The smaller the 62(k), relative to the entire training set, the more meaningful the classification. More importantly, the KNN classification is unique for K , and the determination of K is a simple one-dimensional optimization that has useful constraints. The major disadvantage of the KNN method is computational in nature; in view of computers available today this does not pose a serious problem. Because of the
knowledge that exists on the behavior of the risk of the KNN rule, it has been suggested as “a reference with which other more sophisticated procedures may be compared” (9). RECEIVED for review January 21, 1972. Accepted March 13, 1972. This work was performed under the auspices of the U.S. Atomic Energy Commission.
On-Line Elemental Analysis of Gas-Chromatographic Effluents S. A. Liebman,’ D. H. Ahlstron, and T. C. Creighton Armstrong Cork Company, Research and Development Center, Lancaster, Pa. 17604
G . D. Pruder, R. Averitt, and E. J. Levy Chemical Data Systems, Inc., Oxford, Pa. 19363 A new, relatively simple and inexpensive general tool has been developed which is rapid and accurate for the analysis of gas chromatographic effluents. The experimental equipment is shown in detail and the Pyrochrom Analyzer (registered in U.S. Patent Office by CDS, Inc.) modifications necessary to provide identification of GC peaks are discussed. Data are presented on cornpound s und e r go ing (no ncat a Iy t ic) thermolytic dissociation or (catalytic) elemental C, H, N, and 0 analyses and subsequent empirical formula determination. The catalytic packings in the oxidation-reduction reactors remained effective for over 400 individual C, H, and N determinations. Examples of computer or manual data handling of chromatographic peak areas gave acceptable results within ~ 0 . 5 % of theoretical values on samples as small as 0.01 PI. Linear dynamic range was determined for C, H, and N analyses from 0.01 to 0.5 J , with lower limits predicted to at least 0.005 PI. The elapsed time for these catalytic and noncatalytic processes is approximately 8 minutes from injection, and with a unique stop-flow arrangement, an analysis of a fourcomponent mixture (1.0 MItotal volume) was completed within 45 minutes.
THENEED FOR IDENTIFICATION of trace levels of organics which elute from gas chromatographs (GC) has been discussed for several years. The GC, interfaced with a mass spectrometer, has provided the one major method of obtaining such data, but at the expense of manpower, elaborate hardware, computer availability, and interpretative skill. Various other techniques using infrared and Raman spectrometers, trapping and isolation schemes, or reaction/pyrolysis G C have all been extensively employed. These methods are well documented throughout the current literature and in the compilations by Leathard and Shurlock ( I ) , Ettre and McFadden (2), and Berezkin (3). 1
The ability to determine carbon/hydrogen ratios in G C effluents was shown by Cacace, Cipollini, and Perez ( 4 ) in 1960 by converting the organic species with CuO and reduced Fe catalysts to COS and HS,which were then separated on an analytical column packed with acetonylacetate on Celite C22 at 18 “C with a Nz carrier gas. Other combinations of catalysts, analyzing columns, and equipment parameters have subsequently been employed (1-5). An engineering design and development project by the Armstrong Cork Company and Chemical Data Systems, Inc., has resulted in a new, relatively simple and inexpensive general tool which is rapid and accurate for the analysis of gas chromatographic effluents. On-line determination of the empirical formula of a G C peak has been accomplished in lieu of spectroscopic or trapping equipment. The modification of a Pyrochrom Analyzer, previously established for thermolytic dissociation patterns and functional-group analysis for G C effluents (6-9), included a modular two-stage oxidation-reduction reactor rather than the gold, tubular noncatalytic reactor. When a sample was directly injected into the continuous flow of the Pyrochrom He carrier gas, or a sample was transferred in a He stream from a remote G C unit, the species was quantitatively converted to COZ,H20, and NSfor the C , H, and N elemental analyses. EXPERIMENTAL
The separating chromatograph (GC No. 1) was a HewlettPackard Model 5750 equipped with a thermal conductivity detector. The He carrier gas flow was ca. 20 m1,’min for both the separation analysis, as well as in the Pyrochrom reaction module and its internal analytical column. Sample injections were made with a 0.5-p1 liquid syringe or a solids injector syringe (both obtainable from Chemical Data Systems, Inc., Oxford, Pa.) into either the Pyrochrom unit
Correspondence should be addressed to this author.
