|29a||Last Update: 12/23/99|
A previous application note described the analysis of volatiles in latex base paints utilizing the headspace GC technique in conjunction with the GC Cryo-Trap. This application note extends these techniques to the analysis of volatiles in oil based paints. The combination of the headspace GC technique with the GC Cryo-Trap has proven to be a practical method for the analysis of latex paints as well as the oil based paints. The volatile content in the oil based paints is much higher than the water based paints; therefore, the methods have been modified to permit the analysis of the higher volatile levels in these samples. The following article will demonstrate the accuracy and reproducibility of this method.
The system consists of the LEAP CTC HS500 Automated Headspace Sampler mounted on top of the GC injection port and the SIS GC Cryo-Trap mounted inside the GC oven just under the injection port. The GC Cryo-Trap consists of a small heating/cooling chamber which surrounds the front 4 inches of the capillary or guard 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 and eliminates the need to cool the entire GC oven for cryo-focusing samples on column. During the headspace 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.
Figure 1 - Theory Of Operation Of Headspace Sampler and GC Cryo-Trap
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 LEAP Model CTC HS500 Headspace Autosampler was attached to a HP Model 5890 Series II GC with electronic pressure control. The 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. 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 syringe for injection into the GC. This headspace system uses a heated syringe to directly inject the headspace volatiles from a heated sample into the GC injection port (Figure # 1). The volatiles are subsequently trapped in the GC Cryo-Trap at the head of the GC column.
The SIS GC-Cryo-Trap was attached to the bottom of the GC injection port inside the GC oven. A short 0.53 mm I.D. 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 at -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 65 degrees C at 5 degrees C per minute, then to 90 degrees C at 1 degree/min and finally to 200 degrees C at 10 degrees/min.. The slow temperature ramp between 65 degrees C and 90 degrees C assures the maximum separation of the large number of branched chain hydrocarbons that are present in these oil based paints.
The LEAP Headspace Autosampler was used in the automatic mode. Sample sizes less than 150 milligrams of the liquid oil base 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 10 minutes in the sample block after which 0.30 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 and accurate delivery of the analytes to the injection port and for the accurate and reproducible splitting of the sample to the capillary column. Faster injection would result in less reproducible sample splitting due to the back pressure design of the HP injection port. Depending on the sample size weighed into the headspace vial as well as the gas volume injected into the GC injection port, the split ratio was varied between a 2:1 split up to a 60:1 split ratio. After injection into the cooled GC Cryo-Trap, the trap was maintained at the cryo-cooled temperature (-160 degrees C) 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
A 0.085 gram sample of a clear varnish paint was heated to 90 degrees C in the headspace vial and then 0.30 ml of the headspace volume was injected and analyzed. The GC split ratio was set to 10:1 for this sample. 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 up to the higher boiling hydrocarbons are trapped and subsequently analyzed. In addition to the common aromatics and straight chain hydrocarbons, a large number of branched chain hydrocarbons as well as a large number of substituted cyclohexane type compounds were detected and separated on the GC column. The GC column was slightly overloaded and the peak resolution can be improved if the sample size were reduced or the split ratio increased.
Figure 2 - Headspace / Cryo-Trap Analysis of Clear Varnish, Add 0.085 grams of Red Devil Clear Varnish, Heat to 90 Degrees For 10 min, Inject 0.3 ml of Headspace to GC at 1.5 ml/min. Trap at -160 Degrees For 5.0 min. GC At 30 Degrees For 10 min, To 65 At 5/min, To 90 At 1.0/min, Then To 200 At 10/min.
The headspace volume was injected into the GC injection port relatively slowly (1.5 ml/min). Normally, 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 in a very short time interval since these injectors work off a back pressure 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 split ratio will change rapidly and unevenly over a very short period of time. The accuracy of this splitting can be improved by slowly injecting the sample into the injection port so as to avoid a high back pressure in the injection port.
Figure 3 - Headspace / Cryo-Trap Analysis Of Clear Polyurethane, Add 0.0115 grams of Red Devil Clear Polyurethane, Heat To 90 Degrees For 10 min, Inject 0.3 ml Of Headspace To GC At 1.5 ml/min. Trap At -160 Degrees For 5.0 min. GC At 30 Degrees For 10 min, To 65 At 5/min, To 90 At 1.0/min, Then To 200 At 10/min.
