Dear Sir. Concerning your issue for the GCMC analysis of formalin. GC-MS method is quantitatively analyzes volatile reaction products, particularly formaldehyde and methanol. The reaction volatiles are cryogenically trapped, and then chromatographed with deuterated formaldehyde and methanol as internal standards (Journal of High Resolution Chromatography, Volume 14, Issue 4, pages 241–244, April 1991) , you can visit the following link and the file attached which may help you in your analysis: