Aerosols thermal ionization

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... 

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... 

Nusko, R. and Heumann, K.G. (1997) Crm/CrVI speciation in aerosol particles by extractive separation and thermal ionization isotope dilution mass spectrometry. Fresenius J. Anal. Chem., 357, 1050-1055. 

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. 

Ionization of macro-particles is determined by their work function, which is usually essentially lower than the ionization potential of atoms and molecules. Therefore, thermal ionization of aerosol particles provides high electron density and conductivity at relatively low temperatures, which is apphed in particular in magneto-lydrodynamic generators (Kirillin Sheindlin, 1971) and rocket engine torches (Musin, 1974). Absorption and reflection of radiowaves by the plasma of the rocket engine torches affects and complicates control of the rocket trajectory. Also, the electron density in flames can be high because of thermal ionization of macro-particles (see, for example, Shuler Weber, 1954). 

A low aerosol density, n rl <plasma chemistry. In this case, considered by Fridman (1976), the combination of the Poisson equation with the Boltzmatm distribution leads to equations for the potential cp r) and electron density n r) around a spherical thermally ionized particle ... 

Thermally ionized aerosol particles have strong inherent electric fields (2-137) when their charge Za is high. As a result, the electric conductivity of aerosols at n rl <external electric field E, whereas more electrons can be released from trapping by the inherent electric field of a macro-particle at higher E. The electric conductivity of free space between macro-particles can be presented as... 

As a result, the dependence of electric conductivity of thermally ionized aerosols on the electric field is ... 

Space Distribution of Electron Density and Electric Field Around a Thermo-lonized Aerosol Particle. Consider the thermal ionization of a macro-particle of radius 10 p.m, work function 3 eV, and temperature 1500 K. Find the electron concentration just near the surface of the aerosol particle and corresponding value of the Debye radius, r. Calculate the total electric charge of the macro-particle, the electric field on its surface, and the electric field at a distance 3 p.m from the surface. 

For the determination of Ir in aerosol particulate samples, Heller-Zeisler et al. (2000) compared three methods, INAA, RNAA, and thermal-ionization mass spectrometry. In the RNAA procedure, Ir was separated from the digested sample on an anion-exchange resin as a chlorocomplex. The sensitivities of INAA and RNAA were comparable, i.e., 60-100 fg and both of them were superior to mass spectroscopy. 

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. 

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. 

Some solid materials are very intractable to analysis by standard methods and cannot be easily vaporized or dissolved in common solvents. Glass, bone, dried paint, and archaeological samples are common examples. These materials would now be examined by laser ablation, a technique that produces an aerosol of particulate matter. The laser can be used in its defocused mode for surface profiling or in its focused mode for depth profiling. Interestingly, lasers can be used to vaporize even thermally labile materials through use of the matrix-assisted laser desorption ionization (MALDI) method variant. 

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. If the sample examined is dissolved as a solid solution in a matrix, the rapid expansion of the matrix, often an organic acid, covolatilizes the entrained sample. Proton transfer from the matrix occurs to give protonated molecular ions of the sample. Normally thermally unstable, polar biomolecules such as proteins give good yields of protonated ions. This is the basis of matrix-assisted laser desorption ionization (MALDI). 

To measure an atomic absorption signal, the analyte must be converted from dissolved ions in aqueous solution to reduced gas-phase free atoms. The overall process is outlined in Figure 6.16. As described earlier, the sample solution, containing the analyte as dissolved ions, is aspirated through the nebulizer. The solution is converted into a line mist or aerosol, with the analyte still dissolved as ions. When the aerosol droplets enter the flame, the solvent (water, in this case) is evaporated. We say that the sample is desolvated. The sample is now in the form of tiny solid particles. The heat of the flame can melt (liquefy) the particles and then vaporize the particles. Finally, the heat from the flame (and the combustion chemistry in the flame) must break the bonds between the analyte metal and its anion, and produce free M° atoms. This entire process must occur very rapidly, before the analyte is carried out of the observation zone of the flame. After free atoms are formed, several things can happen. The free atoms can absorb the incident radiation this is the process we want. The free atoms can be rapidly oxidized in the hostile chemical environment of the hot flame, making them unable to absorb the resonance lines from the lamp. They can be excited (thermally or by collision) or ionized, making them unable to absorb the resonance lines from the lamp. The analyst must control the flame conditions, flow rates, and chemistry to maximize production of free atoms and minimize oxide formation, ionization, and other unwanted reactions. While complete... 

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