Optimized Volatiles Analysis Ensures Fast VOC Separations

Optimized methods for the analysis of volatile organic compounds (VOCs) can be time-consuming to develop because compound lists can be extensive and analytes vary significantly in chemical characteristics. For example, target compounds in EPA Method 8260 for solid waste matrices include volatiles that range from light gases (Freon^®) to larger aromatic compounds (trichlorobenzenes). These differences make column selectivity, thermal stability, and inertness critical to resolving volatiles. Often, “624” type columns are chosen for their selectivity, but thermal stability is usually poor, which can result in phase bleed that decreases detector sensitivity. New Rxi^®-624Sil MS columns offer reliable resolution of critical VOC pairs and also provide lower bleed and greater inertness than other columns. In order to provide optimized conditions for labs analyzing VOCs, we established parameters that ensure good resolution, while reducing downtime by syncing purge and trap cycles with instrument cycles. In addition, we present comparative data that demonstrate why Rxi^®-624Sil MS columns are the best choice for volatiles analysis.

Resolve Critical Pairs and Reduce Downtime

In order to achieve desired separations and minimize downtime between injections, several critical pairs were chosen for computational modeling using Pro ezGC software. The temperature program initially determined by the software was 35 °C (hold 5 min.) to 120 °C @ 11 °C/min. to 220°C @ 20 °C/min. (hold 2 min.). While this provided the best resolution of critical pairs, it also extended the analysis time to 19 min. Since the purge and trap cycle time was 16.5 min., we tested other conditions to see if adequate resolution could be maintained, while using a faster instrument cycle time that more closely matched the purge and trap cycle time, in order to maximize sample throughput. In other calculations, the software suggested changing temperature ramps at 60°C; therefore, a program of 35°C (hold 5 min.) to 60°C @ 11 °C/min. to 220°C @ 20 °C/min. (hold 2 min.) was tested. This final program reduced instrument downtime by better synchronizing injection and analysis cycles, and also provided excellent resolution of volatile compounds (Figure 1). Testing of faster conditions determined that the initial hold of 5 minutes at 35°C was critical for the best separation of early eluting compounds, such as the gases, as well as a favorable elution of methanol between gas compounds.

Figure 1  Rxi®-624Sil MS columns resolve methyl ethyl ketone and ethyl acetate, a separation not obtained on other 624 columns.

