Laser, ablation desorption/ionization

The ionization methods reported for IMS included MALDI [41,76-80], Secondary Ion Mass Spectrometry (SIMS) [19, 81-86], Matrix-enhanced (ME)-SIMS [87, 88], Desorption Electrospray Ionization (DESI) [89-99], Nanostructure Initiator Mass Spectrometry (NIMS) [100-102], Atmospheric Pressure Infrared MALDI Mass Spectrometry (AP-IR-MALDI-MS) [103], Laser Ablation-inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) [104-106], Laser Desorption Postionization (LDPI) [107], Laser Ablation Electrospray Ionization Mass Spectrometry (LAESI) [108, 109], and Surface-assisted Laser Desorption/ioniza-tion Mass Spectrometry (SALDI) [110-112], Another method was called probe electrospray ionization (PESI) that was used for both liquid solution and the direct sampling on wet samples. 

Harris, G.A. Graf, S. Knochenmuss, R. Fernandez, F.M., Couphng laser ablation/ desorption electrospray ionization to atmospheric pressure drift tube ion mobility spectrometry for the screening of antimalarial drug quality, Analyst 2012, 137, 3039-4044. 

Several other ionization methods have been developed based on DESI, including desorption atmospheric pressure chemical ionization (DAPCI), desorption atmospheric pressure photo-ionization (DAPPI), laser ablation electrospray ionization (LAESI), and extractive electrospray ionization (EESI). Each technique uses variations of the solvent, how the charged beam is formed, and how the beam is nsed to facilitate the prodnction of analyte ions. Because these are surface methods (except EESI), they are incompatible with LC. 

Until about the 1990s, visible light played little intrinsic part in the development of mainstream mass spectrometry for analysis, but, more recently, lasers have become very important as ionization and ablation sources, particularly for polar organic substances (matrix-assisted laser desorption ionization, MALDI) and intractable solids (isotope analysis), respectively. 

Some solid materials are very intractable to analysis by standard methods and cannot be easily vaporized or dissolved in common solvents. Glass, bone, dried paint, and archaeological samples are common examples. These materials would now be examined by laser ablation, a technique that produces an aerosol of particulate matter. The laser can be used in its defocused mode for surface profiling or in its focused mode for depth profiling. Interestingly, lasers can be used to vaporize even thermally labile materials through use of the matrix-assisted laser desorption ionization (MALDI) method variant. 

For solids, there is now a very wide range of inlet and ionization opportunities, so most types of solids can be examined, either neat or in solution. However, the inlet/ionization methods are often not simply interchangeable, even if they use the same mass analyzer. Thus a direct-insertion probe will normally be used with El or Cl (and desorption chemical ionization, DCl) methods of ionization. An LC is used with ES or APCI for solutions, and nebulizers can be used with plasma torches for other solutions. MALDI or laser ablation are used for direct analysis of solids. 

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. If the sample examined is dissolved as a solid solution in a matrix, the rapid expansion of the matrix, often an organic acid, covolatilizes the entrained sample. Proton transfer from the matrix occurs to give protonated molecular ions of the sample. Normally thermally unstable, polar biomolecules such as proteins give good yields of protonated ions. This is the basis of matrix-assisted laser desorption ionization (MALDI). 

The three techniques — laser desorption ionization, laser ablation with secondary ionization, and matrix-assisted laser desorption — are all used for mass spectrometry of a wide variety of substances from rock, ceramics, and bone to proteins, peptides, and oligonucleotides. 

There are some variants that have emerged in the wake of DESI. By replacing the electrospray emitter by a metal needle and allowing solvent vapor into the coaxial gas flow desorption APCI (DAPCI) can be performed [106], Other versions are atmospheric-pressure solids analysis probe (ASAP) where a heated gas jet desorbs the analyte, which is subsequently ionized by a corona discharge [107], and electrospray-assisted laser desorption/ionization (ELDI) where a laser ablates the analyte and charged droplets from an electrospray postionizes the desorbed neutrals [108],... 

Mass spectrometric measurements of ions desorbed/ionized from a surface by a laser beam was first performed in 1963 by Honig and Woolston [151], who utilized a pulsed mby laser with 50 p,s pulse length. Hillenkamp et al. used microscope optics to focus the laser beam diameter to 0.5 p,m [152], allowing for surface analysis with high spatial resolution. In 1978 Posthumus et al. [153] demonstrated that laser desorption /ionization (LDI, also commonly referred to as laser ionization or laser ablation) could produce spectra of nonvolatile compounds with mass > 1 kDa. For a detailed review of the early development of LDI, see Reference 154. There is no principal difference between an LDI source and a MALDI source, which is described in detail in Section 2.1.22 In LDI no particular sample preparation is required (contrary to... 

There are three types of ion production using lasers as vaporization and ionization sources, laser ablation (LA), direct laser vaporization (DLV), and matrix assisted laser desorption ionization (MALDI). 

An alternative method for producing mass spectra of solid samples which shows promise for the analysis of zeolites and related materials is the plasma desorption technique recently reported by Schwiekert et al. [71,72]. This technique uses a less energetic means of ionizing the solid than laser ablation, and initial indications are that negative ion spectra from materials like zirconium phosphate may reflect the connectivity as well as the stoichiometry of the solid analyzed to a greater degree than laser ablation. 

The three techniques---laser desorption ionization, laser ablation with secondary ionization, and matrix—... 

Interestingly, variations of this cyclic multiphotonic scheme have been independently advanced in the study of the UV ablation of cryogenic films [65] as well as in the matrix-assisted laser desorption/ionization (MALDI) of biomolecules [66]. The reason for giving these results is to illustrate the emergence of common concepts/processes concerning UV ablation of molecular substrates. 

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