Ablation ionization

The large variability in elemental ion yields which is typical of the single-laser LIMS technique, has motivated the development of alternative techniques, that are collectively labeled post-ablation ionization (PAI) techniques. These variants of LIMS are characterized by the use of a second laser to ionize the neutral species removed (ablated) from the sample surface by the primary (ablating) laser. One PAI technique uses a high-power, frequency-quadrupled Nd-YAG laser (A, = 266 nm) to produce elemental ions from the ablated neutrals, through nonresonant multiphoton ionization (NRMPI). Because of the high photon flux available, 100% ionization efflciency can be achieved for most elements, and this reduces the differences in elemental ion yields that are typical of single-laser LIMS. A typical analytical application is discussed below. 

R. W. Odom and B. Schueler. Laser Microprobe Mass Spectrometry Ion and Neutral Analysis, in Lasers and Mass Spectrometry (D. M. Lubman, ed.) Oxford University Press, Oxford, 1990. Presents a useful discussion of LIMS instrumental issues, including the post-ablation ionization technique. Several anal)n ical applications are presented. 

Laser ablation can be used to volatilize samples. An intense laser pulse is focused onto a solid at sufficient pulse power and energy (e.g., millijoule energy in a pulse of 10 nanosecond, or less, duration) to remove material from the surface. Typical conditions result in crater formation after one or a few laser pulses. If performed in flowing argon at atmospheric pressure, the ablated material can be fed into an ICP for ionization. Ions may also be formed by an ablation pulse in a vacuum and directly focused and extracted into a mass analyzer. Laser ablation/ionization is used primarily for analysis of solid compounds and materials. Virtually the entire periodic table can be analyzed with this method. 

In response to this need, aerosol mass spectrometry has developed rapidly and it is now possible to determine both the size (over a limited size range) and qualitative chemical composition of most gas-phase aerosols, with a response time of less than 1 s (see Suess and Prather (1999)). Most of the instruments described in the literature use laser ablation and ionization of the aerosol particles to characterize their chemical composition, but other methods, including thermal vaporization with electron impact ionization, are also used. Here, we first briefly sketch the development of instruments based on laser ablation/ ionization techniques and then describe some of the work that has been done using an aerosol TOP mass spectrometer. 

The first instrument for laser ablation/ionization was described by Sinha et al. (1984). This had most of the features of the present-day instruments unfortunately, the technology available at that time severely limited its development and utility. A major advance was made by McKeown et al. (1991) with the use of TOFMS, which enabled the recording of a complete mass spectrum for each particle. Further developments, notably by Prather s group, have culminated in the first conmiercially available ATOFMS instrument. A schematic diagram of an ATOFMS system is shown in Figure 28.33. 

The third section is the chamber in which a high power laser ablates the particles and ionizes the atoms and molecules that are produced. Finally, in the fourth section, which is maintained at high vacuum, the positive and negative ions formed in the ablation/ ionization process are analysed using dual TOF mass spectrometers. 

Aubriet, F, Maunit, B., Courtier, B., Muller, J.F. (1997) Studies of the chromium oxygenated cluster ions produced during the laser ablation of chromium oxides by laser ablation/ionization Fourier transform ion cyclotron resonance mass spectrometry. Rapid Communications in Mass Spectrometry, 11,1596-1601. 

Anbriet, E, Mannit, B., Mnller, J.F. (2001) Speciation of chrominm componnds by laser ablation/ionization mass spectrometry and a stndy of matrix effects. International Journal of Mass Spectrometry, 209,5-21. 

Aubriet, F, Muller, IF (2002) About the atypical behavior of CrCP, MoCP, and WCP during their UV laser ablation/ ionization. Journal of Physical Chemistry A, 106, 6053-6059. 

