Ablation

Laser ablation systems Flow injection analyzers (FIA) 

Chilled spray chambers and desolvation systems Direct injection nebulizers (DIN) 

Fast automated sampling procedures Chromatographic separation equipment 

Let us now take a closer look at some of these techniques to understand their basic principles and what benefits they bring to ICP-MS. 

The limitation of ICP-MS in analyzing solid materials (without the need for wet chemical dissolution/digestion methods) led to the development of laser ablation. The principle behind this approach is the use of a high-powered laser to ablate the surface of a solid and sweep the sample aerosol into the ICP mass spectrometer for 

Principles and Characteristics Laser ablation is conceptually very simple, but mechanistically complicated. The process involves coupling of the photon energy of a laser pulse (typically about 20-30 ns wide, with an energy of 1-10 Jcm ) into the surface of a solid, resulting in evaporation and ejection of various species from the surface (the so-called plume ) within 10 to 10 s. The first experiments were carried out in 1962 [32]. When focused to a small area, a laser beam provides enormous power densities and electromagnetic fields. The plume , presumably a plasma, is accompanied by shock waves and electrical breakdown. The ejected material may eventually be deposited as a thin film. It is possible, by suitable selection of laser power and focus, to ablate a range of plastic materials in a controlled manner. For some matrices the polymer melts and diffuses away from the centre of the ablation site, leading to the forma- 

Coupling between molecular processes and morphological changes is one of the most unique and important characteristics of laser ablation. Excitation energy relaxation dynamics and primary chemical processes of organic molecules in laser ablation have been investigated by using various time-resolved spectroscopies, such as fluorescence, absorption, Raman and IR spectroscopies. Laser ablation leads to rapid temperature elevation of the polymer matrix and thermal decomposition of the polymer. Ablation causes not only photochemical reactions but also photo-initiated thermal reactions. 

The thermal effect of laser irradiation is obscure. Schawlow [33] has estimated heating rates as high as 10 Ks with temperatures up to 10 K, but others [34] have reported temperature assignments of 4000 to 10,000 K. Much lower estimates (650-1000°C) have also been given [35]. Anyhow, the thermal evaporation component may be deleterious for analytical purposes because elements of high 

Rabek [51] and others [52] have described laser-induced decomposition of polymers. Comprehensive reviews have appeared on the interaction of laser radiation with solid materials and its significance in analytical chemistry [53,53a]. Various reviews cover the subjects of optical and mass spectrometry performed directly on the laser plume [54,55]. Moenke-Blankenburg [38] has described laser ablation for sample introduction. Advances in laser ablation of materials were recently reported [56,57]. 

Laser ablation of solids is of considerably interest in relation to chemical analysis and material fabrication. The main fields of analytical application of laser ablation are (i) microanalysis (ii) local analysis (Hi) distribution analysis with spatial resolution in microregions (migration studies) and (iv) bulk analysis. No firm conclusions have been obtained so far on the most suitable system for bulk analysis, localised analysis or on-line analysis, particularly regarding the different types of existing lasers. Strictly spoken, the use of lasers for sample introduction in inorganic analysis cannot be classified as laser spectroscopy. 

Some of the newer quencher UV stabilizers are nickel-free compounds and do not possess the drawbacks of the nickel complexes. 

Since quencher UV stabilizers are effective in thin sections, they are especially useful in thin films and fibers. 

Ablation is understood to be the loss of surface material through thermal effects. This term was originally used in glaceology. The effect is also important in the construction of space capsule heat shields. In contrast to superficial expectations, polymers exhibit quite minor ablation properties, since they absorb, dissipate, and accumulate heat, with, however, changes in the polymeric material. 

At present, heat shield materials consist of poly(tetrafluorethylene), Si02-filled reinforced phenolic resins, or Si02-filled epoxide/polyamide combinations. Development-stage materials are polyimides, silicones, phosphor nitrilic chlorides, and polyboron phosphorus compounds. 

Staudinger, Disposal of Plastics Waste and Litter, Soc. Chem. Ind., London, 1970. G. Scott, Some new concepts in polymer stabilisation, Brit. Polym. J. 3, 24 (1971). 

An excimer laser was also used to machine a PC chip (6 mm thick) to create 160-pm-wide channels (60 pm deep), [811] or on a polyimide sheet [192,811]. Another UV excimer laser (248 nm) was used to ablate microstructures within PC channels (fabricated by imprinting) [193]. 

FIGURE 2.19 Electron micrograph of (a) the rough channel features on a PMMA master as generated by laser-ablation (b) the smooth channel features on the PDMS replica cast from the PMMA master [367]. Reprinted with permission from the American Chemical Society. 

