Laser-ablation resonance-ionization

Hi) Methods based on mass spectrometry Spark-source mass spectrometry Glow-discharge mass spectrometry Inductively coupled-plasma mass spectrometry Electro-thermal vaporization-lCP-MS Thermal-ionization mass spectrometry Accelerator mass spectrometry Secondary-ion mass spectrometry Secondary neutral mass spectrometry Laser mass spectrometry Resonance-ionization mass spectrometry Sputter-initiated resonance-ionization spectroscopy Laser-ablation resonance-ionization spectroscopy... 

Laser-Ablation Resonance-Ionization Spectroscopy LARIS... 

The closely allied topics of secondary neutral mass spectrometry (SNMS), fast atom bombardment (FAB), and laser ablation SIMS are important, but are beyond the scope of this chapter. SNMS is a technique in which neutral atoms or molecules, sputtered by an ion beam, are ionized in an effort to improve sensitivity and to decouple ion formation from matrix chemical properties, making quantification easier. This ionization is commonly effected by electron beams or lasers. FAB uses a neutral atom beam to create ions on the surface. It is often useful for insulator analysis. Laser ablation creates ions in either resonant or nonresonant modes and can be quite sensitive and complex. 

The principle of CHARISMA is as follows the sample is ablated with a pulsed Nd YAG laser, focused into a spot of micrometer size. Neutral species drift up while ions are suppressed. Two lasers (Ti Sapphire), tuned to resonantly ionize the element of interest, are bred into the ablated, neutral material. The ions are extracted and accelerated and analyzed with a TOF mass spectrometer where mass separation occurs due to different flight times of species with different mass. The useful yield (detected ions/sputtered atoms) is around 1% in the current set-up. A planned modification is aiming at a useful yield of around 30% which would open up new possibilities for the isotope study of trace elements (e.g. the rare earth elements) in presolar grains. With the new set-up it will also be possible to extend the analyses to sub-micrometer-sized grains, which are much more representative of presolar grains than the currently studied micrometer-sized grains. 

Figure 1.10 REMPI-TOF permits selective simultaneous recording of separate spectra for different cluster species or for different isotopomers. The spectra of NiC and NiSi were recorded (by resonant two-photon ionization using an ArF excimer laser to ionize) using laser ablation of a nickel target in a stream of carrier gas containing 3% CH4. No intentional source of silicon was present (but the nickel sample had been roughened using SiC sandpaper) (from Brugh and Morse, 2002, and Lindholm, et al, 2003).

A notable exception regarding cost and complexity is the method of resonant laser ablation (RLA). In RLA, a low-fluence pulsed laser beam is focused onto the solid sample (Eiden et al., 1994). The laser pulse first ablates or desorbs a small amount of sample. On the timescale of the laser pulse (typically a few nanoseconds), the atoms liberated from the surface absorb laser photons and are ionized. By using a laser wavelength resonant with an atomic transition in the atoms of interest, ionization is highly selective. This method has been extensively demonstrated with relatively low-cost YAG pumped dye lasers. 

Inductively coupled plasma mass spectrometry (ICP-MS), laser ablation ICP-MS (LA ICP-MS), thermal ionization mass spectrometry (TIMS), secondary ion mass spectrometry (SIMS), glow discharge mass spectrometry (GDMS), resonance ionization mass spectrometry (RIMS), and accelerator mass spectrometry (AMS) have been used successfully to measure uranimn concentrations and isotope ratios in a wide range of sample matrices. The specific details of the methods are described fully in the relevant sections of this encyclopedia. There are specific advantages associated with each method, which depend on the sample of interest and the information required. 

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. 

