solvent extraction and organic carbon determination in atmospheric particulate matter. organic...

studied) provided the lowest titration errors and highest precision; however, only when the value of 2.303 RTInF was selected to give the best fit to the data did the accuracy and precision approach that attainable using conventional phenolphthalein end points.

4) Table IV lists the various methods used to construct Gran plots in decreasing order of the absolute average titration error obtained with each. The values in Table IV were computed by averaging the values for each method presented in Tables I1 and 111, disregarding signs and ex- cluding trials in which points representing less than 60% of the titration curve were used.

The results of this study indicate that even for well-defined titration systems, of which the strong acid-strong base titration is representative, Gran plots must be used with caution if titration errors comparable to the conventional, visual indicator end-point detection techniques are desired. The two advantages suggested earlier which favor the use of Gran plots can be realized only by wisely se-

lecting the points used, carefully fitting the Gran plots to the points (such as was done with the 2.303 RT/nF corrections in this study), giving close attention to the precision of the measurements, and accepting results which are a t best slightly inferior to those attainable with conventional visual indicators.

LITERATURE CITED (1) C. McCallum and D. Midgley, Anal. Chim. Acta, 65, 155 (1973). (2) G. Gran, Analyst(London), 77, 661 (1952). (3) P. Sorensen, Kern. Maanedsbl., 32, 73, (1951). (4) "Gran's Plots and Other Schemes", Newsletter of Orion Research Incor-

(5) S. L. Burden and D. E. Euler. Proc. lndiana Acad. Sci., 82, 167 (1973). (6) E. L. Bauer, "A Statistical Manual for Chemists". Academic Press, New

(7) J. D. Hinchen, "Practical Statistics for Chemical Research", Methuen and

RECEIVED for review August 19, 1974. Accepted January 15, 1975.

porated, 2, 11 (1970).

York, NY, 1965, pp 85-92.

Co., Ltd., Great Britain, 1969, p 35.

Solvent Extraction and Organic Carbon Determination in Atmospheric Particulate Matter: The Organic Extraction- Organic Carbon Analyzer (OE-OCA) Technique

Daniel Grosjean

W. M. Keck Laboratories of Environmental Engineering, California Institute of Technology, Pasadena, CA 9 1 125

A method is presented for the determination of organic carbon in atmospheric aerosols. It consists of organic solvent extraction of samples collected on glass fiber filters followed by organic carbon analyzer analysis of the concentrated extracts as suspension in water. The organic solvent is removed in the vaporization zone ( T = 100 "C) and the aerosol organic carbon is measured in the combustion zone ( T = 850 "C) of an organic carbon analyzer. Twenty-six solvents and 24 binary mixtures were studied for their ability to extract aerosol organics. We define for this purpose the parameters EF (extraction efficiency) and OCEF (organic carbon extraction efficiency) with benzene as reference solvent. Nonpolar solvents have definite EF's and OCEF's, while polar solvents EF and OCEF vary with the ozone concentration (Le., the smog chemical composition) observed during the sampllng perlod. EF's correlate well with several solvent polarity parameters. Methylene chloride and several polar solvents have higher EF and OCEF than benzene, but none of the single solvents covers all the polarity range of the aerosol organics. Successive extractions using polar solvents, including water, after benzene extraction, indlcate that an important fraction of aerosol organics, up to 48% as organic carbon, is missing using benzene extraction alone. All binary mlxtures of a polar and a nonpolar solvent have higher OCEF than both polar and nonpolar solvents. Successive and blnary mixtures extractions give identlcal OCEF results. Polar-solvent soluble inorganics, mostly nitrates, can be easily measured by dlffer- ence using water extraction after polar solvent extraction. The validlty of the OE-OCA technique Is tested against several others. Among them, the direct OCA analysis of glass fiber filters is suggested. From 95 to 100% of the aerosol

organic carbon is extracted and measured by the means of the proposed method, which seems particularly suitable for routine determlnatlon of atmospheric aerosol organic carbon.

Organic compounds are a significant fraction of the urban aerosols. Their concentration is usually measured by organic solvent extraction of samples collected on glass fiber filters ( I ) . More detailed information is obtained by chemical analysis of the organic extracts: infrared (2, 3 ) and CHN analysis (4, 5 ) , fractionation into classes (5, 6 ) and analysis of each fraction for specific compounds by TLC ( 7 ) , GC (8, 9 ) , UV-fluorescence (10, 11) and mass spectrometry (12). However, all these subsequent analyses depend on the organic solvent extraction efficiency (EF). Hydroxylic and other polar solvents show a good EF for organic particulate matter, but are able to dissolve a significant quantity of inorganics, especially nitrates, as well. Thus, in the absence of a suitable technique for routine organic carbon determination in those polar solvents, nonpolar solvents were most widely used in the past for the extraction of atmospheric organics. For example, benzene has been used for the National Air Surveillance Network (13) and numerous other studies ( 4 , 5 , 8 ) . Among other solvents used &re cyclohexane (12, 14, 15) because its EF is close to that of benzene and it is less toxic; CC14 (16) for its IR transparency and CS2 (17) because of its very low flame ionization detector response in GC.

