Mass spectrometry desorption/ionisation MALDI

Fast atom bombardment (FAB) Plasma desorption (PD) Liquid secondary-ion mass spectrometry (LSIMS) Thermospray (TSP)/plasmaspray (PSP) Electrohydrodynamic ionisation (EHI) Multiphoton ionisation (MPI) Atmospheric pressure chemical ionisation (APCI) Electrospray ionisation (ESI) Ion spray (ISP) Matrix-assisted laser desorption/ionisation (MALDI) Atmospheric pressure photoionisation (APPI) Triple quadrupole (QQQ) Four sector (EBEB) Hybrid (EBQQ) Hybrid (EB-ToF, Q-ToF) Tandem ToF-ToF Photomultiplier... 

Mass spectrometry has assumed great importance in determinations of the molar masses of biological macromolecules, even quite large ones. This is due to developments such as electrospray ionisation (ESI) and matrix assisted laser desorption/ ionisation (MALDI), which have made it possible to determine the molar masses of biopolymers up to several 100 kDa (Pitt 1996 Kellner et al. 1999 Snyder 2000). The combination of MALDI techniques with time-of-flight mass spectrometers (MALDI-TOF) is of particular significance for determination of the molar masses of proteins with high sensitivity (typically pmol quantities, although exceptionally fmol) and precision (proteins up to 100 kDa with precision of about 0.01 %). Mass spectrometry can provide very accurate measurements of protein molar mass that can yield information about even minor structural modifications not readily accessible by other means. 

The classical area of application of mass spectrometry has been with small volatile compounds, although non-volatile samples could be analysed if they were suitably derivatised. The application of mass spectrometry to large complex molecules like proteins has been made possible by the development of novel ionisation techniques which enable large molecules (> 200 kDa) to be introduced into the mass spectrometer in an intact form suitable for analysis (Siuzdak 1996 Dass 2000). Of the various techniques that have been developed, electrospray ionisation (ESI) and matrix-assisted laser desorption ionisation (MALDI) are the ones best suited for use with macromolecules and macromolecular complexes. 

Although FAB has been used in polymer analysis, problems with fragmentation and the relatively low mass limit has made this less popular as new techniques have emerged. Plasma desorption has been used successfully but this too has waned in popularity with commercial spectrometers not really readily available. To a large extent polymer mass spectrometry equates to MALDI time-of-flight and the remainder of this article will bear this in mind. However, the use of electrospray ionisation (ESI) will be considered in conjunction with either quadrupole detectors or ion cyclotron resonance (ICR) N. B. ICR detectors can also be used with MALDI, as this is important and probably not as widely used as it could be. 

Mass spectroscopy [electron ionisation (El), chemical ionisation (Cl), electrospray ionisation (ESI), fast atom bombardment (FAB), matrix-associated laser desorption ionisation (MALDI), inductively coupled plasma-mass spectrometry (ICP-MS)... 

The sensitive requirements for exposure assessment have been met by the recent development of mass spectrometry (MS) techniques for the characterisation of proteins expressed by a genome, tissue or cell. Of particular interest is the matrix-assisted laser-desorption/ionisation (MALDI), which makes it possible to obtain protein mass fingerprinting for a wide range of proteins by MS. This method involves selectively cutting proteins by enzymatic actions, and comparing the fragment masses with theoretical peptides available in bio-informatic databases. 

Experimental considerations Sample preparation and data evaluation are similar to membrane osmometry. Since there is no lower cut-off as in membrane osmometry, the method is very sensitive to low molar mass impurities like residual solvent and monomers. As a consequence, the method is more suitable for oligomers and short polymers with molar masses up to (M)n 50kg/mol. Today, vapour pressure osmometry faces strong competition from mass spectrometry techniques such as matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS) [20,21]. Nevertheless, vapour pressure osmometry still has advantages in cases where fragmentation issues or molar mass-dependent desorption and ionization probabilities come into play. 

Mass spectrometry is used to identify unknown compounds by means of their fragmentation pattern after electron impact. This pattern provides structural information. Mixtures of compounds must be separated by chromatography beforehand, e.g. gas chromatography/mass spectrometry (GC-MS) because fragments of different compounds may be superposed, thus making spectral interpretation complicated or impossible. To obtain complementary information about complex mixtures as a whole, it may be advantageous to have only one peak for each compound that corresponds to its molecular mass ([M]+). Even for thermally labile, nonvolatile compounds, this can be achieved by so-called soft desorption/ionisation techniques that evaporate and ionise the analytes without fragmentation, e.g. matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). 

Below we report methodological studies based upon HPLC, GC/FID, GC-MS, LC-MS, matrix-assisted laser desorption ionisation coupled with time-of-flight mass spectrometry (MALDI-ToF/MS), CE, proton nuclear magnetic resonance ( I INMR), RIA and enzymatic colorimetric techniques. 

The only mass spectrometric methods available during the era of the first cascade synthesis in 1978 [30] were electron impact (El) and field desorption (FD) mass spectrometry [31]. Fast atom bombardment (FAB) mass spectrometry is limited to fairly low mass ranges and not very suitable for compounds of low polarity. It was not until the development of new and gentle ionisation methods such as MALDI (matrix-assisted laser desorption ionization) [32] and ESI (electrospray ionization) [33] that the conditions were fulfilled for the start of intense research in the field of dendrimer chemistry. The following section will present the special features of these mass-spectrometric methods and their importance in dendrimer analysis. 

The mass spectrometry analysis was performed by the matrix assisted laser desorption/ionisation time-of-flight (S8-MALDI) technique using a Voyager-DE PRO Biospectrometry Workstation (Applied Biosystems, USA). Radiation pulses of 0.5 ns and 3 Hz frequency from N2 laser operating at 337 nm were used to desorb the species and negative/positive ions formed were detected in reflectron mode. Sulfur used as a matrix material was also dissolved in toluene and mixed with the samples solution prior to deposition onto a target. 

MALDI-MS Matrix-assisted laser desorption/ionisation mass spectrometry... 

The two additives have heen identified hy matrix assisted laser desorption ionisation mass spectrometry (MALDI-MS) and attenuated total reflection infrared spectroscopy (ATR). 

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