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. T h e values in Table IV were computed by averaging the values for each method presented in Tables I1 and 111, disregarding signs and excluding trials in which points representing less than 60% of the titration curve were used. T h e 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. T h e 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 t o the precision of the measurements, and accepting results which are a t best slightly inferior to those attainable with conventional visual indicators.
LITERATURE CITED C. McCallum and D.Midgley, Anal. Chim. Acta, 65, 155 (1973). G. Gran, Analyst(London), 77, 661 (1952). P. Sorensen, Kern. Maanedsbl., 32,73, (1951). "Gran's Plots and Other Schemes", Newsletter of Orion Research Incorporated, 2, 11 (1970). (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 York, NY, 1965, pp 85-92. (7) J. D. Hinchen, "Practical Statistics for Chemical Research", Methuen and Co., Ltd., Great Britain, 1969, p 35.
(1) (2) (3) (4)
RECEIVEDfor review August 19, 1974. Accepted January 15, 1975.
Solvent Extraction and Organic Carbon Determination in Atmospheric Particulate Matter: The Organic ExtractionOrganic 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 4 8 % 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 dlfference 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 C H N analysis (4, 5 ) , fractionation into classes (5, 6 ) and analysis of each fraction for specific compounds by T L C ( 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. T h e purpose of this study is twofold: to show that extracting 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, M A Y 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. Conditioning 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). Preliminary studies of the optimal extraction time and of the possible loss of organic material during extraction were made by gas chromatographic 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 stirrer (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 tempera798
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
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 difficulty in obtaining homogeneous suspensions. However, the ratios (TOC - VOC)/TOC (=NVOC/TOC) scaled up t o the known initial dilution ratio. permit calculation of the aerosol organic carbon with a precision of f 5 % . 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 Analyzer; c ) direct combustion of a small piece of glass fiber filter, (3he-inchdiameter) 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. Although 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 T H filters). Infrared spectra will be discussed later in this paper.
RESULTS AND DISCUSSION The increasing number of studies dealing with the chemical 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 difunctional compounds, all of which are photochemical products. T h e 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
Table I. Determination of O r g a n i c Carbon in O r g a n i c Solvents E x t r a c t s w i t h the Organic Carbon Analyzer Pure solvent
Solvent: methanol
Dilution r a t i o s (p1,'50 nilH,O) TOC calculateda
voc ib
TOC Ib
AC'IC c
10 59.5 60.0 1.7 60.2 1.6 1.2 0.2
20 119.0 118.9 3 .O 120.9 3.2 1.6 2 .o
50 297.4 299.5 3.6 303.6 3.7 2.1 4.1
Solvent - extractf
100 595 598 11 605 13 1.7 7
200 1190 1202 17
1210 19 1.7 8
50
100
2 00
300.2 6.0 347.8 5.8
601 12 695 11
1208 19 1386 21
...
...
...
...
...
...
TOC - VOC 47.6 94 178 ... ... ... 43.5 87 170 TOC - VOC, correctedd 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 .ratio. Results expressed as mg C/1. C (TOC measured - TOC calculated)/TOC calculated. Subtracting the solvent impurities (TOC VOC) at the same dilution ratio. e 24-hour Hi-Vol sample at 70 SCFM, total aerosol concentration 102 pg/m3. Concentrated to 1 ml after Soxhlet extraction (60 ml solvent, 6 hours).
...
... ...
...
Table 11. Organic Carbon Determination: Comparison of the OE-OCA a n d O t h e r Methods OE-OC.4b E-CHSC EV- OCA^ Sample -*itf mg OC * mg CC *:r r n g orsanic C
r;
CFF -0c.4~ mg OC
f :$
N0.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 5.0 208.2 5.1 4 212.5 3.9 193.4 3.8 197 .O 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 (nonwater miscible, No. 2 and 4 ) , cyclohexane-methanol (No. s),and isooctane-isopropyl alcohol ( S o . 6). OE-OCA: organic extraction, extract concentration, OCA analysis. EV-CHN:organic extraction, evaporation, CHN analysis. EV-OCA: organic extraction, evaporation, OCA analysis. e GFF-OCA: direct OCA analysis of glass fiber filters. Average on 6 runs for each sample and each technique. f
