Laser thermal ionization mass spectrometry

Sr). Over the past 30 years, lead and strontium isotope ratios have been measured with thermal ionization mass spectrometry (TIMS). Elemental salts are deposited on a filament heated to produce ionized particles, which are then sent into a mass spectrometer where they are detected by multiple Faraday cups arrayed such that ions of several masses are collected simultaneously. TIMS is capable of high precision isotope discrimination, but the instruments tend to be large and expensive, and extensive sample preparation is required prior to sample introduction. Newer ICP-MS-based technologies like multi-collector ICP-MS (especially laser ablation) circumvent some of the sample preparation issues while exploiting the precision of simultaneous mass discrimination, but they are still limited by the number and configuration of ion collectors. 

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... 

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. 

Mass spectrometry is the only method that delivers the mass of a molecule, as the ratio of molecular mass to the number z of charges per molecule. Until recently mass spectrometry was of little use for the analysis of polymers, since polymers could not be vaporized in the spectrometer without thermal decomposition. However, newer techniques, such as matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS), which uses a volatile carrier, the matrix, to transport nonvolatile polymers to the magnetic field of the spectrometer, allow the determination of molecular masses of polymers up to 200,000. 

The corabination of an inductively coupled plasma ion source and a magnetic sector-based mass spectrometer equipped with a multi-collector (MC) array [multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS)] offers precise and reliable isotope ratio data for many solid elements. In fact, MC-ICP-MS provides data, the trueness (accuracy) and precision of which is similar to, or, in some cases, even superior to, that achieved by thermal ionization mass spectrometry (TIMS), considered the benchmark technique for isotope ratio measurements of most solid elements [1], The basic strength of ICP-MS lies in the ion source, which achieves extremely high ionization efficiency for almost all elements [2, 3]. Consequently, MC-ICP-MS is likely to become the method of choice for many geochemists, because it is a versatile, user-friendly, and efficient method for the isotopic analysis of trace elements [4-8], The ICP ion source also accepts dry sample aerosols generated by laser ablation [9-16], The combination of laser ablation (LA) with ICP-MS is now widely accepted as a sensitive analytical tool for the elemental and isotopic analysis of solid samples. 

Gunther D, Heimich CA (1999) Enhanced sensitivity in laser ablation-ICP mass spectrometry using helium-argon mixtures as aerosol carrier. J Anal At Spectrom 14 1363-1368 Habfast K (1998) Fractionation correction and multiple collectors in thermal ionization isotope ratio mass spectrometry. Inti J Mass Spectrom 176 133-148... 

A number of other laser spectroscopic techniques are of interest but space does not permit their discussion. A few specialized methods of detecting laser absorption worthy of mention include multiphoton ionization/mass spectrometry (28), which is extremely sensitive as well as mass selective for gas-phase systems optically detected magnetic resonance (29) laser intracavity absorption, which can be extremely sensitive and is applicable to gases or solutions (30) thermal blooming, which is also applicable to very weak absorbances in gases or liquids (31) and... 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Resonance ionization mass spectrometry (RIMS) is much more efficient at getting ions into the detectors. First, atoms are removed from the sample surface with a pulsed laser, which can release the atoms thermally without ionizing them. Then, by using carefully tuned lasers, the element of interest in the resulting gas plume can be ionized at almost 100% efficiency, while other elements are not ionized at all. The ions are extracted into a time-of-flight... 

Resonance ionization mass spectrometry as a combination of resonance laser ionization with mass spectrometry can be performed on gas atoms only. Therefore, in RIMS of solid samples, before resonance ionization, a neutral gas has to be produced using several methods known from solid state mass spectrometry. During the evaporation of solid material, e.g., by laser evaporation, thermal evaporation or by sputtering with a primary ion beam, the formation of ions should to be avoided. In RIMS, mostly the thermal evaporation of sample from a heated W or Re filament is applied. 

At the present time, the most up-to-date methods of ionization in TLC/MS are FAB and secondary ionization mass spectrometry (SIMS) in which organic molecules are sputtered from the surface by impact of a stream of high-energy molecules, and laser desorption (LD) in which sputtering of organic molecules from the surface of a support is due to the influence of high thermal energy produced by a laser beam onto a plate surface. The ions produced this way are usually even-electron (M + H) and appear by means of chemical ionization. The interpretation of the spectrum follows the same lines. 

Mass spectrometry requires that the material being studied be converted into a vapor. Great strides have been taken in recent years to address this problem, especially in enticing large, thermally fragile (bio)molecules into the vapor state. Matrix assisted laser ionization-desorption (MALDI) and electrospray ionization (ESI) are two current forefront methods that accomplish this task. Even components of bacteria and intact viruses are being examined with these approaches. John B. Fenn and Koichi Tanaka shared in the award of a Nobel Prize in 2002 for their respective contributions to development of electrospray ionization and soft laser desorption. 

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