Thermal ionization-mass spectrometry

The first thermal ionization source was developed by Dempster in 1918. The solid material to be analyzed is applied to a hot metal filament and ions are produced by thermal surface ionization at a temperature of 2000 °C. A conunonly used thermal ionization source is the three-filament ion source, developed in 1953 by Inghram and Chupka . This ionization source consists of two parallel filament strips for the sample and an ionization filament in a plane perpendicular to and between the other two filaments. Fig. 4 shows a sectional view of this kind of ion 

The dissolved or suspended sample is applied to the filaments by a syringe and evaporated to dryness. By heating these filaments the sample is vaporized and im- 

The thermal ionization source is useful for an exceedingly large number of metals and metal compounds having first ionization potentials below approximately 9 eV (Table 2). 

Compound Experiment No. Ca-content (ppm) Standard diviation abs Sjel (ppm) (%)  

In the determination of metals from biological and medical matrixes, thermal ionization mass spectrometry is seldom used. Disadvantages of thermal ionization MS are the great fluctuations in the results, caused by different instrumental requirements. Isotope fractionation resulting from vaporization of the sample and the dependence of this process on temperature are the main sources of error. However, the development of computer-controlled sample preparation and measurements have minimized these errors  

The basis of thermal ionization is that when a neutral analyte species approaches a hot metallic surface, its Fermi levels are nearly equal to those of the metal filament, with the consequence that an electron can tunnel from the analyte 

Vice versa, nonmetallic elements with high IE and metal oxides may form negative ions [33,34], The degree of ionization a is then obtained from the modified Saha-Langmuir equation 

The basic principles of thermal ionization mass spectrometry (TIMS) operation were described in Chapter 1 a drop of the liquid sample is deposited on a filament, a low electric current heats the filament, and the solution is evaporated to dryness. The filament current (temperature) is then raised and atoms of the sample are emitted and ionized (either by the same filament or by a second electron emitting filament). The ions are accelerated by an electric field, pass through an electrostatic analyzer (ESA) that focuses the ion beam before it enters a magnetic field that deflects the ions into a curved pathway (in some devices, the ions enter the magnetic field before the ESA—referred to as reverse geometry). Heavy and light ions are deflected by the field at different curvatures that depend on their mass-to-charge ratio. A detector at the end of the ion path measures the ion current (or counts the ion pulses). There are many variations of ion sources, ion separation devices, and detectors that are used in TIMS instruments and specifically adapted for ultratrace or particle analysis. 

Ion separation In TIMS devices, the separation of the ions according to their mass-to-charge ratio is based on the deflection of the ion beam in a magnetic field. 

Total Efficiency (Atoms Loaded to Ions Detected) for Thermal Ionization of Uranium Using Various Sample Preparation and Loading Techniques 

Analytical Element and Sample Form/ Total Efficiency 

Source Adapted from Burger, S. et al., bit J. Mass Spectrom., 286, 70, 2009. With permission. Only ionization efficiency. 

Most particle-analysis methods (SEM, SIMS and FT-TIMS) require the removal of particles from the swipe substrate and deposition on a flat surface. In the case of FT-TIMS, the particles are removed by ultrasonic treatment in a suitable suspension medium such as ethanol or siloxane. The suspension is then mixed with collodion and dried as a thin layer on the Lexan plastic for irradiation. After irradiation, this collodion layer can be peeled off to allow chemical etching of the fission tracks in the plastic. The collodion layer can also be replaced on the Lexan after etching with a slight offset so that the particles and tracks are visible under a Kght microscope at magnification 250-500. Replacement of the collodion layer is not necessary if a comparator microscope is available that allows viewing two objects (i.e., the collodion layer and the Lexan with tracks) simultaneously. 

The basic processes in TIMS, which apply equally to the measurement of particles, have been published elsewhere (Duckworth et al. 1986). In TIMS, the particle is deposited on a pure metal filament typical filament materials are W, Ta, and Re which have been purified by 

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Following the movement of airborne pollutants requires a natural or artificial tracer (a species specific to the source of the airborne pollutants) that can be experimentally measured at sites distant from the source. Limitations placed on the tracer, therefore, governed the design of the experimental procedure. These limitations included cost, the need to detect small quantities of the tracer, and the absence of the tracer from other natural sources. In addition, aerosols are emitted from high-temperature combustion sources that produce an abundance of very reactive species. The tracer, therefore, had to be both thermally and chemically stable. On the basis of these criteria, rare earth isotopes, such as those of Nd, were selected as tracers. The choice of tracer, in turn, dictated the analytical method (thermal ionization mass spectrometry, or TIMS) for measuring the isotopic abundances of... 

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Pickett DA, Murrell MT, Williams RW (1994) Determination of femtogram quantities of protactinium in geologic samples by thermal ionization mass spectrometry. Anal Chem 66 1044-1049 Pietruszka AJ, Carlson RW, Hauri EH (2002) Precise and accurate measurement of Ra- °Th- U disequilibria in volcanic rocks using plasma ionization multicollector mass spectrometry. Chem Geol 188 171-191... 

Volpe AM, Olivares JA, Murrell MT (1991) Determination of Radium isotope ratios and abundances in geologica samples by thermal ionization mass spectrometry. Anal Chem 63 916-919 Volpe AM, Goldstein SJ (1993) Ra- °Th disequilibrium in axial and off-axis mid-ocean ridge basalts. Geochim Cosmochim Acta 57 1233-1241... 

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Figure 4 Measurements of (A) uranium activity ratios, UARs (234U 238U) and U concentrations (B) across a salinity gradient off the Amazon River mouth (1996). UARs were determined by thermal ionization mass spectrometry (TIMS) at Caltech (D. Porcelli) U concentrations by ICPMS...

A. Deyhle. Improvements of Boron Isotope Analysis by Positive Thermal Ionization Mass Spectrometry Using Static Multicollection of CS2BO2 Ions. Int. J. Mass Spectrom., 206(2001) 79-89. 

R. Doucelance and G. Manhes. Reevaluation of Precise Lead Isotope Measurements by Thermal Ionization Mass Spectrometry Comparison with Determinations by Plasma Source Mass Spectrometry. Chem. Geol, 176(2001) 361-377. 

J. L. Mann and W. R. Kelly. Measurement of Sulfur Isotope Composition (834S) by Multiple-Collector Thermal Ionization Mass Spectrometry Using a 33S-36S Double Spike. Rapid Commun. Mass Spectrom., 19(2005) 3429-3441. 

M. D. Schmitz, S. A. Bowring, and T. R. Ireland. Evaluation of Duluth Complex Anorthositic Series (AS3) Zircon as a U-Pb Geochronological Standard New High-Precision Isotope Dilution Thermal Ionization Mass Spectrometry Results, Geochim. Cosmochim. Acta, 67(2003) 3665-3672. 

These methods require that the sample is either a gas or, at least, a volatile substance which can be easily converted into a gas (this explains the utility of mass spectrometry in the field of organic chemistry). In inorganic chemistry it is often more difficult to obtain a gaseous sample, and so other ionization sources have been developed. If the sample is thermally stable, it may be volatilized by depositing it on a filament and heating the filament (thermal ionization mass spectrometry - see below). In restricted cases (e.g., organometallic chemistry), chemical treatment of the sample may give a more volatile sample. 

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