Method for separating anionic, cationic, neutral and zwitterionic arsenic forms

Abstract

A method includes separating a plurality of arsenic forms using a mixed-mode column and a strong acid. The plurality of arsenic morphology includes at least one of each of an anionic arsenic morphology, a cationic arsenic morphology, a neutral arsenic morphology, and a zwitterionic arsenic morphology.

Description

Technical Field

[0001] The present disclosure generally relates to the field of chromatography, including methods for separating anionic, cationic, neutral, and zwitterionic arsenic species. Background Art

[0002] Chromatography, particularly ion chromatography (IC), is a widely used analytical technique for determining anionic and cationic analytes in various sample matrices. Analysis of arsenic species by IC-MS typically uses columns that can be operated with either basic or acidic eluents. When using a basic eluent, neutral and cationic arsenic species generally have poor retention on the column due to poor ionization. When using an acidic eluent, all arsenic species are ionized, making retention possible. However, the acid commonly selected is nitric acid, which is an overly strong anionic eluent (nitrate ion) that results in poor retention and thus poor separation of anionic species. Therefore, there is a need for improved methods for arsenic species analysis.

[0003] In “Separation of organic and inorganic arsenic species by HPLC-ICP-MS” (Fresenius J Anal Chem (1999) 363:577–581), Londesborough et al. disclosed that although it is possible to separate some arsenic species using a nitric acid gradient, monomethylarsonic acid co-elutes with arsenate, and trimethylarsine oxide co-elutes with tetramethylarsonium ion. Londesborough et al. demonstrated that by adding an eluent modifier, it is possible to partially separate trimethylarsine oxide, arsenocholine, and tetramethylarsonium ion.

[0004] In “Metal species determination by ion Chromatography” (Trends in analytical chemistry, Vol. 20, Nos. 6+7, 2001), Sarzanini et al. demonstrated that the separation of arsenite, arsenate, monomethylarsenate, dimethylarsenate, trimethylarsine oxide,

[0005] tetramethylarsonium ion, arsenobetaine, and arsenocholine can be achieved by using an anion exchange column and a cation exchange column. Summary of the Invention

[0006] In a first aspect, a method can include separating multiple arsenic species using a mixed-mode column and a strong acid, the multiple arsenic species including at least one of each of anionic arsenic species, cationic arsenic species, neutral arsenic species, and zwitterionic arsenic species.

[0007] In various embodiments of the first aspect, the anionic arsenic species may be selected from the group consisting of arsenite, methylarsonate, dimethylarsinate, phenylarsonate, and arsenate.

[0008] In various embodiments of the first aspect, the neutral arsenic species may include trimethylarsine oxide.

[0009] In various embodiments of the first aspect, the zwitterionic arsenic species may include arsenobetaine.

[0010] In various embodiments of the first aspect, the cationic arsenic species may be selected from the group consisting of arsenocholine and tetramethylarsonium.

[0011] In various embodiments of the first aspect, the mixed-mode column may include a stationary phase having anionic functional groups and cationic functional groups.

[0012] In various embodiments of the first aspect, the strong acid may have a pKa of less than 2.0.

[0013] In various embodiments of the first aspect, the strong acid includes methanesulfonic acid, ethanesulfonic acid, hydrochloric acid, hydrobromic acid, iodic acid, chloric acid, or any combination thereof.

[0014] In various embodiments of the first aspect, an organic solvent may also be used to separate multiple arsenic species.

[0015] In a second aspect, a method may include separating multiple arsenic species using a chromatographic column and a strong acid having a counterion, the counterion including an anion exchange form weaker than NO3.

[0016] In various embodiments of the second aspect, the multiple arsenic species may include anionic arsenic species selected from the group consisting of arsenite, methylarsonate, dimethylarsinate, phenylarsonate, and arsenate.

[0017] In various embodiments of the second aspect, the neutral arsenic species may be selected from the group consisting of trimethylarsine oxide.

[0018] In various embodiments of the second aspect, the multiple arsenic species may include zwitterionic arsenic species. In a specific embodiment, the zwitterionic arsenic species may include arsenobetaine.

[0019] In various embodiments of the second aspect, the multiple arsenic species may include cationic arsenic species selected from the group consisting of arsenocholine and tetramethylarsonium.

