Ion Chromatographic Estimation of Arsenite and Arsenate at Trace Level

Present method shows simple and specific determination of traces of inorganic arsenic in water. This method enables simultaneous determination of arsenite by electrochemical detection and arsenate by suppressed conductivity detection. The applicability of this method was illustrated by determining the inorganic arsenite and arsenate content from bore-well water and river water samples without any special pretreatment. The present method for direct determination of arsenite and arsenate shows good sensitivity, selectivity, precision and accuracy. Detection limits determined using this procedure was found to be 2.0 μg/L for arsenite and 30.0 μg/L for Arsenate. The simplicity, ease of use, low detection limit and low running cost of this method makes it appealing for increasing capability of testing in the lab.


Introduction
Arsenic occurs naturally often together with other chemicals in soils and minerals. Arsenic and all of its compounds are poisonous in nature. Inorganic arsenic occurs as trivalent arsenite (As 3+ ) and pentavalent arsenate (As 5+ ). The toxicity of arsenic is known to depend on its chemical form. Arsenite is the most toxic of the water-soluble species; arsenate is also relatively toxic, whereas the organic methylated forms, methylarsonic acid and dimethylarsinic acid, are much less toxic 1 . Organic arsenic is mainly found in seafood.
The major cause of arsenic found in ground water is the oxidation of arsenopyrite or arsenic rich ferrous hydroxide present in sediments 2 . Bengal delta region are rich in these ores. The subsequent migration of this arsenic contaminated groundwater through these deltaic sediments may be one of the principal causes of arsenic poisoning in India and Bangladesh. Bore-well water from arsenic prone regions, water leaching from mines etc are also the main sources of arsenic contamination. Various Latin American countries also face the same type of problem. The possible adverse effects on human beings due to arsenicosis are dorsal keratosis, squamous cell carcinoma, gangrene, hyper keratosis, pigmentation on body. World health organization's (WHO) Guideline Value for arsenic in drinking water is 0.01 mg /L 3 . This figure is limited by the ability to analyze low concentrations of arsenic in water. A systematic study is being undertaken by the School of Environmental Studies and School of Water Resources Engg., Jadavpur University since 1989. The maximum permissible limit in India is 0.05 mg/L 3 .
In this method two detectors are used in series for the simultaneous determination of arsenite and arsenate by ion chromatography. The arsenite and arsenate are separated by separation column and eluent containing the analytes flows through electrochemical cell and then through the suppressor and finally flows through a conductivity cell.
The main aim of our present experiment was to determine the inorganic arsenic in different valence forms like As 3+ and As 5+ with simple analytical instrumentation like ion chromatography which almost all labs can easily handle. Hence a relatively simple and precise method was developed using ion chromatographic separation with dionex IonPac AS23 anion exchange column for the simultaneous determination of Arsenite by amperometry and arsenate by conductivity. IonPac AS23 is a high capacity column with dual selectivity of hydroxide and carbonate eluents enabling good resolution. Various ground water samples were analyzed for their arsenic content. Non-ionic impurities present in other type of samples were removed by passing through onguard II RP cartridge. Using this method one can also determine fluoride, chloride, nitrate, phosphate and sulfate on conductivity at low mg/L level in presence of arsenite and arsenate. Resolution of the anions can be customized by changing eluent composition, if required.

Experimental
The equipment used was dionex ion chromatograph ICS 3000 with autosampler(AS) having a 200 µL sample loop, IonPac AG23 guard column (4x50 mm i.d.) and IonPac AS23 separator column (4x250 mm i.d.) were used. The experiment was conducted using a predegassed eluent of 7.0 mM sodium carbonate and 4.0 mM sodium hydrogencarbonate at 1.0 mL/min flow rate. The electrochemical detector consisted of a platinum working electrode and Ag/AgCl as reference electrode and stainless steel body as a counter electrode. Solid electrodes are prone to complicated surface renewal problems. Oxide film formation causes non-reproducible currents during the electrolytic oxidation of arsenic solutions 1,4 . To overcome this problem, platinum working electrode was reconditioned before starting the analysis at a potential of -0.80V and at +0.60V for less than 30 seconds each and then the potential was reset to +0.40 V and then was equilibrated for 20 minutes The suppressor used was DIonex ASRS 300 (4 mm). Software used for data acquisition was dionex chromeleon (version: 6.80 SP2). Chromatograms from both the detectors can be monitored simultaneously during analysis.

Reagents
All solutions were prepared in ultra pure water (18.2 MΩ-cm). Unless otherwise mentioned, reagent grade chemicals were used.

Arsenic standard solution
A standard stock solution of arsenite(1000 mg/L) was prepared from arsenic trioxide (As 2 O 3 ,Sigma Aldrich) by weighing 0.264 g and dissolving in 100 mL of 100 mM sodium hydroxide (NaOH) (prepared from 50% w/w NaOH (sigma Aldrich)). Solution was sonnicated for 20 minutes for complete dissolution of salt. A standard stock solution of arsenate 100 0 mg/L) was prepared from sodium monohydrogen orthoarsenate (Na 2 HAsO 4 .7H 2 O, loba chemie Ltd.) by weighing 0.416g and dissolving in 100 mL of ultra pure water.

Procedure
An autosampler (Dionex AS) was used for the injections. Standard solution containing a mixture of arsenite and arsenate was injected into the ion chromatography system. Subsequently, the standard solution in the sample loop was transferred onto the separator column, on which arsenite and arsenate were separated. After separation on the column, the arsenite and arsenate were detected by amperometric and conductivity detectors, respectively. A sequence containing the blank, standards, samples and recovery samples was run and results were then interpreted. Standard

Sample preparation
Various water samples were taken for analysis. Bore-well water samples; sample 1 (Baghbazar), sample 2 (Barabazar), sample 3 (Uttarpara) were collected from Kolkata, West Bengal region. River water samples; sample 4 (Kalyan), sample 5 (Badlapur), sample 6(Karvai MIDC) and sample 7 (shelu) were collected from various areas of Ulhas River, Maharashtra. Each sample was filtered through 0.22 µ nylon membrane and clear solution was used for direct injection. arsenite and arsenate concentrations were determined from the peak areas using the linearity equation of calibration curve.

Results and Discussion
This is the modified method of Tan et al. 4 , having advantage of better resolution and ease of use. Two individual standard solutions for arsenite and arsenate were prepared and injected separately for peak identification. One individual peak for each standard solution was observed without any interference as shown in Figure 1 and 2.

Sample results
Bore-well water samples (spl_1_Baghbazar, spl_2_Barabazar and spl_3_Uttarpara) and River water samples (spl_4_Kalyan, spl_Badlapur, spl_Karvai and spl_Shelu) were analysed using the linearity calibration method. Samples were injected in duplicate. The results are provided in the Table 1 after typical chromatograms of samples.

Recovery
The sample used for recovery study was spl_1_Baghbazar (Average concentrations were taken for calculation). Recovery test solutions were injected in triplicate. Also for recovery study, sample was spiked with standard at three different levels as shown in Table 2 and 3.  Table 2.

Conclusion
Amperometric electrochemical detection and suppressed conductivity detection are coupled to give specific, sensitive and precise method for simultaneous estimation of arsenite and arsenate respectively. This present method was used for analysis of water samples without any pretreament and it can be used for other type of samples with minimum pretreatment. The detection limits for arsenite was 2.0 µg/L and for arsenate was 30.0 µg/L. This is significantly a low limit for arsenite, which is the most toxic in arsenic species. This technique is cost-effective with respect to analysis required for keeping a check on the limits of inorganic arsenic species in water as prescribed by the water regulatory authorities and various health organizations.