Optimization of Method Parameters for the Evaluation of USEPA Method Using a Purge and Trap with GC/MS

Transcription

1 Application Note Optimization of Method Parameters for the Evaluation of USEPA Method Using a Purge and Trap with GC/MS Objective By: Glynda Smith In this paper, an evaluation of different methods of moisture control will be presented and the Tekmar-Dohrmann 3 Sample Concentrator will be optimized for the analysis of the USEPA compound list using GC/MS. This optimization of method parameters will focus on achieving the optimal chromatographic response by reducing the effect of water on the analysis. Introduction Purge and Trap coupled with gas chromatography offers a simple method for the extraction and concentration of volatile organic compounds (VOCs) from multiple matrices such as waters and soils. USEPA Method monitors a wide range of VOCs in drinking water. Due to the range and number of compounds analyzed in Method 524.2, there are several analytical problems that can arise. One of the most common problems involves the transfer of water to the analytical column. Excessive water transfer can cause problems with both the mass spectrometer and the chromatography. A mass spectrometer is especially sensitive to the presence of water. Various types of vacuum pumps may be incorporated (e.g., roughing pumps, turbomolecular pumps, diffusion pumps, etc.) and the pumping efficiencies can vary with time and with the level of maintenance. The pressure load that water imposes upon a mass spectrometer interferes with the efficiency of electron ionization, shortens the lifetime of the filaments, and results in higher maintenance costs. With respect to the chromatography, excess water often produces unacceptable broadening of early eluting peaks. Removal of water can be facilitated through the use of hydrophobic trapping materials and incorporation of a moisture control system (MCS) which condenses water from the desorb gas during the transfer to the gas chromatograph. The research described in this paper involves the evaluation of the moisture control system and two types of traps under varying conditions of sample mount temperatures, MCS temperatures, dry purge times, and desorb times. Experimental Method Rev. 4 standards were obtained from Restek Corp. (VOA Kit - Cat #3447). Duplicate data runs employed 5 ml samples containing ppb of each of the analytes. The parameters evaluated are as follows: I. Vocarb 3 Hydrophobic Trap A. Moisture Control System (MCS) Installed. Vary 2. Vary Dry Purge 3. Vary Mount Temperature 4. Vary Desorb B. MCS Uninstalled. Vary Dry Purge 2. Vary Mount Temperature 3. Vary Desorb II. #3 (Tenax/Charcoal/Silica Gel) Hydrophilic Trap A. Moisture Control System (MCS) Installed. Vary 2. Vary Mount Temperature 3. Vary Desorb B. MCS Uninstalled. Vary Mount Temperature 2. Vary Desorb 3-.doc; 9-Jun-3 Sales/Support: Main: Socialville Foster Rd., Mason, OH 454

2 The Tekmar-Dohrmann 3 Purge & Trap Sample Concentrator parameters that were held constant throughout all of the data runs are described below: Line Temperature 5 C Trap Pressure Control 4 psi Valve Temperature 5 C Bake 2 min. Purge Ready Temp. 35 C Bake Temperature 27 C (Vocarb);23 C () Purge min. Desorb Temperature 25 C (); 225 C () Desorb Preheat 245 C (); 22 C () Table. Conditions for HP 689GC/5973 MS Injector: Column: Temperature Program: Carrier: C, Split 2: with Vocarb trap; Split 5: with #3 trap Restek Corp. Rtx-VMS, 6m x.25mm x.4µm 45 C (hold min). Increase to 9 C at 2 C/min and hold 2 min. at 9 C. Increase to 225 C at 6 C/min and hold min. at 225 C. Helium,.2 ml/min MS Source Temperature: 23 C MS Quad Temperature: 5 C Electron Multiplier: Mass Range Scanned: Vacuum System: 62 volts amu Turbomolecular pump, x -5 Torr Splitting at the injection port is a good technique for water removal. A 5: split was needed for samples that were run on the hydrophilic #3 trap in order to fully resolve the light gases. When lower split ratios were examined, the higher baselines and broadened peak shapes of the early eluting components resulted in wide variability for the calculated response factors of the light gases. The Vocarb 3 trap, however, is very effective at removing water due to its hydrophobicity. As such, a lower 2: split ratio provided sufficient chromatographic resolution for samples prepared with the Vocarb 3 trap. Results and Discussion Tables 2 through 8 list the response factors obtained for a group of representative compounds under the conditions described in Table. All six of the gas standards are included (dichlorodifluoromethane, chloromethane, vinyl chloride, bromomethane, trichlorofluoromethane) along with three mid-boiling range compounds (methyl acrylate, toluene, pentachloroethane) and three high boiling range compounds (,2,4- trichlorobenzene, naphthalene,,2,3-trichlorobenzene). Vary Table 2.. Vary during Desorb Vocarb 3 Trap Analyte 4 C 5 C 6 C Analyte 4 C 5 C 6 C Dichlorodifluoromethane Dichlorodifluoromethane Chloromethane Chloromethane Vinyl Chloride Vinyl Chloride Bromomethane Bromomethane Chloroethane Chloroethane Trichlorofluoromethane Trichlorofluoromethane Methyl Acrylate Methyl Acrylate Toluene Toluene Bromoform Bromoform Pentachloroethane Pentachloroethane ,2,4-Trichlorobenzene ,2,4-Trichlorobenzene Naphthalene Naphthalene ,2,3-Trichlorobenzene ,2,3-Trichlorobenzene Table 2 shows the results obtained when the MCS temperature was increased during the desorb cycle. The MCS is typically cooled to ambient temperature during the desorb step in order to condense water prior to transfer of 3-.doc; 9-Jun-3 Page 2 of 7

