Speciation of Germanium in Environmental Water Reference Materials by Hydride Generation and Cryotrapping in Combination with ICP-MS/MS

Technological Critical Elements (TCE) are of special interest due to their increasing use and importance in modern technologies and their high supply risk for related industries. Germanium (Ge) is on TCE that has not often been studies because of its presence in the environment at only ultratrace concentrations, related analytical challenges and its relative rare industrial use until recent years.  Because of the analytical challenges to determine Ge and its speciation, very little is known about its geochemical behaviour or the impact of its more recently increased use.

In order to measure Ge concentrations in the aquatic environment at ng/L levels and below, a preconcentration step is typically required. Most of the studies determining ultratrace levels  of Ge relied on hydride generation (HG) followed by cryotrapping (CT).  The cryotrapping can also be used to separate different Ge species by their boiling point during gradual heating. The coupling of HG-CT with ICP-MS has been described by several authors already. A group of scientists from the Czech Republic and Switzerland now aimed at the optimization of the HG-CT method and also at the exploration of the coupling with the state of the art ICP-MS/MS detection for sensitive and interference free analysis of water samples.

Unfortunately, currently there is no CRM with certified Ge values suitable for validation of analytical methods for Ge speciation analysis in environmental water samples. The authors therefore selected recent CRMs of unspiked river and seawater waters as samples for analysis, paying particular attention to possible sources of analytical artefacts.

Standard solutions for the three species inorganic Ge (IGe), monomethyl germanium (MMGe) and dimethyl germanium (DMGe) were prepared daily by serial dilution of a stock standard with deionized water by excluding any glassware.

The buffer solution for hydride generation containing 0.25 mol L-1 Tris-HCl, 0.005 mol L-1 EDTA and 2% (w/v)L-Cys were cleaned-up by hydride generation with NaBH₄. By using such clean-up reagent, blanks corresponding to approximately 0.1 ng/L of iGe and less than 0.01 ng/L of methylated species were achieved.

The concentrations of the buffer and the NaBH4 solution, the pH of the buffer were optimized for maximum signal. Using the optimized conditions, first the standards were analysed in order to study potentially species degradation by demethylation during hydride generation. While some impurities below 1 % were found in the methylated germanium standards, no further degradation occurred during hydride generation. In order to avoid biased calibration as a result of cross-contamination, calibration for iGe was carried out separately.

Detection of Ge by ICP-MS/MS was performed by the oxygen-shift mode that allowed the interference free detection of all Ge isotopes. The combination of these advantages makes the proposed method the most sensitive one reported to this date.

The analytical figures of merit for the HG-CT-ICP-MS/MS were evaluated under optimized conditions. Limits of detection were 0.015, 0.005 and 0.003 ng/L for iGe, MMGe and DMGe respectively. The three times higher detection limit for iGe in comparison to the methylated species is the result of the higher blank. Anyhow, the obtained detection power was appropriate for the analysis of uncontaminated natural fresh water and seawater samples.

Matrix effects were evaluated by comparing results obtained by external calibration with those obtained by the standard addition method. Differences were below 9% for all species in seawater and below 6% in fresh water. The authors therefore considered external calibration to be adequate.

Finally, CRMs of natural seawater and river water were analysed. While there are no certified values for Ge speciation, the obtained results were in the expected concentration intervals and will certainly be helpful for further research in this field.