CARBON-ENHANCED INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRIC DETECTION OF ARSENIC AND SELENIUM AND ITS APPLICATION TO ARSENIC SPECIATION

Addition of carbon as methanol or ammonium carbonate to the aqueous an alyte solutions in combination with increased plasma power input enhan ced the inductively coupled plasma mass spectrometry (ICP-MS) signal i ntensities of arsenic and selenium. In the presence of the optimum 3% v/v methanol concentration the signal intensities achieved were about 4500-5000 counts s-1 per ng ml-1 of arsenic and about 700-1100 counts s-1 per ng ml-1 of selenium (Se-82), corresponding to enhancement fact ors of 3.5-4.5 compared with aqueous solution for the two elements. Di fferences in sensitivity (calculated on the basis of analyte atom) wer e observed between the individual arsenic species and between the sele nium species in aqueous as well as in carbon-added solutions. The pres ence of 3% v/v methanol in the analyte solutions doubled the level of the background signal for arsenic and selenium, but its fluctuation (n oise) was not increased. Therefore, the observed increase in analyte s ensitivity led to a similar increase in signal-to-noise ratio. The add ition of carbon as ammonium carbonate enhanced the arsenic signal by a similar factor but caused severe contamination of the ICP-MS instrume nt by carbon. In the 3% v/v methanol solutions of arsenic and selenium the signal intensity from antimony (internal standard) was enhanced b y a factor of 1.5, which indicates that the enhancement effect of the arsenic and selenium signals by methanol is only to a limited extent c aused by improved sample transport/nebulization efficiency. It is prop osed that an increased population of carbon ions or carbon-containing ions in the plasma facilitates a more complete ionization of analytes lower in ionization energy than carbon itself. The enhanced detection power for arsenic was applied to arsenic speciation by high-performanc e liquid chromatography (HPLC)-ICP-MS, and made possible the detection of the arsenocholine ion (AsC) in extracts of shrimp at the 5-10 ng g -1 concentration level. The limit of detection was improved by a facto r of 3.4 after addition of methanol and was 4.7 ng g-1 as the AsC ion.