- r
United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S4-88/020 August 1988
&EPA Project Summary
Fourier Transform Infrared
Spectrometry of Ambient
Aerosols
Martin Pollard and Joseph Jaklevic
Fourier transform infrared (FTIR)
spectrometry has been evaluated as
a method for determining the con-
centration of selected species pres-
ent in ambient aerosols collected on
Teflon* filters. The filters are analyzed
by transmission measurements after
collection of the fine fraction (< 2.5
urn diameter) of the ambient
particles on 2 pm pore size Teflon
filters. The spectra of the blank filters
are subtracted from the spectra of
the loaded filters and the area of the
specific absorbance bands for am-
monium, nitrate, and sulfate are
measured and compared to calibra-
tion curves. The filter orientation
must be maintained for all measured
infrared spectra.
Calibration curves were devel-
oped for sulfate and ammonium
nitrate from measurements of lab-
oratory-generated particles. The
lower limit of detection (LLD) for
laboratory-generated sulfate, cal-
culated from measurements of the
620 cm'1 absorption band, is 1.7
ng/cm2 (3o). The LLD for the sulfate
band at 1117 crrr1 is 0.2 ng/cm2;
however, this band cannot be used in
the analysis of ambient aerosol
samples because of spectral inter-
ferences. The LLD for laboratory-
generated ammonium nitrate is 1.32
ng/cm2. Multicomponent analysis
methods were developed to analyze
ammonium nitrate in the presence of
ammonium sulfate.
'Mention of product trademark or company
names does not constitute an endorsement or
recommendation for use
Ambient aerosols were collected
during the Carbonaceous Methods
Species Methods Intercomparison
(GARB) study. The filters were
analyzed for sulfate by both FTIR and
Ion Chromatography (1C). There is
good agreement between sulfate
analysis by 1C and analysis of the
FTIR transmission spectra of the air
filters. The LLD for ambient sulfate
was measured to be 5.8 ng/cm2. if
filter orientation is maintained the
LLD is estimated to be 1 - 2 ng/cm2.
The CARB study samples were
also analyzed for elemental carbon
using FTIR. Intercomparisons were
made with results obtained from the
analysis of co-collected quartz filter
samples using a thermal combustion
method of analysis. Good correlation
was found between the infrared
absorbance in the 650 - 666 cm*1
region of particles collected on
Teflon filters and the elemental
carbon determined from analysis of
the quartz filters. Attempts to develop
a direct elemental carbon calibration
standard on Teflon filters were
unsuccessful. It was concluded that
calibrations must be made with co-
collected air samples on quartz filters
analyzed by thermal methods.
The chemical interactions on the
filter between ammonium nitrate and
ammonium sulfate were studied. It
was determined that both com-
pounds can interact to form the
nonvolatile mixed salts 2NH4N03*
(NH4)2S04 and 3NH4N03.(NH4)2S04.
NH4NO3 was found to react on the
filter with NaCI to form NaNO3 and
volatile NH4CI. A procedure based on
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this reaction is proposed as a
method to retain nitrate ion on Teflon
filters.
In order to analyze large
numbers of filter samples, the FTIR
spectrometer was modified by the
addition of an automatic sample
changer. The sample changer com-
partment accepts the filter carousel
from a Sierra/Anderson model 245*
automated dichotomous sampler.
The motion of the sample changer is
completely controlled by the FTIR
computer.
This Project Summary was
developed by EPA's Environmental
Monitoring Systems Laboratory,
Research Triangle Park, NC, to an-
nounce key findings 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 collection and analysis of
ambient aerosols is an important means
of studying the chemistry of the
troposphere. The major constituents of
the fine fraction (< 2.5 \vm diameter) of
the particulate matter in the atmosphere
are ammonium sulfate, ammonium
nitrate, and elemental and organic
carbonaceous particles. Knowledge of
the origins, interaction, and deposition of
these compounds is important in the
study of visibility degradation,
atmospheric chemistry, acid deposition,
and the effects of these particles on
human health and terrestrial ecology.
Ambient aerosols are typically
collected with dichotomous air samplers
which collect particles in two size
fractions. The fine fraction particles are
< 2.5 nm in diameter and the coarse
fraction particles are in the 2.5-15 urn
diameter size range. The size fractions
correspond to the bimodal distribution of
particles sizes found in the atmosphere.
