Development of membrane extraction with a sorbent interface for the analysis of environmental and clinical samples

Abstract

There is an increasing need for simpl.e and rapid sample introduction techniques. This need has driven the development of solvent free sample preparation techniques, like solid phase microextraction (SPME) and membrance extraction with a sorbent interface (MESI). Membrane extraction with a sorbent interface is a single step sample preparation technique, which was developed to enable fast routine analysis, and semicontinuous monitoring of volatile organic compounds in various matrices.

Hollow fiber membranes present the advantage of being self-supported and very easy to couple to the carrier gas line. However, due to their thick wall, they have a slow response time and a long lasting memory. Flat sheet membranes are thinner, but they need to be placed in special holders in order to be connected to the carrier gas line.

A membrane module was constructed to enable the connection of flat sheet membranes in MESI systems. The response time and memory effect of the system were examined using a standard mixture of benzene, toluene, ethylbenzene and xylene. The permeation of analytes through silicone membranes and silicone polycarbonate membranes of different thickness was compared. The analytes permeate faster through thinner membranes. However, due to the higher permeability of the silicon membrane compared to the silicone polycarbonate membrane, the amount of analytes extracted using a silicone membrane of 55 μm thickness is higher than the amount extracted using a silicone polycarbonate membrane of 25 μm thickness.

A new sorbent trap has been designed to allow the use of longer trapping for improved sensitivity. A piece of stainless steel tubing was packed with an appropriate sorbent, and for analytes desorption a voltage pulse was applied to the walls of the sorbent trap. The use of the packed design favors the retention of the analytes by the sorbent, minimizing the channeling effect, making it more convenient for use in field analysis. Even with the use of a short trapping time of one minute, the sensitivity was improved by more than 100 times, versus the sensitivity of a normal injection. The efficiency of the trap was further improved by placing it on a Peltier cooler. A piece of fused silica capillary, having an inner diameter of 100 μm was placed in front of the trap to minimize the back flush of the analytes during the desorption pulse.

The newly designed MESI system was coupled to different detectors and was applied for the analysis of various environmental samples. Complex chromotograms were obtained by analyzing tap water, eucalyptus leaves, and cigarette smoke using the MESI system and a flame ionization detector. The limit of detection obtained for the analysis of benzene by MESI-GC-FID was determined to be 10 ppt. By using a quadrupole mass spectrometer as detector, the peaks from tap water, fume hood air, and parking lot samples were identified. A field portable system was obtained by coupling MESI to a Micro GC equipped with a thermal conductivity detector. Even though TCDs are not very sensitive detectors, a detection limit of 60 ppt was achieved for the analysis of toluene, by preconcentrating the sample with MESI.

MESI was also applied for the analysis of various compounds in human breath. Complex chromatograms were obtained when a flame ionization detector was used as detector in the analysis of breath samples. The permeation of ethanol through skin was monitored using an ion mobility detector. Chloroform was detected in the breath of a swimmer and the variation of acetone in breath over a 4.5 hour period was monitored, using the same detector.