United States Environmental Protection Agency Environmental Monitoring Systems Laboratory Research Triangle Park NC 27711 Research and Development EPA/600/S4-85/062 Jan. 1986 ŁEPA Project Summary Direct Calibration of GC/MS Systems Using SRM Gas Cylinders G. D. Byrd, K. L. Richie, L. T. Sniegoski, and E. White V A cryogenic trapping system has been developed for use in calibrating GC/MS systems for the analysis of volatile organic compounds. This system pro- vides for direct Standard Reference Material (SRM) traceability on data generated on gaseous samples. The cryogenic trap is a coil of stainless steel tubing immersed in a cryogen to trap and preconcentrate organic species present in a gaseous sample. The trap also contains a heated injection port for the addition of isotopically labeled com- pounds for use in isotope dilution measurements. The first part of this research is concerned with the develop- ment of isotope dilution mass spectrom- etry (IDMS) as an independent method for the quantification of analytes in gaseous samples to be used as stand- ards. Results are presented for the determination of bromobenzene in nitro- gen at nominal concentrations of 1 and 25 ppb. In the second part of the re- search, a calibration curve method was developed for using these standards in auditing the performance of GC/MS systems. This method is demonstrated on multicomponent aromatic mixtures in the 10 to 200 ppb concentration range. The system using the calibration curve method was evaluated at an independent laboratory and compared with a static dilution bottle calibration curve method. This Project Summary was developed by EPA's Environmental Monitoring Systems Laboratory, Research Triangle Park, NC, to announce key findings of the research project that is fully docu- mented in a separate report of the same title (see Project Report ordering infor- mation at back). Summary The goal of this project was to develop a trapping system that can be used to introduce volatile organic compounds (VOCs) at low concentrations (ppb) from a gas cylinder into a combined gas chromato- graph/mass spectrometer (GC/MS) in- strument. A trap has been constructed that condenses the gaseous sample from the cylinder using a small coil immersed in a dry ice-ethanol bath. The coil is then electrically heated to revolatize the sam- ple. The gas is introduced into the GC/MS with a syringe needle that is silver soldered to the coil tubing so that the effluent can be injected directly into the instrument. A heated injection port also allows introduction of liquid solutions into the GC/MS. The performance of the trap was evaluated using isotope dilution mass spectrometer (IDMS) measure- ments on samples that had been well characterized by other methods. Re- sponse factor determinations were made using isotopically labeled analytes. A GC/MS method based on establish- ing a calibration curve was developed that permits a direct comparison of the concentrations of trace components from two gas sources. This provided a means of linking data from different laboratories to a common SRM. The cryogenic trap was used for this method, since it can be employed with any GC system that con- tains a septum inlet. The principle of the calibration curve is to establish a plot of response versus amount for a particular analyte. From a measured volume of a sample with an unknown concentration of the analyte and its response, the concentration can be calculated. SRV gas cylinders containing the analytes of interest are well suited as calibrant ------- samples because of their certified values for analytes. In order for this method to work properly, a known relationship, pref- erably linear, must be established over the desired concentration range. This, in turn, requires that reproducible amounts of the sample be trapped and delivered to the GC/MS for analysis. The procedure was validated by measurements at an independent laboratory on two cylinders containing "unknown" concentrations of benzene, chlorobenzene, and bromoben- zene. A cylinder containing known con- centrations of the same compounds was used for calibration. All of the cylinders had been previously prepared and ana- lyzed at the National Bureau of Standards (NBS) using the trapping system. The combined average values for all the measurements made on these cylinders at both laboratories were within two standard deviations of the preoared value for all analytes except bromobenzene in one cylinder. This value was slightly more than three standard deviations outside of the prepared concentration. The components of the gas cylinders were also analyzed by GC/MS using a calibration curve generated by injections of pure gaseous compounds from static dilution flasks. This was the usual method of calibration of the independent labor- atory and afforded a way to compare the results of the two ways to calibrate the analytical system. The differences in the relative standard deviations for the two sets of measurements were not signif- icant, indicating similar precision for both laboratories. The average percent differ- ence between delivered and measured amounts-for the analytes was biased on all cases with 11% to 35% more of the compounds detected than was delivered. G. D. Bryd, K. L. Richie. L T. Sniegoski. andE. White are with the National Bureau of Standards, Gaithersburg, MD 20899. Howard Crist is the EPA Project Officer (see below). The complete report, entitled "Direct Calibration of GC/MS Systems Using SRM Gas Cylinders."(Order No. PB86-110 715/AS; Cost: $9.95. subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Environmental Monitoring Systems Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Official Business Penalty for Private Use $300 EPA/600/S4-85/062 0000329 PS PROTECTION ------- |