The particles are collected on various
filter media including Teflon, Nylon,
quartz, and paper filters. The choice of
filter media depends, among other
things, upon the compounds to be
measured, on the analysis technique to
be used, and the cost of the filters.
There are a number of chemical
analysis techniques available to measure
the concentrations of elements and
compounds collected on air filters.
Sulfate and nitrate ions collected on the
filters are typically measured using ion
chromatography. Ion chromatography is
a very sensitive but destructive analytical
technique for the determination of sul-
fate, nitrate, and a number of other ions.
The method requires liquid extraction,
followed by separate analysis of cations
and anions, and provides no direct
information concerning compound
speciation of the ions. In addition, the
extraction procedure can alter the
species found on the filter.
Another method for determining
sulfate collected on filters is x-ray
fluorescence (XRF) analysis. This
nondestructive method of analysis has
been used in an automated system to
determine elemental sulfur, as well as
other elements, deposited on Teflon
filters. Sulfate concentrations can be
inferred by assuming that all of the sulfur
is in the form of sulfate. Ion chrom-
atography analysis for sulfate has been
compared to XRF analysis for sulfur on
Teflon air filters and the two methods
have been shown to provide equivalent
results for sulfate. X-ray fluorescence
provides no information concerning
nitrate deposition due to the low energy
of the characteristic nitrogen x-ray
emission.
X-ray diffraction is a nondestructive
technique that can provide for compound
identification. It is difficult to obtain
quantitative results using x-ray dif-
fraction for two reasons: (1) The
sensitivity of this technique is relatively
low. Consequently, high particle loadings
are required to obtain adequate
measurements for accurate compound
identification. High particle loadings
introduce the possibility of chemical
reactions between particles concentrated
on the filter. (2) Water in the sampled air
can promote recrystallization of particles
on the filter. This introduces a preferred
orientation in the sample which will
change the peak ratios in the diffraction
signals causing difficulties in calibration.
Recently, another method for particle
analysis on Teflon filters has been
reported by other researchers who have
shown that ammonium and sulfate can
be measured directly on Teflon filters
using FTIR transmission spectrometry.
Their results showed the equivalence of
XRF and FTIR spectroscopy for the
quantitative determination of sulfate.
They also reported quantitative
measurements of the ammonium cation.
In addition to the discrete absorption
bands associated with the vibrational
modes of specific molecular species, the
mid-infrared spectrum of these ambient
aerosols shows continuous absorption
across the entire spectrum which is
characteristic of carbonaceous particles.
To the extent that a relationship between
this absorption and the elemental carbon
content can be established, a method fc
nondestructive automated analysis c
carbon particle concentratio
simultaneously with that for sulfate, arr
monium, and nitrate particles coul
result. Finally, this technique is appealin
because it makes use of a common filte
media, Teflon, is nondestructive, require
no sample preparation, is amenable t
automation, and can provide simul
taneous analysis of compounds tha
typically require different types c
analysis on different filter media.
This report describes a
experimental program intended to buili
on previous work reported in tfv
literature, where a number of suggestion
are proposed to improve lower limits c
detection for sulfate. This study fia
implemented and tested those sugges
tions. Experiments to test analytice
methods to quantitatively determine sul
fate, nitrate, ammonium, and elemente
carbon on laboratory-generated sam
pies and ambient samples are alsi
described.
An FTIR spectrometer was modifie<
with the addition of an automatic sampli
changing compartment. The FTIR in
strument and sample changing com
partment will be described; however, ,
detailed description of the FTIR systen
can be found in the operating manus
accompanying the instrument.
Conclusions
Automated FTIR analysis of aerosc
samples using a Nicolet model 5DXB
Fourier transform infrared spectromete
has been realized with the design an<
construction of a specialized sampli
handling system. The sample com
partment directly accepts the sample
carousel from a commercial air sample
produced by Sierra/Andersen Inc
thereby minimizing sample handling
Software was developed and added t(
the existing data collection and analysii
programs in the Nicolet FTIR to allov
complete control of the position of am
filter. Software was also written t<
analyze ambient air samples by auto
matically providing an optima
background subtraction and integration o
absorption bands of interest of th<
collected particles. This instrument pro
vides automated nondestructive sulfat*
analysis of up to 40 Teflon air filters witt
minimal sample handling and operate
intervention.