[0020] In various embodiments of the second aspect, the chromatographic column may be a mixed-mode column, and the mixed-mode column may include a stationary phase having anionic functional groups and cationic functional groups.

[0021] In various embodiments of the second aspect, the strong acid may have a pKa of less than 2.

[0022] In various embodiments of the second aspect, the strong acid includes methanesulfonic acid, ethanesulfonic acid, hydrochloric acid, hydrobromic acid, iodic acid, chloric acid, or any combination thereof.

[0023] In various embodiments of the second aspect, an organic solvent may also be used to separate multiple arsenic species.

[0024] In a third aspect, a method may include separating multiple arsenic species using a chromatographic column and a strong acid, the multiple arsenic species including at least two anionic arsenic species.

[0025] In various embodiments of the third aspect, the anionic arsenic species may be selected from the group consisting of arsenite, methylarsonate, dimethylarsinate, phenylarsonate, and arsenate.

[0026] In various embodiments of the third aspect, the multiple arsenic species may further include at least one zwitterionic arsenic species, at least one neutral arsenic species, or at least one cationic arsenic species. In a specific embodiment, the neutral arsenic species may include dimethylarsinate. In a specific embodiment, the zwitterionic arsenic may include arsenobetaine. In a specific embodiment, the cationic arsenic species may be selected from the group consisting of arsenocholine and tetramethylarsonium.

[0027] In various embodiments of the third aspect, the chromatographic column may be a mixed-mode column, and the mixed-mode column may include a stationary phase having anionic functional groups and cationic functional groups.

[0028] In various embodiments of the third aspect, the strong acid may have a counterion that is a weaker anion-exchange form than NO3.

[0029] In various embodiments of the third aspect, the strong acid may have a pKa of less than 2.

[0030] In various embodiments of the third aspect, the strong acid includes methanesulfonic acid, ethanesulfonic acid, hydrochloric acid, hydrobromic acid, iodic acid, chloric acid, or any combination thereof.

[0031] In various embodiments of the third aspect, an organic solvent may also be used to separate multiple arsenic species. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] To more fully understand the principles disclosed herein and their advantages, reference is now made to the following description taken in conjunction with the accompanying drawings and presenting schematics, in which:

[0033] Figure 1 is a block diagram of an exemplary chromatographic system according to various embodiments.

[0034] Figure 2is a flow chart showing an exemplary method for analyzing arsenic species in a sample according to various embodiments.

[0035] Figure 3 , Figure 4 and Figure 5 are chromatograms showing the separation of various arsenic species.

[0036] It should be understood that the drawings are not necessarily drawn to scale, and the relationships between objects in the drawings are not necessarily drawn to scale. The drawings are depictions intended to clarify and understand various embodiments of the devices, systems, and methods disclosed herein. Where possible, all drawings will use the same reference numerals to refer to the same or similar parts. Additionally, it should be understood that the drawings are not intended to limit the scope of the present teachings in any way. Detailed Description

[0037] Embodiments of methods for separating anionic, cationic, neutral, and zwitterionic arsenic species are described herein and in the accompanying illustrations.

[0038] The section headings used herein are for organizational purposes only and should not be construed as limiting the subject matter described in any way.

[0039] In the detailed description of the various embodiments, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosed embodiments. However, those skilled in the art will understand that these various embodiments may be practiced with or without these specific details. In other instances, structures and devices are shown in block diagram form. Additionally, those skilled in the art can readily understand that the specific order in which the methods are presented and executed is illustrative, and that the order can be changed and still remain within the spirit and scope of the various embodiments disclosed herein.

[0040] All documents and similar materials cited in this application, including but not limited to patents, patent applications, articles, books, papers, and internet web pages, are hereby expressly incorporated by reference in their entirety for any purpose. Unless otherwise described, all technical and scientific terms used herein have the meaning commonly understood by one of ordinary skill in the art to which the various embodiments described herein pertain.

