A Procedure for Routine Use in Chromatographic Analysis of Automotive Hydrocarbon Emissions Basil Dimitriades and Donald E. Seizinger Bartlesville Petroleum Research Center, Bureau of Mines, U. S. Department of the Interior, Bartlesville, Okla. 74003 ~
m Procedures have been developed for routine analysis of automotive fuels and the unburned hydrocarbon of exhaust emissions. Specific attention is given to the hydrocarbon components that are photochemically reactive. Problems encountered in all phases of the analytical method are identified, and procedures that constitute a reasonable compromise between accuracy and practicality are defined. A packed and a n open tubular column were used to separate the hydrocarbon content of exhaust and gasoline-vapor samples into 165 component groups; analysis time is 30 min. Raw chromatographic data are rapidly processed using a high-speed electronic integrator and a computer technique for automatic interpretation of chromatographic spectra.
I
n automotive emission studies related to the atmospheric smog problem, there is need for a rapid and practical method for routine analysis of the hydrocarbon component of exhaust to permit reliable assessment of its photochemical reactivity. To meet this need, a chromatographic method has been developed to satisfy the demand of both analytical accuracy and practicality. This report describes the procedures developed for sampling and analysis of either exhaust or liquid fuel. Comparison of fuel-composition data from this system with data obtained by other laboratories showed varying levels of disagreement. To explore causes of such disagreement, several fuel-sampling techniques were compared and the results are also reported here. Experimental
Sampling and Analysis of Exhausts and Fuels. EXHAUST SAMPLING AND SAMPLE INJECTION. Exhaust sample is drawn from the automobile's tail pipe using a variable-rate proportional sampling device (Fleming, Dimitriades et al., 1965). The proportional sampler effluent is collected in a 100-liter Tedlar film (2-ml thick) bag that has been precharged with a volume of nitrogen six times that of the exhaust sample to be taken. Sample dilution prevents water condensation and slows down reactions among sample components. The Tedlar bag is kept inside a dark cloth bag to protect the sample from light-induced reactions. Sampling lines and proportional device are kept a t 80°C, to prevent water condensation and subsequent loss of water soluble components. Sample is collected continuously throughout the test, which is typically a 20- to 30-min operation. Following collection, the bag content is mixed by agitation for about 2 min, then sample is immediately injected in the
chromatograph. The injection system includes a multiport valve connected to a sample loop maintained at 110°C and to the chromatographic column. For sample injection, the loop is either evacuated and subsequently filled with the sample mixture, or is purged with about 0.5 liter of sample mixture to ensure capture in the loop of representative sample. FUELSAMPLING AND SAMPLE INJECTION. For fuel analysis, a small amount of fuel is taken from a n unused drum kept a t 0°C into a glass ampule. Helium is used to pressure fuel out of the drum through a stainless steel tube. About 1 liter of fuel is allowed to flow through the tube into a waste container before the uninterrupted fuel stream fills a 20-ml glass ampule kept in Dry Ice. The ampule is then either stoppered and used immediately for analysis or heat-sealed in a nitrogen atmosphere and stored. For chromatographic analysis, about 0.2 ml of fuel is transferred from the Dry Ice-chilled ampule into a n evacuated 16-liter stainless steel cylinder, with a chilled (Dry Ice) 1-ml syringe; nitrogen is pressurized into the tant- to a final mixture with a total hydrocarbon level between 500 and 1000 ppmC. The resultant mixture is heated to 110°C and maintained at that temperature for several hours for mixing, and sample is injected into the chromatograph by purging the GLC sample loop with the cylinder content, as for exhaust sampling. Chromatographic Separation. The separatory system is a modified version of that described by Seizinger (1967). Column design and operating conditions were modified for improved component separation. The system consists of two packed and one open tubular column operated in a Perkin-Elmer Model 900 gas chromatograph. The packed columns are used at room temperature and separate the C1and C?hydrocarbon components; the other components are backflushed and discarded. The open tubular column is used at programmed temperatures and separates the heavier-thanCZcomponents; the C1 and C2 components emerge from the column unresolved. The three columns are connected as shown in Figure 1. Both packed columns, QS and T , are enclosed in a n insulated box and are operated a t 25°C with nitrogen carrier at 80 ml/min. Column QS consists of a 48 by '/s-in. 0.d. Teflon tube packed with 80/'100 Porapak QS and separates the C1 and C, Components from the heavier hydrocarbons. Column T consists of 30 x ]/*-in. 0.d. Teflon tube packed with 80/100 Porapak T, and improves separation of CI and Cz components from each other. Backflush is applied o n column QS only, immediately after the last C?component emerges; foreflush of the C1and C, components through column T is continued using the auxiliary nitrogen stream (see Figure 1). This column combination and arrangement was found to give a reliable cutoff in separation of the CI and Cz components; also it lessens the impact of the flow surge (caused by the backflush step) upon the detector's flame and prevents flame extinction. Volume 5, Number 3, March 1971 223