Quantitative spectral analysis meth
ods were developed to analyze sulfate it
aerosols collected on Teflon filters
Laboratory experiments showed tha
detection limits are eight times lower fo
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analysis using the sulfate absorption
band at 1117 cm-1 than for the band at
620 cm'1. However, spectral inter-
ferences found in ambient samples pre-
vented use of this former band for
routine sulfate analysis. Analysis of air
samples collected in Los Angeles during
the California Air Resources Board GARB
study showed good correlation of the
FTIR method of analysis for sulfate with
ion chromatography (1C) analysis of the
same sample filters.
Small improvements in the detection
limit for sulfate using the 620 cnrr1 band
were realized by using 2 pm pore size
Teflon filters, saving reference spectra of
each blank filter prior to air sampling for
use during background subtraction, and
maintaining the physical orientation of
the filters during infrared spectral
measurements. All of these practices
contribute to more accurate background
subtraction which leads to a more
accurate measurement for sulfate. The
lower limit of detection for sulfate in
ambient aerosols using the 620 cnrr1
absorption band was measured to be 5.8
Vig/cm2 during the GARB study in which
the filter orientation was not maintained
throughout the analysis. If filter orien-
tation is maintained, then the lower limit
of detection is estimated to be 1 - 2
pg/cm2 for ambient sulfate.
Spectral analysis methods were
developed to quantitatively measure
ammonium nitrate in the presence of
interferences due to ammonium sulfate.
Ammonium nitrate measurements were
accurate to within 3 - 5% after am-
monium sulfate spectral interferences
were removed on laboratory-prepared
aerosol samples. The lower limit of
detection for the measurement of am-
monium nitrate was found to be 1.32
ng/cm2 for laboratory-generated sam-
ples.
A high degree of correlation was
found between elemental carbon mea-
surements on infrared spectra of ambient
samples collected on Teflon filters and
elemental carbon measured by thermal
methods on samples collected on quartz
filters. However, attempts to develop a
direct elemental carbon calibration
standard on Teflon filters using lab-
oratory generated soot particles was
unsuccessful. It was concluded that
calibrations must be made with co-
collected air samples on quartz filters
analyzed by thermal methods.
The sensitivity of infrared analysis to
specific compounds represents a unique
advantage for performing quantitative
chemical measurements of ambient
aerosols. The 2:1 and 3:1 mixed salts of
ammonium nitrate and ammonium sulfate
were identified on ambient filters
collected during the GARB study. The
Fourier transform infrared spectrometer
also provides an ideal means for
studying the dynamics of chemical
reactions occurring on the filter.
Laboratory experiments showed that
nitrate ion could be trapped on the filter
by reactions of ammonium nitrate with
NaCI, but experiments with ambient
samples were inconclusive.
Recommendations
It is recommended that this tech-
nique and instrument be incorporated
into an air sampling and analysis network
in order to develop an intercomparison
database to further evaluate this
instrument relative to other methods of
analysis. This instrument can easily be
incorporated into an existing air sampling
network because it uses a common filter
medium, Teflon, performs nondestructive
analysis, and is designed to handle large
numbers of samples in an automated
system. Additional ambient samples are
required to:
1. Determine collection parameters,
such as sampling time and flow
rates, to provide adequate samples
for analysis within the constraints of
the sensitivity of the analytical
method.
2. Determine the type of spectral
interferences that will occur in
various sampling environments.
3. Confirm the correlation for sulfate
measured by Fourier transform
infrared analysis and ion
chromatography.
4. Collect carbon samples on quartz
filters in parallel with Teflon filter
samples to develop a calibration for
elemental carbon.
5. Collect a sufficient number of
samples containing ammonium ni-
trate and carbon to develop an
empirical method to determine the
baseline underneath the ammonium
nitrate absorption band.
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Martin Pollard and Joseph Jaklevic are with Lawrence Berkeley Laboratory,
University of California, Berkeley, CA 94720.
William A. McClenny is the EPA Project Officer (see below).
The complete report, entitled "Fourier Transform Infrared Spectrometry of
Ambient Aerosols," (Order No. PB 88-213 236/AS; Cost: $14.95, 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 Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-88/020
0000329
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