Note 39: Comparison of Sensitivity Of Headspace GC, Purge and Trap Thermal Desorption and Direct Thermal Extraction Techniques For Volatile Organics

John J. Manura and Santford Overton
1999

INTRODUCTION

A variety of techniques are available for the analysis of volatiles and semi-volatile organics in solid, liquid and gas samples. No one method or technique is optimum for all types of samples. The selection of a sample collection and GC introduction technique is dependent upon a large number of variables. One must select the optimum method depending on the sample matrix, boiling point range and concentration of analytes in the sample, interfering compounds and equipment available.

Table I - Variables To Be Considered In the Selection of Analysis Technique

• Sample Matrix - Gas - Liquid - Solid
• Amount of Sample Available for Analysis
• Concentration of Volatiles in Sample Matrix
• Detection Limit of GC Detector
• Amount of Water in the Sample
• Solvents or Major Interfering Compounds
• Thermal Stability of the Analytes
• Boiling Point Range of Analytes in Sample
• Lowest Range of Volatiles Desired in Analysis
• Highest Range of Boilers Desired in Analysis
• Number of Samples to be Analyzed
• Sample Preparation Time
• Use and Disposal of Solvents
• Cost of Analysis

In order to analyze the volatile and semi-volatile organics in various sample matrices, a variety of GC sampling techniques can be utilized (Figure 1). Each of these injection techniques has unique advantages and disadvantages for the analysis of volatile and semi-volatile organics. All of the above factors or variables (Table I) must be considered to determine the optimum method of analysis for a particular sample. In this study, the Headspace GC technique was compared to conventional purge and trap thermal desorption and also to the direct thermal extraction (DTE) technique in order to determine the relative sensitivities of these techniques for the analysis of volatile organics in liquid and solid samples.

Sampling Technique

Figure 1 - Comparison Of GC Injection Techniques

GC Headspace Analysis

GC Headspace sampling is a widely used injection technique. It is most suited for the analysis of the very light volatiles in samples that can be efficiently partitioned into the head space gas volume from the liquid or solid matrix sample. Higher boiling volatiles and semi-volatiles are not detectable with this technique due to their low partition in the gas headspace volume. In addition, the sensitivity of the technique is limited, typically to concentrations in the ppm to ppt range. However, the technique is a preferred method for the analysis of gases and very light volatiles which can not be analyzed by other techniques such as P&T Thermal Desorption. The technique is also a preferred technique when sample automation is required such as in a quality control method or in sample screening.

P&T TD

Purge and Trap (P&T) Thermal Desorption is routinely used for the analysis of volatiles in environmental samples as well as food samples. Through the proper selection of adsorbent resins, as Tenax® TA, water can be eliminated from being introduced into the GC. This is important for the analysis of high water content samples such as food products and water samples. The Purge and Trap technique is more sensitive by at least a factor of 1000 over headspace techniques. Typical sensitivity is in the ppb range. By purging samples at higher temperatures, higher molecular weight compounds can be detected. However, the Purge and Trap technique requires more time for sample preparation and can not normally be automated. In addition, very light volatiles and gases will not be trapped on the adsorbent resins and therefore will be missed in the analysis.

Direct Thermal Extraction

Direct Thermal Extraction (DTE) is a new technique which is unique to the SIS Short Path Thermal Desorption System. In this technique, volatiles and semi-volatiles can be thermally extracted directly from solid matrix samples without the use of any solvents or any other sample preparation. The advantages of this technique are that a wide range of volatiles and semi-volatiles can be analyzed and the high sensitivity of the technique (typically ppb ranges on samples less than 1.0 gram) with little sample preparation. Its main disadvantage 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. Since no sample preparation is required, the sampling time is small. The DTE technique will also detect higher molecular weight volatiles because they are thermally extracted directly from the sample at higher temperatures than can be achieved with P&T sampling. The DTE extraction technique is more sensitive by at least a factor 10 to 1000 as compared to other techniques such as P&T and GC Headspace.

