Anal. Chem. 1982, 54, 1627-1630 Sympaslum on Neutron&,Bpture Qamma-Ray Spectroscopy and Relat-
ed Toplcs, Orenoble, France, 1981. (21) Faiiey, M. P.; Anderson, E). L.; Zolier, W. H.; Gordon, 0. E.; Lindstrom, R. M. Anal. Chem. 1979, 51, 2209-2?21. (22) Chase, 0. D.; Rablnowitz, J. L. Principles of Radioisotope Methcdoioav”. 3rd ed.:- 13uraess - Publishing Ce.: Minneapolis, MN, 1967; pp io4-IO~. (23) Giadney, E. S.; Perrin, E). R.; Baiagna, J. P.; Warner, C. L. Anal. Chem. 1980, 52, 212842132, (24) Rowe, J. J.; Steinnes, E. J . Radlmnal. Chem. 1977, 37, 849-856. (25) Baedecker, P. A.; Rowe, J. J.; Stelnnes, E. J . Radlmnal. Chem. 1077. 40. 115-146. (28) Graham, C. C.; Glascock, M. D., unpublished work, Universlty of Mls-
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sour1 Research Reactor Facillty, Columbla, MO, 1981. (27) Vogt, J. R.,unpublished work, University of Missouri Research Reactor Facility, 1982. (28) Ahmad, S.; Chaudhary, M. S.; Qureshi, I. H. J . Radlmnal. Chem. 1981, 87(1), 119-125. (29) Loebel, R. I n “CRC Handbook of Chemistry and Physlcs”. 57th ed.: CRC Press: Cleveland, OH, 1976; pp 8179-8213.
RECEIVED for review April 12,1982. Accepted May 18,1982. This project was supported in part by National Science Foundation Grant No. BNS 79 15409.
Determination of Organic and Elemental Carbon in Atmospheric Aerosol Samples by Thermal Evolution Roger L. Tanner,” Jeffrey S. Gaffney, and Mary F. Phillips Environmental Chemrstry Dlviklon, Department of Energy and Envlronment, Brookha ven Natlonal Laboratory, Upton, New York 1 1973
A simple apparatus Is described for determlnatlon of volatlle organic and elemental carbon fractlons In fllter-collected atmospherlc aerosol sampleo. The two-step technique Involves rapld heatlng and thermoovolutlon of organlc carbon at 400 OC In helium carrler gas, followed by removal of elemental carbon at 700 O C In 10% O,/He. Evolved carbon Is converted to COSon copper odde catalyst, purlfled, and analyzed by nondlsperslve lnfrareid (NDIR) spectrometry. Flash heatlng of amblent aerosol samples mlnlmlzes organlc-to-elemental carbon conversloni, allowlng determlnatlon of organlc and elemental carbon fractlons wlth relatlva standard devlatlons of f8% and f12%, respectively, for total carbon loadlngs of 6-50 pg of CPcm2 fllter area. Blank fllter varlablllty, sample heatlng duratlon, and the effects of sample heatlng rates and pyrolysltr temperature are evaluated. Ambient aerosol carbon data from both urban and remote sampllng sites are presented and discussed.
Questions remain concerning the atmospheric chemistry of carbon pertaining to gas-to-particle conversion mechanisms, the potential health and geophysical effects (including climate modification), and the ultimate biospheric sinks of natural and man-made organic emissions. The difficulties in evaluating processes undergone by samples containing large numbers of organic species at trace levels in dynamically changing atmospheric conditions are extraordinarily formidable. The results of this complexity me that most atmospheric carbon measurements fall into two types: detailed analysis, by e.g., mass spectrometry (MS) oir gas chromatography/mass spectrometry (GCIMS) of a small number of samples (I),or total-carbon measurements )under a much wider range of atmospheric conditions (2). Methodologies to provide more detailed analyses of carbonaceous aerosol species in large numbers of samples are still being developed (3). Several tschniques have been used to determine organic (i.e., volatilizable) carbon and ellemental (i.e., black, graphitic, or sooty) carbon in ambient and source-dominated aerosol samples. These techniques have been critically evduated by Cadle and Groblicki (4).T o date, analytical methodologies have included use of (a) solvent extraction(s) to se!lectively remove the organic content (5, 6), (b) optical techniques including
laser-Raman and optoacoustic spectroscopy to determine the elemental carbon content usually based on ita high absorptivity (7-9),and (c) thermal methods based on selective evolution of organic and elemental carbon under a variety of pyrolysis conditions (10-16). Thermal evolution techniques have been used in several forms, but basically they all involve programmed thermal vaporization and oxidation steps to differentiate organic carbon compounds from elemental carbon by means of differing temperature profiles and/or by changing carrier gas composition. Catalytic conversion of evolved carbon to C02 or CHI permits quantitative determination of evolved carbon by nondispersive infrared (NDIR) (IO) or flame ionization detector (FID) techniques, respectively ( I I , I 2 ) . We wish to report here a simplified version of thermoevolution techniques which employs flash heating of samples in helium and in 10% 02/He carriers to thereby obtain specific determination of low microgram levels of organic and elemental carbon, respectively, in atmospheric samples, Furthermore, cross-over effects due to “carbonization“, i.e., organic-to-elemental conversions, are minimized by this technique. The limitations in the use of the terminology, organic and elemental carbon, to describe the experimentally speciated quantities will be discussed below.
