Fast, high peak capacity separations in comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry

Peak capacity production is substantially improved for two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC × GC–TOFMS) and applied to the fast separation of a 28 component liquid test mixture, and two complex vapor samples (a 65 component volatile organic compound test mixture, and the headspace of warm ground coffee beans). A high peak capacity is achieved in a short separation time by selecting appropriate experimental conditions based on theoretical modeling of on-column band broadening, and by reducing the off-column band broadening by applying a narrow, concentrated injection pulse onto the primary column using high-speed cryo-focusing injection (HSCFI), referred to as thermal injection. A long, relatively narrow open tubular capillary column (20 m, 100 μm inner diameter (i.d.) with a 0.4 μm film thickness to benefit column capacity) was used as the primary column. The initial flow rate was 2 ml/min (60 cm/s average linear flow velocity) which is slightly below the optimal average linear gas velocity of 83 cm/s, due to the flow rate constraint of the TOFMS vacuum system. The oven temperature programming rate was 30 °C/min. The secondary column (1.8 m, 100 μm i.d. with a 0.1 μm film thickness) provided a relatively high peak capacity separation, concurrent with a significantly shorter modulation period, PM, than commonly applied with the commercial instrument. With this GC × GC–TOFMS instrumental platform, compounds in the 28 component liquid test mixture provided a ∼7 min separation (with a ∼6.5 min separation time window), producing average peak widths of ∼600 ms full width half maximum (FWHM), resulting in a peak capacity on the primary column of ∼400 peaks (at unit resolution). Using a secondary column with a 500 ms PM, average peak widths of ∼20 ms FWHM were achieved, thus providing a peak capacity of 15 peaks on the second dimension. Overall, an ideal orthogonal GC × GC peak capacity of ∼6000 peaks (at unit resolution) was achieved (or a β-corrected orthogonal peak capacity of ∼4400, at an average modulation ratio, MR, of ∼2). This corresponds to an ideal orthogonal peak capacity production of ∼1000 peaks/min (or ∼700 peaks/min, β-corrected). For comparison, standard split/split-less injection techniques with a 1:100 split, when combined with standard GC × GC conditions typically provide a peak capacity production of ∼100 peaks/min, hence the instrumental platform we report provides a ∼7-fold to 10-fold improvement.

► Peak capacity and peak capacity production are significantly improved for GC × GC–TOFMS.
► A peak capacity of ∼6000 was obtained in ∼6 min in a GC × GC–TOFMS separation.
► Primary column peaks were ∼600 ms FWHM and secondary column peaks were ∼20 ms FWHM.
► Cryo-focusing thermal injection was utilized for a narrow injection onto the primary column.
► The instrument was applied to the separation of the headspace of warm, ground coffee beans.