Anal. Chem. 2007, 79, 2940-2944
Analytical Methodology for TEX86 Paleothermometry by High-Performance Liquid Chromatography/Atmospheric Pressure Chemical Ionization-Mass Spectrometry Stefan Schouten,* Carme Huguet, Ellen C. Hopmans, Michiel V. M. Kienhuis, and Jaap S. Sinninghe Damste´
Department of Marine Biogeochemistry and Toxicology, Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB, Den Burg, Texel, The Netherlands
The TEX86 is a recently proposed paleothermometer through which ancient seawater temperatures of up to 120 My ago can be reconstructed. It is based on the relative distribution of glycerol dibiphytanyl glycerol tetraethers as measured by high-performance liquid chromatography/atmospheric pressure chemical ionizationmass spectrometry (HPLC/APCI-MS). The aim of this study was to examine and improve several analytical aspects in the determination of this important proxy in environmental matrices. Comparison of TEX86 analysis using single ion mode (SIM) and mass scanning (m/z 950 to 1450) detection, respectively, revealed that SIM is up to 2 orders of magnitude more sensitive and that the TEX86 can be determined with a reproducibility of (0.004 or (0.3 °C using this method. Comparison of TEX86 values obtained with two different HPLC/APCI-MS setups revealed no significant differences. In addition, analysis of TEX86 of extracts obtained by Soxhlet, ultrasonic, and accelerated high-pressure extraction techniques also showed no significant differences between the methods. Our results suggest that TEX86 analysis by HPLC/APCI-MS is robust and can be determined with analytical errors comparable to those of other temperature proxies. Reconstruction of temperatures of ancient oceans is of crucial importance in understanding past climate changes and generally relies on the chemical analysis of ancient sediments. Several of these chemical temperature proxies are currently used to reconstruct past seawater temperatures and they can be broadly subdivided into proxies based on inorganic or organic fossil remains. Inorganic temperature proxies include, for example, δ 18O and Mg/Ca ratios of foraminifera and transfer functions of foraminiferal assemblages (e.g., ref 1). The U37K′ ratio based on the relative distribution of di- and triunsaturated long-chain C37 alkenones derived from haptophyte algae was, until recently, the only proxy based on organic fossil remains. Originally proposed * Corresponding author. Fax: (+31) 222 319674. E-mail
[email protected]. (1) Lea, D. W. In The Ocean and Marine Geochemistry Vol. 6 Treatise on Geochemistry; Holland H. D.; Turekian, K. K., Eds.; Elsevier-Pergamon: Oxford, 2003; pp 365-390.
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as a temperature proxy by Brassell et al.2 based on sediment core studies, it was later validated by temperature-controlled culture studies of the haptophyte Emiliania huxleyi.3 The U37K′ is typically measured by determining the relative concentrations of long-chain alkenones using gas chromatography (GC) and a flame ionization detector, and an analytical precision of (0.2 °C can be achieved.4 The analytical sensitivity can be further improved by using GC chemical ionization mass spectrometry5 though this potentially also introduces analytical biases in determining this ratio.6 Recently, a second organic seawater temperature proxy based on archaeal glycerol dibiphytanyl glycerol tetraether (GDGT) lipids, the TEX86, was proposed.7 These lipids are biosynthesized by marine Crenarchaeota which are ubiquitous in marine environments, occur throughout the water column, and are one of the dominant prokaryotic groups in today’s oceans.8 Marine Crenarchaeota biosynthesize different types of GDGTs (i.e., GDGTs containing zero to three cyclopentyl moieties; GDGT 0-3; see structures in Figure 1) and crenarchaeol which, in addition to four cyclopentyl moieties, has a cyclohexyl moiety (GDGT 4).9,10 Finally, they also biosynthesize small quantities of a crenarchaeol regioisomer (GDGT 4′). A study of marine surface sediments showed that higher sea surface temperatures result in an increase in the relative amounts of GDGTs with two or more cyclopentyl moieties.7 Thus, measuring the relative amounts of GDGTs present in sediments allows determination of the temperature at which the Crenarchaeota were living when they produced their membranes. The TEX86 ratio was proposed as a means to quantify (2) Brassell, S. C.; Eglinton, G.; Marlowe, I. T.; Pflaumann, U.; Sarnthein, M. Nature 1986, 320, 129-133. (3) Prahl, F. G.; Wakeham, S. G. Nature 1987, 330, 367-369. (4) Herbert, T. D. In The Ocean and Marine Geochemistry Vol. 6 Treatise on Geochemistry; Holland H. D.; Turekian, K. K., Eds.; Elsevier-Pergamon: Oxford, 2003; pp 391-432. (5) Rosell-Mele´, A.; Carter, J. F.; Parry, A. T.; Eglinton, G. Anal. Chem 1995, 67, 1283-1289. (6) Schouten, S.; Hopmans, E. C.; Schefuβ, E.; Sinninghe Damste´, J. S. Earth Planet. Sci. Lett. 2002, 204, 265-274. (7) Chaler, R.; Grimalt, J. O.; Pelejero, C.; Calvo, E. Anal. Chem. 2000, 72, 5892-5897. (8) Karner, M.; DeLong, E. F.; and Karl, D. M. Nature 2001, 409, 507-510. (9) Schouten, S.; Hopmans, E. C.; Pancost, R. D.; Sinninghe Damste´, J. S. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 14421-14426. (10) Sinninghe Damste´, J. S.; Hopmans, E. C.; Schouten, S.; Van Duin, A. C. T.; Geenevasen, J. A. J. J. Lipid Res. 2002, 43, 1641-1651. 10.1021/ac062339v CCC: $37.00
© 2007 American Chemical Society Published on Web 02/21/2007
Figure 1. Base peak chromatogram of GDGTs eluting on (a) an Econosphere Amino column (4.6 × 250 mm, 5 µm) at 1 mL min-1 flow rate and (b) a Prevail Cyano column (2.1 × 150 mm, 3 µm) at 0.2 mL min-1. Insets show mass chromatograms and integrated peak areas of the four GDGTs used in the determination of the TEX86.
