intcrrelationships existed among the variables. I3esides the two arrangements, sequential and categorical described above, a third provocative tabulation was obtained by listing for each variable its correlation coefficients with each of the other 183 variables in descending order of rank by difference between values for days 7-18 and those for days 37-48 without regard to sign. This gave clusters of variables having similar magnitudes of change. For example, the correlation coefficients which changed the most during the experiment (and presumably representing the most labile interrelationships) c,lustered a t the beginning of the tabulaation for each variable, while those which changed least (prcsiimably representing the most stable interrelationships,) were found a t the end of the list. Our studies of these
clusters and the changes which they undergo with respect to their labile and stable components are in progress. These three complementary tabulations, each obtained by means of electronic computers, together constitute a heuristic device for systematically searching out evidence for the existence of hitherto unrecognized nutritional interrelationships. Very recently, in the realm of taxonomy, a similar technique of using computers in claqsifying bacteria was described by Sneath ( I ) . ACKNOWLEDGMENT
The author thanks the Human Sutrition Research Division of the Agricultural Research Service of the U. S. Department of Agriculture for its recognition of the importance of searching out nutritional interrelationships. He is
indebted to Elliot J. Bueche, ,Jr., Carole Rose Anderson, and Maureen Mitchell for their technical assistance in processing the multitudes of data, and to Director B. 13. Townsend and his colleagues a t the LSC' Computer Research Center for their help and advice in programming the computers and operating the machines. LITERATURE CITED
(1) Sneath, P. H. A., Advan. Sci. 2 0 , N o . 88, 572 (1964).
WILLIAM H. JAMES Department of Food Science and Technology Louisiana Agricultural Experiment Station Louisiana State Cniversity and Agricultural and Mechanical College Baton Rouge 3, La. 70803 RECEIVED for review March 9, 1964 cepted June 4, 1964.
Ac-
Bracket Method for Molecular Weight Determination of Pyrolysis Products Using Gas Chromatography with a Gas Density Detector SIR: The Martin gas density balance was first proposed by Liberti, Conti, and Crescenzi (3) for molecular weight determination of components by chromatographing aliquots of the sample with two carrier gases. Accurate area measurements of unknown and standard were required because calculation was such that small ,mea errors were magnified in the molecular weight value. Phillips and T'imms ( 5 ) obtained greater accuracy (19;) by trapping out an individual chromatographic component and introducing the component gas into a calibrated gas density balance by careful pressure volume measurements. We have investigated the Gow-Mac gas density detector '(1,4 ) for molecular weight determination of certain volatile pyrolysis products from polymers as an aid in identification cl-f these substances. By using several carrier gases and in particular a carrier gas of higher molecular weight than the unknown substance and one of lowei- molecular weight, the molecular weight of the unknown may be bracketed with good accuracy. The positive and n1:gative peaks obtained (depending on whether the unknown substance has a higher or lower molecular weight tha,n the carrier gas) often serve to classify several components of a complies mixture in a molecular weight range. This qualitative information is an. advantage when using a selective column where retention
times of polar substances are not related to order of molecular weight or boiling point. Molecular weight determinations on two differentiating peaks present in the pyrolysis products of polyethylene glycol adipate and polypropylene glycol adipate (Figures 3 and 4) illustrate an application of the technique. By obtaining chromatograms in carrier gases with molecular weights greater and less than that of the unknown, the calculation of results is an interpolation and gives greater accuracy of molecular weights than the extrapolation method used by Liberti, where the molecular weights of the two carrier gases were more than 100 units less than the unknown. EXPERIMENTAL
Instrumentation. T h e Gow-Mac gas density detector (hot wire Model 091) was installed in the Podbielniak 9580 chromatograph in place of t h e T / C cell. Operation details are similar to conventional gas chromatographs with one exception: flow rate of the reference gas should always be greater than column flow rate. The instrument has a polarity reversing switch for conveniently obtaining negative peaks. The column was 10 feet long, 1/4-inch diameter stainless steel and packing was 25Tc Paraplex U-148 polyester and 270 phosphoric acid on 60- to 80-mesh Chromosorb P diatomaceous silica. Column temperature was 96" C. and
