Often we hear the argument that people stay away from speciation analysis because the chemistry involved is so complex. However, chemistry is as it is and does not change just because of your interest. Sometimes speciation matters even if the interest is in total element concentration only.
Total element concentration is a kind of "sum parameter", summing up all the different element species that are present as:
- dissolved, colloidal or particular species,
- uncharged, negatively charged or positively charged species,
- free, sorbed, complexed or bound species,
- inorganic, metallorganic or organic compounds.
The prerequisite for total element concentration analysis is that all different species do participate on the generation of the element signal with the same sensitivity. Unfortunately that working hypothesis is more often invalid than analysts believe. Bias may occur in all steps of the analytical procedure:
Sampling: Has to assure that all different species have the same chance to end up in the sample. It is very difficult to create sampling conditions that fulfil this prerequisite. Analyte species can be lost due to sorption to sampling devices, volatilization from the sample or fractionation effects. Main losses occur for colloidal, particular or organic species that get lost by sorption on sampling funnels, container walls or filtration devices. Other losses may occur for dissolved gaseous or volatile species (e.g. Hg(0), methylated species).
Storage:
Optimum sample storage conditions are different for different element species. While total element concentration analysis is not interested in preserving the speciation, storage conditions should be chosen that are optimum for the different element species present. Most often the original sample cannot be preserved for the different species but has to be pre-treated (digestion, preservation) in order to transform the different species into one stable form that can be stored.
Sample preparation:
Sample preparation has to assure that different element species are transformed to a single one that can easily be detected (mineralization). If some species resist such pre-treatment, they may get lost in the final determination step (hidden species).
Detection:
In case that the sample has not been efficiently pre-treated in order to transform the different species into a single one (direct analysis) the sample presentation, sample introduction and the final measurement step must assure that all different species contribute to the final signal with the same sensitivity. Bias occurs because of differences in charges, hydrophilicity, volatility and other characteristics of the different species. The sensitivity of different species might also be influenced by interference effects specific to a special species (cationic, anionic).
The degree of bias is increasing with the chemistry/physics involved for sample presentation and signal generation. In atomic spectrometry colder sources (graphite furnace, flames) are more prone to such effects than hotter plasma sources. Direct sample injection is less problematic than sample nebulisation, simple nebulisation is less problematic than nebulisation/desolvation and chemical vapour generation is even more problematic than nebulisation. Anyhow, even liquid sample introduction by simple nebulisation creates different transport efficiency for different species. Unfortunately, pneumatic nebulisers are "poor pumps and inferior subsamplers". The transport efficiency for different element species is influenced by the interaction with the sample introduction system (sampling capillary, pump tube, valves, transfer lines, nebuliser, spraychamber) mainly because of differences in sorption behaviour (to the different materials in contact with the sample) or different volatility. Especially volatile species have much higher transport efficiency than non-volatile species but also can be lost quite easily on membrane desolvation units. The degree of such effects depends also on the type of nebuliser with pneumatic nebulisers less critical in comparison to ultrasonic nebulisers and thermospray nebulisers.
Effects of the valency state of elemental species on their sensitivity have been observed for Cr(III)/Cr(VI), Fe(II)/Fe(III), V(IV)/V(V), Te(IV)/Te(VI), Ru(III)/Ru(V) in flame AAS, for Se(IV)/Se(VI) in graphite furnace AAS, and for As(III)/As(V), Bi(III)/Bi(V), Se(IV)/Se(VI), Sb(III)/Sb(V), Sn(II)/Sn(IV), Te(IV)/Te(VI) in hydride generation AAS. Sensitivity differences of elemental species of different valency states have also been observed in ICP spectrometry for As(III)/As(V), Os(IV)/Os(VIII), Te(IV)/Te(VII). Sensitivity differences have also been observed for inorganic/organic species with respect to Hg for GFAAS and ETV-ICP-MS, Se for GFAAS and ICP-MS, Iodine in ICP-MS and Si in ICP-OES. The analyte speciation also has a significant influence on direct sample introduction methods such as laser ablation or spark ablation.
To summarize, it is very complex to assure that different species contribute to an element signal with the same sensitivity, making direct analysis (dilute and shoot) for total element determinations very problematic. Validation by a recovery test of a spiked standard is not sufficient to prove trueness of obtained results !
Michael Sperling
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