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.
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