Method miniaturisation is the squeezing of as many instrumental and analytical parameters as possible to optimise efficiency. Variables that can be examined include extraction solvent, extraction technique, injection onto the column, separation on column, quantification via the detector and finally the cycle time of one analytical run. As we strive to make the method more robust we should improve quality, obtain an equivalent if not better Limit of Detection (LOD) and deliver the result more quickly and hence more cheaply.
Quantification via the Detector
Why change the detection technique?
Modern GC is blessed to have a diverse availability of specific detectors (at least sixteen) that respond to a plethora of analytes, atoms and bonds. Specific detectors such as ECD can bring 4 orders of magnitude greater sensitivity than say a FID, NPD has an impressive 105 linear range, PFPD like the two aforementioned can detect fg on column but none may be viable with large analytical suites where functional groups can be present or absent and identification of unknowns may be very important. Gas Chromatography Mass Spectrometry (GCMS) may consequently be required and in moving from SCAN* analysis to SIM**, the higher sensitivity may enable the scaling down of front end extraction volumes.
*SCAN analysis within GCMS spends the finite time available scanning every signal mass across a wide range consequently enabling the identification of unknowns from probability based library matching.
**In comparison SIM analysis searches for 1-12 different ions only, within any given time window, giving more data points per peak, as more cycles per second and longer dwell times on each ion are enabled.
For those with healthy capex budgets, modern instrumentation allows synchronous SIM/SCAN enabling simultaneous qualitative analysis of unknowns with accurate quantitation of targets.
Diagram 1: SIM has less noise and allows more ions of the m/z range through to the MS detector therefore giving higher sensitivity than SCAN