Gas-phase interferences

8 Gas-phase Interferences. Virtually, all hydride forming elements interfere mutually. In some cases even very small concentrations of the interfering element cause severe signal depressions. The extent of the interference depends only on the concentration of the interfering element, not on the interferent-analyte ratio. After sufficient dilution of the sample solution these interferences do not appear. 

The hydride forming elements are normally present at very low concentrations in the sample solutions. When a large excess of sodium borotetrahy-dride is employed, no shortage of reductant should occur. It can thus be assumed that hydrides of all hydride forming elements present in the solution are formed and transported to the atomizer. Atomization of hydrides takes place via collisions with H radicals as explained above. 

As interferes in the determination of selenium at a tenfold lower concentration ( 0.01 mgl ) than As, which only shows an influence above 0.1 mgl Selenium in turn interferes much more in the determination of arsenic at concentrations above 0.001 mgU In addition, the interference is independent of the oxidation state of arsenic. In both cases, the degree of 

The degree of interference of other hydride forming elements on the determination of antimony increases in the following order Pb Bi As Te Ge Se Sn (Table 15). Lead virtually does not interfere and the influence of bismuth is very small and appears only at high concentrations. All other hydride forming elements interfere when their concentration in the sample solution is 100 /t,g/l or more. The interference can be eliminated by a suitable addition of some masking reagent as represented in Table 15. 

---

Electron spectroscopic techniques require vacuums of the order of 10 Pa for their operation. This requirement arises from the extreme surface-specificity of these techniques, mentioned above. With sampling depths of only a few atomic layers, and elemental sensitivities down to 10 atom layers (i. e., one atom of a particular element in 10 other atoms in an atomic layer), the techniques are clearly very sensitive to surface contamination, most of which comes from the residual gases in the vacuum system. According to gas kinetic theory, to have enough time to make a surface-analytical measurement on a surface that has just been prepared or exposed, before contamination from the gas phase interferes, the base pressure should be 10 Pa or lower, that is, in the region of ultrahigh vacuum (UHV). 

Chemical interferences involve a chemical reaction between the analyte and components of the sample matrix that reduces the formation of gaseous atoms during the atomization step. These interferences may be further characterized based on the type of chemical reaction involved. Volatile compound formation involves a reaction between the analyte and a matrix component that produces a volatile molecule that is vaporized out of the furnace during the pyrolysis step. Involatile compound formation involves a reaction between the analyte and a matrix component that produces a molecule that is insufficiently volatile or sufficiently stable to reduce the formation of atoms during the atomization step. Gas-phase interferences refer to a chemical reaction between the analyte and a concomitant in the vapor phase. 

Gas-phase interference effects have been noted due to the mutual interference from the presence of other hydride-forming elements. As above, the extent of the interference is only dependent on the concentration of the interferent and not on the analyte-to-in-terferent ratio. As atomization of the hydrides occurs by collisions with hydrogen radicals, a radical... 

Gas phase interferences due to compound formation of the analyte element with a concomitant should not be very significant in ETAAS because a much longer time is available for dissociation compared to FAAS. It was shown by high-temperature equilibrium calculations that gas phase interferences at the temperatures used in ETAAS should actually be rather insignificant [18], The reason why the literature is nevertheless full of reports on such interferences is largely due to an improper use of this technique. Slavin et al. [19], based on the systematic work of L vov [20], introduced a concept which they called stabilized temperature platform furnace (STPF). It is in essence a package of measures which eliminates most nonspectral interferences in ETAAS by atomization under local thermal equilibrium conditions. 

The gas phase interferences are obviously limited to those species which can be transferred into the vapor phase under these conditions, i.e., the hydride-forming elements and mercury. And if tin(II) chloride is used for the determination of mercury, no other element is volatilized so that no such interferences can occur. The mumal interferences of hydride-forming elements, however, may be quite severe, particularly in batch systems [36]. FI systems have been shown one more time to be superior also in this case as these mutual interferences are one to two orders of magnitude less pronounced compared to batch systems [37]. They should therefore be no problem in the analysis of body fluids or tissues, even in the case of a severe intoxication. 

Since an analyte and interferent are usually in the same phase, a separation often can be effected by inducing a change in one of their physical or chemical states. Changes in physical state that have been exploited for the purpose of a separation include liquid-to-gas and solid-to-gas phase transitions. Changes in chemical state involve one or more chemical reactions. 

Ozone can be analyzed by titrimetry, direct and colorimetric spectrometry, amperometry, oxidation—reduction potential (ORP), chemiluminescence, calorimetry, thermal conductivity, and isothermal pressure change on decomposition. The last three methods ate not frequently employed. Proper measurement of ozone in water requites an awareness of its reactivity, instabiUty, volatility, and the potential effect of interfering substances. To eliminate interferences, ozone sometimes is sparged out of solution by using an inert gas for analysis in the gas phase or on reabsorption in a clean solution. Historically, the most common analytical procedure has been the iodometric method in which gaseous ozone is absorbed by aqueous KI. 

