A human urine standard reference material for accurate assessment of arsenic exposure

Arsenic is a toxic element, and the toxicity of the element is dependent on its molecular form. Accurate assessments of arsenic exposure require the measurement of a complete panel of inorganic, organic, and metabolite arsenic species in urine, including arsenite (As(iii)), arsenate (As(v)), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), trimethylarsine oxide (TMAO), arsenobetaine (AB), and arsenocholine (AC). A certified reference material (CRM) containing the panel of arsenic species in urine is needed for method validation and quality assurance of assessment measurements. Until now, such a CRM was unavailable, due in part to the difficulty in stabilizing arsenic species, especially As(iii). For the first time, O₂ in the ambient atmosphere was determined to be the primary cause for the instability of As(iii) in an aqueous matrix, and a procedure was developed to stabilize the panel of arsenic species in a dark, low temperature (<-70 °C), and oxygen-free environment. Standard Reference Material^® (SRM) 2669 arsenic species in frozen human urine has been developed to meet the needs in arsenic exposure measurements in general and to support National Health and Nutrition Examination Survey (NHANES), in particular. SRM 2669 is certified for each arsenic species mentioned above at two concentration levels intended to proximate the 50^th percentile and 95^th percentile distribution in the US population (concentrations of As(iii), As(v), AC, and TMAO in the SRM are adjusted upward of the target percentiles to be above the method detection limits). The SRM was jointly produced by the National Institute of Standards and Technology (NIST) and the Centers for Disease Control and Prevention (CDC). Measurements leading to the certification were made collaboratively at NIST, CDC, and Rutgers, the State University of New Jersey.