Determination of Free Fatty Acids in Soil and Bryophyte Plants by Precolumn Derivatization via HPLC with Fluorescence Detection and Tandem Mass Spectrometry (HPLC‐MS/MS)

Abstract

A sensitive method for the determination of 30 kinds of free fatty acids (FFAs, C1‐C30) with 1‐[2‐(p‐toluenesulfonate)‐ethyl]‐2‐phenylimidazole‐[4,5‐f]‐9,10‐phenan‐ threne (TSPP) as labeling reagent and using high performance liquid chromatography with fluorescence detection and identification by online postcolumn mass spectrometry with atmospheric pressure chemical ionization (APCI) source in positive‐ion mode (HPLC/MS/APCI) has been developed. TSPP could easily and quickly label FFAs in the presence of K2CO3 catalyst at 90°C for 30 min in N,N‐dimethylformamide (DMF) solvent, and maximal labeling yields close to 100% were observed with a 5‐fold excess of molar reagent. Derivatives were stable enough to be efficiently analyzed by high performance liquid chromatography. TSPP was introduced into fatty acid molecules and effectively augmented MS ionization of fatty acid derivatives and led to regular MS and MS/MS information. The collision induced cleavage of protonated molecular ions formed specific fragment ions at m/z [MH]+(molecular ion), m/z [M′+CH2CH2]+(M′ was molecular mass of the corresponding FFA) and m/z 295.0 (the mass of protonated molecular core structure of TSPP). Fatty acid derivatives were separated on a reversed‐phase Eclipse XDB‐C8 column (4.6×150 mm, 5 µm, Agilent) with a good baseline resolution in combination with a gradient elution. Linear ranges of 30 FFAs are 2.441×10−3 to 20 µmol/L, detection limits are 3.24∼36.97 fmol (injection volume 10 µL, at a signal‐to‐noise ratio of 3, S/N 3:1). The mean interday precision ranged from 93.4 to 106.2% with the largest mean coefficients of variation (R.S.D.)<7.5%. The mean intraday precision for all standards was <6.4% of the expected concentration. Excellent linear responses were observed with correlation coefficients of >0.9991. Good compositional data could be obtained from the analysis of extracted fatty acids from as little as 200 mg of bryophyte plant samples. Therefore, the facile TSPP derivatization coupled with HPLC/MS/APCI analysis allowed the development of a highly sensitive method for the quantitation of trace levels of short and long chain fatty acids from biological and natural environmental samples.