WO2002016928A1

WO2002016928A1 - Method, device and kit for chemical species separation

Info

Publication number: WO2002016928A1
Application number: PCT/US2001/041861
Authority: WO
Inventors: Gregory P. Miller
Original assignee: New Mexico Tech Research Foundation
Current assignee: New Mexico Tech Research Foundation
Priority date: 2000-08-21
Filing date: 2001-08-21
Publication date: 2002-02-28
Anticipated expiration: 2003-02-21
Also published as: AU2001293217A1

Abstract

A method, device, and kit for chemical species separation, such as of arsenic compounds, are provided. Using the kit in the field, a sample is provided, part of which is preserved for total chemical species analysis. The remainder of the sample is introduced into a solid phase that is disposed, for example, in a column (1) and that has been prepared for a chemical species of interest. At least one eluant that is appropriate for separating off the chemical species of interest is then introduced to the solid phase. The sample and eluant effluent are then collected for a mass balance calculation.

Description

METHOD. DEVICE AND KIT FOR CHEMICAL SPECIES

SEPARATION Technical Field

The current invention relates to a method, device, and kit for the separation of arsenic compounds in a setting other than a laboratory.

This invention is also suitable for separation of other compounds amenable to ion exchange chromatography, such as selenium, perchlorate, or chromium although the physical and chemical details of each separation process are dependent on the chemical of interest. The general configuration and intent of the kit is the same regardless of the chemical being separated. Background Art

Review of prior art revealed no patents that address the use of ion exchange chromatography for any separation outside of laboratory settings; the inventive kit is completely novel.

Arsenic is present in the environment as a great number of compounds. The compounds have a great range in toxicities. It has been determined by others that there is no practical way to prevent many of these compounds from transforming into other arsenic compounds when removed from their original location with the intent of transferring the sample to a laboratory for analysis. Since the accurate assessment of the type of arsenic compound present in a sample is equally as if not more important than determining the amount of arsenic present, it is now necessary to have a device readily available that can separate these arsenic compounds in a quantitative manner on site. The device must be rugged, simple, relatively inexpensive, and operable by unskilled personnel. The inventive device will meet these needs.

Arsenic speciation (i.e. species separation) methods were researched to find a procedure suitable for field use. Although there are several separation methods published, all methods failed either in ability to separate all of the environmentally significant arsenic species, provided false positive indications of a species presence, or had no provision for calculating mass balance and preserving the sample. Thus, none of the known processes are able to meet all of the current needs or solve the known problems in arsenic species separation. No method was found that was directly transferable to the field . It is therefore an object of the present invention to provide such a method as well as a device and kit for carrying out same.

Brief Description of the Drawings

The object and advantages of the present invention will appear more clearly from the following specification in conjunction with the accompanying drawings, in which: Fig. 1 depicts the contents of one exemplary embodiment of an inventive arsenic speciation; Fig. 2 depicts the AsK 2 type ion exchange separation column; Fig. 3 depicts the AsK 4 type ion exchange separation column; and

Fig. 4 depicts the AsK 2-4 type ion exchange chromatography columns. Disclosure of the Invention

There are five potential aspects to the current invention: the AsK kit itself; the AsK 2 column configuration; the AsK 4 configuration; a hybrid AsK 2-4 column configuration, and the AsK 2+ process. The arsenic speciation device is characterized primarily by the following functional components: At least one cylindrical column containing materials that sorb, oxidize, or exchange chemicals under predictable conditions. The columns can be singular, stacked in series, or parallel or any combination thereof, as necessary for the intended separation; A reservoir for introduction of samples and the aqueous chemicals necessary to separate chemical species;

An outflow port used to control flow and collect the separated arsenic species from the columns;

A shipping container with multiple use as: a container for the required sample and chemical bottles; documentation and instructions for use of the kit; and, a support structure to hold the column vertically for gravity-driven elution of the chemical species of interest;

A kit prepared to meet the current United States or international standards for shipping integrity; All regulatory-required chain of custody and documentation;

Pre-paid analytical and shipping services in a one-use complete package; and

Pre-cleaned sample containers and high-purity arsenic-free eluants and components. The Arsenic Speciation Kit (AsK) provides the accuracy of laboratory separation of arsenic species in a user-friendly package. It has several embodiments that allow tailoring the analysis to two, three, or four arsenic species. Using the principles of ion exchange and liquid-solid phase sorption the four dominant arsenic species in nature, As³⁺, As⁵⁺, monomethylarsenate, and dimethylarsenate, as their respective oxyanions of arsonic acid, arsenic acid, dimethylarsinic acid (DMA), and monomethylarsenic acid (MMA), can be quantitatively separated for analytical determination. The method provides accuracy and precision for field separation of arsenic species for analysis.

