23 Liquid CO2 for PTV inlet Gerstel CIS4 Peltier system JAS Unis Peltier system
Diagram 3: Evolution of Large volume injection
pass through the column and be resolved. Benefits can be a smaller initial sample volume, smaller extraction solvent volume and then additionally no 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 a 500ml water sample, with 100ml 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 time and money on sample bottles, solvent and time from extraction to vialling/running.
Historically it would have been normal practice to use a cryogenic gas to get sufficient temperature differential between the solvent of use and the first eluting target compound 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.
The payback in method miniaturisation is rapid even for small environmental labs (~£1 million turnover), a period of seven months
Table 4: Cost benefit analysis / cost per sample table
analyst cost £ standards
solvents etc. £ vessels £
extraction cost per sample £
35 PAH waters / day LLE 1.5
0.25 0 1.75
would not be untypical to see the payback for a modern LVI capability.
Why change the column dimensions or carrier gas?
By using a shorter, narrower column, with a thinner film and increasing gas flows and oven temperature ramp rates, it is possible to shorten run times considerably with little or no loss in resolution or signal-to-noise ratio. Method translation software has been developed [4] and is freely available to assist analysts.
Further possible improvements include using more efficient carrier gases like hydrogen, which compared to helium has
35 PAH waters / hr LVI 0.18
0.01 0.14 0.33
6 PAH soils / hr soxtherm 1.1
0.5 0 1.6
60 PAH soils / hr sonicate 0.1
0.1 0.14 0.34
a larger linear velocity. Whilst hydrogen is known to be reactive to certain compounds, using a PTV inlet with cool injection reduces the likelihood of artefact production, leaving the major gains of improved efficiency, better signal to noise ratios and further reduced run times.
Why change the detection technique?
Specific detectors such as Electron Capture Detectors (ECD) can bring 1-2 orders of magnitude greater sensitivity but may not be viable with large analytical suites where functional groups can be present or absent and identification of unknowns may be
Diagram 5: Moving to a shorter column but keeping phase ratio the same reduces run time but preserves selectivity
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