I AIDS FOR ANALYTICAL CHEMISTS
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In-Situ Sampling System for Thermal Volatilization Analysis of Nonvolatile Materials by Gas Chromatography-Mass Spectrometry Jacob Shen The Standard Oil Company, Warrensville Heights Ohio 44 128 I
While literature reports on the on-column flash pyrolysis have been extensive in the past decade (1-7), they have been few on the on-line GC/MS analysis of the gases evolved from nonvolatile materials a t lower temperatures (8,9). The latter analysis is not concerned with the pyrolyzates resulting from the high-temperature flash pyrolysis of a very small amount of solid sample but is concerned with the organic volatiles evolved from a bulk of nonvolatile materials at temperatures of practical importance. For example, it is often of interest to know what are the off gases evolved from a resin a t its processing temperature. This provides invaluable information as to whether thermodegradation of the resin or its additives has taken place as well as whether any hazardous material has evolved from the resin during its processing. We report in this paper an in-situ sampling system for the analysis of the gases evolved from nonvolatile materials by a GC/MS data system. Using the in-situ sampling system, the off gases can be directly prepared in a GC injection port and transferred in-situ onto a GC column without using a gas syringe. The sampling system can also be used as a regular GC injection port as well as a pyrolysis/GC interface. It is capable of self-cleaning during the run by backflushing with a stream of nitrogen. The atmosphere under which a solid sample is heated can be controlled.
EXPERIMENTAL The In-Situ Sampling System. The in-situ sampling system mainly consists of: (1)a heated chamber where a solid sample can be heated up to 500 "C; (2) a quartz probe for introducing the solid samples into the heated chamber; and (3) a valve system for transferring the off gases from the heated chamber onto a GC column, for backflushing, and for controlling the atmosphere in the heated chamber. Figure 1A shows the diagram of the in-situ sampling system. The heated chamber used is essentially a modified CDS-100 pyrolysis/GC interface (Chemical Data System, Oxford, Pa.). It is so modified that the distance between the chamber outlet and a Carle 5521 minivolume switching valve (C) is minimized. Both the heated chamber and the switching valve are enclosed in a metal case (A) which can be mounted onto a Varian 1200 gas chromatograph at a position normally occupied by the injection port. The valve (C) is insulated from both the environment and the heated chamber by rigid insulation boards (I) and heated by a short cartridge heater (H). The details of the quartz sampling probe (P) are shown in in.) Figure 2. It is basically a piece of quartz tubing (3 f t X connected to a piece of quartz rod. The nonvolatile materials are placed in the middle of the quartz tubing section. Silinated glass wool plugs can be used to keep the solid sample in place. The full length of the tubing section can be inserted into the heated chamber and a silicon O-ring (0)is used to seal the probe at the rod section. There is a 3 / 4 in. long slit in the tubing section to aid the cleaning of the sampling probe after each run. For sample preparation, a solid sample is introduced through the quartz probe when valve C is at a bypass position (Figure 1B). When desired the atmosphere in the chamber can be controlled by sweeping the chamber with a stream of inert gas such as nitrogen or helium. The sample is then heated at a chosen temperature for a fixed length of time. A t the end of the heating 886
ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
period, the valve (C) is switched to the sampling mode shown in Figure 1A to allow the off gases to be swept into the column and analyzed by a GC/MS data system. The sampling period can be varied according to the amount of sample used and the amount of gases evolved under the experimental conditions used. After the sampling period, the valve (C) is again switched to the bypass position to isolate the heated chamber. The quartz sampling probe can then be removed from the heated chamber and the later can be cleaned by backflushing with nitrogen while the run is proceeding. For satisfactory transferring of the off gases, it is best to heat the switching valve and the transferring line so that the off gases will not be condensed during the transferring process. In our experience, it is usually adequate to keep the valve body and the transferring line at 200 "C for satisfactory transferring of the off gases. Since the Carle valve used has a maximum temperature limit of 250 "C, and since the heated chamber sometimes has a temperature higher than 250 "C, the switching valve and the heated chamber are insulated from each other and heated separately. GC/MS Data System. A Varian 1200/Finnigan 3000/System 150 GC/MS data system is used in this work. Since the in-situ sampling system can also serve as a regular GC injection port, the second port of the chromatograph can be used for other equipments such as a capillary injection port. The System 150 data system used to control the Finnigan 3000 mass spectrometer has a dual disk configuration, and both disks can be used for data acquisition during the same run. Thermal Volatilization Analysis of Polymers. Thermal volatilization analyses of several commercial grade flame retarding polystyrene resins were analyzed using the in-situ sampling system. in. Carbowax For GC separation, a stainless steel 10% 10 ft X 20M (on Chromosorb WAW, 60/80 mesh) column was used. The helium flow rate was 20 mL/min and the oven temperature was programmed from room temperature to 200 "C at 4 "C/min. Typically, about 15-20 pellets can be placed in the middle of a quartz probe in between two glass wool plugs. The heating period was so chosen that it was similar to the resin's travelling time in a molding machine. The sampling period (the length of time the switching valve was switched from bypassing mode to sampling mode) was usually 10 s. The residual polymer sample in the quartz probe can be either pushed out by a metal wire through the slit or dissolved by a suitable solvent such as methyl ethyl ketone. If the total weight loss of a sample is to be determined, the sample probe can be weighed both before and after the run. Other techniques such as TGA can also be easily used to provide the weight loss information when necessary.
RESULTS AND DISCUSSION The performance of the in-situ sampling system is shown in Figure 3 which is a typical chromatogram from the thermal volatilization analysis of a flame retarding polystyrene resin carried out a t 250 "C in the presence of air. The chromatogram shows good separation and there is no significant amount of peak tailing observed. The GC peaks were identified through their respective mass spectrum acquired by the data systems. Figure 3 shows that styrene and ethylbenzene are the two major components in the off gases. Other compounds observed include aromatics (such as benzene, xylenes, pro-
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Flgure 1. (A) The in-situ sampling system in its sampling mode. (B) The in-situ sampling system in its bypassing (isolation or backflushing) mode
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Flgure 3. The gas chromatogram of the off gases from the thermal volatilization analysis of a flame retarding polystyrene resin (Lot A) In the presence of air
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Table I. The Normalized GC Peak Area of the Major Components in the Off Gases of Polystyrene Off gas components
C,2C,2BD Acrolein CH,Cl, C6 H6 4-Vinylc yclohexene
Toluene CH,ClBr Ethylbenzene Propylbenzene Styrene + methylstyrenes Others
Lot D
Lot E
Lot F
0.6 15.9 4.0 1.5
0.4 4.1 2.8
0.4 2.6 2.6 0.4
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