Membrane Extraction with a Sorbent Interface (MESI)
A new concept, MESI, has been developed to allow rapid routine analysis and long-term on-line semi-continuous monitoring of volatile organic compounds (VOCs) in various matrixes (biological, environmental or industrial) . A MESI system, in general as shown right, consists of a membrane extraction module, a sorbent interface, and a gas chromatograph (GC). This technology has been patented, US Patent number 5492838, issued Feb. 29, 1996. The MESI system offers many advantages including simplicity, low cost, durability, elimination of solvents, and easy automation. The use of a non-polar (and non-porous) membrane prevents moisture from entering the carrier gas. The sorbent interface concentrates the analytes and takes the place of the GC injector. The sensitivity of the system is tunable, controlled by the mass transfer rate of analytes through the membrane and the sorbent interface concentration time. The limit of detection is low and the analysis results are reproducible. Three mathematical models have been derived to describe the extraction process in air, water and headspace.
A low-pressure carrier gas stream can be used as the stripping phase for removal and transfer of the permeated components (e.g., VOCs). Membrane extraction modules can be made in different configurations depending on the application. The schematic of the membrane extraction (as shown right) illustrates how a simple membrane probe can be applied for headspace VOC analysis with different types of sample matrices. Water and solids can also be monitored directly at the sample source by placing the membrane probe above a lake, soil, a river bed, or an industrial process stream. The membrane probe consists of a piece of hollow fiber membrane and two pieces of stainless steel needle which facilitate the stripping gas transfer. Tight seals are formed between the membrane and the needle by the shrinkage of the membrane at the junctions. Carrier gas flows through the lumen to transport the analytes into the GC.
The sorbent interface performs two major functions:
- concentrates permeated analytes from the membrane extraction module;
- thermally desorbes the accumulated compounds from the trap to the separation column for analysisis.
Use of the thermal desorption technique eliminates the need for a sampling valve and allows easy automation. A typical design of such an interface (as shown right) consists of a sorbent trap, an electrical heating coil, and a solid-state relay. For thermal desorption, a heating coil designed for the sorbent trap generates an electrical heat pulse. The electrical current supply to the coil can be a capacitor or a regulated electrical source. A solid-state relay switches the electrical power supply to the heating coil on and off according to a computer signal. The sorbent in the trap should have low thermal capacity to enable rapid and reproducible desorption. When the electrical current passes through the coil, heat is generated and the analytes are desorbed and form a total organic concentration pulse at the inlet of the separation column. When electrical power is turned off, the temperature of the sorbent trap drops very rapidly to the ambient level and the interface begins to trap the analytes for the next analysis cycle. Two operating modes: a) trapping and b) concentration modulation in a multiplex process, have been investigated using this type of interface for different applications. Although the operating procedures of the two modes are different, both methods allow high sample throughput and ensure high sensitivity for the MESI system.