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PROCESS AND ENVIRONMENTAL APPLICATIONS OF
GAS CHROMATOGRAPHY WITH THE REDUCTION GAS DETECTOR
Marco Succi
SAES Getters S.p.A.,
Milano, Italy
James G. Moncur
Independent Consultant
KEYWORDS: REDUCTION GAS DETECTOR, PPB, PPT, TRACE GAS ANALYSIS, H2, CO, HYDROCARBONS, ETHYLENE OXIDE, ETHYLENE, PROPYLENE, ARSINE, PHOSPHINE,
CARBONYL SULFIDE
ABSTRACT
The Reduction Gas Detector (RGD) is routinely
used for gas chromatographic analysis of volatile compounds that are readily oxidized. The RGD has a tiny
amount of HgO in a heated assembly that reacts with volatile compounds as they pass over this bed of material. This
causes traces of Hg to be released into a low volume flow
cell with photometer for quantitative determination of the
analyte. The traces of Hg are then scrubbed with activated
charcoal. The HgO bed lasts for six months to two years
and is easily changed. Impurities in nitrogen, oxygen, argon, helium, ambient air, carbon dioxide, and ammonia
are readily analyzed by GC as the RGD does not react
with these relatively stable gases. Hydrogen, carbon monoxide, arsine, phosphine, carbonyl sulfide are measured in
ethylene and propylene for catalyst protection purposes.
Ethylene, ethylene oxide, arsine, phosphine, nitric oxide,
ketones, aldehydes, isoprene, 1,3-butadiene, and benzene
are analyzed in ambient air for industrial hygiene and environmental reasons.
This paper discusses examples of GC/RGD
analyses of ambient air and in chemical process monitoring and the comparison between the flame ionization
detector (FID) and the RGD for organic compounds detection.
INTRODUCTION
The Reduction Gas Detector (RGD) is a unique,
ultrasensitive detector developed for the analysis of trace
hydrogen, carbon monoxide and reducing species. In the
last ten years it has been widely used in several applications where its specificity, reliability, and sensitivity has
been a major advantage over more traditional detectors,
such as thermal conductivity detectors (TCD) or discharge
ionization detectors (DID).
This paper discusses some of the various ambient air
and process monitoring1-7 applications of the Reduction
Gas Detector for gas chromatography.
The difference between the RGD and the flame ionization detection (FID) of organic compounds is also
discussed.
OPERATING PRINCIPLE OF THE RGD
The basic operating principle of the detector is
based upon the strong absorption of UV light by mercury
vapor. Compounds eluting from the column pass through
a mercuric oxide bed that is heated. Reducing components, i.e. compounds that can combine with oxygen, react
with the mercuric oxide and release mercury vapor. The
free vapor evolved in the reaction is then detected by
measuring the absorption of UV light in a photometric
cell. The absorption is directly proportional to the concentration of the mercury vapor which is proportional to the
concentration of the reducing analyte from the column.
The following general reaction takes place:
X + HgO (solid) → XO + Hg (vapor)
For H2 and CO the reactions are:
H2 + HgO
→
H2O + Hg
CO + HgO → CO2 + Hg
This technique of sensing mercury vapor lends itself to part-per-trillion (ppt) and part-per-billion (ppb)
detection limits of reducing compounds. Since the detector reacts only to reducing compounds, there is no
interference from the matrix such as O2, Air, He, N2, Ar,
CO2, etc. The actual detection limits are related to the
reaction efficiency of different compounds in the mercury
oxide bed as well as the sample volume injected into the
column, the path length of the photometer cell, and the
signal to noise level achieved through the system.
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A cross sectional drawing of the detector is shown in
Figure 1. The UV source is a mercury lamp with an optical filter and a reference photo diode detector to
monitor lamp intensity fluctuations at 254 nm.
Figure 1. Reduction Gas Detector schematic.
The rest of the light travels via the light guide
through the cell and out the other light guide through the
254 nm optical filter to the signal photo diode detector.
