GC-ICP-MS determination of dimethylselenide in human breath after ingestion of ⁷⁷Se-enriched selenite: monitoring of in-vivo methylation of selenium

The amount of volatile dimethylselenide (DMSe) in breath has been monitored after ingestion of sub-toxic amounts of selenium (300 μg ⁷⁷Se, as selenite) by a healthy male volunteer. The breath samples were collected in Tedlar bags every hour in the first 12 h and then at longer intervals for the next 10 days. The samples were subjected to speciation analysis for volatile selenium compounds by use of cryotrapping-cryofocussing-GC-ICP-MS. Simultaneously, all urine was collected and subjected to total selenium determination by use of ICP-MS. By monitoring m/z 82 and 77, background or dietary selenium and selenium from the administered selenite were simultaneously determined in the urine and in the breath--dietary selenium only was measured by monitoring m/z 82 whereas the amount of spiked ⁷⁷Se (99.1% [enriched spike]) and naturally occurring selenium (7.6% [natural abundance]) were measured by monitoring m/z 77. Quantification of DMSe was performed by using DMSe gas samples prepared in Tedlar bags (linear range 10-300 pg, R ²=0.996, detection limit of Se as DMSe was 10 pg Se, or 0.02 ng L-¹, when 0.5 L gas was collected). Dimethylselenide was the only selenium species detected in breath samples before and after the ingestion of ⁷⁷Se-enriched selenite. Additional DM⁷⁷Se was identified as early as 15 min after ingestion of the isotopically-labelled selenite. Although the maximum concentration of ⁷⁷Se in DMSe was recorded 90 min after ingestion, the natural isotope ratio for selenium in DMSe (77/82) was not reached after 20 days. The concentration of DMSe correlated with the total Se concentration in the urine during the experiment (R ²=0.80). Furthermore, the sub-toxic dose of 300 μg selenium led to a significant increase of DMSe and renal excretion of background selenium, confirming that selenium ingested as selenite is homeostatically controlled by excretion. The maximum concentration of DMSe resulting from the spiked selenite was 1.4 ng Se L-¹ whereas the dietary background level was less than 0.4 ng Se L-¹. Overall excretion as DMSe was calculated to be 11.2% from the ingested selenite within the first 10 days whereas urinary excretion accounts for nearly 18.5%.