Tech Tip 17: Miniaturisation in GC laboratories – Part III

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.

Injection onto the column

Why change the injection technique?

For sample matrices where large extraction volumes have traditionally been necessary, i.e. waters, something fundamentally more intelligent has evolved as an injection technique, as laboratory space quickly becomes prohibitive in such instances. To miniaturise environmental water analysis to the same scale as soil analysis, the technique of Large Volume Injection [1] (LVI) is often used. This enables the user to get more analyte onto the column by controlling the  injection of a hundred times more sample dissolved in solvent but then venting almost all of the latter leaving the concentrated target compounds to pass through the column and be resolved. Benefits can be a smaller initial sample volume, smaller extraction solvent volume and then additionally no need for solvent evaporation, the latter can lead to analyte loss through sample transfers.

Traditionally Polycyclic Aromatic Hydrocarbon (PAH) analysis from water has been performed by liquid/liquid extraction of 500ml water sample, with 100ml of dichloromethane (DCM), evaporated to 1ml and 1ul injected; with LVI 50ml of water is extracted with 2ml pentane and 100µl of solvent injected in solvent vent mode with no subsequent requirement for evaporation, saving money on sample bottles / solvent and time from extraction to vialling.

Historically it would have been normal practice to use a cryogenic gas to cool the inlet to a required starting temperature where solvents and analytes were in the liquid form but recent advances have moved to Peltier cooling using an ethanol/water mix (still not ideal as the control equipment is cumbersome and the solution needs periodic replacement) and since on to modern solvent-free Peltier cooling elements.

Liquid CO2 for PTV inlet
Liquid CO2 for PTV inlet Gerstel CIS4 Peltier system control JAS Unis Peltier system

Diagram 1 : Evolution of Large volume injection

The payback in method miniaturisation is rapid even for small environmental labs (~£1M turnover), a period of seven months would not be untypical to see the payback for a modern LVI capability.

  35 PAH waters / day LLE 35 PAH waters / hr LVI
analyst cost £ 1.5 0.18
standards solvents etc. £ 0.25 0.01
vessels £ 0 0.14
extraction cost per sample £ 1.75 0.33

Table 1: Cost benefit analysis / cost per sample table





[1] Large Volume Injection with Solvent Venting - Application to Trace Detection of Analytes in Water - A. Hoffmann, K. MacNamara, Gerstel GmbH & Co. KG, Eberhard-Gerstel-Platz 1, D-45473 Mülheim an der Ruhr, Germany