I
Nitroge~i
,
Poropak O g j n t column Sample l o o p (5ml1
-
Bockflush
Somple in
I Nitrogen
t
1 Helium
Sample out
Figure 1. Chromatographic system for analysis of exhaust and fuel hydrocarbons
The open tubular column (purchased from Perkin-Elmer) consists of a 150-ft X 0.01-in. i.d. X 0.02-in. 0.d. stainless steel tubing, coated internally with Versilube F-50 (chlorophenylsilicone oil). The column is operated from -100“ to 150°C with a helium carrier flow that, a t room temperature, is 10 ml/min. The temperature program design and the time schedule of all operational steps for one complete analysis are shown in Figure 2. Because such temperature changes affect carrier flow rate and cause undesirable baseline drift, a flow-equalizing system was devised and installed (Figures 1 and 3); the device provides makeup nitrogen when the carrierflow rate is reduced by column temperature increase. A complete analysis consists of two chromatographic runs in which two separate portions, 5 and 10 ml, of the gaseous sample are chromatographed, respectively, on the two packed and the open tubular columns. The two runs are timed for use of the same detector. Total analysis time is 30 min. Figure 4 shows a chromatogram of a typical exhaust sample with a total hydrocarbon level of 114 ppmC. Detector Calibration. The flame-ionization (FI) detector is calibrated at the beginning of every emission-measurement program with standard mixtures of 22 hydrocarbons and daily with a standard mixture containing eight hydrocarbons. Hydrocarbons were selected to represent those typically present in automobile exhaust and fuel. Standard mixtures of C1to C 5 hydrocarbons were prepared manometrically. Standard
I .~ Lz
2
f
,
,Oven
mixtures of C G to CILhydrocarbons were prepared gravimetrically in the liquid phase, and samples were injected into the chromatographic system by the procedure described for fuels. Results from measurements of detector response to 22 hydrocarbons are listed in Table I. Data in Table I show that the detector response is different for methane, aliphatics except methane and heavy olefins, and aromatics plus heavy olefins; but the response is uniform within each of these hydrocarbon groups. These data led to formulating three calibration factors for use in converting response data into concentrations. Data Processing. The FI detector signal is fed either directly into a n Infotronics Model cRs/llO integrator o r into a magnetic tape recorder; tapes from several chromatographs are played back in a centrally located console connected to an Infotronics Model CRS-11 HSB/40 integrator. Output from each integrator is shown o n a tape printout and consists of two numbers per chromatographic peak, retention time in seconds, and peak area in “integrator counts.” Both integrator units are equipped with automatic baseline drift corrector (ABDC). In routine operation, the ABDC accessory is used during the first 2 to 3 min of the analysis to establish the baseline and, in the last 15 min, to correct peak areas for baseline drift caused by column bleeding; for the remainder of the run, the ABDC accessory is switched off after having locked in the initially established baseline level. Of the operating parameters affecting the ABDC function, the speed with which a shifting baseline is re-established is apparently the most important. The Infotronics system provides separate controls for updrifting and downdrifting baselines. Optimum settings for these controls-“tracking rate up” and “tracking rate down” -are defined as those that yield peak-area values which agree with values obtained from manual measurements; for the samples and instrument used in this program, these settings were found to be 0.3 pV/sec and 1.0 pV/sec, respectively. For systems in which chromatograms are first recorded on magnetic tapes and then played back for integration, optimum recording and playback speeds were found to be ‘“116 and 1’18 in. of tape per second, respectively. Further processing of the retention-time and peak-area data to give sample composition is accomplished semiautomatically using computer methods. Of these methods, the one dealing with the identification of the chromatographic peaks is described.
