Desorption/ablation

Knockenmuss, R. (2009) Laser desorption/ablation plumes from capillary-like restricted volumes. Eur. J. Mass... 

In addition to DESI and AP-MALDI, a large variety of other, sometimes closely related, atmospheric-pressure desorption ionization techniques have been introduced in the past decade, connected to a huge number of acronyms. Van Beikel et al. [76] tried to classify these emerging techniques into four categories, i.e., (1) thermal desorption ionization, (2) laser desorption/ablation ionization, (3) liquid-jet and gas-jet desorption ionization, and (4) hquid extraction surface sampling probe ionization. 

The MALDI process formally consists of the following steps (i) formation of a crystalline solid out of the solution (ii) incorporation of analyte molecules into matrix crystals (Hi) desorption-ablation of material from crystals by the laser beam and (iv) gas phase processes with transfer of charges to analyte molecules (cfr. Fig. 3.15). The mechanisms involved in MALDI remain poorly understood a detailed discussion of the possible MALDI mechanisms is available [276]. Contrary to laser desorption, where the analyte is irradiated directly by laser light, in matrix-assisted laser desorption the analyte is assumed to... 

When they exposed a surface sample of organic molecules to intense UV radiation pulses, Antonov et al. (1980a) observed the formation of molecular ions without any noticeable fragmentation, which had the character of a nonthermal process. A detailed investigation led to an understanding of the two-stage character of the process in the form of thermal desorption of the molecules and their subsequent photoionization. A study of the laser thermal-desorption (ablation) process revealed the possibility of soft chemoionization in the dense cloud of laser-desorbed molecules (Karas and Hillenkamp 1987). This process was called matrix-assisted laser desorption/ionization... 

Laser Desorption/Ablation Ionization Methods. Matrix-assisted laser desoiption ionization (MALDI) relies on the use of a solid chromophQric matrix, chosen to absorb laser light, which is co-mixed with the analyte (4). Typically, a solution of a few picomoles of analyte is mixed widi a lOO-to-5000 fold excess of the matrix in solution. A few microliters of the resulting solution are deposited on a mass spectrometer solids probe, and the solvent is allowed to evaporate before inserting the probe into the mass spectrometer. When the pulsed laser beam strikes the sample surface in the spectrometer, the sample molecules are desorbedAonized at high efficiency. The various mechanisms for matrix and non-matrix assisted laser desorption have been discussed (5,6). 

Pulsed lasers with relatively short pulse widths (<50 ns) are typically used for laser desorption/ablation techniques because of their high peak powers and because short pulses reduce sample consumption and minimize laser-induced pyrolysis of the sample. Only mass spectrometers that can measure ions of all mass-to-charge values near simultaneously (corresponding to a single short laser pulse) or mass spectrometers that can trap all the ions produced in a single laser pulse are compatible with this pulsed ionization teclmique. Time-of-flight (2,7-9) and Fourier transform mass spectrometers (9-74) are commercially proven for laser desorption/ablation mass spectrometry. The TOFMS detects all ions near simultaneously, and the FT/MS is a ion trapping mass spectrometer that detects all ions simultaneously. 

An issue of Chemical Reviews (volume 103, number 2,2003) covered laser ablation of molecular substrates and included several articles on MALDl. A microscopic view of desorption/ablation was provided by the molecular dynamics work of the Zhigilei and Garrison groups, while Dreisewerd presented an overview of MALDl desorption, and one recently developed cluster model was summarized and further developed by Karas and Kriiger. Secondary mechanisms were examined in detail by Knochenmuss and Zenobi. ... 

As described below, there is ongoing discussion about initial (primary) ion formation, which may vary with laser wavelength or other variables. Secondary reactions of these ions with neutrals in the desorption/ablation plume are generally either invoked or not ruled out, making this aspect of MALDl uncontroversial since the early work of Liao and Alhson. In addition, it is now rarely challenged that local thermal equilibrium is often approached in the plume. This allows secondary reactions to be treated with conventional thermodynamics and has motivated extensive efforts to measure or calculate the corresponding thermodynamic quantities for MALDI-relevant molecules. Since the two-step model is fundamentally a consequence of the characteristics of the MALDl ablation/desorption event, this will be briefly discussed next. 