Experimental

An H.P. 5890 Series II GC with Electron Pressure Control attached to the HP 5989A Engine Mass Spectrometer was used for the analysis and identification of the analytes. A J&W DB5-MS capillary column was used (60 meter x 0.32 mm x 0.25u film thickness) at a flow rate of 1.8 ml/min. The analytes were detected using the mass spec in the EI mode to scan the mass range from 25 to 400 daltons.

The SIS GC-Cryo-Trap was attached to the bottom of the GC injection port inside the GC oven. The GC injection port was maintained at 250 deg C and the GC Cryo-Trap was set for -100 deg C during the trapping phase and 200 deg C during the sample release phase and GC run time. The GC oven was held at 30 deg C for 5.0 minutes during the injection and cryo-focusing phase and for an additional 5.0 minutes after the sample was released from the cryo-trap. The column oven was then temperature programmed to 80 deg C at 5 per minute and then to 200 deg C at 10 deg /min.

For the headspace sample analysis, the LEAP Model CTC HS500 Automated Headspace Autosampler was attached to the injection port of the GC. This headspace system uses a heated syringe to directly inject the headspace volatiles from a heated sample into the GC injection port. Samples to be analyzed are placed into 10 ml glass vials with crimp tops which are sequentially inserted into a heated oven and agitated during the minute equilibrium step. Headspace volumes between 0.1 ml and 2.5 ml can be accurately removed from the sample into a heated syringe for injection into the GC.

The SIS Short Path Thermal Desorption System Model TD3 was used for both the P&T and Direct Thermal Extraction Techniques. This system mounts directly over the GC injection port to permit the direct thermal extraction of desorbed samples into the GC injection port for subsequent cryo-trapping and GC/MS analysis. For the Direct Thermal Extraction technique, solid matrix samples were placed directly into the desorption tube and subsequently thermally extracted into the GC injection port. For the direct thermal extraction of the liquid Olive Oil sample, 25 milligrams of Tenax TA was placed into the desorption tube and 10 l of the olive oil was injected directly onto the resin bed. For the P&T techniques, 100 milligram of Tenax TA was placed into the desorption tube and the volatiles from the liquid or solid samples was purged onto the Tenax resin bed using the SIS Purge and Trap System.

The desorption tubes were then attached to the Scientific Instrument Services Short Path Thermal Desorption System and thermally desorbed at 250 deg C, at a desorption flow rate of 4 ml per minute for 5 minutes into the GC injection port. The GC injection port was maintained at 250 deg C, and the volatiles were cryo-focused at the front of the GC column using the Scientific Instrument Services GC Cryo-Trap at a temperature of -100 deg C. After the desorption process was complete, the GC Cryo-Trap was ballistically heated to 200 deg C to release the trapped volatiles in a narrow band for chromatography.

Samples of gasoline, olive oil and black tea were studied via these three injection techniques in order to determine the range of volatiles that could be detected via each technique. The sensitivity of analysis was also compared for each of the injection techniques.

Results and Discussion

Analysis of Gasoline Samples

Figure 2 - Comparison Of Injection Techniques For the Analysis of Gasoline Samples

In order to compare the various injection techniques, a sample of regular gasoline was analyzed via the three techniques. (Figure 3). In the first chromatogram, approximately 0.1 nl of gasoline was injected directly into the GC via a syringe and cryo-trapped at the front of the GC column for 5.0 minutes. The sample was then released by heating the cryo-trap and the analytes were chromatographed through the GC column. The entire range of volatiles from butane through the naphthalenes were detected. The headspace analysis was performed by analyzing gasoline in 5.0 ml water at a concentration of 0.1 l/ml. The technique proved quite useful for detecting the very volatile analytes and most of the aromatics, but it was not very sensitive for the less volatile analytes. In the Purge and Trap technique 5.0 ml of water containing gasoline at a concentration of 0.1 nl/ml was purged with 200 ml of Nitrogen at 80 deg C and the analytes trapped on a Tenax TA adsorbent resin bed. The trapped analytes were subsequently thermally desorbed at 250 deg C, cryo-trapped at the front of the column and then analyzed as the other samples. The P&T technique proved to be more sensitive by a factor of 1000 for the less volatile aromatics and semi-volatiles. However, it did not prove to be much more sensitive for the very volatile compounds, probably due to breakthrough through the Tenax trap during sample preparation.

