Note 77: The Determination of Volatile Organic Compounds in VacuumSystem Components

Presented at ASMS Meeting, Portland, OR., June 1997

INTRODUCTION

High vacuum systems are routinely employed in many manufacturing processes. These systems are expected to provide a contaminant-free environment. In many cases, small amounts of impurities within these systems can have major effects on product quality. This poster demonstrates a technique for detecting volatile and semi-volatile compounds. As an example, we examine a series of materials that are commonly found in moderate and low vacuum systems. However, the technique may be applied to many different materials and has previously been used in the examination of samples as wide ranging as foodstuffs and magnetic recording media. Numerous application notes can be found at http://www.sisweb.com

Direct Thermal Extraction (DTE) is performed using a Short Path Thermal Desorption System. The short path system is unique in that the sample is trapped or placed in a glass lined tube that becomes part of the injection syringe. This results in a very short transfer line (the needle) between the heated sample and the GC injector. The technique of Direct Thermal Extraction permits the analysis of solid samples without any prior solvent extraction or other sample preparation. The solid sample to be analyzed is inserted directly into the Glass Lined Stainless Steel (GLT) Thermal Desorption Tube between two quartz wool plugs. The Desorption Tube with the sample enclosed is purged with carrier gas to remove all traces of oxygen, after which the sample is heated to a predetermined set temperature and sparged into the GC injection port. The sample is then cryo-trapped at the front of the GC capillary column for a period of 5 to 10 minutes. When this is completed, the components are eluted from the GC capillary column via a temperature program cycle and are detected via a GC detector or mass spectrometer. The Direct Thermal Extraction technique is useful for identifying trace residues of volatiles and semi-volatiles in a wide variety of solid samples including vegetation, packaging and building materials, food products, pharmaceuticals, and other manufactured products. Using alternative techniques these samples would normally be analyzed after extraction with a suitable solvent such as chloroform, ether, benzene, etc. The exposure of laboratory staff to these solvents has become of major concern to both employees and health officials. In addition, the disposal of used solvents has become a serious problem and is rapidly becoming quite expensive. The use of direct extraction also prevents the dilution and contamination of low level analytes during the solvation step. Sensitivity in the ppb range can often be achieved from samples on the order of 1 to 5 mg. The main disadvantage of Direct Thermal Extraction is the analysis of high water content samples which results in the extraction of water into the GC column which will form an ice plug when cyro-focusing is used.

In this work, several types of o-rings were evaluated using Direct Thermal Extraction. Qualitative and quantitative information about compounds which may evolve from samples can be obtained by heating them directly in the Short Path Thermal Desorption system followed by gas chromatography/ mass spectrometry (GC/MS) analysis.

The relationship between vapor pressure and temperature allows the volatility of potential impurities under vacuum to be approximated by heating the matrix from which they arise. While this estimate may not be strictly quantitative, it can give a good idea of the types of contaminants that are likely to arise in a given application, as well as provide a basis for comparison of the impurities generated by different materials. In processes where both high temperature and low pressure are present, the volatility of a given contaminant is expected to be further exaggerated.

Experimental

Figure 1 - Direct Thermal Extraction of Silicone O-Rings

O-rings were chosen for testing because of their wide use as sealing components and their availability in a variety of different materials. Sample preparation consisted of simply cutting a sample of appropriate size from the o-ring and placing it in a conditioned, glass-lined Thermal Desorption tube packed with a small amount of glass wool. Sample size was dependent on the amount of volatile material generated in trial analyses, but was generally 20 mg. DTE was performed at various temperatures for each material tested, and extraction time was constant at 5 minutes per sample.

Figure 2 - Direct Thermal Extraction of Buna O-Rings

Instrumentation included an S.I.S. model TD-3 Short Path Thermal Desorption unit, an S.I.S. model 961 Cryotrap, an HP 5890 series II GC and HP 5989A MS Engine running HP ChemStation software. The analytical column was a 60 meter BPX35 capillary , 0.25 mm ID, 0.25m m film thickness (SGE Co.). The cryotrap was held at -70° C for the duration of the extraction step and for an additional 1 minute, after which it was ballistically heated to the injection port temperature of 250° C and the run was started. Oven temperature was ramped immediately from an initial value of 40° C at 8° C /min to a final temperature of 280° C where it was held for five minutes. The mass spectrum was scanned from 35-600 AMU at 1.2 scans/second during the first fifteen minutes, and from 36-700 AMU at 0.6 scans/second for the remainder of each run.

Figure 3 - Direct Thermal Extraction of Viton® O-Rings

The analysis of a sample of recording media from a used 3.5 inch floppy disk was also performed. For this sample, 20 mg of the disk was placed directly in the sample tube and the analysis was performed as described above. Volatiles were extracted at a temperature of 225° C.

Figure 4 - Direct Thermal Extraction of PTFE® O-Rings

Results/Discussion

Figure 5 - Direct Thermal Extraction of Kalrez® O-Rings

Figures 1-5 illustrate the effect of elevated temperature on extraction of volatiles from o-ring materials. Silicone clearly generates the greatest number of volatile and semi-volatile compounds while Kalrez® provides the cleanest system. A direct comparison between Viton® and Kalrez® is shown in Figure 6. Unfortunately, Kalrez® o-rings are roughly 100 times more expensive than the alternate materials. One viable option, which we did not examine is the use of PTFE® coated Viton®. We would expect this material to generate results similar to PTFE® at a reduced cost. The principal components released from each material are noted in the figures. Material that can be expected to interfere with manufacturing processes, such as siloxanes and phthalates, are observed.

Figure 6 - Comparison of Viton® and Kalrez® Via Direct Thermal Extraction

Figure 7 - Analysis of PC Floppy Disk Via Direct Thermal Extraction

Figure 7 shows an example of the results obtained from the DTE of a piece of floppy disk. Numerous volatile compounds are easily observed. These include: Acetaldehyde and Taetrahydrofuran (THF). Other compounds are also present including Hindered Phelol-type antioxidants and their degradation products.

We are interested in examining other materials and would be happy to accept suggestions.

For related references please see http://www.sisweb.com.

Viton® is a registered trademark of DuPont Dow Elastomers

Kalrez® is a register trademark of DuPont Dow Elastomers