Improved quality control in gas chromatography interfaced to stable isotope ratio mass spectrometry by application of derivative chromatography

Compound-specific isotope analysis using gas chromatography interfaced to isotope ratio mass spectrometry (GC-IRMS) is a versatile technique for applications ranging from source appointment and the elucidation of biochemical pathways. When δ¹³C values are going to be determined, the sample is combusted to CO₂ and the resulting gas is analyzed relative to a standard with known stable carbon isotope ratio. With the combustion step any information on the identity of a peak is lost. Co-eluting compounds can no more be identified which can lead to significant alterations of the δ¹³C value of the analyte. For improvement of the QA/QC protocols in GC-IRMS, we used first, second, and third order derivative chromatography. The suitability of the technique was studied using mixtures of 2,3,3′,4,4′,5,5′-heptachloro-1′-methyl-1,2′-bipyrrole (Q1) and 2,2′,4,5,5′-pentachlorobiphenyl (PCB 101). By application of different GC oven programs four scenarios ranging from baseline separation to full co-elution were obtained. Derivative chromatography enabled identification of the interference of Q1 with PCB 101 even when both peaks fully co-eluted. Although the δ¹³C values could not be determined from interfered scenarios, the use of derivative spectroscopy will help to prevent acceptance of incorrect data due to co-elutions. Derivative chromatography was finally used to study the peak purity of 2,2′,3,4,4′-pentabromodiphenyl ether (BDE 85) in technical pentabromo diphenyl ether (DE-71). Already the first order derivative demonstrated that this key-BDE congener was interfered by a compound identified as 2,2′,4,4′,6,6′-hexabromodiphenyl ether (BDE 155).