Analysis of Olive Oils

Figure 3 - Comparison Of Injection Techniques For the Analysis Of Olive Oils

In order to compare the various injection techniques, a sample of olive was oil was analyzed via the three techniques. (Figure 3). The headspace analysis on 5.0 ml of olive oil produced only two minor peaks at the low end of the chromatogram (not shown). In the purge and trap technique, 1.0 ml of oil was purged at 40 ml per minute for 30 minutes to purge the volatiles from the oil and trap them on a Tenax TA thermal desorption trap. For the direct thermal extraction technique, 10.0 l of the oil was injected directly onto the Tenax trap and subsequently thermally desorbed into the GC for analysis. The DTE technique proved to be 100 times more sensitive than the purge and trap technique and was able to detect a wider boiling point range of volatiles than the other techniques. The DTE technique was more than 1000 times more sensitive than the headspace technique in addition to detecting a much higher range of volatile organics.

Analysis of Black Tea

Figure 4 - Comparison Of the Direct Thermal Extraction Technique and Purge and Trap For Teas

Experimental

Samples of Black Tea were also analyzed via the same techniques. In the Headspace technique, 200 milligrams of black tea was added to 5.0 ml of water, heated to 90 deg C for 10 minutes and the 2.0 ml of the headspace volume was injected into the GC. No significant peaks were detected (not shown). In the purge and trap technique, 200 milligrams of Black Tea was weighed into a 10.0 ml purge and trap tube and 5.0 ml of distilled water was added. The purge and trap vessel was heated to 80 deg C in a hot water bath, and the volatiles were extracted using high purity helium in the Purge and Trap system at a flow rate of 22.5 ml per minute and sampled for 20 minutes (450 ml total). The volatiles were purged onto a desorption tube containing 100 milligrams of Tenax TA which had been previously conditioned. The desorption tube was then desorbed at 250 deg C into the GC column, cryo-focused and subsequently analyzed. A wide range of volatiles and semi-volatiles responsible for the aroma and taste of the tea were detected. In the Direct Thermal Extraction technique, 20 milligram of black tea was weighed into the desorption tube and then the volatiles were thermally extracted at 100 deg C from the solid sample matrix directly into the GC injection port, cryo-focused at the front of the GC column and then chromatographed through the GC column. Many of the same compounds detected in the P&T technique were also detected in this technique but at lower levels. In addition, several higher boiling compounds were also detected with this technique. This range could have been extended even further by extracting at higher temperatures.

Conclusion

A variety of techniques are available for the analysis of volatiles and semi-volatiles in various sample matrices. The Headspace technique has proved very useful for the analysis of the very volatile organics at the ppm level. However, it was less useful for the analysis of the higher boiling analytes. The Purge and Trap Thermal Desorption Technique proved more sensitive by a factor of 100 to 1000 over the headspace technique for the analysis of volatile and semi-volatile organics with detection limits in the ppb range. However, it is not as useful for the detection of low levels of very volatile organics due to breakthrough on the adsorbent resin beds. The Direct Thermal Extraction technique is the most sensitive technique for the analysis of a wide range of volatile and semi-volatile organics. It can detect higher boiling organics than any of the other techniques. In addition, this technique has the advantage that it is quick and no sample preparation is required. Additional studies still need to be done to compare these techniques further and to also study other techniques such as SFE and SPME.