Wavelength, desorbing laser

In a MALDl experiment, the sample is mixed or dissolved in a matrix material that has an absorption spectrum matching the laser wavelength of energy, The sample may not have a matching absorption peak (a), but this is not important because the matrix material absorbs the radiation, some of which is passed on to the dissolved sample. Neutral molecules and ions from both sample and matrix material are desorbed (b). 

The photoablation behaviour of a number of polymers has been described with the aid of the moving interface model. The kinetics of ablation is characterized by the rate constant k and a laser beam attenuation by the desorbing products is quantified by the screening coefficient 6. The polymer structure strongly influences the ablation parameters and some general trends are inferred. The deposition rates and yields of the ablation products can also be precisely measured with the quartz crystal microbalance. The yields usually depend on fluence, wavelength, polymer structure and background pressure. 

In this approach material is laser desorbed from a sample probe in front of a pulsed nozzle. The desorption laser is typically (but not exclusively) a Nd YAG laser operated at its fundamental wavelength of 1064 nm. At this wavelength one does not expect photochemical interaction with any of the nucleobases. Laser desorption involves heating of the substrate, rather than the adsorbate. Therefore it is typically... 

Figure 10.3 shows the IR-LDI mass spectrum of the peptide bradykinin (Mr = 1060.2) desorbed and ionized from the PDMS cover. The bradykinin was loaded into the chip at a concentration of 2.5 mM and injected into the chip channel using a field of 150 Y cm-1 for 9 min. The spectrum is the result of 10 laser shots at 2.95 pm wavelength. The off-line capillary gel microfluidic mass spectra are obtained after electrophoretic transport through a closed chip channel. Operation with a closed channel requires that the cover be removed from the chip prior to analysis. When removing the cover, sections of the gel tended to adhere to the surface of the PDMS. With some amount of care the entire gel lane could be extracted intact from the chip and, in many cases, irradiation of the PDMS cover resulted in the best mass spectra. The base peak... 

A notable exception regarding cost and complexity is the method of resonant laser ablation (RLA). In RLA, a low-fluence pulsed laser beam is focused onto the solid sample (Eiden et al., 1994). The laser pulse first ablates or desorbs a small amount of sample. On the timescale of the laser pulse (typically a few nanoseconds), the atoms liberated from the surface absorb laser photons and are ionized. By using a laser wavelength resonant with an atomic transition in the atoms of interest, ionization is highly selective. This method has been extensively demonstrated with relatively low-cost YAG pumped dye lasers. 

Among the various mass spectrometry techniques, MALDI is probably the most important as it provides an absolute method for molar mass determination and molar mass distribution, as well as information on end groups and copolymer composition. The MALDI process consists of the ablation of the polymer molecules dispersed in a matrix typically made up of aromatic organic acids. The matrix needs to be able to absorb at the wavelength of a laser (usually 337 nm). This process excites the matrix molecules, which vaporize at the same time, the polymer molecules desorb into the gas phase, where they are ionized. Thus, the role of the matrix is that of transferring the laser energy to the polymer molecnles. 

Fig.1 Schematic depictions of (a) ESI and (b) MALDI mass spectrometry, hi (a), the electric field causes an accumulation of positive charges at the liquid meniscus. When coulombic repulsion forces overcome the forces associated with surface tension, a spray is formed. Additional mechanisms (see text) lead to the release of analytes from the droplets (Reprinted with permission from Kebarle et al. [2], Copyright 1993 American Chemical Society). In (b), analytes are co-crystallized with a matrix which has an absorption maximum near the wavelength of the laser used to desorb the analytes for analysis by MS.

A secmid popular soft ionization method is MALDI. In this technique, analytes are co-crystallized with a matrix (typically a small, acidic, organic molecule), with an absorption maximum close to the wavelength of a laser used to irradiate the substrate. This process is typically performed in vacuum (although it has been shown to be feasible at atmospheric pressure) after the analytes have been desorbed and ionized (in MALDI, it is believed that fast heating caused by the laser pulse desorbs analytes into gas phase however, the process by which... 

A precondition for PI is that the analyte molecule must be present in the gas phase. Solid samples must be desorbed or vaporized prior to photoionization. Thermal vaporization [8] or laser desorption (LD) [9] are convenient means to convert the solid samples into a gas-phase plume. A combination of LD and laser ionization has been used successfully to analyze organic contantinants in water and soils [9]. A proper choice of the wavelength will exclude ionization of the bulk components of a real-world sample (e.g., N2, O2, CO2, or water). 

In early mass spectrometry applications of lasers, the sample was irradiated directly by a laser beam to desorb intact sample-related ions [27]. In this direct mode, termed laser desorption/ionization (LDI), the extent of energy transfer is, however, difficult to control and often leads to excessive thermal degradation. Also, not all compounds absorb radiation at the laser wavelength and thus are not amenable to LDI. Only those compounds that have mass below 1000 Da can be analyzed by LDI. Analytical sensitivity is also poor. A key contribution of LDI experiments is the observation that the desorption efficiency of amino acids and peptides that absorb the laser fight beam is greater than those without the chromophore [28]. IR lasers (e.g., an Nd YAG laser at 1.06 p m and a pulsed CO2 laser at 10.6 pm) and UV lasers (frequency-quadrapled Nd YAG laser at 266 nm) have aU been used. The detection of malaria parasites in blood by LDI with an N2 laser has been demonstrated [29]. 

The technical principle of MALDl imaging is summarized in Figure 4.1. A pulsed laser beam is focused to the size of the aspired lateral resolution. To date, mainly lasers with ultraviolet wavelengths (337, 355, and 266 nm) and pulse lengths of a few nanoseconds have been used. The focused laser beam is directed to the surface of the sample, which is then moved in steps in order to scan the sample according to the intended lateral resolution of the system. Before analysis, the sample must be prepared in such a way that the biomolecular ions can be desorbed and ionized by the laser beam, as in regular MALDl analyses. To achieve this, the sample (e.g., a biological tissue sample) must be covered with a suitable matrix, such as 2,5-dihydroxybenzoic acid (DHB), sinapinic acid (SA) or... 

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