SOLAS HALOCARBON INSTRUMENTATION AND CALIBRATION
Measurement of the important halogenated trace gases relevant to ozone depletion
and climate change are measured using the MADS GC-MS (Modified Adsorption
Desorption System) instrument. At the heart of the MADS is the Peltier cooler which
maintains a temperature of 40C, even with a substantial heat load, using two simple
three-stage Peltier coolers. The microtrap can independently be heated resistively to any
temperature from 40C to about +210C in about four seconds. The use of a microtrap
permits the desired analytes from 3-litre air samples to be pre-concentrated and
effectively separated from more-abundant gases that would otherwise interfere with
chromatographic separation or ion generation in the GC-MS source, such as N2, O2, Ar,
H2O, CO2, CH4, Kr and Xe. Importantly, the capillary column dimensions of the
microtrap, without the need for internal packings mean that the flow rate through the trap
is unimpeded, blank and artifact formation are negligible and also means that the
resulting injections to the Agilent 5973 GC-MS are sharp and reproducible. By trapping
and eluting analytes at low temperatures, the range of compounds that can be measured
include a number of important volatile compounds. The MADS system uses a highprecision integrating mass flow controller (MFC) for accurate measurement of sample
volumes. In addition, significant advances have been made in the software to control
and acquire data from the MADS and the GC-MS itself, so that the entire system has
programmability, versatility and ease of operation comparable to that of the Advanced
Global Atmospheric Gases Experiment (AGAGE) GC-MD instruments. Each ambient
air measurement is bracketed by the measurement of fixed loop injection of halocarbons
from an oven containing permeation tubes of halocarbon standards. For compounds not
contained in permeation tubes, a secondary method of calibration utilises calibration
standards, which are generated at the University of Bristol from high concentration
cylinders diluted using a specially developed system. The field standards (contained in
pressurized 35L Essex Cryogenic stainless steel canisters) are inter-compared, before
and after use, by reference to gold standards maintained at the University of Bristol and
Scripps Institution of Oceanography (SIO). Gold standards have a lineage, which is
linked to primary calibration standards maintained at SIO.
Figure 1
The MADS measures 3-liter samples, at a frequency of 60 minutes per measurement.
A list of the currently calibrated halocarbon gas species it measures is given in Table
1. The microtrap consists of a 75 cm long length of ultimetal tubing (PoraPLOT Q),
0.53mm I.D. Cooling of the coiled microtrap (Figure 2) is achieved by holding it in an
anodized block between two triple stage thermoelectric coolers (Peltier coolers).
Figure 2
Species
C2H5I
CHBrCl2
CH2ClI
2-C3H7I
1-C3H7I
CHBr2Cl
CHBr3
CH2I2
Table 1
The flow scheme of the MADS (Figure 3) permits direct concentration and focusing
of analytes prior to GC-MS analysis. The “strip chart” shows (Figure 4) how key
operating parameters are changed over the course of the 60-minute measurement
cycle. A sample is taken from any of 6 pressurized inlets flows (V1) through a
counter-purged Nafion drier before passing through the microtrap (V4) to a filter and
MFC (set to 100 scc/min) where the sample volume is determined before exhausting
to vacuum. After trapping, the microtrap is post-flushed with He from an electronic
pressure controller to remove residual N2, O2, Ar and CH4. The microtrap is then
heated to 210C and the sample is back-flushed into the GC/MS. Separations are
performed on a PoraBOND-Q capillary column, which is ramped from 40C to 220°C
during the main analysis to allow less-volatile analytes to elute sharply.
Figure 3
Figure 4
In routine operation of the MADS, there are substantial investments in optimization
and data collection and processing. The Agilent GC-MS control, tuning and
acquisition software has been completely replaced by custom software under the
Linux operating system which runs both the MADS “front end” and the GC-MS in
selective ion mode (SIM). This software builds upon the base “gccontrol” developed
for the AGAGE program, but now includes the mass/charge ratio as a variable, as
well as the many control and diagnostic parameters of the MADS. Blanks and
instrument linearity’s are measured routinely. This software is continually being
updated in response to the needs of the station operators and the data processors.
An important advance in the MADS is its ability to check its linearity by injecting a
wide range of sample loops (from permeation tubes) or standard gas volumes (from
pressurized cylinder). Such measurements were used throughout the development
process to optimize trapping materials and operating parameters, and they are now
part of routine diagnostics which confirm that the MADS is linear over wide ranges of
sample volume, concentration, and composition. Such linearity and compositionindependence are critical to accurate calibration, especially when propagating
synthetic primary standards or when measuring samples spanning wide concentration
ranges. By using quantifier and qualifier ions for each measured species, the MADS
also offers improved peak identification and immunity from interference by co-eluting
species.
As noted above, using custom AGAGE software has had many benefits, including
complete source-code control over all instrument operation software, integration and
data processing algorithms, and the ability to improve the software interactively. Its
success is borne out by the fact that portions of the software have been adopted by
other leading laboratories engaged in atmospheric and oceanic trace gas
measurements, including NOAA/GMD, CSIRO and the European SOGE program.
Chromatograms are acquired and displayed in real time, and are stored with a 4:1
compression ratio. The software allows the operator to quickly review and batchintegrate chromatograms, and produce time series and diagnostic plots of integration
results to assess instrumental performance.