|28a||Last Update: 12/23/99|
Headspace GC is a well-established technique for the analysis of volatiles in solid and liquid samples. It is widely used for applications, as blood alcohol analysis and arson analysis. Headspace GC can often be used in place of thermal desorption techniques. The main advantage over thermal desorption is the ability of the headspace technique to be automated. Its main disadvantage is its lack of sensitivity in comparison to thermal desorption. However, for applications in which large quantities of samples need to be analyzed and the volatiles are present in the samples at concentrations above the ppb level, headspace GC is an excellent technique.
The GC Cryo-Trap is a useful accessory to expand the capabilities of the headspace GC technique. The GC Cryo-Trap consists of a small heating/cooling chamber which surrounds the front 4 inches of the capillary or guard column. It is installed inside the GC oven, just under the GC injection port to permit the trapping of volatile organics at the front of the column. Liquid Nitrogen is utilized to permit the trapping of volatiles down to -180 degrees C. The GC Cryo-Trap permits the injection of large gas sample volumes via techniques such as Headspace and Thermal Desorption. It eliminates the need to cool the entire GC oven for cryo-focusing samples on column. During the entire injection process, the volatiles in the gas sample when injected into the GC injection port are concentrated in a narrow band at the front of the GC capillary column. When the GC Cryo-Trap is subsequently heated, the trapped volatiles are released in a narrow band to be chromatographed via the GC capillary column. This results in the generation of sharp, well resolved GC peaks for large gas volume injections. It also permits the injection of multiple samples when weak concentrations of volatiles are present in the samples to be analyzed.
The purpose of this study was to evaluate the usefulness of the GC Cryo-Trap in conjunction with the headspace injection technique for the analysis of volatile organics in water based latex paints utilizing capillary GC columns. Water-based latex paints contain a large number of volatile organics at concentrations in the ppm level. The combination of the headspace GC technique with the GC Cryo-Trap is a practical method for the analysis of these samples.
Figure 1 - Theory Of Operation Of Headspace Sampler and GC Cryo-Trap
The LEAP Model CTC HS500 Headspace Autosampler was attached to the injection port of a HP Model 5890 Series II GC with electronic pressure control. This headspace system uses a heated syringe to directly inject the headspace volatiles from a heated sample into the GC injection port (Figure # 1). No long transfer lines or rotary valves are used in this system. The system is automated to permit the automatic injection and analysis of up to 50 samples unattended by the operator. Samples to be analyzed are placed into 10 ml glass vials with crimp tops which are sequentially inserted into a heated oven which can be agitated during the timed 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 HS500 Headspace Autosampler is mounted directly over the injection port of the GC in order to permit the direct injection of the sample into the GC injection port where the volatiles are subsequently trapped in the GC Cryo-Trap at the head of the GC column.
An HP 5890 Series II GC with electronic pressure control and a split/splitless injector was used for the following experiments. The HP Engine mass spectrometer was used as the detector in the EI mode. A J&W DB5-MS capillary column, 60 meters long by 0.32 mm I.D. x 0.25 micron film thickness was used in the GC oven.
The SIS GC-Cryo-Trap was attached to the bottom of the GC injection port inside the GC oven. A short 0.53 mm uncoated deactivated fused silica guard column was used from the GC injection port and through the GC Cryo-Trap. At the exit of the guard column, an SGE zero-dead volume union was used to join the uncoated guard column to the DB5-MS capillary column. The GC injection port was maintained at 200 degrees C and the GC Cryo-Trap was set for -160 degrees C during the trapping phase and 200 degrees C during the sample release phase and GC run time. The GC oven was held at 30 degrees C during the 5.0 minute injection phase and cryo-focusing 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 degrees C at 5 degrees per minute and then to 200 degrees C at 20 degrees /min.
