United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-041 July 1981
Project Summary
Technique for Measuring
Reduced
-orms of
Sulfur in Ambient Air
Robert S. Braman and J
A new technique for measuring low
concentrations of volatile sulfur com-
pounds in ambient air is
The technique consists of
tration of sulfur compoun
isorption on gold metal co ited sand or
gold foil surfaces followec
mes M. Ammons
discussed.
ireconcen-
s by chem-
by thermal
desorption, separation, and detection
by flame photometry. Breakthrough
capacities are on the order of 1 micro-
gram total sulfur compounds. The
unique aspect of this research is the
derivatization of thiol type compounds
principally H2S and CH3SH, by ethyl
iodide for partial separation of H2S,
DMS, SOz. COS, and CH3SH. Best
suited for the 0.01 - 5 ppb range, the
technique has been used to detect
trace sulfur compound concentrations
as low as 0.001 ppb by volume with
100L and larger sample volumes.
Sample size detection limits depend
upon the type of flame photometric
detector used but are generally in the
0.1 ng range. Repeatability of mea-
surements is ±5-8% relative standard
deviation. Accuracy depends upon the
compound. DMS and SO2 are detected
as individual compounds. H2S, CS2
and COS are detected as a single
compound on gold foil. Methylmer-
captan and dimethyldisulf ide appear
as a single compound. Field studies at
two sites. Cedar Island, N.C. and
Prairie View, Texas, have demonstrated
that the technique is practical for field
use and for determining vertical pro-
files up to 10 meters above ground
level. Hydrogen sulfide, dimethylsul-
fide, sulfur dioxide and carbonylsulfide
were the principal reduced forms of
sulfur detected at these sites.
This Project Summary was devel-
oped by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park. NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The most widely used commercial
instrumentation for sulfur compounds
in air employs the methods of Stevens,
Mulik, O'Keeffe and Krost (1) and
Stevens, O'Keeffe and Ortman (2) or
modifications and improvements there-
of. These methods are readily applicable
for direct sulfur analyses at or above 1
ppb, and capable of separating several
sulfur compounds in air in GC applica-
tions. Ambient concentrations of H2S,
(CHafeS and CH3SH compounds in non-
polluted locations are in the 0.01 -1 ppb
range. Consequently, these methods
have only marginal detection capability
and oxidation losses of H2S and CH3SH
can be substantial.
This study was undertaken to further
explore the use of gold surface adsorp-
tion as a preconcentration approach. In
the improved methods reported here
reduced sulfur compounds are adsorbed
onto gold metal coated silica sand or
gold foil. Chemisorbed and preconcen-
trated sulfur compounds are derivatized
to a Iky I sulf ides by ethyl iodide, desorbed
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by heating, separated on a short column
and then detected by means of a flame
photometric detector.
Preconcentration by means of gold
coated silica sand was developed first,
studied for interferences and used in
the two field studies reported here.
Nitrogen dioxide and ozone interfere if
large sample volumes over 20-30 liters
are used or if present in elevated con-
centrations. These interferants are
removed by use of a sodium carbonate
filter.
The method was used in field studies
at Cedar Island, North Carolina and at
Prairie View, Texas where H2S, DMS,
SOz and COS were the major reduced
compounds of sulfur detected. Diurnal
variations and vertical profiles of sulfur
compounds are reported.
After the field studies, further methods
development was carried out including
investigation of gold foil preconcentra-
tion and investigation of high molecular
weight secondary amines for COS and
CS2 preconcentration. Gold foil precon-
centration was found to be less sensitive
to oxidation than gold coated silica. The
use of high molecular weight amines for
COS and CS2 preconcentration was not
successful.
Preconcentration Tube
Analysis
Preconcentration tubes adsorb SOz,
H2S, CH3SH (CH3)2S, COS and other
sulfur compounds from air quantitatively
at the flow rates used in sampling.
These compounds are desorbed from
the tubes in a two step analysis, trapped
in a short liquid nitrogen cooled U-trap,
separated during the heating of the U-
trap and detected by means of a flame
photometric detector. Figure 2 shows
the analysis apparatus arrangement.
Separation of sulfur compounds is
accomplished in the U-trap part of the
analysis system during heating.