The purpose of this study is twofold: to show that ex- tracting with benzene (or cyclohexane, or other nonpolar solvents) alone may lead to a serious underestimation of aerosol organics, and to describe a simple, accurate method

ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975 797

' I voc ~ NVOC I voc i NVOC

Figure 1. Aerosol organic carbon determination using the Organic Carbon Analyzer

VOC = Volatile organic carbon, vaporization zone (I 125 "C). NVOC = Nonvolatile organic carbon, combustion zone (850 "C). Left: water sample: (a) pure water: (b) water extract. Middle: direct combustion of glass filters: (a) pure water; ( b ) unloaded filter: (c) loaded filter. Right: OE-OCA technique, organic solvent extracts: (a) pure water; (6) pure organic solvent in water; (c) concentrated organic solvent extract in water

for the determination of organic carbon in atmospheric particulate matter. This technique consists of organic solvent extraction (OE) followed by organic carbon determination using an Organic Carbon Analyzer (OCA). We will discuss the extraction efficiencies of several solvents and binary mixtures. In addition, the potential applications of the Organic Carbon Analyzer to air pollution analyses will also be discussed.

EXPERIMENTAL Extraction. Two-, 4- and 24-hour atmospheric samples were

collected at Pasadena, CA, during the summer and fall of 1973 on two calibrated high volume samplers (General Metal Works) in parallel using 8-X 10-inch Gelman type A glass fiber filters. Condi- tioning of the filters to minimize carbon blanks, weight corrections related to the carbon blanks, and the effects of relative humidity and gas adsorption were described elsewhere (18). Six-hour extractions of % of each filter were performed using Soxhlet extractors with 60 ml of solvent (spectro quality or Reagent grade). Prelimi- nary studies of the optimal extraction time and of the possible loss of organic material during extraction were made by gas chromato- graphic and OCA analysis of extracts of known mixtures representative of aerosol organics. Because of the variations in the aerosol composition from one sampling period to another, '16 of each sample was extracted with benzene chosen as reference solvent. This allowed the direct comparison of 11 solvents. In the same way, direct comparison of all the binary mixtures was made by taking two one-week Hi Vol samples in parallel. One-sixteenth of each filter was extracted with one of the 24 binary mixtures (50-50% by volume) or one of the four polar solvents studied. The four remaining pieces of filter were extracted together with benzene, and then each one extracted again with one of the four polar solvents for comparison of the successive extraction with the corresponding binary mixture. Total soluble material (organics and others) were measured by weight of the filter before and after extraction [with appropriate equilibration at 50% RH (relative humidity) for 24 hr] and by weight of an aliquot of concentrated extract obtained by partial evaporation of the solvent using a rotating evaporator (Rinco). Results obtained from weighing the aliquots and the filters agree within 5%. Water extractions using double distilled water were performed in the same way.

Organic Carbon Determination. Water extracts were analyzed for volatile organic carbon, VOC, (if any, vaporization below 100 "C) and nonvolatile organic carbon, NVOC (vaporization I 8 5 0 "C) using a Dohrmann Envirotech Organic Carbon Analyzer Model DC 50. Organic carbon concentrations are measured with a flame ionization detector after combustion (850 "C + C0304) and reduction of COz to CHI (350 "C + Ni). Total organic carbon, TOC, is the sum of VOC and NVOC. Concentrated organic extracts were analyzed as a solution or a suspension (for immiscible solvents and partially miscible binary mixtures) in water.

Homogeneous suspensions were obtained using a magnetic stir- rer (21 hour). A typical run is shown on Figure 1. Most of the solvents and binary mixtures studied having a boiling point below 100 "C appear as VOC (first peak, vaporization zone). For a few solvents having higher boiling points, the vaporization zone tempera-

ture was set up at 125 "C. The second peak, NVOC, corresponds to the combustion of aerosol organics. Blank determinations were made using pure solvents and corrections were made for possible water and solvent organic impurities. Peak areas were recorded (Recorder Varian Model A 50) and measured using an electronic integrator (Varian Model 477) or the OCA readings after calibration with known organic standard solutions. Proper dilution ratios were calculated to fall within the ranges (0-200 and 0-2000 mg C/ 1.) of the Carbon Analyzer. For water-miscible solvents, the precision is that of the apparatus (f2%). Measured and calculated carbon concentrations agree over a wide range of dilution ratios (Table I). For non-water-miscible solvents, the reproducibility is less satisfying (up to f10% for 6 runs), probably due to the diffi- culty in obtaining homogeneous suspensions. However, the ratios (TOC - VOC)/TOC (=NVOC/TOC) scaled up to the known initial dilution ratio. permit calculation of the aerosol organic carbon with a precision of f5%.

Organic Carbon Determination: Alternate Approaches. In order to check the internal consistency of the OE-OCA technique, the results were compared with those obtained by: a) CHN analysis (F and M Scientific Corp. Model 180) after solvent evaporation (EV-CHK); b) complete evaporation of the solvent and direct combustion of a weighed amount of the extract in the Carbon Ana- lyzer; c ) direct combustion of a small piece of glass fiber filter, (3he-inch diameter) after aerosol sampling, in the Carbon Analyzer, using the Dohrman solid injection port adaptor kit No. 899816.

For b) and c), 30 pl of water were added to ensure a good contact between the samples and the combustion catalyst in the sample boat. Results for several samples are compared in Table 11. Al- though a) and b) are in good agreement, the values were always found to be 10 to 20% lower than that measured by the OE-OCA technique: this is probably due to a significant loss of material during the evaporation step. Organic carbon measured by c) is higher than that measured by a) and b) and agrees reasonably with the OE-OCA measurements. However, because of the organic blank of the filters the precision obtained by c) depends on the amount of collected aerosol, Le., on the sampling time and sampling flow rates. Errors introduced by the carbon content of the filters were calculated and are listed in Table 111. They are in good agreement with the total carbon blanks measured by Patterson (19) for the same type of filters by CHN analysis. Nonetheless, the direct analysis of carefully cleaned glass fiber filters with the OCA seems to be promising and is currently used in our laboratory for kinetic measurements of organic carbon aerosol formation in smog chamber experiments (20).

