Surface-assisted laser desorption/ionization

Koichi Tanaka performed the first successful LDI experiments on proteins. Since the original 1999 nanostructured silicon SALDI was introduced as a promising method with potential applications in systems biology, particularly metabolomics. The introduction of nanomaterials as SALDI substrates attracted researchers in analytical chemistry. Such materials include carbon nanotubes (CNTs), metallic nanoparticles like Ag, Pt, Au, and nanostructured surfaces. This development of substrates allowed for further development of SALDI. The development of desorption/ionization on silicon (DIOS)-MS and nano-assisted laser desorption/ionization (NALDI), has also attracted the attention of analytical scientists. These methods have since become a benchmark for semiconductor-based SALDI research. == Basic principles ==

The main principle of SALDI relies on a medium that absorbs energy from a laser and then transfers the energy to the target sample. This class of techniques where the bulk of energy goes to the substrate instead of the sample molecules is known as soft ionization techniques. The development of SALDI started as a modification of matrix-assisted laser desorption/ionization (MALDI). The former technique suffered from ionization interference from the matrix molecules of MALDI. SALDI substituted an active surface of specific substrates, usually made of inorganic components, for the organic matrix of MALDI. 1) The optical absorption coefficient: as this increases the ability of the substrate to absorb and generate more heat when absorb energy increases. 2) The heat capacity: as this decreases, the same amount of heat induces a larger temperature increase. 3) The heat conductivity: as this decreases, the substrate is better able to maintain the high temperature; therefore, the adsorption, desorption and ionization of the analytes occur more rapidly and effectively. There are three classes of nanomaterials that are utilized in SALDI-MS. Namely, the carbon-based, semiconductor-based and metallic-based. Carbon nanotubes and carbon-based SALDI The term carbon nanotube refers to a cylinder with a rolled graphene sheet. CNT can be single walled (SWNT) or multi-walled (MWNT). The SWNTs are perfect simulators of an ideal blackbody in the electromagnetic radiation ranging from the UV to far infrared. They exhibit better performance than former materials like super black, (a chemically etched nickel-phosphorus alloy). This makes the CNT's a desired material for laser mass spectrometry applications. That's why they attracted the researchers since discovery in the year 1991. Graphene as a surface material Graphene is a type of popular carbon nanomaterial discovered in 2004. It has a large surface area that could effectively attach the analyte molecules. On the other hand, the efficiency of desorption/ionization for analytes on a layer of graphene can be enhanced by its simple monolayer structure and unique electronic properties. Polar compounds including amino acids, polyamines, anticancer drugs, and nucleosides can be successfully analyzed. In addition, nonpolar molecules can be analyzed with high resolution and sensitivity due to the hydrophobic nature of graphene itself. Compared with a conventional matrix, graphene exhibits a high desorption/ionization efficiency for nonpolar compounds. The graphene substrate functions as a substrate to trap analytes and it transfers energy to the analytes upon laser irradiation, which allows for the analytes to be readily desorbed/ionized and the interference of matrix to be eliminated. It has been demonstrated that the use of graphene as a substrate material avoids the fragmentation of analytes and provides good reproducibility and a high salt tolerance. Nanostructured semiconductor-based SALDI Porous silicon as a substrate material Porous silicon acted as an effective substrate for SALDI, its porous structure helped in trapping the analytes and its unique optical activity transferred effectively the laser energy to the adsorbate. It was effective for analyzing wide range of biological small molecules. recently, a new technique named nanostructure Imaging mass spectrometry (NIMS) was introduced as a result of using explosive vaporization for desorption. The mechanism for porous silicon surface as a SALDI substrate involves three steps: 1)Adsorption: the analyte is adsorbed by porous silicon through Hydrogen bond formation using the silanol groups. 2) Electronic excitation:laser pulse excite the silicon producing free electrons and positive charges in the surface layer.this increase the acidity of silanol groups which donate the proton easily to analytes. 3) Thermal Activation: analytes are activated thermally and dissociated from the surface. == Instrumentation ==

SALDI as an improvement of MALDI, used the very similar instrument to that of MALDI. It employs a laser source for pulsed laser generation which is responsible for excitation of the sample mixture, which consists of the analyte and substrate materials. On the other side of the instrument, the mass analyzer which separates the analytes according to their mass-to-charge ratio (m/z) and the detector are located. Analytes are accelerated to the analyzer by applying potential difference. Ambient SALDI Recently, researchers were able to analyze in ambient conditions as a result of the involvement of DART ion source into the SALDI-MS system. == Applications ==

Forensics Forensic investigation owes DIOS the favor of producing evidence in contraceptive polymers in an alleged sexual assault case that could've never been made by any other analytical technique. Moreover, Pihlainen K. et al. reported that the technique showed great promise in the forensic analysis of illicit drugs. They also reported that the interference was diminished by using this technique. In addition, another report stated that DIOS identified 11 impurities. Profiling the impurities was expected to lead to their origin. Eight years later, the authors published another report and mentioned that the technique identified the catecholamines in a human peripheral blood lymphocyte extract. Also the quantitation of salicylate in human serum was proved using the DIOS-MS in negative ion mode. Biomedical Thomas et al. worked on a group of enzyme systems, were able to achieve monitoring and direct analysis of enzyme-catalyzed reaction by DIOS-MS. One famous result was the reaction of acetylcholineesterase (AChE) with acetylcholine producing choline. This approach gained more fame when showed the ability to detect the selectivity of different enzyme inhibitors. The study started with hyperzine A, tacrine, and 2,6-dimethoxyphenyl-N-butylcarbamate, which are all inhibitors of AChE. The Inhibitor constant (ki) value of each of the inhibitors was found to be an important factor of their inhibition potentials. DIOS-MS has another advantage over MALDI, it can detect additional information in the low-mass region of mass spectrum as it can detect peptide peaks, and also identify post-translation modifications. These capabilities have great applications in protein identification with more confidence. This was the first achieved SALDI-MS. As in SALDI, laser has to penetrate through the tissue and be absorbed by the layer underneath, thickness would be a limiting factor, and researchers were able to overcome this factor by introducing an organic matrix onto the tissue section. This was named matrix enhanced surface-assisted laser desorption/ionization mass spectrometry (ME-SALDI-MS) to account for the different processes employed in the technique and refer to the modification that enhanced the technique. More work was done for imaging of drug molecule distribution in brain tissue, then the cholesterol distribution in brain tissue and the sucrose distribution in Gerbera jamesonii flower stem. Also biofluids for direct analysis of drug molecules and their metabolites has been investigated. == See also ==