[0041] It should be understood that there is an implicit "about" preceding temperatures, concentrations, times, pressures, flow rates, cross-sectional areas, etc. discussed in this teaching, such that minor and non-substantive deviations are within the scope of this teaching. In this application, unless otherwise specifically specified, the use of the singular includes the plural. Additionally, the use of "comprising", "including", and "containing" is not intended to be limiting. It should be understood that both the foregoing general description and the following detailed description are merely exemplary and illustrative, and not limiting of the present teachings.

[0042] As used herein, "a" or "an" may also refer to "at least one" or "one or more". In addition, the use of "or" is inclusive, such that the phrase "A or B" is true when A is true, B is true, or both A and B are true. In addition, unless the context requires otherwise, singular terms shall include the plural and plural terms shall include the singular.

[0043] A "system" describes a set of physical or abstract components, including a whole, where each component interacts or is related to at least one other component within the whole.

[0044] Figure 1 An embodiment of a chromatographic system 100 is shown. The chromatographic system 100 may include a pump 102, an electrolytic eluent generator 104, a continuously regenerable capture column 106, a degassing device 108, an injector 110, a chromatographic separation device 112, an electrolytic suppressor 114, a detector 116, and a microprocessor 118. The chromatographic separation device 112 may take the form of a capillary column or an analytical column.

[0045] The pump 102 may be configured to pump a liquid from a liquid source 132 (such as deionized water) and be fluidly connected to the electrolytic eluent generator 104. The pump 102 may take the form of a high performance liquid chromatography (HPLC) pump.

[0046] An eluent is a liquid containing an acid, a base, a salt, or a mixture thereof, and may be used to elute an analyte through a chromatographic column. In addition, the eluent may include a mixture of a liquid and a water-miscible organic solvent, where the liquid may include an acid, a base, a salt, or a combination thereof. The electrolytic eluent generator 104 is configured to generate an eluent form. An eluent form refers to a particular form of an acid, a base, or a salt that may be added to the eluent. In one embodiment, the eluent form may be a base, such as potassium hydroxide, or the eluent form may be an acid, such as carbonic acid, phosphoric acid, acetic acid, methanesulfonic acid, or a combination thereof. The eluent may contain an organic solvent, such as acetonitrile and methanol.

[0047] Referring Figure 1 , the eluent generator 104 may be configured to receive the liquid from the pump 102 and then add the eluent form to the liquid. The liquid containing the eluent form may be output from the eluent generator 104 to the inlet of the continuously regenerable capture column 106.

[0048] The continuous regeneration capture column 106 is configured to remove cationic or anionic contaminants from the eluent. The continuous regeneration capture column 106 may include an ion exchange bed having electrodes at the eluent outlet. An ion exchange membrane stack may separate the eluent from the second electrode, and contaminant ions may be swept through the ion exchange membrane stack towards the second electrode. The ion exchange membrane stack may include one or more ion exchange membranes. In various embodiments, anion removal may utilize an anion exchange bed where the cathode at the eluent outlet is separated from the anode by an anion exchange membrane. Alternatively, cation removal may utilize a cation exchange bed where the anode at the eluent outlet is separated from the cathode by a cation exchange membrane.

[0049] The degassing device 108 can be used to remove residual gases. In one embodiment, the residual gases may be generated electrolytically, such as hydrogen and oxygen. The degassing device 108 may include a gas-permeable and liquid-impermeable tubing section, such as amorphous fluoropolymer or more specifically Teflon AF. The flowing liquid can be output from the degassing device 108 to an injector 110 where a significant portion of the gas has been removed.

[0050] The injector 110 can be used to inject a large dose of liquid sample into the eluent stream. The liquid sample may include a variety of chemical components (i.e., matrix components) and one or more target analytes. The injector 110 may include an autosampler 134, a sample loop 136, and a multiport valve 138. The autosampler 134 can draw a sample from a sample container. The multiport valve 138 can be in a first position to allow the sample to fill the sample loop 136. After filling the sample loop 136 to the desired level, the multiport valve can be switched to a second position, and the eluent stream can drive the sample onto the chromatographic separation device 112.

[0051] The chromatographic separation device 112 can be used to separate the various matrix components present in the liquid sample from the target analytes. Generally, the chromatographic separation device 112 may take the form of a hollow cylinder containing a packed stationary phase. As the liquid sample flows through the chromatographic separation device 112, the matrix components and target analytes may have a range of retention times to elute from the chromatographic separation device 112. Depending on the characteristics of the target analytes and matrix components, they may have different affinities for the stationary phase in the chromatographic separation device 112. The output of the chromatographic separation device 112 can be fluidly connected to an electrolytic suppressor 114.

