Anal. Chem. 1981, 53, 801-805
that the method here presented is quite flexible, allowing the simultaneous determination of compounds present in very different concentrations. No correlation apparently exists between the concentrations of CC&Fand CHClzF within the same sample. An evaluation of these data implies a discussion that is out of the scope of this work. The data obtained by using this method in several monitoring campaigns have been presented and discussed in a paper recently published (16).
L0cation:RED S E A Sample Volume: 30 L F12
I
F11
84.96
102.93 116.90
88.86 83.96 68.87 68.97 m/e
1
!*2
1
.
IJ i;d
100.93
JL94.04 96.96 m/e
Flgure 6. Analysis of an air sample collected over the Red Sea.
Table 111. Selected Values of the Concentrations of CHC1,F (F-21) and CCl,,F (F-11) in Some Air Samples
sample
location Central Red Sea Indian Ocean Porto Marghera S. Marco Platform
801
no. 3 8 42 51-53
amtof amtof CC1,F CHC1,F (F-11), (F-21), P,PtV PPtV 135
11
188
36
485
153
0.0 0.7
Figure 6 shows the analysis of a 30-L air sample, collected over the Red Sea where CHClzF is present a t higher concentration (36 ppt). In Table I1 a typical concentration pattern for several halocarbons determined by the method here degcribed is shown. Some uncertainty is in the value found for CC14, but it is known that this halocarbon presents difficulties due to possible decomposition during storage or other processes. In Table I11 the concentrations of CC13F and CHClzF determined within the same air sample are reported, showing
ACKNOWLEDGMENT The authors are indebted to A. R. Mastrogiacomo for the preparation of permeation tubes and to the Mass Spectrometry Service for the CNR Rome research area for the use of the instrument.
LITERATURE CITED (1) Moiina, M. J.; Rowiand, F. S. Nafure (London) 1974, 249, 810-812. (2) “Stratospheric Ozone Depletion by Halocarbons: Chemlstry & Transport”; National Academy of Science: Washington DC, 1979. (3) Vidal-Madler, C.; Gonnord, M. F.; Benchah, F.; Guiochon, G. J. Chromafogr. Sci. 1978, 16, 190-194. (4) Bruner, F.; Bertoni, G.; Crescentinl, G. J. Chromafogr. 1978, 167, 399-407. ___ (5) Crescentinl, G.; Bruner, F. Ann. Chim. (Rome) 1978, 66,343-348. (6) Loveiock, J. E.; Maggs, R. J.; Wade, R. J. Nature(Lor0‘on)1973, 241, 194-199. (7) Su, C. W.; Goidberg, E. D. Nafure (London) 1973, 245, 27-29. (8) Westberg, H. H.; Rasmussen, R. A.; Hoidren, M. Anal. Chem. 1974, 46, 1852-1854. (9) Crescentini, G.; Mangani, F.; Mastroglacomo, A. R.; Bruner, F. J. ChrOmatOgr. 1981,204, 445-451. (IO) Grlmsrud, E. P.; Rasmussen, R. A. Atmos. fnvlron. 1975, 9 , 1010-1015. (11) Cronn, D. R.; Harsch, D. E. Anal. Left. 1979, 12, 1489-1492, and references therein.
(12) Crescentini, G.; Bruner, F. Nature (London) 1979,279, 311-312. (13) O’Keeffe, A. E.; Ortman, G. C. Anal. Chem. 1966, 38, 760-785. (14) Slng, H. B.; Salas, L.; Lillian, D.; Arnts, R. R.; Appleby, A. fnviron. Sci. Techno/. 1977, 77, 511-513. (15) Penkett, S. A,; Prosser, N. J. D.;Rasmussen, R. A,; Khaiii, H. A. K. Nature (London) 1980, 286, 793-798. (16) Crescentlni, G.;Bruner, F. Ann. Chlm. (Rome) 1980, 70, 631-636.
RECE~VED for review October 20,1980. Accepted January 22, 1981. This work was partially supported by Chemical Manufactures Association under Research Project CF 78-256 R and, in the early stage, by Commission of the European Communities under Contract No. 214-77-1 ENV I.
Effects of Normalization on Feature Selection in Pyrolysis Gas Chromatography of Coal Tar Pitches Matthew S. Klee, Alice M. Harper,
and L. B. Rogers”
Department of Chemistry, IJniversity of Georgia, Athens, Georgia 30602
Computerized calculationi of variance weights sometimes detected important features that were not obvious from visual Inspections of chromatograms for any two of three classes of coal tar pitches. I n addition, normalizations by one or more peak areas or of peak heights, procedures often resorted to as a means of minimizing the effect of dlfferences In size or sample heterogeneity, generally resulted in decreasing the welghts of all features. F:lnally, a successful means was devised for “correcting” the chromatograms obtained using a column that had gradually degraded durlng a long series of runs.
