Table VI.
Quantitative Analyses with Capillary Column and Flame Ionization Detector
Mixture No. 55 Integrated Area Values
D. *Heptane
E. Methylcyclohexane
Weighed in Per Cent b.w.
A. B.
C. D. E.
16.38 18.02 24.47 18.68 22.45
2 6427 6768 9814 6942 8348
1 6318 6643 9615 6776 8141
A . 2,ZDimethylbutane B. %Methyl-1-pentene C. Benzene
Found Uncorrected Per Cent b.w. 1 16.85 17.72 25.65 18.07 21.71
2 16.78 17.67 25.62 18.13 21.80
3 16.78 17.71 25.63 18.11 21.77
4 16.79 17.71 25.62 18.10 21.78
5 16.84 17.71 25.68 18.05 21.72
3 6398 6749 9773 6905 8298
Area Factors 0.963 1.ooo 0.949 1.000 1.008
4 6287 6626 9593 6776 8156
5 6570 6910 10022 7043 8477
Found Corrected Per Cent b.w. 1 16.52 18.03 24.77 18.40 22.28
The question whether the concentration ratios are in fact falsified if a bypaw is included in the system, is not answered by this paper, which restricted its findings t o the statement that under constant operating conditions the normalization factors are affected with practically the same errors as those obtained with packed columns
model mixtures gave analogous results. I t may, therefore, be presumed that our bypass system complies with the basic requirements for quantitative analyses with a capillary column and flame ionization detector. It should, nevertheless, be attempted to develop an unbranched system permitting a reproducible introduction of samples aa
and thermal conductivity cells. Detailed investigations are underway to find whether such falsifications do, in fact, occur. A similar series of experiments on
small as 0.05 to 1 pg.
2 16.45 17.99 24.75 18.45 22.36
3 16.45 18.02 24.76 18.44 22.33
4 16.46 18.02 24.75 18.42 22.35
5 16.50 18.03 24.81 18.37 22.29
W. Scott, ed., p. 46, Butterworths, (2)London, Halhz, 1960. I., Schneider, W., 2. anal. C h m G 175,94(1960); in “Gas Chroma-
tography,” R. P. W. Scott, ed., p. 104, Butteworth, London, 1960. (3)Tedr. 32, 675 I*,(1960f er, G*, Ins* (4) InformalSymposium on Gas Chr+ matogra hy, Atlantic City, N. J., ANAL.(%EM. 32,339 (1960). (5) McWilliam, I. G., J . Appl. Chem. (London)6, 379 (1959).
LITERATURE CITED
e.
(1) Dqsty, D. H., Geach, J., Goldu A., in ‘‘Gas Chromatography,” R.
8;
RECEIVED for review December 20, 1960. Accepted March 24, 1961.
Compact Two-Stage Gas Chromatograph for Flash Pyrolysis Studies STANLEY B. MARTIN and ROBERT W. RAMSTAD
U. S. Naval Radiological Defense Laboratory, b A compact, portable, two-stage gas chromatography system which permits the direct measurement of the complex reaction products o f flash pyrolyses has been constructed and successfully operated as part of a program dealing with the high temperature behavior o f materials. A unique feature of this system i s that the flash pyrolysis reaction i s carried out directly in the carrier gas stream just prior to its entering the first of two initially coupled stages. Products of the reaction mixed with the helium carrier go directly onto the liquid partition column of the first stage, and pass onto the absorption column of the second stage. The two stages are then uncoupled. The first stage proceeds to analyze the higher molecular weight products while the second stage concurrently analyzes the fixed gases. To achieve the pressure balance necessary to perform such a valving 982
ANALYTICAL CHEMISTRY
San Francisco 24, Calif.
operation, a precise electrical flow control was developed. Maximum component density was achieved by constructing each stage to fit inside o f a small Dewar flask. Details of construction, performance, and utility o f the instrument are described.
A
ideally suited for investigating the mechanisms of some reactions is a combined reactorchromatograph in which the reactants and/or products are carried through the reacting zone by the chromatography carrier gas (1). This approach lends itself particularly well to the study of the rapid degradation of nonvolatile solids or liquids when these reactions can be induced by some external stimulus as in the case of flash pyrolysis. The subject of flash pyrolysis is really beyond the scope of this paper. The interested reader should TECHNIQUE
refer to the work of Nelson and Lundberg (3). The application of chromatographic techniques to such a reacting system has been used with some succew and has been reported elsewhere by the author (9). This application made use of adsorption chromatography to resolve and determine the amounts of the fixed gases and liquid partition chromatography to analyze the condensable substancea which had been trapped out of the line ahead of the adsorption column. This procedure was necessarily laborious, time-consuming, and not completely reliable. The principle of staging two or more chromatography systems seemed the most natural solution to this problem, but the multiple-stage systems commercially available were much too large. Compactness and portability were qualities deemed essential to the program which involves the use of several different sources of intense radiant energy,
E.RRIE*
0.s
e E r I R E N C c DLTICTO"'
Figure 1.