Infrared Analysis. IR spectra of concentrated extracts were recorded using a Beckman Model IR-5 spectrophotometer with NaCl cells for organic solvent extracts and Irtran-2 cells for water extracts. Extracts spectra were compared with those of aerosol collected at the same time on infrared-transparent filters (matched Millipore type TH filters). Infrared spectra will be discussed later in this paper.

RESULTS AND DISCUSSION The increasing number of studies dealing with the chem-

ical composition of urban aerosols provide a better picture of the organic fraction (18, 21, 22): besides nonpolar compounds (aliphatics, aromatics, polynuclear aromatics) associated with primary emissions, there is a significant amount of polar, oxygenated species: carbonyl compounds, organic nitrates, mono- and dicarboxylic acids and difunc- tional compounds, all of which are photochemical products. The relative amounts of polar and nonpolar organics depend on the extent of photochemical conversion compared to primary emissions, and the resulting complex organic fraction contains numerous compounds having different polarities. The best EF is obtained when the polarities of the solvent and the extractable compound are similar. However, no one organic solvent covers the complete polarity range of the aerosol organic compounds. Successive extractions, using water after benzene extraction, show that a significant amount of aerosol organics is missing with nonpolar solvent extraction alone (Table IV). Significant amounts of organics were also recovered using water extraction after cyclohexane (18) and other polar solvents

798 ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975

Table I. Determinat ion of Organic Carbon in Organic Solvents Ext rac ts wi th the Organic Carbon Analyzer Pure solvent Solvent - extractf

Solvent: methanol

Dilution ra t ios (p1,'50 nilH,O) 1 0 20 50 100 200 50 100 2 00 TOC calculateda 59.5 119.0 297.4 595 1190 . . . . . . . . .

i b 1.7 3 .O 3.6 11 17 6.0 12 19

Ib 1.6 3.2 3.7 13 19 5.8 11 21 AC' IC c 1.2 1.6 2.1 1.7 1.7 . . . . . . . . . TOC - VOC 0.2 2 .o 4.1 7 8 47.6 94 178 TOC - VOC, correctedd . . . . . . . . . . . . . . . 43.5 87 170 pg/m3 solvent -soluble

organic carbon, as C" . . . . . . * . . . . . . . . 15.3 15.3 14.9 a From the molecular weight, carbon '?& by weight and density of the solvent, and the dilution ratio. * Average 6 runs for each dilution

Subtracting the solvent impurities (TOC - Concentrated to 1 ml after

voc 60.0 118.9 299.5 598 1202 300.2 601 1208

TOC 60.2 120.9 303.6 605 1210 347.8 695 1386

.ratio. Results expressed as mg C/1. C (TOC measured - TOC calculated)/TOC calculated. VOC) at the same dilution ratio. e 24-hour Hi-Vol sample at 70 SCFM, total aerosol concentration 102 pg/m3. Soxhlet extraction (60 ml solvent, 6 hours).

Table 11. Organic Carbon Determination: Comparison of the OE-OCA a n d Other Methods OE-OC.4b E-CHSC EV- OCA^ CFF -0c.4~

*itf mg OC * ;r mg CC *:r mg OC f :$ Sample --

r n g orsanic C N 0 . a 1 12.4 2.4 11.2 5.0 11 .o 6.0 12.6 12.3 2 74.3 3.5 60.5 3.4 63.7 3.5 70.2 10.2 3 126.3 2.7 100.7 3.1 104.6 3.9 135.0 6.5 4 212.5 3.9 193.4 3.8 197 .O 5.0 208.2 5.1 5 52.5 3.3 44.3 5.2 45 .O 6.0 50.2 10.5 6. 170.0 5 .O 151.0 7.2 148.5 7.9 173.5 7.5

Samples collected on Hi-Vol samplers and extractedwith: isopropyl alcohol (water miscible solvent, samples No. 1 and 3) benzene (non- OE-OCA: organic extraction, extract

EV-OCA: organic extraction, evaporation, OCA water miscible, No. 2 and 4) , cyclohexane-methanol (No. s), and isooctane-isopropyl alcohol ( S o . 6). concentration, OCA analysis. analysis. e GFF-OCA: direct OCA analysis of glass fiber filters. f Average on 6 runs for each sample and each technique.

EV-CHN:organic extraction, evaporation, CHN analysis.

___ -. Table 111. -Carbon Content of Glass Fiber Fi l ter as Measured

47-mm i. d. 3 x 10 inch Readi.ngs (mg OC/l. H,O) Sample l3 42.5 t 3.2' 23.3 i 8.2'

11.2 * 7.0' Sample 2" 25.2 z 3.0" 12 .Filters, averageb 25.7 + 20.2' 26.3 i 13.7' 12 Fil ters, highest measured value 45.7 40.0

Highest measured value, pg OC pe r sample (3/16-in. i.d. f i l ter) 1.37 1.20 pe r cm2 of fi l ter 7.75 5.50 per total f i l ter 74 2238

EquiIralent sampling t ime of 100 ,ug/m3 acmospheric aerosol,

sampling flow ra t e 80 I . /min 9.25 min. . . . 20 1. /min (cascade impactor) 37 min. . . .

