole, triple-quadrupole, time-of-flight (TOF), ion trap, Orbitrap or magnetic sector. Finally, the separated ions are guided towards a detector which, e.g., is one of a photo-ion multiplier, ion-electron multiplier, Faraday collector, Daly detector, microchannel plate or a channeltron.

The ions generated during the ionization of a sample by means of inductively coupled plasma ionization (ICP) can be analyzed with optical spectrometers, e.g., inductively coupled plasma optical emission spectrometer (ICP-OES).

The inductively coupled plasma is often sustained with Argon gas. Recently, an alternative ionization method has been described which uses Nitrogen gas for sustaining the plasma, the so called MICAP (microwave inductively coupled atmospheric plasma) which can be applied for mass spectrometry as well as for optical spectrometry (see U.S. Pat. No. 9,706,635 B2). MICAP generates a plasma which is also sustainable with air. As described in unpublished U.S. application Ser. No. 18/466,925, the nitrogen-sustained plasma can even be used to aspirate the sample into the mass spectrometer and towards the plasma.

However, both mass spectrometers and optical spectrometers need to be calibrated with the help of a standard or reference material which contains a known concentration of a chemical element. Whereas calibration is well-established for liquid samples, calibration of solid samples remains a problem. Due to small sample inconsistencies during entering of the sample into the plasma, e.g., due to differences in particle sizes or densities within the sample, the signal intensity obtained at the detector of the plasma spectrometer is not stationary but fluctuates.

SUMMARY

Therefore, it is an object of the present disclosure to provide a method and a system with which calibration of a plasma spectrometer is possible in an easy manner.

The object is achieved by a method for analyzing a sample by means of a system, wherein the sample is a solid sample and comprises a first chemical element and a second chemical element, wherein the system comprises a plasma spectrometer and an analytical device, wherein the plasma spectrometer is configured to ionize the sample by means of a plasma such that ions and/or photons are generated and to analyze the generated ions and/or photons, wherein the analytical device is configured to determine a concentration of the first chemical element, wherein the sample is provided to the plasma spectrometer in the form of an aerosol, wherein the method comprises at least the following steps:determining the concentration of the first chemical element by means of the analytical device;determining a sensitivity of the plasma spectrometer to the first chemical element and to the second chemical element;measuring the signal intensity of the first chemical element by means of the plasma spectrometer;measuring a signal intensity of the second chemical element by means of the plasma spectrometer; andcalculating the concentration of the second chemical element by means of the determined concentration of the first chemical element, the sensitivities to the first chemical element and to the second chemical element and the signal intensities of the first chemical element and the second chemical element.

According to the present disclosure, the concentration of the first chemical element determined by means of the analytical device is used to calibrate the signal intensities of the plasma spectrometer. Although the signal intensities of the solid sample will fluctuate, all signal intensities will fluctuate in the same order. Consequently, the signal intensities are calibrated by assigning the determined concentration to one of the signal intensities, i.e., the first chemical element. The concentration of the second chemical element (and of further chemical elements) is then calculated.

Generally, each chemical element is detected by the plasma spectrometer with its respective sensitivity, which needs to be considered during the calculation of the concentration of the second chemical element. For example, the respective sensitivities of the first chemical element and the second chemical element are determined during an initial setup of the plasma spectrometer, e.g., using reference standards with known concentrations of the first and second chemical element.

There are different ways to calculate the concentration of the second chemical element. One way is to determine the ratio of the signal intensities of the first chemical element and the second chemical element, to determine the ratio of the sensitivities to the first chemical element and to the second chemical element, to multiply the determined concentration of the first chemical element with the ratio of the sensitivities, and by dividing the result by the determined ratio of the signal intensities.

Another way is to determine the ratio of the signal intensities of the first chemical element and the second chemical element, to determine the ratio of the sensitivities to the first chemical element and to the second chemical element, to divide the determined concentration of the first chemical element by the signal intensity of the first chemical element, and then to multiply the result with the signal intensity of the second chemical element and with the ratio of the sensitivities to obtain the concentration of the second chemical element.

Other ways to calculate the concentration of the second chemical element also fall within the scope of the present disclosure.

Typically, the plasma spectrometer is configured to determine the concentration of (at least) the second element with much higher precision than the analytical device. Determining the concentration of the second chemical element is therefore preferred by means of the plasma spectrometer.

There are many methods to produce an aerosol from a solid sample which all fall under the scope of the present disclosure.

In one embodiment, the sample is a powder or dust or a rock sample or a soil sample or a drug or a food sample.

