Thermal ionization mass spectrometr

Figure 15. Error in age vs. age, both on log scales (after Edwards et al. 1997). Each data point represents data from a particular sample analyzed by thermal ionization mass spectrometric techniques. Solid circles represent Pa ages. Contours of aniytical error in Pa/ U pertain to the Pa data points. Shaded squares represent °Th ages. See text for discussion.

Thermal ionization mass spectrometry (TIMS) is one of the oldest mass spectrometric techniques, first applied by Dempster in 1918.114 The thermal emission of positivly charged ions emitted from a salt on a heated surface was first observed by Gehrcke and Reichenheim 12 years before.115 The thermal surface ionization source is a very simple ion source and operates under high vacuum conditions. TIMS is mostly useful for elements with relatively low ionization energy ( )) - in... 

A main characteristic of TIMS is that positively or negatively charged ions of the analyte are also formed and used for mass spectrometric analysis. In negative thermal ionization mass spectrometry (NTIMS) elements or molecules with a relatively high electron affinity (/iea > 2eV) can be... 

The method requires mass spectrometric measurements of ratios of stable or long lived isotopes and can be applied to about 75 elements, from which about 50 can be measured as solid compounds by thermal ionization mass spectrometry. 

Although several different mass spectrometric methods have been deployed to determine enriched stable isotopes in human studies of nutrient mineral metabolism, thermal ionization mass spectrometry (TIMS) and particularly ICP-MS are now used almost exclusively. ICP-MS is rapid, very sensitive, and sample preparation and introduction is often simplified. Furthermore, ICP-MS can be coupled directly to separation techniques such as size-exclusion chromatography (SEC), high-performance liquid chromatography (HPLC), or CE so that speciation, the determination of the chemical form of particular elements, may also be studied. The two major drawbacks of ICP-MS, low precision relative to TIMS and interference from polyatomic ions in the argon plasma, have largely been overcome by new generations of instruments equipped with multiple collectors and collision/reaction cells, respectively. 

A variety of mass spectrometric approaches have been used for determining the isotopic composition and concentration of trace elements in biological matrices. The more commonly used are thermal ionization-mass spectrometry (TI-MS) [5,8], inductively coupled plasma-mass spectrometry (ICP-MS) [7,9], fast atom bombardment-mass spectrometry (FAB-MS) [10-12], and gas chromatography-mass spectrometry (GC-MS) [4]. 

Once a sample is collected, the isotopic composition of uranium must be determined as the content is one of the main factors that determine the price of the product. Several mass spectrometric techniques have been developed for direct isotope analysis of gaseous UFg and for indirect analysis (usually after hydrolysis) of liquid and gaseous UFg samples. The use of a thermal ionization mass spectrometer (TIMS), nowadays equipped with several detectors (i.e., multicollector TIMS), has been the method of choice for many years, but the sample must be hydrolyzed to liquid form (uranyl fluoride or uranyl nitrate solutions) and the uranium must be purified (usually not a problem for UFg samples), as mentioned, for example, by ASTM (C1413 2011). The method is used for hydrolyzed samples of UFg (UOjFj (uranyl fluoride)) or for... 

Thermal ionization mass spectrometry (TIMS) is a sensitive mass spectrometric technique that has been deployed in some cases to measure trace amounts of uranium in urine and its isotopic composition (Kelly et al. 1987). The authors report measurement of one freeze-dried urine standard sample (SRM 2670) and two actual urine samples collected from children. For TIMS measurements, chemical separation has to be performed prior to the analysis. In an earlier work by the same author (Kelly and Fassett 1983), a spike of was used to implement isotope dilution measurements of picogram quantities of uranium in biological tissues. As mentioned earlier, a single urine sample tested by TIMS gave 3.4 ng L (Wrenn et al. 1992). 

Bombick DD, Allison A. Desorption/Ionization mass spectrometric technique for the analysis of thermally labile compounds based on thermionic emission materials. Anal Chem. 1987 59 458-66. 

Table 11-1 list-s the most important types of atomic mass spectrometry. Historically, thermal ionization mass spectrometry and spark source mass spectrometry were the first mass spectrometric methods developed for qualitative and quantitative elemental analysis, and these types of procedures still find applications, although they are now overshadowed by some of the other methods listed in Table 11-1, particularly inductively coupled plasma mass. spectrometry (fCPMS). 

The structures 42 and 43 (Rgs. 19 and 106) are confirmed by chemical ionization mass spectrometric data. For endo and exo 42 the most important ions are the quasi-molecular ion (100% in both cases) and the fragment-ion, obtained by loss of water (60% for exo 42, 78% for endo 42). In the spectra of exo 43 and endo 43 intense ions occur at m/z 433, 417 and 399 for the quasi-molecular ion and fragments, derived from successive losses of an oxygen atom and water, respectively. While the molecular ions are very intense (70% for exo 43 and 100% for endo 43), the ions at m/z 417 (11% in both cases) can be ascribed to (thermal) decomposition of the peroxides in the ion source. The resulting alcohols yield via dehydration the ions at m/z 399 (15% for exo 43 and 10% for endo 43). 

To achieve sufficient vapor pressure for El and Cl, a nonvolatile liquid will have to be heated strongly, but this heating may lead to its thermal degradation. If thermal instability is a problem, then inlet/ionization systems need to be considered, since these do not require prevolatilization of the sample before mass spectrometric analysis. This problem has led to the development of inlet/ionization systems that can operate at atmospheric pressure and ambient temperatures. Successive developments have led to the introduction of techniques such as fast-atom bombardment (FAB), fast-ion bombardment (FIB), dynamic FAB, thermospray, plasmaspray, electrospray, and APCI. Only the last two techniques are in common use. Further aspects of liquids in their role as solvents for samples are considered below. 

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. 

Several original papers must be mentioned that deal with mass spectrometric techniques which the numerous reviews do not comprise. Kaufmann and coworkers268,288 studied the mass spectrometric analysis of carotenoids and some of their fatty acid esters using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry and its post-source-decay (PSD) variant. Some advantages concerning the thermal instability and limited solubility were discussed, but the fragmentation paths of the carotenoid cations were found to be essentially the same as those observed with conventional techniques. 

Temperature-programmed vacuum pyrolysis in combination with time-resolved soft ionization mass spectrometry allows principally to distinguish between two devolatilization steps of coal which are related to the mobile and non-mobile phase, respectively. The mass spectrometric detection of almost exclusively molecular ions of the thermally extracted or degraded coal products enables one to study the change of molecular weight distribution as a function of devolatilization temperature. Moreover, major coal components can be identified which are released at distinct temperature intervals. 

Scheme 16.18. Pyrolysis with mass spectrometric detection of 1,3-dibromo- and 1,3-dinitronaphthalene indicates that 51 is not thermally stable at 900 C. The measured ionization potential suggests that the ring opened product 65 is formed under these conditions (cf. Scheme 16.10). ...

Different mass spectrometric techniques can be classified according to the evaporation and ionization methods applied. Evaporation of solid samples can be performed, for example, by thermal (e.g., on a hot tantalum filament or in a heated graphite furnace) or laser-induced evaporation, and by electron or ion bombardment. Electron ionizaton (El), ionization during the sputtering process with a primary ion beam, resonant or non-resonant laser ionization or thermal surface ionization... 

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