Filter
The U-trap is constructed of 6 mm o.d.
borosilicate glass and is 15 inches long
packed with OV-3, 15% on Anakrom C-
22, 40/50 mesh, Analabs, Inc., New
Haven, Conn.
The FPD was constructed using an
interference filter, quartz burner and
MacPhearson Co. monochrometer. A
logarithmic amplifier was used to lin-
earize detector response to sulfur on
one of the two analyzers assembled.
The first desorption step involves
derivatization of sulfur compounds on
the preconcentration tube. Ethyl iodide
vapor is injected into the nitrogen car-
rier gas stream with the liquid nitrogen
p
f*
-
6 mm O.D.
Quartz
\ Polystyrene
Analytical Procedures
Sampling
Air sampling was accomplished using
quartz absorption tubes packed with
gold metal coated white beach sand.
Tubes were 10 mm o.d. and 20 cm long
having a 3 cm packed section as shown
in Figure 1. The sand was gold coated by
first mixing the sand with auric chloride,
and then taking it to dryness with
stirring on a hot plate. This was followed
by hydrogen reduction at elevated
temperature. Enough gold chloride was
used to give a 5%-10% gold coating by
weight on the sand. The packed section
was in 2 1cm lengths separated by
quartz wool to prevent cracking of the
quartz because of thermal expansion
during heating in the analysis procedure.
Flow rates through a filter and absorp-
tion tubes were generally in the 1-3
L/mn range and were determined for
each set of sampling pump, tube and
filter used in air sampling.
Air sampling was generally done at 2
meters above the ground. In certain
instances sampling was done with
towers at several distances above the
ground at generally, 10, 5, 2, 1,0.5 and
0.2 meters.
Figure 1. Preconcentration tube design.
Hi-Air Burner
Sampling Tube "" —I
PTFE Connector T° autotransformefi
Adaptor /
To autotransformer
Cold-trap
PM
Module
Filter
To Amplifier and
Strip Chart Recorder
Dewar
Figure 2. Analysis apparatus arrangement.
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Dewar in place and passes through the
preconcentration tube which is then
heated to 300-400°C for 2-4 minutes.
Ethyl iodide, far in excess of the sulfur
present, is adsorbed on the gold surface
and the excess is collected in the cold U-
trap. Upon heating, some of the ethyl
iodide converts the RSH and H2S present
to the ethyl derivatives, likely by two
steps.
C2HSI + Au - Aul" + CaH6+
2 C2H5+ + H2S - (C2Hs)zS + 2H+
Aul~ + H+ - HI • Au
Alkylthiols are converted to the ethyl
derivative by the same reaction.
C2H5I + C4H9SH - C4H»SC2H5 + HI
Sulfur dioxide and dimethylsulfide do
not react. Carbonyl sulfide and carbon
bisulfide, if present and adsorbed on the
gold, are also converted to diethyl sul-
fide (DES).
In natural samples COS is observed to
be adsorbed onto the quartz wool part of
the preconcentration tube where it is
not converted in the alkylation process.
It is removed from the quartz wool
during the first heating step and is
recovered as COS in the analysis of the
U-trap.
The second step in analysis deter-
mines S02. The carrier gas is switched
to hydrogen and the U-trap is cooled.
The preconcentration tubes are then
heated to 600-700°C. Sulfur dioxide
still adsorbed onto the gold is converted
to H2S which is desorbed and trapped by
the U-trap. The U-trap is allowed to
warm up, and H2S is detected.
Response calibration is done using
permeation tubes (for SOa,"H2S, DMS)
and an air mixing manifold. Permeation
rates determined gravimetrically are
corrected for temperature effects. Re-
sponse curves are prepared by absorbing
known amounts of the low concentra-
tions of diffusion tube standards onto
blanked preconcentration tubes followed
by their analysis.
The typical reproducibility of standards
on absorption tubes has been deter-
mined from several sets of standardiza-
tion experiments and was found to be ±
4.5 - 5% for DMS, ± 8 -10% for SO* all
in units of relative standard deviation.
Gold Foil Preconcentration
Because of low recoveries of H2S
occasionally noted when using Au
coated sand, work was initiated using
gold foil packed tubes.
