Thermal ionization efficiency

Traditional thermal ionization mass-spectrometry efficiently ionizes Ra (>1%) (Cohen and Onions, 1991 Volpe et al., 1991). Elements with higher first ionization energies, such as Th and U, give significantly lower ion yields, typically ionization efficiencies up 5%o for U. Yet, even given typical thermal ionization efficiencies of 0.5-0.03%o for 10-100 ng Th loads... 

Fig. 15.5. Thermal ionization efficiency a vs. filament temperature. The curves for Mo reflect the dependence of a on the filament material. The advantage of the Re over the W filament follows fromEqs. 15.1-2. Reproduced from Ref. [7] with permission. John Wiley Sons Ltd, 2008.

Samples to be examined by inductively coupled plasma and mass spectrometry (ICP/MS) are commonly in the form of a solution that is transported into the plasma flame. The thermal mass of the flame is small, and ingress of excessive quantities of extraneous matter, such as solvent, would cool the flame and might even extinguish it. Even cooling the flame reduces its ionization efficiency, with concomitant effects on the accuracy and detection limits of the ICP/MS method. Consequently, it is necessary to remove as much solvent as possible which can be done by evaporation off-line or done on-line by spraying the solution as an aerosol into the plasma flame. 

In contrast to thermal ionization methods, where the tracer added must be of the same element as the analyte, tracers of different elemental composition but similar ionization efficiency can be utilized for inductively coupled plasma mass spectrometry (ICPMS) analysis. Hence, for ICPMS work, uranium can be added to thorium or radium samples as a way of correcting for instrumental mass bias (e g., Luo et al. 1997 Stirling et al. 2001 Pietruszka et al. 2002). The only drawback of this approach is that small inter-element (e g., U vs. Th) biases may be present during ionization or detection that need to be considered and evaluated (e.g., Pietruszka et al. 2002). 

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... 

Wayne DM, Hang W, McDaniel DK, Fields RE, Rios E, Majidi V (2002) The thermal ionization cavity (TIC) source elucidation of possible mechanisms for enhanced ionization efficiency. Inti J Mass Spectrom 216 41-57... 

Thermal ionization has a mass-dependent efficiency and the intensity Im measured at mass m by the DVM relates to the actual number of moles Nm through the unknown function f(m)... 

Advances in TIMS-techniques and the introduction of multiple collector-ICP-MS (MC-ICP-MS) techniques have enabled the research on natural variations of a wide range of transition and heavy metal systems for the first time, which so far could not have been measured with the necessary precision. The advent of MC-ICP-MS has improved the precision on isotope measurements to about 40 ppm on elements such as Zn, Cu, Fe, Cr, Mo, and Tl. The technique combines the strength of the ICP technique (high ionization efficiency for nearly all elements) with the high precision of thermal ion source mass spectrometry equipped with an array of Faraday collectors. The uptake of elements from solution and ionization in a plasma allows correction for instrument-dependent mass fractionations by addition of external spikes or the comparison of standards with samples under identical operating conditions. All MC-ICP-MS instruments need Ar as the plasma support gas, in a similar manner to that commonly used in conventional ICP-MS. Mass interferences are thus an inherent feature of this technique, which may be circumvented by using desolvating nebulisers. 

The filament material most commonly used in thermal ionization is rhenium. There are several properties that dictate its choice. It has a high enough melting point (3180°C) that it can withstand the temperatures required for efficient ionization (up to about 2200°C). It has the highest work function of any metal with a high enough melting point like all metals, its work function varies with the crystal... 

A long-standing and still-current challenge in thermal ionization mass spectrometry is to improve ionization efficiency. This is usually defined experimentally as the ratio of ions collected to the number of atoms loaded it thus includes all aspects of the ionization, extraction, transmission, and collection processes. 

High efficiency denuders that concentrate atmospheric S02 were coupled to an ion chromatograph to yield detection limits on the order of 0.5 ppt (106). A newer approach has been introduced for the quantitative collection of aerosol particles to the submicrometer size (107). When interfaced to an inexpensive ion chromatograph for downstream analysis, the detection limit of the overall system for particulate sulfate, nitrite, and nitrate are 2.2,0.6, and 5.1 ng/m3, respectively, for an 8-min sample. A two-stage membrane sampling system coupled with an ion trap spectrometer has been utilized for the direct analysis of volatile compounds in air, with quantitation limits to low ppt levels (108). Toluene, carbon tetrachloride, tricholoroethane, and benzene were used in these studies. The measurement of nitrogen dioxide at ppb level in a liquid film droplet has been described (109) (see Air pollution). A number of elements in environmental samples have been determined by thermal ionization ms (Table 6). The detection limit for Pu was as low as 4 fg. 

The transfer of ions from atmospheric pressure to the vacuum of a spectrometer necessarily induces ion losses. But these losses are compensated by the higher total ion yield in the API source due to fast thermal stabilization at atmospheric conditions. Indeed, when the sample ionization is performed under atmospheric pressure [50,51 ], an ionization efficiency 103 to 104 times as great as in a reduced-pressure Cl source is obtained. 

Thermal ionization from hot surfaces has been used extensively to produce positive ions for isotopic analysis The efficiency of positive ion formation for selected alkali and other elements is controlled by the Saha- Langmuir expression ... 

---