Yuan Ma

We report the association of inkjet and electrospray ionization MS to detect picoliter droplet, where the liquid volume and its position onto the tip can be precisely controlled to form ultrafine droplets for successive ionization of the analyte. Single rectangle pulse was applied to piezoelectric device on inkjet microchip for the ejection of each picoliter droplet, and it was controlled by a computer. The voltage and width of driving pulse for the inkjet were optimized to make reproducible ejection of the solvent with low viscosity. The volume of each droplet was about 600 pl, and a trigger of 10 droplets was selected as the best inlet mode taking relative standard derivation of the droplets into consideration. The target substrate used with high voltage to form ionization was graphite, after several attempts with some materials. High‐speed camera was used to capture the breaking‐up process of a droplet. The distance between the inkjet nozzle and the tip was set at 2 cm to avoid short circuit. The influences on the mass intensity of the diameter of the tip, the volume and the concentration of the sample were examined. The tip with a small diameter performed greater intensity, and the limit of detection decreased, whereas the small volume of liquid played high ionization efficiency. Linear regression in the range between 1 and 200 ppm for caffeine was conducted, where internal standard theobromine was used. Some real samples were also detected with the instrument. Copyright © 2013 John Wiley & Sons, Ltd.

The double-stranded DNA or circular plasmid DNA can be cleaved by the Ser-His dipeptide by hydrolysis of the DNA-phospho-diester bond. The proper sequential order of the amino acids serine and histidine is apparently crucial in its unique cleavage activity as compared to the other di- or tri-peptides containing one of these amino acids. An inverted sequence of this dipeptide to a His-Ser linkage renders the peptide ineffective in the cleavage of DNA. In addition to the DNA cleavage function, Ser-His is also capable of cleaving other molecules, e.g., proteins, esters and RNAs. The cooperative actions of the hydroxyl group and the basic groups in the serine and histidine or related amino acids can be found in contemporary enzymes, such as DNase, serine proteases, lipases, esterases, chymotrypsin, trypsin, and elastase, etc. The Ser-His and related oligopeptides might have played important roles in the evolution of enzyme functions.

The mass spectral behavior of 2,3‐dihydro‐4 H ‐1,3‐oxazin‐4‐ones has been investigated using electrospray ionization multi‐stage mass spectrometry (ESI‐MS ⁿ ). All compounds showed a predominant retro‐Diels‐Alder (RDA) fragmentation pathway, and a novel ring contraction reaction by loss of isocynates was also found. The fragmentation mechanisms proposed for 4 H ‐1,3‐oxazin‐4‐ones are supported by ESI‐MS/MS/MS spectra. Copyright © 2004 John Wiley & Sons, Ltd.

The mass spectrometric behavior of eleven 1‐( N ‐benzyloxycarbonyl(Cbz)amino)alkylphosphonate diesters was studied under positive ion electrospray ionization (ESI) conditions. Their fragmentation pathways are depicted and supported by tandem mass spectrometry. Besides the common eliminations of ether, benzyl alcohol, phosphite and an ether plus benzyl alcohol from molecular ions, the title compounds show a tendency to undergo an interesting intramolecular benzyl rearrangement to yield benzylphosphonate ions. The fragmentation patterns do not depend on the substituent attached to the α ‐carbon atom. Copyright © 2003 John Wiley & Sons, Ltd.