flow rate was approximately 40 ml. per minute; reference flow rate was 75 ml. per minute and detector current was 140 ma. Pyrolysis was carried out in an evacuated quartz tube, Q, a t approximately 450" C., as shown schematically in Figure 1. The tube (25 cm. x 1-cm. diameter) had ball joints, B, on both ends. One end was attached to a stainless steel socket joint', B , with a syringe needle, N , attached so that the needle could be inserted into a collector syringe, S Y . The other half of the quartz tube remained outside the oven (furnace, F , hinged; inside length, 4 inches; catalog KO. F-9285, Scientific Glass Apparatus Company, Inc., Bloomfield, N. J.) and had a socket joint attached with a suitable cutoff valve, V , to the vacuum pump. The sample, S, contained in a small porcelain boat or wrapped in aluminum foil, was inserted in the cold part, of the tube and pushed near the furnace entrance with a quartz poker, P . An iron ball, d d , placed after the poker was used to move the sample into the hot furnace with an external magnet,, AfG, after the system was evacuated. The collector syringe (50-ml. hypodermic, B-D, glass tip) had a septum attached to the end with a polytctrafluoroethylene sleeve, T, and a small serum stopper, SP. 'The syringe was wrapped with a heating t'ape, H , a n d kept a t about 85' C. for the present work. X porcelain perforated disk was 111 aced in the syringe to aid in mixing the gasps. A small amount of Xliiezon grease was used on the upper portion of the syringe VOL. 36, NO. 9, AUGUST 1964
1849
Figure 1 .
Schematic of pyrolysis apparatus
V , Valve to vacuum; B, boll ond socket joints; M, iron ball; MG, magnet; P, poker; S, sample; F, furnace; Q, quartz tube; N , needle 20-gauge; SP, septums (flat and serum types); SY, collection syringe; H, heating tape; T, polytetrafluoroethylene
barrel for vacuumtight seal. The collector syringe was connected t o the pyrolysis tube by inserting the needle, N , into the syringe. For pyrolysis, sample (50 mg. of polymer) was placed in the quartz tube, and the tube was evacuated. The vacuum valve was closed, and the sample was moved into the furnace (450" C.) with the external magnet. After 3 to 4 minut'es, the collector syringe plunger was pulled out about half the syringe length and the needle connected to the pyrolysis tube was withdrawn. About 5 to 10 ml. of pyrolysis gases were collected. The standard, 1 pl. of benzene, was injected into the sample syringe and mixed with the pyrolysis gap by shaking the syringe so that the tumbling action of the porcelain disk helped to mix the gases. A sample of the pyrolysis gases (1 ml.) was withdrawn with a warm 2-ml. hypodermic syringe and injected into the chromatograph. The 2-ml. sampling syringe was jacketed and kept in an oven a t 85" C. until needed. The carrier gas was changed after a chromatogram was obtained and another sample of the same pyrolysis gases was injected. The ratio of unknown and standard in the collector syringe, S Y , should remain constant over 2 hours or time for obtaining chromatograms in several carrier g a m . For example, the ratio of propionaldehyde and benzene was constant to within &4y0 (standard deviation) relative for a 2-hour period; this precision is adequate with the bracket technique.
04t 0.6
1
Figure 2. gas (MJ 0
A
Response ( Y ) vs. molecular weight of carrier
Peak E in polypropylene glycol adipote pyrolysate Peak 2 in polyethylene glycol adipate pyrolysate
from a plot of response value for constant sample size, Y , against molecular weight of carrier gas, M,; intersection of the .M, axis gives .1fz (Figure 2). The response value of the unknown, Y , as a function of carrier gas molecular weight is derived as follows:
can be divided into the right side of Equation 2. Then if
A,
=
A,
(4) (A) *If, - M,
the resulting equation may be expressed as a function of carrier gas XC.
(5) A , = area of unknown peak; A , = area of standard peak; K = instrument constant; qs and pz = area responses of
qz/qa is constant for a given sample. Therefore, the right side of Equation 3
II
I
COMPUTATIONS
Molecular weight of the unknown peak .tfSis computed from the areas of the unknown (Az, A=') and standard (.I8, .13')in each carrier gas according to the following Equation 1 for two carrier gases :
A,'(Ma - JL) A,'
... ...
ANALYTICAL CHEMISTRY
... ...
..
i j 2
.. .. .. .. ... ...
.... .... .. .. .. .. .. .. ... ... .. .. : . ... ... .. .. ... ...
