Matrix Assisted Laser Desorption Ionization

Matrix assisted laser desorption ionization (MALDI) has grown from the efforts to analyze macromolecules by mass spectrometry. Two groups were able, in the late 1980s, to obtain mass spectra of proteins. The first group was led by T. Tanaka [39] (Nobel Prize 2002) and developed MALDI where the analyte is mixed in a matrix of glycerol and cobalt and ionized with a laser. The second group formed by M. Karas and F. Hillenkamp [13] developed MALDI where the analyte is mixed with a matrix solution containing UV-absorbing molecules (Table 1.2). A few microliters of solution are spotted onto a MALDI target where the sample crystallizes.

After introduction of the target into the vacuum, an UV laser pulse is used to desorb and ionize the sample. Nitrogen laser emitting at 337 nm and Nd:YAG laser emitting at 355 nm are the most widely used. MALDI is a very powerful technique for the analysis of synthetics and natural biopolymers. It has completely replaced former techniques such as fast atom bombardment (FAB). In

Table 1.2 Commonly used matrices for matrix assisted laser desorption ionization.

Sinapinic acid a-Cyano-4-hydroxycinnamic acid 2,5-Dihydroxybenzoic acid

Proteins, imaging Peptides, small molecules Proteins

Sinapinic acid a-Cyano-4-hydroxycinnamic acid 2,5-Dihydroxybenzoic acid

Proteins, imaging Peptides, small molecules Proteins

Time Flight
Fig. 1.16 Mass spectra of a recombinant protein obtained by: (A) matrix assisted laser desorption ionization-time of flight, (B) electrospray-quadrupole time of flight. cps Counts per second.

most cases singly charged ions are predominantly detected while very little fragmentation or multiply charged ions are observed. MALDI is commonly used for the analysis of high molecular weight compounds such as peptides and proteins [40], synthetic polymers [41], DNA [42] and lipids [43].

MALDI has the intrinsic advantage over ESI-LC-MS in that it can achieve a high sample throughput. Sample preparation and separation can also be decoupled from the mass spectrometric analysis. The MALDI target plate can be easily archived, which allows simple reanalysis of selected samples. MALDI or ESI are suitable for the analysis of proteins, as depicted in Fig. 1.16. One of the key advantages of ESI over MALDI is the formation of multiply charged ions which allows the analysis of proteins on almost any type of mass analyzer while MALDI requires a time of flight mass analyzer in the linear mode to cover the high mass range.

The high throughput capability of MALDI and the different ionization mechanisms make this technique also an attractive alternative to electrospray ionization for the analysis of low relative molecular mass compounds (LRMM) [44]. However, interferences of matrix ions and the ionization of the low relative molecular mass compounds are the challenges of this technique [45, 46].

Desorption/ionization on porous silicon (DIOS) without any matrix has been described for the analysis of LRMM compounds with no chemical background [47, 48]. The use of MALDI for the analysis of small molecules was recently reported. Particularly attractive is the coupling of a MALDI source with a triple quadrupole mass analyzer for quantitative analysis in the selected reaction monitoring (SRM) mode due to very high analysis speed.

Surface enhanced laser desorption/ionization (SELDI) is a distinctive form of laser desorption ionization where the target plays an active role in the sample preparation procedure and ionization process [49]. Depending on the chemical or biochemical treatment, the SELDI surface acts as solid phase extraction or an affinity probe. Chromatographic surface is used for sample fractionation and purification of biological samples prior to direct analysis by laser desorption/ ionization. SELDI is mainly applied for protein profiling and in biomarker discovery by comparing protein profiles from control and patient groups.

Because MALDI is a desorption technique, it is particularly suited for the analysis of surfaces such as biological tissues [50]. In this application, the matrix is applied on the complete surface of the tissue. The laser resolution is about 100 mm and complete analyte distribution images (low molecular weight compounds, peptides, proteins) can be recorded [51, 52].

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