Supplies and Services for Mass Spectrometers, Gas Chromatographs
and Liquid Chromatographs
 |
8 | Last Update: 01/31/00 |
INTRODUCTION
Short Path Thermal Desorption is a versatile technique for the analyses of solid, liquid and gaseous samples. Previous articles in this newsletter have described the theory and operation of short path thermal desorption (1) and have demonstrated its versatility in the analysis of volatile organic chemicals (VOC's) and semi-volatile organic chemicals in solid matrices (2 & 3). The detection and analysis of VOC's in water is routinely performed utilizing Purge and Trap techniques with standard EPA methodology (4). The detection of VOC's in other water-based matrices presents its own unique problems. The purpose of this article is to demonstrate the versatility of the short path thermal desorption technique to the analysis of VOC's in commercial water based products including liquid formulations, colloidal suspensions, and liquid pastes. Using a newly designed liquid purging system for the collection of the VOC's followed by trapping on an adsorbent trap and subsequent analysis via the Short Path Thermal Desorption technique, it is possible to detect and identify the various flavors, fragrances, off-flavors, off-odors, and manufacturing by-products in this wide diversity of liquid samples. This technique can easily be incorporated into a troubleshooting technique to detect problems in a wide variety of liquid products, to compare various competing manufacturers products, as well as a quality control program.
Volatile organic compounds are present in commercial liquid formulations, a diverse range of colloidal suspensions, and liquid pastes. Flavors and fragrances are added for their aesthetic values and to increase consumer appeal. The flavor/fragrance qualities of liquid commercial products is greatly dependent on the plethora of volatile and semivolatile organic compounds contained both in the liquid matrix and the headspace aroma (5). Volatile compounds are also used in the manufacturing process to obtain the desired physical properties. Trace residues of these manufacturing by-products are often present in the final products. Analytical techniques are needed to profile and identify flavors, fragrances, off-flavors, off-odors and potential contaminants that may be present as flavor and fragrance additives, residual solvents from the manufacturing process or as impurities in raw materials. Results of such analyses may be used for developmental research as well as production quality control.
The detection of VOC's from liquids can be accomplished by using a headspace sampling technique combined with gas chromatography (GC) and/or gas chromatography/mass spectrometry (GC/MS). The most sensitive of the techniques is the dynamic headspace technique. Dynamic headspace analysis depends on the vapor phase extraction of the sample liquid achieved by purging it with an inert gas. VOC's present will diffuse into the vapor phase and are collected on a solid sorbent material, as Tenax. Older desorption type purge and trap methodology and apparatus (such as utilized by the EPA) is only applicable to very volatile components, as solvents.
However, the Short Path Thermal Desorption system permits the analyses of liquid samples by desorbing the samples previously collected on adsorbent resins directly into the GC injection port for subsequent analyses by conventional GC detectors or via mass spectrometers. Due to its short path of sample flow, this new system overcomes the shortcomings of previous desorption systems by eliminating transfer lines which are easily contaminated by samples and by providing for the optimum delivery and therefore maximum sensitivity of samples to the GC injector via the shortest path possible (1). By making the transfer path as short as possible, the maximum sample size is delivered to the GC, samples are not lost or destroyed in hot transfer lines, and no memory effects occur due to contamination of transfer lines from previous samples. The high recovery rates, time savings for sample preparation and the high sensitivity of short path thermal desorption make this not only an attractive technique to the analyst, but an essential detection apparatus because of toxicology and quality control as well as a means for determining human health impacts.
The objective of this investigation is to demonstrate the feasibility of combining a headspace sampling technique with the technique of short Path Thermal desorption to analyze VOC's that can be purged from diverse liquids, colloidal suspensions, or water based pastes. Commercial beverages including fruit drinks, carbonated colas, and wine coolers as well as olive oil, shampoo, latex paint, toothpaste and a spiked water sample were analyzed by thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) using a Scientific Instrument Services (S.I.S.) model TD-1 Short Path Thermal Desorber accessory.