The LEAP Headspace Autosampler was used in the automatic mode. In fact, the entire process including sample equilibration, injection, cryo-trapping, release of volatiles and chromatography of the compounds was fully automated. Three (3.00) gram samples of the liquid Latex paints were weighed into 10 ml headspace vials on an analytical balance, sealed with PTFE lined septa, inserted into the headspace sampler tray and then consecutively inserted into the headspace heater block. They were heated to the required temperature between 60 degrees C and 120 degrees C with agitation for 15 minutes in the sample block after which 2.0 ml gas samples were injected into the GC injection port. The samples were injected very slowly (25 ul/sec or 1.5 ml/min). This slow injection assures the full delivery of the analytes to the capillary column. Faster injection would result in sample splitting due to the back pressure design of the HP injection port. After injection into the cooled GC Cryo-Trap, the trap was maintained at the cryo-cooled temperature for at least another 5.0 minutes before the volatiles were released and the GC program begun. This cryo cooling equilibration time after the injection is complete permits the entire contents of the GC injection port to be passed onto the Cryo-Trap guard column and flushes the injection port of any remaining sample thereby eliminating peak tailing and broadening. When the cryo-trap is rapidly heated, the released volatiles are eluted in a sharp band which produces highly resolved chromatography peaks.
Results and Discussion
Three grams of a typical water based latex paint was heated to 90 degrees C in the headspace vial and then 2.0 ml of the headspace volume was injected and analyzed. The resulting chromatogram is shown in Figure # 2. Because the headspace volume is directly injected into the GC injection port and all the volatiles are cryo focused at the front of the column, a large number of compounds from very light volatiles such as acetaldehyde up to the higher boiling plasticizers are trapped and subsequently analyzed. Other techniques such as solvent extraction and even thermal desorption would have resulted in the loss of the very light volatiles.
Figure 2 - Headspace / Cryo-Trap Analysis Of Latex Flat Wall Paint 3.00 Grams Of Fresh White Latex Paint, Equilibrate 10 min, 2.0 ml Headspace At 90 degrees C. Trap at -160 Degrees For 5 min.
The headspace volume is injected into the GC injection port relatively slowly (1.5 ml/min). This is contrary to normal GC injection methods in which the user wants to inject as quickly as possible in order to obtain narrow highly resolved GC peaks. However, with the GC Cryo-Trap, speed of injection is not critical, since all the compounds injected are cryo-trapped at the front of the GC column or guard column inside the GC Cryo-Trap. When injecting large gas volumes into GC injection ports, as the HP split/splitless injector, quick injections would result in significant sample splitting since these injectors work off a backpressure regulated system. If a large sample is injected quickly, the head pressure at the front of the GC column will rise rapidly. The GC injection port electronics will attempt to restore the pressure back to its original value by splitting the sample out the split vent. The result is a significant splitting of the sample. This can be avoided by slowly injecting the sample into the injection port so as to avoid a high backpressure in the injection port. This minimized splitting of the sample and assures the maximum delivery of sample to the GC capillary column. Despite the long injection time (1.33 minutes), the GC peaks are highly resolved, even the very volatile compounds such as acetaldehyde.
Effect Of Headspace Temperature
For this study, the same latex paint sample (3.0 grams) was weighed into a headspace vial. The headspace vial was heated to various temperatures and 2.0 ml of the headspace gas was injected into the GC injection port and then cryo-trapped at -160 degrees C (Figure # 3). As the headspace temperature is increased, the intensity of the higher boiling plasticizers increases. Only minimal increase in the intensity of the lower boiling volatiles is seen below the n-Butylether peak (peak # 11). Depending on the range of volatiles requested, the user can select the headspace temperature to achieve the desired results. For the remainder of our study, we selected 90 degrees C as our headspace temperature. This enabled us to detect all the very light volatiles and still detect the higher boiling plasticizers.
