In a recent experiment, we made an effort to identify kairomonal compounds from plant volatile samples through suction in a closed arena. The sample when subjected to GC-MS detected many saturated fatty acids like myristic and palmitic acid. Is it possible for saturated fatty acids to get detected through GC-MS, as they do not have volatility?
Hi Nitin Kulkarni,
No its not possible to analyse fatty acids by GC or GCMS directly without derivatization.
If you are checking with GCMS library then check how much similarity index is there
and what sort of difference in mass spectrums.
you also need to check the traetments given to plant extracts before inejction on GCMS.
sometimes free fatty acids in acidic condition with certain solvents may be detected by GC/GCMS.
With the high temperature GC and the develepment of stabel columns a detection of fatty acids by GC is possible. See ISO method for milk fat (ISO 17678 /I DF 202) or the analysing of fatty acids of cacao butter with a 5 m long and nonpolar capillary column.
I agree with Dr. Santosh, fatty acids must be derivatized first (methyl esters) whether saturated or unsaturated and then run on GC or GC/MS. But please try to explore the way to extract the triglycerides from the plant material.
You asked "Is it possible for saturated fatty acids to get detected through GC-MS..?" The answer is unequivocally yes, they can be detected. Their vapor pressure rises very rapidly with temperature, making them volatile at elevated GC temperatures. They are often parts of ultrafine atmospheric particles, are often associated with humidity and moisture, are frequently emitted from plants and microbes, and can be generated through the thermal, hydrolytic, and oxidative degradation of related compounds, especially in the atmosphere. Others have stated you must derivatize them for GC. If you want reliable quantitation that too is true. It is also true that saturated fatty acids' intense alkane structures are prone to EI-MS library mis-identification due extreme MS similarity between all alkane type spectra. Not only can one saturated fatty acid be misidentified by EI-MS as another fatty acid, but other alkane structures can be misidentified as fatty acids. Finally fatty acid amides, esters, MAGs, DAGs, TAGs and other related compounds can either degrade to free fatty acids, or their alkane structures can be mis-id'ed as fatty acids. MS similarity indexes using forward AND reverse matching can help, but retention index examinations, and peak shape can also quickly tell you if the peak in question might really be a free fatty acid.
Thank you Luke. What characters in peak structure should be looked for ?
I routinely analyse free fatty acids up to C18 in biological materials by GC-MS without derivatization, using a polyethylene glycol (RTXwax) column from Restek. As long as the inlet and column are properly deactivated they elute as sharp, symmetrical peaks.
Direct determination of FA's by GLC have some problems and require great care as stated by Experts themselves, these are reliable quantitation ? wrong identification ? specialized column, extreme temperature, outlet and inlet deactivation and possibly capillary column deterioration after prolong use.
The analysis of FAME's is the most quick, simple, clean and reliable qualitative and quantitative analysis in oils and fats.
Standard methods are available for oils and fats (which mostly consist of truglycerides not FFAs) based on derivatization to FAMES. With other matrices, such as the urine and faeces that I analyse, or the emissions from plants that the OP is interested in, the presence of other important compounds that would be degraded by the esterification dictates that the FFAs be analysed in their free acid form.
The structural similarities among free acids that make their mass spectra similar and their identification solely by MS problematic are carried through to their methyl esters. For both esters and FFAs identification of chain length requires comparison on elution time with standards.
Modern capillary columns and inlet liners are deactivated well enough for the elution of nanograms of free acids are symmetrical peaks. This does not require specialised columns, or extreme temperatures.
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