Contents
Calibration standard preparation and internal standards
All dilutions, preparation of calibration standards etc. are performed gravimetrically and the weights recorded in a laboratory notebook or directly to a spreadsheet. The calibration standards are prepared by addition of a mass of a commercial primary traceable ICP-MS standard (or a diluted sub stock of the primary standard). The calibration standards cover a concentration range of 0 -50 µg l-1, and include a blank and six concentration standards, which are prepared in an equivalent acid matrix to the standards. Inclusion of higher calibration standards or further dilution of samples may be required for certain analytes. All reported results will be from data within the calibration range. Instrument recalibration will be conducted during analysis of the sample batch, usually after analysis of 30 samples to correct for matrix-induced instrument drift. To further monitor and correct for drift, the samples (and standards) are mixed with an internal standard solution through a ‘tee’ prior to being nebulized into the ICP-MS. The internal standard solution contains Ga, Ge, Y,Rh, In, and Bi at around 200 ppb in 1% HNO3 and 5% butanol to equalize any carbon-enhanced ionization.
ICP-MS method set-up
We run all samples that require bulk (conventional) elemental analysis on the 8900 ICP-MS with the PrepFast M5 autosampler/autodilution system.
No gas mode analytes: Li, Be, B, Al, Hg, Tl, Pb, Th, U
He mode analytes, Al, Na, Mg, Al, P, K, Ca, V, Mn, Fe, Co, Ni, Cu, Zn, As, Rb,Sr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, REEs,
Hydrogen mode: Ca, Fe, Se, Cd
Oxygen mode (8900 only), Si, P, S, As, Se, REEs
Integration times are 100 msecs – 500 msecs, generally minimum of 3 reps and 100 sweeps per reading.
We use more than one mode for some elements where we expect there might be interferences or detection limit issues or linear range issues….we do our best to get the right answer.
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.
Quality Control
The Trace Element Analysis Laboratory follows quality control procedures outlines in EPA SW 846 (http://www.epa.gov/osw/hazard/testmethods/sw846/online/index.htm) and EPA method 6020 (http://www.epa.gov/osw/hazard/testmethods/sw846/pdfs/6020a.pdf) . These procedures are outlined below.
Calibration: The ICP-MS is calibrated using NIST traceable single and multi-element standards containing the analytes of interest. Multi-point calibration curves (n ≥ 5) are constructed for each analyte with correlation coefficient criteria > 0.995. The calibration is followed by an Initial Calibration Blank (IBC) and an Initial Calibration Verification (ICV). The ICV (which is also the solution for subsequent Continuing Calibration Verifications (CCV)) is made from a second source of single and multi-element standards and contains all the analytes at concentrations at or below the mid-point calibration range. Acceptance criteria for the ICV and CCV are ± 10%. The CCV is run after every 10 samples.
Laboratory Control Samples (LCS) and Standard Reference material (SRM): The TEA generally uses Standard Reference Materials (SRM) as LCS’s. For solid samples the SRM is chosen to match the sample matrix (soil/sediment/animal tissue/plant tissue) as closely as possible. The LCS is taken through all sample preparation steps and handled in an equivalent manner to a sample. For aqueous samples the LCS is a Certified Water Sample such as SLRS 6 or similar. Acceptance criteria for LCS are ± 20% of the certified or accepted value.
Duplicates and Spikes: Unless sample is limiting, duplicate and spike analysis are performed on all sample batches. Duplicate solid samples are digested at a frequency of one duplicate per 20 samples. Analytical duplicates and spikes are performed at a frequency of one each per 20 samples. Recovery criteria for all analytes are 80%-120% of the spike amount.