Effects of GC temperature and carrier gas flow rate on on-line oxygen isotope measurement as studied by on-column CO injection.

Abstract

Although deemed important to δ18 O measurement by on-line high-temperature conversion techniques, how the GC conditions affect δ18 O measurement is rarely examined adequately. We therefore directly injected different volumes of CO or CO-N2 mix onto the GC column by a six-port valve and examined the CO yield, CO peak shape, CO-N2 separation, and δ18 O value under different GC temperatures and carrier gas flow rates. The results show the CO peak area decreases when the carrier gas flow rate increases. The GC temperature has no effect on peak area. The peak width increases with the increase of CO injection volume but decreases with the increase of GC temperature and carrier gas flow rate. The peak intensity increases with the increase of GC temperature and CO injection volume but decreases with the increase of carrier gas flow rate. The peak separation time between N2 and CO decreases with an increase of GC temperature and carrier gas flow rate. δ18 O value decreases with the increase of CO injection volume (when half m/z 28 intensity is <3 V) and GC temperature but is insensitive to carrier gas flow rate. On average, the δ18 O value of the injected CO is about 1‰ higher than that of identical reference CO. The δ18 O distribution pattern of the injected CO is probably a combined result of ion source nonlinearity and preferential loss of C16 O or oxygen isotopic exchange between zeolite and CO. For practical application, a lower carrier gas flow rate is therefore recommended as it has the combined advantages of higher CO yield, better N2 -CO separation, lower He consumption, and insignificant effect on δ18 O value, while a higher-than-60 °C GC temperature and a larger-than-100 µl CO volume is also recommended. When no N2 peak is expected, a higher GC temperature is recommended, and vice versa. Copyright © 2015 John Wiley & Sons, Ltd.