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

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 'vater from the analyte vapor strean. Data are reported for two instruments; in one the membrane/jet separator system is interfaced to a single ijuadrupolc 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 2–5 min at ambient temperature, and Mow injection methods are used for sample delivery. Direct comparisons, made under identical instrumental conditions, show that the jet separator system displays even lowsr detection limits than a conventional direct‐insertion membrane probe. Detection limits in the range 30 parlis 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.