In another embodiment, a surface of the sample is mechanically machined such that the sample is partially brought into the form of an aerosol. The surface of the sample can be machined such that dust or powder is created from the surface, e.g., by drilling or scratching or grinding. The dust or powder may form an aerosol together with the surrounding atmosphere, e.g., air.

Further, the sample may be provided in the form of a solid cylinder. The solid cylinder may be obtained by surface drilling. For example, to find metals or other precious elements in the ground, especially underground, long cylinders, up to 1 km of length and longer, are drilled into the ground. The cylindrical sample is brought up to the surface and analyzed with regard to its chemical elements.

In another embodiment, the sample is brought into the form of an aerosol by means of exposure to mechanical machining, light, electric power and/or sound waves. The sample can be exposed to strong light or laser pulses such that dust or powder is created from the surface of the sample. Similarly, the sample can also be exposed to focused ultrasound waves such that dust or powder is created from the surface of the sample. Again, the aerosol may be formed in air and provided as such to the plasma spectrometer.

The object of the present disclosure is further achieved by a system for analyzing a sample comprising a plasma spectrometer and an analytical device, wherein the sample is a solid sample and comprises a first chemical element and a second chemical element, wherein the plasma spectrometer is configured to ionize the sample by means of a plasma such that ions and/or photons are generated and to analyze the generated ions and/or photons, wherein the analytical device is configured to determine a concentration of the first chemical element, wherein the sample is provided to the plasma spectrometer in the form of an aerosol, wherein the system is configured to execute the method according to the present disclosure.

According to the present disclosure, an easy calibration of the plasma spectrometer with respect to solid samples is possible, which enables a quick and straightforward sample analysis. In one embodiment, the plasma spectrometer and the analytical device are arranged such that a same area or portion of the sample is analyzed by either device. The concentrations of the first chemical element and/or the second chemical element may differ throughout the sample, e.g., in a case of a large sample. Therefore, it can be necessary to analyze the same area or portion of the sample with both the plasma spectrometer and the analytical device.

In another embodiment, the system comprises a sample unit which is configured to move the sample along a direction, wherein the analytical device and the plasma spectrometer are arranged along the direction such that the same area or portion of the sample is continuously analyzed by either device. In such an embodiment, continuous analysis of the sample is enabled.

Further, the system may comprise a cleaning unit which is configured to clean and/or dry the sample prior to providing the sample to the analytical device and/or the plasma spectrometer. The sample may be wet and/or contain dirt or other material on its surface which needs to be removed prior to analyzing the sample by means of the plasma spectrometer and/or the analytical device.

Another embodiment comprises that the analytical device is configured to determine the concentration of the first chemical element with a high precision, e.g., with a precision of less than 3% relative standard deviation. To ensure a precise calibration of the plasma spectrometer by means of the analytical device, the analytical device is ideally able to determine the first chemical element with a high precision.

It is further advantageous that the plasma spectrometer is configured to determine the concentration of the second element in a concentration range which is not available or only partially available by means of the analytical device. Often, the analytical device is not able to determine the concentration of the second element at all or only with a rather low precision. By determining the concentration of the second chemical element by means of the plasma spectrometer, a much higher precision may be obtained or may be measured at all.

In at least one embodiment, the plasma spectrometer is configured to aspirate the sample in the form of an aerosol, e.g., by means of the plasma. The plasma spectrometer may comprise a plasma torch configured to produce a plasma. The plasma may be configured to aspirate the sample into the plasma.

In one embodiment, the plasma spectrometer is a microwave inductively coupled atmospheric plasma mass spectrometer, a microwave inductively coupled atmospheric plasma optical emission spectrometer, a radio-frequency inductively coupled mass spectrometer, a radio-frequency inductively coupled optical spectrometer, a glow discharge mass spectrometer, or a glow discharge optical spectrometer.

In another embodiment, the analytical device is an X-ray fluorescence spectrometer, a laser induced breakdown spectrometer or an X-ray diffractometer.

In the figures, same elements are provided with the same reference numbers.

DETAILED DESCRIPTION

FIG.1schematically illustrates a schematic plasma spectrometer21, which may be used with the present disclosure. The plasma spectrometer21may comprise a sample preparation unit3with a sample inlet4and a plasma torch5, an interface7, a detector8, and an evaluation unit9. The sample inlet4may be arranged upstream of the plasma torch5, as shown inFIG.1. The components of the plasma spectrometer21may also be arranged differently.