EXPERIMENTAL SECTION The apparatus devised for determination of microgram levels of organic and elemental carbon in ambient aerosol samples using a selective, thermoevolution technique is shown in Figure 1. The operational features of this apparatus consist of the following: (a) The small quartz tube (0.6 cm o.d., 12 cm in length) is divided into two zones, the pyrolysis zone and the copper oxide catalyst zone, each 4 cm in length and wrapped in 20-gauge Nichrome wire. A disk (0.4-1.0 cm diameter) cut from the particulate-laden quartz filter is placed into the pyrolysis zone through a Swagelok tee, after which the system is closed and flushed with 100 mL/min helium at room temperature. (b) During analysis the sample is first heated rapidly to 400 “C by electrically heating the pyrolysis zone, and a helium gas stream is passed through the system. The copper oxide catalyst, heated to 650 “C, converts evolved “organic” carbon to carbon dioxide. Subsequent conversion of remaining “elemental”carbon to C02 occurs when a 10% Oz/He stream is passed over the sample at 700 “C. (c) Gas chromatography on a Carbosieve B (32/40 mesh) column at 155 5 “C is used to purify the COz in the mixture
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0003-2700/8~/0354-1627$01.25/0 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
Table I. Blank Variability of Treated Quartz Filters IA Storage Method storage methoda
organic carbon blank, pg/cm2
A B
2.9 2.1 2.3
C D
std dev (s) 0.3 0.2 0.5 0.8
5.0
IB Experimental Filter Blanks storage method
filter type quartz fired to 700 "C HC1-treated quartz fired t o 700 "C HC1-treated quartz Barrow, AK, sampling HC1-treated quartz, Ft. Wayne, IN, sampling untreated quartz, Warren, MI, sampling
organic carbon blank, pg/cm2 (i )
elemental carbon blank, pg/cm2 ( i s )
no. of values
E F
0.70 (+-0.06) 1.37 (k0.35)
0.21 (k0.23) 0.27 (i0.27)
3 5
G
8.7 (i2.2)
1.26 (k0.50)
7
G
21.1 (k3.6)
2.62 (10.74)
5
1.31 (k0.47)
5
H
9.8 (k0.65)
a A = open laminar flow hood overnight; B = as A except evaculated €or 2 h at 25 "C; C = as A except evaculated 1 h at 50 "C; D = polyethylene bag open to lab air. E = wrapped in A1 foil in Petri dish over soda lime; F = vacuum oven at 25 "C for 72 h; G = sealed in polyethylene bag for 2-6 months; H = samples collected by GMRL for intercomparison study, storage method unknown.
n
on untreated quartz fiber filters.
RESULTS AND DISCUSSION U FLOW
METER
PYROLYSIS ZONE
I
CATALYST ZONE
*VENT
U
VENT
MASS FLOW METER
NDIR Cop ANALYZER
GC
[ RECORDER 1 BLOCK DIAGRAM OF CARBON MICROANALYSIS SYSTEM
Flgure 1. Block diagram of carbon microanalysis system: NDIR = nondisperslve infrared analyzer for CO,.
from other evolved gases (e.g., SOz, NOz). Cryogenic pretrapping (liquid Nz)of the COz in the elemental carbon volatilization step (10% Oz/He carrier) is used to prevent oxidative damage to the GC column packing; pretrapping is optional for the organic carbon volatilization step. (d) A nondispersive infrared (NDIR)detector (Beckman, Model 865) is used to quantify the evolved C02 from the sample. The full scale ranges of this device are 100 and 500 ppm COz,which is adequate in accuracy and sensitivity for this work, even after dilution of the sample to 500 mL/min (minimum flow for NDIR) with He. The system is calibrated by admission, just prior to the pyrolysis zone, of microliter levels of C02 from a pressurized tank source using a gastight syringe. Admitted COz is determined with the pyrolysis temperature and carrier gas used for the organic mode or the elemental mode and with the peak height from the NDIR response used to construct the calibration curve. No differences in response to admitted COzare observed for organic mode and elemental mode conditions. Determination of the amount of evolved COzin a sample is achieved by measuring the peak heights on the chart recording of the NDIR output and referring to the appropriate COz calibration curve, correcting for filter blank. In nearly all cases reported, carbon analyses were performed on aerosol samples collected on treated quartz filters (17). Blanks were determined for all samples on unexposed filter sections from the same batch of treated quartz stored under identical conditions. Samples for the intercomparison study described below were taken
Several aspects of method validation will be considered below, followed by inclusion of some ambient data obtained for Barrow, AK, and Warren, MI, sampling sites. Blank Variability. Major consideration was given to determination of the variability of filter blanks with respect to establishing proper storage conditions. Treated quartz filters will apparently adsorb ambient gaseous organic compounds and/or carbon dioxide in significant amounts when not properly stored. This is illustrated in Table IA in which treated quartz filters, freshly fired to 700 OC, were stored as indicated and duplicate segments analyzed for organic carbon. Storage in polyethylene bags does increase the organic carbon blanks; furthermore, these blanks and their variability are not significantly reduced by simple vacuum oven treatment. At the suggestion of Huntzicker (13) filters were stored wrapped in aluminum foil in glass Petri dishes over soda lime. The filter blanks of quartz filters so treated are compared in Table IB with freshly fiied, HC1-treated quartz filters and with unexposed filters shipped to and from sampling sites in polyethylene bags. Again it is clear that proper storage conditions are essential for low, reproducible blanks. Storage method E is preferred for detection of elemental and organic carbon a t low levels in ambient samples for which loadings on 24-h, 20 cm X 25 cm high volume samples may frequently be