3 the analytes from the trap to the GC. As the data indicates, the overall responses are higher with the Vocarb 3 trap relative to the #3 trap. Specifically, the average responses of the lighter components are about 8% better with the Vocarb trap. This is not surprising since the early eluting components are adversely affected by the presence of water. But for either trap, there is little variation in the analyte responses as a function of MCS temperature and there seems to be no analytical advantage in operating the system at elevated MCS temperatures. Graph provides a brief synopsis of this information showing the effect on two of the gas standards, a mid-boiling range compound and a high boiling range compound. Graph Vary - Dichlorodifluoromethane 5 Vary - Chloroethane Vary - Toluene Vary - Naphthalene Vary Dry Purge Hydrophobic traps such as the Vocarb 3 trap can be purged with dry gas prior to desorption of the sample. This step facilitates water removal. Hydrophilic traps, however, do not benefit from incorporation of a dry purge since water will preferentially remain in the silica gel. Thus, the data in tables 3 and 4 only refer to the Vocarb 3 trap. Table 3.. Vary Dry Purge (Applies to Vocarb 3 Trap Only) Vocarb 3 Trap Analyte min. 2 min. 4 min. 6 min. 8 min. Dichlorodifluoromethane Chloromethane Vinyl Chloride Bromomethane Chloroethane Trichlorofluoromethane Methyl Acrylate Toluene Bromoform Pentachloroethane ,2,4-Trichlorobenzene Naphthalene ,2,3-Trichlorobenzene doc; 9-Jun-3 Page 3 of 7

4 Table 4.. Vary Dry Purge Vocarb 3 Trap Analyte min. 2 min. 4 min. 6 min. 8 min. Dichlorodifluoromethane Chloromethane Vinyl Chloride Bromomethane Chloroethane Trichlorofluoromethane Methyl Acrylate Toluene Bromoform Pentachloroethane ,2,4-Trichlorobenzene Naphthalene ,2,3-Trichlorobenzene When a moisture control system is installed along with a hydrophobic trap such as the Vocarb 3, a dry purge step may or may not be beneficial and the length of time employed should be determined by the application. In the work described in this paper, the split ratios employed minimized the dry purge effect. When the MCS is bypassed inside the valve oven with a short piece of /6 Silcosteel tubing, however, a long dry purge step is needed in order to attain the same analyte responses as when the MCS is incorporated in the sample pathway (see Graph 2). Graph 2. Vary Dry Purge - Dichlorodifluoromethane Vary Dry Purge - Vinyl Chloride Vary Dry Purge - Bromoform Vary Dry Purge - Naphthalene doc; 9-Jun-3 Page 4 of 7

5 Vary Mount Temperature Tables 5 and 6 compare the results obtained at different mount temperatures when the MCS is installed and removed, respectively. Table 5.. Vary Mount Temperature Vocarb 3 Trap Analyte Ambient 75 C C Analyte Ambient 75 C C Dichlorodifluoromethane Dichlorodifluoromethane Chloromethane Chloromethane Vinyl Chloride Vinyl Chloride Bromomethane Bromomethane Chloroethane Chloroethane Trichlorofluoromethane Trichlorofluoromethane Methyl Acrylate Methyl Acrylate Toluene Toluene Bromoform Bromoform Pentachloroethane Pentachloroethane ,2,4-Trichlorobenzene ,2,4-Trichlorobenzene Naphthalene Naphthalene ,2,3-Trichlorobenzene ,2,3-Trichlorobenzene Table 6.. Vary Mount Temperature Vocarb 3 Trap Analyte Ambient 75 C C Analyte Ambient 75 C C Dichlorodifluoromethane Dichlorodifluoromethane Chloromethane Chloromethane Vinyl Chloride Vinyl Chloride Bromomethane Bromomethane Chloroethane Chloroethane Trichlorofluoromethane Trichlorofluoromethane Methyl Acrylate..6. Methyl Acrylate Toluene Toluene Bromoform Bromoform Pentachloroethane Pentachloroethane ,2,4-Trichlorobenzene ,2,4-Trichlorobenzene Naphthalene Naphthalene ,2,3-Trichlorobenzene ,2,3-Trichlorobenzene Elevated sample mount temperature eliminates a cold spot at the top of the glassware. On average, increasing the mount temperature from ambient conditions to 75 C results in a 6% increase in the response factors for the heavier components without sacrificing the response of the early eluting gases. Furthermore, the data clearly indicates that removal of the MCS results in the loss of analyte response on the hydrophilic #3 trap. For example, with the MCS removed, the response factor for naphthalene drops by 5% as shown in Graph 3. Graph 3. Vary Mount Temperature - Naphthalene s Temperature ( C) - Vocarb - Vocarb doc; 9-Jun-3 Page 5 of 7

