High precision retention time measurements in gas chromatography. I. A simple differentiating time printer for large sample sizes

Summary

A differentiating time printer (DTP) apparatus has been constructed from standard laboratory equipment by connecting a differentiating CR circuit via an operational amplifier to an electronic temperature regulator that triggers the print out of a line frequency driven register. The electronic resolution is 0.1 s, giving a relative standard deviation σ_rel, which is determined by the stability of the line frequency at sufficiently long retention times. In Lund this corresponds to a best σ_rel = 0.02% and to a mean σ_rel = 0.03%. For late and broad peaks in real GC the noise caused the print-out of extra time values, especially with fronting peaks from ordinary 1/8″ columns. However, a comparison with the simultaneously recorded chromatograms was generally sufficient to pick out the proper retention times. For large sample sizes the corrected retention time t′ increased with peak height, the later the peak in the chromatogram. The In t ^′_n+1 /t ^′_n values from a series of n-paraffins showed good linear correlations with peak height h_n+1 and peak area A_n+1, giving standard deviations around the line approaching that expected from σ_rel. The accuracy of the system was mainly determined by the effect of a non-zero triggering level on the derived signal and by the time displacement expected from a large time constant. A rough estimate of the time error was made by simulating GC peaks with a cosine wave generator.