Ozone in the gas phase can be deterrnined by direct uv spectrometry at 254 nm via its strong absorption. The accuracy of this method depends on the molar absorptivity, which is known to 1% interference by CO, hydrocarbons, NO, or H2O vapor is not significant. The method also can be employed to measure ozone in aqueous solution, but is subject to interference from turbidity as well as dissolved inorganics and organics. To eliminate interferences, ozone sometimes is sparged into the gas phase for measurement. 

Clearly, the presence of NO caused a decrease in the concentration of CHj- radicals and, consequently, in the formation of products. The nonlinear decrease in CHj-radicals suggests that at low concentrations the NO may interfere with the production of the radicals at the surface, and at all concentrations, reactions with NO in the gas phase may destroy CHs- radicals. The decrease in Cj products is consistent with the results reported at higher pressures [4]. 

Experimental limitations initially limited the types of molecular systems that could be studied by TRIR spectroscopy. The main obstacles were the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long pathlengths and/or multipass cells could be used without interference from solvent IR bands. Pimentel and co-workers first developed a rapid scan dispersive IR spectrometer (using a carbon arc broadband IR source) with time and spectral resolution on the order of 10 ps and 1 cm , respectively, and reported the gas phase IR spectra of a number of fundamental organic intermediates (e.g., CH3, CD3, and Cp2). Subsequent gas phase approaches with improved time and spectral resolution took advantage of pulsed IR sources. 

A particularly interesting question which remains unanswered is whether dinuclear photoproducts are produced directly from the photoexcited parent molecule or whether they are formed by reaction of free radicals within the solvent cage. In principle this question can be answered by making time-resolved IR measurements on the molecules in the gas phase, where no solvent cage can interfere. Thus, it may transpire that a full understanding of the photolysis of these dinuclear compounds will require complementary experiments in solution and in the gas phase. 

A great deal of success was attendant on the early application of PM-IRRAS to the gas/solid interface. Golden et ai (1981) reported the development of instrumentation, using conventional dispersive optics, able to record detailed infrared reflection-absorption spectra from molecules adsorbed on single-crystal Pt without any interference from the gas-phase species. 

Transmission infrared spectra of species adsorbed on the catalyst were taken with a Digilab FTS-10M Fourier-transform infrared spectrometer, using a resolution of 4 cm-l. To improve the signal-to-noise ratio, between 10 and 100 interferograms were co-added. Spectra of the catalyst taken following reduction in H2 were subtracted from spectra taken in the presence of NO to eliminate the spectrum of the support. Because of the very short optical path through the gas in the reactor and the low NO partial pressures used in these studies, the spectrum of gas-phase NO was extremely weak and did not interfere with the observation of the spectrum of adsorbed species. 

A strong point of Raman spectroscopy for research in catalysis is that the technique is highly suitable for in situ studies. The spectra of adsorbed species interfere weakly with signals from the gas phase, enabling studies under reaction conditions to be performed. A second advantage is that typical supports such as silica and alumina are weak Raman scatterers, with the consequence that adsorbed species can be measured at frequencies as low as 50 cm-1. This makes Raman... 

Another approach that has been reported is GC linked to IR spectroscopy linked to MS. The gas phase components from a GC analysis can easily be directed into an IR gas cell for analysis. Note that the carrier gases used in GC will not generally interfere with or be detected by IR spectroscopy and thus... 

A series of compounds that has been studied extensively, both theoretically and experimentally, is the poly phenyls (38-40). In considering these compounds it is very helpful if one has at least a little understanding of the significance of the X-ray crystallographic results. Biphenyl, the first of the series, is known to be nonplanar in the gas phase, with a torsional angle about the central bond of 42°. This is interpreted in terms of relief of steric interference between the... 

Acid-induced wagner-meerwein rearrangements in chiral alcohols. In view of the considerable interest on ion-molecule complexes involved in gas-phase analogues of solvolytic reactions," ° " a sustained research effort has been directed to the study of Wagner-Meerwein rearrangements in cationized )8-arylaIkyl systems, under conditions excluding nucleophilic assistance by the solvent which in these systems normally interferes with anchimeric assistance of groups adjacent to the... 

A diagram of a typical gas-phase (ozone-ethylene) chemiluminescent ozone analyzer is shown in Figure 6-10. The detector responds linearly to ozone concentrations between 0.003 and 30 ppm no interferences were initially observed. More recently, however, it has been established that, as the relative humidity goes from 0 to 60% and the temperature from 20° to 25° C, water vapor produces a small positive signal that results in an increase of about 8% in the ozone concentration measurement. This potential source of error can be minimized by using humidified, rather than dry, ozone in air streams when calibrating. 

---