Fast gas chromatography combustion isotope ratio mass Spectrometry

We report here the first coupling of fast GC to IRMS, in a system capable of 250 ms peak widths (fwhm) at 1 mL/min flow rates, one-fifth as narrow as any previously reported GCC-IRMS system. We developed an optimized postcolumn interface that results in minimal peak broadening, using a programmable temperature vaporization injector in place of a rotary valve or backflush system to divert solvent, a narrow capillary combustion reactor followed by a cryogenic water trap with narrow-bore (< 0.20 mm We transfer lines, and a narrow W. open split to the IRMS directly inserted into the column effluent. Quantitative combustion was demonstrated with CH4 injections. A comparison of CO2 injections with different fwhm peak widths (250, 2500, and 7500 ms) showed similar precisions, SD(delta C-13) = 0.2-0.3 parts per thousand, for injections of > 600 pmol C on column; precision for the narrow peaks (250 ms) was considerably better for injections < 150 pmol C on column. SD(delta C-13) < 1 parts per thousand was achievable for injections of 5-15 pmol on column for 250 ms wide peaks, 10-fold better precision than 2500 ms wide peaks, and within a factor of 3 of the counting statistics Emit For a mixture of 15 fatty acid methyl esters (FAME), 1.5 nmol C of each on column yielded typical SD(delta C-13(pdb)) = 0.4 parts per thousand for fast GC and 0.5%o for conventional GC. For 14 of the 15 FAME, delta C-13 values between the two systems were within 1.5 parts per thousand and not significantly different Fast GCC-IRMS required one-third the run time (450 s vs 1400 s) to achieve comparable resolution. Mean peak widths for fast GCC-IRMS of the FAME were 720 ins, compared to 650 ms by fast GC with flame ionization detection. At a 15-fold dilution (100 pmol C on column for each FAME), fast GCC-IRMS achieved -2-fold better precision and accuracy than similar injections on conventional GCC-ERMS. Finally, a mixture of 10 steroids (similar to 7 nmol C (100 ng) each on column) was analyzed with mean precision of SD)(delta C-13) = 0.2 parts per thousand in 620 s by fast GCC-IRMS, while conventional GCC-IRMS required 1200 s and achieved poorer resolution. delta C-13 values for the two system were similar (Delta delta C-13 <=:5 2 parts per thousand for all steroids), indicating that accuracy is not compromised. In summary, fast GCC-IRMS can achieve similar precision to conventional GC with considerable time savings for standard sample sizes (> 1 nmol C) and achieves modest precision (similar to 1 parts per thousand) near the counting statistics limit on low level components.