Laser desorption time-of-flight mass spectrometry

• Main Author: Costello, Kevin Francis
• Published: University of Edinburgh 1991
• Subjects: 543.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.645018

The techniques of supersonic molecular beam cooling and laser multiphoton ionisation (MPI) spectroscopy have been combined to give a potentially powerful analytical technique. Two methods of sample introduction into the molecular beam have been employed, namely resistive heating and laser desorption. The resistive heating method allowed 2-colour MPI spectra of naphthalene, anthracene and perylene to be recorded in a simple free jet apparatus. A sensitivity for anthracene of 600 ppb is estimated. A time-of-flight (TOF) mass spectrometer has been developed, incorporating both linear and reflecting-geometry (reflectron) flight tubes, to allow laser desorption MPI (LD-MPI) mass spectra to be recorded for a number of involatile and thermally unstable compounds. Mass resolutions of 300 (linear) and 850 (reflectron) have been obtained for aniline. The constraints affecting the mass resolving power of both spectrometer designs are discussed. Finally, the potential of LD-MPI mass spectrometry as a sensitive, selective analytical technique is evaluated. The mass spectra of a number of polynuclear aromatic hydrocarbons, porphyrins and amino acids are presented, along with those of a simple mixture of the three aromatic amino acids tryptophan, tyrosine and phenylalanine. A sub-nanomole detection limit is estimated for tryptophan. Means to improve the sensitivity of the technique are discussed. The wider analytical applications of LD-MPI mass spectometry are considered.