Efficiency, 79 ionization

In the ideal case for REMPI, the efficiency of ion production is proportional to the line strength factors for 2-photon excitation [M], since the ionization step can be taken to have a wavelength- and state-mdependent efficiency. In actual practice, fragment ions can be produced upon absorption of a fouitli photon, or the ionization efficiency can be reduced tinough predissociation of the electronically excited state. It is advisable to employ experimentally measured ionization efficiency line strengdi factors to calibrate the detection sensitivity. With sufficient knowledge of the excited molecular electronic states, it is possible to understand the state dependence of these intensity factors [65]. 

The degree of ionization increases with temperature, and at 6000-8000 K, where ionization efficiencies are 90 to 95%, nearly all atoms exist as ions in the plasma. Operation of the plasma torch under conditions that produce a cooler flame (cold plasma) has important advantages for some applications, which are discussed after the next section. 

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. 

Ionization efficiency. The ratio of the number of ions formed to the number of electrons, photons, or particles that are used to produce ionization... 

To increase the radiation absorption and ionization efficiency of IR YAG-Nd laser (wave length - 1064 nm) used in for analysis of synthetic oxide single crystals Bi Ge O, Li MoO, LiB O, the samples were grounded and pills of 010 mm were pressed and analyzed by LIMS EMAL-2 (Uki aine) ... 

Because SIMS can measure only ions created in the sputtering process and not neutral atoms or clusters, the detection limit of a particular element is affected by how efficiently it ionizes. The ionization efficiency of an element is referred to as its ion yield. The ion yield of a particular element A is simply the ratio of the number of A ions to the total number of A atoms sputtered from the mixing zone. For example, if element A has a 1 100 probability of being ionized in the sputtering process—that is, if 1 ion is formed from every 100 atoms of A sputtered from the sample—the ion yield of A would be 1/100. The higher the ion yield for a given element, the lower (better) the detection limit. 

The material evaporated by the laser pulse is representative of the composition of the solid, however the ion signals that are actually measured by the mass spectrometer must be interpreted in the light of different ionization efficiencies. A comprehensive model for ion formation from solids under typical LIMS conditions does not exist, but we are able to estimate that under high laser irradiance conditions (>10 W/cm ) the detection limits vary from approximately 1 ppm atomic for easily ionized elements (such as the alkalis, in positive-ion spectroscopy, or the halogens, in negative-ion spectroscopy) to 100—200 ppm atomic for elements with poor ion yields (for example, Zn or As). 

Surface analysis by non-resonant (NR-) laser-SNMS [3.102-3.106] has been used to improve ionization efficiency while retaining the advantages of probing the neutral component. In NR-laser-SNMS, an intense laser beam is used to ionize, non-selec-tively, all atoms and molecules within the volume intersected by the laser beam (Eig. 3.40b). With sufficient laser power density it is possible to saturate the ionization process. Eor NR-laser-SNMS adequate power densities are typically achieved in a small volume only at the focus of the laser beam. This limits sensitivity and leads to problems with quantification, because of the differences between the effective ionization volumes of different elements. The non-resonant post-ionization technique provides rapid, multi-element, and molecular survey measurements with significantly improved ionization efficiency over SIMS, although it still suffers from isoba-ric interferences. 

Resonant (R-) laser-SNMS [3.107-3.112] has almost all the advantages of SIMS, e-SNMS, and NR-laser-SNMS, with the additional advantage of using a resonance laser ionization process which selectively and efficiently ionizes the desired elemental species over a relatively large volume (Eig. 3.40 C). Eor over 80% of the elements in the periodic table, R-laser-SNMS has almost unity ionization efficiency over a large volume, so the overall efficiency is greater than that of NR-laser-SNMS. Quantification is also simpler because the unsaturated volume (where ionization is incom-... 

The energy of the electron beam (the potential difference between filament g and the area of impact) is adjusted to 70 ev for maximum ionization efficiency, but it can be varied by appropriate choice of potential difference between the heated filament g and anode h ... 

Figure 2. Ionization efficiency curves in the Cermak-Herman operation of an ion source. Relative ion intensity normalized at 40 volts for CHA+ and CH +. Voltage between filament and ionization chamber constant at 8 volts...

This reaction was considered the only reaction channel because it is the only known channel which is exothermic with ground state CH4+ ions. Reactions yielding C2H5+ and C2H4 + have been observed and are the least endothermic of the possible reactions of CH4+ with CH4. However, ionization efficiency curves establish CH3 + rather than CH4 + as the reactant ion. Reaction 14 ... 

There should be high sample transfer to the mass spectrometer or, if this takes place in the interface, ionization efficiency. This is of particular imporfance when frace-level componenfs are of inferesf or when polar and/or labile analytes are involved. 

