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.