United States
Environmental Protection
Agency
Research and Development
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park, NC 27711
EPA/600/SR-92/050 April 1992
EPA Project Summary
Evaluation of the Fourier
Transform Infrared Spectrometer
for Particle-Associated
Ammonium Sulfate
Determination
Kenneth John Krost
A commercial Fourier transform In-
frared (FTIR) spectrometer, modified for
automated analysis of particle-associ-
ated sulfate, was used to obtain trans-
mission spectra of samples of partlcu-
late matter collected from the ambient
air onto Teflon filters. An evaluation of
this Instrumentation was undertaken to
determine its applicability for measur-
ing sulfate and the correlation of re-
sults with those of ion chromatogra-
phy and X-ray fluorescence. Particle
samples from six geographical areas
were evaluated in this study. Results
from different filter sets, for which Indi-
vidual filter blanks were used, show
values of the coefficient of determina-
tion of 0.90 or higher using any two
analytical techniques, with one notable
exception. This exception Is due to dis-
placement of sulfate absorption from
the wavelength Interval used for analy-
sis, and it apparently indicates the ef-
fect of co-collected compounds in
chemically shifting the spectra. For the
best analytical results, the spectrum of
Individual Teflon filters should be used
for subtracting the contribution of Te-
flon absorption. Orientation of the filter
in the FTIR system must be maintained
to avoid errors In determining the zero
loading values of sulfate. The detec-
tion limit of 1.2 ng/cm2was determined
by establishing the uncertainty (3o)
value for replicate measurements of
blank filters. This corresponds to the
sample loading obtained by sampling
an ambient air concentration of approxi-
mately 0.4 jig/m3 of ammonium sulfate
collected over 24 hours as the fine par-
ticulate fraction in a dlchotomous sam-
pler.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search Triangle Park, NC, to announce
key findings of the research project
that Is fullly documented In a separate
report of the same title (see Project
Report ordering Information at back).
Introduction
There are several techniques available
to measure or estimate sulfate concentra-
tions of the various particulate species
collected on air filters. Among these tech-
niques are Ion chromatography (1C), X-
ray fluorescence (XRF) and Fourier trans-
form infrared (FTIR) transmission spec-
trpmetry. Of these three the FTIR tech-
nique is the only one that can provide an
unambiguous, nondestructive analysis of
sulfate. For this reason and because other
functional groups can also be measured,
EPA has a continuing interest in this tech-
nique and has pursued the development
and evaluation of instrumentation and pro-
cedures appropriate for optimization. In
particular, a: commercially available FTIR
spectrometer was previously modified and
demonstrated for the automated analysis
of particle-associated sulfate by transmis-
sion spectrometry. In the current report,
sample analysis results for sulfate by the
FTIR technique are compared with those
using XRF and 1C analysis for samples
obtained at six different field sites.
Printed on Recycled Paper
-------
Procedure
A commercial (FTIR) spectrometer,
modified with the addition of an automatic
sample-changing compartment, was ap-
plied to the measurement of sulfate in the
form of ammonium sulfate. The sample
compartment directly accepts the sample
carousel from a Sierra/Anderson, Inc.
Model 245* dtehotomous air sampler. To
perform an analysis, the spectrum of a
blank Teflon filter is recorded and subse-
quently subtracted from the spectrum ob-
tained after loading the filter with particu-
lato matter. This results in a spectrum due
solely to absorption by substances in the
partfculate matter. Software was written to
analyze ambient air samples by automati-
cally providing an absorbance spectrum
with optimal background subtraction of the
blank Teflon filter contribution and inte-
gration of the absorbance peak over the
595- to 635-cnv1 spectral range used for
sulfate detection and quantitation. The
sample filters were 37-mm diameter, 2
u.m pore size PTFE Teflon (Gelman Sci-
ences, Incorporated, Ann Arbor, Ml),
mounted on a polypropylene ring holder.
Results and Discussion
The inherent detection sensitivity for the
current instrumentation and procedure was
determined to be 1.2 ng/cm2of particle -
associated ammonium sulfate. The error
in ammonium sulfate measurement result-
ing from a rotation of blank filters by 90°
averaged 0.7 ng/cm2for six filters while
slight vertical displacement of the filters
had negligible effect.