The plasma spectrometer21may be configured to allow introduction of the sample2into the plasma6along a downwards-pointing vertical direction and/or to allow ions to be extracted from the plasma6along the downwards-pointing vertical direction. The downwards-pointing vertical direction points in the same direction as gravity. The plasma torch5may comprise a vertically oriented injector tube configured to introduce the sample2into the plasma6. The plasma torch5may be arranged vertically. In such an embodiment, a carrier gas which is used to introduce the sample2into the plasma torch5has a lesser impact on sample introduction. It is even possible to omit the carrier gas such that the sample2is introduced into the plasma6by gravity alone. Obviously, other arrangements of the plasma torch5also fall under the scope of the present disclosure.

The evaluation unit9can also be arranged apart from the other components. The plasma spectrometer21or some of its components may be housed in a housing (not shown). In embodiments in which the plasma spectrometer21is embodied as a mass spectrometer, the interface7may be a mass-analyzer, and the detector8may be an ion-electron multiplier, a Faraday collector, a Daly detector, a microchannel plate or a channeltron or other suitable, equivalent detector. In embodiments in which the plasma spectrometer21is embodied as an optical spectrometer, the interface7may be a wavelength-selector and the detector8a photomultiplier tube or a CCD camera other suitable, equivalent detector.

FIG.2illustrates an exemplary embodiment of plasma spectrometer21of the present disclosure as a microwave inductively coupled atmospheric plasma mass spectrometer (MICAP-MS). The sample2may be aspirated by means of the plasma6through the sample inlet4into the mass spectrometer21. The plasma6is generated by means of a MICAP source27in combination with a waveguide28specialized for MICAP. Upon entering the plasma torch5, the sample2is ionized by the plasma6such that ions and/or photons are generated. To detect the generated ions with the detector8, the ions31are guided by means of an ion optics30and mass analyzer35towards the detector8.

After the plasma6, a skimmer cone33is arranged to skim and focus the ions into an ion beam31. A collisional/reaction gas32may be introduced into the ion beam31to remove interfering ions through ion/neutral reactions. The ion beam31is then directed and focused by means of ion mirrors30towards the mass analyzer35in which the ions are separated according to their mass-to-charge ratio (m/z). The ions are then detected by the detector8. Pumps29can be arranged to generate vacuum conditions after the plasma torch5.

FIG.3illustrates an embodiment of the system1according to the present disclosure, comprising a plasma spectrometer21and an analytical device22. The plasma spectrometer21is configured to ionize the sample2by means of a plasma6such that ions and/or photons are generated and to analyze the generated ions and/or photons. The analytical device22is configured to determine a concentration of the first chemical element. The sample2is a solid sample, e.g., a powder or dust or a rock sample or a soil sample or a drug or a food sample, and is provided to the plasma spectrometer21in the form of an aerosol. There are many ways to produce an aerosol from a solid sample, including mechanical, electrical, photonic or acoustic methods. Often, particles or dust or powder is created from the sample which then forms an aerosol together with the surrounding gas, e.g., air.

To perform the method according to the present disclosure, the plasma spectrometer21and the analytical device22may be connected via a cable (illustrated as a dotted line inFIG.3) such that the concentration of the first chemical element is sent from the analytical device22to the plasma spectrometer21, e.g., to the evaluating unit9of the plasma spectrometer21. The plasma spectrometer21may be a microwave inductively coupled atmospheric plasma mass spectrometer, a microwave inductively coupled atmospheric plasma optical emission spectrometer, a radio-frequency inductively coupled mass spectrometer, a radio-frequency inductively coupled optical spectrometer, a glow discharge mass spectrometer, or a glow discharge optical spectrometer or other suitable, equivalent spectrometer. The analytical device22may be an X-ray fluorescence (XRF) spectrometer, a laser induced breakdown spectrometer or X-ray diffractometer.

XRF spectrometers are typically used to determine the concentration of gold or other (precious) chemical elements in soil samples because XRF spectrometers are simple to use and can be used on-site, i.e., where the sample has been found or drilled, etc. Samples with high enough gold concentrations are then sent to a laboratory in which the gold concentration is determined more precisely by means of plasma spectrometers. Mining companies are nowadays interested in gold concentrations down to 1 ppm. However, XRF spectrometers cannot measure the gold concentration under 10 ppm with high precision. Clearly, there is a lack of on-site accessibility of the gold concentration of the soil samples.

In the present disclosure, the system1provides the gold concentration (or the concentration of other precious metals or chemical elements) on-site with high precision, even down to very low gold concentrations, e.g., down to 1.2 ppb. Whereas XRF may not be used to measure the gold concentration at all or only at low precision, it is able to measure other chemical elements with high precision which may be abundant in the sample2. For example, the XRF spectrometer may be used to determine the concentration of aluminum or silicon (e.g., as the first chemical element) in the sample2. The concentration of aluminum or silicon determined by XRF can be used to calibrate the plasma spectrometer21. The gold concentration in the sample2can then be determined with high precision. A further example is provided inFIG.4.

