Method Translation and Retention Time Locking in Partition GC

Abstract

Method translation in gas chromatography (GC) is a variation of components (columns, carrier gases, detectors, etc.) and parameters (pressures, temperature programs, etc.) of a method in a way that maintains the peak elution pattern. The concept is based on the fact that the void time can be viewed as a universal time unit in GC; method translation is the scaling of the time axis of the temperature program relative to the void time. Method translation can be used to reduce analysis time, improve resolution, and adopt a method to a different carrier gas or to a different outlet pressure (vacuum for mass spectrometers, ambient pressure for conventional detectors, etc.). It can also be used for retention time locking (RTL). Theoretical and practical aspects of method translation and RTL are analyzed. It is shown that a constantpressure method can be translated between the columns that have the same stationary-phase type and phase ratio. Method translation also implies that optimum temperature ramp rate (expressed in °C per void time) can be the same for a broad class (maybe all) of temperatureprogrammed GC analyses. For partition GC, this rate is ∼10 °C/void time.Suppose that there exists a gas chromatography (GC) method that provides a satisfactory separation of a given mixture, but takes too much time, or uses a column with too low sample capacity, or maybe, there is a need to use a different carrier gas or to change outlet pressure as might be required due to the change from, say, a flame ionization detector (FID) to mass spectrometric detector (MSD). In another scenario known as retention time locking (RTL), it might be necessary to lock retention times in several GC systems, i.e., to make sure that all the systems yield the same retention times for the same solutes. These and similar results can be achieved via method translation 1,2 which, in essence, is a rescaling of the pace of the temperature programming in proportion with the change in the void time or, vice versa, rescaling of void time in proportion with the change in the pace of temperature programming.Originally, 1 method translation was described as a set of rules to preserve resolution of all peaks in GC while porting a GC method to a smaller column in order to substantially reduce the analysis time. Since then, the technique gained in popularity, several revisions of method translation software 3 have been released, and many applications [4][5][6][7][8][9][10][11][12][13] have been developed.The scope of method translation has also been expanded. As the following theory shows, the underlying property of the technique is not the preservation of resolution, but the preservation of the peak elution pattern. That means that, while improving or reducing peak resolution and/or analysis time as required, method translation preserves the ratios between retention times of all pairs of peaks corresponding to the same pairs of solutes. This broader view of the technique allowed expansion of its applications to column optimization for the best resolution and r...

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