24 February / March 2016
Diagram 6: Utilising further variables gives even more productivity gains
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.
How can we further shorten the cycle time?
The longest time delay in any GC cycle is the oven cooling down. As the typical GC oven heating process is very inefficient (fundamentally it is heating up air in an oven), improvements have been steadily introduced over history varying from the futuristic (encapsulating the capillary column in a heated metal shield thereby reducing the air volume that is required to be heated and cooled) to the less mechanically challenged (high efficiency secondary cooling fans blowing cold air through the GC oven flap at the end of the cycle). Other developments have included thermally stable oven pillows to reduce the volume of air in the oven to secondary heating elements within
ovens that accelerate the GC ramp rate. Some broad-minded laboratories have vented the heat from the GCs through chimneys that feed into ductwork and exhausted it from the building courtesy of a robust fan. All GC analysts are extremely thankful in the summer for the latter technology but strangely less so in the winter. All of the above solutions have one thing in common, they shorten cycle times and therefore save money by improving delivery.
How do consumable savings then arise?
Looking at miniaturising water analysis methods for PAHs or Total Petroleum Hydrocarbons (TPH) from liquid/liquid extraction to large volume injection, small disposable glass vials (60ml) will replace large cumbersome separating funnels (one litre), saving time, reducing courier costs and negating the need for washing glassware. Similarly, certified EPA vials used in soil analysis - whilst being excellent quality and the industry vessel of choice - are most certainly not the cheapest, other cheaper fit-for-purpose alternatives exist.
Diagram 7: SIM has less noise and allows more ions of the m/z range through to the MS detector there- fore giving higher sensitivity than SCAN
Diagram 8: Cumbersome extraction to miniaturised high cost to miniaturised low cost
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