Peaks RT (min.) 1. Dichlorodifluoromethane (CFC-12) 2.198 2. Chloromethane 2.459 3. Vinyl chloride 2.659 4. Bromomethane 3.226 5. Chloroethane 3.434 6. Trichlorofluoromethane (CFC-11) 3.876 7. Diethyl ether (ethyl ether) 4.44 8. 1,1-Dichloroethene 4.909 9. 1,1,2-Trichlorotrifluoroethane (CFC-113) 4.998 10. Acetone 5.029 11. Iodomethane 5.195 12. Carbon disulfide 5.323 13. Acetonitrile 5.637 14. Allyl chloride 5.715 15. Methyl acetate 5.723 16. Methylene chloride 5.981 17. tert-Butyl alcohol 6.234 18. Acrylonitrile 6.451 19. Methyl tert-butyl ether (MTBE) 6.509 20. trans-1,2-Dichloroethene 6.512 21. 1,1-Dichloroethane 7.315 22. Vinyl acetate 7.359 23. Diisopropyl ether (DIPE) 7.407 24. Chloroprene 7.429 25. Ethyl tert-butyl ether (ETBE) 7.97 26. 2-Butanone (MEK) 8.193 27. cis-1,2-Dichloroethene 8.193 28. 2,2-Dichloropropane 8.193 29. Ethyl acetate 8.265 30. Propionitrile 8.276 31. Methyl acrylate 8.318 32. Methacrylonitrile 8.476 33. Bromochloromethane 8.507 34. Tetrahydrofuran 8.521 35. Chloroform 8.651 36. 1,1,1-Trichloroethane 8.843 37. Dibromofluoromethane 8.848 38. Carbon tetrachloride 9.026 39. 1,1-Dichloropropene 9.037 40. 1,2-Dichloroethane-d4 9.246 41. Benzene 9.262 42. 1,2-Dichloroethane 9.334 43. Isopropyl acetate 9.34 44. Isobutyl alcohol 9.421 45. tert-Amyl methyl ether (TAME) 9.421 46. Fluorobenzene 9.598 47. Trichloroethene 9.976 48. 1,2-Dichloropropane 10.243 49. Methyl methacrylate 10.29 50. 1,4-Dioxane (ND) 10.299* 51. Dibromomethane 10.326 52. Propyl acetate 10.346 Peaks RT (min.) 53. 2-Chloroethanol (ND) 10.368* 54. Bromodichloromethane 10.496 55. 2-Nitropropane 10.698 56. cis-1,3-Dichloropropene 10.904 57. 4-Methyl-2-pentanone (MIBK) 11.026 58. Toluene-D8 11.148 59. Toluene 11.21 60. trans-1,3-Dichloropropene 11.407 61. Ethyl methacrylate 11.435 62. 1,1,2-Trichloroethane 11.585 63. Tetrachloroethene 11.662 64. 1,3-Dichloropropane 11.729 65. 2-Hexanone 11.749 66. Butyl acetate 11.837 67. Dibromochloromethane 11.921 68. 1,2-Dibromoethane (EDB) 12.035 69. Chlorobenzene-d5 12.412 70. Chlorobenzene 12.44 71. Ethylbenzene 12.507 72. 1,1,1,2-Tetrachloroethane 12.507 73. m-Xylene 12.612 74. p-Xylene 12.612 75. o-Xylene 12.935 76. Styrene 12.949 77. n-Amyl acetate 13.018 78. Bromoform 13.118 79. Isopropylbenzene (cumene) 13.226 80. cis-1,4-Dichloro-2-butene 13.268 81. 4-Bromofluorobenzene 13.385 82. 1,1,2,2-Tetrachloroethane 13.456 83. trans-1,4-Dichloro-2-butene 13.496 84. Bromobenzene 13.515 85. 1,2,3-Trichloropropane 13.526 86. n-Propylbenzene 13.565 87. 2-Chlorotoluene 13.657 88. 1,3,5-Trimethylbenzene 13.699 89. 4-Chlorotoluene 13.751 90. tert-Butylbenzene 13.965 91. Pentachloroethane 14.007 92. 1,2,4-Trimethylbenzene 14.01 93. sec-Butylbenzene 14.14 94. 4-Isopropyltoluene (p-cymene) 14.254 95. 1,3-Dichlorobenzene 14.263 96. 1,4-Dichlorobenzene-D4 14.321 97. 1,4-Dichlorobenzene 14.34 98. n-Butylbenzene 14.579 99. 1,2-Dichlorobenzene 14.635 100. 1,2-Dibromo-3-chloropropane (DBCP) 15.252 101. Nitrobenzene 15.407 102. 1,2,4-Trichlorobenzene 15.935 103. Hexachloro-1,3-butadiene 16.04 104. Naphthalene 16.196 105. 1,2,3-Trichlorobenzene 16.396 * ND = not detected; retention time determined by wet needle injection GC_EV1169 Column Rxi®-624Sil MS, 30 m, 0.25 mm ID, 1.40 µm (cat.# 13868) Sample 8260A Surrogate Mix (cat.# 30240) 8260A Internal Standard Mix (cat.# 30241) 8260B MegaMix® Calibration Mix (cat.# 30633) VOA Calibration Mix #1 (ketones) (cat.# 30006) 8260B Acetate Mix (Revised) (cat.# 30489) California Oxygenates Mix (cat.# 30465) 502.2 Calibration Mix #1 (gases) (cat.# 30042) Conc.: 25 ppb in RO water Injection purge and trap split (split ratio 30:1) Inj. Temp.: 225 °C Purge and Trap Instrument: OI Analytical 4660 Trap Type: 10 Trap Purge: 11 min. @ 20 °C Desorb Preheat Temp.: 180 °C Desorb: 0.5 min. @ 190 °C Bake: 5 min. @ 210 °C Interface Connection: injection port Oven Oven Temp: 35 °C (hold 5 min.) to 60 °C at 11 °C/min. to 220 °C at 20 °C/min. (hold 2 min.) Carrier Gas He, constant flow Flow Rate: 1.0 mL/min. Detector MS Mode: Scan Transfer Line Temp.: 230 °C Analyzer Type: Quadrupole Source Temp.: 230 °C Quad Temp.: 150 °C Electron Energy: 70 eV Solvent Delay Time: 1.5 min. Tune Type: BFB Ionization Mode: EI Scan Range: 36-260 amu Instrument Agilent 7890A GC & 5975C MSD Notes Other Purge and Trap Conditions: Sample Inlet: 40°C Sample: 40°C Water Management: Purge 110°C, Desorb 0°C, Bake, 240°C

Not all "624s" are Equivalent

While optimizing instrument conditions can improve sample throughput, obtaining adequate resolution depends largely on column selectivity, thermal stability, and inertness. Rxi^®-624Sil MS columns are optimized across these parameters, and therefore provide reliable separation of critical VOCs.

Lower Bleed Means Improved Sensitivity and Longer Column Lifetime

While 624 type columns generally provide good selectivity for most volatiles, they are limited by their low thermal stability. Poor thermal stability results in phase bleed that can reduce column lifetime, decrease detector sensitivity (especially ion trap mass spectrometers), and interfere with the quantification of later eluting compounds. Rxi^®-624Sil MS columns have the highest thermal stability and lowest bleed among 624 type columns due to the incorporation of phenyl rings in the polymer backbone (Table I, Figure 2). The conjugated ring system of this silarylene phase provides a more rigid structure that increases thermal stability compared to nonsilarylene phases.