___ -.
Table 111. -Carbon
Content of Glass Fiber Filter as Measured
4 7 - m m 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 Filters, highest measured value 45.7 40.0 Highest measured value, pg OC p e r sample (3/16-in. i.d. filter) 1.37 1.20 7.75 5.50 p e r cm2 of filter 74 2238 p e r total filter EquiIralent sampling time of 100 ,ug/m3 acmospheric aerosol, sampling flow r a t e 9.25 min. ... 80 I./min 20 1. / m i n (cascade impactor) 37 min. ... 60 scfm (Hi-Vol) 13.1 min a t 80 l . / m i n : at 60 scfm: E r r o r due to the carbon impurities 1-h.our samples 15% = 22% 24-hour samples 2 0,65%# = 0.9% 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 extraction. The aerosol organic fraction, usually expressed in t h e past as benzene soluble, is seriously underestimated for samples taken in urban areas where oxygenated organics, produced by photochemical reactions, are a significant fraction of t h e atmospheric aerosol. This is of considerable importance for control strategies dealing with visibility degradation a n d adverse health effects associated with par-
ticulate organic matter. I n order t o improve t h e determination of aerosol organic carbon, we propose t h e following scheme: 1) successive extraction using a polar solvent after a nonpolar one, or alternatively, "one-step" extraction using a binary mixture of a polar and a nonpolar solvent; and 2 ) organic carbon determination using a Carbon Analyzer, as described in the Experimental section. Several solvents and binary mixtures were studied for ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 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 CL&in 0 , = IO0 OC,/'O, 70 73 70 73 77 78 75 75 77 79 0.060 0.052 0.070 0.085 0.038 0.026 0.046 Water soluble organic carbon OC,"* 0.105 0.073 0.038 0.176 0.192 0.199 0.169 0.152 0.210 Organic carbon fraction OCF = 0.301 0.236 0.166 0.185 oc, + OC," 29.6 36.4 42.6 22.4 17.1 30 35 31 22.8 24.8 Missing organic carbon, as % of OCF, using benzene alone Missing organic carbon with benzene alone, average 10 samples: 29.2% 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 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 Solvent (0)
Literature data (b)
(C)
(d)
O C E , this work
(e)
(f)
(b)
Silicon tetrachloride 69 ... ... ... ... ... Carbon disulfide 70 ... ... *.. ... 69 #?-Pentane 71 ... ... *.. 66 Freon 113 76 ... ... ... ... 76 Cyclopentane 77 ... ... ... ... ... a-Hexane 78 ... 71 ... 78 Carbon tetrachloride 81 ... 87 ... *.. 80 Diethylether 82 77.5 ... 100 80 Cyclohexane 84 ... 62 72.5 75, 81.5 84 10. Isooctane 95 ... ... 93 94 11. BENZENE (reference) 100 (100) (100) (100) 100 12. Toluene 102 ... 100 133 94 ... 13, Trichlorethylene 109 ... ... ... ... 108 112 ... ... ... ... ... 14. Diethylamine 118 ... ... ... 112, 107 114 15. Chloroform 16. Triethylamine 120 ... ... ... ... 116 1 7 . Methylene chloride 126 *.. 107 104.5 106 120 18. Tetrahydrofuran 162. 100-260 ... ... 84 54-115 ... ... 82 47-110 170 112-262 19. l,4-Dioxane 20. Pyridine 185 115-295 259 ... ... 75 52-116 224 70-327 222 *.. ... 105 61-167 21. 2-Propanol 250 71-388 239 197 194, 1 4 1 112 6+170 22. Acetone 23. Ethanol 281 95-421 234 ... 350 113 64-190 362 85-560 272 520 135 69-200 24. Methanol 25. Dimethyl sulfoxide 364 90-520 ... ... ... k) ... 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). 1. 2. 3, 4. 5. 6. 7. 8. 9.
...
...
...
...
...
...
...
