Analysis of alkenone unsaturation indices with fast gas chromatography/time-of-flight mass spectrometry

Extensively purified C-37 alkenone references and mixtures thereof were analyzed by gas chromatography/flame ionization detection (GC/FID) and fast gas chromatography/time-of-flight mass spectrometry (GC/TOF-MS), to establish the latter as an alternative, fast, and reliable analysis method for alkenone unsaturation indices (U-37(k')). This index is a tool for past sea surface temperature reconstructions with extensive use in paleoclimate and paleoceanographic research. TOF-MS was chosen because of its unique capability to acquire full-range spectra at high data rates (up to 500 spectra s(-1)) and to produce homogeneous spectra across a gaschromatographic peak, allowing faster separations than conventional GC/MS and the employment of enhanced peak deconvolution algorithms. Analysis time per sample could be reduced to run times of < 10 min, i.e., by a factor of similar to 10 compared to conventional GC/FID (90-100 min) methods. However, U-37(k') values from GC/TOF-MS showed deviations from those obtained by GC/FID, resulting from sensitivity differences between the C-37:2 and C-37:3 alkenone when analyzed by GC/TOF-MS. A solution to this bias is presented by determining compound-specific linear response factor equations to derive sensitivity ratios (S-R) that allow conversion of GC/TOF-MS values into calibrated GC/FID data. Using alkenone mixtures of known composition and a variety of samples from natural environments, the applicability of this approach is demonstrated.