title: Quantification of citrulline by parallel fragmentation monitoring--a novel method using graphitized carbon nanoparticles and MALDI-TOF/TOF mass spectrometry.
abstract: BACKGROUND
  Selected reaction monitoring (SRM) is a reliable mass spectrometry (MS)-based technique for quantification of small molecules. However, it is not applicable to matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem MS (MALDI-TOF/TOF MS) instruments. This work presents a novel comparable MALDI-TOF/TOF MS technique, "Parallel Fragmentation Monitoring" (PFM), for high-throughput quantification of citrulline.
  
  
  METHODS
  Calibrator/sample solutions were spiked with internal standard that was a stable isotopic analog with 1 mass unit heavier than citrulline. Both citrulline and internal standard were isolated and fragmented in parallel by MALDI-TOF/TOF MS in the presence of graphitized carbon nanoparticles as matrix. The ratio of the peak intensities of the selected fragment of citrulline to that of internal standard was used to calibrate/calculate the concentrations of citrulline in samples.
  
  
  RESULTS
  Linear calibration curves were obtained in the range of 10-250 μmol/l citrulline with correlation coefficients ≥0.997. Stored calibration curve and batch-specific calibration curve produced highly similar measurement values. Within- and between-day CVs were 3.1-8.7% and 3.5-10.6%, respectively, illustrating the reliability and robustness of PFM.
  
  
  CONCLUSIONS
  Using citrulline for proof-of-concept, we have developed the PFM technique with tremendous potential for high-throughput quantification of amino acids and other small molecules.
authors.affiliation: Li Ka Shing Institute of Health Sciences, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, NT, Hong Kong SAR, China.
authors.forename: Eddy Wing Yin
authors.initials: EW
authors.lastname: Ng
authors.forename: Hugh Simon
authors.initials: HS
authors.lastname: Lam
authors.forename: Pak Cheung
authors.initials: PC
authors.lastname: Ng
authors.forename: Terence Chuen Wai
authors.initials: TC
authors.lastname: Poon
mesh_terms: D002244:Carbon / Q000737:chemistry*; D002956:Citrulline / Q000032:analysis*; D006108:Graphite / Q000737:chemistry; D006801:Humans; D053758:Nanoparticles / Q000737:chemistry*; D015203:Reproducibility of Results; D053719:Tandem Mass Spectrometry
chemical_list: D002956:Citrulline; D002244:Carbon; D006108:Graphite