Using IC-ICP/MS Method to Determine Organic and Inorganic Arsenic Species

Arsenic (As) is a metallic element that occurs naturally in soil and ores in the environment. It can exist in both inorganic and organic forms, but inorganic arsenic, whether introduced anthropogenically or naturally occurring, is typically in the form of arsenite As(III), which is partly reduced, or arsenate As(V), which is completely oxidized.

Chronic exposure to inorganic arsenic is associated with excess lung, skin, liver, kidney, and bladder cancers in humans. Both arsenite and arsenate are genotoxic and capable of inducing sister chromatid exchange and chromosome aberrations in human and rodent cells. Arsenite, in this context, is more potent than arsenate, by approximately one order of magnitude.

The two forms of inorganic arsenic affect the function of pulmonary alveolar macrophage at noncytotoxic concentrations, with arsenite being more potent than arsenate. Following intratracheal instillation in hamster lungs, both forms produce tumors (Saranko, C. J.; 1998).

Inorganic arsenic species in dietary supplements using gradient ion chromatography and IC-ICP/MS

Dietary supplements are generally regarded as beneficial to health and free of toxic side effects, but studies have demonstrated that high levels of toxic substances are present in certain supplements. Arsenic is regarded as one of the elements of primary concern due to its toxicity. Arsenic contamination of dietary supplements mainly occurs as a result of the plant materials used in the ingredients.

Both anthropogenic and natural activities cause arsenic to be released into the environment. It is taken up by plants, where it builds up in edible parts of the plants. Arsenic should be assessed based on the determination of its individual species, because it is the chemical form that influences the toxicity of the element.

In this analysis, a microwave-enhanced protocol was used to simultaneously extract organic and inorganic arsenic species from dietary supplements and the species were then determined by the IC-ICP/MS method. Baseline separation of the species was achieved using high-pressure-gradient IC with a Hamilton PRP-X200 cation exchange column and these species were then injected into an Agilent 7700 ICP/MS instrument.

Using an 800 Dosino, a post-column internal standard was injected. The IC and ICP/MS and IC instruments were synchronized using remote signal. The sample loading and gradient program were controlled by MagIC Net software, and data manipulation and handling were controlled by Agilent MassHunter software.

Results

Arsenobetaine (AsB), As(III), As(V), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA) and trimethylarsine oxide (TMAO) were separated under chromatographic conditions. Although TMA and AsC were isolated from the other species, they were not resolved from one another, as depicted in Figure 1. The optimized protocol was used to analyze 10 widely used prenatal and children’s dietary supplements.

It was observed that the supplements had total arsenic contents in the range of 59–531 ng/g concentration. IC-ICP/MS analysis showed that that DMA and As(III) were present in all the supplementary products. Two of the supplements contained As(V) and one product contained an unknown species of arsenic, as shown in Figure 2.

Figure 1. Standard solution containing 10.0 ng/g arsenic per species, conditions as below (Figure 2).

Figure 2. Microwave extract generated from a prenatal supplement prepared from plant materials. The supplement was available in hard-pressed powder (tablet) form. Column: Hamilton PRP-X200 cation-exchange column; eluent A 1.0 mmol/L HNO₃ (pH 2.5); eluent B: 2.0 mmol/L HNO₃, 20.0 mmol/L NH₄NO₃ (pH 2.5); flow rate: 0.9 mL/min; m/z 75 9.

Mercury and arsenic speciation analysis by IC-ICP/MS

This article describes the application of the IC-ICP/MS method for determining inorganic and organic mercury and arsenic compounds. Traditional speciation analysis is used to determine arsenic species (monoisotopic), as these are not prone to interconversion.

Results

The IC-ICP/MS method can be used to separate and accurately identify different arsenic species in both organic and inorganic organic forms, as illustrated in Figure 3.

Figure 3. Separation and detection of arsenite, dimethylarsenate, monomethylarsenate, and arsenate. Column: Metrosep A Supp 15 - 150/4.0; eluent: 8 mmol/L NH₄NO₃ (pH 8); flow rate 0.7 mL/min; m/z 75

Simultaneous speciation of arsenic and selenium species in petroleum refinery aqueous streams

The study investigated the quantitative speciation of As and selenium (Se) in the streamwaters of a refining process.

Results

Three inorganic Se species - selenocyanate (SeCN^-), selenate Se(VI) and selenite Se(IV) - and four arsenic species - As(V), As(III), DMA and MMA - were isolated in a single run by IC. This was done by using gradient elution with 100 mmol/L NH₄NO₃, pH 8.5, adjusted by addition of NH₃ as eluent, as shown in Figures 4 and 5.

It was observed that repeatabilities of peak position and peak area evaluation were better than 1% and about 3%, respectively. Defined as 3× baseline noise, detection limits were 56, 75, and 81 ng/L for Se(VI), SeCN^-, Se(IV), respectively, and 16, 19, 22, and 25 ng/L for DMA, As(V), As(III), and MMA, respectively.

Figure 4. Separation of the arsenic species As(III), DMA, MMA, and As(V), and the selenium species Se(IV), Se(VI), and SeCN− (5 ng of each species). Conditions as Figure 5.

Figure 5. Arsenic species in a typical process wastewater inlet sample. Column: Metrosep A Supp 10 - 250/4.0; eluent: 100 mmol/L NH₄NO₃ (pH 8.5, adjusted by addition of NH3); flow rate: 1 mL/min using gradient elution; m/z 75–83.

Simultaneous determination of arsenic and selenium species in fish tissues using IC-ICP/MS

For simultaneous extraction of Se and arsenic As species from fish tissues, a microwave-assisted enzymatic extraction (MAEE) method was developed.

Results

Two reference materials, BCR- 627 and DOLT-3, were analyzed to validate the accuracy of the extraction protocol developed. In these reference materials, the extraction recoveries ranged between 82% and 94% for As. The major species detected in fish tissues were selenomethionine (SeMet) and AsB. In the fish extracts that were examined, the total number of arsenic species detected was in good agreement with the total As extracted, as shown in Figure 6.

Figure 6. IC-ICP/MS profile of enzymatic extracts of (left) BCR-627 and (right) DOLT-3 obtained by MAEE. Peak identification of the right-hand chromatogram: (1) AsBet, (4) DMA, and (6) As(V). Column: Metrosep Anion Dual 3 - 100/4.0, Metrosep Anion Dual 3 guard; eluent A: 5 mmol/L NH₄NO₃; eluent B: 50 mmol/L NH₄NO₃, 2% (v/v) methanol (pH 8.7); flow rate: 1 mL/min; m/z 75, 77, 82.

Arsenic – further applications with IC-ICP/MS

• Study on analytical methods for inorganic arsenic speciation in soil by IC-ICP/MS. Mun, S. H.; Lee, B. J.; Kim, H. S.; Cho, M. K.; Sung, J. Y. (2015)
• Sulfur redox chemistry governs diurnal antimony and arsenic cycles at Champagne Pool, Waiotapu, New Zealand. Ullrich, M. K.; Pope, J. G.; Seward, T. M.; Wilson, N.; Planer-Friedrich, B. (2013) J. Volcanol. Geotherm. Res. 262, 164–177 Determination of arsenic and selenium species in drinking water applying IC-ICP/MS Application Note AN-M-010

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About Metrohm

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Metrohm is the global market leader in analytical instruments for titration. Instruments for ion chromatography, voltammetry, conductivity, and stability measurement make the Metrohm portfolio for ion analysis complete. Instruments for Near-infrared and Raman spectroscopy are another, strongly growing segment of the Metrohm portfolio.

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