Investigation of the speciation of arsenic in the environment requires the ability to preserve the arsenic species prior to analysis, or to separate the species before transformation takes place. Given the uncertainties in the preservation of arsenic species, field separation is necessary. Separation is initiated within minutes of sample collection. Uncertainties associated with sample preservation are eliminated by the method of the present invention.

All prior known methods for arsenic separation have one or more of the following shortcomings.

The method requires a laboratory environment or is not field deployable. By not separating the arsenic species at time of collection the method cannot prevent arsenic species transformation between sampling and analysis. Is not suitable for a broad range of aqueous media chemistries or arsenic concentrations.

The separation produces false positives for one or more of the four arsenic species of interest herein.

Does not account or correct for the accumulation of arsenic in the columns from trace quantities in the reagents and water used for preparation of the columns and elution of the arsenic species.

Cannot provide an assessment of mass balance for the separation of species. This results in data of unknown quality.

Requires very expensive equipment and facilities. Requires highly trained personnel for its operation.

Must be assembled from bulk-packaged components by the user, validated by the user, and analysis performed or arranged by the user. All of this is very costly and time consuming. The inventive AsK device has none of these shortfalls. The complete AsK 2, AsK 4, AsK 2-4, or AsK 2+ kit is shipped to the user. This includes step-wise instructions, pre-paid shipping, and pre-paid analytical. The shipping container holds the user- chosen column configuration ready for use. To use the AsK kit a filtered or unfiltered pH adjusted liquid sample of known volume is introduced to the top of an AsK column. Samples can be drinking water, groundwater, surface water, wastewater and effluent, or soil extracts. Chemical species changes, i.e. oxidation and reduction, can occur within minutes of sample collection. There is simply no time for arsenic species transformation prior to initiation of separation using the inventive method. Supplied pre- measured reagents are added to the top of the column and measured quantities of effluent are collected at intervals at the bottom of the column. The ion chromatography method depends on the property of arsenic species to change charge depending on pH. Samples are collected in supplied bottles and all paperwork necessary to satisfy EPA analytical and custody protocol are included to be filled out by the user. The device is simple to use and tolerant of rough handling. Detailed Description of Preferred Embodiments AsK Kit

Figure 1 depicts the AsK column 1 in its shipping container 2. The container can be made of any suitable material, preferably durable enough for reuse and constructed of easily recyclable materials, such as paperboard, plastic, and suitable polymeric materials. The shipping container also doubles as a support for the AsK device during use and as a billboard for information important to the user. It has two compartments, the first 3 containing one of the columns detailed in Figure 2 or 3, or a combination thereof (see Fig.4). This column can be located on the left or right side of the container as needed for shipping or balance considerations. The column is supported at the top via a sliding mechanism 4 that surrounds and grips the column, is made either of cardboard, plastic or another suitable material, and that when the slide is partially withdrawn from the shipping box places the AsK column in position so that it is easy to place a sample container beneath the outflow 5. The slide 4 can be configured to support one or multiple columns. The bottom of the column is supported during shipping by a U-shaped bracket 6 located in the lower portion of the shipping container. Rubber, neoprene or a similar material for grip on the column in a semi-flexible or rigid frame forms the slide and U bracket. The lid 7 of the shipping container functions as a brace for the kit when the column is deployed and holds the instructions for use in an easy to read location. The second compartment 8 is used for shipment and storage of sample containers, ion exchange eluants, documentation, and sample handling, packaging, measuring, and filtering supplies and apparatus. It would also be possible to provide more than two compartments. AsK Columns Ask 2 Figure 2 depicts the AsK 2 column configuration. The AsK 2 column 1 comprises a reservoir 10, ion exchange resin 11 , upper and lower confining (e.g. 0.35 urn pore size ultra high density polyethylene) bed supports or frits 12, a lower flow adapter and holder 13 for the lower frit, a plastic or borosilicate glass barrel 14, an upper flow adapter 15 to the reservoir, and an outflow valve 16. By way of example, with the glass barrel 14 in a 1.5 cm by 10 cm configuration it can be used to separate arsenate from arsenite in liquid samples using 0.12 M HCI as an eluant. Columns are shipped saturated with distilled-deionized water. As an appropriate resin or solid phase adsorbent, the strong base anion exchange resin (anion resin) AG 1-X850 -100 mesh (Bio-Rad, Hercules, CA) may be used. Anion resin is a strongly basic anion exchanger with quaternary ammonium functional groups (positively charged) attached to the styrene divinylbenzene copolymer lattice. Functional groups have a counterion of opposite charge adsorbed to them. AG 1 refers to a particular formulation of lattice and functional groups. X8 refers to the degree of polymer crosslinking (8%), which in turn determines the structural properties of the resin, and thus its ion exchange properties. Resins must be extensively preconditioned or prepared prior to use.

