Facilities & Laboratories
Arsenic Speciation Laboratory
The Battelle Marine Sciences Laboratory (MSL), has two independent methods for conducting determinations of arsenic (As) speciation in water, biological tissues and other matrices. One method is based on separation of As species using high performance liquid chromatography (HPLC) and detection by Inductively Coupled Plasma Mass Spectrometry (ICPMS). The other method uses hydride generation (HG) of arsenic species as arsines, cryogenic trapping (CT) and detection by atomic absorption spectrometry (AAS). A description of the methods capabilities is given below.
The MSL has the capability to conduct As speciation measurements of water, sediment, and biological tissues using separation of As species by high performance liquid chromatography (HPLC) with detection by Inductively Coupled Plasma Mass Spectrometer (ICPMS). For this method, we have interfaced a Dionex HPLC system (AS50 autosampler, GP50 gradient pump, and AS50 thermal compartment) with a Perkin-Elmer ELAN DRC-e ICPMS equipped with a dynamic reaction chamber (DRC). We have developed an isocratic method that uses a Hamilton PRP-X100 anion exchange column to separate and quantify these As species: inorganic As (III), inorganic As (V), arsenobetaine, DMA (V) and MMA (V). An example chromatogram showing separation of these species is shown to the right. The current method detection limit for As species in water is given in Table 1 below. We are currently conducting MDL studies for these arsenic species in tissue and sediments.
The MSL has the capability of conducting As speciation determinations using EPA method 1632a, which is based on procedures developed by Eric Crecelius, a staff scientist at the Battelle Marine Sciences Laboratory in Sequim, Washington. (US EPA, 2001). EPA method 1632a is a hydride generation - cryogenic trapping - atomic absorption spectrometry (HG-CT-AAS) method for arsenic speciation in water and tissues. Hydride generation is used to convert As species to their corresponding hydrides, which are purged from the sample, cryogenically trapped, separated chromatographically by their boiling points, pyrolized and atomized in a quartz hydrogen flame atomizer and detected in the gas phase using AAS. Because large volumes of sample can be used for analysis, the technique can yield detection limits in the low part per trillion (around 1 ng L-1) range (USEPA, 2001). The normal HG approach described in EPA 1632 does not distinguish oxidation states of inorganic arsenic, MMA or DMA. However, inorganic As (III) can be isolated and quantified using this approach by adjusting the pH used for the HG step. Inorganic As (V) can then be determined as the difference between a Total Inorganic As measurement and an Inorganic As (III) measurement. The 1632 method does not have a procedure for separating the different oxidation states of MMA or DMA.
Comparison of Methods
Detection Limits for Aqueous Solutions
Both the HPLC-ICPMS and the HG-CT-AAS method have comparable detection limits for water given the volume of sample used for analysis. The HPLC-ICPMS method uses an injection volume of 100 µL. In addition, prior to analysis the samples are typically diluted with carrier phase by 4- to 10-fold prior to injection. A minimum of 150-200 µL of diluted sample is necessary for a single analysis and approximately 500 µL of diluted sample is required if replicate analyses are to be conducted. For the HG-CT-AAS method, the volume of sample used for a water analysis is typically 10 mL. More sensitive detection is possible using a larger sample volume for analysis. However, the reverse is true if limited sample volumes are available. For example, if only 100 µL of sample is available, then the corresponding detection limit for an HG-CT-AAS analysis would be 100x that shown in Table 2. Hence, if sample volumes are limited, then the HPLC-ICPMS method has a significant sensitivity advantage over that of the HG-CT-AAS method. Conversely, if significant sample volumes are available for analysis (> 10 mL), then the HG-CT-AAS method has a detection limit advantage since larger volumes can be used for analysis.
ReferencesU. S. E. P. A. (2001). Method 1632 - Chemical Speciation of Arsenic in Water and Tissue by Hydride Generation Quartz Furnace Atomic Absorption Spectrometry. EPA-821-R-01-006.