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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