Keywords: 
• monomethylalkanes(MMAs) /
• hydrocarbon source rock /
• characteristic ion /
• Kovats index /
• homomorphy factors

Abstract: Saturated hydrocarbons in source rock from member 8 of Yanchang Formation(Upper Triassic) in Ordos Basin were analyzed by GC-MS. Different isomers of monomethylalkanes (MMAs)( carbon number ranging from 12 to 31) have been measured on a rock sample from Well Wu 56 located in Wuqi area. MMAs, with different substituted methyl ranging from the second carbon to the middle carbon, have shorter retention time than the corresponding n-alkanes with identical carbon number.2-methylalkanes and 3-methylalkanes are the most abundant compounds among MMAs, while 4-methylalkanes and other MMAs are relatively low. The positions of the substituted methyl in the MMAs can be identified through typical fragment ions, recombination pattern for chemical bonds, and retention time by mass spectrometry. For example, it has been found that the substituted methyl positions of C₂₂-MMAs are C₂-C₁₁.The mass spectrometry is similar to the n-alkane with the same carbon number, m/z57 or m/z 71 is their base peak. They also show a predominant M⁺ peak and (M-15)⁺peak, suggesting a branched methyl. Moreover, the relative abundance decreases as the total carbon increases. When the methyl position is exceeding the fifth carbon, in comparison with n-alkanes, their major characteristic fragment ions are such m/z126,140,154,as even C_nH_2n,which are important to identify MMAs. Generally, a table about the characteristic fragment ions of MMAs are concluded. As well as gas chromatographic unseparated monomethylalkane isomers were resoluted by mass spectrometric deconvolution. Finally, by using of the Kovats index and the homomorphy factors for precise retention prediction and identification of the monomethylalkanes.