1 -h.our samples 15% = 22% 24-hour samples 2 0,65%# = 0.9%

60 scfm (Hi-Vol) 13.1 min E r r o r due to the carbon impurit ies a t 80 l . /min : at 60 scfm:

a Samples consist of a piece of filter (3hs-in. i.d.) + 30 1 1 double distilled HzO. The filters were washed with cyclohexane and water, heated for 24 hr at 450 "C and allowed to equilibrate for 24 hr at 50 f 2% relative humidity prior to analysis. Without conditioning the filters show much higher organic carbon blanks. Taken from 3 different lots. Maximum deviation for 6 runs. Note the larger deviations observed for 8 x 10-in. filters due to non-uniform carbon repartition on the filters and the high filter/sample area ratio. Maximum observed deviation.

such as alcohols or acetone after nonpolar solvent extrac- ticulate organic matter. In order t o improve the determination. The aerosol organic fraction, usually expressed in the tion of aerosol organic carbon, we propose the following past as benzene soluble, is seriously underestimated for scheme: 1) successive extraction using a polar solvent after samples taken in urban areas where oxygenated organics, a nonpolar one, or alternatively, "one-step" extraction produced by photochemical reactions, are a significant using a binary mixture of a polar and a nonpolar solvent; fraction of the atmospheric aerosol. This is of considerable and 2 ) organic carbon determination using a Carbon Ana- importance for control strategies dealing with visibility lyzer, as described in the Experimental section. degradation and adverse health effects associated with par- Several solvents and binary mixtures were studied for

ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975 * 799

Table IV. Missing Organic Carbon Using Benzene Extraction: Consecutive Extractions with Water after Benzene Benzene soluble organics 0," 0.280 0.223 0.183 0.191 0.170 0.162 0.188 0.165 0.158 0.144 Benzene soluble organic carbon OC,"'b 0.196 0.163 0.128 0.139 0.131 0.126 0.141 0.124 0.122 0.114

Water soluble organic carbon OC,"* 0.105 0.073 0.038 0.046 0.038 0.026 0.060 0.052 0.070 0.085 Organic carbon fraction OCF = 0.301 0.236 0.166 0.185 0.169 0.152 0.210 0.176 0.192 0.199

Missing organic carbon, as % of OCF, 35 3 1 22.8 24.8 22.4 17.1 3 0 29.6 36.4 42.6

CL& in 0 , = IO0 OC,/'O, 70 73 70 73 77 78 75 75 77 79

oc, + OC,"

using benzene alone

a 0,, OC,, OC,, and OCF are expressed as fractions of the total collected aerosol. 2-, 4-, or 24-hr Hi-Vol samples, Pasadena, summer and Missing organic carbon with benzene alone, average 10 samples: 29.2%

