Gas Chromatographic Analysis of c8-c,8 Hydrocarbons in Paris Air Alain Raymond and Georges Guiochon' Laboratoire de C h i m i e Analytique Physique, E c o l e Polytechnique, 17, rue D e s c a r t e s , Paris tjeme, F r a n c e
Gas chromatographic analysis of the Ca-Cia organic pollutants of Paris atmosphere were made during fall 1972. T h e sampling procedure involved trapping of the vapors over graphitized carbon black followed by heat desorption for direct injection of the pollutants on long efficient capillary columns. T h e separations were made over capillary columns in temperature programming. More than 70 peaks were identified using a mass spectrometercomputer system coupled t o t h e capillary columns; they are mainly aromatics a n d paraffinic hydrocarbons. Quantitative determinations of some pollutants were made and compared with similar results found in the literature.
This work describes a fast and efficient system for gas chromatographic analysis of atmospheric hydrocarbons, especially in t h e Ca-Cla range. along with the results obtained. Though many powerful analytical systems are now available for the analysis of organic pollutants in the a t mosphere. a too large time interval between t h e sampling and analysis periods strongly decreases the validity of the results. Indeed, it is well-known t h a t changes in the concentration of atmospheric pollutants may be rapid. A better use of t h e air pollution monitoring systems requires a decrease in t h e time between sample conditionning and its injection into t h e analytical system. We chose to investigate the Ca-Cla hydrocarbons because few studies have been published so far dealing with the individual analysis of these atmospheric pollutants, in contrast to t h e Cl-Ca compounds (Eggersten and Nelsen, 1958; Altshuller and Bellar, 1963: Williams. 1965) and to the polynuclear hydrocarbons and related compounds (Liberti et al., 1964: Sawicki et al.. 1965; Matsushita et al.. 1970; Lao et al.. 1973). T h e Ca-Cla hyclrocarbons are present in relatively large concentrations in the fuels used for industrial or domestic heatings. for diesel or internal combustion engines, in the exhaust gases of these engines, and in many other pollution sources, so t h a t it does not seem surprising to find them in a n urban atmosphere. Our method involves the choice of graphitized carbon black for t h e trapping of organic vapors. In a previous paper (Raymond and Guiochon, 1973), we studied the efficiency of this adsorbent. Its main advantages are t h a t it is a nonselective adsorbent, which adsorbs strongly all organic compounds, independently of their structure; its chemical inertness makes any reaction with known pollutants or catalysis of reactions. improbable: its thermal stability is excellent, so t h a t heat desorption is possible u p t o more t h a n 400°C-necessary for the highest boiling compounds; its physical structure is well-known, and its surface is very homogeneous; at ambient temperature, a d sorption of water and light gases-like methane, carbon monoxide, and carbon dioxide-is very small so t h a t a dehydrating mass is not needed in the sampling line. even during a rainy period. T h e Cg-Cia atmospheric hydrocarbons represent a very To whom correspondence should be addressed.
complex mixture where hundreds of compounds are present. Consequently, t h e use of very efficient capillary columns is needed for such separations, and the identification of more t h a n 100 peaks is possible only if a mass spectrometer is coupled to the chromatograph.