The column effluent enters the HgO reaction bed which is
heated to about 265°C, where the reducing compounds
react and release mercury vapor. The light intensity will
be different between the two photo diodes, depending
upon the amount of mercury vapor in the cell. Signal
processing electronics measure the difference between the
two photodiodes, automatically canceling lamp flicker
noise, and displaying the mercury vapor concentration.
This is directly proportional to the concentration of a reducing compound eluting from the column system.
The low level of mercury vapor in the carrier gas
flowing from the photo cell is trapped with a scrubber
before entering the ambient atmosphere.
For the ultimate detection limit, it is important to
have a carrier gas with a reliable negligible concentration
of reducing species to optimize the detector signal to noise
ratio.
ENVIRONMENTAL APPLICATIONS
H2 and CO Analysis in the Atmosphere
The RGD is ideally suited for the determination
of ppb to part-per-million (ppm) levels of H2 and CO in
atmospheric studies. CO is involved in ozone depletion,
even at very low concentrations.
The RGD was mounted in an aircraft and was
used by Dr. Paul Crutznen for his analyses of CO in the
upper stratosphere. In these analyses, the Reduction Gas
Detector is used to provide rapid and reproducible quantitative measurements 8,9.
This system was designed for trace levels of H2
and CO in the atmosphere, where there is a possibility of
other high molecular weight pollutants that could interfere
with the determination.
Air carrier gas is preferred because it gives a
minimum of upset in the baseline from the injection, is
inexpensive, and readily available.
A 10-port valve is used to inject the sample from
the sample loop to the stripper column. Lighter compounds, such as H2 and CO, elute quickly from the
stripper column into the analytical column, while higher
molecular weight components such as hydrocarbons and
moisture elute very slowly from the stripper column. After the light components have had sufficient time to elute
from the stripper column into the analytical column, the
10-port valve returns to the load position. In addition to
preparing the analyzer for the next sample injection, this
valve position is used to backflush the higher molecular
weight components from the stripper column to vent.
Backflushing of the stripper column accomplishes two objectives. It shortens the analysis time and it
vents contaminants which can produce an unstable baseline away from the detector. Oxygen and nitrogen from
the air will elute through the analytical column. However
these components do not interfere with the determination
of H2 and CO because of the chromatography and the selectivity of the RGD.
Ethylene Oxide in Ambient Air
The presence of ethylene oxide in ambient and
indoor air should be monitored at less than the OSHA
regulatory limit of one ppm to reduce the possibility of
cancers in workers exposed to it.
A Reduction Gas Detector, with appropriate injection system and columns, responds to ethylene oxide at
20-30 ppb in ambient air (Table I and Figure 1).
In the RGD determination of ethylene oxide, a
10-port valve and a 1 mL sample loop is in the flow path
of continuously flowing ambient air. The valve rotates to
inject the sample to a stripper column, (inorganic phase).
Light components, such as oxygen, are diverted from the
stripper column to vent using a 4-port valve located
downstream from the 10-port valve. After the oxygen
elutes from the stripper column, this 4-port valve is programmed to return to its original position to transfer
ethylene oxide to the analytical column (HayeSep). In this
way, the 10-port valve and the 4-port valve divert the
oxygen, carbon monoxide, and hydrogen to vent and away
from the analytical column and RGD detector. The 10port valve is programmed to return to its original sampling
position once the ethylene oxide has been transferred to
the analytical column, and at the same time, puts the stripper column in backflush mode to prevent the build up of
moisture. The total cycle time between sequential injections in this work is 500 seconds (8.3 minutes).
Table I summarizes the results obtained generating different concentrations.
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Table I EtO in Air Analytical Data
EtO (ppb)
Peak Area
2100
481,313
2100
468,380
2100
450,875
700
149,658
700
146,872
700
155,444
50
11,568
50
13,864
50
10,856
Average
%SD
466,856
3.27
150,658
2.90
12,096
13.00
Figure 2 shows a typical spectrum with less than
1 ppm EtO.
A 10-port valve is used to inject sample from the
sample loop to the stripper column. Lighter compounds
such as H2 and CO, elute quickly from the stripper column
into the analytical column while higher molecular weight
components, such as hydrocarbons (principally ethylene
and propylene) and moisture remain in the stripper column. After the light components have had sufficient time
to elute from the stripper columns into the analytical column, the 10-port valve returns to the load position. In
addition to preparing the analyzer for the next sample
injection, the valve in this position is used to backflush the
higher molecular weight components from the stripper
columns to vent.
