Suppression of ionization and optimization of assay for 3-hydroxy fatty acids in house dust using ion-trap mass spectrometry.

Background: 3-Hydroxy fatty acids (3-OHFAs), components of lipid A of gram-negative bacteria are useful chemical markers of endotoxin.

Methods: We analyzed 3-OHFAs in house dust as trimethylsilyl (TMS) methyl ester derivatives in the electron impact ionization mode using gas chromatography ion-trap mass spectrometry. Linear calibrations with r > 0.995 were observed for all the 3-OHFA methyl ester external standards.

Results: Recovery efficiency experiments with house dust demonstrated that accurate quantification requires calibration curves to be subjected to phase separation and solid phase extraction (SPE) because of differing clean-up losses according to chain length of 3-OHFAs. Recovery experiments also demonstrated interference with detection of C16:0 by the ion trap, which may be due to suppression of ionization by a constituent in house dust. Interference was overcome by injecting 1:4 dilutions of derivatized samples. The range of recoveries was 89.3%-111.5% for 3-OHFAs added to house dust. The reproducibility of injections was high (CV for C14:0 = 2.36%). The limit of detection (LOD) was 0.15 ng/mg for each 3-OHFA.

Conclusions: The modifications we made included: use of 3-hydroxy C11:0 and C13:0 methyl esters as internal standards, subjecting calibration standards to phase separation and SPE; addition of water to phase separation; addition of 1-pentadecanol as a carrier; injecting 1:4 diluted TMS derivatives of 3-OHFAs; and monitoring both m/z 131 and 133 ions to improve stability of area measurements for product ions. This method of optimization establishes an appropriate technique for quantification of 3-OHFAs in house dust.