Thermal ionization source

Thermal ionization is based on the production of atomic or molecular ions at the hot surface of a metal filament [95,96]. In this ionization source, the sample is deposited on a metal filament (W, Pt or Re) and an electric current is used to heat the metal to a high temperature. The ions are formed by electron transfer from the atom to the filament for positive species or from the filament to the atom for negative species. The analysed sample can be fixed to the filament by depositing drops of the sample solution on the filament surface followed by evaporation of the solvent to complete dryness, or by using electrodeposition methods. 

Analysis of inorganic compounds by mass spectrometry. Top El spectrum of orthorhombic sulfur (Ss). Bottom ESI spectrum of Cr(CO)2(dpe)2. The last spectrum is redrawn from data taken from Traeger J.C. and Colton R., Adv. Mass Spectrom., 14, 637-659, 1998. 

Single, double and triple filaments have been broadly used in thermal ionization sources. In a single filament source, the evaporation and ionization process of the sample are carried out on the same filament surface. Using a double filament source, the sample is placed on one filament used for the evaporation while the second filament is left free for ionization. In this way, it is possible to set the sample evaporation rate and ionization temperature independently, thus separating the evaporation from the ionization process. This is interesting when the vapour pressure of the studied elements reaches high values before a suitable ionization temperature can be achieved. A triple filament source can be useful to obtain a direct comparison of two different samples under the same source conditions. 

Positive and negative ions can be obtained by the thermal ionization source. High yields of positive and negative ions are obtained for atoms or molecules with low ionization potential and with high electron affinity, respectively. Owing to the ionization process and 

Schematic representation of a thermal ionization cavity source. 

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Plasma torches and thermal ionization sources break down the substances into atoms and ionized atoms. Both are used for measurement of accurate isotope ratios. In the breakdown process, all structural information is lost, other than an identification of elements present (e.g., as in inductively coupled mass spectrometry, ICP/MS). 

Dacheux N, Aupiais J (1997) Determination of uranium, thorium, plutonium, americium, and curium ultratraces by photon electron rejecting alpha liquid scintillation. Anal Chem 69 2275-2282 Duan YX, Chamberlin EP, Olivares JA (1997) Development of a new high-efficiency thermal ionization source for mass spectrometry. Inti JMass Spectrom IonProcessesl61 27-39 Edwards RL, Chen JH, Wasserburg GJ (1987) systematics and the precise... 

Glow discharge source (GDMS) Laser ion source (LIMS) Secondary ion source (SIMS) Sputtered neutral source (SNMS) Thermal ionization source (TIMS) Inductively coupled plasma ion source (ICP-MS)... 

Fig. 1. UNISOR thermal -ionization source. 1) target/window, 2) graphite felt catcher, 3) tungsten filament, 4) Ta target retainer, 5) graphite felt heat shielding, 6) Ta heat shield, 7) Ta support for graphite felt.

In thermal ionization sources, the most abundant ions are usually the singly charged atomic ions. No multiply charged ions can be observed under normal ionization conditions. Cluster ions occur very seldom. However, some metal compounds lead to abundant metal oxide ions. 

The studies of calcium metabolism in normal children are underway with 15-20 children to be studied in the near future The data generated from these studies will be used as the basis for studies of diseases of calcium metabolism in children We anticipate being able to study a number of children with FOP and possibly contribute to innovative therapies for these children by monitoring changes in calcium kinetics Improvements in the thermal ionization source for the quadrupole mass spectrometer are underway a substantial improvement in sesitivity is expected from this effort The model used for data analysis is under continuing investigation in order to improve its predictive and experimental design capabilities ... 

Blakely and Vestal [24] employed the thermospray system with the quadrupole mass spectrometer and demonstrated that it could provide stable vaporization and ionization at flow rates up to 2 ml/min with an aqueous mobile phase. If the mobile phase contained a significant amount of ions in solution ca. 10 to 1.0 M) no extra thermal ionization source is required to achieve detection of many non-volatile solutes at the sub-microgram level. They found that with weakly ionized mobile phases, a conventional electron beam needs to be used to provide gas-phase reagent ions for the chemical ionization of the solute. The thermospray system has been effectively used by Voyksner et al. [25] in the analysis of cancer drugs. 

Electron ionization usually gives some molecular ions and sometimes these are abundant. Often it is difficult to be certain which ion, if any, is the molecular ion. For this rea.son, it is advantageous to obtain both El and Cl spectra, the first giving good structural information and the second good molecular mass information. g Plasma torches and thermal ionization sources break down the substances into atoms and ionized atoms. Both are used for measurement of accurate isotope ratios. In the breakdown process, all structural information is lost, other than an identification of elements present (e.g., as in inductively coupled mass spectrometry, ICP/MS),... 

FIGURE 17.1. Thermal ionization source. Figure from Smith (2000), pg. 9. 

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... 

Table 1. Melting points and work functions of metals which are often used as filament strips in thermal ionization sources ...

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). 

Figure 7.1. Double-filament assembly thermal ionization source.

Which situation will produce more positive-ion current in a thermal ionization source ionization of sodium salt on (a) rhenium (( ) = 4.98 eV) filament heated to 2200°C or (b) tungsten (( ) = 4.58 eV) filament heated to 2100°C ... 

The first commercially available multiisotope ratio mass spectrometers offered by Finnigan MAT and VG Isotopes were fitted to thermal ionization sources. Thermal ionization of the purified target element from a metal filament, heated resistively in vacuum, is used for sensitive measurements of the alkali, alkaline earth and rare earth elements (REEs). Surface ionization techniques are typically element-specific and can produce relatively high ionization yields (> 10%) with low backgrounds. Early MC instruments were equipped with... 

In experiments aiming at a search for emitters, which surface-ionize organic compounds more efficiently [20] (based on the understanding that the work func tion of the surface is greatly increased by the chemisorption of electronegative gases [21]), a considerable increase in alkali metal ion currents is always observed, when chlorine (or fluorine) is introduced into a mass spectrometer with a thermal ionization source. This phenomenon is well interpreted as the so-called stimulated surface ionization [22] in which the exothermic reaction on the surface is responsible for the ionization. This is another interesting example of RSI. 

Thermal ionization Thermal ionization sources produce ions by heating sample-coated metallic filaments. Samples in solution are first deposited as microlitre drops on the filaments and dried at low temperatures forming thin salt or oxide layers (Figure 4). The prepared filament is transferred into the mass spectrometer source, the source is evacuated, and the filament is then heated to temperatures ranging from 800 to 2000"C. The probability of forming an ion in thermal equilibrium with the surface of the hot metal filament depends on the temperature (T), work function of the filament (W), and the ionization potential associated with the production of a given ion species (IP). This is described by the Langmuir- Saha equation as the relative number of ionized particles (N+) to neutral species (N ) evaporated from the filament (Eqn [4])... 

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