Correlating the quality of a thermic graphite electrode with
a property of the coal tar pitch from which it was fabricated has been difficult. It has often been necessary to make the actual electrode and assess its quality, rather than to estimate its quality using a physical or chemical measurement of the pitch prior to fabrication. Recently, an attempt at correlating the profiles of the pyrograms of some pitches to their coking values (a measure of pitch quality derived after graphitization), has been moderately successful (I). Those comparisons were based solely on criteria that could be detected easily by eye. The present study had two goals. The first was to determine if pyrolysis gas chromatographic analysis of the coal tar pitches could be optimized to aid in the estimation of the quality of a given coal tar pitch. The second was to determine if algorithms commonly used as preliminary steps in some pattern recognition schemes could significantly aid in the
0003-2700/81/0353-0801$01.25/00 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 6 , MAY 1981
selection of important peaks or sections of the pyrograms of different quality coal tar pitches, so that differentiating between said pyrograms would be facilitated. In addition, because a normalization step using the area of a peak or section of the chromatogram is often performed to minimize the differences between replicate pyrolyses, its effect on the importance of distinguishing features was explored. Because column performance gradually changed during a series of chromatographic runs, correction of retention times was required in order t o avoid frequent repacking of a column and the large errors in the calculated variance weights. This paper describes a simple method, which incorporates an extraction step prior to pyrolysis, for estimating the quality of a coal tar pitch. In addition, the effects of different normalization steps on the feature selection for pyrograms in three general classes and the successful development of a procedure t o correct for column degradation are also reported.
EXPERIMENTAL SECTION Chemicals. Tetrahydrofuran (THF) (J. T. Baker Chemical Co., Phillipsburg, NJ) for extracting pitches was used without further purification. The chromatographic column packing was Tenax-GC, 60/80 mesh (Applied Science Laboratories, State College, PA). A second column prepared from a 3% loading of SE-30 (Alltech Associates, Inc., Arlington Heights, IL) on GasChrom Q, 80/100% mesh (Applied Science, Inc.), was also used so that major peaks in pyrograms of the coal tar pitches could be identified by comparing their retention times to those of phenanthrene, fluoranthene, pyrene, and dibenzonaphthalene standards (Eastman Kodak Co., Rochester, NY). Coking values for the 10 coal tar pitches used in this study were provided by the supplier (2) and had been used in establishing the quality of the individual pitches (four good, four intermediate, two poor). The glass columns were silanized by an approximately 50/50 mixture of dimethyldichlorosilane and trimethylchlorosilane (Applied Science Laboratories,State College, PA) which had been diluted to 10% in toluene (J.T. Baker Chemical Co., Phillipsburg, NJ). Helium, hydrogen, and air used for the operation of the gas chromatograph were purchased from Selox, Inc. (Atlanta, GA). Helium, which was passed through silica gel and 13X molecular sieve to remove water, was used as a carrier gas. Apparatus. A Chemical Data Systems Model 190 pyrolysis unit equipped with ribbon pyroprobe was used for pyrolysis. A 2 m X 6 mm i.d. silanized glass column was packed with Tenax-GC and mounted in a Perkin-Elmer Model 3920 gas chromatograph. The regular injection port was replaced by a Kovar-glass injection port made by the instrument shop of the University of Georgia. The effluent from the column was split between a flame photometric detector (FPD) in the sulfur mode and a flame ionization detector (FID) with a glass-lined stainless steel splitter (Scientific Glass Engineering, Inc., Austin, TX). The hydrogen and air lines for the FPD were reversed so as to increase the sensitivity to sulfur compounds by lowering the flame temperature. The signals from both detectors were recorded simultaneously on a chart for visual comparisons, but only the FID signal was amplified, converted from analog to digital form, and stored on magnetic tape. The core limitations (24K) of the PDP 11/20 minicomputer restricted simultaneous analysis of pyrograms to 15 at any one time. Procedures. A 1.000-g amount of a pitch was added to 10.0 mL of THF in a centrifuge tube which had an aluminum foil lined screw cap. The mixtures were then shaken manually for 5 rnin followed by a 20-min centrifugation at 2500 rpm. No significant differences were found between the pyrograms of samples which had been ground thoroughly for 10 min to a very fine powder using a mortar and pestle prior to extractionand those of samples which had been ground for only a few seconds prior to extraction. By use of a 10-pL syringe, 6.0 pL of an extract was transferred from the centrifuge tube t o a ribbon probe. The extract was applied in approximately 1-pL portions, allowing a few seconds for the THF to evaporate between each addition. Pyrolyses were carried out in the injection port of the GC by stepping the probe at its maximum rate of 75 Co/ms to 375 "C, where it was held
W
m
2 0
a m W
E
I
'
Figure 1. Fyrogram divided into three sections prior to compression. The three sections correspond to times before, during, and after the temperature ramp.