fall 1973). Measured by the OE-OCA technique.

~~~ - - ~ ~ ~ _ _ _ Table V. Extraction Efficiencies (EF) and Organic Carbon Extraction Efficiencies

(OCEF) of Various Solvents for Atmospheric Aerosol Samples EF

This ,work Literature data O C E , this work

(0) ( b ) ( C ) ( d ) ( e ) (f) ( b ) Solvent

1. Silicon tetrachloride 69 . . . . . . . . . . . . . . . 2. Carbon disulfide 70 . . . . . . * . . . . . 69 3, #?-Pentane 7 1 . . . . . . * . . 66 . . . 4. Freon 113 76 . . . . . . . . . . . . 76 5. Cyclopentane 77 . . . . . . . . . . . . . . . 6. a-Hexane 78 . . . 71 . . . . . . 78 7 . Carbon tetrachloride 8 1 . . . 87 . . . * . . 80 8. Diethylether 82 . . . 77.5 . . . 100 80 9. Cyclohexane 84 . . . 62 72.5 75, 81.5 84

10. Isooctane 95 . . . . . . . . . 93 9 4 11. BENZENE (reference) 100 (100) (100) (100) 100 12. Toluene 102 . . . 100 133 94 . . . 13, Trichlorethylene 109 . . . . . . . . . . . . 108 14. Diethylamine 112 . . . . . . . . . . . . . . .

17. Methylene chloride 126 * . . 107 104.5 106 120

15. Chloroform 118 . . . . . . . . . 112, 107 114 16. Triethylamine 120 . . . . . . . . . ... 116

18. Tetrahydrofuran 162. 100-260 . . . . . . . . . 84 54-115 . . . . . . . . . 82 47-110 19. l,4-Dioxane 170 112-262

20. Pyridine 185 115-295 259 . . . . . . 75 52-116 21. 2-Propanol 224 70-327 222 * . . . . . 105 61-167 22. Acetone 250 71-388 239 197 194, 1 4 1 112 6+170 23. Ethanol 281 95 -421 234 . . . 350 113 64-190 24. Methanol 362 85-560 272 520 135 69-200

26. Water 407 183-594 . . . . . . . . . 123 74-230 (a), ( b ) Measured in this work. ( b ) EF range of polar solvents. (c) Gordon, Ref. (24). ( d ) , ( e ) Stanley, Ref. ( 2 1 ) . ( e ) atmospheric samples

enriched with 3 polynuclear aromatics; ( e ) , ( d ) , and ( e ) are scaled u p to benzene. ( j ) OE-OCA technique, results scaled up to benzene. (g ) DMSO is not suitable for OCA analysis (bp = 189 "C).

25. Dimethyl sulfoxide 364 90-520 . . . . . . . . . k) . . .

their ability to extract particulate organics. Results for ambient atmospheric samples are presented in Table V for extraction with single solvents and Table VI1 for extraction with binary mixtures.

Extraction Efficiencies of Individual Organic Sol- vents. Twenty-six solvents were studied. Most of them were chosen for their particular use in further chemical analysis of the extracts: isooctane and CHZC12 as TLC solvents, CC14 as IR solvent, CC14 and CS2 for their very low response to the flame ionization detector. Freon 113 has been used for the extraction of hydrocarbons in water (16, 23). Sic14 contains no carbon atom and, therefore, seemed promising as solvent for GC and OCA. THF and DMSO are good solvents for organic polymers. Basic solvents were studied because of the important acidic fraction in aerosol organics (18). Other solvents (benzene, cyclohexane, halo- genated hydrocarbons, alcohols) were widely used in the past for the extraction of aerosol organics.

Results are listed in Table V, as relative extraction ef-

ficiencies (EF) with benzene as reference solvent:

x 100 t o t a l ex t r ac t ed ma te r i a l , solvent i

- total ex t r ac t ed material, benzene , E F , -

s a m e sample Depending on the sampling period, benzene-soluble material varied from 9 to 28% of the total collected aerosol. Stanley e t al. ( 1 1 ) have compared several solvents for their ability t o extract atmospheric polycyclic aromatic hydrocarbons. Gordon (24) has compared the efficiency of 17 organic solvents for Los Angeles aerosol samples. Their results are included in Table V for comparison purpose. The organic carbon content of most of the extracts was measured with the OCA, as described in the Experimental section. Results, scaled-up to benzene, are also listed in Table V, as organic carbon extraction efficiencies (OCEF):

ex t r ac t ed organic carbon, solvent i

s a m e sample

x 100 O C E F , - - ex t r ac t ed organic carbon, benzene,

800 ANALYTICAL CHEMISTRY, VOL. 47, NO 6, MAY 1975

Table VI. Parallel Extractions with Benzene and Water oc Lla O C w b CCEF, water'

0.205 0.187 9 1 0.098 0.094 96 0.107 0.191 179 0.162 0.140 86.5 0.088 0.106 120 0.192 0.149 77.5 0.093 0.214 230 0.150 0.236 157 0.096 0.125 130 0.131 0.110 84 0.158 0.1 22 77.5 0.103 0.088 101 0.225 0.194 103 0.100 0.069 82 0.211 0.226 107 0.138 0.150 109 0.128 0.280 219 0.107 0.145 135 0.068 0.138 2 04 0 .120 0.089 74

Average 123 O , * See definition in'rable IV. ' Reference OCEF benzene = 100.

5 0 3 ,

EF Scale. I t can be seen from Table V, first column, tha t nonpolar solvents have definite EF's, while the EF's of polar solvents vary considerably. Among the nonpolar solvents, CS2, CC14, Freon 113 and Sic14 have a poor EF. The EF's of aliphatic hydrocarbons increase with the carbon atom number: the solubility increasing with the tempera- ture, high molecular weight aerosol organics are expected to be more soluble when the boiling point of the solvent increases. Our results agree well with those of Gordon and Stanley. Benzene is more efficient than cyclohexane, but methylene chloride and chloroform are even better and their use should be recommended instead of tha t of benzene when single-solvent extraction is required. Polar solvents have generally higher EF's than nonpolar solvents, because of their ability to extract inorganic material as

L O C -

30c - E F

?OC c

CC -

" 2 3 C 2 3 3 2 4 0 5 C6 :::-e F P ~

Figure 2. Solvents extraction efficiencies (EF) as function ozone concentration averaged over the sampling period.

of the

The highest ozone concentration was observed for samples collected on 7- 25-73.

well. Infrared spectra of polar solvents extracts show a strong absorption band due to the presence of the nitrate ion (830 cm-l), while this band is absent in the IR spectra of nonpolar solvent extracts. IR spectra of polar solvents extracts show also higher carbonyl (1720 cm-') and organic nitrates (1630 and 1280 cm-')/C-H (2950 cm-l) absorption bands ratios, indicating that numerous oxygenated organics are more soluble, as expected, in polar solvents.

Moreover, the EF's of polar solvents vary in a broad range depending on the sampling period. Figure 2 shows the EF's of CH2C12, isooctane, ethanol, and acetone for samples taken a t different ambient ozone concentrations (i.e., for different oxygenated organics and inorganic nitrates concentrations, both formed by ozone-gas phase hydrocarbons photochemical reactions). The EF's of the two polar solvents increase with the ozone concentration. Therefore, because of the variable concentrations of polar organics and nitrate ion in urban aerosols, i t is not possible to define an E F scale for polar solvents based on a few short period samples. The E F scale of Gordon ( 2 4 ) based

