The presumptive detection of benzene in water in the presence of phenol with an active membrane-UV photo-ionisation differential mobility spectrometer

Abstract

Studies with a new technique, active membrane-differential mobility spectrometry, with aqueous standards of benzene and phenol are described. The atmospheric pressure photo-ionisation chemistries of benzene and phenol in the presence of oxygen are similar in that benzene forms phenol radicals that subsequently react to yield diphenylether and 4-phenoxyphenol products. Further phenol sequesters charge from benzene ions leading to a significant loss of sensitivity. This is an important consideration in the development of screening techniques for the presence of benzene in environmental water samples. This challenge was addressed by including a pre-separation stage prior to photo-ionisation, and a 10 cm long polydimethylsiloxane active membrane inlet using nitrogen as a carrier gas was used to sample, concentrate and deliver low resolution separations to the 10.6 eV UV-ionisation region of a differential mobility spectrometer. Acetone was also proposed as a charge carrier for the UV photo-ionisation source; to promote phenol protonation and inhibit charge sequestration from benzene. The responses of the system to aqueous standards of benzene and phenol with and without acetone doping at 10.2 mg m⁻³ were evaluated and four to five-fold increases in sensitivity were obtained with acetone doping. With a sampling time of 60 s and a total measurement cycle of 180 s it was possible to obtain quantitative responses to single standards over the concentration range 6 to 177 µg cm⁻³ with linear correlations with R² values ranging from 0.97 to 0.99. The effects of the heating rate of the membrane and the dispersion field strength of the differential mobility spectrometer on sensitivity and the differentiation of benzene from phenol responses were optimised, leading to a configuration where a voltage heating programme of 4.75 V s⁻¹ was applied to a 124 µm stainless steel wire heating element within the active membrane, and a dispersion field strength of 22 kV cm⁻¹ was used to test a mixture of benzene (14 µg cm⁻³) and phenol (6 µg cm⁻³) in water. The presence of benzene was identified through the presence of a peak corresponding to a benzene response, V_C = −9 V FWHM = 1 V, that followed the thermal desorption profile of benzene.