Every effort is made here to achieve the highest possible absolute power of detection. Microdistribution analysis represents the primary field of application for microprobe techniques based on beams of laser photons, electrons, or ions, including electron microprobe analysis (EPMA), electron energy-loss spectrometry (EELS), particle-induced X-ray spectrometry (PIXE), secondary ion mass spectrometry (SIMS), and laser vaporization (laser ablation). These are exploited in conjunction with optical atomic emission spectrometry and mass spectrometry, as well as various forms of laser spectrometry that are still under development, such as laser atomic ab.sorption spectrometry (LAAS), resonance ionization spectrometry (RIS). resonance ionization mass spectrometry (RIMS), laser-enhanced ionization (LEI) spectrometry, and laser-induced fluorescence (LIF) spectrometry [36]-[44],... 

Lasers have been used in mass spectrometry for many years. Trace elements in biological samples [90] can be determined by using laser microprobes (LAMMA, laser microprobe mass analyzer) or a combination of laser ablation with ICPMS. For the analysis of bulk materials, techniques such as resonance ionization mass spectrometry (RIMS) and laser ablation MS (LAMS) are employed for a review see [91]. 

Figure 1.13 Selected analytical techniques used for metallomics studies. ICP-OES, inductively coupled plasma optical emission spectroscopy, ICP-MS, inductively coupled plasma mass spectrometry LA-ICP-MS, laser ablation ICP-MS XRF, X-ray fluorescence spectroscopy PIXE, proton induced X-ray emission NAA, neutron activation analysis SIMS, secondary ion mass spectroscopy GE, gel electrophoresis LC, liquid chromatography GC, gas chromatography MS, mass spectrometry, which includes MALDI-TOF-MS, matrix-assisted laser desorption/ ionization time of flight mass spectrometry and ESI-MS, electron spray ionization mass spectrometry NMR, nuclear magnetic resonance PX, protein crystallography XAS, X-ray absorption spectroscopy NS, neutron scattering.

The UV-irradiated spot is typically 1-3 pm in TOP LMMS so that power densities between 10 and 10 W cm 2 can be attained with mJ pulses of 10-15 ns. Refractive objectives allow spot sizes down to the diffraction limit of 0.5 pm. Reflective optics (as in Figure 1) allow larger working distances and are free from chromatic aberrations so that use of other wavelengths is facilitated. Ionization with a tunable dye laser allows resonant one-step Dl or postionization of the laser ablated neutrals. Specificity and sensitivity can thus be improved substantially but wavelength selection becomes cumbersome for unknown compounds. 

In order to investigate intact neutral ablated products one has to ionize the molecules as gently as possible to avoid secondary decomposition by laser. In this sense, often used MPI techniques are not suited, as mentioned briefly in Section 9.2 intense electric fields of MPI pulses tend to cause extensive fragmentation of the ablated products. Moreover, accidental resonance enhancement of some specific mass peaks may distort the true mass distribution pattern. 

The most common LA-ICP/MS work to date is with flash-lamp pumped Nd YAG lasers that produce a light pulse of 3-10 nanoseconds in width and are relatively inexpensive. Shorter pulse width lasers (picosecond to femtosecond) are widely available but more expensive. A wavelength other than IR or UV is used only for resonant laser ionization or ablation. In this case, the goal is to excite or ionize a specific element preferentially, and the laser wavelength is chosen to correspond to a particular transition of an element, as discussed in Section 17.7.4. 

Dissociation and ionization by means of one or several lasers. Sensitive detection of fragments and ions with resonance photoionization and mass spectrometric techniques (tunable lasers). Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) with pulsed ablating laser sources. High lateral resolution enables molecular microprobing of biological cell compounds... 

L2MS has been used to directly analyze additives in a range of polymers [41]. The L2MS method decouples the desorption and the ionization step in order that each step can be optimized. An infrared laser at 10.6 p,m is used to irradiate the polymer and cause ablation through a pyrolysis mechanism. During this step, the polymer is thermally decomposed and ejected while the additives remain intact. With a delay time of about 20 p,s, the additives are then subjected to a selective ionization with a UV laser at 266 nm through a resonant two-photon ionization step. Finally, the ions are mass separated, and recorded in a time-of-flight mass spectrometer. 

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