“LET’S ANALYZE IT BY GC/MS”
The suggestion is often heard in troubleshooting or, proactively, in validating critical cleaning processes. The term GC/MS is familiar to most analytical scientists and engineers and also, due to popular TV crime shows, to much of the general public. GC/MS consists of two analytical techniques used in tandem: gas chromatography (GC) followed by mass spectrometry (MS).

CHROMATOGRAPHY
In chromatography, molecular species in a mobile phase are separated by elution through a column containing a stationary phase. GC terminology dates to early chromatographic analysis that involved a transparent tube or column packed with a solid sorbent, the stationary phase. Classically, the unknown material is placed at the top or inlet to the column. A liquid, the mobile phase, is passed through the column, carrying the unknown with it.

The time it takes for a molecule to move through the column is a function of the interaction with the mobile and stationary phases. Components of the unknown mixture are eluted (washed) from the column at different times. In early chromatography, colored bands (thus the root “chroma”) would be distinguishable in the column, each band associated with a specific molecule.

In a GC, the mobile phase is a gas such as He, H, Ar, or N. Most GCs now use an internally coated, narrow bore capillary tube, frequently coiled, instead of a packed column as the stationary phase. Separation of molecules occurs via different adsorption (stickiness) characteristics between the carried molecule and the stationary phase, so some molecules are washed, or elute, from the column sooner than others.

MASS SPECTROMETRY
After the initial chromatographic separation, mass spectrometry further separates molecules by charge-tomass ratio. Ionized molecules are spatially deflected, either by magnetic or electric fields, as they pass through. The higher the charge-to-mass ratio, the greater the deflection.