The Construction of a Simple Pyrolysis Gas Chromatograph Jack L. Hedrick Elizabethtown College, Elizabethtown, PA 17022
Since its introduction some 20 years ago, pyrolysis gas chromatography (PGC) has proved useful in a wide variety of applications. The proceedings of a recent international symposium on pyrolysis include, among others, presentations of PGC in the areas of forensic and soil on the andieation .. chemistry, geochemistry, biochemistry, polymers, and reaction mechanisms r l 1. Articles in m e iournal alonr since 1975 ttstifv t o the versatility of PGC, primarily as a tool for qualitative analysis. These include studies on or analysis of vinyl acetatelolefin copolymers (2), saccharin (3), penicillins and ceohalosnorins (4). ethvlene feedstock (5). enzvmes (6). h o ~ t u l i s m ' ~and ~ ) , aroma.tic polymers (81. A recell; paper is devoced to current I'GC ~nstr~nnentation and the awli(:atiun of the method to polymer analysis (9).Historically, qualitative analysis by PGC has proved more successful than quantitative analysis due to the lack of interlaboratory reproducibility, hut this problem has been attacked by ASTM (10). Despite the importance and versatility of PGC, only two articles have appeared in THIS JOURNAL that describe it to any extent (11,12). Perhaps part of the reason for this seeming lack of interest on the part of contributors involves the cost of such a unit, a sum that may be prohibitive for many institutions. The purpose of this paper is to describe a simple and inexoensive PGC svstem that can beconstructed from items available in many undergraduate institutions, particularly those that have an outmoded eas chromatomanh stashed awav in a remote corner. The sy&m is somewhat limited in that it accepts only Liquid samples and pyrolyzes "on the fly" rather than statically as is usually the case. In addition, it does not allow for reductive pyrolysis, a desired feature in many instances. Nevertheless, it does lend itself to a number of ap~lications.The aualitv and ape of manv of the items incorporated into the system will h&omc ohvious as it is described further. Some of the items are IS year8 old and older. h s p i t e this, well defined and reproducible pyrograms are -ohtained.
Construction Two different svstems have been constructed. and exoerimcms utilizing vari~u;design features have been carried out with hoth. This section summarizes these ellorls. The reader will recognize that apparatus other than that mentioned here could be quite suitable. A block diagram of the PGC system is shown in Figure 1. Aluminum or copper tuhing, %-in. o.d., is used to make the necessary connections while stainless steel (SS) tuhing is used in the pyrolyzer, oxidizer, and gas chromatograph. Standard commercially available "tees" and needle valves are used to split and regulate the carrier gas flow. a
Carrier Gas Both helium and nitrogen have been used with equal success. Purifier If a relativelv, hieh .. nuritv. carrier eas is emnloved. . . . a nurifier . is trnnecessar).; however, both an Ascarite/Dricritecolumn and a S u ~ e l c oCarrier Gas Purifier have been used for this nurpose.
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CARRIER
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PURIFIER SAMPLE INJECTION
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OXIDIZER
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RECORDER
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Figure 1. Block diagram of the PGC system.
Sample injection The sample is introduced by means of a 1or 10 fiL Hamilton syringe. Pyrolyzer A common muffle furnace (MF) of the Thermolyne 1300 series is used. Two holes are drilled through the door of the MF to allow for connections to the pyrolyzer tuhing. At the. entrance to the MF, the connector tuhing is fitted with a "tee," one end of which is fitted with a septum and acts as the injection port. The other end of the "tee" is connected to a coil of SS tubing, -100 cm in length, which resides in the cavity of the MF. The process of pyrolysis occurs as the sample passes through this coil. Oxidizer
A section of SS tubing, -40 cm in length, is filled with copper(I1) oxide, wire form, and placed in the cavity of a heating device of linear design. Both a Fisher microcomhustion furnace and tube furnace have filled this role. While successful to some extent, experience has shown that the temperature to which the CuO is usually heated (-400°C) is too low and the carrier gas flow is usually too fast (-60 mL sec-l) to allow an appreciable amount of oxidation of the pyrolysis products to occur. The CuO is usually allowed to remain on line, however, since its presence frequently leads t o pyrograms of better definition and resolution.
