[Fundamental knowledge and master of mass spectrometry: important reminder and pitfall in quantitative analysis]

Recently, mass spectrometry has become frequently applied in various areas of research because of the easy handling of instruments. However, its use without an adequate understanding of the characteristics of mass spectrometry and/or with an unreliable analytical method often lead to the misinterpretation of data. Sample pretreatments such as deproteinization, liquid-liquid extraction, and solid-phase extraction are very important to improve detection sensitivity. In addition, during the electrospray ionization process, the components of droplets affect the ionization efficiency of analytes, and, therefore, the elimination of unwanted materials is essential for the development of a reliable analytical method. We experienced marked deviations in measurement values for the sirolimus blood concentration in the same sample. Post-column infusion experiments were very useful for identifying ionization suppression factors in analytical samples, and mass spectrometric analysis was effective for their identification. In consequence, lysophosphatidylcholines suppressed the ionization of sirolimus and an internal standard, ascomycin, and we removed them from analytical samples using a HybridSPE-pretreatment cartridge, which is a solid-phase extraction cartridge for capturing phospholipids from biological samples. The incorporation of this step into the sample pretreatment procedure led to the development of a reliable analytical method for the quantitative determination of sirolimus in human blood using liquid chromatography/electrospray ionization tandem mass spectrometry. In clinical practice, rapid and reliable analytical methods are required for the improvement of medical services, and, therefore, users of mass spectrometers should understand their characteristics.