[0052] The suppressor 114 can be used to reduce the eluent conductivity background and enhance the analyte response by effectively exchanging the eluent counterions for regenerant ions. One type of suppressor is the electrolytic suppressor 114, which may include an anode chamber, a cathode chamber, and an eluent suppression bed chamber separated by an ion exchange membrane. The anode chamber and / or the cathode chamber can generate regenerant ions or deliver supplied regenerant ions. The eluent suppression bed chamber can include a flow path for the eluent that is separated from the regenerant by an ion exchange barrier, and the eluent counterions can exchange with the regenerant ions across the ion exchange barrier. The output of the electrolytic suppressor 114 can be fluidly connected to the detector 116 to measure the presence of separated chemical components of a liquid sample. The suppressor 114 can also be a chemical species that requires a chemical regenerant to operate. Any suppressor in the prior art is suitable for the present application having multiple channels as configured.

[0053] The detector 116 can be in the form of an ultraviolet-visible spectrometer, a fluorescence spectrometer, an atomic fluorescence detector, an atomic emission spectrometer, a refractive index detector, a radioactive flow detector, a chiral detector, an electrochemical detector, a conductivity detector, a mass spectrometer, a flame ionization detector, or a combination thereof.

[0054] The electronic circuit can include a microprocessor 118, a timer, and a memory section. Additionally, the electronic circuit can include a power supply configured to apply control signals, respectively. The microprocessor 118 can be used to control the operation of the chromatography system 100. The microprocessor 118 can be integrated into the chromatography system 100 or be part of a personal computer that communicates with the chromatography system 100. The microprocessor 118 can be configured to communicate with and control one or more components of the chromatography system, such as the pump 102, the pump 130, the eluent generator 104, the injector 110, and the detector 116. The memory section can be used to store instructions to set the magnitude and timing of the current waveform relative to the switching of the injector 110 for injecting a sample.

[0055] Figure 2FIG. 200 is a flow chart showing a method for analyzing a sample containing arsenic species. At 202, the sample is injected into a chromatographic column. Depending on the sample, various pre-injection sample preparation steps may be taken. In various embodiments, a solid or semi-solid sample may be ground and suspended in a solution to extract arsenic species. The liquid solution may be separated from the solid material and injected into the column. In other embodiments, the sample may be a liquid sample, such as a water sample, and may be directly injected onto the column without extensive pre-injection sample preparation. The sample may include multiple arsenic species, including anionic arsenic species, cationic arsenic species, neutral arsenic species, and zwitterionic arsenic species. In various embodiments, the sample may include at least two anionic arsenic species. In various embodiments, the sample may include an anionic arsenic species and any combination of cationic arsenic species, neutral arsenic species, and zwitterionic arsenic species. For example, the sample may include one or more anionic arsenic species and at least one of cationic arsenic species, neutral arsenic species, and zwitterionic arsenic species. In another example, the sample may include at least one of each arsenic species of anionic arsenic species, cationic arsenic species, neutral arsenic species, and zwitterionic arsenic species.

[0056] Anionic arsenic species may behave as anions at all pH values. Examples of anionic arsenic species include arsenite, methylarsonate, dimethylarsinate, phenylarsonate, and arsenate. Neutral arsenic species may behave as cations at low pH. Trimethylarsine oxide is an example of a neutral arsenic species. Zwitterionic arsenic species include anionic and cationic groups and may behave as cations at low pH when the anionic group is neutralized. Arsenobetaine is an example of a zwitterionic arsenic species. Cationic arsenic species may behave as cations at all pH values. Examples of cationic arsenic species include arsenocholine and tetramethylarsonium.

[0057] The column may include a mixed-mode stationary phase. The mixed-mode stationary phase includes functional groups having two or more different properties, such as cationic functional groups, anionic functional groups, polar groups, etc. In a specific embodiment, the mixed-mode stationary phase includes an anionic functional group and a cationic functional group. The arsenic species may be retained on the column by binding to the functional groups.