Gas Chromatograph A Carle 9000 GC has been utilized in one instance, the study of aqueous suspensions of bacteria, to take advantage of the
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Minutes Figure 2. Pyrograrn of methanol.
Figure 3. Pyrogram of ethanol
flame ionization detector. Studies of simple organic compounds have been carried out using a Fisher oven, 0.7 cu. ft. capacity, in which is placed the column and a Gow-Mac GK-001 thermal conductivity cell. In this case, a lead storage battery provides the bridge current and the control box is placed on top of the oven (13).An-75 cm length of S S tubing packed with Porapak Q, 50-80 mesh, serves as the column.
1. Most obviously, it can intraduce the student to the basic concepts of PGC. 2. By keeping theMFat a lower temperature than necessary far pyrolysis to occur, the instrument reverts to being a simple GC. 3. The unique pyrograms produced can be used in the identification
Recorder
plication which the author as yet has not seriously attempted. The fact that peak heights and areas have been determined to he a function of flaw rate and the rate of sample injection would tend to make this application more complicated than it may appear. 5. The dependence of pyrolysis product formation as a function of pyrolysis temperature can he clearly demonstrated. For example, the pyrogram of an aqueous methanol sample with the MF set at 100°C and using a thermal conductivity detector shows two peaks as expected due to the CHjOH and HzO. With the MF set at 350eC, the height of the CH30H peak a diminished and the presence of the C02 peak is noted. At 600°C, the CO peak is apparent and at 650°C, the CH, peak arises. Also at 650°C, the CHsOH peak is reduced in size so as to be barely diseernihle.The H 2 0 peak remains virtually the same throughout this change in pyralyzer temperature. 6. Preliminary investigationsof aqueous suspensionsof bacteria indicate some limited success in producing characteristic pyrograms, thereby opening the door to the possible use of the PGC system in microbiological studies.
The exact recorder employed depends upon the particular PGC system chosen and the desired sensitivity. A T I Servo1 riter 11, a Health EUW-20A, and a Varian 20 have all been used successfully a t one time or another. Experimental Condltions and Typlcal Results T h e set of conditions for a typical analysis includes a pyrolvzer temperature of from 700 to 80OoC. an oxidizer tempeiature of ~OO'C,and column temperatuie of lOO0C with a flow rate of 60 mL min-I and a sample size of 0.5 pL. A number of simple organic compounds including aliphatics and aromatics with various functional m o u ~ have s been analwed under these conditions, each prod;cing its own unique iyropram. For example, consider the analvsis of two simple alcohols when usinga thermal conductivity detector. he pyrogram of aqueous methanol is shown in Figure 2. The four peaks, in order of increasing retention time,are carhon monoxide, methane, carbon dioxide, and water. The pyrogram of aaueous ethanol disnlavs eight neaks as shown in Fieure 3. he first three peak:, as in the case of methanol, areittributable to carbon monoxide. methane. and carhon dioxide while water appears as the sixth peak. o f the remaining peaks, the fourth has been identified as ethvlene. Peak identification is made from the infrared jpectrum of thc wrolssis . .~roductscollertedin a Wilks .Mini-Gas cell. Infrarkd spectrophotometers ranging in age, price, and quality from a Perkin-Elmer 700 to a Perkin-Elmer 283 all have performed this function satisfactorily. Applications There are a number of ways in which the PGC system described can be incorporated into the undergraduate curriculum, six of which follow.
of compounds, e.g., at the introductory organic level. 4. Identification of all of the peaks of a pyrogram could lead to the use of the PGC system in performing elemental analysis, an ap-
Acknowledgment The author wishes to acknowledge the work of Sharon Newcomer Minney, Michael Siena, David Hunsberger, Virginia Zuern Indivero, and Janet Pollard, all of whom contributed to this project as student research assistants. The author is also grateful for financial aid to support certain aspects of this work in the form of a n Elizabethtown College Seed Grant. Literature Cited (11 Jones,C. E. R., and Cmrners. C. A,. (Editorri. "Analytical Pyrolysis: Elsevier. New
(19781.
( 5 ) Greeo. M . J . Chmrnolngr. Sci. 16.36 (61 Danielmn, N. D.,Glajeh, J. L., and Rodgers. (19781.
L. R., J.
Chromotogr S e i , 16, 45
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