4
temperalure
IOO-
-I00 --c
Step I
TIME,minuies
I
F l odr n e Ionization k
Backpressure reguiotor
Inject sample i n t o Porapak column Open tubular column
maintained 01 - 100“ C Step 2 Bockflush Poropak column
t 4 1 r
Step 3 Inject sample into open tubular column Step 4 Raise oven temperature to
- 20” C
NZ
-1
Step 5 S t a r t temperature p r o g r a m Step 6 Switch i o isothermal operotion a t 150°C Step 7 Run terminated A u t o m a l i c cooldown to - i O O ° C
Figure 2. Oven temperature schedule and sequence of operations 224 Environmental Science & Technology
Figure 3. Carrier flow-equalizing device
2
0
4
12
IO
8
6
peaks has been continuously used in this laboratory for the past year. Manual checks made on 200 samples that had been treated automatically showed that only 0 to 3 of 165 peaks were misidentified ; this margin of error was judged unimportant. Further, automation has reduced time expenditure in processing GLC data and has provided more uniform interpretation of chromatographic spectra. Success with the automatic peak-identification method was possible primarily because of the chromatographic system’s low retention-time variability. This variability, best expressed in terms of variation in retention time difference between two successive peaks, is approximately 1 2 5 % for retention time differences near the 2-sec minimum value and * l o % for those near 10 sec. Evaluation
I6
I8
20
22
24
RETENTION T I M E , minutes
-
26
28
30
Figure 4. Chromatogram of typical exhaust sample with a total hydrocarbon level of 114 ppmC
The computer method for automatic peak identification is based on the assumption that the same peak identity assignments can be made for all exhaust samples originating from one fuel (but different automobiles). With this assumption, peak identities can be established in all chromatograms representing exhausts from one fuel, through comparison to one “master chromatograph,” for which peak identity assignments have been established experimentally. Experimental evidence for the validity of the assumption and for preparation of such a master or reference chromatogram will be reported separately. Comparison of the unknown chromatogram to the reference chromatogram is made by the computer, using retention-time data. Specifics of this computer method are as follows: CohwuTER INPUT.Input information pertains to the unknown chromatogram and consists of ( a ) peak areas in integrator counts, (b) corresponding retention times in seconds, and ( c ) retention time of nine “indicator” peaksthat is, peaks which are located in strategic positions in the chromatogram and whose identities can be established readily by the analyst. The Infotronics integrator provides data for a and b, and the analyst provides data for c. STOREDINFORMATION. Stored information pertains to the reference chromatogram and consists of ( a ) emission components named in terms of serial numbers and (b)corresponding retention times in seconds. COMPUTATION DIAGRAM. Computation involves the following steps: (1) From retention-time data, the computer computes time distances, (AT,),, of successive indicator peaks in sample chromatogram and compares with corresponding distances, (AT,),, in the reference chromatogram, (2) By use of the ratios (ATJ,/(AT,),as adjustment factors, the computer adjusts distances among peaks in the reference chromatogram so that they are comparable to those in the sample chromatogram. (3) By use of the adjusted reference chromatogram as a model, the computer divides sample chromatogram into sections, each one of which is centered around a peak of known identity. (4) The computer sums all peak areas (if more than one) within each section of sample chromatogram, and this sum is reported under the name of the section’s peak. COMPUTER OUTPUT.Printouts list component concentrations in desired units and corresponding peak identities, This computer method for identifying chromatographic
The following describes experimental evidence of analytical errors associated with the various procedural steps. The evidence pertains to some problems that, from the authors’ experience, are common and potentially important. Exhaust Sampling and Sample Injection. The exhaust sample may be affected by component losses on or through the sample bag walls, by chemical reactions occurring during and after sample collection, and by adsorption-desorption phenomena o n GLC sample loop walls. These possibilities were explored experimentally. Hydrocarbon loss on or through the Tedlar bag wall was investigated by use of a synthetic mixture of eight hydrocarbons in nitrogen. The hydrocarbons were similar to those
