Fast and Efficient Volatiles Analysis by Purge and Trap Gc/ms

Recent changes in environmental regulatory paradigms, such as EPA’s performance-based measurement systems (PBMS), are lowering method compliance barriers for laboratories working under the Resource Conservation and Recovery Act (RCRA). One of the stated goals of PBMS is to educate the regulators and the regulated community on the inherent and intended flexibility of SW-846 methods. Operating under EPA’s PBMS guidelines, laboratories could employ the flexibility of SW-846 methods to simplify and improve purge and trap GC/MS volatile organic analyses (P/T GC/MS VOAs). Laboratories performing Method 8260B for P/T GC/MS VOAs have two basic GC configuration options: wide bore columns connected to the mass spectrometer through a jet separator or narrow bore columns directly interfaced to the mass spectrometer. SW-846 methodology recognizes both approaches as valid. The narrow bore column/direct interface approach is the better of the two techniques for most analyses when certain modifications are made. When newer purge and trap concentrator designs are employed and when several Method 8260B instrument parameters are modified dramatic performance benefits result. This “enhanced” narrow bore column/direct interface approach produces results such as reduced susceptibility to column contamination by high level samples, improved chromatographic behavior of early eluting and closely eluting compounds, analysis times under 20 minutes, and improved hardware ruggedness. The outcome is better quality data, higher sample throughput, and fewer instrument mechanical failures. INTRODUCTION Connecting the purge and trap concentrator to the GC inlet is one of the major challenges in P/T GC/MS VOAs. The challenge stems from vastly different flow rate requirements of the purge and trap concentrator, the capillary column, and the mass spectrometer. Method 8260B describes GC/MS systems equipped with either cryogenic cooling devices attached to narrow bore (0.25 mm and 0.32 mm) capillary columns or wide bore (0.53 mm) capillary columns connected to enrichment devices such as jet separators. Many laboratories choose wide bore capillary columns with jet separators when running Method 8260B because they can easily accept the high flow rates required to efficiently desorb the trap. The jet separator provides the necessary decrease in carrier gas flow rate prior to entering the mass spectrometer. The wide bore column/jet separator approach has been the traditional approach to P/T GC/MS VOAs for some time. The wide bore column/jet separator approach has a host of problems. The problems include susceptibility to column contamination by high level samples, poor chromatographic behavior of early eluting and closing eluting compounds, long analysis times (run times approaching 40 minutes), and frailty of the jet separator. Narrow bore capillaries, which potentially offer better chromatography, have not been used as much for volatiles analysis primarily because they cannot easily handle the relatively high flow rates coming from the purge and trap concentrator. Method 8260B suggests cryofocusing the analytes on a capillary pre-column interface situated between the purge and trap concentrator and the GC capillary column. This device condenses the desorbed sample components and focuses them into a narrow band that can be transferred to the analytical capillary column. However, this is an additional capital expense and it adds to the total analysis time. Newer purge and trap concentrator designs allow a much simpler interface. A conventional split/splitless injector usually already installed on the GC/MS system can be plumbed in series with the purge and trap concentrator. The operating principle is quite simple: the excess flow coming from the purge and trap is vented at the column inlet allowing a reduction in carrier gas flow rate to one more suitable for high resolution chromatography. Feyerherm and Neal have described how this is done with a Hewlett Packard 5890 GC. Aside from this instrument modification, the concentrator desorb time and the GC oven temperature program should be optimized to improve the chromatographic behavior of method compounds and shorten analysis time. The concentrator desorb time may be as short as 30 seconds depending on the trap material. Shortening the desorb time reduces the amount of water transferred to the GC system and thus improves chromatography. The GC oven temperature program for P/T GC/MS VOAs must accommodate compounds with a relatively wide boiling point range. The initial oven temperature will determine how well-behaved the gases (Dichlorodifluoromethane, Chloromethane, Vinyl Chloride, Bromomethane, and Chloroethane) are. Once the compounds are on the GC column, the higher boilers are not difficult to resolve. Fast (50-60°C/min.) GC oven temperature ramps can be used to save time WTQA '99 15th Annual Waste Testing & Quality Assurance Symposium