1. 2-Methyl-1-Propene 14. Ethylbezene 2. Acetaldehyde 15. 1-Hexanol 3. Acetone 16. n-Butylether 4. 2-Methyl-2-propanol 17. Xylenes 5. 2-Butanone 18. Nonane 6. Ethylacetate 19. Butylpropanoic Acid 7. 2-Butenal 20. alpha-Pinene 8. 1-Butanol 21. Camphene 9. Methylisobutylketone 22. Decane 10. Di-tert-butyl peroxide 23. Undecane 11. Touene 24. Alcoxy-phenoxy plasticizer 12. Butylacetate 25. Alcoxy-phenoxy plasticizer 13. Isobutylether
Figure 3 - Headspace / Cryo-Trap Analysis Of Latex Flat White Paint, 3.00 grams Of Fresh White Latex Paint, Headspace Analysis Of Paint At 60 degrees C
Analysis Of White Latex Wall Paints
Using the above described headspace method at a headspace heater temperature of 90 degrees C, a series of 4 different manufacturers white flat latex wall paints were analyzed. Three (3.00) grams of each paint were weighed into the headspace vial for subsequent analysis. The results are shown in Figure # 4. With the exception of the Glidden Spred 2000 paint, all the latex paints contained the two high boiling plasticizers. The last latex paint had relatively high concentrations of terpenes in the paint. This was probably added for their fragrance properties. These terpenes were not present in any of the other paints analyzed. As can be seen from Figure # 4, this technique could readily be utilized as a quality control method or a method for the comparison of different manufacturers paints. The results have proven to be quantitative and reproducible. Previous studies on water and soils have demonstrated that correlation coefficients for quantification have been close to 1.000 for most volatiles and the relative standard deviation of repetitive sampling is better than 5%. These results are quite good, especially since this is a headspace method with no internal standard used in the analysis method.
Figure 4 - Headspace / Cryo-Trap Analysis Of Flat White Latex Wall Paints
In a likewise manner, four different manufacturers of white latex gloss and semi-gloss wall paints were analyzed as shown in Figure # 5. The results are similar to those for the flat wall paints. With the exception of the Glidden paint, all the paints contained the two plasticizers. The types and concentrations of volatiles varied with the manufacturer.
Figure 5 - Headspace / Cryo-Trap Analysis of Latex Gloss & Semi-Gloss Paints
The use of the GC Cryo-Trap in conjunction with the automated headspace system has proven to be a powerful technique for the analysis of volatiles in water base latex paints. This technique could readily be adapted for either a quality control method for the analysis of volatiles in paints or other manufactured products and could also be used as a comparison tool. This combination of hardware permits the quantitative analysis of volatiles in various matrix samples over more than 4 decades of sensitivity utilizing capillary GC columns.
Most previous GC headspace methods have been done using packed GC columns and megabore capillary columns. However, it is now possible to inject large gas volumes (0.5 ml to +100 ml) into a microbore capillary column by concentrating the analytes in a narrow band at the front of the GC column over a time period from 1.0 minute to >30 minutes, then rapidly releasing the analytes for chromatography. The relatively slow injection of large sample volumes at injection rates of more than 1.0 ml/min eliminates the normal splitting of samples which would occur with faster sample injection. All of the volatiles are passed onto the capillary column and cryo-trapped and concentrated in a narrow band at the front of the GC column or guard column inside the GC Cryo-Trap. The resulting GC peaks are highly resolved and the retention times and the areas of the GC peaks are very reproducible.
The techniques permit the adjustment of a number of variables such as the sample temperature, the gas volume injected, the trapping temperature and the release temperature. This will permit the development of experimental conditions to fit the needs of the chemist. In addition to the analysis of volatiles in latex paints, this technique can be used for applications such as the analysis of volatiles in soils, residual solvents in pharmaceuticals, blood alcohols and other headspace techniques that could profit from the analysis of compounds on a capillary column or which could be enhanced by the injection of larger gas sample volumes or repetitive injections onto the GC column. The use of the GC Cryo-Trap can expand the usefulness of the GC headspace technique.
Yttria coated filament at start
Yttria coated filament after 16,000 cycles