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

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.