Measurement of Organic Nitrates
and Selected Trace Gases from Whole Air Samples during TRACE-P
Elliot Atlas
Atmospheric Chemistry Division
National Center for Atmospheric Research
Boulder, CO
The goal of the research is to understand the
chemical composition of air masses in the TRACE-P study region
through targeted analysis of selected organic trace gases and with
exploratory studies of chemical composition of whole air samples
using gas chromatography/mass spectrometry. This
project is a collaborative research project with the Blake/Rowland
research group in the Dept. of Chemistry at the University of California,
Irvine. Blake et al.
describe the whole air sampling procedures from the TRACE P aircraft. In
the UCI laboratory, we have collaborated on several of the analytical
channels, and provide standardization for organic nitrates and
selected other compounds. In
addition to the collaboration at UCI, air samples are reanalyzed
in the laboratory at NCAR for verification and additional analyses. Since
the UCI canisters need to be recycled after analysis, aliquots
of selected samples are transferred to NCAR canisters. When
recycling is no longer required, UCI sample canisters are shipped
to NCAR for the additional analyses. Procedures
for sample handling and analysis of canisters by NCAR are given
below.
NCAR canisters are cleaned under mild heating
(about 80C) by repeated evacuation and flushing with moist air
free of organic trace gases (zero air). The
2.6-liter, electropolished stainless steel canisters are stored
with 20 psia of wet zero air. Prior
to transfer in the UCI laboratory, NCAR canisters are evacuated
to <1 torr. The
transfer system incorporates an all metal vacuum line with cryogenic
filters. To accomplish
the transfer, the NCAR canister is connected to the vacuum system
along with the UCI canisters. After
evacuation of the connecting lines, the NCAR canister is immersed
in liquid nitrogen. The
valves on the NCAR and UCI canisters are opened and gas is transferred
while the flow rate is > 1 lpm.
In the NCAR laboratory, canisters from TRACE-P
were analyzed on two analytical systems, both using GC/MS techniques. The
system for target compound analysis was a Hewlett Packard 5971
GC/MSD, which was operated in the selected ion mode. Approximately
300 std cc of air was introduced into a 30 m x 0.25 mm id. DB-1
capillary column using cryogenic preconcentration on a glass bead
filled sample loop. Target ion signals were recorded over predetermined time windows
during the temperature programmed analysis. A
calibrated whole air sample was used as a secondary standard, and
this was analyzed before and after 3 to 4 sample canisters.
A second GC-MS system was used for full scan
MS analysis. A VG
TRIO-1000 GC/MS was operated with a 60 m DB-624 column under constant
flow conditions. A
cryogenic enrichment system for sample preconcentration employed
a Teflon water trap (-20C) that was not used in the HP system. Full
scan spectra were obtained for semiquantitative analysis of selected
compounds, and for comprehensive, but qualitative, identification
of compounds from selective regions covered in TRACE-P.
Trace gas standards are based on both laboratory
and commercially prepared mixtures. The
high concentration mixtures are characterized using gas chromatography
with flame ionization and/or atomic emission detection. Comparisons are made to certified reference materials when
appropriate. Flow
dilution of the high concentration mixtures are used to calibrate
working standards near ambient mixing ratios.
|