the relative abundance of GDGTs (see also Figure 1):
TEX86 ) [GDGT 2] + [GDGT 3] + [GDGT 4′] (1) [GDGT 1] + [GDGT 2] + [GDGT 3] + [GDGT 4′] The TEX86 was calibrated using over 40 sediment core-tops obtained from 15 different locations with the following resulting equation:
TEX86 ) 0.015 × T + 0.28
r2 ) 0.92
(2)
This equation allows the conversion of the calculated TEX86 values into SSTs (see, for details, ref 7).
This initial work has been followed by other studies which investigate the validity of the TEX86 as a SST proxy. For example, mesocosm experiments confirmed that marine Crenarchaeota change their membrane composition with growth temperature,11 and a survey of particulate organic matter showed that TEX86 values correlated well with in situ temperature at depths 0.7 is expected to be similar or even better due to the relatively high amounts of the different GDGTs used for determination of the TEX86, while for samples with TEX86 values 100 ng) concentrations down to 0.05 ng of GDGTs injected on column in the case of the Arabian Sea sample though with decreasing reproducibility (Figure 2). Thus, SIM yields reproducible TEX86 values at two orders (Arabian Sea) and one order (Drammensfjord) of magnitude lower concentrations than the scanning mode. At the lowest amounts of injected GDGTs where peaks could still be detected with SIM, TEX86 values start to significantly deviate Analytical Chemistry, Vol. 79, No. 7, April 1, 2007
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Figure 3. Cross-plot of TEX86 values determined using the TSQ Quantum Ultra (Thermo Inc.) and an Agilent 1100 MSD HPLC/APCIMS. Dashed line indicates the 1:1 line.
from those at higher concentrations, suggesting that care has to be taken when analyzing TEX86 close to the limit of detection of the GDGTs. Comparison of Mass Spectrometers. A selected set of sediment extracts, with a range of TEX86 values, were analyzed by HPLC/APCI-MS in SIM mode using two different mass spectrometers (i.e., the Agilent 1100 MSD SL and the Thermo Inc. TSQ Quantum Ultra Triple Quadrupole MS). The resulting TEX86 values from single analysis of similar quantities of GDGTs injected on column are cross-plotted in Figure 3 which shows that the TEX86 values are close to the 1:1 correlation line, suggesting little systematic instrumental bias. Thus, it seems that the TEX86 is not strongly dependent on APCI interface design and the mass spectrometer. However, it should be noted that this method was only tested in SIM mode on a quadrupole mass spectrometer, and it is unknown if the use of other mass spectrometry techniques such as ion trap mass spectrometers cause deviations. Recently, Escala et al.19 reported the analysis of TEX86 with an HPLC/APCI/ion trap-MS and a reproducibility of 0.012 or 0.8 °C, suggesting that this index can be determined on different types of instruments. Effect of Extraction Techniques. We also investigated whether the use of different extraction techniques could potentially bias the TEX86. Sediment from Drammensfjord was extracted in triplicate using ultrasonic extraction, Soxhlet extraction, and accelerated solvent extraction at high pressure and temperature. The results show that the TEX86 values obtained through the different techniques are identical within error (Figure 4). The average error is 0.010 or 0.7 °C which is twice as high as that of (19) Escala, M.; Rosell-Mele´, A.; Masque´, P. Org. Geochem. 2007, 38, 161-164. (20) Forster, A.; Schouten, S.; Moriya, K.; Wilson, P. A.; Sinninghe Damste´, J. S. Paleoceanography 2007, in press.
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Figure 4. TEX86 values of composite sediment of Drammensfjord extracted using ultrasonic, Soxhlet, and accelerated solvent extraction techniques. Error bars indicate (1σ standard deviation of triplicate extractions and triplicate analysis. Line represents the average TEX86 of all the experiments.
the analytical reproducibility (see above). However, this error now also includes errors due to different extraction techniques and the workup procedure (e.g., column chromatography, filtration) of the extracts. Thus, it seems that only relatively minor errors are introduced due to the workup procedures and that all these different extraction techniques can be used. Importantly, this includes accelerated solvent extraction which allows rapid extraction (
