MEDUSA Gas Chromatography with
Mass Spectrometry (Medusa GC-MS)
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 A photograph of the AGAGE Medusa GC-MS system,
showing the thermostatted enclosure that contains its valves, integrating
precision flow controller, and other flow components appears in left (Figure 1).
Also in this figure is an inset showing the interior of the Medusa’s
cryogenic vacuum chamber.
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Figure 1, Front
view of the Medusa sample module mounted on an Agilent 5973 GC-MS, with the two
front service panels at the module removed. Temperature and Valco valve
controllers are mounted on the left. The opening in the center shows the
cryogenic vacuum chamber, and the insert shows the interior of this chamber with
the two traps mounted on the copper cold plate. The opening on the right shows
the thermostated enclosure housing the Valco valves and other flow components.
The ADS GC-MS system described
earlier have been extremely successful as the first automated GC-MS instruments
to be deployed long-term at remote stations. Because it depends on Peltier
thermoelectric devices for cooling, trapping of analytes with the ADS system can
not be done below a temperature of -500C. At this temperature there are
unavoidable trade-offs in the design of the trapping system. Increasing trap
size and the amounts of absorbents reduces the bleed-through of the most
volatile analytes and increases the size of air samples that can be analyzed.
However, with large amount of absorbents in the trap it also becomes
increasingly difficult to maintain sharp and reproducible injections into the
GC-MS over the wide range of volatilities of the compounds being measured. The
current ADS preconcentration system represents an optimal solution within these
constraints, but because of its limited trapping temperature, we have been
unable to find absorbents that would permit the system to retain quantitatively
the most volatile perfluorinated compounds that are of importance climatically
and in the context of the Kyoto Protocol.
In order to address these
concerns, the SIO AGAGE laboratory, with strong input from the University of Bristol group, has developed a new GC-MS
cryogenic preconcentration system, which we call Medusa (Figure
1) after the appearance of
its early prototypes. At
the heart of the Medusa is a Polycold "Cryotiger" cold end which maintains a
temperature of
-1750C,
even with a substantial heat load, using a simple single-stage compressor with a
proprietary mixed-gas refrigerant. This cold end conductively cools multiple
traps to about
-1650C.
By using standoffs of limited thermal conductivity to connect the traps to the
cold head, each trap can independently be heated resistively to any temperature
from -1650C
to +2000C
or more, while the cold end remains cold. The use of two traps with
extraordinarily wide programmable temperatures ranges, coupled with the
development of appropriate trap absorbents, permits the desired analytes from
2-liter air samples to be effectively separated from more-abundant gases that
would otherwise interfere with chromatographic separation or mass spectrometric
detection, such as N2, O2, Ar, H2O, CO2,
CH4,
Kr and Xe. Importantly, the dual traps also permit the analytes to be purified
of interfering compounds by fractional distillation and re-focusing from the
larger first-stage trap (T1) onto a smaller trap (T2) (Figure
2) at very low temperatures,
so that the resulting injections to the Agilent 5973 GC-MS are sharp and
reproducible. By trapping and eluting analytes at very low temperatures, the
range of compounds that can be measured is greatly extended to include a number
of important volatile compounds, and problems with reaction of analytes on the
traps at higher temperatures are avoided. The Medusa system uses a
high-precision integrating mass flow controller (MFC) for improved measurement
of sample volumes. In addition, significant advances have been made in the
software to control and acquire data from the Medusa and the GC-MS itself, so
that the entire system has programmability, versatility and ease of operation
comparable to that of the AGAGE GC-MD instruments.
As with the
ADS, the Medusa measures 2-liter air samples, but at a
frequency (60 minutes per measurement) that is twice that of the ADS. A
list of the 40 trace gas species it currently measures, ranging in concentration
from a few tenths of a ppt (part-per-trillion) to about 600 ppt, is given in
Table 1, together with typical measurement precisions. The two traps, T1 and T2
(Figures 2a and 2b) are packed with HayeSep-D, a relatively inert
high-surface-area polymer, but the less-volatile analytes never contact this
adsorbent because they are trapped in the wraps of
-1650C
open tubing before they reach the absorbent and are subsequently transferred by
backflushing.
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(b) |
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Figure 2.
Top view
(a) and front view (b) of the Medusa cryogenic vacuum chamber with
the cover removed. The two traps T1 and T2 mounted to the continuously cooled
copper baseplate using thin aluminum thimbles which permit the two traps to be
heated independently while the baseplate remains cold.
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(a)
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(b)
Figure 3.
The flow scheme of the
Medusa (a) allows samples from the 6 inlets on the left to be dried,
trapped and fractionally distilled using the two traps, and then injected for
GC-MS analysis. EPCs are electronic pressure controllers, the Vs are multiport
valves, the MFC is a mass flow controller, and the MSD is a mass-selective
detector. The "strip chart" (b) shows how pressures, temperatures and
sample flow are controlled during the trapping and analysis.