(1) D. A. Leathard and B. C. Shurlock, “Identification Techniques in Gas Chromatography,” Wiley-Interscience, New York, N.Y., 1970. (2) L. S. Ettre and W. H. McFadden, Ed., “Ancillary Techniques of Gas Chromatography,’’ Wiley-Interscience, New York, N.Y., 1969. (3) V. G . Berezkin, “Analytical Reaction Gas Chromatography,” Plenum Press, New York, N.Y., 1968.
(4) F. Cacace, R. Cipollini, and G. Perez, Scie~ce,182,1253 (1960). (5) R. S. Tse, J. Chem. Educ., 48,550 (1971). (6) E. J. Levy and D. G. Paul, J . Gas Clzromatogr.,5,136 (1967). (7) H. Kwart, S. F. Sarner, and J. H. Olson, J. Phys. Chem., 73, 4056 (1969). (8) S. F. Sarner, G. D. Pruder, and E. S. Levy, Amer.. Lab., 3 ( l l ) , 57 (1971). (9) E. J. Levy, S. F. Sarner, G. D. Pruder, and S. A. Liebman, Eastern Analytical Symposium, New York, N.Y., Nov. 1971. ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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VENT
5
7
TWO-STAGE O X / R E D
1
CONDUCTIVITY
SOLENOID TIMER CONTROLLED
Figure 1. Detailed schematic of Pyrochrom elemental analyzer ( 6 4 , showing two-stage reactor module
INJ P O R T S
VENT
Pyridine
Benzene
Table I. Computer Output for Samples Handled by On-Line Computer or Manual Peak Area Determinations Benzene Cd-I,, On-Line Areas Peak area Volume cos HzO
CP2
0.05 0.05 0.05 0.05 0.05
14286.00 15354.00 15467.00 17464.00 16910.00 Mean 15896.20
coz
8mlnutes
I
Figure 2. Pyrochrom C,H,N elemental analysis of benzene and pyridine First peak left is due to total gaseous products; analyzed peaks emerge subsequently from left to right
directly or, for peak transfer studies, into GC No. 1 in the usual manner. Figure 1 shows the flow diagram, discussed in detail elsewhere (6-8) and the two-stage reactor unit located in the Pyrochrom unit. The oxidation reaction quartz tube (5-in. x 3/s-in.) was filled with Coboxide, a cobalt oxide catalyst, obtainable from Coleman Instruments, Maywood, Ill. The reduction quartz tube (Sin. X 3/s-in.)was filled with reduced copper wire (Hewlett-Packard, Avonddle, Pa.) and also plugged with quartz wool (Perkin-Elmer Corp., Norwalk, Conn.). The effluents from G C No. 1 were directed into the oxidation reactor maintained at 800 "C, then into the reduction reactor held at 400 "C. Alternatively, for the noncatalytic studies, an empty quartz reactor tube was placed 1412
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
2085.00 1952.00 2202 00 2008,00 1792.00 2007 80 ~
I
HzO 0.750 7.70 7.48 0.22
Response factors 1.172 Theoretical % 92,30 Unknown % 92.52 Difference of av unknown - 0 . 2 2 tert-Butylbenzene CI0Hl4,Manual Areas Peak area Volume coz Hz0 0.05 0.05 0.05
66.05 73.08 67.20 Mean 68,78
12.81 14.20 14.00 13.67
cos
H20 0.750 10.45 11.28 -0.83
Response factors 1,172 Theoretical 89,55 Unknown 88.72 Difference of av unknown 0.83
in the oxidation reactor, operated at 800 "C, and the empty quartz tube in the reduction reactor was held at 400 "C. The catalytic conversions of effluents to CO for oxygen analysis were accomplished by Pt on carbon packing (ROC/RIC Corp., 11686 Sheldon St., Sun Valley, Calif.) in the quartz oxidation reaction tube at 750 "C; the empty quartz reduction tube was held at 400 "C. Thereby, G C effluents undergoing catalytic conversions passed through the oxidation-reduction reactors for conversion to COS,
5z
Table 11. Molecular Formula Program (IO) Results with High Error Limit Using Representative C,H Data Molecular Formulae Program c, H, Z Mol wt
z
92.30 1 .oo
7.70 1.OO
error permitted
Structures allowed I8
c 6 C7
Hs
C8
H8
c9 c9
H8 Hio Search complete through Clo.
