Lasers desorption ionization

Positive ions are usually generated in these techniques, as in mass spectrometry, by electron bombardment. Photo-ionization, laser desorption and chemical ionization have also been used for specific applications. When the ionizing energy is above the ionization potential of the parent compound, this procedure may result in the formation of several fragment ions. Collisions of parent and fragment ions with neutral molecules may then give rise to a sequence of chemical reactions. 

Laser desorption ionization Laser desorption postionization Levofloxacin... 

Desorption of analytes by a pulsed CO2 laser from a TLC plate, followed by chemical ionization (laser desorption-chemical ionization, LD-CI), was already reported in 1983 [46]. Later, desorption by an infrared (IR) laser and two-photon ionization were... 

Solvent-free separation is especially powerful in parmership with solvent-free ion-ization.( Here we discuss a few milestones critically influencing the production of ions of the entire sample to be separated and mass detected. We demonstrated that solvent-free and matrix-ionized laser/desorption (MILD) meth-... 

However Zare has pointed out in [32] that the nature of the recorded mass spectra (intensity and occurrence of the PAH signals, fragmentation...) highly depends on various parameters involved in this method (e.g. energy, beam geometry and wavelength of the desorption and - in case of two-steps configuration - ionization lasers, desorption/ionization delay, etc...). 

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). 

FigureBl.7.2. Schematic representations of alternative ionization methods to El and PI (a) fast-atom bombardment in which a beam of keV atoms desorbs solute from a matrix (b) matrix-assisted laser desorption ionization and (c) electrospray ionization.

Karas M and Hlllenkamp F 1988 Laser desorption Ionization of proteins with moleoular masses exoeedlng 10,000 Daltons Anal. Chem. 60 2299-301... 

Laser desorption is commonly used for pyrolysis/mass spectrometry, in which small samples are heated very rapidly to high temperatures to vaporize them before they are ionized. In this application of lasers, very small samples are used, and the intention is not simply to vaporize intact molecules but also to cause characteristic degradation. 

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

Until about the 1990s, visible light played little intrinsic part in the development of mainstream mass spectrometry for analysis, but, more recently, lasers have become very important as ionization and ablation sources, particularly for polar organic substances (matrix-assisted laser desorption ionization, MALDI) and intractable solids (isotope analysis), respectively. 

A further important property of the two instruments concerns the nature of any ion sources used with them. Magnetic-sector instruments work best with a continuous ion beam produced with an electron ionization or chemical ionization source. Sources that produce pulses of ions, such as with laser desorption or radioactive (Californium) sources, are not compatible with the need for a continuous beam. However, these pulsed sources are ideal for the TOF analyzer because, in such a system, ions of all m/z values must begin their flight to the ion detector at the same instant in... 

On the other hand, there are some ionization techniques that are very useful, particularly at very high mass, but produce ions only in pulses. For these sources, the ion extraction field can be left on continuously. Two prominent examples are Californium radionuclide and laser desorption ionization. In the former, nuclear disintegration occurs within a very short time frame to give a... 

Although the above has considered only the use of a continuous main ion beam, which is then pulsed, it is not necessary for the initial beam to be continuous it too can be pulsed. For example, laser desorption uses pulses of laser light to effect ionization, and the main ion beam already... 

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. 

Some solid materials are very intractable to analysis by standard methods and cannot be easily vaporized or dissolved in common solvents. Glass, bone, dried paint, and archaeological samples are common examples. These materials would now be examined by laser ablation, a technique that produces an aerosol of particulate matter. The laser can be used in its defocused mode for surface profiling or in its focused mode for depth profiling. Interestingly, lasers can be used to vaporize even thermally labile materials through use of the matrix-assisted laser desorption ionization (MALDI) method variant. 

El = electron ionization Cl = chemical ionization ES = electrospray APCI = atmospheric-pressure chemical ionization MALDI = matrix-assisted laser desorption ionization PT = plasma torch (isotope ratios) TI = thermal (surface) ionization (isotope ratios). 

Mass-Analyzed Laser Desorption Ionization (MALDI)... 

The above direct process does not produce a high yield of ions, but it does form many molecules in the vapor phase. The yield of ions can be greatly increased by applying a second ionization method (e.g., electarn ionization) to the vaporized molecules. Therefore, laser desorption is often used in conjunction with a second ionization step, such as electron ionization, chemical ionization, or even a second laser ionization pulse. 

Ionization can be improved in many cases by placing the sample in a matrix formed from sinapic acid, nicotinic acid, or other materials. This variant of laser desorption is known as matrix-assisted laser desorption ionization (MALDI). The vaporized acids transfer protons to sample molecules (M) to produce protonated ions [M + H]+. 

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. If the sample examined is dissolved as a solid solution in a matrix, the rapid expansion of the matrix, often an organic acid, covolatilizes the entrained sample. Proton transfer from the matrix occurs to give protonated molecular ions of the sample. Normally thermally unstable, polar biomolecules such as proteins give good yields of protonated ions. This is the basis of matrix-assisted laser desorption ionization (MALDI). 

The three techniques — laser desorption ionization, laser ablation with secondary ionization, and matrix-assisted laser desorption — are all used for mass spectrometry of a wide variety of substances from rock, ceramics, and bone to proteins, peptides, and oligonucleotides. 

TOF mass spectrometry is ideally suited to those ionization methods that inherently produce ions in pulses, as with pulsed laser desorption or Cf-radionuclide ionization. 

Laser-desorption mass spectrometry (LDMS) or matrix-assisted laser desorption ionization (MALDI) coupled to a time-of-flight analyzer produces protonated or deprotonated molecular ion clusters for peptides and proteins up to masses of several thousand. 

MALDI. matrix-assisted laser desorption ionization... 

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