GC-MS Interface With Enrichments Devices

GC-MS interface and its Enrichment Devices

Presented By:Surendra P Gupta

Chromatography is the separation of a mixture of compounds (solutes) into separate closely related components from the mixtures.

Gas Chromatography (GC) is one of these techniques, derived from the fact that the mobile phase is gaseous.

How Does GCMS Work?

1. Sample is transported via an inert gas called mobile phase. The only function of mobile phase is to sweep the analyte mixture through the length of the column of the mobile phase.2. For separation to occur , the stationary phase must have an affinity for the analytes in the sample mixture, the analyte must strongly retained in the stationary phase and move slowly related to the flow of the mobile phase.3. The contrast components that have a lower affinity for the stationary phase travel through the column at a faster rate.4. As a consequence of the differences in mobility, sample components separate into discrete bands that can analysed qualitatively and quantitatively.

Gas Chromatography – Instrument Layout

Interfacing Gas Chromatography and Mass Spectroscopy (GC/MS)

Transports the effluent from the gas chromatograph to the mass spectrometerAnalyte must not condense in the interface Analyte may not decompose before entering the mass spectrometer ion sourceThe gas load entering the ion source must be within pumping capacity of the mass spectrometer

After separation of our sample components by the GC, we need a way of introducing it into our MS - an interface. An ideal interface should:1.Permits and analyze at 1 to 10 nano gram range sample. 2.Quantitatively transfer all analyte3. Reduce pressure/flow from chromatograph to level MS can handleFor appropriate performance, M.S analyzer has to be maintained at very low pressure , 10-6 to 10-7 Torr.This low pressure drop can easily implanted by simple needle valve or by a capillary restrictor.

Enrichment devices

– Jet Separators – Membrane Separator– Effusion separator

Jet separators are most commonly used, because enough sample can be introduced into the ion source

The major goal of the enrichment device is to remove most of the carrier gas - the majority of the effluent.

Jet Separators In these separators, the GC flow is introduced into an evacuated chamber through a restricted capillary.In an expanding jet, high molecular mass compounds are concentrated in the core flow whereas the lighter and more diffusive carrier molecules are dispersed away, in part through collisions. Thus, sampling of the core flow produces an enrichment of the analytes.The yields are about 25% under normal operating condition.

Advantages

Any gas producing source can be used i.e He gas.

Relative low cost and easy to use.

Membrane Separatordeveloped by Llewellyn and Littlejohn

It is made of a silicone-rubber membrane ( size .025-.04mm) that transmits organic non-polar molecules and acts as a barrier for (non-organic) carrier gases.

Despite being a very effective enrichment procedure, it also suffers from discrimination effects with more polar analytes and produces significant band broadening of their chromatographic peaks. A semi-permeable membrane is placed

between the GC effluent and the MS.

Major problems with this approach: Membrane selectivity based on polarity & high MW compound Slow to respond Only a small fraction of analytes actually permeates through membrane

Effusion separator It is based on the molecular filtering / diffusion of the gas effluent by means of a porous glass frit.The column effluent passes through a fritted tube situated in a vacuum chamber.Small molecules traverse the microscopic pores in the tube walls and are evacuated whereas high molecular mass molecules are transferred to the ion source.Yield is about 80-90%.

Advantage

Relative simple and inexpensive approach.

Disadvantages

Rate of diffusion is molecular weight dependent - selectivity based on MW.

the principal drawbacks of this interface are the high dead volume added and its high surface area.

GENERAL USES identification and quantitation of volatile and semi-volatile organic compounds in complex mixtures .

Determination of molecular weights and elemental compositions of unknown organic compounds in complex mixtures

Structural determination of unknown organic compounds in complex mixtures both by matching their spectra with reference spectra and by a prior spectral interpretation

SPECIFIC APPLICATIONS Identification of unknown organic compounds in hazardous waste dumps Identification of reaction products by synthetic organic chemists Analysis of industrial products for quality controlAnalysis of Anabolic Steroids in Biological materials.