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.
The extraction solvent
It is not set in stone that a solvent used for a method written c1970 should be used forever. If you could inject more solvent with no adverse chromatographic side effects, would you? Yes, of course, it’s good for business to have the best detection limits in the industry. If you’re more concerned with saving money you may just enjoy the benefit of extracting less initial sample. Another benefit will be that you can start the analytical run at a higher temperature meaning shorter cycle times.
Old soil semi-volatile methods that traditionally used dichloromethane, BP 39.6˚C, can easily be replaced with a mixture of hexane/acetone (90:10 v:v), the BP of which is nearer 68˚C. 1ul of the former has an expansion volume of ~400ul in a hot splitless inlet, the latter is ~200ul which in a liner of volume 1000ul can make a big difference. The hexane/acetone also has some polarity if required, extraction efficiency will be sufficient (and can be performance-checked by a Proficiency Testing scheme if need be) and if anything it brings out less matrix, yet the starting oven temperature for solvent focussing can be increased from 35˚C to 60˚C, significantly shortening cycle times. Added benefits of the proposed mixture are that it is eliminating a chlorinated solvent simultaneously improving health and safety and the environment.
Table 1: Polarity indices
First published in Chromatography Today Feb/Mar 2016 issue.