Validation of a LC–MS Method for the Determination of Urea Contamination in Market Teas

A universal surrogate matrix assay for urea measurement in clinical pharmacokinetic studies of respiratory diseases

In pharmacokinetic studies for respiratory diseases, urea is a commonly used dilution marker for volume normalization of various biological matrices, owing to the fact that urea diffuses freely throughout the body and is minimally affected by disease states. In this study, we developed a convenient liquid chromatography-tandem mass spectrometry (LC-MS/MS) surrogate matrix assay for accurate urea quantitation in plasma, serum and epithelial lining fluid. Different mass spectrometer platforms and ionization modes were compared in parallel. The LC method and mass spectrometer parameters were comprehensively optimized to reduce interferences, to smooth the baseline and to maximize the signal-to-noise ratio. Saline was selected as the surrogate matrix, and its suitability was confirmed by good parallelism and accurate quality control sample measurements. Reliable and robust assay performance was demonstrated by precision and accuracy, dilution integrity, sensitivity, recovery and stability, all of which met bioanalysis requirements to support clinical studies. The assay performance was also verified and better understood by comparing it with a colorimetric assay and to a surrogate analyte assay. The newly developed surrogate matrix assay has the potential to be further expanded for urea quantitation in numerous physiological matrices.

Simultaneous determination of ethyl carbamate and urea in Korean rice wine by ultra-performance liquid chromatography coupled with mass spectrometric detection

In this study, a rapid method for simultaneous detection of ethyl carbamate (EC) and urea in Korean rice wine was developed. To achieve quantitative analysis of EC and urea, the conditions for Ultra-performance liquid chromatography (UPLC) separation and atmospheric-pressure chemical ionization tandem mass spectrometry (APCI-MS/MS) detection were first optimized. Under the established conditions, the detection limit, relative standard deviation and linear range were 2.83μg/L, 3.75-5.96%, and 0.01-10.0mg/L, respectively, for urea; the corresponding values were 0.17μg/L, 1.06-4.01%, and 1.0-50.0μg/L, respectively, for EC. The correlation between the contents of EC and its precursor urea was determined under specific pH (3.5 and 4.5) and temperature (4, 25, and 50°C) conditions using the developed method. As a result, EC content was increased with greater temperature and lower pH. In Korean rice wine, urea was detected 0.19-1.37mg/L and EC was detected 2.0-7.7μg/L. The method developed in this study, which has the advantages of simplified sample preparation, low detection limits, and good selectivity, was successfully applied for the rapid analysis of EC and urea.

Development of a multi-matrix LC-MS/MS method for urea quantitation and its application in human respiratory disease studies

In human respiratory disease studies, liquid samples such as nasal secretion (NS), lung epithelial lining fluid (ELF), or upper airway mucosal lining fluid (MLF) are frequently collected, but their volumes often remain unknown. The lack of volume information makes it hard to estimate the actual concentration of recovered active pharmaceutical ingredient or biomarkers. Urea has been proposed to serve as a sample volume marker because it can freely diffuse through most body compartments and is less affected by disease states. Here, we report an easy and reliable LC-MS/MS method for cross-matrix measurement of urea in serum, plasma, universal transfer medium (UTM), synthetic absorptive matrix elution buffer 1 (SAMe1) and synthetic absorptive matrix elution buffer 2 (SAMe2) which are commonly sampled in human respiratory disease studies. The method uses two stable-isotope-labeled urea isotopologues, [¹⁵N₂]-urea and [¹³C,¹⁵N₂]-urea, as the surrogate analyte and the internal standard, respectively. This approach provides the best measurement consistency across different matrices. The analyte extraction was individually optimized in each matrix. Specifically in UTM, SAMe1 and SAMe2, the unique salting-out assisted liquid-liquid extraction (SALLE) not only dramatically reduces the matrix interferences but also improves the assay recovery. The use of an HILIC column largely increases the analyte retention. The typical run time is 3.6min which allows for high throughput analysis.