[0058] At 204, an acidic eluent may be used to elute the bound arsenic species from the column. The acidic eluent may include a strong acid, such as an acid with a pKa less than 2.0. The strong acid may include hydronium ions and a weak anionic eluent form, such as an anionic eluent form weaker than NO3 - weaker anionic eluent forms. Examples of strong acids with weak anionic eluent forms may include methanesulfonic acid, ethanesulfonic acid, hydrochloric acid, hydrobromic acid, iodic acid, and chloric acid.

[0059] At 206, the output of the detector can be recorded over time, and at 208, the detector output can be used to identify or quantify the arsenic species present in the sample. In a specific embodiment, the detector can be an arsenic-selective detector, such as a mass spectrometer. For example, method 200 can be performed using ion chromatography-mass spectrometry (IC-MS).

[0060] While the present teachings are described in connection with various embodiments, it is not intended to limit the present teachings to such embodiments. On the contrary, the present teachings cover various alternative forms, modifications, and equivalents, as will be understood by those skilled in the art.

[0061] Moreover, in describing various embodiments, this specification may present methods and / or processes as a particular sequence of steps. However, insofar as the method or process does not depend on a particular sequence of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As will be understood by those of ordinary skill in the art, other sequences of steps are possible. Accordingly, the particular sequence of steps set forth in the specification should not be construed as a limitation on the claims. Additionally, the claims directed to the method and / or process should not be limited to performing their steps in the order written, and those skilled in the art can readily understand that the order can vary and still remain within the spirit and scope of the various embodiments.

[0062] Any of the operations forming part of the embodiments described herein are useful machine operations. The embodiments described herein also relate to apparatus or devices for performing these operations. The systems and methods described herein may be specially constructed to achieve the desired purposes or they may be a general-purpose computer selectively activated or configured by a computer program stored in the computer. In particular, various general-purpose machines may be used with a computer program written in accordance with the teachings herein, or it may be more convenient to construct a more specialized device to perform the desired operations.

[0063] Example

[0064] Figure 3 is a chromatogram showing the separation of a sample comprising nine arsenic species. The separation was performed using an IonPac AS7 column with methanesulfonic acid as the eluent. During the period from 0 to 5 minutes, the concentration of methanesulfonic acid was 2.0 mM, increased to 20 mM during the period from 5 to 10 minutes, and then maintained at 20 mM during the period from 10 to 25 minutes. The sample included (1) arsenite, (2) methylarsonate, (3) dimethylarsinate, (4) phenylarsonate, (5) arsenate, (6) arsenobetaine, (7) trimethylarsine oxide, (8) arsenocholine, and (9) tetra methylarsonium.

[0065] Figure 4It is a chromatogram showing the separation of a sample containing 17 arsenic species. The separation was carried out using an IonPac AS7 column with methanesulfonic acid as the eluent. During the period from 0 to 5 minutes, the concentration of methanesulfonic acid was 2.0 mM, increased to 20 mM during the period from 5 to 10 minutes, and then maintained at 20 mM during the period from 10 to 25 minutes. The sample includes (1) arsenite, (2) phenylarsine oxide, (3) methylarsonate, (4) dimethylarsinate, (5) phenylarsonate, (6) 2-nitrophenylarsonate, (7) 2-aminophenylarsonate, (8) 4-hydroxyphenylarsonate, (9) p-aminophenylarsonate, (10) carbarsone, (11) arsenate, (12) arsenobetaine, (13) nitarsone, (14) roxarsone, (15) trimethylarsine oxide, (16) arsenocholine, and (17) tetramethylarsonium.

[0066] Figure 5 It is a chromatogram showing the separation of a sample containing nine arsenic species. The separation was carried out using an IonPac CS5A column with methanesulfonic acid as the eluent. During the period from 0 to 2 minutes, the concentration of methanesulfonic acid was 2.0 mM, increased to 6 mM during the period from 2 to 8 minutes, and then increased to 80 mM during the period from 8 to 20 minutes. The sample includes (1) arsenite, (2) methylarsonate, (3) phenylarsonate, (4) arsenate, (5) dimethylarsinate, (6) arsenobetaine, (7) trimethylarsine oxide, (8) arsenocholine, and (9) tetramethylarsonium.