inclutiing refocused carbon arcs a t several fixed sites. This report describes how some of the design and construction problems were handled and presents an evaluation of the utility of the resulting system. DESCRIPTION
OF SYSTEM
To achieve a high degree of compact-
ness, each stage was constructed so that i t would fit within a standard 665-ml. Dewar flask. Figure 1 is a cutaway drawing of the Dewar and the stage contained therein. The central a r t is a massive cylindrical brass lock which was drilled out to form the passageways for the gas and to take the tungsten filament detectors along with a normally closed thermal switch (Thermoswitch manufactured by Fenwall, Ashland, Mass.). The columns were wound around the outside of this brass block along with a preheater line for bringing the temperature of the helium up to the temperature of the column before it passes over the reference elements. The heating wire was then wound around the outside and the entire assembly was wrapped with asbestos to fit snugly inside a Dewar flask which lies on its side, as shown. Figure 2 shows the manner in which two of these units were connected together to make up the two-stage system. The Dewar assemblies are represented by the two circles labeled GLPC (gasliquid partition chromatography) stage and GSAC (gas-solid adsorption chromatography) stage. Only one of the two columns in each stage is indicated in the drawing. Either of the two columns can be put into use by turning a four-way Teflon plug valve (not shown). The three valves marked VI, VS, and V S are also four-way Teflon valves (Republic Manufacturing Co., Cleveland, Ohio). V I conneck the reactor into the system or bypasses it; Vn is the coupling valve between the two stages; and V , allows either of the two streams to be monitored by the flowmeter, or, alternatively, t o run t o the collection manifold for the collection of samples. The pressure balance necessary for smooth performances during the coupling-uncoupling operation is conveniently provided by electrically controlled, variable-flow resistances (F1and F , in Figure 2). These unusual devices were constructed from 10-inch len ths of stainless steel needle tubing embefded in asbestos. Precise control of the re-
E
Figure 2.
Assembly drawing of one stage
sistance to carrier gas flow is achieved by heating the needle tubing with an electric current conducted directly through it and controlled by a panelmounted Variac. I n this way, when a column is changed or its temperature is varied, the carrier flow resistance at a corresponding point in the other stream can be quickly and precisely matched. T o ' increase the range of control, precision needle valves ( I / r inch orifice metering valve from Hoke Inc., Cresskill, N. J.) were connected in series with the adjustable flow resistance. The carrier gas, normally helium, is supplied by a pressure regulator a t about 30 p.s.i.g. and split into two equal streams of 100 ml. per minute flow rate each. InitiaJly, with the stages coupled, one stream runs through the reactor and the two columns in turn while the other stream is directed to the reference detector elements and through the adjustable flow resistances. After uncoupling, one stream supplies the GLPC and then drops through the flow resistance corresponding to the GSAC, while the other stream (still bathing the reference elements) drops through the resistance matched to the GLPC and then supplies virgin carrier gas to the GSAC to develop its chromatograph. When the material to be pyrolyzed has been introduced into the reactor, the valve VI is turned so that the carrier gas flows through the reactor, which is heated to about 90" C. Sufficient time is allowed for any moisture contained in the sample to be driven out and for the system t o attain equilibrium. The reaction is
Flow diagram of two-stage system
initiated by flashing with intense radiant energy from either a carbon arc source or a xenon flash tube (the time of reaction is generally of the order of a second down to a millisecond in duration). The products which are formed are immediately swept out of the reactor and onto the head of the gas-liquid partition column. As the fixed gases emerge from the gas-liquid partition column unresolved, they are carried immediately to the head of the gas-solid absorption column and will proceed to separate from one another in the usual manner. At this time the two stages are uncoupled by turning valve V?. From this point on, the materials which emerge from the GLPC stage will flow through the variable flow resistance corresponding to the GSAC column but will be diverted from the GSAC stage. They will then pass to valve V3 and can be collected one a t a time in the sample collection manifold. The fixed gases (usually hydrogen, carbon monoxide, methane, carbon dioxide, ethane, and the unsaturated Czhydrocarbons) which have already gone onto the GSAC column will emerge one a t a time and they, too, can be collected in the conventional manner. Most of the connecting lines were made of 1/8-inch soft annealed thinwalled stainless steel tubing of the type used in aircraft. I t bends sharply without constricting, takes up very little room, and connects easily and reliably with Swagelok connectors (Crawford Fitting Co., Cleveland, Ohio) to make a leak-free system. Figure 3 shows the schematic wiring diagram. The variable transformers T
I
Figure 3.
Schematic wiring diagram VOL. 33, NO. 8, JULY 1961
983
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T .. .. "
moo. . m u , . . . . H a . .h S ] .. .. .. .. -01. .
984
ANAlYllCAL CHEMISTRY
. u , . . . S]z :%q: :2: ,. .. :. 00
,010.
.
'
.o . . .cD . .
.. .. ..
' 0 "
'
.