Table VII. EF and OCEF of Binary Mixtures

Nonpolar solvent E F I O C E F ~ D

Freon 113 76 2.4

Cyclohexane 84 2 .o

Isooctane 95 1.9

Benzene 100 2.4

Chloroform 118 4.8

Methylene 126 8.9 chloride

Polar solventd

E F O

O C E F ~

Db

E F OCE F D e E F OCEF D E F OCEF D E F OCE F D E F OCEF D E F OCE F D

2-Propanol 230

107

18.3

157 119

210 125

220 131

225 135

218 131

219 140

7.8

7.4

9.6

7.0

9.5

11.5 "Measured for the sampling period. See Table V for EF and OCEF ranges and Y

EF and OCEF are the same for nonpolar solvents. Binary mixtures, 50-50% bv volume. e Measured ( 4 2 ) or estimated (see text).

Acetone Ethanol Methanol 260 291 374

120 122 144

20.7 24.3 32.6

191 198 220 131 134 147

230 242 252 136 130 144

241 255 281 132 13 0 144

240 268 301 140 13 5 145

248 260 312 137 139 149

249 260 320 144 142 151

9.6 12.6 15.8

9.0 11.8 15.0

11.2 14.8 18.3

10.0 12.0 15.9

14.7 13.7 17 .O

12.4 14.0 18.2 averaged values. * D = dielectric constant.


I 430 c / / - I

,/ ’ 1 3 23 30 43 50 60 3 :-!,ET!-) 0 2 4 6 e r ?‘!*I

IC I 5 8 (-1

Figure 3. Solvent EFs as function of the dielectric constant D, the solubility parameter 6, and the polarity parameters ET and P’

on yearly averaged samples collected in the L.A. basin is the most significant comparison available a t this time. For all the polar solvents common to the two studies, Gordon’s values are in the E F range measured for our samples. Our averaged EF’s of polar solvents are higher than Gordon’s values, probably because of the higher concentration of ox- ygenates during our sampling period.

EF Scale and Solvent Polarity Parameters. Measured EF’s depend upon various types of solvent-solute interactions: dipole orientation, dispersion forces, and hydrogen bonding for either proton donor or proton acceptor solvents. As pointed out by Gordon ( 2 4 ) , the efficiency of the extraction process depends on solvent-solute interactions combined with the ability of the solvent to desorb the organics from the filter. Thus, our measured EF’s should be related to physical parameters characterizing the strength of the solvent (dipole moments (16, 25), dielectric constant (261, solubility parameter (27), solvent strength (28)), and its efficiency in the desorption step, as evaluated by semiempirical parameters used in liquid chromatography (polarity ET (29) and P‘ (30) parameters, Rohrschneider (31, 32) or McReynolds (33) constants). Linear relations are obtained between EF’s and both dielectric constants, solubility parameters 6 (compiled from reference 34) and ET and P’ polarity parameters (Figure 3). This indicates tha t both desorption and solubility are important in the extraction process. EF’s of isooctane (because of its high boiling point) and acetone (which promotes condensation reactions) are higher than predicted from their S , ET , and P’ parameters.

Most of the solvents studied fall into distinct categories that depend on dispersion forces (CS2, CClI, a-dispersion for benzene and toluene), dipole orientation (CH2C12, acetone, alcohols, water), or hydrogen bonding for both basic (di- and triethylamine, pyridine and, to a lesser extent, dioxane and diethyl ether) and proton-donor solvents (alcohols, water, CHCls). Thus, an attempt was made to find out if one specific type of solute-solvent was predominant. Hansen’s modified solubility parameters for dipole, dispersion and hydrogen bonding interactions (35-37) and Sny- der’s xd (proton donor), x e (proton acceptor), and x , (dipole interactions) (30) were used. No relation was found between EF’s and any type of specific interaction. This is due to the complexity of the aerosol organic fraction, which contains numerous functional groups. For example, ethanol may form hydrogen bonds with carboxylic acids or basic compounds, or act as a dispersion force-type solvent with nonpolar aerosol organics and as a dipole orientation-type solvent with polar neutral compounds like ketones. There- fore, EF’s are functions of the “overall” polarity of the solvent, which is simply the net effect of the various solvents- solute interactions.

I 1 1 0 50 IO0

Moles % polor solvent

Figure 4. EF of solvents and binary mixtures as function of the molar fraction of the polar solvent

OCEF Scale. Extracted organic carbon fractions, as measured by the OE-OCA technique, are expressed as OCEF (Table V, last column, reference benzene = 100). Here again, polar and nonpolar solvents differ considerably.

OCEF’s of nonpolar solvents follow the same trend as their EF’s, as expected for solvents which most likely extract only organics. For these solvents, the organic carbon/ total extracted ratios were found to be 75 f 10% (See Table IV for benzene data). This agrees well with the carbon percent by weight of nonpolar solvents soluble aerosol organics as measured by CHN analysis (18) . Thus, either E F or OCEF scales can be used for nonpolar solvents. On the con- trary, OCEF’s of polar solvents are much lower than their EF’s and in most of the cases more than 50% of the polar solvents extracts are inorganics, mostly nitrates. As for their EF’s, polar solvents OCEF’s vary with ozone concentrations (Le., with the amount of oxygenated organics and inorganic nitrates) from values lower to values higher than those of nonpolar solvents. For example, ethanol or water are more efficient than benzene when severe photochemical smog conditions are encountered or simulated in smog chamber experiments. As for their EF’s, polar solvents OCEF ranges, indicated in Table V, are those experimen- tally measured and may be different for different sampling periods.

EF and OCEF of Water. High E F values were found for water, as expected, because of the solubility of inorganics. Water soluble nitrates and sulfates always account for a significant fraction of urban aerosols (18) . The high water OCEF values found in this work need some comments. I t might seem surprising to extract organics with water, although water has been used among other polar solvents in early studies for the extraction of urban aerosols. Renzetti and Doyle (3) measured about 25% of particulates in auto- mobile exhaust to be water soluble. A large fraction of benzene-soluble aerosol organics has been found to be also soluble in water (38). I t can be seen from solubility data that even nonpolar hydrocarbons are not strictly insoluble in water. For example, 1.7 grams of benzene are soluble in 1 liter of water a t 22 “C (39). Although small, this quantity is much higher than the measured concentration of benzene in urban aerosols (21), even if 24-hour or more Hi-Vol samples are collected. Numerous other hydrocarbons, like ole- fins and diolefins, are also slightly soluble in water (40).


/ 2ooL

T+-- I

LL- 20 10 I O 0

D

