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 ------- |