Although previous experiments have reported signal improvements when 13% methanol is aded to the mobile phase4, this application note demonstrates that the Varian ICP-MS can produce low detection limits without this step. The addition of methanol can also cause problems if other elements such as Cr (VI) are measured, as the formation of 40Ar13C leads to substantial degradation of the signal-to-noise ratio, and higher limits of detection. Additions above 1% methanol can also cause problems, as the chromatographic separation between AsB and As(III) may be degraded to such an extent that base line resolution is not achievable.
Analysis of reference materials
Given the lack of reference materials that are certifi ed for all fi ve arsenic species, a number of reference materials were spiked with known quantities of each species and the % recovery recorded, to test for robustness of the Gradient method.
The reference materials analyzed included two water reference materials, NIST 1640 and HPS TMDW, two biological tissue reference materials, HPS OT and HPS TMF, and a urine reference material. Each reference material was spiked with 11 μg/L of each As compound and analyzed. The HPS TMF and the Seronorm urine reference material were diluted by a factor of 100 prior to injection. The percentage spike recoveries for each reference material can be viewed in Table 5. Very good recoveries were achieved, especially considering that no internal standard was used.
This work has demonstrated that the Varian ICP-MS is an excellent detector when coupled with liquid chromatography for elemental speciation. Low ng/L detection limits can be routinely obtained and very good recoveries were obtained on a variety of spiked referenc