Temperature requirements for thermal modulation in comprehensive two‐dimensional gas chromatography

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Abstract
Temperature requirements for trapping and release of compounds in a cryogenic gas loop‐type GC×GC modulator were determined. Maximum trapping temperatures on the uncoated, deactivated modulator capillary were determined for compounds from C4 (bp –0.5°C) to C40 (bp 522°C). The liquid‐nitrogen cooled gas flow rate was reduced from a high of 15.5 to 1.5 SLPM over the range to achieve the required trapping temperature. Excessive cold jet flow rates caused irreversible trapping and peak tailing for semi‐volatile compounds above C26. Alternate cold jet coolants were investigated. An ice water‐cooled jet was able to trap compounds with boiling points from C18 (bp 316°C) to C40 and a room temperature air‐cooled jet was able to trap compounds from C20 (bp 344°C) to C40. The hot jet produced launch temperatures approximately 40°C hotter than the elution temperature with heating time constants of 8 to 27 ms. Modulated compound peaks were symmetrical with half‐height peak widths of 43 to 56 ms for compounds with little second column retention, and 70 to 75 ms for compounds with more second column retention. The liquid nitrogen‐cooled loop modulator with gas flow programming was used to produce a GC×GC chromatogram for a crude oil that contained compounds from C7 to C47.