Isotope fractionation during gas chromatographic separations

Physical interaction between the sample and stationary phase during GC separations may result in severe isotopic fractionation of the effluent, making it essential to collect it totally in order to obtain quantitative results. When carbon dioxide was eluted through a Porapak column the front portions of the elution peak was enriched in ¹³C. When methane was eluted through molecular sieve, both the heavier species were eluted faster than the lighter ones. However, when the carbon dioxide was eluted through silica gel the first CO₂ eluted was enriched in ¹³C and the last CO₂ eluted was enriched in ¹⁸O. This behavior of the isotopes probably indicates the presence of two different interaction mechanisms.