A n Injection System for Gas Chromatography Wassef W. Nawar, F. Miles Sawyer, Ernest0 G. Beltran, and Irving S. Fagerson, Department of Food Technology, University of Massachusetts, Amherst, Mass.
methods have been used to S introduce vapor samples into gas chromatographic instruments (1-3, 6). EVERAL
I n sampling for trace components where external concentration is required, these methods suffer from numerous disadvantages. A considerable amount of the sample condenses and is adsorbed on the surface of the tubing before it reaches the column, which complicates results of subsequent samplings. I n samples containing several constituents which vary widely in volatility it is difficult to remove all of the sample from the trap in one injection. As many as 20 consecutive injections from the same trap with a conventional sampling system may be required to obtain complete transfer of a mixture of volatiles. A nonuniform distribution of components over the series of injections has been observed, with concentration of lower boilers being highe in the first injections than in the last the converse being true for the higher boilers. The system (Figure 1) was developed in this laboratory and is efficient particularly in application to food volatiles. It can be constructed easily in the laboratory a t small cost from readily available components and adapts to almost all types of gas chromatographic instruments. A fairly high vacuum can be achieved to enable rapid volatilization of high boiling compounds. KO appreciable sample loss occurs when pressure is applied. The proximity of the unit to the column minimizes sample condensation and adsorption and allows a n almost complete transfer of volatiles to the column.
Figure 1 . A.
Injection system
1 /4-lnch S. S. trap
6, C.
Toggle valves fittings with O-rings E. Lead F. Filament transformer G. Varloble transformer 1. Inlection block
D. S. S.
Initially valve B is closed and all parts remain in place except trap A , in which the volatiles from a food sample may be separately collected. Sample collection is achieved by a recycling sweep and liquid nitrogen trapping (4) Yhile still immersed in liquid nitrogen, trap A is fitted to the sample introduction assembly by Swagelock fittings D1 and D2. Serum caps are used to avoid foreign contaminants when A is returned a t liquid nitrogen temperatures. Valve C is opened and vacuum applied a t E (from a mechanical pump) to reduce the pressure to about 50 microns, Valve C is then closed, the liquid nitrogen removed, and trap A heated to bring the sample components into the vapor state. Fast and uniform heating is achieved by passing an electric current through the trap with the aid of filament I
transformer F and variable transformer G. The setting for the desired temperature can be determined with a thermocouple. Time and temperature of heating will vary with the nature of the sample. The volatilized sample is then injected by allowing a stream of helium to pass through lead E a t a pressure 5 p.s.i.g. higher than the column pressure. Valves C and B are opened to inject the sample into the gas chromatographic instrument. Twelve seconds of injection time are generally sufficient to place the bulk of the sample on the column. It is advisable to make a control run for each sample by using the same tubing and trap m-ithout the sample. Recoveries of approximately 90% have been obtained with a test mixture containing nine components varying in their boiling points from 27' to 173.5' C. Results have also been satisfactory with samples from various food products. LITERATURE CITED
(1) Craats, F. van de, Anal. Chim. Acta 14 136 (1956). (2) blew, D. K., Young, D. hl., ANAL. CHEY.30,1890(19%). (3) Harvey, D., Chalkley, D. E., Fuel 34, 191 (1955). (4) Xawar, W. W., Fagerson, I. S., ANAL. CHEX32,1534 (1960). ( 5 ) Rhodes, J. W., Food Research 23, 254 (1958).
CONTRIBUTIOH No. 1231 from the Cniversity of Massachusetts College of -4 riculture Experiment Station, Amherst, %ass. Work supported in part by a research grant RG-5848 from the Division of General Medical Sciences of the Sational Institutes of Health, Public Health Service.
Technique for Collection of Food Volatiles for Gas Chromatographic Analysis W. W. Nawar and I. S. Fagerson, Department of Food Technology, University of Massachusetts, Amherst, Mass. HE collection of volatiles from food T s a m p l e s in forms and quantities suitable for gas chromatographic analysis is usually difficult. Most of these compounds are present as traces; thus even slight losses during collection or concentration are critical. The high ratio of water to organic volatiles in food samples further complicates collection and separation. Conventional distillation and extraction with immiscible solvents have not proved universally satisfactory because of loss of
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ANALYTICAL CHEMiSTRY
sample during solvent removal, contamination introduced by solvents, difficulty in transferring small samples for injection without dilution, and unavailability of a single immiscible solvent suitable for the wide range of composition of volatiles generally encountered. Sweeping the sample with a gas or vapor stream (4) and condensing the volatiles from the stream in a cold trap offer advantages, but require several serial traps for efficient collection. Use of multiple traps is a serious
disadvantage in subsequent sample injection. The collection system shown in the figure is simple and efficient for the collection of food volatiles. The food sample (about 500 ml.) is placed in a 1-liter round-bottomed Bask, A , equipped with a sinteredglass tip (coarse) to aid in sweep gas dispersion. B is a tube packed with anhydrous K2C03 to remove moisture from the sweep gas and has minimal adsorption for sample volatiles (1).