90 104 116
118
t ~
Table 111. Same as Table 11, with Low Error Limit Molecular Formulae Program Benzene c, H, Z Mol wt
z
92.30 0.50
00
a2
0.4
a6
MICROLITERS
Figure 3a. Linear dynamic range for benzene
7.70 0.50 % error permitted
Structures allowed
c6 -0.04
78 0.04
C8
H 8
-0.04
0.04
104
H20, and N2; or alternatively, for CO conversion or functional group study. Initially, the internal Pyrochrom analytical column was 9-ft x 1/8-in. stainless steel filled with Poropak Q, 80-100 mesh (Waters Associates, Inc., Framingham, Mass.), and maintained at 60 "C to provide adequate separations and peak shapes. However, subsequent work has shown an 11-ft X lis-in. stainless steel column with 10-ft Poropak Q and 6 inches at either end with Poropak T added (80-100 mesh) has given superior results. The digital computer for on-line area calculations was a Honeywell 610, and additional computer programs were developed by C. D. Nauman, Armstrong Cork Co., on the IBM 360 Model 44. RESULTS AND DISCUSSION
Benzene and pyridine were used as reference compounds to establish the accuracy and reproducibility of the vapor-phase oxidation-reduction process (Figure 2). A solids microsyringe was also used to inject solid organic compounds which could be volatilized in the injection port to provide accurate C and H analysis. Manual or on-line computer measurements of chromatographic peak areas resulted in absolute errors of 1 0 . 5 routinely (Table I). A computer program allowed direct conversion of the experimental areas of COZ,H20,and N ? to C, H, and N content, respectively, and compared these percentages to theoretical values if known species had been examined. If unknown species were examined, a molecular-formula program (IO) showed reasonably allowed structures within the error limits established for the analysis (Tables I1 and 111). Linear dynamic ranges for the C, H, and N standards were determined from 0.01 to 0.50 p1 (Figure 3, a and b), using measured areas of C 0 2 ,HZO,and Nz us. amount (10) D. A. Usher, J. G. Gougoutas, and R. B. Woodward, ANAL. CHEW,37, 330 (1965).
0 00
02
04
06
MICROLITERS
Figure 3b. Linear dynamic range for pyridine OXYGEN ANALYSIS 5% PtICharcoal Reactor 750'C Acetone
DMF
MeOH
Figure 4. Elemental analysis for oxygen content in acetone, dimethylformamide, and methanol ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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GC 1
Pyrochrom-Elemental Reactor
G C1
CPZ
Pyrochrom-Elemental Reactor
BENZENE
I
Figure 5a. Elemental analysis for benzene transferred from GC No. 1 during separation analysis from tert-butyl benzene
Figure 5b. Same as 5a, except tert-butyl benzene peak transferred for elemental analysis
of sample injected. The predicted lower limit for obtaining elemental analysis is at least 0.005 p1, or for 1 to components in a mixture of ca. 0.5 p1 total volume. The oxidantreductant packings in the catalytic reactors were those of conventional microanalytical procedures for C, H, and N determinations and gave excellent results even after more than 400 individual samples had been analyzed.
Replacement of the quartz oxidation reactor tube with 5 Pt/carbon catalyst resulted in the conversion of oxygencontaining compounds to CO (Figure 4), as in the usual microchemical procedures (which generally employ 1 :1 Pt/ carbon catalysts). Evidently, from the large number of samples processed in the C, H, and N series and the performance of less-active
Zz
GC 1
n
Reference IuXtlKL PENTANE PEAK 1
c2 H4 Pyrochrom PEAK 2
Figure 6. Analysis of a four-component organic mixture utilizing stop-flow, transfer, and Pyrochrom functional group analysis
t
rK) MNUTES
Pyrochrom PEAK 3
co
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
I
Pyrochrom PEAI 4
5 % catalyst for oxygen analysis, vapor-phase conversions for these elemental species are highly efficient. Also, the extremely small sample sizes capable of being analyzed lengthen the lifetimes of reactor packings in this system. Modification of the two-stage catalytic reactors with noncatalytic surfaces (quartz or Au-coated stainless-steel foil) permitted functional-group analysis for unknowns by reference to previous Pyrochrom thermolytic-dissociation patterns (6-9). Hydrocarbons resulted in only C and H fragments; ketones, in addition, give CO; alcohols, CO and HzO; esters, CO, Cot,and H 2 0 ;chlorohydrocarbons, HCI. Demonstration of the ability to transfer sequential G C peaks from G C No. 1 to the Pyrochrom and accurately determine their elemental constituents is shown in Figure 5 , a and 6, for benzene and tert-butyl benzene. The calculated and experimental C and H values are indicated for these representative compounds and are within the desired accuracy limits of +0.5 %. In