. . 0 0 . . r : . :. 0 3 m :, :, ': : :
:uJ
'
5 L O W F'YROLYSIS
"FLASH PYROLYSIS
Figure 4. Two-stage chromatograph with external exposure chamber located in focus of corbon-arc thermal radiation source I
and T,are used to control the power to the heaters in the Dewar flasks. Variable transformen Ts and Tg control the current to the needle tubing flow controllers RIsand RM. Switches S2 and Sa provide an initial fast heating of the blocks in one position and a much reduced heating rate in the other position. H, and H, are the Fenwall thermal switches. L, and La are pilot lights which serve a double function. When the unit is first turned on, switches S, and Ssare turned to the "fast heat" position. As soon aa the temperature of each block reaches the desired level, as preset by the Fenwall switches, the pilot lights Lr and Ls indicate this fact, at which time S, and S, are turned to the "operate" position. The pilot tights also act as ballast resistors to reduce the current through the heating elements (indicated as RI and &) after the Fenwall switch opens. I n this way, there is always some current supplied to the heaters, reducing the rate of fall of the block temperature and minimizing the temperature fluctuations. I, and I2 are iron-constantan thermocouples connected to the brass blocks indicated here as BI and B1. These can be connected to the recorders through switch S, allowing the temperature to he recorded. This permits adjustment of TI and TI to their optimum settings (minimum temperature fluctuations) and also indicates when the device has reached equilibrium after warming up. The bridges are indicated in the conventional manner and the filaments are shown as Ex through Es. Initially, large dry cells (18 volts nominal) were used to supply the bridge current, but because of the extreme dependence of sensitivity on bridge current, a small regulated power supply has heeu suhstituted for the battery. Figure 4 is a photograph of the overall unit as it looks in operation. Some reactions produce large quantities of tarry substances and heavy viscous liquids having low vapor pressures. To preclude the introduction of these materials into the chromatography system, a short stripping column is provided between the reaction chamher and the port of entry into the unit; because the carrier gas is allowed to flow through the reactor and the strip-
Figure 5. Droducts
Chromatograms of
ping column for only a few minutes after the exposure is completed, these materials are efficiently retained by the stripping column. This column (generally a couple of inches long) is packed with a weighed amount of column packing such as silicone oil on crushed firebrick. After the exposure is completed the stripping column is set aside for subsequent analysis of the materials retained. The system has heen provided with a means of back-flushing the columns to remove from them components which require a prohibitively long t i e to elute from the column in the normal direction. These 'components can be frozen out as they emerge during backflushing and subsequently can be analyzed at a higher column temperature or on a different type column.
cellulose
pyrolysis
slower pyrolysis occurred, at .. tures between 250' and 350" C . resulting from exposure to a much less intense radiant input of several seronds duration. I
~~
~
Table
II.
Performance Characteristics of GSAC Stage
6foot activated charcoal column at 120' C.. 100 ml./min.
Sample Size Com- (GasVol. pound in MI.) 0 . 1 -1 0.034.3 0.034.2 0.0443 0.1 4 . 2 0.1
0.034.3
OPERATING CHARACTERISTICS OF SYSTEM
The sensitivity of the system is such that, in general, it can provide quantitative values for sample sizes down t o about 10-8 mole. The characteristics of the system are shown in Tables I and 11. The resolution of the columns is equivalent to about 2400 theoretical plates. An example of the type of analysis afforded by this system is shown in Figure 5. In this case the GLPC column was a polypropylene glycol column 6 feet in length, and the GSAC colunin was an activated charcoal column also 6 feet in length. The polypropylene glycol column was maintained at 70" C. and the activated charcoal column at 120' C. The figure compares the low molecular weight products formed during a flash pyrolysis of cellulose to those produced by relatively slow pyrolysis. The flash pyrolysis was induced hy intense radiant power of millisecond duration which produced momentary temperatures in excess of 600' C. in the sample. The
a b
Sensitivity, M1.Mv.1
Retention. Relative
Mg. 270b
1470 2280 768 1060 1680 1570
to
Carbon Dioxide -0.17 +0.07 0.39 1.00 2.45
3.67 5.3
Measured from air peak. Value good only for rough estimation.
This compact two-stage chromatograph provides a convenient and reliable means for studying the rapid degradative reactions of organic solids. Detailed analyses of the stable flash pyrolysis products of cellulose and cellulose-like compounds will appear in subsequent publications. LITERATURE C I T L
(1) Kokes, R. J., Tohin, H.. Jr., Emmett, P. H., 6. A m Chem. Soe. 7 7 , 5860 (1955). (2) Martin, S., J . Chromatog. 2 , 272 (1959). (3) Nelson, L. S., Lundherg, J. L., J . Phys. Chem. 63, 433 (1959).
RECEIVED for review December 5, 1960. Accepted April 12, 1961. Division of Andytieal Chemistry, 137th M e e h g , ACS, Cleveland, Ohio, April 1960. VOL. 33, NO. 8, JULY 1961
.
98s
