FGCMS Instrument Description

A gas chromatograph/mass spectrometer (GC/MS) instrument was used to measure £ C5 carbonyl compounds, methanol and ethanol on the NASA DC-8 aircraft during the TRACE-P Experiment.

Air samples are drawn into the introduction system, via ¼” fused-silica coated stainless steel tubing (Silinite™), where preconcentration occurs. A three-stage trapping sequence is used to prepare the sample prior to separation and detection. Helium carrier gas transfers the preconcentrated compounds to a custom-built, miniaturized gas chromatograph fitted with a HP-624 column. The carbonyl and alcohol compounds of interest elute from the column and into the detector, a Hewlett-Packard 5973 mass spectrometer operating in the single ion mode. A dedicated laptop computer controls the mass spectrometer via an Ethernet connection and contains the software for data analysis. The GC/MS method provides unambiguous identification because the compounds are chromatographically separated and mass selected. The limit of detection is estimated to be between 5 and 30 pptv depending on the compound.

In-flight calibration and zeroing (system blanks) are necessary for quality MS-based carbonyl and alcohol measurements. The calibration system consists of a custom-built compressor/zero air generator/dilution system unit. High-efficiency Teflon diaphragm pumps are used to draw in ambient air. The zero air generator scrubs the air free of VOCs while maintaining ambient humidity. For calibration, standard alcohol/carbonyl mixtures are added to the scrubbed diluent gas stream. The system is capable of diluting the standard mixtures by factors of 100 to 10,000 and is very accurate because it contains only two, previously calibrated, flow controllers. The zeros and diluted standard samples follow a path identical to the ambient air samples through the analytical system. To help ensure the precision of our alcohol and carbonyl measurements we also analyze one or two long-lived CFCs present in the atmosphere during each chromatographic run. By analyzing atmospherically stable compounds such as CFC-11, CFC-12, CFC-113 and carbon tetrachloride - which have retention times within our chromatographic window - we can account for small variations in mass spectrometer response. This augments our on-board analysis of diluted alcohol and carbonyl standards.