The flow scheme of the Medusa (Figure
3a) permits several stages
of analyte purification and refocusing prior to GC-MS analysis. The "strip
chart" (Figure
3b) shows how key operating parameters are changed over the
course of the 60-minute measurement cycle. A sample from any of 6 pressurized
inlets flows through two counterpurged Nafion driers before passing through T1
to a filter and MFC (set to 100 scc/min) where the sample volume is determined
before exhausting to vacuum. After trapping, T1 is postflushed with He from
electronic pressure controller EPC-4 to remove residual N2, O2, Ar and CH4.
T1 is then warmed to
-800C
and CF4
is transferred to T2 using He, being careful to leave most CO2 behind on T1. T2 is then postflushed at about
-1250C
to further reduce N2,
O2
and Ar. Then T2 is heated to +1000C and CF4
is backflushed into the GC-MS through a MS-4A and HiSiv-3000 packed micro-precolumn
which separates CF4
from residual O2, N2, Ar, Kr, and CO2. While
CF4
is analyzed, T1 is further postflushed at about
-700C
to reduce CO2
and Xe. T1 is then heated to +1000C and backflushed with He to transfer the
remaining analytes to T2, which is then postflushed at
-650C
to further reduce CO2 and Xe. Finally, T2 is again heated to +1000C and the remaining analytes
are backflushed into the GC-MS, this time bypassing the precolumn which is
backflushed to vent. Separations are performed on a Porabond-Q capillary
column, which is ramped from 400C
to 2000C during the main analysis to allow less-volatile analytes to elute
sharply.
In routine
operation of the Medusa systems, there are substantial investments in
optimization and data collection and processing. The Agilent GC-MS control,
tuning and acquisition software was not up to this task, and has been completely
replaced by custom software under the Linux operating system which runs both the
Medusa "front end" and the GC-MS in selective ion mode (SIM). This software
builds upon the base "gccontrol" developed earlier for the GC-MD, but now
includes the mass/charge ratio as a variable, as well as the many control and
diagnostic parameters of the Medusa. Blanks and instrument linearities are
measured routinely. This software is continually being updated in response to
the needs of the station operators and the data processors. As in the case of
the GC-MD, the Medusa instruments can now be run from anywhere in the world, and
the data are available to all AGAGE investigators.
A number
of significant technological hurdles had to be overcome to make the Medusas
operational. The design of traps T1 and T2 to have good thermal connection to
the cold end baseplate while retaining the electrical isolation necessary for
low-voltage high-current resistive heating was overcome by anodizing the Al
thimbles on which they are wrapped, and by using thermally conductive silicone
paste. The Cryotiger cold ends have an inherent problem with oil vapor blockage
which has been greatly ameliorated, but not eliminated, by working closely with
the manufacturer to install oil traps in the refrigerant lines. Because of the
importance of measuring CF4, a great deal of effort has gone into
reducing CO2 interferences with this measurement, and we believe the
problem has now been solved. Because of the high rate of Medusa standard gas
consumption (24 liters/day), a quaternary level of whole-air calibration gas was
added to the normal tertiary level of calibration used by the GC-MD
instruments. The quaternary working gases are calibrated over the course of
their use in the field by analyses against the tertiary standards sent from SIO
for both the Medusas and GC-MDs at each station. Otherwise, the calibration
hierarchy is the same as is used for the GC-MDs (Prinn, Weiss, et al.,
2000), including the practice of alternating ambient air and calibration gas
analyses to obtain the highest precision measurements.
An important advance in the Medusa is its ability
to check its linearity by injecting a wide range of standard gas volumes. 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 Medusas are linear over wide ranges of sample volume,
concentration, and composition. Such linearity and composition-independence
are critical to accurate calibration, especially when propagating synthetic
primary standards or when measuring samples spanning wide concentration ranges.
As
Table 1 demonstrates, the Medusa systems are
producing exceptional routine precisions which rival those of the GC-MD systems
for the more abundant species, and greatly exceed them for the less abundant
ones. By using quantifier and qualifier ions for each measured species, the
Medusas also offer improved peak identification and immunity from interference
by co-eluting species. This is especially important in the case of methyl
chloroform, which should be measurable down to ~1 ppt with a precision of ~2%,
and thus remain valuable in determining global OH for roughly another two
decades.
The original ADS GCMS instruments
at Mace Head and Cape Grim were retired in December 2004. The new generation of
Medusa GCMS began routine operation in November 2003 at Mace Head and January
2004 at Cape Grim. Three additional Medusa GCMS instruments were also installed and
started routine measurements at Trinidad Head, California and Barbados in May
2005, and at American Samoa in May 2006, respectively.
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