Laser-desorption mass spectrometer

As indicated above, there are many advantages to LDMS analysis of peptide peaks that are candidates for Edman degradation. Since we have previously sent numerous LDMS targets for analysis by an "outside" facility, we know it is quite possible to use this approach even if a laser desorption mass spectrometer is not immediately available. 

The main features of the laser desorption mass spectrometer have been presented in detail elsewhere [33-36] and is schematized in Fig.l. Briefly, a solid soot sample is placed in a UHV chamber, separated from the mass-spectrometer chamber by a gate valve, on a liquid nitrogen cooled sample holder (to prevent sublimation of light PAHs due to their high vapour pressure at ambient temperature [37]). Once the pressure in the transfer chamber is low enough (<10 Torr), the gate valve is opened and the sample is translated into the desorption zone of the mass spectrometer (residual pressure 10 Torr). 

Concentration detection limits in CE-MS with the ESI interface are similar to those with UV detection. Sample sensitivity can be improved by using ion-trapping or time-of-flight (TOE) mass spectrometers. MS analysis can also be performed off-line, after appropriate sample collection, using plasma desorption-mass spectrometry (PD-MS) or matrix-assisted laser desorption-mass spectrometry (MALDI-MS). 

Figure 1.14 Laser desorption mass spectrum of Compound 2, the carbon-black filled vulcanised rubber compound, obtained using the LAMMA 1000 spectrometer Reproduced with permission from Waddell and co-workers. Rubber Chemistry and...

Finally, the mass analyzers used for laser desorption mass spectrometry experiments usually need to be synchronized to the laser s emission. The synchronization accuracy required varies from instrument to instrument. With time-of-flight instruments, the laser emission triggers the oscilloscope trace and must be known to within a few picoseconds. With ion trap, orthogonal time-of-flight, and Fourier transform mass spectrometers, usually the laser emission must be known to within a few microseconds at best. Thus, the electronics that are used to control the instruments can vary considerably. 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

There are two common occasions when rapid measurement is preferable. The first is with ionization sources using laser desorption or radionuclides. A pulse of ions is produced in a very short interval of time, often of the order of a few nanoseconds. If the mass spectrometer takes 1 sec to attempt to scan the range of ions produced, then clearly there will be no ions left by the time the scan has completed more than a few nanoseconds (ion traps excluded). If a point ion detector were to be used for this type of pulsed ionization, then after the beginning of the scan no more ions would reach the collector because there would not be any left The array collector overcomes this difficulty by detecting the ions produced all at the same instant. 

Tandem mass spectrometry (MS/MS) is a method for obtaining sequence and structural information by measurement of the mass-to-charge ratios of ionized molecules before and after dissociation reactions within a mass spectrometer which consists essentially of two mass spectrometers in tandem. In the first step, precursor ions are selected for further fragmentation by energy impact and interaction with a collision gas. The generated product ions can be analyzed by a second scan step. MS/MS measurements of peptides can be performed using electrospray or matrix-assisted laser desorption/ionization in combination with triple quadruple, ion trap, quadrupole-TOF (time-of-flight), TOF-TOF or ion cyclotron resonance MS. Tandem... 

The penultimate example of macrocycles based on phenyl and acetylenic units has been the very recent report by Tobe [801 and Rubin [81] of cyclophane 134. Both groups generated 134 in the mass spectrometer by laser desorption of hexa-protected polyynes 135 (robust) and 136 (unstable), respectively (Scheme 31). 

The usual method of detecting the desorbed molecules in TPR and laser desorption is with a quadrupole mass spectrometer placed a few centimeters from the surface of the crystal. The use of a quadrupole mass spectrometer limits the experiment in several... 

The chemical compositions of the isolated Au SR clusters were investigated by mass spectrometry [15,16,18, 22,32-35]. TEM was used to confirm that the species detected by the mass spectrometer represents the clusters in the sample. Figure 3a is a schematic representation of the top view of the mass spectrometer, which consists of five stages of differentially pumped vacuum chambers. The apparatus accommodates two t5 pes of ion sources, electrospray ionization (ESI) and laser-desorption ionization (EDI), and a time-of-flight (TOE) mass spectrometer with a reflectron. Details of the apparatus and the measurement protocols are described below. 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

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