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 other chief comptment of a mass spectrometer, the ion source, determines the types of ions that can be examined when starting fi om a specific sample (Gross and Caprioli, 2007). Laser-based methods, variably dubbed ablation, ionization, and desorption/ionization supposedly depending on the involved laser power, have played an important role fi om initial studies of bare metal ions to the widespread sought-after production of cluster ions for these latter species, the development of the so-called cluster sources was key to progress (Duncan, 2012). In more recent years, electrospray ionization (ESI) has played a central role due to its capacity to transfer/produce ions from solutions under mild conditions. Besides yielding new types of ions for chemical probing, ESI became a method of choice for the identification of solution species (speciation) and for direct observation of reaction... 

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). 

Much of the energy deposited in a sample by a laser pulse or beam ablates as neutral material and not ions. Ordinarily, the neutral substances are simply pumped away, and the ions are analyzed by the mass spectrometer. To increase the number of ions formed, there is often a second ion source to produce ions from the neutral materials, thereby enhancing the total ion yield. This secondary or additional mode of ionization can be effected by electrons (electron ionization, El), reagent gases (chemical ionization. Cl), a plasma torch, or even a second laser pulse. The additional ionization is often organized as a pulse (electrons, reagent gas, or laser) that follows very shortly after the... 

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. 

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. 

Modern commercial lasers can produce intense beams of monochromatic, coherent radiation. The whole of the UV/visible/IR spectral range is accessible by suitable choice of laser. In mass spectrometry, this light can be used to cause ablation, direct ionization, and indirect ionization (MALDI). Ablation (often together with a secondary ionization mode) and MALDI are particularly important for examining complex, intractable solids and large polar biomolecules, respectively. 

Aerosols can be produced as a spray of droplets by various means. A good example of a nebulizer is the common household hair spray, which produces fine droplets of a solution of hair lacquer by using a gas to blow the lacquer solution through a fine nozzle so that it emerges as a spray of small droplets. In use, the droplets strike the hair and settle, and the solvent evaporates to leave behind the nonvolatile lacquer. For mass spectrometry, a spray of a solution of analyte can be produced similarly or by a wide variety of other methods, many of which are discussed here. Chapters 8 ( Electrospray Ionization ) and 11 ( Thermospray and Plasmaspray Interfaces ) also contain details of droplet evaporation and formation of ions that are relevant to the discussion in this chapter. Aerosols are also produced by laser ablation for more information on this topic, see Chapters 17 and 18. 

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 previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

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. 

Laser based mass spectrometric methods, such as laser ionization (LIMS) and laser ablation in combination with inductively coupled plasma mass spectrometry (LA-ICP-MS) are powerful analytical techniques for survey analysis of solid substances. To realize the analytical performances methods for the direct trace analysis of synthetic and natural crystals modification of a traditional analytical technique was necessary and suitable standard reference materials (SRM) were required. Recent developments allowed extending the range of analytical applications of LIMS and LA-ICP-MS will be presented and discussed. For example ... 

A somewhat related technique is that of laser ionization mass spectrometry (LIMS), also known as LIMA and LAMMA, where a single pulsed laser beam ablates material and simultaneously causes some ionization, analogous to samples beyond the outer surface and therefore is more of a bulk analysis technique it also has severe quantiBaction problems, often even more extreme than for SIMS. 

Direct sampling of solids may be carried out using laser ablation. In this technique a high-power laser, usually a pulsed Nd-YAG laser, is used to vaporize the solid, which is then swept into the plasma for ionization. Besides not requiring dissolution or other chemistry to be performed on the sample, laser ablation ICPMS (LA-ICPMS) allows spatial resolution of 20-50 pm. Depth resolution is 1-10 pm per pulse. This aspect gives LA-ICPMS unique dit nostic capabilities for geologic samples, surface features, and other inhomogeneous samples. In addition minimal, or no, sample preparation is required. 

The sensitivity, accuracy, and precision of solid-sample analysis have been greatly improved by coupling LA with ICP-OES-MS. The ablated species are transported by means of a carrier gas (usually argon) into the plasma torch. Further atomization, excitation, and ionization of the ablated species in the stationary hot plasma result in a dramatic increase in the sensitivity of the detection of radiation (LA-ICP-OES) or of the detection of ions (LA-ICP-MS). 

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