Mercury Speciation Analysis of Dried Blood Spot Samples

Dried blood spot (DBS) sampling involves the collection and storage of a small volume of blood obtained via a simple prick (heel, finger, toe, or tail) or other means from a human or study animal. The blood sample is typically collected on a paper card made of cellulose or polymer materials. DBS sampling has been used in human medicine since the 1960s, predominantly for screening in-borne metabolic disorders and, more recently, for toxicology. The advantages of the method are a) easy blood collection, b) small volume sampling, c) uncomplicated transport and storage.  The disadvantages and assay variables potentially affected when using filter paper samples are complex. Concerns include sample volume, humidity, haematocrit, chromatographic effects (homogeneity), analyte recovery, specimen source, anticoagulant, and filter paper characteristics.

Photo: Sampling of capillary blood DBS from a two weeks old baby

When this sampling and sample presentation techniques are used for quantitative analysis, the most critical parameters are the inhomogeneous distribution of the analyte across the spot, the quantitative recovery from the spot, the analyte loss during the drying and storage and possible contamination during sampling and by the paper.

Mercury is a global pollutant of high concern both for the ecosystem and human health worldwide. To monitor pollution trends in the population, cost-effective sampling procedures are required.

A group of researchers from North America now investigated, whether dried blood spot sampling is a feasible approach for mercury speciation analysis in human blood. For this reason, the authors investigated major questions that need to be answered for obtaining a reliable method for speciation analysis.

To answer these questions, two sources of human blood (i.e., human whole blood reference materials from the Institute National de Santé Publique du Québec with reported THg and MeHg concentrations; whole blood from a consenting volunteer that was Hg-spiked) were analysed both directly and after volume-controlled subsampling on filter paper.  Capillary blood was also collected from each participant following the protocol outlined by the “USAID Biomarker Manual” for DBS collection.

Whole blood and DBS samples were extracted with 4 M HNO₃ and 0.02% L-Cysteine at 60°C for 24 hours. Mercury species in the extract were determined by GC-CVAFS after ethylation with 1% sodium tetraethyl borate according to US EPA 1630 method. Quality control measurements during method development indicated that the method is both accurate and precise for both whole blood and DBS samples. The analytical precision was below 13% for the DBS samples and below 5% for whole blood samples. Background contamination by the cards were below the detection limit of the method.  Also, there was no significant difference in MeHg concentration between capillary and venous blood. For InHg, only 8% of the whole blood samples and 16% of the DBS samples were above the MDL, meaning that for most samples a confident measure could not be realized. The question, whether the measurement of MeHg in capillary DBS reflects the gold standard of venous whole blood analysis was overly simplified answered with  "yes". The stability of MeHg in DBS was investigated by analysing stored samples after 1 year. These results suggest that the MeHg (like THg) is relatively stable in SDBS after one year of storage at room temperature. The authors concluded, that the method detection limit for InHg was not adequate for monitoring human populations at background concentration levels.

The original publication:

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