Figure 5. Binary mixtures EFs and dielectric constant D (0) 2-propanol. (0) acetone. (A) ethanol. (A) methanol.

Most of the polar organics show low, but sufficient in re- gard to their very low concentrations, or good solubilities in water. Table VI shows, along with other recent studies (18) , tha t significant amounts of organics are extracted using water. Organic carbon was found to account for 16 to 44% of the water soluble aerosol (average 25.7% for 26 samples studied). As pointed out before, inorganic ions (Nos-, NH4+, S042-, and, to a lesser extent, Na+ and C1-) account for the remaining water soluble aerosol.

Extraction with Binary Mixtures. Results presented in the previous sections of this paper show tha t aerosol organics are not fully recovered by single-solvent extraction (polar organics are missing using nonpolar solvents and vice-versa) and suggest that the extraction should be im- proved using binary mixtures of a polar and a nonpolar solvent. For this purpose, 24 mixtures were studied using 6 nonpolar solvents: benzene, cyclohexane, Freon 113, isooctane, chloroform, and methylene chloride and 4 polar solvents: acetone, 2-propanol, ethanol, and methanol. E F values of polar solvents were found to be slightly higher for the sampling period than the averaged values listed in Table V. E F of benzene was also measured and EF's of other nonpolar solvents were assumed to be those listed in Table V. Both E F and OCEF were measured and are listed in Table VII.

EF's of Binary Mixtures. Only two binary mixtures have been studied in the past for aerosol organics extractions: benzene-diethylamine (11) and benzene-methanol ( 2 4 ) . The latter mixture was also used to extract organics from water (41) . The following trends are observed: All binary mixtures have E F values much higher than tha t of the nonpolar solvent (NP) but slightly lower than tha t of the polar solvent (P):

For a given nonpolar solvent, E F increases with the polarity of the polar solvent:

We have seen (previous section and Figure 3) that EF's of individual solvents correlate well with polarity parameters. The behavior of binary mixtures can also be explained in

3001 300 -

E F

200 -

Figure 6. Binary mixtures EF's and solubility parameter 6 (0) isooctane mixtures. (A) acetone mixtures. The arrow indicates the acetone-isooctane mixture. ( W ) all other binary mixtures

terms of polarity. I t can be seen (Figure 4) tha t EF's increase with the molar fraction of the polar solvent, i.e., with the dielectric constant of the binary mixture. Dielectric constants D of binary mixtures were calculated from available data ( 4 2 ) or estimated. Dielectric constants of Freon 113 and isooctane binary mixtures were estimated from the corresponding cyclohexane data. Solubility parameters 6 of binary mixtures were also calculated (from reference 43) . As for individual solvents, EF's of binary mixtures vary lin- early with D (Figure 5 ) and 6 (Figure 6). Actually the molar fractions, dielectric constants, and solubility parameters of binary mixtures should be estimated from the liquid composition in the Soxhlet extractor rather than from tha t of the initial composition of the binary mixture. The initial volume of binary mixture was chosen so that i t exceeded the volume of the Soxhlet extractor by only a few cm3, this excess allowing for possible loss by evaporation. Thus in the case of azeotrope-forming binary mixtures, the liquid composition in the extractor in one extraction cycle varied from tha t fixed initially by the azeotrope composition to tha t very close to the boiling flask composition. Figures 4 and 5 , based on boiling flask composition, should be re- garded as qualitative in this respect.

Systematic deviations were observed again, as for the individual solvents, for all isooctane and acetone binary mixtures, whose EF's were found to be higher than predicted from their 6 parameters. The highest deviation was found for the isooctane-acetone mixture.

OCEF of Binary Mixtures. All binary mixtures were found to be more efficient for organic carbon extraction than benzene alone. This justifies the choice of binary mixtures for aerosol organics extraction. Using mixtures instead of nonpolar solvents alone permits the recovery of an important additional fraction of aerosol organics: up to 60% for Freon 113, 35-40% for benzene, and 1 5 2 0 % for the more efficient methylene chloride. Even the mixture of the less efficient polar (2-propanol, OCEF = 107) and nonpolar (Freon 113, OCEF = 76) solvent showed a higher OCEF ( = 119) than benzene. OCEF of all other binary mixtures vary in a small range (-130 to 150), without showing a marked influence of the nature of the solvent. For example, low OCEF increases were observed when replacing 2-propanol by methanol in benzene-polar solvent mixtures (from 135 to 145) or replacing cyclohexane by CH2C12 in methanol- nonpolar solvent mixtures (144 to 151). OCEF's of mixtures were found to be also higher than those of polar


Table VIII. Extractions with Binary Mixtures Compared to Successive Extractions Binary mixtures extractions, benzene +:

2 -Propanol Acetone Ethanol Methanol OCEF (from Table VII) 135 140 135 145 Water extraction, after binary

mixture extraction; OCEF 1 1.5 < 1 2 Successive extractions

F i rs t solvent Benzene Benzene Benzene Benzene

Second solvent 2-Propanol Acetone Ethanol Methanol

OCEF, + OCEFZ 137 138 133 149

OCEFl 100 100 100 100

OCEFZ 37 38 33 49

OCEF, < 1 1.5 2 < 1 Third solvent, water

solvents: 12-30% for 2-propanol, 10-24% for acetone, 12- 20% for ethanol, and a few percent for methanol. Higher differences between mixtures and polar solvents OCEF’s are expected for different sampling periods with lower oxygenated organics concentrations.

As shown in Table VII, binary mixtures have lower EF’s but higher OCEF’s than polar solvents. For example, the organic carbon accounts for 57% of the cyclohexane-metha- no1 extract compared with only 38.5% for the methanol extract. This indicates tha t inorganic species, most likely nitrates, are less soluble when lowering the dielectric con- s tant of the medium. This is consistent with the known solubilities of ammonium nitrate (and, in a lesser extent of sodium nitrate), which decreases from water to methanol, ethanol ( 4 4 ) , and pyridine ( 4 5 ) .