Claims

1. A method, the method comprising: Separating a plurality of arsenic species using a mixed-mode column and a strong acid, the plurality of arsenic species including at least one of each arsenic species of anionic arsenic species, cationic arsenic species, neutral arsenic species, and zwitterionic arsenic species.

2. The method according to claim 1, wherein the anionic arsenic species is selected from the group consisting of arsenite, methylarsonate, dimethylarsonate, phenylarsonate, and arsenate.

3. The method according to claim 1, wherein the neutral arsenic species includes trimethylarsine oxide.

4. The method according to claim 1, wherein the zwitterionic arsenic species includes arsenobetaine.

5. The method according to claim 1, wherein the cationic arsenic species is selected from the group consisting of arsenocholine and tetramethylarsonium.

6. The method according to claim 1, wherein the mixed-mode column includes a stationary phase having anionic functional groups and cationic functional groups.

7. The method according to claim 1, wherein the strong acid has a pK of less than 2.0 a .

8. The method according to claim 1, wherein the strong acid includes methanesulfonic acid, ethanesulfonic acid, Hydrochloric acid, hydrobromic acid, iodic acid, chloric acid, or any combination thereof.

9. The method according to claim 1, wherein an organic solvent is also used to separate the plurality of arsenic species.

10. A method, the method comprising: Separating a plurality of arsenic species using a chromatographic column and a strong acid having a counterion, the counterion including an anion exchange form weaker than NO3.

11. The method according to claim 10, wherein the plurality of arsenic species includes an anionic arsenic species selected from the group consisting of arsenite, methylarsonate, dimethylarsonate, phenylarsonate, and arsenate.

12. The method according to claim 10, wherein the neutral arsenic species is selected from the group consisting of trimethylarsine oxide.

13. The method according to claim 10, wherein the plurality of arsenic species includes a zwitterionic arsenic species.

14. The method according to claim 13, wherein the zwitterionic arsenic species includes arsenobetaine.

15. The method according to claim 10, wherein the plurality of arsenic species includes a cationic arsenic species selected from the group consisting of arsenocholine and tetramethylarsonium.

16. The method according to claim 10, wherein the chromatographic column is a mixed-mode column, and the mixed-mode column includes a stationary phase having anionic functional groups and cationic functional groups.

17. The method according to claim 10, wherein the strong acid has a pK of less than 2 a .

18. The method according to claim 10, wherein the strong acid includes methanesulfonic acid, ethanesulfonic acid, hydrochloric acid, hydrobromic acid, iodic acid, chloric acid, or any combination thereof.

19. The method according to claim 10, wherein an organic solvent is also used to separate the plurality of arsenic species.

20. A method, the method comprising: Separating a plurality of arsenic species using a chromatographic column and a strong acid, the plurality of arsenic species including at least two anionic arsenic species.

21. The method according to claim 20, wherein the anionic arsenic species is selected from the group consisting of arsenite, methylarsonate, dimethylarsonate, phenylarsonate, and arsenate.

22. The method according to claim 20, wherein the plurality of arsenic species further comprises at least one zwitterionic arsenic species, at least one neutral arsenic species, or at least one cationic arsenic species.

23. The method according to claim 22, wherein the neutral arsenic species comprises dimethylarsinate.

24. The method according to claim 22, wherein the zwitterionic arsenic comprises arsenobetaine.

25. The method according to claim 22, wherein the cationic arsenic species is selected from the group consisting of arsenocholine and tetramethylarsonium.

26. The method according to claim 20, wherein the chromatographic column is a mixed-mode column, and the mixed-mode column comprises a stationary phase having anionic functional groups and cationic functional groups.

27. The method according to claim 20, wherein the strong acid has a counterion, and the counterion is an anion-exchange form weaker than NO3.

28. The method according to claim 20, wherein the strong acid has a pK of less than 2 a .

29. The method according to claim 20, wherein the strong acid comprises methanesulfonic acid, ethanesulfonic acid, hydrochloric acid, hydrobromic acid, iodic acid, chloric acid, or any combination thereof.

30. The method according to claim 20, wherein an organic solvent is further used to separate the plurality of arsenic species.

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