Elemental speciation analysis

The toxicity of trace elements in the environment is in many cases dependent on the chemical form in which they occur. Total element concentrations, which have traditionally been reported, no longer provide sufficient information to allow risk-based toxicity assessments as prescribed in the ANZECC 2000 Water Quality Guidelines.

One of the most powerful techniques available to separate and identify elemental species is the interfacing liquid chromatography (LC) with inductively coupled plasma mass spectrometry (ICP/MS). Under specific conditions, elemental species elute one by one from the LC column, directly to the ICP via a nebuliser into a high temperature argon plasma, where they are degraded, atomised and ionised. The resultant ions are transported through a sampling interface into a mass spectrometer, where they are detected and quantified.

ALS Environmental (ALSE) has released a metal speciation capability in Australia. Robust methods have been developed to separate arsenic and selenium species using liquid chromatography linked to ICP/MS.

Arsenic speciation analysis

Arsenic can be found in the environment as arsenite [As(III)], arsenate [As(V)], and some organoarsenic compounds such as monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenobetaine (AsB) and arsenocholine (AsC). Of these, As (III) and As (V) are the most toxic, DMA and MMA less toxic, AsB and AsC the least toxic. The separation and detection of these species is vitally important for accurate pollution monitoring and toxicological assessment. The following chromatogram provides an example of the output from liquid chromatography - ICP/MS analysis.

ALSE method detection limits for As species range from 0.1 to 0.2 µg/L in fresh water and 0.7 to 1.9 µg/L in saline water.

The ANZECC 2000 trigger values for As (III) and As (V) are 24 and 13 µg/L in fresh water (95% level of protection). Insufficient data is available for derivation of statistically-based trigger values in marine waters, but 'low reliability' levels of 2.3 and 4.5 µg/L for As (III) and As (V) respectively are tabled.

Selenium speciation analysis

Selenium may also be speciated using the same anion exchange column as arsenic, but with an alternate mobile phase. ALSE method detection limits for Se (IV) are 0.2 in fresh water and 0.4 µg/L in saline water.

The ANZECC 2000 'low reliability' (due to insufficient data) trigger values for Se (IV) are 11 µg/L in fresh water and 3 µg/L in marine water.

Sampling and storage

It is recommended the water samples should be preserved with dilute hydrochloric acid for both arsenic and selenium speciation work. Samples should be field filtered prior to acidification using a 0.45 µm membrane. Aeration of the sample must be minimised during the filtration process. For 'mass balance' purposes, it is useful to undertake 'Total' element analysis. This involves the collection of an additional field filtered sample - preserved with nitric acid.