Comparison of Binary Mixtures Extraction with Successive Extractions. As indicated in the experimental section, OCEF’s of benzene-polar solvents mixtures were compared with successive extractions using the same polar solvents after benzene extraction. The same amounts of organic carbon were recovered, within the experimental precision, using either binary mixtures or successive extraction (Table VIII). As expected, more inorganic nitrates were found in the polar solvents extracts (successive extraction, second step) than in the mixture extracts. Nitrates were not measured in the organic extracts but by difference in water extracts, after mixture or successive extraction, using a colorimetric technique ( 4 6 ) . Water soluble nitrates after organic extraction were in the following order in respect to the solvents (or mixtures) used: benzene > benzene-2 propanol > benzene-ethanol >

benzene-methanol > 2-propanol > ethanol > methanol

OCA analyses were also made on water extracts, and the organic carbon concentrations were found to be very low, less than 2% of the samples organic carbon fraction (Table VIII). Very low organic carbon concentrations were recovered replacing water by benzene, CHZC12, triethyl amine, or methanol for numerous other samples after binary mixtures or successive extractions. From the results presented here, it is estimated that 95 to 100% of the aerosol organics are extracted using a polar solvent and a nonpolar one, together or in sequence.

CONCLUSION We have demonstrated a simple, reliable method for the

determination of aerosol organic carbon: organic solvent extraction followed by organic carbon analyzer measure- ment using concentrated extracts.

By comparison of numerous solvents, we have shown tha t solvent extraction efficiencies correlate well with polarity parameters, but tha t a single solvent is usually un- able to extract all the aerosol organics. Binary mixtures or successive extractions, using a nonpolar and a polar solvent, were much more efficient. Their use is strongly recommended instead of the widely accepted benzene extraction. All the concentrated extracts are suitable for OCA analysis without modification of the apparatus. Each OCA determination requires only a few minutes.

Another feature of the proposed method is its flexibility. Several pairs of solvents having similar OCEF’s can be chosen depending on requirements for further chemical analysis of the extracts (IR, GC, TLC, etc.). One might also use either binary mixtures (one-step) or successive extractions. In the latter case, OCA analysis can be made on the polar solvent extracts only. Assuming that organic compounds only are recovered in nonpolar solvents, and that their carbon content by weight is 75 f 5%, a simple weight determination is sufficient for nonpolar solvent extracts. Aerosol organics can be then expressed either as organic carbon or as organics (assuming now that polar solvent soluble organics contain 65 f 5% of carbon). Inorganic nitrates and sulfates can be measured in water extracts. A small piece of the filter can also be used for OCA analysis by direct combustion allowing the determination of total and inorganic carbon. The OE-OCA technique, combining a high extraction efficiency of aerosol organics with a rapid, accurate determination of organic carbon, is suitable for both laboratory, smog-chamber aerosols, and organics in atmospheric aerosols.

ACKNOWLEDGMENT I am grateful to S. K. Friedlander for his continuing in-

terest and helpful discussions during the course of this work.

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RECEIVED for review November 8, 1974. Accepted January 15, 1975. This work was supported by Environmental Pro- tection Agency Grant No. R802160. The contents do not necessarily reflect the views and policy of the Environmen- tal Protection Agency.

Response Characterization of the Tritium Ionization Cross- Section Detector

Ewan R. Colson

Scientific Services Department, Gas & Fuel Corporation of Victoria, No. 7 Liardet Street, Port Melbourne, Victoria-3207, Australia

Seven binary gas mixtures were fed to either, or both, of two tritium ionization cross-section detectors in order to find a general characterizing relation. A relation was found between a defined detector response parameter (v) and molar composition ( X ) , of the form

X y = - A. - (A - B - l)X - BX2

where A and B were the coefficients of a linear regression. A single regression coefficient relation was also proposed. The root mean square of the percentage difference between observed and calculated detector response, over the 130 more accurately blended mixes of the 291 observation pairs reported, was 0.32 YO. Under favorable conditions, the detector responded to composition changes of 50 to 100 PPm.

The ionization cross-section detector of Lovelock et al. ( I ) , using a tritium ionization source, was inferred to be linear to a t least 50% vapor concentration by volume. These authors, and Shoemake ( 2 ) , presented calibration curves for a micro parallel plate detector plotted on a log/log scale from data collected using an exponential decay cell, as described by Lovelock ( 3 ) .

I t has also been stated ( I ) that the ionization cross-section detector of Pompeo and Otvos ( 4 ) is linear to 100% gas or vapor concentration.

Washbrooke ( 5 ) reported tha t the responses of both the tritium and strontium 90/yttrium 90 detectors were linear over many orders of magnitude.

Published experimental evidence to adequately support these claims seems to be lacking. In fact, Deal, Otvos, Smith, and Zucco (6) presented calibration data with nitro- gen-heptane blends, and Boer (7) showed curves for nitro- gen-butane blends which indicated a diminishing response per unit of concentration change as the concentration of the heavier component increased. This detector used com- paratively higher energy &particle sources of strontium 9O/yttrium 90 in larger volume cells than the detector of ( I ) . The paper of Deisler e t al. (8 ) , showed nonlinear calibration curves for several binary gas mixtures in a gas analysis cell using, as the main ionizing species, a-particles produced by the decay (in three stages) of radium D.

The choice of radiation source for these detectors has in par t been based on the consideration tha t the mean energy of the ionizing species should be nearly uniform within the measuring zone. The cell geometries have been designed to make the possible path lengths of the energetic particles a smal1,proportion of their mean range in the medium (6). Thus, the use of weak @-particles from tritium was practical only with the micro version of the detector ( I ) .

This paper describes experimental observations of ob- viously nonlinear binary mixture responses of two versions of the tritium ionization cross-section detector. A characterizing relation is then developed which seems to adequately conform to the experimental data and so enable the wider practical application of the detector.


solvent extraction and organic carbon determination in atmospheric particulate matter. organic...

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