Experimental Air pollutants are sampled and concentrated by aspirating air with a n oilless vacuum p u m p (Bell and Gosset, BLV model), through a t r a p made of a short tubing (4 m m i.d., 5 cm long) packed with graphitized carbon black, Sterling F.T.G. (kindly supplied by W . R. Smith, Cabot Co., Boston, Mass.). Each t r a p contains 0.25-0.40 gram of 200-250 p sieved black, held in place by small silica wool plugs. They may be used for several ( u p t o 10) sampling-desorption cycles, without any decrease in their performances. Because the particles of graphitized carbon black are. in fact. aggregates of fine dust. the permeability however decreases slowly, and the tubes have to be changed periodically, otherwise sampling becomes too slow. T h e sampling syst e m also involves a gas-counter and a flow-rate controller. No attempt to remove aerosols or particulates by a filter is made, and t h e sampling tube is used alone a t ambient temperature; the air flow is set a t 0.5 l./min. Lsually, sample volumes of 0.2-2 m3 are used for the analysis of Ca-Cls organic pollutants. After the sampling period is completed, the tubing t r a p is carried to the gas chromatograph and connected by leakproof metal fittings to the analytical column and to the carrier gas (argon) supply. After the carrier gas is set u p at about 1 cm3/min. the t r a p is heated to 400°C within 15 sec by a n electric. wire (Thermocoax) soldered around it. During this operation, the analytical column ( a capillary column) is kept a t ambient temperature, so that the compounds vaporized from the sampling tube are trapped a t the column inlet. The desorption step is usually ended within 5 min. After this step is achieved, the sampling tube is removed and the gas chromatographic analysis is started. The column used (100 meters long. 0.4 m m i.d.) is coated with silicon fluid OV 101 (Supelco). Its efficiency is about 200,000 theoretical plates for n CZo at 200°C ( k ' = 1.21).Analyses are made at a programmed temperature, from ambient to 250°C, a t a rate of 2"Cjmin. T h e detector is a flame ionization detector, but for identification purposes the column can be coupled to a T S N 203 S E mass spectrometer (Thomson). The sweeping rate of this instrument is one mass decade per second, the resolution is about 400 a t mass 170, and the ionization voltage is 70 eV. T h e mass spectrometer is directly connected t o a Hewlett-Packard 2116 B computer for real-time, acquisition of the spectra. The intensity and masses of the main peaks of the mass spectra are calculated and typed immediately (Guilbot et al., 1972). This system allows one t o save considerable time in experiments during which more t h a n 50 mass spectra are usually generated during each analysis, which takes about 1 hr. Some experiments have also been made. using a iXi63 electron capture detector (Microtek. Tracor) but we did not succeed in obtaining useful chromatograms due to detector overloading. Volume 8, Number 2 , February 1974 1 4 3
Results and Discussion Samples were all taken on the flat roof of our laboratory. about 20 meters above a busy street (Monge Street) in central Paris. This place is windy and well-ventilated. A typical chromatogram obtained in the conditions described above, on Tuesday, October 24, 1972, is shown in Figure 1. The sample volume was 200 1. in 24 hr. and the ambient temperature was about 10°C. T h e identified peaks are listed in Table I. All these substances are hydrocarbons, mainly alkanes and aromatics. This fact might seem surprising but it has been verified many times. There are probably many oxygen derivatives. but at much lower concentrations, so they are concealed by the hydrocarbons. Using our system t o prepare standard atmospheres with known amounts of organic vapors (Raymond and Guiochon, 1973), we have demonstrated t h a t reactive compounds, such as benzaldehyde, are not destructed on graphitized carbon black or reduced during the desorption step, but eluted unreacted. No peak can be identified before toluene on Figure 1. This can be explained by losses during the trapping and transfer steps. First, vapors are trapped from t h e air sample only in a gas volume equal to the retention volume of the corresponding compound ori the t r a p used (Raymond and Guiochon, 19'73). T h e n the gas exiting from the column has the same concentration as the original sample and no more trappings occur. In addition, some competitive adsorption might occur a t the disadvantage of the lighter compounds. Moreover, during the desorption step, t h e lighter compounds either are not retained a t the inlet of the column or they give large bands resulting in the broad peak appearing early on the chromatogram. We did not attempt to analyze light pollutants in this work, however. If such analyses have to be made. it would be easy to cool the adsorption tubing during the sampling. and the beginning of the capillary column during the desorption step; this gives much flexibility to our method. Most of the peaks have been identified using the mass spectrometer-computer system, but there are some limitations to the results obtained:
In fact, even with a very efficient capillary column, most peaks in the chromatograms of atmospheric pollutants correspond to a mixture of substances where isoparaffinic compounds are always present. Consequently, even by using a computer, the examination of the mass spectra is difficult. T h e alkanes. one of the two main groups of substances present in the Paris atmosphere, give very closed mass spectra, and their complete identification by mass spectrometry is almost impossible. Especially is it not possible to determine exactly which isomers are present. Last, the same problem arises for the isomeric alkyl benzenes, the mass spectra of many such compounds also being very much closed. The identification of such isomers is possible by a chromatographic method if they are resolved. We have used the Kovats indices (Kovats. 1965). The indices of the gas chromatographic peaks are measured and compared to those calculated from chromatograms obtained for the pure products in the same conditions. The calculation of the Kovats indices is easy in our analysis because the nalkanes are always present in large amounts in our samples. If pure products are not available, and in the case of hydrocarbons it is still possible t o provide for Kovats indices since. on a nonpolar liquid phase like OV 101, hydrocarbons are roughly eluted according to their boiling points. Moreover, some methods of prediction of Kovats indices are available, like the Robinson and Odell's one (1971), which involves the use of the boiling points d a t a of pure compounds. Kovats indices measurements and mass spectrometry data are usually sufficient to make sure the identification of most gas chromatographic peaks in unknown mixtures. Figures 2 and 3 show chromatograms of premium grade automobile gasoline and of fuel for diesel engines. The analytical conditions are exactly t h e same as for Figure 1, but in these experiments, the solutes (0.02 and 0.04 p1. of each mixture) have been directly put down on the sampling tubing by means of an Hamilton microsyringe. Figure 2 is similar to the beginning of Figure 1. except in the case of the n-alkanes above n-Cs (peaks no. 7, 19.