The desorption/ablation aspect of MALDl has been the object of an excellent review by Dreisewerd. Among the most basic functions of the matrix is absorption of the laser energy and conversion of most of it to heat. Subsequent matrix vaporization is sufficiently forceful that it entrains and ejects analyte that has been cocrystaUized in or on the matrix. The time scale is not as short as in fast atom bombardment (FAB) or secondary ionization mass spectrometry (SIMS) because the typical MALDl lasers used emit pulses of a few to hundreds of nanoseconds duration (e.g., N2 337 nm, 3 ns or tripled Nd YAG, 355 nm, 4—7 ns, Er YAG, 2.98 pm, 200 ns, although a range of pulse lengths has been studied ). This is slow compared to intramolecular motions. In UV MALDl, the energy conversion step is... 

Macroscopic models have been applied to the MALDI plume, including a hydrodynamic approach and that of a pre-accelerated adiabatic expansion. These cannot directly account for mixed gas/clusters in the ablation regime, but are still useful as a first approximation. They also cannot account for changes in plume development as the desorption/ablation crater shape changes.In contrast, the mixed-phase aspect of ablation is a natural part of molecular dynamics, even though it is not computationally possible to include the full temporal and spatial extent of a real experiment. Nevertheless, molecular dynamics has illuminated numerous aspects of the phase transition aspect of MALDI. " - ... 

If models involving in-plume charge separation by solution-like mechanisms are inadequate, a next logical step might be to ask what happens to the ions that do exist in a typical sample preparation solution. MALDI samples are often prepared from polar or aqueous solution, using acidic matrices. Formic or trifluoroacetic acids may be added. In such solutions many analytes, such as typical proteins or peptides, will be protonated. If these survive the drying process intact as preformed ions, perhaps they merely need to be liberated in the desorption/ablation event. 

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

Laser desorption to produce ions for mass spectrometric analysis is discussed in Chapter 2. As heating devices, lasers are convenient when much energy is needed in a very small space. A typical laser power is 10 ° W/cm. When applied to a solid, the power of a typical laser beam — a few tens of micrometers in diameter — can lead to very strong localized heating that is sufficient to vaporize the solid (ablation). Some of the factors controlling heating with lasers and laser ablation are covered in Figure 17.2. 

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. 

Analyte desorption Laser ablation, thermal desorption LA-AAS, LA-ICP-MS, TD-GC-MS... 

Nondestructive radiation techniques can be used, whereby the sample is probed as it is being produced or delivered. However, the sample material is not always the appropriate shape or size, and therefore has to be cut, melted, pressed or milled. These handling procedures introduce similar problems to those mentioned before, including that of sample homogeneity. This problem arises from the fact that, in practice, only small portions of the material can be irradiated. Typical nondestructive analytical techniques are XRF, NAA and PIXE microdestructive methods are arc and spark source techniques, glow discharge and various laser ablation/desorption-based methods. On the other hand, direct solid sampling techniques are also not without problems. Most suffer from matrix effects. There are several methods in use to correct for or overcome matrix effects ... 

In non-highly focussed laser desorption ionisation, employing spot sizes in the range of 50-200 pm in diameter, the surface is deformed by an ablation volume of about 1 pm3 per pixel per laser pulse. But this ablated volume is spread over a large desorption area leading to ablation depths of the order of a few nanometres. In laser microprobing, the same ablation volume leads to ablation crater depths in the micrometer range. 

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). 

Unfortunately, some of the analyzed molecules, as most biologically related molecules (e.g., amino acids), are solids with extremely low vapor pressures at room temperature and rapidly decompose when they are heated. For these molecules, which cannot be thermally vaporized, laser ablation or desorption have been alternatively used to produce neutral species in the gas phase.Both methodologies refer to laser-induced particle removal (laser sputtering) from a surface under the two extremes of massive and negligible rates of surface erosion, respectively. 

Miller, J.C. and Haglund, R.F. (eds) (1998). In Laser Ablation and Desorption, Experimental Methods in the Physical Sciences, vol. 30. Academic Press, London... 

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