(I)
.Ifz - J12
J6, equals molecular weight of the standard and M Iand M 2are molecular weights of the carrier gases. For several carrier gases, the molecular weight of unknown .lI, is obtained
1850
... ...
4
Figure 3. 1,
Pyrolysate of polyethylene glycol adipate
Combined volatile peok; 2, acetaldehyde; 3, benzene (standordl Nitrogen carrier gas Difluoroethane carrier gas
-
......
+ W u)
z
0
Q v)
W
a
I
Figure 4. I’yrolyslate of polypropylene glycol adipate in difluoroethane carrier gas A, combined volatile peak; B, C, D, F, not identified; E, propionaldehyde; G, benzene (standard)
standard and unknown converted to a weight basis (Equations 2 and 3); and
molecular wt. of std. molecular wt. of std. - molecular wt. of carrier gas Rigorous derivations of Equations 1, 2 , and 3 are given in previous papers ( 2 , 3, 5). DlSCUSljlON
Chromatograms for volatile pyrolysis products of polyethylene glycol adipate obtained in nitrogen and difluoroethane carrier gases with benzene added as a standard are shown in Figure 3. Polypropylene glycol adipate volatile pyrolysis products in difluoroethane are shown in Figure 4 ; all peaks were positive in nitrogen carrier gas. Molecular weight of peak 2 in Figure 3 and peak I:’ in Figure 4 was determined by the preceding method. These two peaks were confirmed as acetaldehyde and propionaldehyde by infrared spectra of gaseous samples collected from the column effluent with a large-scale pyrolysis.
I n Figure 3, the broad negative peak coming after benzene with difluoroethane does not show as a peak when run in nitrogen. Also in Figure 4, peak C has a higher rrolecular weight than the three peaks with longer retention times. Molecular weight data for peaks 2 and E obtained from chromatograms in several carrier gases and computed according to Equation 1 and the graphical method (Figure 2) are presented in Table I. The bracket method is considerably more accurate. S o t e the erroneous value of 7 5 for molecular weight obtained with nitrogen and carbon dioxide carrier gases. These same data obtained in difluoroethane (negative peak) gave a value of 59.5. I n another calculation with nitrogen and argon carrier gases, a 10% assumed error in one area value changed the molecular weight from 59 to 66 units, whereas with difluoroethane, the molecular weight was changed only from 59.0 to 58.7. The fluoro and chloro hydrocarbon gases are available up to molecular weights of 200 (octafluorocyclobutane) and several of these gases have been evaluated as carrier gases for molecular weight determination of known substances. Molecular weight values are presented in Table I1 using the chromatographic conditions previously mentioned except that the sample was injected as a liquid. Liberti, Conti, and Crescenzi ( 2 )
Table I. Molecular Weights of Pyrolysis Products by the Bracket Method
Substance Computation Mol. wt. pyrolyzed Carrier gas method Pol ethylene Nitrogen, difluoroethane Equation 1 or 43 0 yycol adipate graphical Po?yeth y lene Sitrogen, difluoroethane Graphical 44.5 glycol adipate Polye thy lene Xitrogen, carbon dioxide Graphical 44.5 glycol adipate Av. 44 05 Pol propylene Nitrogen, argon, carbon Graphical 59.0 gKycol adipate dioxide, difluoroethane Polypropylene Nitrogen, carbon dioxide, Graphical 58.5 glycol adipate difluoroethane Pol propylene Nitrogen, difluoroethane Equation 1 59.4 gfycol adipate 59 5 Polypropylene Nitrogen, difluoroethane Equation 1 glycol adipate Av. 59.1b Pol propylene Nitrogen, carbon dioxide Equation 1 75.0 grycol adipate b Identified by infrared as propionaldehyde. a Identified by infrared as acetaldehyde. Table II.
Molecular Weights of Various Substances
Substance Standard Carrier gases CClzFza, C4Fsb Carbon Benzene tetrachloride CClzFZ’, CIF, * Chloroform Acetone CClzFz, CF4 Toluene Benzene Acetone Nz, .4, COz, CzH4Fzd Cyclohexane Dichlorodifluoromethane. * Octafluorocyclobutane. c Standard deviation 1 0 .4y0 for four determinations. 1,l-Difluoroethane.
