Sensitive determination of arsenite and arsenate in plasma by electrospray ionization tandem mass spectrometry after chelate formation

Abstract

Inorganic arsenite (As³⁺) and arsenate (As⁵⁺) are well-known poisons, and the toxicity of As³⁺ is about ten times that of As⁵⁺. In this study, a simple, rapid, and sensitive method was developed for As³⁺ in plasma using electrospray ionization (ESI) tandem mass spectrometry (MS-MS). After washing plasma with trichloroethylene (TCE), As³⁺ in the aqueous layer was reacted with pyrrolidinedithiocarbamate (PDC, C₄H₈NCSS_-), and the produced As(PDC)3 was extracted with methyl isobutyl ketone (MIBK); a 1-µl aliquot of the MIBK layer containing As(PDC)₃ was introduced into the MS-MS instrument in the direct-flow injection mode. Other arsenic compounds such as As⁵⁺, monomethyl arsonic acid, dimethyl arsinic acid, arsenobetaine, arsenocholine, and tetramethyl arsonium did not produce As(PDC)₃. Therefore, without liquid chromatographic separation, As³⁺ alone could be detected after washing with TCE followed by solvent extraction of As(PDC)₃ with MIBK. Thus, inorganic As⁵⁺ was reduced to As³⁺ with thiosulfate, and then the total inorganic As was quantifi ed as As³⁺; As⁵⁺could be calculated by subtracting As³⁺from the total inorganic As. The MS-MS quantification was performed by selected reaction monitoring using a peak at m/z 114 of a product ion (C₄H₈NCS)⁺ formed by collision-induced dissociation from the precursor ion As(PDC)₂ ⁺ at m/z 367. The mass spectral identification on MS-MS spectrum was possible even at 1 ng As³⁺/ml plasma. The calibration curve for As³⁺ showed linearity from 0.5 to 100 ng/ml plasma. The limits of detection by selected reaction monitoring were 0.3 ng/ml in water and 0.2 ng/ml in plasma. The analysis could be completed in less than 15 min, because chromatographic separation was not necessary before the MS-MS detection.