No abstract

As recently shown in Fourier transform ion cyclotron resonance mass spectrometry studies (Rapid Commun. Mass Spectrom. 11, 1602, 1997), photochromic systems can be used as molecular probe for the study of matrix-assited laser desorption ionization (MALDI) processes inducing the typical formation of the triply hydrogenated molecules [M 2H H] simultaneously with the color change. In this paper, a novel photochromic system blocked in its open form allows the proposal of a possible mechanism for protonated ions. The results suggest that H-atom transfer from a matrix molecule to an analyte plays an important role in the ionization step. The transferring H-atom may be derived from electronically excited states of matrix and analyte molecules via the triplet states. Among several ionization processes described in the literature, we observe that H-atom transfer following analyte photoionization is a possible ionization mechanism in MALDI. # 1998 John Wiley & Sons, Ltd. Received 15 July 1998; Revised 1 September 1998; Accepted 1 September 1998 Since its development, extensive efforts have been focused on understanding the mechanism of desorption and ionization in MALDI processes. Up to now, several different models have been proposed to explain the mechanism of desorption of large molecules in MALDI.1 However, the process by which ionized analytes are produced is still not fully understood and no adequate quantitative model for the complete process exists, despite extensive studies. 2-6In MALDI, evidence exists that analyte ionization occurs both in the surface and in the expanding plume. Initiated by the laser shot, ions may form by ion-molecule reactions (a collisional mechanism) between matrix molecular ions and the analyte with the participation of radical ions, 7 or may exist after excited state proton transfer (ESPT) between electronically excited matrix molecules and neutral analyte molecules, 8,9 or most probably they may originate from a combination of both processes. Due partly to the experimental conditions and partly to the character of the analyte itself, different ionization mechanisms can be activated. Proteins and peptides ionize almost exclusively by proton transfer, for other classes of compounds such as carbohydrates and synthetic polymers the dominant ionization process is the attachment of sodium, potassium ions. Recently, Allwood et al. 10 proposed a scheme in which thermionic electron emission from excited singlet states provide the main ionization route for species in MALDI. Thus, ionization processes in MALDI appear far more complex than were first believed.In our earlier paper, 11 the MALDI processes were studied by the structural change of photochromic probe incorporated in a matrix. The photochromic probes are constituted by a series of substituted spirooxazines exposed to the UV light. These compounds rapidly change color (transparent to blue in the time scale`10 À9 s) and return to the original color if the irradiation is stopped. The blue color is due to the opening of the spiromoiety t...

In a search for a better understanding of the desorption/ionization process in matrix-assisted laser desorption/ionization (MALDI), photochromic systems such as spirooxazines have been studied by Fourier transform ion cyclotron resonance mass spectrometry coupled with a laser interface. These compounds become intensely blue upon exposure to ultraviolet light and return to the original color if the radiation is stopped. The blue color is due to the opening of the carbon-oxygen bond, creating a delocalized π-electron photomerocyanine structure which absorbs light in the 500-650 nm range. The spirooxazines appear to be an excellent probe for the study of MALDI processes because the opening of the spiro structure induces the typical formation of the triply protonated molecules [M + 3H] + simultaneously with the color change. The source of hydrogen in the matrix has been deduced by using nicotinic acid, methyl nicotinate and deuterated nicotinic acid as matrices. The differences between the direct laser desorption (LD) (detection of molecular ion, M + .) and the MALDI process are discussed. A detailed mechanism of spiro-compound fragmentation is proposed.

Different compositions of micrometric particles of powdered iron oxides (Fe,-,O, Fe,O, and Fe,O,) are examined by time-of-flight laser microprobe mass spectrometry. The positive-ion cluster distributions show a strong correlation with the stoichiometry of iron in the oxide. This correlation is more striking when the resonant process of iron at 293.7 nm is used. The bond dissociation energy of Fe---0 in the bulk Fel-,O also affects the intensity distributions. It is very important to study and to identify the iron oxides because they could play a role in the toxicity of certain molecules which could be removed on the oxides.There is tremendous interest in gas-phase studies of well-characterized transition-metal systems'-4 to enhance our understanding of the chemistry and sometimes of the toxic, carcinogenic effects of transition metal oxide clusters and compounds.s The analysis of inorganic solids by time-of-flight laser microprobe mass spectrometry (TOF-LMMS) shows the formation of clusters in the mass spectra which can be used for the identification of inorganic particles.'-' A systematic study of the types of cluster induced and of their relative abundances is needed to indicate to what extent the mass spectra can be used as 'fingerprints' of the analysed compound. io-is Recently, resonance ionization spectroscopy coupled with mass spectrometry detection (RIMS) has been revealed in several laboratories as an interesting analytical technique.The outstanding features of resonance ionization spectroscopy (RIS) are sensitivity and selectivity with respect to the element or isotope under study, which make it a valuable tool for many applications in trace detection and in analysis of toxic and radioactive materials.''-23 The idea was then developed to use lower wavelengths (UV range) to ionize atoms in the gas phase by a two-photon ionization process (A(oi, ~o ' e -) A + )~~ for which two photons of the same laser pulse are successively absorbed by the atoms in the laser plume (screen effect) exciting them into their own ionization continuum. Increased sensitivity is consequently obtained with this kind of process. More recently, Nogar et al. showed the potential use of RIMS as a tool for the detection of copper in isolated plant cells. 2sIn this paper, we have applied resonant ionization of iron to study the non-stoichiometric Fe,-,O (to 1200 "C : 0.852 < (1 -x) < 0.953), Fe,04 and Fe203 compounds in positive mode by TOF-LMMS. The positiveion mass spectra are characterized by many differences between the clusters detected and their intensities. Critical parameters of the ionization are the laser wavelength, the energy of the pulse and the resonant character of the ionization. The determination of iron oxidation states has been performed by this technique which needs only the use of one UV-tunable laser for both desorption and resonant photoionization. The present investigation has been performed instrumen-* Author for correspondence.CCC 095 1-4 198/95/030225-05

The complexing abilities of two macrocycles, namely a 18-C-6 crown ether (CE 1) and a 20-C-6 crown ether (CE 2) towards one organic guest [protonated phenethylamine (PEAH +)], were investigated with help of matrix-assisted laser desorption/ionization (MALDI) Fourier transform mass spectrometry. A qualitative correlation between the complexation behavior of CE 1 in solution and in the gas phase of the spectrometer could be established. We concluded from these results that a fine control of laser irradiance and the use of a suitable sample protocol allowed the preservation of weak molecular interactions throughout the MALDI process. The observed positive ions can be considered as being already present in the deposited solid solution and merely liberated by the laser pulse.