QuantitativeAnalysis by Isotopic Dilution Using Mass Spectroscopy The Determination of Caffeine by GC-MS Devon W. Hill, Brian T. McSharry, and Larry S. Trzupek Furman University. Greenville, SC 29613 The develonment of reliable.. easv-to-use. and reasonahlv " priced systems for coupled gas c h r o m a t ~ ~ r ~ ~ h ~ -spec-mass troscow (GC-MS) has led to the nroliferation of these sophisticated instruments, even in academic settings with a predominantly undergraduate emphasis. For example, the NSF-CSIP program made a t least 12 grants to colleges in 1985 for GC-MS systems t o he used for instructional purposes, a number large enough that a special section on the incorporation of GC-MS in the undergraduate curriculum was included in a program on "NSFICSIP Catalyzed Innovations in the Undergraduate Laboratory" a t the 1987 National ACS meeting in New Orleans. In many cases, the most straightforward use of these instruments in undergraduate experiments involves qualitative analysis of mixtures or structure determination of reasonably pure compounds using the mass spectrometer component in its "scan" mode. However, many of the most interesting practical applications of GC-MS involve its use in analyscsof components in rumplex mixtures, using the "specific ion monitoring", or "SIM" mode. In this configuration, the mass spectrometer samples the effluent GC stream only for ions of specific, designated masses, typically cycling through a small number of such ions many times per second. In this way, both the sensitivity and the analvtical snecificitv of the technioue are enhanced. The most sophisticated of these procedures use quantitation through isoto~icdilution,in which the intensitv o i a snecific analyie ion is compared to that of an analogous ion of a suitably labelled form of the compound of interest, added to the analytical matrix in a known amount. s u c h methods have been extensively used in hoth environmental and forensic analysis. For example, EPA Method 8280 describes the GC-MS analysis for polychlorinated dibenzo-p-dioxins, a procedure which reouires the addition of C'3-labeled TCDD'S as internal standards for quantitation. In addition, the most reliable methods for urinalvsis of drues and drue metahdites are those which involvbGC-MS and isotopic dilution; a twical case int,olves the GC-MS analvcisof urine for 1 I -nor-d&a-9-tetrahydrocannabinol-9-rarbox!.lic acid. the major metabolite of (-)-trans-(lelta-9-tetrahydrorannahinol (THC,, an active component of marijuana ( 1 3 ) . Ilespite the technical elegance of such methods, practical problems (such as the expense of the standards