Resin is fully converted to the acetate form using greater amounts of reagents than specified by the manufacturer and completeness of acetate conversion verified. Trace metal grade reagents may not be of sufficient quality to achieve the lowest possible quantification limits (sub-ppb). In this case the reagents must be additionally purified using iterative distillation and extremely pure water for dilution. Trace metal grade is sufficient for higher ambient arsenic concentration separation. A 25.0 ml sample (pH adjusted to ~2.5 with H₂S0₄ and checked with indicator) is introduced to the top of the column either using a pipette or a syringe with or without a filter cassette. The stopcock 16 is opened and the effluent from the column is collected in a pre-cleaned sample vessel. 25 ml of 0.12 M HCI is used as an eluant following the sample to elute the total of 50 ml of sample and acid into the first sample container. This is the arsenite sample. Another 50 ml aliquot of 0.12 M HCI is passed through the column and collected in a second sample container. This is the arsenate sample. A sample is also collected of the source. Mass balance is calculated as the difference between the analytically determined total arsenic species concentration and the sum of the analytically determined concentrations of the individual species As³⁺ and As⁵⁺. It is known that the device will provide false positives for arsenite if MMA and/or DMA are present. AsK 4 Figure 3 depicts the AsK 4 column configuration. The AsK 4 column 1 comprises a reservoir 18, ion exchange resin, cation 19 and anion 20, upper and lower confining e.g. 0.35 urn pore size ultra high density polyethylene frits 21 , plastic or borosilicate glass barrels 22, a flow adapter and holder 23 for frits on all but the uppermost barrel, an upper flow adapter 24 to the reservoir, a capillary tube 25 between the upper and lower columns, and an outflow valve 26. The configuration is essentially the same as two AsK 2 columns joined by a rigid or semi- flexible capillary tube. The cation-anion resin columns in series configuration allows the separation of four arsenic species, As³⁺, As⁵⁺, MMA, and DMA, quantitatively. The method uses 10 cm of anion exchange resin (50 -100 mesh, e.g. AG 1-X8, chloride form, lowermost column) and 25 cm of cation exchange resin (50 -100 mesh, e.g. AG 50W-X8, hydrogen form, uppermost column) in two separate columns. Cation resin is a strongly acidic cation exchanger with sulfonic acid functional groups (negatively charged) attached to the styrene divinylbenzene copolymer lattice. Functional groups have a counterion of opposite charge adsorbed to them. AG 50 refers to a particular formulation of lattice and functional groups. X8 refers to the degree of polymer crosslinking (8%), which in turn determines the structural properties of the resin, and thus its ion exchange properties. An alternate embodiment of AsK 4 is a longer single glass barrel with a frit separating the dissimilar resins. Resin is slurried into the columns, frits attached and is rinsed with 100+ ml of distilled deionized water and 50 ml ultra-clean 0.5 M HCI followed by 25 ml of 0.006 M TCA. Columns are shipped saturated with 0.006 M TCA.

The AsK 4 elution process is as follows. The sample (pre- filtered and pH adjusted to ~2.5 if desirable) is gently introduced to the top of the cation exchange resin 19 using a 2 ml class A pipette (±0.006 ml).

The sample is then eluted using 55 ml of 0.006 M TCA, 20 ml of 0.05 M TCA, 20 ml of 0.2 M TCA, 55 ml of 3.0 M NH₄OH, and 50 ml of 0.2 M TCA that are gently decanted into the reservoir from plastic containers containing the pre-measured solutions. Since gravity flow is used, flow rates are variable as the head on the column changes. All reagents need to be specially cleaned for trace arsenic as described previously here. Four eluant fractions are collected: 0-30 ml, As³⁺; 30-60 ml, MMA; 60-100 ml, As⁵⁺; and, 160-200 ml, DMA. Mass balance is calculated as the difference between the analytically determined total arsenic species concentration and the sum of the analytically determined concentrations of the individual species As³⁺, As⁵⁺, MMA, and DMA.

