The Use of Subtle Differences in Selectivity to Improve Separation of Structurally Diverse Analytes

Peter Dawes, Naza Lahoutifard, Roy Hibbert and Paul Wynne, SGE Analytical Science, Ringwood, Victoria, Australia.

Introduction

The separation of structurally diverse analytes is often complicated by chance coelutions with other analytes or with matrix related compounds. Often the column is blamed, but while such coelutions make analysis difficult they do not necessarily indicate a faulty column, poor chromatography or method design. No single column or method will resolve all compounds that can be chromatographed, so selecting a column that matches the needs of the application is an important first step. Rather than attempting to modify a method to resolve coeluting peaks, selecting a column with subtly different selectivity can achieve this aim without significant changes to established methods, elution orders or run times.

Experimental Detail

A mixed pesticide standard PL-3-1 containing 20 μg/mL of each component in acetone was obtained from Wako Pure Chemical Industries (Osaka, Japan) and diluted with acetone prior to use. Gas chromatography mass spectrometry was performed on a 6890GC-5973N MSD (Agilent Technologies, California, USA) and one of a BP5, BPX5 or HT8 column (30 m × 0.25 mm i.d., 0.25 μm film thickness, SGE Analytical Science, Victoria, Australia). BPX5 is a high temperature silphenyl 5% equivalent, BP5 is a standard 5% Phenyl column and HT8 is a modified, high temperature 5% equivalent.

Injections of 1 μL standards in acetone were fast and splitless at a temperature of 250 °C. Purge flow was 50 mL/min and a nominal inlet pressure of 64 kPa. The initial oven temperature was 40 °C (held for 4 min) and heated at 10 °C/min to 300 °C and the carrier gas was helium at a flow-rate of 1.2 mL/min in constant flow mode. EI mass spectra were collected over the range 50–550 Da at 2 scan/sec. The quadrapole temperature was 150 °C and the source was 230 °C and the transfer line was 260 °C.

Results and Discussion

In Figure 1, the separation of a mixture of pesticides is shown on three different 5% phenyl equivalent phases using columns of the same dimensions and the same conditions. This figure highlights the selectivity differences between BP5, BPX5 and HT8. These differences enable the selection of a column that delivers the optimal resolution solution for the compounds that are particularly important for an application — without a significant change to elution order or the retention time.

Figure 1

Conclusion

Subtle differences in selectivity exist between 5% phenyl equivalent phases. Careful exploitation of these differences can provide optimal resolution for a particular application without either dramatic changes to the elution order, or significant increases in run times.

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