A XeCl excimer laser (308 nm) was used to photoablate biodegradable polymers (PDLA and PVA) into channels (10-50 pm deep) [196], 

A molecular fluorine excimer laser (157 nm) was also used to ablate PMMA chips to a depth of 500 pm [197], It was also reported that acrylic (or PMMA) plates were processed using a laser-cutting machine [198], 

X-ray micromachining (7-9 A synchrotron radiation) was used to ablate PMMA channel structures (20 pm wide and 50 pm deep) [199]. Bonding was achieved by first heating both PMMA plates on a hot plate (150°C) for 5-10 min, 

This limitation led to the development of laser ablation as a sampling device for atomic spectroscopy instrumentation, where the sampling step was completely separated from the excitation or ionization step. The major benefit is that each step can be independently controlled and optimized. These early devices used a high-energy laser to ablate the surface of a solid sample, and the resulting aerosol was swept into some kind of atomic spectrometer for analysis. Although initially used with atomic absorption - and plasma-based emission techniques, it was not until 

FIGURE 12.12 Representative reentry trajectories for various flight vehicles. ICBM, intercontinental ballistic missile IRBM, intermediate-range ballistic missile. (Data from Schmidt, D. L., Mod. Plast., 37, 131, November 1960 147, December 1960.) 

Polymers have been used in ablative systems for a combination of reasons. These have been snmmarized by Schmidt [25] as follows  

High heat absorption and dissipation per unit mass expended, which ranges from several hundred to several thousand Btu/lb of ablative material 

Excellent thermal insulation, which eliminates or reduces the need for an additional internal cooling system 

Useful performance in a wide variety of hyperthermal environments 

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Other methods of sample introduction that are commonly coupled to TOP mass spectrometers are MALDI, SIMS/PAB and molecular beams (see section (Bl.7.2)). In many ways, the ablation of sample from a surface simplifies the TOP mass spectrometer since all ions originate in a narrow space above the sample surface. 

Figure Bl.22.7. Left resonant seeond-hannonie generation (SHG) speetnimfrom rhodamine 6G. The inset displays the resonant eleetronie transition indueed by tire two-photon absorption proeess at a wavelength of approximately 350 mn. Right spatially resolved image of a laser-ablated hole in a rhodamine 6G dye monolayer on fiised quartz, mapped by reeording the SHG signal as a fiinetion of position in the film [55], SHG ean be used not only for the eharaeterization of eleetronie transitions within a given substanee, but also as a mieroseopy tool.

Akkermans R P, Wu M, Bain C D, Fidel-Suerez M and Compton R G 1998 Electroanalysis of ascorbic acid a comparative study of laser ablation voltammetry and sonovoltammetry E/eofroana/ys/s 10 613... 

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... 

A laser pulse strikes the surface of a specimen (a), removing material from the first layer, A. The mass spectrometer records the formation of A+ ions (b). As the laser pulses ablate more material, eventually layer B is reached, at which stage A ions begin to decrease in abundance and ions appear instead. The process is repeated when the B/C boundary is reached so that B+ ions disappear from the spectrum and C+ ions appear instead. This method is useful for depth profiling through a specimen, very little of which is needed. In (c), less power is used and the laser beam is directed at different spots across a specimen. Where there is no surface contamination, only B ions appear, but, where there is surface impurity, ions A from the impurity also appear in the spectrum (d). 

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. 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

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. 

These data are typical of lasers and the sorts of samples examined. The actual numbers are not crucial, but they show how the stated energy in a laser can be interpreted as resultant heating in a solid sample. The resulting calculated temperature reached by the sample is certainly too large because of several factors, such as conductivity in the sample, much less than I00% efficiency in converting absorbed photon energy into kinetic energy of ablation, and much less than 100% efficiency in the actual numbers of photons absorbed by the sample from the beam. If the overall efficiency is 1-2%, the ablation temperature becomes about 4000 K. 

Suffice it to say at this stage that the surfaces of most solids subjected to such laser heating will be heated rapidly to very high temperatures and will vaporize as a mix of gas, molten droplets, and small particulate matter. For ICP/MS, it is then only necessary to sweep the ablated aerosol into the plasma flame using a flow of argon gas this is the basis of an ablation cell. It is usual to include a TV monitor and small camera to view the sample and to help direct the laser beam to where it is needed on the surface of the sample. 

With a typical ablated particle size of about 1 -pm diameter, the efficiency of transport of the ablated material is normally about 50% most of the lost material is deposited on contact with cold surfaces or by gravitational deposition. From a practical viewpoint, this deposition may require frequent cleaning of the ablation cell, transfer lines, and plasma torch. 

Solid samples can be analyzed using a plasma torch by first ablating the solid to form an aerosol, which is swept into the plasma flame. The major ablation devices are lasers, arcs and sparks, electrothermal heating, and direct insertion into the flame. 

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 solid to be examined must be vaporized in some way. This vaporization can be done by using the heat of the plasma flame or, more usually, the solid is ablated separately and the resulting aerosol is mixed with argon gas and swept into the center of the flame. 

The major methods used for vaporization (ablation) include lasers, electrically heated wires, or sample holders and electrical discharges (arcs, sparks). 

A laser beam is capable of putting so much energy into a substance in a very short space of time that the substance rapidly expands and volatilizes. The resulting explosive shock wave travels through the sample, subjecting it to high temperatures and pressures for short times. This process is also known as ablation. 

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). 

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