Negative photoionization chloride ion attachment ion mobility spectrometry for the detection of organic acids

Abstract

A novel negative photoionization chloride ion attachment ion mobility spectrometry (NP-CA-IMS) instrument has been developed. In this technique, chloride ion attachment technology is first applied to photoionization ion mobility spectrometry in a negative detection mode. The reactant ions are chloride ions, which are generated from the reaction between carbon tetrachloride (CCl₄) and low energy electrons induced by the photoionization of acetone (CH₃COCH₃) with a commercial vacuum ultraviolet Krypton lamp. The generation efficiency of chloride ions was investigated. Subsequently, the performance of the instrument was investigated. A series of volatile straight chain organic carboxylic acids were detected in the order of magnitude of ppb, including acetic acid (CH₃COOH), propionate acid (CH₃CH₂COOH), butyric acid (CH₃(CH₂)₂COOH), valeric acid (CH₃(CH₂)₃COOH), isovaleric acid (2-CH₃(CH₂)₃COOH), hexanoic acid (CH₃(CH₂)₄COOH), heptanoic acid (CH₃(CH₂)₅COOH), and octanoic acid (CH₃(CH₂)₆COOH). Besides, the concentration of acetic acid was calibrated and a mixture of the investigated acids was also studied. Finally, the new method's capability to detect acetic acid in five different brands of edible white vinegar was evaluated. The experimental results show that negative photoionization chloride ion attachment is an excellent nonradioactive source for IMS.