Table I  The Rxi^®-624Sil MS column has the highest thermal stability of any 624 column.

Figure 2  The Rxi®-624Sil MS column has the lowest bleed of any column in its class and provides true GC/MS capability.

Peaks 1. Fluorobenzene GC_GN1147 Column Rxi®-624Sil MS (see notes), 30 m, 0.25 mm ID, 1.4 µm (cat.# 13868) Sample Fluorobenzene (cat.# 30030) Diluent: methanol Conc.: 200 µg/mL Injection Inj. Vol.: 1 µL split (split ratio 20:1) Liner: 4mm Split Liner with Wool (cat.# 20781) Inj. Temp.: 220 °C Oven Oven Temp: 40 °C (hold 5 min.) to 60 °C at 20 °C/min. (hold 5 min.) to 120 °C at 20 °C/min. (hold 5 min.) to 200 °C at 20 °C/min. (hold 10 min.) to 260 °C at 20 °C/min. (hold 10 min.) to 300 °C at 20 °C/min. (hold 20 min.) Carrier Gas He, constant flow Linear Velocity: 40 cm/sec. Detector FID @ 250 °C Instrument Agilent/HP6890 GC Notes Columns are of equivalent dimensions and were tested after equivalent conditioning.

Better Peak Shape Means More Accurate Results
Rxi^®-624Sil MS columns are the most inert 624 column available. Figure 3 shows the differences between vendor columns using primary amines, which are good indicators of column activity. The unique Rxi^®deactivation results in symmetric peaks with minimal tailing, which improves quantitative accuracy. Minimizing tailing is especially important with concentration techniques, such as purge and trap, since the act of desorbing analytes off of the packing material results in some tailing. If a column is not inert, additional tailing due to column activity can magnify this problem. The sharp, symmetric peaks seen on Rxi^®-624Sil MS columns allow greater resolution, higher signal-to-noise ratios, and more accurate results for active volatiles such as alcohols (Figure 4).

Figure 3  Highly inert Rxi®-624Sil MS columns provide better peak shape and more accurate results for active compounds.

Peaks Conc. (...) 1. Isopropylamine 100 2. Diethylamine 100 3. Triethylamine 100 GC_PH1162 Column Rxi®-624Sil MS, 30 m, 0.32 mm ID, 1.8 µm (cat.# 13870) Sample Diluent: DMSO Conc.: 100 µg/mL Injection Inj. Vol.: 1 µL split (split ratio 20:1) Liner: 5mm Single Gooseneck with Wool (cat.# 22973-200.1) Inj. Temp.: 250 °C Oven Oven Temp: 50 °C (hold 1 min.) to 200 °C at 20 °C/min. (hold 5 min.) Carrier Gas He, constant flow Linear Velocity: 37 cm/sec. Detector FID @ 250 °C Instrument Agilent/HP6890 GC

Figure 4  Obtain more accurate results for active volatiles, such as alcohols, by using highly inert Rxi®-624Sil MS columns.

Peaks 1. tert-Butyl Alcohol GC_EV1175 Column Rxi®-624Sil MS, 30 m, 0.25 mm ID, 1.40 µm (cat.# 13868) Sample Conc.: 25 ppb in RO water Injection purge and trap split (split ratio 30:1) Inj. Temp.: 225 °C Purge and Trap Instrument: OI Analytical 4660 Trap Type: 10 Trap Purge: 11 min. @ 20 °C Desorb Preheat Temp.: 180 °C Desorb: 0.5 min. @ 190 °C Bake: 5 min. @ 210 °C Interface Connection: injection port Oven Oven Temp: 35 °C (hold 5 min.) to 60 °C at 11 °C/min. to 220 °C at 20 °C/min. (hold 2 min.) Carrier Gas He, constant flow Flow Rate: 1.0 mL/min. Detector MS Mode: Scan Transfer Line Temp.: 230 °C Analyzer Type: Quadrupole Source Temp.: 230 °C Quad Temp.: 150 °C Electron Energy: 70 eV Solvent Delay Time: 1.5 min. Tune Type: BFB Ionization Mode: EI Scan Range: 36-260 amu Instrument Agilent 7890A GC & 5975C MSD Notes Other Purge and Trap Conditions: Sample Inlet: 40°C Sample: 40°C Water Management: Purge 110°C, Desorb 0°C, Bake, 240°C

Conclusion

Labs interested in optimizing resolution and sample throughput can adopt the conditions established here on Rxi^®-624Sil MS columns to maximize productivity and assure accurate, reliable results.