Table I. Response of Flame-Ionization Detector to Hydrocarbons Relative hydrocarbon Hydrocarbon response0 1.077 Methane
Aliphatics except methane and heavy olefins Ethylene Acetylene Propylene Isobutane n-Butane cis-2-Butene n-Pentane n-Heptane n-Octane &Octane n-Nonane n-Decane n-Undecane
1.001 1.001 1.009 1.026 1.005 1.000 0.981 1.022 0.986 0.984 1.023 0.989 0.998 0.988
Heavy olefins and aromatics 1-Octene 1-Nonene Benzene Toluene p-Xylene Ethylbenzene 1,3,5-Trirnethylbenzene n-Butylbenzene
0.944 0.946 0.957 0.963 0.933 0.923 0.937 0.933 0.962
a Response per ppmC of hydrocarbon relative to response per ppmC of n-butane.
Volume 5, Number 3, March 1971 225
--I
9-
liI
i:
6
0
-
I
0
'
--
-
;
2)
'
20
I
I 40
I 60
I
I
P I
80
l
!
I
IO0
STANDING T I M E , m i n u t e s
Figure 5. Stabilities of hydrocarbons in unconditioned Tedlar bag 0 Toluene
A Benzene A
n-Undecane
H 1-Octene X p-Xylene ~3 Iso-octane
0 n-Propylbenzene
typically present in exhaust samples and were considered susceptible to wall losses. The test blend was prepared in a 16-liter stainless steel tank and transferred into a n unused 10-liter Tedlar bag to yield a surface-to-volume ratio equal to 9 m-l; this ratio is used in routine exhaust analysis. Hydrocarbon loss was determined every 30 min by chromatographic analysis. Results are shown in Figure 5. Concentration values a t zero time represent component concentrations in the test blend prior to charging the test bag. Similar tests were made with a 10-liter bag that had been conditioned by charging the bag with 10 liters of test blend and allowing the sample to stand overnight. Results from these tests showed the same loss levels as those depicted in Figure 5 . Also, tests were made to determine hydrocarbon loss in the presence of water vapor at levels comparable to those in typical exhaust-plus-N2 mixtures; results showed no humidity effect on hydrocarbon loss. These data suggest that in routine exhaust analysis, where sample collection time is 20 to 30 min, maximum loss of individual hydrocarbon o n or through the bag wall does not exceed 5 to 6%, if analysis is made immediately after sample collection. Furthermore, bag conditioning is deemed unnecessary. Hydrocarbon loss due to chemical reactions was investigated by use of exhaust samples of widely varied compositions. Successive chromatographic measurements upon each of these exhaust samples (diluted with nitrogen and kept in the dark) showed that certain components disappear a t rates considerably higher than those associated with wall effect. Also, other components increase in concentration with time. Such rapid component disappearance and buildup phenomena were attributed to chemical reactions, presumably involving hydrocarbons, nitrogen oxides, and oxygen. Of the approximately 200 peaks present in a typical exhaust chromatogram, 16 peaks, representing about 6 % of total hydrocarbon content, are affected by reaction losses; 5 to 10 peaks, representing about 2.5z of total hydrocarbon content, are affected by component buildup. Diolefins and aromatic olefins generally react most rapidly. These components may be lost by as much as 10 with a 15-min delay in sample analysis. One unidentified component's lifetime was less than 20 min. On the average, observed total hydrocarbon loss resulting from chemical reactions in samples allowed to stand for 0.5 hr equals about 226 Environmental Science & Technology
2 %. This percentage was calculated from individual hydrocarbon loss data of 20 to 100%. Because such individual hydrocarbon loss levels are well outside the limits of experimental error, the total hydrocarbon loss of 2z also is outside the error limit. A few. tests were conducted to investigate the stability of oxygenated hydrocarbon derivatives in the collected exhaust sample. Oxygenate to levels of 5 to 10 ppm was injected in a bag charged with exhaust plus nitrogen, and chromatographic measurements were made every 30 min. Results showed acetone to disappear a t the rate of 3 0 z per 0.5 h r ; calculations, involving permeability data for acetone and Tedlar film, showed that such disappearance rate may be caused by permeation alone. Acetaldehyde and butyraldehyde levels did not change significantly, whereas benzaldehyde disappeared completely within 0.5 hr. Epoxides of ethylene and 1-butene were somewhat unstable; however, analytical data were not sufficiently conclusive to establish stability levels. Problems in sample injection may be caused by adsorptiondesorption phenomena occurring in the GLC sample loop. For example, adsorption may cause low results when the evacuated-loop technique is used. When the loop is charged by purging with sample, adsorbable