The speciation of the chemical forms of arsenic in the biological monitoring of exposure to inorganic arsenic

Total As content may be determined in blood and urine by means of an AAS method that involves reduction of As to its volatile hydride and ashing at 600°C with MgO and Mg (NO₃)₂. Separation of inorganic As (InAs), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMAA) by ion-exchange chromatography, followed by direct AAS analysis, allows the determination of each As species in the urine.In a reference population of 148 subjects with only normal environmental exposure to As, total As concentration in the urine averages 17.2 ± 11.1 μg/l. Urinary As consists of 10% each of InAs, MMAA and DMAA, the remaining 70% consisting of other forms of organic As.Blood As concentration averages 5.1 ± 6.9 μg/l and correlates significantly with the urinary concentration of InAs and the sum of its metabolites (InAs + MMAA + DMAA).Inorganic arsenic undergoes methylation in the organism. After ingestion of high quantities of As₂O₃, the time course of excretion of its metabolites indicates that As methylation occurs by a saturable mechanism.In workers exposed to As₂O₃, InAs, MMAA and DMAA are the only chemical forms of As excreted in the urine that are relevant to a study of occupational exposure. Blood As concentration is proportional to exposure and correlates only with urinary DMAA excretion; DMAA seems to be the most appropriate single indicator of exposure. At high levels of exposure (total As excretion above 200 μg/l), As accumulates in the organism and DMAA excretion reflects its accumulation. At low levels of exposure (total As excretion below 50 μg/l) a short-term accumulation does not occur and the best biological indicator of exposure is InAs excretion.Seafood ingestion brings about a marked increase in urinary excretion of total As that lasts for 24–48 h and is not accompanied by any increase in InAs, MMAA or DMAA excretion. Organic As from seafood does not mix with the pool of inorganic As in the organism and may be separately detected in urine. In the biological monitoring of human exposure to As, particularly in the case of high urinary values, the speciation of the chemical forms of As in urine is necessary in order to establish with certainty the source, industrial or alimentary, of exposure.