Sets of 10-11 particulate (<2.5-u.m)
samples wore collected from five sites in
fivo states in the eastern U.S.A. and ana-
lyzed for sulfate concentrations by FTIR,
XRF, and IC. For the most part, good
agreement among the three systems was
achieved. For four of the state sites, the
linear least squares fitting (y=mx+b) of
data between FTIR-determined values (Y)
and IC-determined values (X) gave slopes
at ±95% uncertainty values of 1.04 ± 0.08
(NY), 1.08 ± 0.04 (VA), 0.77 ± 0.13 (TN)
and 0.93 ± 0.07 (KY), while the coefficient
of determination exceeded 0.90 in each
case and averaged 0.97. Figure 1 shows
a typical comparison using samples taken
at a NY site. Figure 2 shows the equiva-
lency obtained for four of the five sites
when the values from FTIR measurements
were plotted against 1C values. As seen
the agreement is quite good. The one
noticeable exception to good agreement
existed for the samples collected at a site
in Massachusetts for which the slope val-
ues were 0.19 ± 0.23 with a coefficient of
determination of 0.18. In this set FTIR
results were often significantly different
from those obtained from the IC and XRF
analyses. The FTIR spectra for this set of
filters were closely examined. In every
instance where a major discrepancy ex-
isted in the reported sulfate values, a dis-
placed and skewed sulfate absorption band
was evident. Similar spectral effects have
been produced in the laboratory when high
humidity samples were collected, exposed
to water vapor and analyzed. There was a
noticeable loss of peak symmetry and lack
of absorption at 870 cm'1. The presence
of bisulfate in the sample can also cause
a change in the spectral features being
used for sulfate determination; however
bisulfate was not present in the samples
from Massachusetts. Intercept values other
than the Massachusetts filter set for the
FTIR/IC comparative studies were all less
than 6.50 u.g/cm2, and averaged -2.02 u.g/
cm2.
Conclusions
FTIR transmission spectrometry was
shown to be a viable technique for mea-
surement of sulfate in the form of ammo-
nium sulfate, such as that occurring in c
collected ambient air particulate matter.
The detection limit of 1.2 ug/cm2 is equiva-
lent to the loading obtained by sampling
an ambient air concentration of approxi-
mately 0.4,u,g/m3 of ammonium sulfate
collected over 24 hours as the fine par-
ticulate fraction in a dichotomous sampler.
This detection limit is low enough to allow
measurement of background ammonium
sulfate concentrations (these are routinely
above 3 u.g/m3). Intercomparison of the
FTIR method with X-ray fluorescence and
ion chromatography show values of the
coefficient of variations of 0.90 or better
using any two analytical techniques, with
one exception. This exception is due to
shifting of absorption features caused by
co-collected compounds or to the pres-
ence of ammonium bisulfate. In either case
this problem is easily identified by exami-
nation of the individual spectra.
Intercept =7.38 ± 1.44 -
Slope = 0.85±0.05 _
Pf = 0.985
!
0 5 10 15 20 25 30 35 40 45 50
Intercept = -2.93 ± 2.23 -
Slope » 1.22 ±0.10
Ff = 0.985
-5 0 5 10 15 20 25 30 35 40 45 50
XRF-pg/crr?
50
45
40
35
30
25
20
15
10
5
0
Intercept = -1.37 ± 2.05-
Slope= 1.04 ±0.08 _
P? = 0.985
-505 10 15 20 25 30 35 40 45 50
IC—^g/crrf
Figure 1. New York intercomparative
analysis of particulate sulfate.
Calculated for± two standard
errors.
' Mffttbn of trtcSa names or common:!*! products does not
cootltut* •odoreoment or rocommondalkxi for use.
-------
Kentucky
O New York
Tennessee
Virginia
Intercept = 0.12 ± 1.05
Slope = 0.91 ± 0.05
Ff = 0.962
Calculated for ± Two Standard Errors
Figure 2. Results from sulfate analysis for samples from composite of Kentucky, New York, Tennessee, and Virginia.
•fru.S. GOVERNMENT PRINTING OFFICE: 1992 - 648-080/40234
-------
The EPA author Kenneth J. Krost (also the EPA Project Officer, see below) is
with tho Atmospheric Research and Exposure Assessment Laboratory,
Triangle Park, NC 27711
The complete report, entitled "Evaluation of the Fourier Transform Infrared
Spectrometer for Particle-Associated Ammonium Sulfate Determination,"
(Order No. PB92-160084/AS; Cost: $19.00, 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:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/SR-92/050
------- |