-._ . ...
i
1
~
L e -
Bo
Figure 1. Chromatogram of Sample volume 200 I Numbers refer to substances listed
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Environmental Science & Technology
Table I. Air Pollutants Identified N u m b e r s correspond t o those i n Figures 1, 2, and 3 No.
1 2 3 4 5 6
7 8 9 10 11 12 13
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
-_1
Identification
Compound
+
Toluene 1-octene n-Octane 1-nonene Ethylbenzene m. a n d p-Xylene Styrene isononane 0-Xylene n- No na rie lsodecane Isodeca ne n-Propyl benzene lsodeca ne m- a n d p.Ethyltoluene 1-3-5.Trimethylbenzene lsodecane 0.Ethyltoluene tert-Butylbenzene 1-2-4-Trimethylbenzene sec- Bu tyl benzene n-Decane 1-2-3-Trimethylbenzene p-cymene i sou n d eca n e lndane lsou ndecane Indene Diethylbenzene lsoundecane n-undecane n.Butylbenzene n-undecane lsoundecane Ethyldimethyl benzene Ethyldimethylbenzene lsou ndecane I sou ndecane I sou ndeca ne U ndeca ne Deca hydronaphthalene n.U ndecane 1-2-4-5-Tetramethylbenzene
Ms M S-gc M S-gc M S-gc M S-gc M S-gc M S-gc Ms Ms Gc Ms M S-gC M S-gc
+
+
+
+
+
~~
~
-
-
Ms M s-gc MS-gc M s-gc
+
M S-gC M S-gc M S-gc M S-gc Ms Gc Gc Ms MS-gc Ms M S-gC M S-gC Ms Ms Ms Ms Gc M S-gc M s-gc
No.
Corn po u nd
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57
1-2-3-5-Tetram et hy Ib e nze n e lsododecane lsododecane Do d ece ne lsododecane Dodecene lsododecane tetrahydronaphthalene lsododecane lsododecane Naphthalene lsododecane Dodecene n-Dodecane lsotridecane Isotridecane tridecene Isotridecane I sotridecane I sotridecane I sotri deca ne Tridecene Pentamethylbenzene 2methylnaphthalene n-Tridecane 1-methylnaphthalene lsotetradecane Diphenyl isotetradecane 2 ethylnaphthalene n-Tetradecane 1-Et hy I na p ht ha le n e 1-6-Dimethylnaphthalene I-4-Dimethyl na p hthalene 2-3-Dimethylnaphthalene Hexamethylbenzene Ace nap hthene n-Pentadecane diphenylpethane n-Hexadeca ne Fluorene n-Heptadecane n.0ctadeca ne
58 59 60 61 62 63 64 65 66 67 68 69 70 71 72
+
+
+
+
+
+
+
Identification
M S-gC
Ms Ms Ms
Ms MS
M s-gc Ms Ms M S-gC Ms Ms MS-gc
Ms M S-gC Ms
Ms Ms Ms
Ms M S-gC M s-gc
Ms M S-gc
M S-gC Ms M s-gc M S-gc M S-gc Gc Gc Gc-ms GC Gc GC GC
I
--I
60
n
LO
sample of polluted atmosphere in Table I Straight line gives the column temperature
Volume 8. Number 2, February 1974
145
20
1
Figure 2. Chromatogram of premium Sample volume, 0.04 11.
