Dear Editor, Recovery of organic matter from mineral-rich sediment and soils for stable isotope analyses using static dense media Light stable isotopes in organic matter (e.g., d 13 C, d 15 N, d 18 O, dD, d 34 S) enable a breadth of investigations in the environmental sciences. 1–3 However, organic matter is often highly dispersed in mineral-rich sediment and soils, and is thus difficult to reliably isolate for analysis. Moreover, both aquatic and terrestrial sediments contain abundant resistant polymers such as cellulose, keratin, and chitin which can be potentially recovered for multi-isotope proxy measurements for use in investigating past environmental change. Isolation of organic material without mineral contamination for further chemical purification would thus be beneficial, particularly for dD and d 18 O, where minerals may serve as contaminants during measurement by pyrolysis. In soils, modelling the carbon and nitrogen cycling of soil organic matter (SOM) requires the separation and apportioning of the organic matter to appropriate soil pools of differing residence time with minimal sample loss or cross-mixing. 4 Dense media separation is a relatively common technique for isolating heavy minerals 5 such as Zircon, Monazite and Apatite for chemical analysis and dating. It has also been suggested as an alternative to harsh chemical treatments for isolating pollen grains in mineral-rich sediment, 6 and more recently for recovering cellulose from lacustrine sediments for d 18 O analyses. 7 These techniques all use a dense media for separating the matrix from the analyte of interest. In the case where organic matter is of interest, ZnCl 2 , LST (lithium heteropolytungstates) or SPT (sodium polytungstate) is commonly used as these salts are non-toxic and can be mixed with DI-H 2 O (deionised water) to achieve a specific gravity between 1 and 2. Separation usually takes place using light centrifugation, and the organic floatant is either poured off or aspirated. 6,7 However, it is often difficult to retrieve all of the floatant as it tends to stick to the centrifuge tube walls, while at the same time not disturbing and remixing the separated fractions. Herein, we report a modified technique to quickly and effectively concentrate organic matter from mineral-rich sediments and soils using static dense media for stable isotope analysis. This method is modified from previously described techniques for extracting pollen, cellulose, chitin, and partitioning soils using dense media. Two examples are provided: (1) insect cuticles are recovered for stable carbon and nitrogen isotope analysis from Quaternary sub-fossil guano deposits, and (2) tropical soil is partitioned into light and heavy fractions for quantification of their carbon abundance and determination of carbon stable isotopic composition. Sufficient SPT (Sometu-Europe TM , Berlin, Germany) was dissolved in DI-H 2 O to achieve a density of 1.87 g/cm 3 , as most organic matter has a density <1.6 g/cm 3 , and most minerals of >2.0 g/cm 3 . Depending on the organic content of the sample, about 5–10 g of material is placed in a 50 mL centrifuge tube with 5 mL of the SPT solution. SPT was then added using a wash bottle to 15–20 mL and the solution vortexed until well mixed with enough SPT solution added to enable the organic material to float on the surface. The centrifuge tubes were placed in an ultrasonic bath for 20 min. After sonication, samples were centrifuged for 15 min at 1800 rpm and left overnight for further settling. After the sample had separated in the dense medium, it was carefully placed in a freezer in an upright position. When frozen, the sample was removed from the freezer and the surface material immediately either scraped off, or washed off using DI-H 2 O and placed in a beaker. DI-H 2 O from a wash bottle thawed the surface, enabling all surface material (material whose density is <1.87 g/cm 3 ) to be recovered. Depending on the application, a consistent amount of frozen SPT solution and surface material can be readily recovered. To remove SPT from the now organic rich sample, the floatant was placed on a filter and rinsed using DI-H 2 O. As fine filtration (e.g., 0.45 mm) was desired, it helped to add the sample in a number of aliquots with 10 mL of DI-H 2 O, and then let the DI-H 2 O filter through before adding more with assistance from a vacuum pump. Usually, only 200–250 mL of water was needed to completely filter the sample free of SPT. To isolate insect cuticles from cave guano sediment, we combined the dense media separation described above with a modified technique of Schimmelmann et al. 8 developed to purify natural chitin. Approximately 6 g of sub-fossil guano sediment is lipid-extracted using 2:1 dichloromethane/ methanol (v/v) in an accelerated solvent extractor (Dionex TM ASE 100; Dionex, Sunnyvale, CA, USA). Samples were extracted in three 5-min cycles at 1008C. Solvent-extracted sub-fossil guano was then density-separated using the above method. SPT is eliminated from the floatant by filtering through a 0.45 mm polycarbonate filter. The sample was washed free from the filter into a 50-mL centrifuge tube using 1 N NaOH, capped and placed in a water bath at 1008C for 0.5 h then neutralised by re-filtering with DI-H 2 O. Using 2 N HCl, the sample was washed from the filter paper into a beaker and left for 3 h to remove carbonate, filtered again using DI-H 2 O, washed free from the filter paper using DI- H 2 O, and freeze-dried. For tropical soils, we followed the size fractionation protocol aimed at evaluating SOM turnover in models like Roth-C. 9 It is not essential to use this particular methodology, and the static dense media technique will work in a range of soil fractionation methods. Particulate organic matter (POM) (herein defined as the light fraction >63 mm) is separated by SPT according to the above protocol. The light fraction was collected and passed through a 20 mm filter, and rinsed free of SPT. A homogenous sample of the heavy fraction was filtered using a pre-weighed 0.45 mm filter. Both fractions RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2010; 24: 165–168 Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rcm.4358 RCM Letter to the Editor Copyright # 2009 John Wiley & Sons, Ltd.