6 Vary Desorb Tables 7 and 8 compare the results obtained at different desorb times when the MCS is installed and removed, respectively. Table 7.. Vary Desorb Vocarb 3 Trap Analyte 2 min. 4 min. 6 min. 8 min. Analyte 3 sec. min. 2 min. 4 min. Dichlorodifluoromethane Dichlorodifluoromethane Chloromethane Chloromethane Vinyl Chloride Vinyl Chloride Bromomethane Bromomethane Chloroethane Chloroethane Trichlorofluoromethane Trichlorofluoromethane Methyl Acrylate Methyl Acrylate Toluene Toluene Bromoform Bromoform Pentachloroethane Pentachloroethane ,2,4-Trichlorobenzene ,2,4-Trichlorobenzene Naphthalene Naphthalene ,2,3-Trichlorobenzene ,2,3-Trichlorobenzene Table 8.. Vary Desorb Vocarb 3 Trap Analyte 2 min. 4 min. 6 min. 8 min. Analyte 3 sec. min. 2 min. Dichlorodifluoromethane Dichlorodifluoromethane Chloromethane Chloromethane Vinyl Chloride Vinyl Chloride Bromomethane Bromomethane Chloroethane Chloroethane Trichlorofluoromethane Trichlorofluoromethane Methyl Acrylate Methyl Acrylate Toluene Toluene Bromoform Bromoform Pentachloroethane Pentachloroethane ,2,4-Trichlorobenzene ,2,4-Trichlorobenzene Naphthalene Naphthalene ,2,3-Trichlorobenzene ,2,3-Trichlorobenzene When the hydrophobic Vocarb 3 trap is installed, variations in desorb time between 2 8 minutes do not significantly affect analyte response. The hydrophilic #3 trap, however, shows a better overall response with a shorter desorb time. Longer desorb times yield poor chromatography with the early eluting components, i.e., higher baselines with broadened peaks due to the excess water that is also desorbed onto the column. The result is wide variability in the calculated response factors. Thus, it was deemed unnecessary to examine desorb times longer than 4 minutes with the #3 trap. Also, a longer desorb time in conjunction with removal of the moisture control system further compounds the problem for the #3 trap because the responses of the later eluting analytes begin to show significant variability. Graph 4 provides a graphical summary of this information. 3-.doc; 9-Jun-3 Page 6 of 7

7 Graph 4. Vary Desorb - Vinyl Chloride Vary Desorb - Vinyl Chloride #3 - #3 Vary Desorb - Toluene Vary Desorb - Naphthalene In general, incorporation of a moisture control system (MCS) appears to help with water removal. When the MCS is installed and a Vocarb 3 trap is used, a dry purge step can be minimized. With the MCS removed, however, a dry purge is needed in order to obtain responses similar to those observed when it is installed. The MCS also provides better consistency in the data obtained with the #3 trap. When the MCS is removed, the responses tend to drop because this trap is holds water in the silica gel layer and allows the water to be desorbed to the GC. The data further indicates that a heated mount feature, such as that provided on the Tekmar 3 Sample Concentrator, provides excellent responses for the heavier components in the Compound List when it is maintained at an elevated temperature. Conclusions Excess water can pose difficulties in purge & trap analyses. But using the combination of a dry purgeable trap, a moisture control system, split injection, and a heated mount provides maximum reduction of water that gets transferred to the column. Figures and 2 provide quick references to the best conditions for water removal with a Vocarb 3 trap and a #3 trap, respectively. Figure. Purge & Trap Conditions for Vocarb 3 Trap Figure 2. Purge & Trap Conditions for 3-.doc; 9-Jun-3 Page 7 of 7