Laser desorption/ionization applications

An application of surface-assisted laser desorption-ionization (SALDI) method for practical, ultrahigh sensitivity detection of aromatic amines by GC-MS is reported. The prototype analytical device for trace detection of different organic compounds is created. 

APPLICATION OF SURFACE-ASSISTED LASER DESORPTION IONIZATION TO THE DETECTION OF BIOMOLECULES... 

Matrix-assisted laser desorption/ionization (MALDI) is widely used for the detection of organic molecules. One of the limitations of the method is a strong matrix background in low-mass (up to 500-700 Da) range. In present work an alternative approach based on the application of rough matrix-less surfaces and known as surface-assisted laser desoi ption/ionization (SALDI), has been applied. 

Two relatively new techniques, matrix assisted laser desorption ionization-lime of flight mass spectrometry (MALDI-TOF) and electrospray ionization (FS1), offer new possibilities for analysis of polymers with molecular weights in the tens of thousands. PS molecular weights as high as 1.5 million have been determined by MALDI-TOF. Recent reviews on the application of these techniques to synthetic polymers include those by Ilantoif54 and Nielen.555 The methods have been much used to provide evidence for initiation and termination mechanisms in various forms of living and controlled radical polymerization.550 Some examples of the application of MALDI-TOF and ESI in end group determination are provided in Table 3.12. The table is not intended to be a comprehensive survey. 

Matrix-assisted laser desorption ionization (MALDI) is not yet a technique that has been used extensively for LC-MS applications. It is included here because it often provides analytical information complementary to that obtained from LC-MS with electrospray ionization, as illustrated later in Chapter 5. 

Sugiura Y, Shimma S, Setou M. Two-step matrix application technique to improve ionization efficiency for matrix-assisted laser desorption/ionization in imaging mass spectrometry. Anal. Chem. 2006 78 8227-8235. 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Also for MALDI, there is a special case worth mentioning. Surface-enhanced laser desorption/ionization (SELDI) is a technique that utilizes special sample plates [196, 197]. These have different modified surfaces, for example, hydrophobic, anionic, or antibody treated. Which type of surface to select depends on the application. After application of analyte the surface is washed according to a protocol leaving only the desired components on the target. Finally, a MALDI matrix is applied before analysis in the spectrometer. See Chapter 12 for an application example of SELDI. 

The development of soft ionization methods (electrospray ionization and matrix-assisted laser desorption ionization, and others not discussed here) has contributed to the remarkable progress seen in mass spectrometry applied to biochemistry and molecular biology research progress, and is beginning to find applications in archaeology. 

Zhou, G. H., Luo, G. A., Zhou, Y, Zhou, K. Y, Zhang, X. D., and Huang, L. Q. (1998). Application of capillary electrophoresis, liquid chromatography, electrospray mass spectrometry and matrix-assisted laser desorption/ionization time of flight mass spectrometry to the characterization of recombinant human erythropoietin. Electrophoresis 19, 2348—2355. 

Petkovic, M., Muller, J., Muller, M., Schiller, J., Arnold, K., Arnold, J. Application of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for monitoring the digestion of phosphatidylcholine by pancreatic phospholipase A2. Anal. Biochem. 2002, 308, 61-70. 

Kang, M.-J., Tholey, A., Heinzle, E. Application of automated matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for the measurement of enzyme activities. Rapid Commun. Mass Spectrom. 2001, 15, 1327-1333. 

This proposal describes the development of a new, systematic approach for qualitatively and quantitatively studying surface-biomolecule interactions by matrix-assisted laser desorption ionization (MALDl) mass spectrometry (MS). This methodology is being developed because of the profound importance that surface-biomolecule interactions play in applications where biomaterials come into contact with complex biological fluids, it can readily be shown that undesired reactions occurring in response to surface-biomolecule contact (protein adsorption, biofouling, immune response activation, etc.) lead to enormous economic and human costs. Thus, the development of analytical methodologies that allow for efficient assessment of the properties of new biomaterials and/or the study of detailed fundamental processes initiated upon surface-biomolecule contact are of critical value ... 

The exception to this is the application of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). In 1981, Barber and Liu and coworkers independently introduced the concept of employing MALDI where the absorption of the matrix is chosen to coincide with the wavelength of the employed laser to assist in the volatilization of materials. In 1988, Tanaka, Hillenkamp, and coworkers employed the laser as the energy source, giving birth to MALDI-MS. 

Nonier, M.F. et al., Application of off-line size-exclusion chromatographic fractionation-matrix assisted laser desorption ionization time of flight mass spectrometry for proanthocyanidin characterization, J. Chromatogr. A, 1033, 291, 2004. 

Tholey, A. and Fleinzle, E., Ionic (liquids) matrices for matrix-assisted laser desorption/ionization mass spectrometry - applications and perspectives. Anal Bioanal. Chem., 386, 24-37, 2006. 

The third focus is the use of IL as supports for MS. The application of the IL enabling electrospray ionization (ESI) using nonpolar solvents will be demonstrated. A major emphasis will be put on the use of ILs as matrices for matrix-assisted laser desorption/ionization (MALDI) MS, a field gaining emerging interest in the last few years. 

Nicola, A. J. et al.. Application of the fast-evaporation sample preparation method for improving quantification of angiotensin II by matrix-assisted laser desorption/ionization. Rapid Commun. Mass Spec., 9, 1164,1995. 

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