As mentioned previously, backflushing of the
stripper column to vent shortens the analysis time and
vents contaminants which can produce an unstable baseline.
Figure 3 shows a typical sample of propylene
when the backflush is set after the ethylene peak; in the
case of high ethylene concentrations, the backflush should
be set before the ethylene peak to avoid excessive
consumption of the mercury oxide bed.
Figure 2. Typical chromatogram of about 0.8 ppm EtO in
air.
PROCESS APPLICATIONS
H2 and CO in Ethylene or Propylene
Making polyethylene or polypropylene from ethylene or propylene requires a catalyst, such as the ZieglerNatta polymerization catalyst. The performance of this
catalyst is adversely affected by CO, and it is of interest to
determine this component at the ppb levels. In this analysis, the RGD is used to provide rapid and reproducible CO
measurements 10.
Unsaturated hydrocarbons give strong response
in the RGD. When ethylene or propylene is a major component in the sample, it can overload the RGD as well as
the analytical column, producing a long tailing peak that
requires a long time to return to baseline. This application
package is designed to overcome interference from the
major unsaturated component by using special plumbing
and column configuration.
Nitrogen carrier gas is used because it gives a
minimum of upset in the baseline from the injection, is
inexpensive, and readily available.
Figure 3. Propylene sample with trace level of H2, CO,
and ethylene.
The RGD is also sensitive to phosphine, arsine,
and carbonyl sulfide, additional impurities that can adversely affect catalyst performances, as shown in Figure 4.
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Even better results, in terms of sensitivity, can be obtained for less reactive matrix gases such as Ar, N2, H2,
O2, and CO2. Figure 7 shows a chromatogram of about
1 ppb H2 and CO in a sample of ultra pure nitrogen 11-13.
CO
150
COS
100
PH3
18 ppb
50
50,000
1 ppb
AsH3
27 ppb
Detector Auto Zero
CO
49,000
Normal Valve Pressure Sw itch
48,000
RGD Signal (counts)
RGD millivolt response
200
0
0
100
200
300
retention time (sec)
H2
47,000
46,000
45,000
Figure 4. CO, PH3, AsH3, and COS in propylene
44,000
H2 and CO in Ammonia
Ammonia purity used in the production of light
emitting diodes or diode lasers is becoming more and
more important to a successful manufacturing operation.
Setting the injection system in a way similar to
the one mentioned previously, letting H2 and CO pass
over a stripper column and trapping NH3, allows the
analysis of H2 and CO at trace level to be easily performed.
As shown in Figures 5 and 6, the calibration performed generating various concentrations in the
10-1000 ppb range provided good linear results and allowed detection in the range of 10 ppb.
peak area (counts)
1.E+05
y = 79.265x
R2 = 0.999
8.E+04
6.E+04
4.E+04
2.E+04
0.E+00
0
200
400
600
800
1,000
1,200
inlet concentrations (ppb)
Figure 5. H2 calibration in an ammonia sample.
3.E+06
peak area (counts)
3.E+06
y = 3569.5x
R2 = 0.9994
2.E+06
2.E+06
1.E+06
5.E+05
0.E+00
0
100
200
300
400
500
600
700
concentration (ppb)
Figure 6. CO calibration in an ammonia sample.
800
900
43,000
0
50
100
150
200
250
300
Tim e (seconds)
Figure 7. One ppb H2 and CO in ultra pure N2.
CONCLUSIONS
Some applications of a Reduction Gas Detector
for gas chromatography have been briefly discussed.
Even if the RGD is not commonly used for the
detection of organic compounds, it offers several advantages over the more traditional FID, such as a sensitivity
about 50-100 times better and the lack of a requirement
for service gases such as air or hydrogen.
The data presented in this paper were obtained
with packed column; however, the low carrier gas flow
rate required for its operation, 10 mL/minute, makes it
suitable for use in combination with capillary columns,
therefore allowing use for other applications.
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