Gold foil tubes gave 50% higher
response than the Au coated sand
indicating a lower loss of H*S. Because
of the better response of the gold foil
tubes several were prepared and used
in subsequent experiments. The detec-
tion system was recalibrated using a
gold foil tube as the preconcentration
standard tube. The resulting calibration
curves had less point scatter than
previous curves using the gold coated
sand tubes, especially for small sample
sizes in the 0.5-10 ng range.
Dimethyl sulfide seems to be quite
stable towards oxidation loss of sample.
Both the gold foil and gold sand tubes
exhibited good scrubbing and recovery
efficiencies. No DMS was detected on
the quartz wool and silica tubes and
essentially all of the DMS was observed
to pass.
Gold foil tubes under ambient sam-
pling conditions have given reproducible
results. Standards checked on the foil
tubes following field use have consist-
ently been on the calibration curve
showing outside use dots not degrade
the tubes. After about eight months and
approximately 500 analyses one original
Au foil tube showed a decrease in re-
covery of about 15%. The foil was
removed and placed in a new quartz
holding tube. The response returned to
normal. Microscopic examination of the
now somewhat cloudy original quartz
holding tube showed that the surface
had become etched. It is believed that
the vastly increased surface area due to
the etching caused partial loss of the
sample in a mechanism similar to that
observed for the silica tube.
A set of three gold foil tubes were
used for 15 samples of H2S of 4.0 ng
each. The repeatability of analyses was
4.5% r.s.d. This rose to approximately
8% near the detection limit. The detec-
tion limit as 2 x RMS noise for H2S using
the foil tube was 0.115 ng H2S as H2S.
For DMS it was 0.33 ng as DMS.
Conclusions and
Recommendations
The preconcentration of reduced
compounds of sulfur on gold metal
surfaces has been studied, improved
and applied in field analyses. The major
improvement made was the addition of
a derivatization step in the procedure.
The derivatization of thiol compounds
by ethyl iodide eliminates sulfur dioxide
and sulfate interferences in preconcen-
tration prior to desorption and analysis.
Sample volumes up to 50 liters can be
preconcentrated with minimal inter-
ference from ozone or nitrogen oxides
copresent in air so long as sodium
carbonate scrubbers are employed.
Carbonyl sulfide is collected and identi-
fied separately on gold coated sand but
carbon disulfide is derivatized and not
distinguished from hydrogen sulfide.
On gold foil H2S, CS2 and COS all appear
as the same derivative. Within these
limitations the method is useful for
preconcentrating H2S, (CHa)2S, COS,
CHaSH and CH3-S-S-CH3 at concentra-
tions as low as 1 ppt.
Gold foil preconcentrators appear to
have the advantage of easier adaptability
to automated use than the gold coated
sand tubes.
Polymeric secondary amines of several
types preferentially absorb COS and CS2
while not absorbing HaS. Unfortunately,
thermal desorption of the COS and CS2
is incomplete and the approach was not
successful.
Field use of the method at Cedar
Island, N.C. and Prairie View, Texas
indicated that COS, H2S, S02 and DMS
exhibit a diurnal variation with increased
production in salt marshes during the
day. These sulfur compounds are pro-
duced in the ground over land and dif-
fuse into ambient air.
Although suitable for qualitative ex-
amination of vertical profiles, the sam-
pling time required and precision of the
method are probably not sufficient for
accurate calculation of dry deposition
rates.
Further study of the analytical ap-
proach should include modification to
selectively separate and determine COS
and CS2.
Gas to particle conversion at the
ground level in air and further verifica-
tion of the diurnal variation at locations
other than over salt marshes are also
needed.
References
1. R.K. Stevens, J.D. Mulik, A.E.
O'Keeffe, and K.J. Krost, Anal.
Chem., 43, 827(1971).
2. R.K. Stevens, A.E. O'Keeffe, and G.C.
Ortman, Environ. Sci. Technol., 3,
6.52(1969).
« UAOOVCRNtKNT mOTM OFFICC: 1»1 .757-012/7215
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Robert S. Braman and James M. Ammons are with the University of South
Florida, Tampa. FL 33620.
William McClenny is the EPA Project Officer (see below).
The complete report, entitled "Technique for Measuring Reduced Forms of
Sulfur in Ambient Air," (Order No. PB 81-179 772; Cost: $8.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United'States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
. PS 0000329
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