A literature search will show that SOM has been studied extensively, including fractionations of different types . Not my field, but most extractions use polar, often highly alkaline solvents.  Do you have a particular reason for suggesting organic solvents?  

Thank you for your answer, @Paul. I want to quantify organic materials of the SOM in a LC-MS. My colleague told me I need to extract these organic materials using a organic reagent but not water or salt solutions. So I aks for a help here.

SOC factions can be quantified by many physical and chemical methods but the goal results cannot be got by using these methods. Alkaline solvents can extract parts of SOM as described by previous literature. I want to know whether a preferable method is available to extract the SOM for quantification in the LC-MS.

Not my field, but last year I had an opportunity to listen to a seminar by Dr. Myrna Simpson (University of Toronto) on this subject.  Her research team has published extensively on these methods.  As an example, please see the following paper:

 Analysis of soil organic matter biomarkers by sequential chemical degradation and gas chromatography – mass spectrometry

http://onlinelibrary.wiley.com/doi/10.1002/jssc.200600243/abstract;jsessionid=7244A1085203492BF51707A21BB6C99D.f03t01

Another useful paper (open access) can be found in the following link:

Organic matter source and degradation as revealed by molecular biomarkers in agricultural soils of Yuanyang terrace (http://www.nature.com/articles/srep11074)

Thank you for the information, @Susantha. These papers are helpful to me.

Dear Wei, a large number of extractants have been used to recover organic substances from the soil. The organic matter extracted from soil using reagents like 0.5N NaOH and 0.1M Na4P2O7 is usually fractioned on the basis of solubility characteristics. Sequential fractionation scheme outlined is used to get different fractions of organic matter. The SOM is sub-divided into three classes of humic materials: 1. Fulvic acid (FA), which is soluble in alkalis and acids, 2. Humic acid (HA), which is soluble in alkali but insoluble in acid and 3. Humin, insoluble in both alkali and acid. The other fractions include hymatomelanic acid and the gray and brown humic acids. It is emphasized that FA, HA and humin are not the compounds of definite chemical composition but are a group of compounds having similar solubility characteristics. Reagents generally used as a extractant of organic constituent from the soils are: Strong bases like NaOH, Na2CO3: Neutral salts like NaF, Na4P2O7: Acid salts like organic chelates, acetylacetone, cupferron, 8-hydroxyquinoline, formic acid, acetone-H2O-HCL solvent, Hot 6N HCL, Hot 1N H2SO4 etc.

You could characterize organic matter, especially the stable fraction by techniques such as TGA, NMR spectroscopy or IR spectroscopy. NMR analysis provides reliable quantification and has the advantage of requiring little or no pretreatment to alter the characteristics of organic matter. To illustrate new techniques, I will recomine the following papers:

 Nebbioso, A., & Piccolo, A. (2011). Basis of a humeomics science: chemical fractionation and molecular characterization of humic biosuprastructures. Biomacromolecules, 12 (4), 1187-1199.

Nebbioso, A., & Piccolo, A. (2012). Advances in humeomics: enhanced structural identification of humic molecules after size fractionation of a soil humic acid. Analytica Chimica Acta, 720, 77-90.

Nebbioso, A., Mazzei, P., & Savy, D. (2014). Reduced complexity of multidimensional and diffusion NMR spectra of soil humic fractions as simplified by Humeomics. Chemical and Biological Technologies in Agriculture, 1 (1), 24.

Thanks for the response, @JC and Nelson. Actually I will analyze my samples using NMR to quantify different C functional groups. Combining with this method, I also hope to quantify different organic materials in the SOM. Therefore I ask for a help here to find a effective solvent which can be used to extract SOM to run in the LC-MS. I fastly read the suggested literature by you Nelson and find these papers are very useful to meet my need. Thank you all very much.

Dr. Wei, please remember that organic matter in the soil can be partitioned into different forms or pools. The size of individual pools varies seasonally and is dependent on soil type, vegetation and climate. The concept of pools of organic matter that differ in their susceptibilities of microbial decomposition and their longevity in soil provides basis for understanding the dynamic nature of soil organic matter and how nutrient availability is influenced by management practices and changes in soil environment. The important pools of organic matter are: Litter, Organic residues in various stages of decomposition, Microbial biomass, Faunal biomass, Below-ground plant constituents, Water soluble organics, Stable humus and Soil enzymes.