The AsK 4 arsenic species separations work in the following manner. In 0.006 M TCA (trichloroacetic acid), pH ~ 2.5, the As³⁺ and MMA exists in solution as a neutral species, and the As⁵⁺ exists primarily as an anionic species; DMA is dominantly a cation. Most of the DMA is retained at this pH on the cation exchange resin. Elution of the neutral As³⁺ is not retarded by cation and anion exchange resins and elutes in the first 30 ml. The second fraction of the eluted solution (30 to 60 ml) contains the MMA. Although the MMA is also predominantly neutrally charged it elutes behind the As³⁺. It is probable that the MMA is weakly retained due to non-polar interactions with the resin. The resin column now contains the As⁵⁺ species strongly bound on the anion exchange resin 20 and the DMA species strongly bound on the cation exchange resin 19. The addition of the 20 ml of 0.05 M TCA and the 20 ml of 0.2 M TCA gradually lowers the pH to less than 1.0. The DMA is retained strongly on the cation exchange resin. As the more concentrated TCA enters the anion resin As⁵⁺ is converted into its neutral form and is collected in the 60 to 100 ml fraction. The only arsenic species left in the chromatographic column is the DMA on the cation resin. Addition of the solution of 3.0 M NH₄OH (pH~12) converts the DMA into its anionicform, strips it from the cation exchange and carries it into the anion exchange resin at the bottom of the column where it is strongly retained. Finally, the addition of 0.2 M TCA strips the DMA species by converting it to its neutral and cationic forms to be collected in the 160 to 200 ml fractions. AsK 2-4

Figure 4 depicts the AsK 2-4 configuration. The shipping container 2 has the same components and purpose as depicted in Figure 1 and described herein. An AsK 4 column 30 is used in parallel with a modified AsK 2 column 31. The standard AsK 4, as described above (Figure 3 and herein), is used but not eluted past the As⁵⁺ aliquot. The modified AsK 2 column contains cation resin only and is eluted only for DMA. The AsK 2 style column is set with cation resin (AG 50W-X8 resin 50 - 100 mesh, hydrogen form) in a column configuration of 0.79 cm by

25+ cm, the same as the upper portion of the AsK 4 column. The cation column is treated with an identical sample aliquot and rinsed with 50 ml

0.006 M TCA. The DMA is subsequently eluted with 30 ml 3.0 M NH₄OH.

In this manner DMA recovery is enhanced and DMA false positives from As⁵⁺ are eliminated. Mass balance is calculated as the difference between the analytically determined total arsenic species concentration and the sum of the analytically determined concentrations of the individual species As³⁺, As⁵⁺, MMA, and DMA. AsK 2+

The Ask 2+ configuration, which is not illustrated separately, uses a total of two resin columns (of the Fig.2 type) in parallel assembled in a manner similar to that depicted in Figure 4. The shipping container has the same components and purpose as depicted in Figure 1 and described herein. The standard AsK 2 column is as described above.

The separation is accomplished by sequestering As⁵⁺ from the sample and eluting As³⁺, MMA and DMA using the AsK 2 separation. A portion of the eluant containing As³⁺, MMA and DMA is treated with a drop of commercial bleach (2.5% sodium hypochlorite), mixed and allowed to react for 1 minute. This process converts As³⁺ to As⁵⁺ without changing MMA or DMA. The sample treated this way is then re-acidified with H₂S0₄ to pH ~ 2.5. The eluant from this step is then passed through an AsK 2 column where the newly formed As⁵⁺ is retained and the organic arsenic species MMA and DMA are co-eluted. The normal AsK 2 procedure is completed on column 1 for quantification of As⁵⁺. In this manner As³⁺, As⁵⁺, and organic arsenic species MMA and DMA are separated. Four samples are processed, 25 ml arsenite, 50 ml arsenate, 50 ml total methyl arsenic, and an aliquot of the original filtered or unfiltered sample. Additional intermediate samples can also be analyzed to improve error checking, precision, and accuracy if necessary. In this manner MMA and DMA are separated from the arsenite and are determined as total organic arsenic.

