Supported capillary membrane sampler (SCMS)
Description

The SCMS was originally developed by Wolcott and Westlake at Dow Chemical (US Patent 5,317,932).  When Wolcott joined Global FIA, we obtanied an exclusive license to the technology and supply this powerful sampling device to companies in predominantly in the petrochemical industry.

 The SCMS is a unique sampling probe that allows sampling of various compounds from complex process solutions. The SCMS may be submerged in a sample vessel, inserted directly into process lines, or used in a flow cell / fast loop arrangement, as shown in the following figure. The in-process configuration allows the SCMS to be inserted or removed without shutting down process flow.

 

The unique feature of the SCMS is that it allows the use of a long (typically 2 meters), thin-walled membrane tubing, which provides mass transfer kinetics that reach vapor-liquid equilibrium inside the tubing, simplifying calibration and yielding accurate quantisation. This feature makes the SCMS sampling equivalent to head space and purge-and-trap sampling, but with a much simpler, less troublesome, and more direct sampling system. It can also replace solvent extraction of organics as a step in the GC analysis of aqueous samples. 

This membrane, which would usually be quickly damaged, in a process stream is protected in the SCMS in the threads of the underlying support.  Problems arising from high levels of suspended particulates, dissolved solids, high levels of salts, etc., which have been the bane of reliable on-line sampling, are eliminated. Typical lifetimes of SCMS membranes in continuous service are a year or better. 

The SCMS can be used with a number of different types of analyzers, such as GC, HPLC, and FIA. The widest application of the SCMS to date has typically been as a sampling system for volatile and semi-volatile analytes with analytical finish by GC.  In practice, a carrier gas (typically nitrogen) is purged through the inside of the tubular membrane of the SCMS into the sample loop of the GC injection valve.  The constant-volume gas phase aliquot is injected onto the GC column for separation and analysis

Examples of organic analytes sampled in process streams with the SCMS include organic halides, 1,3 butadiene, acetaldehyde, acetone, and benzene.