Surface-assisted laser desorption/ionization (SALDI) is a soft laser desorption technique used for mass spectrometry analysis of biomolecules, polymers, and small organic molecules. In its first embodiment Koichi Tanaka used a cobalt/glycerol liquid matrix and subsequent applications included a graphite/glycerol liquid matrix as well as a solid surface of porous silicon. The porous silicon represents the first matrix-free SALDI surface analysis allowing for facile detection of intact molecular ions, these porous silicon surfaces also facilitated the analysis of small molecules at the yoctomole level. At present laser desorption/ionization methods using other inorganic matrices such as nanomaterials are often regarded as SALDI variants. As an example, silicon nanowires as well as Titania nanotube arrays (NTA) have been used as substrates to detect small molecules. SALDI is used to detect proteins and protein-protein complexes. A related method named "ambient SALDI" - which is a combination of conventional SALDI with ambient mass spectrometry incorporating the direct analysis real time (DART) ion source has also been demonstrated. SALDI is considered one of the most important techniques in MS and has many applications.
History Koichi Tanaka performed the first successful LDI experiments on proteins. Subsequently, Sunner and Chen used graphite particles of 2-150 μm in size as a substrate and solutions of analytes in glycerol to analyze low molecular weight analytes, peptides, and small proteins by surface-assisted laser desorption/ionization MS (SALDI-MS). The technique was soon picked up by the Siuzdak lab who used nanostructured silicon surface for analyses. Subsequent work on nanostructures included the addition of fluorinated "initiator" molecules onto the porous surface to enhance desorption/ionization, mass range, surface robustness, and sensitivity. Other surface-based SALDI-MS approaches have also developed, including in 2000 where a thin layer of activated carbon particles fixed on aluminum support Since the original 1999 nanostructured silicon experiments, the research has largely focused on introducing novel nanomaterials as substrates, to enhance the sensitivity, broaden the mass range and expand the categories of samples that can be analyzed using this technique. 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 in particular, and subsequently nanostructure-initiator mass spectrometry (NIMS) 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. SALDI is a three-stage process. The first stage is mainly concerned with mixing the samples with the substrate. In the second stage, the laser pulses are applied to the mix where the substrate absorbs the laser energy and transfers it to the sample molecules. In the final stage desorption and ionization occur and the potential difference accelerates produced ions into the mass analyzer.
Substrates The substrate surface is playing a big role in adsorption, desorption, and ionization of the analyte molecules. This role is affected mainly by the chemical and physical properties of the substrate. Among these physical properties are the optical absorption coefficient, heat capacity and heat conductivity. 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.
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