Jet separator/membrane introduction mass spectometry for on‐line quantitation of volatile organic compounds in aqueous solutions

Abstract

A new technique is described for the direct detection of volatile organic compounds present in aqueous solutions at levels in the parts per trillion range. The sample is enriched in analyte in two consecutive stages; one utilizes a semi-permeable membrane interface and the other a jet separator. The analyte solution is sampled as it flows coaxially over a semi-permeable capillary membrane, the interior of which is continuously purged by helium. The permeate is pneumatically transported to the mass spectrometer via a jet separator, which is used to remove excess helium and water from the analyte vapor stream. Data are reported for two instruments; in one the membraneljet separator system is interfaced to a single quadruple mass spectrometer via a custom-built metal jet separator with a variable capillary gap. In the second, an ion-trap mass spectrometer is used in conjunction with a conventional fixed-gap quartz jet separator. Typical analyte response times are 2-5min at ambient temperature, and flow injection methods are used for sample delivery. Direct comparisons, made under identical instrumental conditions, show that the jet separator system displays even lower detection limits than a conventional direct-insertion membrane probe. Detection limits in the range 30 parts per trillion to a few parts per billion are observed for selected volatile organic compounds and the response is linear over 3 orders of magnitude.Analysis of environmental samples for organic pollutants is traditionally carried out by gas chromatography (GC) combined with detection based on electron capture, the Hall effect, photoionization, or mass spectrometry. ' Although these methods yield reproducible quantitative results, the effort and costs involved in sample preparation and the time required to complete a temperature-programmed GC analysis are considerable. Furthermore, environmental concerns dictate that measurements be made on complex mixtures with a high degree of chemical specificity and with a minimun of preliminary sample work-up.2The combination of flow-injection analysis (FIA) sample handling methods with membrane-introduction mass spectrometry (MIMS)3 is an alternative procedure which addresses these requirements. In MIMS, the organic compounds in a sample are separated from the matrix by selective permeation across a membrane by the process of pervaporation. This results in a substantial increase in concentration of the organic compounds relative to the sample matrix in the permeate vapor. The sample handling system and membrane interface are both relatively simple and they can be used with low-cost mass spectrometers (single quadrupole or iontrap types). Such combinations have been shown to provide a linear response, exhibit high sensitivity, and allow sufficient chemical speciation for rapid and continuous screening of trace amounts of organic compounds in aqueous s~l u t i o n .~ In earlier versions of the membranelmass spectrometer interface, the membrane was placed in the analyte solution and was far removed from the ion...

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