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Analysis of C37 alkenone mixtures by gas chromatography (GC) with flame ionization detection (FID) and GC coupled to mass spectrometry (GC/MS) in the chemical ionization mode (CI) shows that the later is useful for paleotemperature estimation when ammonia is used as reagent gas. Conversely, the use of isobutane gives rise to Uk'37 readings that are dependent on the amount of C37 alkenones introduced in the system, being unreliable for paleoclimatic studies. However, ammonia CI GC/MS may produce Uk'37 measurements that deviate from those obtained by GC-FID, the method calibrated for temperature estimation from algal cultures and marine sedimentary data. The differences result from changes in relative sensitivity between the di- and triunsaturated alkenones and depend on the instrument used and operational conditions. This problem is solved in the present study by determination of the response factor linear equations for each alkenone and their average relative sensitivity (R) using mixtures of known composition. These parameters allow the transformation of the GC/MS readings into the GC-FID equivalents using the following equation: UG = R x UM/(1 - UM(1 - R)). Examples of the suitability of this approach are given.

Ambers from Peñacerrada (Basque Country, North Spain) have been analysed by IR spectroscopy and GC-MS in order to study the polymeric and solvent-soluble GC-amenable fractions, respectively. All samples showed branched monoalkybenzenes, bicyclic sesquiterpenoids and tricyclic diterpenoids related to pimaric acids precursors, suggesting a paleobotanic origin from araucarian species. Molecules containing oxygenated polar functionalities were not found, which is in agreement with the Cretaceous origin of the samples and in good correspondence with the IR spectra and with the low oxygen content estimated from elemental composition analysis. In addition, no IR bands for exocyclic double bonds and very few GC-amenable monounsaturated molecules were found, which provides further evidence of the loss of olefinic groups. Theses features show that the age-dependent loss of oxygenated and olefinic functions is a general process affecting the whole amber structure, not only the solvent soluble fraction.

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