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positive charge for each part of the molecules on ionization. The data are presented with the NH2 groups being treated as single units. This allows for the fact that removal of electrons from the nitrogen is compensated for by increased polarization of the N-H bonds. For urea the oxygen gains only 0.17+ whereas the sulphur of thiourea gains 0.71+. By contrast each amino group in urea gains 0.48+ whereas each amino group in thiourea gains only 0.12+. This is in broad agreement with our earlier proposal that the charge is localized on N in ureas and on S in thioureas.The fact that both amino groups gain identical amounts of charge suggests a delocalized orbital extending over both nitrogens and the carbon. Ionization of urea is largely by removal of an electron from this orbital, weakening it and causing the N-C-N angle to reduce and the N-C bonds to lengthen. At the same time the carbonyl bond is strengthened as is shown by a small reduction in the C-0 bond length. Ionization of thiourea is largely by removal of a non-bonding electron from sulphur. This weakens the thiocarbonyl bond, which shows an increase in length, and allows the N-C-N orbital to increase its influence as the bond angle increases and the N-C bonds shorten.We plan to employ more complex calculations in further studies on this system, although we do not anticipate arriving at substantially different conclusions. We also intend extending this study to the methylated ureas and thioureas, and to amides and thioamides. AcknowledgementsWe are grateful for the use of the computer facilities at the University of London Computer Centre and the assistance of the advisory staff. We also acknowledge the assistance of Dr D. G. Peacock of the School of Pharmacy. We thank a referee for helpful comments. References1.

A new laser desorption mass spectrometer has been Interfaced to a liquid chromatograph using a moving stainless steel belt. Samples are sprayed on-line onto the belt under partial vacuum with a thermospray vaporizer. These samples are transported through a differentially pumped vacuum lock and ionized In the source of the mass spectrometer with 45 ns, 10s W/cm2 laser pulses from a Q-swltched Nd:YAG laser. Data on the performance of this new LC/LDMS are presented for several classes of nonvolatile, thermally labile blomolecules.An area of great analytical importance to organic and biomedical research is the development of mass spectrometric techniques for determination of high molecular weight (>1000 daltons), nonvolatile, thermally labile biomolecules (see reviews in ref 1-3). Some of the most successful approaches to this problem involve the use of desorption-ionization (DI) techniques where energetic beams of fission fragments, ions, atoms, or photons are used to desorb molecular ions from solid samples present in the source of a mass spectrometer. These DI techniques include plasma desorption mass spectrometry (PDMS) (4), secondary ion mass spectrometry (SIMS) (5,6), fast atom bombardment (FAB) (7, 8), and laser desorption mass spectrometry (LDMS) (9-19).

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