Matrix-assisted laser desorption spectrometers

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). 

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... 

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... 

Figure 3.14. Schematic diagram of the scanning microprobe matrix-assisted laser desorption ionisation (SMALDI) mass spectrometer. (Spengler and Hubert 2002.)...

Moyor, S. C. Marzilli, L. A. Woods, A. S. Laiko, V. V. Doroshenko, V. M. Cotter, R. J. Atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) on a quadrupole ion trap mass spectrometer. Int. I. Mass Spectrom. 2003, 226,133-150. 

Chien, B. M. Michael, S. M. Fubman, D. M. Enhancement of resolution in matrix-assisted laser desorption using an ion-trap storage/reflectron time-of-flight mass spectrometer. Rapid Comm. Mass Spectrom. 1993, 7,837-843. 

Brown, R. S. Lennon, J. J. Mass resolution improvement by incorporation of pulsed ion extraction in a matrix-assisted laser desorption/ionization linear time-of-flight mass spectrometer. Anal. Chem. 1995,67,1998-2003. 

S.T. Hsiao, M.C. Tseng, Y.R. Chen and G.R. Her, Analysis of polymer additives by matrix-assisted laser desorption ionisation/time of flight mass spectrometer using delayed extraction and collision induced dissociation, J. Chinese Chem. Soc., 48 (2001) 1017-1027. 

There is a recent hybrid between AP-MALDI and ESI, matrix-assisted laser desorption electrospray ionization (MALDESI) [202], where species desorbed from a MALDI target are subjected to an electrospray before entering the mass spectrometer. The method is similar to ELDI except that the analyte is embedded in a matrix as in MALDI. 

C. K. G. Piyadasa, P. Hakansson, and T. R. Ariyaratne. A Fligh Resolving Power Multiple Reflection Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometer. Rapid Commun. Mass Spectrom., 13(1999) 620-624. 

R. S. Brown and J. J. Lennon. Mass Resolution Improvement by Incorporation of Pulsed Ion Extraction in a Matrix-Assisted Laser Desorption/Ionization Linear Time-of-Flight Mass Spectrometer. Anal. Chem., 67(1995) 1998-2003. 

R. M. Whittal, L. M. Russon, S. R. Weinberger, and L. Li. Functional Wave Time-Lag Focusing Matrix-Assisted Laser Desorption/Ionization in a Linear Time-of-Flight Mass Spectrometer Improved Mass Accuracy. Anal Chem., 69(1997) 2147-2153. 

R. S. Annan, H. J. Kochling, J. A. Hill, and K. Biemann. Matrix-Assisted Laser Desorption Using a Fast-Atom Bombardment Ion Source and a Magnetic Mass Spectrometer. Rapid Commun. Mass Spectrom., 6(1992) 298-302. 

The development of mass spectroscopic techniques such as matrix assisted laser desorption (MALDI) and electrospray mass spectrometry has allowed the absolute determination of dendrimer perfection [7,8], For divergent dendrimers such as PAMAM and PPI, single flaws in the chemical structure can be measured as a function of generation to genealogically define an unreacted site of or a side reaction producing a loop at a particular generation level. Mass spectromet-ric results on dendrimers, not only demonstrate the extreme sensitivity of the technique, but also demonstrate the uniformity of the molecular mass. The polydispersity index of Mw/Mn for a G6 PAMAM dendrimer can be 1.0006 which is substantially narrower than that of living polymers of the same molecular mass [7],... 

Mass spectral data were obtained either on electrospray infusion (EDI) Model TSQ 700 mass spectrometer (Finnigan MAT) or a Matrix Assisted Laser Desorption Time-of-Flight (MALDI-TOF) (Finnigan, Model 2000). 

Fig. 3. Schematics of a matrix-assisted laser desorption time-of-flight mass spectrometer. Ions are desorbed from the target, accelerated into the machine, and left to drift freely in the flight tube. Heavy ions are slower than light ions and reach the detector later. The flight time is converted into mass using an appropriate calibration.

Fig. 6. Protein identification using a peptide map measured with a matrix-assisted laser desorption time-of-flight mass spectrometer. All the peptide extracted from the gel is measured and the set of masses is used in the database search. The mass resolution is in the order of 10,000. Individual isotopes of a 2.5 kDa peptide are clearly resolved.

The two membranes most used for protein work are nitrocellulose and polyvi-nylidene fluoride (PVDF). Both bind proteins at about 100 pg/cm2. Nitrocellulose is the best membrane to use in the initial stages of an experiment. PVDF is used when proteins are to be sequenced or placed into a (matrix-assisted laser desorption ionization) mass spectrometer. PVDF can withstand the harsh chemicals of protein sequenators and the heat generated by mass spectrometer lasers, whereas nitrocellulose cannot. 

Loboda A.V., Krutchinsky A.N., Bromisrski M., Ens W., and Standing K.G. (2000), A tandem quadrupole/time of flight mass spectrometer with a matrix-assisted laser desorption/ionization source design and performance, Rapid Commun. Mass Spectrom. 14, 1047-1057. 

Selby, D.S. Mlynski, V. Guilhaus, M. A 20 KV Orthogonal Acceleration Time-of-Flight Mass Spectrometer for Matrix-Assisted Laser Desorption/Ionization. Int. 

Matrix assisted laser desorption ionization time-of-flight (MALDI-TOE) mass spectrometry was carried out with a PerSeptive Biosystems Voyager-DE-RP MALDl-TOF mass spectrometer. A 337-nm UV nitrogen laser producing 3-ns pulses was used in the reflectron mode. The samples were prepared by mixing 10 pi of a 0.1 M HAc solution of the sample with 20 pi of a solution of 3 mg/1 a-cyano-4-hydroxy cinnamic acid in wafer. One pi of that solution was loaded on the gold-sample plate. 

Martin, R. L. Branda, E. L. Analysis of high mass peptides using a novel matrix-assisted laser desorption/ ionisation quadrupole ion trap time-of-flight mass spectrometer. Rapid Commun. Mass Spectrom. 2003, 17, 1358-1365. 

Figure 14.1 Schematic view of a mass spectrometer. Its basic parts are ion source, mass analyzer, and detector. Selected principles realized in modern mass spectrometers are assigned El—electron impact. Cl—chemical ionization, FAB—fast atom bombardment, ESI—electrospray ionization, MALDI—matrix-assisted laser desorption/ionization. Different combinations of ion formation with mass separation can be realized.

In 1974, Comarisov and Marshall60 developed Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). This technique allows mass spectrometric measurements at ultrahigh mass resolution (R = 100000-1000000), which is higher than that of any other type of mass spectrometer and has the highest mass accuracy at attomole detection limits. FTICR-MS is applied today together with soft ionization techniques, such as nano ESI (electrospray ionization) or MALDI (matrix assisted laser/desorption ionization) sources. 

ToF mass spectrometers as dynamic instruments gained popularity with the introduction of matrix assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) as effective pulsed ion sources for the soft ionization of large biomolecules (up to 10s dalton) due to their high ion transmission.38 ToF mass spectrometers, quadrupole analyzers and/or magnetic sector fields can be combined in tandem mass spectrometers (MS/MS) for the analysis of organic compounds. 

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