Numbers refer to substances
Figure 3. Chromatogram Sample volume, 0.02 PI. Numbers refer to substances
35, . . .), these compounds' being absent from automobile gasoline. On the other hand, these compounds are present in large amounts in diesel engine fuel, as we can see on Figure 3, where they are the main components (peaks no. 7, 19, 35. 49, 58, 61). Consequently, the main source of nalkanes in t h e Paris atmosphere, a t least in the c8-ClS range, is due to truck traffic while aromatics are mainly emitted by automobiles; both truck and automobile traffic is by far t h e largest source of air polluants in t h e CSCIS range through evaporation and emissions of crank and exhaust gases. This fact is not surprising when we consider the importance of traffic in center Paris and especially near the place where samplings were made. For quantitative determinations of pollutants, we must be sure t h a t no substances to be analyzed passed through the tubing during the sampling period. As it is passed 146
Environmental Science & Technology
through by a roughly constant concentration mixture, the tubing may be regarded as a chromatographic column working in frontal analysis. At a given temperature, any vapor diluted in a gas stream continually flowing through the tubing will be quantitatively adsorbed inside the t r a p until t h e total sample volume is equal to the retention volume of the studied substance a t this temperature, less the half-base width of t h e particular peak obtained for this compound in elution analysis (Figure 4 ) . When this volume is reached, the gas and the adsorbent are in equilibrium for the corresponding substance, no more trapping occurs, and the system operates according to the S o v a k and coworkers' method (Fovak et al., 1965). For trimethyl phosphate, we have determined (Raymond and Guiochon, 1973) that the maximum sample volume for quantitative trapping in one of our graphitized carbon
t
grade gasoline (in C S - C , range) ~ listed in Table I . Column temperature is given in abscissa
Start
/ ! A of diesel
I
engine fuel
listed in Table I . Column temperature is given in abscissa
black tubing is 6.5 liters a t 25°C. This value is certainly lower than t h a t which would be obtained with commonly used adsorbents--- i.e., charcoal, silica gel, porous polymers. Conversely heat desorption of heavier compounds becomes possible with graphitized carbon black. I n practice, a n easy way to verify if quantitative determination is possible consists in putting two sampling tubings in series and in looking for t h e pollutants which have been trapped on t h e second tubing. With this procedure we could note t h a t quantitative determination is possible from peak eluted after no. 4. We have carried out t h e quantitative analysis after the peak heights using calibration curves determined for some representative compounds. In t h e case of trace analysis. this method is as accurate as t h e area measurements (Guiochon e t al. 1970).
Table I1 shows the range of variations in concentrations of some atmospheric pollutants measured in Paris between October 2 i , 1972 and November 8, 1972. Unfortunately, there are few concentration d a t a in the literature for individual compounds in t h e Cs-Cls range, and we only can compare our results t o those obtained by Lonneman et al. (1968) in Los Angeles and by K . and G. Grob (1972) in Zurich; though our d a t a are approximate, they lead to similar values. We also note t h a t the range of variation of these concentrations is large; it depends much on meteorological conditions and on traffic volume.
Conclusion We have shown that qualitative and quantitative tleterminations of CS-ClS individual atmospheric pollutants are Volume 8, Number 2, February 1974
147
A
Table II. Concentrations of Some Atmospheric Pollutants in Paris, Zurich, and Los Angeles Concentration, p g / m a
Los
N0.
4 6 7 12
13
8
15
17 19 35 36 37 46 49 69 Figure 4.
Profile of concentration at outlet of chromatographic
a
Compounds m.
+ p.Xylene
o.Xylene n-Nonane rnp.Ethyltoluene 1-3-5-Trimethylbenzene 0.Ethyltoluene 1-2-4-Trimethylbenzene n-Decane n-Undecane 12-4-5.Tetramethylbenzene 1-2-3-5-Tetrarnethylbenzene Naphthalene n-Dodecane n.Hexadecane
Data given
+
I 1
Paris
2.2-6.5 1.0-2.5 0.5-3.1
An gel e s Zurich (Lonne(Grob a n d m a n e t Grob, 1972) al., 1968)
93a 33a
2.20.