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 Solvents. 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 T L C 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, halogenated hydrocarbons, alcohols) were widely used in t h e past for the extraction of aerosol organics. Results are listed in Table V, as relative extraction ef800
ANALYTICAL CHEMISTRY, VOL. 47, NO 6, M A Y 1975
ficiencies ( E F ) with benzene as reference solvent: t o t a l e x t r a c t e d m a t e r i a l , solvent i x 100 EF, - total e x t r a c t e d material, b e n z e n e , 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 t h e efficiency of 17 organic solvents for Los Angeles aerosol samples. Their results are included in Table V for comparison purpose. T h e organic carbon content of most of the extracts was measured with t h e OCA, as described in t h e Experimental section. Results, scaled-up to benzene, are also listed in Table V, as organic carbon extraction efficiencies (OCEF): e x t r a c t e d o r g a n i c c a r b o n , solvent i O C E F , - e x t r a c t e d o r g a n i c c a r b o n , benzene, x 100 s a m e sample
503,
Table VI. Parallel Extractions with Benzene and Water
O,*
oc Lla
OCwb
0.205 0.098 0.107 0.162 0.088 0.192 0.093 0.150 0.096 0.131 0.158 0.103 0.225 0.100 0.211 0.138 0.128 0.107 0.068 0.120
0.187 0.094 0.191 0.140 0.106 0.149 0.214 0.236 0.125 0.110 0.1 22 0.088 0.194 0.069 0.226 0.150 0.280 0.145 0.138 0.089
CCEF, water'
91 96 179 86.5 120 77.5 230 157 130 84 77.5 101 103 82 107 109 219 135 2 04 74 Average 123
LOC
-
30c
-
EF
?OC c
CC -
"2
Polar solventd
76
D
2.4
05
24
C6
F P ~
The highest ozone concentration was observed for samples collected on 725-73.
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, it is not possible t o 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
EFIOCEF~
33
Figure 2. Solvents extraction efficiencies (EF) as function of the ozone concentration averaged over the sampling period.
EF Scale. I t can be seen from Table V, first column, t h a 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. T h e EF's of aliphatic hydrocarbons increase with the carbon atom number: the solubility increasing with the temperature, 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 t h a 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
Nonpolar solvent
C 2
:::-e
See definition in'rable IV. ' Reference OCEF benzene = 100.
Freon 113
3
2-Propanol
Acetone
Ethanol
Methanol
EFO
230
260
291
374
OCEF~
107
120
122
144
Db
EF OCE F De EF OCEF D EF OCEF D EF OCE F D EF OCEF
18.3
20.7
157 119 7.8 210 125 7.4 220 131 9.6 225 135
191 131 9.6 230 136 9.0 241 132 11.2 240 140 10.0 248 137 14.7 249 144 12.4
24.3
32.6
198 220 134 147 12.6 15.8 Cyclohexane 84 2 .o 242 252 130 144 11.8 15.0 Isooctane 95 1.9 255 281 13 0 144 14.8 18.3 Benzene 100 2.4 268 301 13 5 145 7.0 12.0 15.9 Chloroform 118 4.8 260 312 218 131 139 149 D 13.7 17 .O 9.5 219 Methylene 126 8.9 EF 260 320 chloride 140 142 151 OCE F D 11.5 14.0 18.2 "Measured for the sampling period. See Table V for EF and OCEF ranges and averaged values. * D = dielectric constant. EF and OCEF are the same for nonpolar solvents. Binary mixtures, 50-50% bv volume. e Measured ( 4 2 ) or estimated (see text). Y
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
801
430
c
I
/ / - I
,/
13 0
23 2
30 4
43 6
50
e
60 r IC
’
3 :-!,ET!-) ?‘!*I
I5
8 (-1
EFs as function of the dielectric constant D,the solubility parameter 6, and the polarity parameters ET and P’ Figure 3. Solvent
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 oxygenates 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 t h a 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 , E T , 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 Snyder’s x d (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. Therefore, EF’s are functions of the “overall” polarity of the solvent, which is simply the net effect of the various solventssolute interactions. 802
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
I 0
1
1
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 contrary, 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 experimentally 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 automobile 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 olefins and diolefins, are also slightly soluble in water (40).
