Sample Analysis
Samples are analyzed by ICP-QQQ-MS (8900 Triple Quadrupole ICP-MS, Agilent, Wilmington, DE) using a prepFAST M5 autosampler (ESI, Omaha, NE). Two primary standards (S-Hi and S-Lo) are prepared from NIST-traceable custom multi-element calibration standards and single element standards for Sn and Hg (Inorganic Ventures, Christiansburg, VA). S-Hi and S-Lo are prepared gravimetrically using a four-place Mettler balance. S-Lo is a 100X manual dilution of S-Hi. A seven-point calibration is created using the prepFAST dilution feature by running a calibration blank (Level 1), 25X dilution of S-Lo (Level 2), 5X dilution of S-Lo (Level 3), S-Lo (Level 4), 25X dilution of S-Hi (Level 5), 5X dilution of S-Hi (Level 6), and S-Hi (Level 7).
The Internal Calibration Verification (ICV) solution is prepared from NIST-traceable second source calibration standards. This solution is run after every calibration and after every 10 samples; passing quality control criteria for this standard are 90% – 110% recovery of known analyte concentrations. The 8900 ICP-QQQ is operated in helium (single quadrupole) and oxygen (MS/MS) modes. A sample element method is listed below. This can be customized to your analyte interests.
Arsenic Speciation
Our standard approach is to use anion exchange chromatography (50 mm Hamilton PRPX-100) with a 50 mM (NH4)2CO3 gradient. We re using a fast method which we developed in our lab and it allows us to separate arsenobetaine (AsB), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), and total inorganic arsenic in 1.7 minutes. It can also be used to separate arsenic(III) (As3) and arsenic(V) (As5) in water samples where the organo compounds are not expected. AsC,AsB, As3, DMA, MMA and As5 can be separated in 3.5 minutes with a gradient separation. We use the Agilent 1260 LC interfaced to the 8800 ICP-QQQ and use mass shifting of As to AsO. Detection limits for As species are about 10 ng/L except of AsV which is about 20 ng/L.
For more complex speciation such as arsenosugars and thiol analogs then we use anion and cation ion exchange and reversed phase methods.
More in depth descriptions of our As speciation methods are published in:
Jackson, BP. 2015. Fast Ion Chromatography-ICP-QQQ for arsenic speciation. Journal of Analytical Atomic Spectroscopy. DOI: 10.1039/C5JA00049A
Taylor, V.F., Jackson, B.P, Siegfried, M. Francesconi, K., Kirshtein, J. Voytek, M. 2012. Multiple Arsenic speciation and cycling in food chains from mid-Atlantic hydrothermal vents. Environmental Chemistry, 9(2):130-138.
Jackson, B.P, Taylor, V.F., Punshon, T. Cottingham, K.L. (2012) Arsenic concentration and speciation in infant formulas and first foods. Pure and Applied Chemistry 84: 215-224.