material may be concentrated on the surface and subsequently injected as part of the sample, thus causing high results (McEwen, 1966; Papa, Dinsel et al., 1968). Occurrence and magnitude of such sorptive phenomena were established in a brief investigation of the effect of loop temperature upon measurements. Sample loop in this investigation was charged by the sample-purging technique; test blends were made of nitrogen and C6 to CII hydrocarbons, each a t about 100 ppmC, and were kept at room temperature. Results, corrected for loop temperature variation, are illustrated in Figure 6. A few tests were made also with the test blends maintained at 110°C to prevent possible cooling of sample loop during purging. Results showed a similar, though somewhat less pronounced, effect of loop
c
20
I
40
60
eo
100
L O O P TEMPERATURE,'C
Figure 6. Effect of sample-loop temperature on sampled hgdrocarbons 0
Toluene
A n-Butylbenzene A n-IJndecane
0 n-Decane yr 2,3-DirnethjIbutene-2
H n-Octane X p-Xjlene E ii-Xonane # if-Hexane
temperature. These data indicated that adsorption and accumulation of adsorbable material o n loop walls does occur when the loop is filled by the sample-purging technique and that when the loop is maintained at llO"C, such adsorption is minimal. Further tests t o compare the evacuated-loop and sample-purging techniques, keeping the sample loop a t 110°C, gave results illustrated in Figure 7; these results indicate that the two techniques are equally good. Fuel Sampling and Sample Injection. The fuel-sampling and sample-injection technique used in this laboratory was evaluated by comparison with three other techniques commonly used o r thought to be inherently more reliable. This comparison involved parallel application of the four techniques o n three fuels-a typical currently used premium fuel (Fuel A), a highly volatile fuel (Fuel B), and a highly aromatic fuel (Fuel C). All fuel samples were kept in sealed 20-ml glass ampules. Of the four techniques evaluated, the Bureau of Mines method has been previously described in this report. The remaining three techniques are described below. I m E r SPLITTERMETHOD.With a 10-pl syringe prechilled to -1OO"C, about 2 p1 of sample is drawn from the chilled (Dry Ice) glass ampule and injected through a septum into the heated inlet splitter of a Perkin-Elmer Model 900 chromatograph. Split ratio is 150:l. This method (with possible minor variations) is in common use. DIRECTINDIUMENCAPSULATION METHOD. Pressurization with nitrogen forces sample out of the chilled ampule and through a piece of indium tubing. While the fuel flows, a section of the indium tubing is crimped, resulting in encapsulation of 0.01 to 0.02 pl of fuel. The indium tube containing the encapsulated sample is rinsed successively in methylene chloride and acetone, dried, and placed in the injector from where the fuel sample is thermally released into the carrier stream of the PE-900. INDIRECT INDIUM ENCAPSULATION METHOD.Fuel sample ( = 3pl) is encapsulated, as described in the procedure above, and released thermally into a nitrogen stream that is received in a n evacuated 150-ml stainless steel cylinder. Additional nitrogen is then pressurized in the cylinder to a final mixture with total hydrocarbon level between 500 and 1000 ppmC. The pressurized mixture is heated and maintained a t 110°C several hours for mixing, and is sampled for GLC analysis following the same procedure as for exhaust. Results of the GLC analyses are summarized in Table 11. Data represent mole levels of Ca and C5olefins and paraffins in the fuels; each entry is the result from a single determination unless otherwise specified. The (C, CJ-total values obtained from replicated analyses ranged within k 1 % for fuels nos. 1 and 8, and + 2 % for fuel no. 2 (most volatile of the three fuels). Data in Table I1 illustrate the agreement in analytical results that may be expected frqm application of the four procedures. The following comments express the investigators' impressions regarding relative merits of tested procedures: All procedures except those involving indium tubing encapsulation appear to be equally good in design. I n practice, accuracy of results probably depends o n operator skill rather than o n sampling procedure, Procedures involving indium tubing encapsulation are, in concept, superior to the others because they require no intermediate sample-handling steps. I n practice, however, fuel losses were found to occur in some of the encapsulated samples, presumably as a result of inadequate sealing. Therefore, the procedure cannot be recommended until the technique for sealing indium tubing is perfected.