In a likewise manner, samples of clear polyurethane (Figure # 3) and oil base enamel paint (Figure # 4) were also analyzed. Both of these paints contained less of the aromatic compounds, but both contained a wide variety of the branched chain hydrocarbons and the substituted cyclohexane type compounds. The largest molecular weight hydrocarbon detected was C13. This range could be extended by increasing the headspace temperature as will be demonstrated below.
Figure 4 - Headspace / Cryo-Trap Analysis Of Oil Base Enamel Paint, Add 0.0115 grams Of Martin Oil Base Black Enamel, Heat To 90 Degrees For 10 min, Inject 0.3 ml Of Headspace To GC At 1.5 ml/min. Trap At -160 Degrees For 5.0 min. GC At 30 Degrees For 10 min, To 65 At 5/min, To 90 At 1.0/min, Then To 200 At 10/min.
Effect of Headspace Temperature
For this study, the clear varnish paint (0.030 grams) was weighed into a headspace vial. The GC split ratio was left at 10:1 for this study. The headspace vial was heated to various temperatures and 0.30 ml of the headspace gas was injected into the GC injection port and then cryo-trapped at -160 degrees C. As the headspace temperature is increased, the intensity of the higher boiling compounds increased (Figure # 5) as expected. Only minimal increase in the intensity of the lower boiling volatiles is seen below the C9 hydrocarbons as the headspace sample temperature increased. 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 a wide range of aromatics, straight chain and branched chain hydrocarbons and the substituted cyclohexanes.
Figure 5 - Headspace / Cryo-Trap Analysis of Varnish at Various Headspace Temperatures, 0.030 grams Of Clear Varnish, equilibrate For 10 minutes At Indicated Headspace Temperature Then Inject 0.30 ml Of Headspace Volume Into GC Cryo-Trap At -160 Degrees C, Hold 5 min, Heat To 200 and Chromatograph.
Reproducibility of Sample Analysis
Questions often arise as to the accuracy and reproducibility of the headspace injection technique. Using the above described headspace method at a headspace heater temperature of 90 degrees C, a series of 0.119 gram samples of the clear varnish paint were analyzed. The split ratio was increased to 60:1 for these larger samples. As can be seen from Figure # 6, this technique is very reproducible. This is quite exemplary for the headspace injection technique since a large split ratio was used in the GC injection port and no internal standard was used to normalize the data. This method could readily be utilized as a quality control 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%.
Figure 6 - Reproducibility At a 60:1 split of 0.119 gram sample, Headspace / Cryo-Trap Analysis Of Varnish At 120 Degrees C, Add 0.0119 grams Of Clear Varnish, Equilibrate For 10 minutes At 120 Degrees, Inject 0.30 ml Of Headspace Volume At a 60:1 Split Into GC Cryo-Trap At -160 degrees C, Hold For 5.0 min., Heat To 200 and Chromatograph.
Effect of Evaporation of Paints
Next, a study was undertaken in order to study which volatiles are most easily removed from a varnish sample during the normal drying process. A quantity of clear varnish (0.120 grams) was weighed into the headspace vial and each of the vials was purged with clean nitrogen at room temperature to simulate the process of normal drying of the clear varnish samples. The samples were subsequently analyzed as described above using a GC split ratio of 60:1 in the GC injection port. As the evaporation of the sample progressed, the amount of the lower boiling volatiles decreases as would be expected (Figure # 7). However, the levels of the higher boiling compounds increased in intensity as the degree of evaporation increased. These are the organic volatiles that would take the longest to evaporate in a normal drying process. Their increased intensity in the more highly evaporated samples can best be explained by the decreased competition for partition into the sealed headspace volume in the sample vial due to the absence of the lighter volatiles.
Figure 7 - Headspace / Cryo-Trap Analysis Of Varnish At Various Degrees of Evaporation, Add 0.120 grams Of Clear Varnish, Equilibrate For 10 minutes At 120 degrees C, Inject 0.30 ml of Headspace Volume Into GC (60:1 split) Cryo-Trap At -160 degrees C, Hold For 5.0 min., Heat To 200 and Chromatograph.
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 oil based paints and varnishes. This technique could readily be adapted for either a quality control method for the analysis of volatiles in all types of paints or it could also be used as a comparison tool. Depending on the level of volatiles in the paints, the sample size can be varied as well as the GC split ratio so as not to overload the GC capillary column or GC detector. This combination of a headspace sampler and the GC Cryo-Trap and the versatile sampling technique 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 (0.1 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 results in an even delivery of sample to the injection port and the accurate and reproducible splitting and delivery of sample to the GC column. 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 oil base and 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.
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