2ooL
3001
I
/
T+--
300
-
200
-
EF
IO0
LL-
20 10
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 regard to their very low concentrations, or good solubilities in water. Table VI shows, along with other recent studies ( 1 8 ) , t h a 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 t h a 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 improved 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 ) . T h e latter mixture was also used to extract organics from water ( 4 1 ) . The following trends are observed: All binary mixtures have E F values much higher than t h a t of the nonpolar solvent ( N P ) but slightly lower than t h a 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. T h e behavior of binary mixtures can also be explained in
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) t h a 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 4 3 ) . As for individual solvents, EF's of binary mixtures vary linearly 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 t h a t of the initial composition of the binary mixture. The initial volume of binary mixture was chosen so that it 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 t h a t fixed initially by the azeotrope composition to t h a t very close to the boiling flask composition. Figures 4 and 5 , based on boiling flask composition, should be regarded 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 methanolnonpolar solvent mixtures (144 to 151). OCEF's of mixtures were found to be also higher than those of polar ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
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Table VIII. Extractions with Binary Mixtures Compared to Successive Extractions Binary mixtures extractions, benzene
OCEF (from Table VII) Water extraction, after binary mixture extraction; OCEF
2 -Propanol 135
1
+:
Acetone
Ethanol
Methanol
140
135
145
1.5
2
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 measurement using concentrated extracts. 804
Benzene
100
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
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By comparison of numerous solvents, we have shown t h a t solvent extraction efficiencies correlate well with polarity parameters, but t h a t a single solvent is usually unable 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 a m grateful to S. K. Friedlander for his continuing interest and helpful discussions during the course of this work.
LITERATURE CITED (1) G. A . Jutze and K. E. Foster, J. Air Poliut. ControiAssoc., 17, 17 (1967). (2) P. P. Mader. R. D. McPhee, R. T. Lofberg, and G. P. Larson, h d . Eng. Chern., 44, 1352 (1952). (3) N. A. Renzetti and G. J. Doyle, J. Air Pollut. Control Assoc., 8, 293 (1959). (4) P. Cukor, L. L. Ciaccio, E. W. Lanning, and R. L. Rubino. Environ. Sci. Techno!., 6, 632 (1972). (5) E. Sawicki, T. W. Stanley, T.R. Hauser, H. Johnson, and W. Elbert. Int. J. Air WaterPoiiut.. 7, 57 (1963). (6) E. C. Tabor, T. R. Hauser. J. P. Lodge, and R. H. Burtschell, AMA Arch. lnd. Health, 17, 58 (1958). (7) E. Sawicki, T. W. Stanley, S. McPherson. and M. Morgan, Talanta, 13, 619 (1966). (8) T. R. Hauser and J. N. Pattison, Environ. Sci. Techno/., 6, 549 (1972). (9) S. P. McPherson, E. Sawicki, and F. T. Fox, J. Gas Chrornatogr.,4, 156 (1966).
E. Sawicki, S. P. McPherson, T. W. Stanley, J. Meeker, and W. C. Elbert, lnt. J. Air Water Pollut., 9, 515 (1965). T. W. Stanley, J. E. Meeker, and M. J. Morgan, Environ. Sci. Techno/., I, 927 (1967). > , RTC. Lao, R. S. Thomas, H. Oja, and L. Dubois, Anal. Chem., 45, 908 (1973). "Air Quality Data for Organics 1969 and 1970 from the National Air Surveillance Networks" Report APTD-1465, Environmental Protection Agency, Research Triangle Park, NC, June 1973. lntersociety Committee: Methods of Air Sampling and Analysis, American Public Health Association, Washington, DC, 1972, p 173. A. Liberti, G. P. Cartoni, and V. Cantuti, J. Chromatogr., 15, 141 (1964). M. C. Goldberg. L. DeLong, and M. Sinclair, Anal. Chem., 45, 89 (1973). K. Grob and G. Grob, J. Chromatogr., 62, 1 (1971). D. Grosjean and S. K. Friedlander, 67th Air Pollution Control Association Annual Meeting, Paper No. 74-154, Denver, CO, June 9-13, 1974. R. K. Patterson, Anal. Chem., 45, 605 (1973). D. Grosjean and S. K. Friedlander, in preparation. D. Schueltze, A. L Crittenden, and R. J. Charlson. J. Air Pollut. Control Assoc., 23, 704 (1973). D. Schueltze, D. R. Cronn, A. L. Crittenden, and R. J. Charlson. 