order to perform these above processes of thermolytic dissociation, C, H, N , and 0 determinations on a series of G C peaks emerging from G C No. 1 more rapidly than ca. 8 minutes apart (the time needed for complete C, H, N , or 0 data for any one peak), it was necessary to provide a stop-flow arrangement. A six-port microvalve (Carle Instruments, Inc., Fullerton, Calif.) and two matched analytical columns were installed in G C No. 1 to accomplish this requirement. Several trials determined that stop-flow of more than 10 minutes showed no adverse effects in peak shapes or relative peak relationships. With this arrangement, a 1-microliter mixture of four compounds of differing functionality was injected into GC No. 1 and sequentially dissociated within the Pyrochrom noncatalytic reactor, then directly analyzed on T internal analytical column. The the 11-ft Poropak Q results are shown in Figure 6. It is evident that the stop-flow system has enabled each peak to be separated, transferred,
dissociated, and individually analyzed as a hydrocarbon, ester, ketone, or alcohol species. Should the (catalytic) elemental reactors be utilized, the stop-flow arrangement allows ample time (ca. 8 minutes) for the C, H, N, or 0 data to be obtained for each peak in sequence as it is separated in the GC No. 1 analytical column. Alternatively, if desired, only one or certain peaks may be removed from a multicomponent mixture and analyzed as above. Therefore, for a mixture of five compounds, each species could be analyzed for C, H, and N content in 8 minutes and one would be provided with data within cu. 40 minutes for determining the likely empirical formula (C,H,N,) for each peak. CONCLUSIONS
It is felt that further direct application of this arrangement to complex problems of organic reaction mechanisms, kinetics, catalysis, and trace-product analysis will be possible. Extension of the method will be attempted to detect and quantitatively determine at these trace levels additional elements, such as sulfur and halogens, by selective combustion procedures. Organic compounds with more complex structures and combinations of heteroatoms will be studied. Development of a systems approach to problem-solving in organic analysis will continue. ACKNOWLEDGMENT
We appreciate the helpful discussion and advice provided by S. A. Sarner, CDS, Inc., and the aid in computer programming by C. D. Nauman and D . C. Messersmith, Armstrong Cork Company.
+
RECEIVED for review November 5 , 1971. Accepted March 17,1972.
Predicting Gas-Chromatographic Resolution for Pairs of N-Alkane Homologs Robert S. Swinglel and L. B. Rogers Department of Chemistry, Purdue UniGersity, Lafayette, Ind. 47907 Using values for relative retention, number of theoretical plates, and relative peak widths determined for two members of a homologous series, reasonably good predictions can be made for resolutions at a given value of capacity ratio for other pairs of homologs in a relatively well behaved chromatographic system. The limitations of the assumptions, upon which some of the calculations were based, have been examined using as an example the n-alkanes on SE-30.
ANALYSIS TIME is often critical in process-control applications ( I ) where analysis lag must be minimized and where many samples are to be analyzed. With the increasing use of digital computers as data processing devices for chromatographic Present address, Central Research Department, Experimental Station, E. I. DuPont de Nemours and Co., Wilmington, Dela. 19898. (1) I. G. McWilliam, in “Advances in Chromatography,” Vol. 7,
J. G. Giddings and R. A. Keller, Ed., Dekker, New York, N.Y., 1968, pp 163-220.
instruments ( 2 , 3), the total analysis time should no longer be limited by calculation of the experimental data but by the separation time itself. The problem of separation time in gas chromatography has been considered from the standpoints of minimum time for a prescribed resulution (4-6) and for normalized-time analysis (7-9) in which resolution is maximized for a prescribed analysis time. Hawkes (IO) has used computer (2) A. W. Westerberg, ANAL.CHEM., 41,1595 (1969). (3) P. P. Briggs, ControlEng., 14(9), 75 (1967). (4) B. 0. Ayres, R. J. Loyd, and D. D. DeFord, ANAL.CHEM., 33, 986 (1961). (5) J. C. Giddings, Am/. Clzem., 34, 314 (1962). (6) I. Halasz and E. Heine, in “Advances in Analytical Chemistry,” Vol. 6, J. H. Purnell, Ed., Interscience, New York, N.Y., 1968, pp 153-208. (7) B. L. Karger and W. D. Cooke, ANAL. CHEM., 36,985 (1964). (8) Ibid.,p 991. (9) G. Guiochon, ANAL. CHEM., 38,1020 (1966). (10) S . J. Hawkes, J . Chromatogr. Sci., 7, 526 (1969). ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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