An inter-laboratory investigation of the Arctic sea ice biomarker proxy IP₂₅ in marine sediments: key outcomes and recommendations

Abstract. We describe the results of an inter-laboratory investigation into the identification and quantification of the Arctic sea ice biomarker proxy IP₂₅ in marine sediments. Seven laboratories took part in the study, which consisted of the analysis of IP₂₅ in a series of sediment samples from different regions of the Arctic, sub-Arctic and Antarctic, additional sediment extracts and purified standards. The results obtained allowed 4 key outcomes to be determined. First, IP₂₅ was identified by all laboratories in sediments from the Canadian Arctic with inter-laboratory variation in IP₂₅ concentration being substantially larger than within individual laboratories. This greater variation between laboratories was attributed to the difficulty in accurately determining instrumental response factors for IP₂₅, even though laboratories were supplied with appropriate standards. Second, the identification of IP₂₅ by 3 laboratories in sediment from SW Iceland that was believed to represent a blank, was interpreted as representing a better limit of detection or quantification for such laboratories, contamination or mis-identification. These alternatives could not be distinguished conclusively with the data available, although it is noted that the precision of these data was significantly poorer compared with the other IP₂₅ concentration measurements. Third, 3 laboratories reported the occurrence of IP₂₅ in a sediment sample from the Antarctic Peninsula even though this biomarker is believed to be absent from the Southern Ocean. This anomaly is attributed to a combined chromatographic and mass spectrometric interference that results from the presence of a di-unsaturated highly branched isoprenoid (HBI) pseudo-homologue of IP₂₅ that occurs in Antarctic sediments. Finally, data are presented that suggest that extraction of IP₂₅ is consistent between Accelerated Solvent Extraction (ASE) and sonication methods and that IP₂₅ concentrations based on 7-hexylnonadecane as an internal standard are comparable using these methods. Recoveries of some more unsaturated HBIs and the internal standard 9-octylheptadecene, however, were lower with the ASE procedure, possibly due to partial degradation of these more reactive chemicals as a result of higher temperatures employed with this method. For future measurements, we recommend the use of reference sediment material with known concentration(s) of IP₂₅ for determining and routinely monitoring instrumental response factors. Given the significance placed on the presence (or otherwise) of IP₂₅ in marine sediments, some further recommendations pertaining to quality control are made that should also enable the two main anomalies identified here to be addressed.