High throughput analysis of atmospheric volatile organic compounds by thermal injection--isothermal gas chromatography--time-of-flight mass spectrometry.

ABSTRACT

Sixty one volatile organic compounds (VOCs) from a standard gas mixture were separated via isothermal gas chromatography coupled with time-of-flight mass spectrometry (GC-TOFMS) in a ≈ 35s separation time window (≈ 45 s separation). The VOCs in the standard gas mixture were selected based on the EPA TO-15 methodology. The high throughput separation was achieved with a relatively high total peak capacity (n(c) ≈ 114), by simultaneously minimizing both on-column and off-column peak width broadening. The on-column contributions to peak width broadening were minimized by taking into account and applying GC separation theory for the selection of column dimensions and carrier gas flow rate conditions. Both fast cryogenic focusing and re-injection of compounds (implemented via a commercially available thermal modulator and referred to herein as thermal injection (TI) and fast TOFMS detection (100 scans/s)) were applied to reduce off-column sources of peak width band broadening (sometimes referred to as off-column band broadening). Cryogenic focusing during TI and minimal band broadening-based dilution during separation resulted in preconcentration factors for the detected peaks ranging from 78 (1,4-dichlorobenzene) to 420 (propylene). Since the injected volume for preconcentration was 500 µl, and based on the detected noise levels at selected m/z for each analyte compound, the concentration limit of detection (LOD) ranged from 67 ppbv (parts per billion by volume) for propylene, to 4 ppbv for freon-12. While application of standard VOC analysis conditions leads to separation times typically ranging from ≈ 30 to 50 min, the isothermal GC-TOFMS method reported herein represents a 40-fold improvement in analysis time while maintaining peak capacity and detection sensitivity that is comparable to traditional GC-MS VOC analysis.