Mass accuracy improvement of reversed-phase liquid chromatography/electrospray ionization mass spectrometry based urinary metabolomic analysis by post-run calibration using sodium formate cluster ions

RATIONALE Typically, a batch metabolomics analysis using liquid chromatography/electrospray ionization time-of-flight mass spectrometry (LC/ESI-TOF MS) takes 2 to 3 days. However, the mass accuracy - which has an important influence on metabolite identification - can drift by as much as about 17 ppm in such a time period. In an untargeted urinary metabolomics analysis by reversed-phase liquid chromatography (RPLC)/ESI-MS, the signals of sodium formate cluster ions were detected at the column-washing step. The cluster ions were used to calibrate the mass spectrometer for more accurate detection. METHODS The spectra were calibrated post-run by the sodium formate cluster ions, which were used as the internal standard, in order to improve the mass accuracy. RESULTS In the analysis of urine samples, we calibrated the spectra acquired by the micrOTOF with the sodium cluster ions. In positive mode ESI, the average errors of these cluster ions were improved to ±0.48 ppm and in negative mode ESI, to ±0.94 ppm after calibration. The mass accuracy remained within ±0.01 ppm over the duration of 6.25 days. An error window of 4 ppm appears to be suitable for metabolite identification when using post-calibration. CONCLUSIONS The results showed that sodium formate cluster ions could be utilized for the calibration of LC/ESI-TOF MS and the average instrumental errors could be maintained at low levels for long-term analyses. This method could be applied not only to urine sample, but also to low sodium samples, such as saliva, by dissolving the sample in 1μM sodium formate solution. This method provides a good solution for accurate mass detection of metabolomic analysis.