Exova now offers speciation of As(III), As(V), MMA, and DMA by ion chromatography coupled with ICPMS.

Introduction

Recent studies indicate that we are at a higher risk of cancer from arsenic ingestion than previously thought.1 Since drinking water is one potential source of arsenic, the EPA has to lowered the Maximum Contaminant Level (MCL) from  50 ug/L to 10 ug/L, effective January 23, 2006.  For the latest information click on EPA.

 ICPMS Plasma, Arsenic Analysis
Established analytical methods for arsenic in water currently have detection limits in the 1-5 ug/L range. Because of the anticipated reduction in the MCL, the EPA and water agencies would like new methods for arsenic with detection limits as low as 0.02 ug/L. Based partly on the work we had already done, the American Water Works Association Research Foundation (AWWARF) selected WCAS as a member of a team to develop these methods. Arsenic occurs naturally in ground water from inorganic arsenite (As(III)) and arsenate (As(V)) dissolved from rock as well as organic forms of arsenic from metabolism (methyl- and dimethylarsenic, and other forms) and from agricultural and industrial pollution. The toxicity characteristics will of course vary with the species of arsenic. Even so, EPA will probably regulate total arsenic in drinking water. Because treatment and removal alternatives will vary with species, both total arsenic as well as arsenic speciation methods are needed in the future.

Arsenic Methods

ICPMS with traditional sample introduction (direct nebulization) can determine total arsenic to approximately 0.2 ug/L. The sample is simply acidified and sprayed (via a nebulizer) into an argon plasma. The high temperature of the plasma atomizes and ionizes all forms of arsenic so that response does not vary by species as with more traditional atomic absorption (AA) and graphite furnace atomic absorption (GFAA) methods which require thorough digestion prior to analysis. The sensitivity of the mass spectrometer allows detection down to

approximately 0.2 ug/L. One problem with ICPMS using direct nebulization is that high levels of chloride may interfere with the analysis due to the formation of argon chloride (ArCl) in the plasma which has the same mass as arsenic (mass 75). This interference may cause the arsenic levels to be biased high by as much as 1 ug/L for each 100 mg/L of chloride present. Even if corrections are being made to the results using the chlorine isotope ratio, these values may be inaccurate at the ug/L level.

Newer instruments, like our our Agilent 7500ce, are equipped with reaction or collision cells to eliminate this chloride interference.

 

Hydride Generation

Arsenic forms a volatile hydride (arsine, AsH3) when reduced with sodium borohydride. The traditional hydride generation AA method uses this chemistry to determine arsenic. However, the batch wise production of the hydride is not a precise technique, therefore most laboratories favor the furnace (GFAA) method of analysis for this reason. On the other hand, continuous formation of the hydride is a very precise technique (RSD<3% at 

 0.05 ug/L). Reagents are mixed on-line as the sample is introduced to generate arsine. With the inherent sensitivity of ICPMS, continuous HG-ICPMS can detect total inorganic arsenic to approximately 0.02 ug/L. This technique also minimizes the chloride interference discussed above since only the volatile hydride is entrained in the argon, and the dissolved chloride does not reach the plasma. The sample does require pretreatment to convert all inorganic arsenic (III and V) to As(III). Methyl- and dimethylarsenic species also respond using this method.

Arsenic Speciation - 
update 4/02

We have used the following arsenic speciation techniques: (1) hydride generation - ICPMS and (2) ion chromatography - ICPMS.  Presently we offer only IC-ICPMS.

Hydride Generation - ICPMS

Within a certain pH range, arsine is produced only from the As(III) species, not from As(V). First As(III) is determined under the controlled pH conditions.  Then total As is determined.  As(V) is then determined by difference.  Because DMA and MMA also form volatile hydrides, this speciation technique should not be used when these species may be present.  The advantage of the hydride technique is sensitivity.  The detection limit is less than 0.02 ug/L.

IC - ICPMS

Ion chromatography can be used to separate the common As species.2  The chromatogram below shows the determination of As(III), dimethylarsinate (DMA), monomethylarsonate (MMA), and As(V).  Coupling the IC column directly to the ICPMS results detection limits near 0.5 ug/L in clean water.  

This method should be used whenever the organic species are expected, i.e. organic vegetation.  Under these chromatographic conditions, arsenobetaine (AsB) is not separated from As(III), so this method should not be used for shell fish or urine.  However the AsB interference can be removed by using IC-HG-ICPMS or under different chromatographic conditions.  For more information on this method, click on Arsenic Speciation.

IC-ICPMS Chromatogram of 50 ug/L Arsenic Species

For a quotation...

1. Smith, A. Env. Health Perspective, 1992, 97, 259-267.  For more information on environmental and health hazards, click on arsenic.

2. Heitkemper, D.T., et al., J. Anal. Atomic Spectrom., 2001, 16, 299-306.

 

For a full listing of Exova services, please visit our new Exova web site,  Exova Pharmaceutical Solutions web site and our new Santa Fe Springs laboratory home page.

Exova

9240 Santa Fe Springs Rd
Santa Fe Springs, CA 90670
562.948.2225   Fax 562.948.5850 info400@exova.com

Exova

Formerly West Coast Analytical Service (WCAS) and Bodycote Testing Group