172nd National Meeting, ACS, Chicago, IL, August 27, 1973. M. Gruenfeld, Environ. Sci. Techno/., 7, 636 (1973). R. J. Gordon, Atmos. Environ., 8, 189 (1974). A. L. McClellan, "Tables of Experimental Dipole Moments", Freeman, San Francisco, CA, 1963. A. A. Maryott and E. R. Smith, National Bureau of Standards Circular No. 54, Washington, DC, August 10, 1951. J. H. Hildebrandt and R. L. Scott, "The Solubility of Non-Electrolytes". 3rd ed., Dover Publications, New York, NY, 1964. L. R. Snyder, "Principles of Adsorption Chromatography", Marcel Dekker, New York, NY, 1968, Chap. 8. C. Reichardt and K. Dimroth, Fortschr. Chem. Forsch., 11, 1 (1968). L. R. Snyder, J. Chromatogr., 92, 223 (1974). L. Rohrschneider. J. Chromatogr., 22, 6 (1966). L. Rohrschneider, Anal. Chem., 45, 1241 (1973). ~~
(33) (34) (35) (36) (37) (38)
(39) (40) (41) (42) (43) (44) (45) (46)
W. 0. McReynolds, J. Chromatogr. Sci., 8, 685 (1970). K. L. Hoy, J, Paint Techno/., 42, 76 (1970). C. Hansen. lnd. Eng. Chem., Prod. Res. Dev., 8, 2 (1969). R. A. Keller, E. L. Karger, and L. R. Snyder, "Gas Chromotography 1970", R. Stock and S. G. Perry, Ed., Institute of Petroleum, London, England, 1971, p 125. A. Hartkopk, J. Chromatogr. Sci., 12, 113 (1974). J. Cholak, L. J. Schaefer, D. W. Yaeger, and R. A. Kehoe, "The Nature of the Suspended Matter", Section Vlll in "An Aerometric Survey of the Los Angeles Basin, August-November 1954", Air Polution Foundation, Los Angeles, CA, 1955. A. Anusiem and P. A. Hersch, Anal. Chem., 45, 592 (1973). C. McAuliffe, J. Phys. Chem., 70, 1267 (1966). E. J. Gallegos. Anal. Chem., 45, 1399 (1973). J. Timmermans, "The Physico-Chemical Constants of Binary Systems in Concentrated Solutions". Vol. 1 and 2, Interscience, New York, NY, 1959. H. M. N. H. Irving, "Ion Exchange and Solvent Extraction", Vol. 6, J. A. Marinsky and Y. Marcus, Ed., Marcel Dekker. New York, NY, 1974, Chap. 3. p 139. W. F. Linke, "Solubilities, inorganic and Metal-Organic Compounds". American Chemical Society Pub., Washington, DC, 1965, Vol. 2, 4th ed., pp 709-727. H. Stephen and T. Stephen, "Solubilities of Inorganic and Organic Compounds", Macmillan, New York, NY, 1963, Vol. 1, Part 1, p 745. lntersociety Committee on Methods for Ambient Air Sampling and Analysis No. 3, E. E. Saltzman, Chairman, Health Lab. Sci., 7, 267 (1970).
RECEIVEDfor review November 8, 1974. Accepted January 15, 1975. This work was supported by Environmental Protection Agency Grant No. R802160. T h e contents do not necessarily reflect the views and policy of the Environmental Protection Agency.
Response Characterization of the Tritium Ionization CrossSection Detector Ewan R. Colson Scientific Services Department, Gas & Fuel Corporation of Victoria, No. 7 Liardet Street, Port Melbourne, Victoria-3207,
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
y = -
X
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.
T h e 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.
Australia
Washbrooke ( 5 ) reported t h a 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 nitrogen-heptane blends, and Boer (7) showed curves for nitrogen-butane blends which indicated a diminishing response per unit of concentration change as the concentration of the heavier component increased. This detector used comparatively higher energy &particle sources of strontium 9O/yttrium 90 in larger volume cells than the detector of ( I ) . T h e paper of Deisler et 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. T h e choice of radiation source for these detectors has in part been based on the consideration t h a 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 @-particlesfrom tritium was practical only with the micro version of the detector ( I ) . This paper describes experimental observations of obviously 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. ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
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