+
II
7 -
~-
1
L l n e o f I I correspondence Sec- bulylbenzene-
Yfi-
Heptane
1
I
7 a 9 10 NUMBER O F CARBON ATOMS I N H Y D R O C A R B O N MOLECULE
5
6
Figure 7. Comparison of flow-through and evacuated-loop techniques for filling sample loop 0 Loop purging technique X Evacuated-loop technique F o r analyzing high-volatility fuels, the splitter procedure may be somewhat inferior to the others in that it requires extreme care and skill. Occasionally, solid material precipitated in fuel samples kept in Dry Ice baths. Such precipitates dissolved shortly after the samples were removed from the bath. The phenomenon was attributed to crystallization of hydrocarbons. I n view of this, samples should be cooled with Dry Ice only if necessary for handling and then only with great care. Baseline Drift. Operating the open tubular column under the specified temperature conditions caused excessive and undesirable baseline drift. Such drift was found to be caused by column bleeding and by change in carrier-flow rate as a result of helium viscosity change attendant to column temperature variation. Baseline drift due to column bleeding begins in the last 15 min of the analysis and becomes pronounced in the
Table 11. Results from GLC Analyses of Fuels following Different Sampling and Sample-Injection Procedures
Inlet splitter method C4 C5unsaturates Ca C5saturates Total Direct indium method C4 Cs unsaturates C4 Cs saturates Total Indirect indium method C4 C5 unsaturates Ca C5 saturates Total Bur. Mines method C4 C5unsaturates C4 C 5saturates Total
+ +
+ + + +
+ +
4.5. 29.0 33.5
4.0~ 34.2 38.2
4.6. 25.4 30.0
4.7. 33.0 37.7
4.4. 35.1 39.5
4.9. 28.0 32.9
4.4 28.7 33.1
4.3 34.2 38.5
2.2b 16.1 18.3
4.9. 30.6 35.5
4.5c 35.5 40.0
5.0~ 27.3 32.3
a Average of three replicated analyses. b L o results ~ were attributed to loss of volatile components from inadequately sealed indium tubing. c Average of two replicated analyses.