1.1-4.1 1.3-5.2
2.Ja
Xa
0.7-2.6
4.5-15.3
11.6=
4.3-11.2
2.1a
5.2-8.8
1.24
1.5-3.9 1.6-5.3
3.8-11.2 2.3-5.1
1,7a
0.16-1.0
p p b or ppm/vol by t h e authors.
column A Frontal
analysis. B
Elution
analysis
made easier and quicker by trapping on graphitized carbon black followed by heat desorption directly to the capillary column inlet and analysis in temperature programming. Particularly, heat desorption and transfer to the column inlet present many advantages: Because no solvent has t o be used to recover the pollutants, all the sample can be injected in the capillary column without splitting. The sensitivity is increased by a factor of 102-103.Moreover, this procedure avoids severe damages to the column, which occur when large amounts of solvent are concentrated in small portions of the column. Because sample concentration by solvent evaporation is no more necessary, the main source of losses by destruction or parasitical reactions disappears; all the pollutants sampled on graphitized carbon black, in spite of their possible different chemical structures, are injected in the capillary column, provided they can withstand the heat desorption process. There is no possible contamination by solvent impurities. Finally, much time is gained between the sampling and the analysis periods. Thus, the evolution of concentrations of a great number of atmospheric pollutants can be followed with a greater precision since more analyses can be made during the same period. Inversely, we verified t h a t no destruction or parasitical reactions occur on graphitized carbon black with atmospheric pollutants, even if the analysis is made some hours or some days after the trapping period. Consequently, it is possible to make samplings far from the analytical system and, by example, to obtain a geographical distribution of atmospheric pollutants a t the same time, if several sampling systems are simultaneously used.
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Environmental Science & Technology
A t last, as it is easy to change some working conditions -i.e., the time of sampling and desorption periods, the temperature of the tubing during sampling, the nature of the capillary column, and so on-it is possible to use our analytical system for the analysis of other categories of organic pollutants; for example, we are now trying to extend our method to the study of the composition of the heaviest organic compounds (above C18) in the atmosphere. Indeed, most of these substances are mainly in the form of particulates, but our sampling tubes are also efficient with vapors and aerosols by adsorption and with particulates by mechanical filtration. Literature Cited Altshuller, A. P., Bellar, T. A , , Air Poliut. Contr. Ass., 13,81 (1963). Eggersten. F. T.. Selsen. F. M.,Anal. C h e m . . 30, 1040 (1958). Grob. K.. Grob, G.. J . Chromatog., 6 2 , l ( 1 9 7 2 ) . Guiochon, G., Guilbot. .J. F.. Pahaut. P.. .Jacob. L.. Sellier. N,, C h i m . Ind.. 105,985 (1972) (Fr.). Guiochon, G.. .Jacob, L..Goedert, M., “Gas Chromatography,” 1970. R. Stock ed.. p 166, The Institute of Petroleum, London, 1971. Kovats. E.. “Advances in Chromatography,” .J. C. Giddings. R. A . Keller, Eds., Dekker. Vol. I, p 224, New York, N.Y., 1965. Lao. R. C., Thomas, R. .J.. Oja. H., Dubois. L., Anal. Chem., 45, 908 (1973). Lonneman. W. A., Bellar, T. A , . Altshuller. A . P.. Enciron. Sei. Techno/.,2, 1017 (1968). Matsushita, H.. Esumi. U..Yamada. K.. Bunsehi Kagahu, 19, 951 (1970) (.Japan.); CA, 73, 101766s (1970). S o v a k , J..Vasak. V..Janak, .J.,Anal. Chem., 3 i , 660 (1965). Raymond A , , Guiochon, G.. Analusis, 2,357 (1973) (Fr.1. Robinson. P. G.. Odell. A . L., J . Chromatog., 57, 1(1971), Sawicki. E.. McPherson. S. P.. Stanley. T. W..Mecker, .J. E., Elbert, W . C., Int. J. Air Water Pollut., 9, 515 (1965). Williams. I. H.. Anal. Chem., 3 i , 1723 (1965).
R e w i r e d for r e c i e u Ma? I?. 1973. A c c e p t e d October 19. 197.9.