A 25 ml sample (pH adjusted to -2.5 with H₂S0₄) is introduced to the top of the standard AsK 2 column either using a pipette or a syringe with or without a filter cassette. The stopcock is opened and the effluent from the column is collected in a pre-cleaned sample vessel. 25 ml of 0.12 M HCI is used following the sample to elute the total of 50 ml into the same sample container. This is the arsenite sample. 50 ml of 0.12 M HCI is then used to elute the As⁵⁺ from the column. This is the arsenate sample. The procedure to this point is identical to AsK 2.

The arsenite sample from the column is split with 25.0 ml reacted with one drop of bleach, mixed for 10 seconds by swirling, and allowed to stand and react 1-2 additional minutes, 50 additional seconds being the minimum reaction time following mixing. The remainder of the split is retained for analysis of arsenite plus organicarsenic as DMA and MMA. The pH of this reacted split is lowered to pH ~2.5 using H₂S0₄ and checked using an indicator.

The 25.0 ml split of the pH adjusted eluant is introduced into the second AsK 2 column. The As⁵⁺ in the sample is retained on the resin. A 25 ml aliquot of 0.12 M HCI is used to complete the first sample elution, exactly as in AsK 2. This sample no longer contains inorganic arsenic and if the sample was obtained from natural waters the arsenic is dominantly MMA and/or DMA.

Another 50 ml aliquot of 0.12 M HCI is passed through the final column and collected in a third sample container. This is the sample of As³⁺ that has been converted to arsenate and is used as a check for completeness of the oxidation reaction and mass balance of the separation. A sample is also collected of the source and acidified with HN0₃. Mass balance is calculated as the difference between the analytically determined total arsenic species concentration and the sum of the analytically determined concentrations of the individual species As³⁺, As⁵⁺, and Total Organic Arsenic.

The present invention is, of course, in no way restricted to the specific disclosure of the specifications and drawings, but also encompasses any modifications within the scope of the appended claims.

Claims

CLAIMS:

1. A method of chemical species separation and preservation that allows for calculation of mass balance, said method characterized by the steps of: providing a sample; preserving a portion of said sample for total chemical species analysis; providing a container that contains a solid phase which has been prepared for a chemical species of interest; introducing the remainder of said sample to said solid phase; introducing at least one eluant to said solid phase, wherein said at least one eluant is appropriate for separating off said chemical species of interest; and collecting the fraction or fractions of sample and eluant effluent that contain said chemical species of interest and are necessary for performing a mass balance calculation.

2. A method according to claim 1 , characterized in that said step of introducing at least one eluant comprises introducing successive eluants.

3. A method according to claim 1 , characterized in that said sample and eluant effluent are collected together or in separate containers.

4. A method according to claim 1 , characterized in that a further container that contains a different solid phase is provided, and said sample remainder and said at least one eluant are introduced to said further container as effluent from said first-mentioned container.

5. A method according to claim 1 , which, prior to or after said step of introducing eluant, includes the step of adding a chemical oxidant or reductant to said remainder of sample to change chemical speciation thereof for enhanced separation, or the step of adding a pH modifier or buffer to said remainder of sample to change chemical speciation thereof for enhanced separation.

6. A device for accomplishing chemical species separation and preservation that allows for calculation of mass balance, said device characterized by: a column (1) having an inlet and outlet and containing a solid phase that has been prepared for a chemical species of interest; a porous lower bed support (12) of chemically inert material disposed in said column (1) for retaining said solid phase in position therein and for providing an even flow distribution to said solid phase; a closure device (16) for said outlet of said column

(1) to regulate flow at said outlet; and a reservoir (10) disposed between said inlet and an upper bed support (12) for containing a sample volume necessary for accomplishing separation of the chemical species of interest and for containing an eluant volume necessary for accomplishing separation of the chemical species of interest, wherein said reservoir (10) serves for delivery of said sample and said eluant to said solid phase.

7. A device according to claim 6, characterized in that, a cap is provided for said reservoir (10) and/or in that a further column is provided that contains a different solid phase and is disposed downstream of said first-mentioned column.

8. A kit for accomplishing chemical species separation and preservation that allows for calculation of mass balance, said kit characterized by: a container (2); the device of claim 6 provided in said container; means (4, 6) provided in or as part of said container for holding said device upright; at least one eluant container; at least two initially empty sample containers; and a sample measuring and delivery device for delivering sample to said device.

9. A kit according to claim 8, characterized in that said means (4, 6) for holding said device upright is connected to said container (2), which can include a first compartment (3) for said device, and a second compartment (8) for said containers, and can also include at least one further compartment.

10. A kit according to claim 8, further characterized by instructions for use disposed on or in said container (2), required documentation, and shipping labels.