Volume 5, Number 3, March 1971 227
Table IV. Variability of Results from Chromatographic Measurement of Exhaust Hydrocarbons Mean value of CoeK. of concn, ppmCQ varlation,b Hydrocarbon 95.9 2.5 Methane 152.3 6.1 Ethylene Ethane 17.6 2.0 79.5 3.0 Acetylene 61.6 2.0 Propylene 4.8 trans-2-Butene 5.2 18.3 cis-2-Butene 3.0 1.9 47.1 Isopentane 10.4 2,3-Dimethyl-2-butene 3.5 0.8 8.7 Isooctane 2.0 26.7 n-Heptane 3.9 47.9 Toluene 4.6 1.6 Unknown olefin 8.2 2.1 Unknown paraffin Unknown paraffin 9.7 1.7 13.1 Compositec 104.1
W
Y)
0 z 0, e
W
e Y e P 0
e W
5
6
8
TIME, minutes
+- ~ O O O C+-200c
9
-.++Temp
TEMPERATURE , 'C
51arts
program
4
Figure 8. Baseline obtained with and without carrier flow-equalizing device (FED)
Table 111. Variability of Flame-Ionization Detector Response to Hydrocarbons Response, integrator counts Concn, Mean Coeff. of Component PPmC value" variation, Z b Methanec 20.58 5,647 5.1 Ethanec 46,65 13,409 4.2 Propaned propylene 94.21 160,333 3.6 Isobutaned 65.94 110,990 3.7 n-Butaned 80.01 136,539 3.7 trans-2-Butened 107.12 189,643 4.0 3-Methyl-1-butened 113.27 197,383 4.1
+
Data represent results from 20 replicated determinations. Defined as ( S i x ) X 100, where S and X are standard deviation and mean, respectively. c Chromatographed by packed columns. Chromatographed by open-tubular column. a
b
last 10 min, when column temperature exceeds 100°C. Such drift affects peak area measurement; however, as discussed, the effect is partly offset by use of the automatic baseline drift corrector. Baseline drift due to carrier-flow instability begins and is most pronounced when column temperature is raised rapidly from - 100" to -20°C (see Figure 2, step 4); this temperature change causes a 50% reduction of the heliumcarrier flow. This baseline drift was eliminated by installing the flow-equalizing device (Figures 1 and 4). Extent of baseline drift, before and after the flow-equalizing device was connected, is shown in Figure 8; a n exhaust chromatogram is shown in two portions, one produced without use of the device (solid baseline) and one produced with the device (broken line). I n Figure 8, the area associated with baseline drift can be compared to that of a peak representing 6.5 ppmC of propylene. Precision. While accuracy of results from chromatographic analyses of exhausts and gasoline fuels cannot be determined directly, precision can. Data of Table 111 comprise results 228 Environmental Science & Technology
Values represent results from 20 replicated determinations. Defined as (SIT) X 100, where S and 2 are standard deviation and mean, respectively. Total of hydrocarbons emerging from column after n-decane. a
from repeated (daily) calibrations with an eight-component standard blend for one month. The precision levels shown represent the composite of variability associated with all phases of the chromatographic measurement-namely, sampling, column separation, hydrocarbon measurement, and peak integration. I n investigating causes of such variability, a contributing factor was found to be oxygen-level variation in the detector's combustion air. Oxygen content of the air varied 15 to 28% from cylinder to cylinder. Results from pertinent tests showed that increasing the oxygen concentration in the detector's combustion air from 22 to 27.5% caused a 9% increase in absolute hydrocarbon response but had no effect o n relative response. Variability of results from actual analyses of exhaust samples is shown by the data of Table IV; data were obtained from replicated analyses of a n exhaust-plus-N2 mixture that had been allowed to stand for several weeks to a stable composition. Hydrocarbons were selected to represent sample components at widely different concentration levels and in widely different positions in the chromatographic spectrum. Discussion The appropriateness of the analytical procedures is judged considering the purpose for which these procedures were developed-that is, to estimate reactivity content of hydrocarbon emission samples in a routine fashion. Therefore, the procedures are judged mainly o n the basis of accuracy of obtained reactivity estimates and practicality. Thus, the importance of each hydrocarbon component in the emission sample is rated on the basis of not only its concentration level, but also its reactivity; procedural inadequacies, in turn, are evaluated from their effects upon those components that contribute relatively more to the samples' reactivity. Using this perspective, losses of exhaust components during sampling and sample handling may be of much more consequence than the value for the total hydrocarbon loss suggests. This is illustrated from the following data, obtained when a n exhaustplus-nitrogen sample was allowed to stand for about 1 hr. Total hydrocarbon loss in the sample was 2.7%; correspond-
