Most interfaces between a gas chromatograph (GC) and a Fourier transform infrared (FT-IR) spectrometer for the on-line measurement of the spectra of eluting peaks have been designed for use with 1/8 in. o.d. packed GC columns, and the light-pipes are too large for GC/ FT-IR measurements of peaks eluting from capillary columns without degrading the GC resolution. The volume of a light-pipe for GC/FT-IR should be large enough to ensure that as much of the sample as possible is present in the cell during the measurement of the spectrum, but small enough that the GC resolution is not seriously degraded. It has been shown that the optimum GC/FT-IR sensitivity is obtained when the volume of the light-pipe, <i>V</i><sub>cell</sub>, is equal to the volume of carrier gas containing a GC peak between its half-height points, <i>V</i><sub>1/2</sub>. For peaks eluting from lightly loaded 1/8 in. o.d. packed columns, <i>V</i><sub>1/2</sub> is usually about 5 ml or greater, while for peaks eluting from capillary columns <i>V</i><sub>1/2</sub> is considerably smaller than this. If a support-coated open tubular (SCOT) column is used, a typical value for <i>V</i><sub>1/2</sub> is 0.5 ml, and if wall-coated open tubular (WCOT) columns are used, <i>V</i><sub>1/2</sub> may be 50 μl or even lower. Thus, if GC/FT-IR techniques are to be used for the identification of components of mixtures separated by capillary columns, light-pipes of smaller volumes than the 3 ml light-pipes used on commercial GC/FT-IR systems must be used.

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