FIG.4shows a table with experimental results from the system according to the present disclosure. Three reference standards from NIST were used for testing the system, each containing 12% calcium and different concentrations of gold. In the first column, the respective reference standard and the concentration of the chemical element to be analyzed is given. The second column gives the concentration as determined by XRF. For the gold concentrations, the respective error has been added. Whereas the error from XRF in the reference standard containing 25 ppm gold is somewhat low with ca. 8%, the error in the reference standard containing 5 ppm gold is as high as 60%. XRF was not able to detect any gold in the last reference standard containing 0.18 ppm gold. The third column gives the signal intensities for the respective chemical element of the respective reference standard as obtained by a plasma mass spectrometer, such as shown inFIG.2. The fourth column gives the sensitivity of the plasma spectrometer to calcium and gold. The calculated concentration of the second chemical element, e.g., gold, is shown in the last column.

The ratio of the signal intensities of calcium (the first chemical element) and gold (the second chemical element) of the NIST 610 sample is about 1,653. The ratio of the sensitivities to the first chemical element and to the second chemical element is about 0.345. The concentration of the second chemical element can be obtained by multiplying the determined concentration of the first chemical element with the ratio of the sensitivities and by dividing the result by the determined ratio of the signal intensities. In another way, the concentration of the second chemical element can be obtained by dividing the determined concentration of the first chemical element by the signal intensity of the first chemical element and then to multiply the result with the signal intensity of the second chemical element and with the ratio of the sensitivities. After calibration of the plasma spectrometer with the concentration of the first chemical element, the concentration of gold can be determined with high precision, even in the reference standard containing only 0.18 ppm gold.

FIG.5shows another embodiment of the system1in which the sample2is provided as a solid cylinder. The solid cylinder may be obtained by surface drilling. The plasma spectrometer21and the analytical device22may be arranged such that a same area or portion of the sample2is analyzed by either device21,22. Using a drilling unit23the surface of the sample may be machined such that an aerosol is formed (in air), which is provided to the plasma spectrometer21. The analytical device22may be configured to determine the concentration of the first chemical element based on the solid sample or the aerosol. The system1may further comprise a sample unit36, which is configured to move the sample2along a direction such that the plasma spectrometer21and the analytical device22can analyze the sample2continuously. Furthermore, the system1may comprise a cleaning unit26configured to clean and/or dry the sample2prior to providing the sample2to the analytical device22and/or the plasma spectrometer21.

The plasma spectrometer21may further comprise additional components such as a sample introduction unit10and/or a classifier16, which are illustrated inFIG.6.

The sample introduction unit10may be configured to introduce the sample2into the sample inlet4of the plasma spectrometer21. The sample introduction unit10may comprise a transport means11, e.g., a moving belt14, which is configured to transport the sample2towards the sample inlet4, and a connecting unit12, which is configured to be connectable with the sample inlet4. The sample introduction unit10, as shown inFIG.6, further comprises a transfer tube25through which particles13, e.g., dust or powder, are guided towards the moving belt14, which is being loaded with the particles13and moves the particles13towards the sample inlet4by means of the rollers24. A dosage controller15is arranged and configured to control the amount of particles13entering the sample inlet4through the connecting unit12. The sample inlet4or the sample introduction unit10can be open to the ambient atmosphere to aspirate air together with the particles thus forming the sample2. The sample introduction unit10may be connected directly with the sample inlet4(not shown).

The plasma spectrometer21may further comprise a classifier16as shown inFIG.6which is configured to separate smaller particles from larger particles within the sample2and to guide particles with a mass below a predefined upper mass limit to the sample inlet4. The classifier16may comprise a container17with an inlet18, which is connected to air or gas, and an outlet19, which is connectable to the sample inlet4. Moreover, a tube20may be inserted partially into the container17such that the flow of the sample2is opposed to the flow of the particles with a mass below the predefined upper mass limit towards the outlet19. Particles with a mass above the predefined upper mass limit will sediment towards the lower end of the container17. The suction property of the plasma6can be used to aspirate the particles13together with air as the sample2into the plasma spectrometer1. In at least one embodiment, the classifier16is arranged between the sample introduction unit10and the sample inlet4.

The position of the analytical device22may vary. For example, it can be arranged next to the transport means11or next to the connecting unit12or between the classifier16and the sample inlet4.