ing loss in reactivity content computed by the linear summation method (Dimitriades, Eccleston et al., 1970), was 7 . 0 x . This reactivity-loss value was obtained from rate-of-NOzformation reactivity data (Jackson, 1967); use of other reactivity scales may yield higher or lower values. These results suggest that component losses during sampling and sample handling could cause significant error. However, under the conditions used in this study, including the provision that sample-collection time be constant and that the sample be analyzed immediately after collection, effect from component loss is judged to be acceptably small and reproducible. I n regard to exhaust sample injection, although the two described techniques gave identical results, these investigators consider the evacuated-loop technique less desirable because of disadvantages such as its vulnerability to leaks, and occasional problems with requisite valves. The chromatographic system’s performance in terms of component resolution is judged t o be adequate for intended uses. Component resolution is necessary, but only to the extent that permits calculation of the sample’s reactivity content. More detailed separation-e.g., separation of hydrocarbons of similar reactivity and molecular size-is of little benefit and it introduces, as a rule, performance demands that penalize practicality. The described separatory system has been tested extensively and apparently provides adequate component resolution for obtaining usable estimates of reactivity content. For example, emission reactivities computed from compositional data correlated in value usefully with reactivities as measured with a smog chamber (Dimitriades, Eccleston et ul., 1970). The system’s performance, in terms of retention time reproducibility, was sufficiently good to permit semiautomatic identification of chromatographic peaks from retention time data. This reduced considerably (but did not eliminate) the time and skill requirements of this data processing phase; these requirements constitute the most serious obstacles in using chromatographic analysis for routine assessment of emissions for reactivity. The separatory system performed satisfactorily for about six months (eight analyses per working day, on the average), after which time, the open tubular column began to show signs of deterioration, manifested by peak broadening and attendant loss of resolution. Although the column was usable after six months, it almost certainly cannot meet the usage and performance demands of current studies for longer than one year. The Porapak columns did not deteriorate significantly with time,
Difficulty was encountered in preparing Porapak columns with reproducible performance-presumably because of difficulty in packing and conditioning Porapak columns reproducibly, and because of variability of Porapak material from batch to batch. Considering that Porapak columns are not affected by usage, the problem of column reproducibility is unimportant. The developed system is also judged to be acceptably practical. The analysis time of 30 minis reasonably short. Columns and requisite instrumental controls are commercially available. Column operating conditions are controlled automatically and a n inexperienced person can be trained to operate the chromatographic system for routine applications within a few days. Conclusions Procedures for routine analysis of automobile emissions in photochemical air pollution studies are described and discussed with emphasis on practical workings. All phases of the analytical method-sampling, sample injection, chromatographic separation and measurement, and processing of raw chromatographic data-are treated. Adequate separation of hydrocarbon components in a n ordinary exhaust or fuel sample can be accomplished chromatographically within 30 min, and rapid processing of the raw chromatographic data can be accomplished with use of automatic equipment and computer techniques. Literature Cited Dimitriades, Basil, Eccleston, B. H., Hurn, R. W., J . Air Pollut. Contr. Ass. 20, 150 (1970). Fleming, R. D., Dimitriades, B., Hurn, R . W., J . Air Pollut. Contr. Ass. 15, 371 (1965). Jackson, M. W., “Effects of Some Engine Variables and Control Systems on Composition and Reactivity of Exhaust Hydrocarbons,” SAE Progress in Technol., Vol. 12, Vehicle Emissions, Part 2, (selected papers, 1963-66), Society of Automotive Engineers, New York City, 1967, pp 241-67. McEwen, D., J . Anal. Chem. 38, 1047 (1966). Papa, L. J., Dinsel, D. L., Harris, W. C., J. Gas Chromatogr. 6, 270 (1968). Seizinger, D. E., Perkin-Elmer Znstrum. News 18, 11 (1967). Received for review October 8 , 1969. Accepted November 2, 1970. The work was done in cooperation with the National Air Pollution Control Administration. Trade names in this report are used for identification only and do not necessarily imply endorsement by the Bureau of Mines. This work was supported by the National Air Pollution Control Administration, V.S. Department of Health, Education, and Welfare.
Volume 5, Number 3, March 1971 229