for 20 s. After a chromatogram had been finished, the film left on the ribbon was flashed off (outside the injection port), at >lo00 "C. The starting temperature for the chromatogram was 245 "C where it was held for 16 min. Then, a 4 Co/min ramp increased the temperature to 290 "C, where it remained for the last 75 min. The injection port and detectors were held at 280 and 330 "C, respectively. The temperature program for a column of SE-30 on Gas Chrom-Q started at a temperature of 150 "C which was held for 16 min followed by a 2 "C/min ramp to 280 "C where it was held for 64 min. Standards used for peak identification were dissolved in THF and injected via syringe. The helium flow rate was 25 mL/min. When the Tenax-GC column was used to confirm peak identification, it was operated under the same conditions as those for pyrograms. The flow rate of helium for both cases was 25 mL/min. Data Handling. For the calculation of meaningful variance weights for the chromatograms, several prior data-processing steps were necessary. In other studies, these steps have included correction and normalization by either the peak height or the area of one or more peaks (3-6). In the present study, the retention times of the components in a given sample gradually increased with the number of runs so that accurate calculation of variance weights was impaired even though successful visual classification of a pyrogram was still possible. For reduction of the variation in retention times due to degradation of the column, a program was developed to adjust all pyrograms with respect to another pyrogram which had been run prior to column degradation. For this, each pyrogram was split into three sections as shown in Figure 1. The first section was from the start of the pyrogram to the onset of the temperature ramp, the second was during the period of the ramp, and the third section was from the end of the ramp to the end of the pyrogram. A correction factor was calculated for each section of a chromatogram on the basis of the difference in the time between the peak maxima of the two bordering peaks compared to the smallest difference found for that section. The equation Sj = integer
Dj - (Do- 1)
where Diis the difference between peak maxima in section j of the ith pyrogram and Do is the smallest difference found in the jth section of dl pyrograms, permitted calculation of a factor that was used as a counter in a loop to remove every Sj point in that section of the pyrogram. After a pyrogram had been compressed, the time axis was shifted, if necessary, so that the first major peak common to each chromatogram (phenanthrene) had the same retention time. The average of the middle 65 of the 75 lowest data points was sub-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981 803 I
a
b'
A
A
I
b r
W I
I
0
I
I
T IIR E t s )I
'
I
I
6908
Figure 2. Simultaneous FIC) and FPD (top trace) responses for pyrograms of (a) poor-, (b) medium-, and (c)good-quality pitches recorded under the same Conditions.
tracted from each data point in each pyrogram, in order to correct their base lines to zero. To assess the effects of normalization on the calculated (pattern recognition) weights, we normalized the corrected pyrograms either by area or by the height of the fluoranthene peak at 1200 s. The area was calculated for the pyrograms by summing the 257th through 5120th out of 61414 data points. This avoided the large variations in the beginning and end of the pyrograms due to incomplete evaporation of solvent and column degradation, respectively. The data were autoscaled so that the responses a t each time could be directly compared (7). Briefly, autoscaling adjusted the data at each time to have a mean of zero and a variance of unity. The autoscaled data were ithen used to calculate variance weights for each time period (8). The greater the value of the weight, the greater the importance of the feature from which it was calculated. Variance weights may go from 0.5 upward as the feature increases in importance for distinguishing between two classes. All weights greater than 1.5 were printed out for inspection. The weights increased and decreased in a peaklike manner and were centered upon an important feature. In classification routines, the features which corresponded to the highest weights would have been used to classify unknown chromatograms. In this study, however, the weights were used only as numerical indicators of the differences in the features between the three classes of pitches. Preliminary Experiments. Several of the peaks on the simultaneous FID and FPCl traces of the pyrograms of the T H F extracts of coal tar pitches were identified by comparing their retention times with those of standards by using a Tenax-GC column and a 2-m glass column packed with 3% SE-30 on 80/100 mesh Chromosorb W. These peaks are indicated in Figure 2a which shows the simultaneous FID and FPD pyrograms for a poor-quality pitch. The identified peaks are polynuclear aromatic and heterocyclic hydrocarbon compounds. Typical FID and FPD traces for medium- and good-quality pitches are shown in Figure 2b,c. Since the same sample size, pyrolysis conditions, and detector sensitivites were used for all pitches, the pyrograms could be directly compared. The differences between the magnitudes of the peaks were so pronounced between the three classes that classification of the pyrograms was possible by visual comparison, the pyrograms having the larger amounts of pyrolyzates corresponded to the poorer pitches. Ordering of some of the pitches within the classes was also possible by visually sequencing the pyrograms in the order or increasing height of the major peaks. A previous attempt a t visual classification of the pyrograms of coal tar pitches relied on the use of peak-height ratios and area ratios of certain sections of the pyrograms (I). In that study, 25-mg amounts of the solid pitcheu were pyrolyzed in a quartz tube and coil pyroprobe which was at >800 "C. Similarities existed between the later portion of the pyrograms acquired by that method and the pyrograms acquired using the current method. Specifically, phenanthrene, fluoranthene, and pyrene were major pyrolyzates in both studies and, in both cases, the peak profiles of the py-
I1
1
11
11
Ill
Figure 3. (a) FID responsed for pyrograms of the same pitch showing decrease in resolution of the column. The top trace was recorded 17 runs after the bottom trace. (b) The same two runs as in (a) after the chromatograms had been corrected.
Table I. Calculated Variance Weights for Unnormalized Pyrograms of Classes of Coal Tar Pitches time of feature, s 630 950 1200 1300
1485 1946
1vs. 2
1vs. 3
2 vs. 3
