A coaxially heated membrane introduction mass spectrometry (MIMS) sampling interface is presented that demonstrates improved on-line performance for the direct measurement of semi-volatile organic compounds (SVOCs) in air and water samples at parts-per-trillion levels. The device is based on a polydimethylsiloxane (PDMS) capillary hollow fibre membrane (HFM) in a pneumatically assisted "flow-over" configuration that is resistively heated on the membrane interior via a coaxial nichrome wire, establishing a thermal gradient counter to the analyte concentration gradient. This arrangement allows for continuous and/or pulsed heating modes, affording excellent sensitivity for the on-line measurement of SVOCs while retaining sensitivity for volatile organic compounds (VOCs). In addition, the signal response time for SVOCs is reduced substantially over conventional "flow-over" MIMS interfaces. Separation and quantitation of analytes are achieved using quadrupole ion trap tandem mass spectrometry.

Membrane introduction mass spectrometry (MIMS) is emerging as an important technique for on-line, real-time environmental monitoring. Because MIMS interfaces are simple and robust, they are ideally suited for operation in MS instrumentation used for in-field applications. We report the use of an on-line permeation tube to continuously infuse an isotopically labeled internal standard for continuous quantitative determinations in atmospheric and aqueous samples without the need for off-line calibration. This approach also provides important information on the operational performance of the analytical system during multi-day deployments. We report measured signal stability during on-line deployments in air and water of 7% based on variation of the internal standard response and have used this technique to quantify BTEX (benzene, toluene, ethylbenzenes, and xylenes), pinenes, naphthalene and 2-methoxyphenol (guaiacol) in urban air plumes at parts-per-billion by volume levels. Presented are several recent applications of MIMS-MS-MS for on-line environmental monitoring in atmospheric and aqueous environmental samples demonstrating laboratory, remote and mobile deployments. We also present the use of a thermally assisted MIMS interface for the direct measurement of polyaromatic hydrocarbons, alkylphenols, and other SVOCs in the low ppb range in aqueous environmental samples and discuss improvements in both the sensitivity and response times for selected SVOCs. The work presented in this paper represents significant improvements in field deployable mass spectrometric techniques, which can be applied to direct on-site analytical measurements of VOC and SVOCs in environmental samples.

Membrane introduction mass spectrometry (MIMS) provides direct measurement of volatile and semivolatile analytes in condensed and gas-phase samples without sample preparation steps. Although MIMS has numerous advantages that include direct, on-line, real-time analysis with low detection limits, current applications of MIMS are predominantly limited to volatile and semivolatile analytes that permeate hydrophobic membranes (e.g., polydimethylsiloxane; PDMS). We report the first enzyme modified PDMS membrane for use with MIMS. This was achieved by immobilizing Candida rugosa lipase directly onto the surface of oxidized PDMS. These surface immobilized enzymes catalyze ester hydrolysis, releasing an alcohol product at the membrane interface that is readily detected. We have successfully used an enzyme modified membrane for the analysis and quantification of low-volatility and hydrophilic esters. We report the quantification of several carboxylic acid esters in dilute aqueous solutions, including a phthalate monoester carboxylate that is not readily detected by conventional MIMS. This new interface demonstrates the potential for extending the range and versatility of MIMS.