The buffer concentration also directly affects the size of droplets produced - the higher the buffer concentration, then the smaller they are, and this is desirable. The buffer concentration, however, has an effect on the ionization efficiency and at high buffer concentrations (>10 M) the relationship between detector response and analyte concentration is not linear. As indicated earlier in Figure 2.6, this situation must be avoided for precise quantitative measurements. 

The effect of the buffer on the efficiency of electrospray ionization was mentioned earlier in Section 4.7.1. This is a good example of the dramatic effect that this may have - good chromatographic separation and ionization efficiency with formic, acetic and propionic acids, and good separation, although with complete suppression of ionization, with trifluoroacetic acid (TFA), the additive used for the protein application described previously. Post-column addition of propionic acid to the mobile phase containing TFA has been shown to reduce, or even... 

Fragmentor voltage Over 220 V Under 90 V 110-150V Collision-induced dissociation effect Poor ionization efficiency Good ionization efficiency... 

Method development is important. LC-MS performance, probably more than any other technique involving organic mass spectrometry, is dependent upon a range of experimental parameters, the relationship between which is often complex. While it is possible (but not always so) that conditions may be chosen fairly readily to allow the analysis of simple mixtures to be carried out successfully, the widely variable ionization efficiency of compounds with differing structures often makes obtaining optimum performance for the study of all components of a complex mixture difficult. In such cases, the use of experimental design should be seriously considered. 

Most reported triazine LC applications are reversed-phase utilizing C-8 and C-18 analytical columns, but there are also a few normal-phase (NH2,CN) and ion-exchange (SCX) applications. The columns used range from 5 to 25-cm length and from 2 to 4.6-mm i.d., depending on the specific application. In general, the mobile phases employed for reversed-phase applications consist of various methanol and/or acetonitrile combinations in water. The ionization efficiency of methanol and acetonitrile for atmospheric pressure chemical ionization (APcI) applications were compared, and based on methanol s lower proton affinity, the authors speculated that more compounds could be ionized in the positive ion mode when using methanol than acetonitrile in the mobile phase. 

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). 

Another material based on the crown ether extractant 4,4 (5 )-bis(t-butyl-cyclohexano)-18 crown-6, marketed under the name Sr-Spec, is useful for separations involving divalent cations including Pb, Ba, and Ra (Horwitz et al. 1991). For Ra analysis by TIMS, Ra-Ba separations are required because the presence of Ba drastically decreases the ionization efficiency of fg Ra samples from 10% to <1%. This material has been widely used for separations of Ra from Ba (e.g., Chabaux et al. 1994 Lundstrom et al. 1998 Rihs et al. 2000 Joannon and Pin 2001 Pietruszka et al. 2002) and is a complement or alternative to cation exchange separations for EDTA complexes of these elements (Volpe et al. 1991 Cohen and O Nions 1991). Sr-Spec material would also be useful for °Pb analysis, since Pb has a greater distribution coefficient than Sr with this extractant. 

Carburization of rhenium filaments has been used to optimize Th and Pa ionization efficiency for TIMS analysis on single filaments (Esat 1995). ReC has a greater work function than Re metal, and elemental oxidation state is maintained in the reduced or metal state by the presence of carbon in the filament. Using this method and a mass spectrometer with improved ion optics, Esat (1995) was able to improve Th transmission and ionization efficiency by about a factor of 30 over conventional methods. Using more conventional mass spectrometry, Murrell et al. (personal communication) were able to improve ionization efficiency for Pa and Th by a factor of 5-10 over conventional graphite sandwich loads on Re filaments (Goldstein et al. 1989 Pickett et al. 1994). For Pa analysis, one drawback is that Pa and U ionization commonly overlap using this... 

Resonance ionization methods (RIMS) have also been explored for improving Th ionization efficiency for mass spectrometric measurement (Johnson and Fearey 1993). As shown in Figure 3, two lasers are required, a continuous resonant dye laser for resonance of thorium atoms, and a continuous UV argon laser for transition from resonance to ionization. Consequently, sophisticated laser instrumentation is required for these methods. 

The weakness of MC-ICPMS lies in the inefficiency by which ions are transferred from the plasma source into the mass spectrometer. Therefore, despite very high ionization efficiencies for nearly all elements, the overall sensitivity (defined as ionization plus transmission efficiencies) of first generation MC-ICPMS instruments is of the order of one to a few permil for the U-series nuclides. For most, this is comparable to what can be attained using TIMS. 

Ion extraction. The aspirated or laser ablated sample is transported from the sample introduction system into the center of the torch by a 1 1/min flow of Ar carrier gas where it is immediately dissociated and ionized by energy transfer with the hot -6000 K temperature of the surrounding Ar plasma. Ionization efficiencies are >95% for U and Th (Jarvis et al., 1992). For laser ablation sampling, helium may be employed as the carrier... 

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