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Presented at EAS, Somerset, NJ., November 1997
Direct Thermal Extraction
Extraction is achieved using the S.I.S. Short Path Thermal Desorption system, placing the sample of interest (usually less than 0.1 gram) directly into the apparatus where it is heated in a stream of carrier gas. Volatile and semi-volatile components are deposited into the GC inlet. Cryotrapping and MS detection aid in separating and identifying the components.
Direct heating of some samples raises concerns of thermal decomposition. This is particularly a problem in the analysis of natural products and food items, where the analytes of interest reside in complex matrices that may already include reactive components. Using an increasing temperature program during extraction may preserve labile analytes in these types of samples by allowing lower boiling compounds to volatilize before destructive temperatures or reaction rates are reached .
We have subjected several samples to Direct Thermal Extraction using both temperature ramping and ballistic heating techniques in order to assess the effectiveness of temperature ramping at minimizing thermal decomposition. The results of these experiments show that temperature ramping can mitigate thermal decomposition and improve the qualitative analysis of complex samples such as spices.
Samples of black pepper and dried basil flakes were thermally extracted in both ramped and isothermal modes. In addition, a single black pepper sample was fractionated by extracting it at successively higher temperatures. A purge and trap sample of volatiles from green tea was also evaluated for differences in the chromatographic result when the trap was desorbed isothermally as opposed to using a temperature program.
The samples tested were generic consumer items purchased within the last year. Direct Thermal Extraction was performed on an S.I.S. Short Path Thermal Desorption system model TD-3. A Hewlett-Packard HP 5890 series II GC equipped with an S.I.S. model 971 micro Cryotrap using liquid CO2, and an HP 5989A MS Engine were used for analysis.
Samples for Direct Thermal Extraction were weighed directly into glass-lined thermal desorption tubes and attached to the desorption unit. Those that were extracted isothermally were heated ballistically to the desired temperature and were held there for five minutes. Samples extracted using a temperature ramp were brought from the initial temperature to the final temperature at a rate of 20° C per minute, and only remained at the final temperature instantaneously. The purge-and-trap tea samples were collected by placing 250 mg of green tea and 5 ml water into a 20 ml purge and trap vessel. The vessel was immersed in a water bath and kept at 80 ° C. Nitrogen was purged through the liquid at a rate of 40 ml/minute and a dry nitrogen purge was also applied at an equal rate for thirty minutes. The effluent was trapped on conditioned glass lined tubes containing 100 mg of Tenax® TAÒ adsorbent resin. After sampling, water was stripped from the traps with dry nitrogen. Ramped and isothermal desorption of the traps proceeded as above.
Figure 1 - Direct Thermal Extraction of Black Pepper
Direct Thermal Extraction of the same 2 mg black pepper sample at various temperatures yielded chromatograms with some interesting differences (Figure 1). Extraction at 150 ° C resulted in weak signals from a few cyclic terpenes and Caryophyllene, as well as trace amounts of other sesquiterpenes and their oxygenates. When the sample was extracted again at 200 ° C , the higher boiling sesquiterpenes dominated the chromatogram, with the largest peak corresponding to Copaene. When a temperature of 250 ° C is used, even higher boiling compounds appear, as expected. However, there is also a disproportionate increase in some lighter, more volatile compounds, as acetic acid and 1-hydroxy-2-propanone. The appearance of these low-boiling compounds only at high temperature and after extensive thermal extraction indicates that they are in fact products of thermal degradation, and are not representative of the true volatile profile of black pepper. Possible sources of these compounds include hydrolysis of esters in the sample as well as accelerated Maillard type reactions of reducing carbohydrates and amines.
Also illustrated in Figure 1 are chromatograms of two additional pepper samples. Chromatogram D shows the result of ballistically heating 2mg of black pepper to 250 ° C, and chromatogram E is the product of ramping the temperature from 100 ° C to 250 ° C. Among the noteworthy differences between the two plots are the lower abundance of degradation products in the ramped sample, as well as better resolution of the main groups of terpene, sesquiterpene, and piperidine compounds and a much lower baseline. Because the ramped sample did not remain at the higher temperature as long, there are fewer high boiling compounds eluting between 30 and 35 minutes, and the peaks are generally smaller because the sample had not been as exhaustively extracted. Note that the key flavor compounds of the pepper (terpenes and piperidine substances) are proportionally similar in both extractions.
Figure 2 - Ballistic (A) and Ramped (B) Direct Thermal Extraction of Volatiles From Dried Basil
Two samples of dried basil flakes were also subjected to ballistic and ramped thermal extraction. Figure 2 illustrates the main differences between techniques, namely that the sample that was ballistically heated to 200 ° C (A) shows a larger acetic acid peak which may be indicative of degradation, and larger peaks from higher boiling compounds, again an indication of more time spent at higher temperature. In addition to a cleaner baseline, the sample that was ramped from 80 ° C to 200 ° C (B) displays markedly higher extraction efficiency with respect to bergamotene and a and b pinenes. Although as with the pepper samples, most of the character impact compounds of the basil were extracted in similar proportions, higher recoveries of these three terpenes at generally lower temperatures is significant, and may be of interest from a quality control standpoint.
Figure 3 - Ballistic (A) and Ramped (B) Desorption of Volatiles From Green Tea By Purge and Trap GC/MS
Purge and trap samples of green tea volatiles were analyzed by thermal desorption in order to assess the effect of temperature ramping and possible decomposition of analytes. Because the samples were immobilized on an adsorbent bed and were, therefore, removed from the complex matrix of the tea leaves as well as any water present, it was expected that evidence of thermal decomposition , if any, would be meager. As can be seen in Figure 3, no decomposition was in fact observed. Chromatogram A resulted from the ballistic desorption of the trapped sample at 250 ° C, and chromatogram B depicts the result of ramped desorption from 80 ° C to 250 ° C. Comparison of the chromatograms reveals that all of the peaks present in trace A are accounted for in trace B. The significant difference between the samples is in the lower recovery of the higher-boiling compounds when the temperature program was used. This is due to the fact that these compounds have a generally higher affinity for the adsorbent and, therefore, require a larger volume of carrier at a given temperature to be driven off of the trap. Since there was no evidence of thermal decomposition at higher temperatures, nothing is gained by using a temperature program for purge and trap samples and, in this instance, sample recovery actually suffered.
Use of controlled temperature ramping with the Direct Thermal Extraction technique can mitigate the appearance of analytical artifacts that arise due to extended exposure of the samples to high temperatures. In some cases, extraction efficiency can be improved by using a temperature program to extract samples rather than ballistic heating. Use of temperature programming with adsorbent traps has not been shown to effect thermal decomposition of samples.
Future investigations in this area may include recovery studies and a quantitative assessment of extraction efficiency using temperature programs. Additionally, the use of temperature ramping may be investigated with respect to different types of samples including polymers, packaging materials and pharmaceutical products.
Identities of some major peaks from the analyses are given below in order of elution.
Figure 1. Black Pepper
Yttria coated filament at start
Yttria coated filament after 16,000 cycles