Mol. wt. found 155 2O 120 0 93 5 59 0
VOL. 3 6 , NO. 9, AUGUST 1964
Error +1 4 +O 6
+14
+I 0
1851
suggested in another paper that a carrier gas of high molecular weight would improve accuracy, but no experimental data were presented. ACKNOWLEDGMENT
The author thanks Alfred Pollara for assistance in making the infrared identifications and W. B. Prescott and
J. T. Woods for their encouragement and many helpful suggestions.
(4) Nerheim, A. G., ANAL. CHEM. 35, 1640 (1963). ( 5 ) Phillips, C. S. G., Timms, P. L., Chromatog. 5 , 131 (1961). JAMESS. PARSONS
LITERATURE CITED
( 1 ) Guillemin, C. I,., Aurcourt, F., J . Gas Chromatog. 1 , 24 ( 1963). ( 2 ) Liberti~ Conti, L., Crescenzi, V., Accad. Nazl. Lincei Rend. 20,623 (1956). ( 3 ) Liberti, A , , Conti, L., Crescenzi, V., Nature 178, 1067 (1956).
organicChemicals ~
i
~
i
~
i
American Cyanamid Co. Bound Brook, N. J. RECEIVEDfor review March 20, 1964. Accepted May 25, 1964.
Detection of Traces of Polynuclear Aromatics in Hydrocarbons by Gas Chromatography SIR: Traces of polynuclear aromatic hydrocarbons in petroleum products are usually determined by ultraviolet spectrometry (3) or gas chromatography ( 4 ) . Isolation of the polynuclear aromatics and elimination of interferences is time-consuming. With an electron capture detector, however, certain polynuclear aromatics may be determined by gas chromatography with few or no prior separations, because this type of detector is insensitive to most other hydrocarbons (6). An electron capture detector for detecting polynuclear aromatics in heavy oil samples is connected in series with a hydrogen flame ionization detector, which responds to all hydrocarbons, for determining the boiling point distribution of the entire sample. Two parallel detectors have been used t o analyze portions of the effluent from a gas chromatography column ( 1 , 6); Lovelock calls this “twochannel” gas chromatography. To differentiate the two modes of operation, we suggest the terms “parallel detector” and “series detector” gas chromatog,H2-FLAME
raphy. The number of detectors need not be limited to two. One requirement of series operation is that all detectors except the last must be nondestructive. Series detector systems are simpler, more sensitive, and more accurate than parallel systems. The entire column effluent passes through both detectors, giving greater response in both. No correlation of the ratio of the flows through the detectors is necessary; there is no danger that the ratio will change, or that the gases entering the detectors will have different compositions because of fractionation in a stream splitter. Our specific application has an additional advantage-e.g., the hydrogen flame detector establishes that the sample has passed through the electron capture detector, which gives no response if there is no material present that captures electrons. A disadvantage of series operation is that the components separated by the column may be partially scrambled in the first detector. I n our electron capture
detector, which has a volume of about 1 ml., little remixing occurs. The time lag between responses of the two detectors is about 3 seconds; it could be reduced by introducing dilution gas upstream of the first detector. EXPERIMENTAL
A Barber-Colman Series 5000 modular gas chromatograph is used. I t consists of a temperature-programmable column oven, a parallel-electrode electron capture detector and a hydrogen flame ionization detector in an isothermal oven, two electrometers, and a dual-pen 5 m v . recorder. The two detectors are connected by sections of ‘/je-inch stainless steel tubing joined by a short section of Teflon tubing for electrical insulation. The chromatograph was modified to give on-column injection to achieve proper vaporization of high-boiling samples. Auxiliary heat is supplied to the dilution gas line entering the bottom of the electron capture detector to prevent condensation of sample a t that point. A Moore 63-BCL flow controller maintains the eluting gas flow rate constant.
DETECTOR
BENZO(a)PYRENE
BENZ(a)ANTHRACENE
3- METHYLCHOLANTHRENE HOLANTHRENE
I
I 0
IO
Figure 1 .
I
I 20
I
I
30
MINUTES Chromatograms of white oil containing added polynuclear aromatics
4-ft. X 3-mm. 0.d. gloss column; 0.5% SE-30 on nonacid-worhed 30- to 60-mesh Chromororb W; flow rote, 20 ml. Nn/min.; no dilution go,; 1.0 #I, som ple column temp. progrommed from 130’ to 260’ C. a t 4’ C./min.; 33 volts on electron copture detector; 300 volts on hydrogen Rome detector
1852
ANALYTICAL CHEMISTRY
~
~
