MALDI disadvantages

This desorption ionisation technique leads to weak fragmentation. The analyte is incorporated into a solid organic matrix (such as hydroxybenzoic acid) and the mixture is placed on a sample holder that is irradiated with UV laser pulses (e.g. N2 laser, A = 337 nm, pulse width = 5 ns). The laser energy is absorbed by the matrix and transferred to the analyte, which becomes desorbed and ionised (Fig. 16.18c). Although MALDI is considered to be a soft ionisation technique, a substantial amount of energy is involved. Because the technique involves pulsed ionisation, it is well suited for time-of-flight mass analysis of biomolecules. The analysis of small molecules (M < 500 Da) is limited because the matrix decomposes upon absorption of the laser radiation. However, solid supports such as silicone can be used as the matrix to overcome this disadvantage. 

One of the biggest disadvantages of MALDI-TOF MS is its low resolution. For small proteins, the resolution of a linear TOF MS is around 1 500 (i.e., an ion with m/z of 10,000 can be distinguished from an equally abundant ion with a m/z of 10,020). The low resolution is due partly to the fact the ions produced by MALDI do not have uniform kinetic energy, but a distribution of kinetic energies. Thus, ions with the same m/z ratio arrive at the detector at different times giving rise to broad peaks and hence reducing the observed resolution. There are two ways in which the resolution can be increased. 

The major disadvantages of MALDI are (a) the strong dependence of the quality of data obtained on the sample preparation, (b) the low resolution of linear TOF instruments (without DE), and (c) the inability to interface the MALDI ion source directly to HPLC or CE. The advantages of MALDI are ... 

MALDI suffers from some disadvantages such as low shot-to-shot reproducibility and strong dependence on the sample preparation method. Each laser shot ablates a few layers of the deposit at the spot where the laser irradiates. This can produce variation in the shot-by-shot spectrum. Also, the impact position on the surface of the deposit can lead to spectral variations. Improvement of the deposit homogeneity gives a better reproducibility of the signal. This is very important if precise quantitative results must be obtained. A given... 

Disadvantages of MALDI-MS. Intense matrix background below 800 Da. 

More success was obtained using on-line solid phase extraction before offline analysis by capillary electrophoresis with matrix-assisted laser-desorption ionization (MALDI) MS [18]. Because microdialysis is a method used for analysis of small molecules and peptides, MALDI is not used frequently with microdia lysis sampling. Another disadvantage is that MALDI cannot be used on-line with the separation because it is a vacuum ionization technique. However, if the pep-tide is large enough (0 1000 Da) MALDI can be useful. For the analysis of peptides in dialysate an appropriate separation is important before mass spectrometric detection. In a comparison with direct sampling of dialysate in MALDI, capillary electrophoresis provides the high efficiency separations necessary to resolve all... 

Three soft ionisation methods are in use for earbohydrates, fast atom bombardment (FAB), eleetrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI). FAB is the oldest and involves directing a high-energy beam of Cs" ions or Xe atoms at the sample dissolved in a nonvolatile solvent such as m-nitrobenzyl alcohol. The atoms sputter the sample and matrix [M + H] or [M + Na]" ions are commonly observed. With an upper limit of M of about 2000, FAB is not that soft, and is usually used for small oligosaccharides it has the further disadvantage that the sample is prepared and then directly introduced into the mass spectrometer, so that it cannot be combined with liquid chromatography. 

Several modifications of MALDI have been developed to couple additional sampling and reaction capabilities to this technique. Surface-enhanced laser desorption ionization (SELDI) is one type of modified MALDI and describes an ionization process that involves reacting a sample with an enhanced surface. With SELDI, the sample interacts with a surface modified with some chemical functionality prior to laser desorption ionization and mass analysis. For example, an analyte could bind with receptors or affinity media on the surface, and be selectively captured and sampled by laser desorption. A SELDI surface can be modified for chemical (hydrophobic, ionic, immunoaffinity) or biochemical (antibody, DNA, enzyme, receptor) interactions with the sample. This technique can act as another dimension of separation or sample cleanup for analytes in complex matrices. As discussed before, one disadvantage of MALDI is that the matrix (usually a substituted cinnamic acid) that is mixed with the sample can directly interfere with the analysis of small molecules. There have been several areas of research to overcome this issue.Direct ionization on silicon (DIOS) is an example of a modification of MADLI that eliminates the matrix. In this case, analytes are captured on a silicon surface prior to laser desorption and ionization. Other examples of matrix-free laser desorption techniques include the use of siloxane or carbon-based polymers. 

ESI and MALDI have distinct advantages and disadvantages, such that they are complementary for many applications. Of the two techniques, ESI is the softer one, allowing for ionization of intact multi-molecule complexes however, in the presence of high concentrations of salts or other unwanted constituents, the formation of ions can be suppressed (making analysis impossible). This effect is less pronounced in MALDI, which can produce ionized products even in the presence of salts, but MALDI spectra tend to be very noisy in the mass range below 500 daltons (Da), due to the presence of matrix ions. MALDI usually... 

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