36 May / June 2014
number of valves increases the complexity of the tubing but does give the user greater flexibility or allows for parallelisation of the separation process. Figure 1 has two examples of the simpler but more common valve configurations using a single 2 position, 10 port valve, comprising of a first dimension column, 2 sample loops and two second dimension columns. This approach has been proven to be the most popular; however there are inherent issues with this. Van der Horst [22] has demonstrated that the direction of the flow in the sample loop can have an effect on the retention time of the compounds eluting from the sample loops, and hence would advise against the use of the asymmetrical valve arrangement in particular where the first dimension is operated very slowly.
and thus ensuring that the sampling rate of the second dimension is commensurate with the elution rate of the first dimension column. The sampling rate also dictates the use of two second dimension columns, as this will allow all of the eluant from the first column to be analysed. The development valve configurations which utilise two second dimension columns in combination with sample loops or a trapping column of some description offers some advantages. In particular the use of two second dimension columns allows for the separation time to be double that of the fractionation time which effectively doubles the peak capacity of the second dimension chromatographic system [24-27], dramatically increasing the overall separation capability of the chromatographic system.
and second dimension columns presents many challenges since any retention that is observed in the trapping media has to be negated in a manner that will not result in the elution of the analytes from the second dimension column. There are different approaches that can be employed to ensure that the compounds are retained on the second dimension column, but can still be eluted from the trapping cartridge. One approach is to add a diluent stream to the eluant from the trapping column. This approach has several disadvantages, in that;
• the peak is diluted,
• the transfer time from the trapping column to the second dimension column is typically increased in accordance with the flow rate ratio of the eluant from the trapping column to that of the diluents stream,
• the flow rate of the second dimension column will mean that a larger id column will have to be used or a UHPLC system will have to be employed which may not be compatible with the valve configurations being used.
• Requires more complex plumbing
There are other approaches that could be employed but as of yet there is limited academic literature to support these ideas.
Another approach that has seen some success is to effectively thermally desorb the analytes form the trapping column, in a manner similar to that employed when utilising GCxGC [28]. This implies that the compounds are thermally labile, how for the field of comprehensive GCxGC. The concept here is that the eluant from the first dimension is passed through a thermal modulator, which cycles between two temperatures, to initially trap components and then release these components into the second dimension column. This approach works well with the field of GC but is harder to apply to LC for several reasons;
• LC columns have a greater thermal mass
• Utilising an appropriate stationary phase that is thermally stable
• Some compounds are thermally labile
Figure 1 Common valve arrangements for 2D LC, showing a – Symmetrical, and Asymmetrical configurations
As has been previously stated it is important to consider not only the stationary phases being employed but also the dimensions of the columns that are employed. In general the second dimension column will be short and also have a wider diameter. This allows the flow rates to be optimised
The use of a focussing device between the first dimension column and the second dimension column is very popular and this can come in a variety of formats but typically will be a trapping column, or a sample loop packed with a suitable stationary phase. The use of a trapping device between the first
Developments in the field of high temperature LC are already addressing many of these issues [29,30], which will result in new approaches to the 2D LC.
Another approach that could be employed is to use an electric field to retain charged compounds. This technique would be comparable to capillary electrophoresis, where analytes are retained based on their charge. This approach presents many
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