Solute volatilization interferences

Chemical interferences are usually specific to particular analytes. They occur in the conversion of the solid or molten particle after desolvation into free atoms or elementary ions. Constituents that influence the volatilization of analyte particles cause this type of interference and are often called solute volatilization interferences. For example, in some flames the presence of phosphate in the sample can alter the atomic concentration of calcium in the flame owing to the formation of relatively nonvolatile complexes. Such effects can sometimes be eliminated or moderated by the use of higher temperatures. Alternatively, releasing agents, which are species that react preferentially with the interferent and prevent its interaction with the analyte, can be used. For example, the addition of excess Sr or La minimizes the phosphate interference on calcium because these cations form stronger phosphate compounds than Ca and release the analyte. 

Ionization interferences may be suppressed in two ways. First, a cooler flame may be employed, for example, the alkali metals are little ionized in the cooler air-hydrogen flame. However, this approach is not suitable for the majority of the elements since they are either not determined in cool flames (e.g., the lanthanoids) or subject to solute-volatilization interferences (e.g., barium). The second approach is to shift the ionization equilibrium on the basis of the law of mass action by producing a large excess of electrons in the flame or by charge transfer. In practice, this is simply achieved by adding a large excess of an easily ionized element (e.g., potassium) to both the sample and reference solutions. The effect of this... 

Nonspectral interferences are either nonspecific or specific. Nonspecific interferences affect the nebulization by altering the viscosity, surface tension, or density of the analyte solution, and consequently the sample flow rate. Certain contaminants also decrease the desolvation and atomization efficiency by lowering the atomizer temperature. Specific interferences are also called chemical interferences because they are more analyte dependent. Solute volatilization inter-... 

Determinations via ternary complexes between n(lll) - CI/Br/SCN - cationic dye can be made quite selective, because of preceding separation, and sensitive, because of the possibility to choose a very strong absorbing dye. The ternary complex has nearly the same spectrum as the dye, but it is separable from the excess dye by extraction into an organic solvent from acid solution. Cross interferences are usually similar, whether the dye is measured by photometry or fluorimetry. If preceding enrichment by volatilization, or at least solvent extraction, is done, environmental background levels can be reached with simple equipment. Prior to the determination, Tl has to be oxidized. Chlorine, bromine, Ce(IV), peroxydisulfate are suitable in the cold. 

For interferences other than spectral, the analyte itself is directly affected. The nonspectral interferences are best classified according to the stage at which the particular interference occurs, i.e. solute-volatilization and vapour-phase interferences. A nonspectral interference is found when the analyte exhibits a different sensitivity in the presence of sample concomitants as compared to the analyte in a reference solution. The difference in the signal may be due to analyte loss during the thermal pretreatment stage in the electrothermal atomizer analyte reaction with concomitants in the condensed phase to form compounds that are atomized to a lesser extent, analyte ionization or change the degree of ionization caused by concomitants. 

A major advantage of this hydride approach lies in the separation of the remaining elements of the analyte solution from the element to be determined. Because the volatile hydrides are swept out of the analyte solution, the latter can be simply diverted to waste and not sent through the plasma flame Itself. Consequently potential interference from. sample-preparation constituents and by-products is reduced to very low levels. For example, a major interference for arsenic analysis arises from ions ArCE having m/z 75,77, which have the same integral m/z value as that of As+ ions themselves. Thus, any chlorides in the analyte solution (for example, from sea water) could produce serious interference in the accurate analysis of arsenic. The option of diverting the used analyte solution away from the plasma flame facilitates accurate, sensitive analysis of isotope concentrations. Inlet systems for generation of volatile hydrides can operate continuously or batchwise. 

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. 

Resin is frequently found in cassia oil. It interferes with the accurate determination of the aldehyde by making it difficult to read off the uncombined oil. It may be detected by adding a solution of lead acetate in 70 per cent, alcohol to a solution of the oil in alcohol of the same strength. The presence of resin increases the amount of non-volatile residue, and also increases the acid value of the oil. 

ESI-MS has emerged as a powerful technique for the characterization of biomolecules, and is the most versatile ionization technique in existence today. This highly sensitive and soft ionization technique allows mass spectrometric analysis of thermolabile, non-volatile, and polar compounds and produces intact ions from large and complex species in solution. In addition, it has the ability to introduce liquid samples to a mass detector with minimum manipulation. Volatile acids (such as formic acid and acetic acid) are often added to the mobile phase as well to protonate anthocyanins. A chromatogram with only the base peak for every mass spectrum provides more readily interpretable data because of fewer interference peaks. Cleaner mass spectra are achieved if anthocyanins are isolated from other phenolics by the use of C18 solid phase purification. - ... 

If we add a known amount of a compound to our solution, we can use it to quantify the material of interest. This is great except that we may not want to contaminate our material with some other compound. A number of people have looked at using standards that are volatile so that they can be got rid of later (TMS is an example that we have seen published). The problem with this approach is that if the sample is volatile then you need to run it quickly before it disappears. TMS disappears really quickly from DMSO so it is probably not a good idea in this case. TMS also suffers from the fact that it has a long relaxation time so you have to be very careful with your experiment to ensure that you do not saturate the signal. The last major problem with TMS is that it comes at the same part of the spectrum as silicon grease which can be present in samples. Choosing a standard so that it has a short relaxation time, is volatile and comes in a part of the spectrum free of interference is really tricky. In fact, we wouldn t recommend it at all. 

After the reaction, freeze the solution and lyophilize to remove excess ammonium carbonate. Complete removal of volatile salt can be accomplished by re-dissolving the solid in warm methanol. After the completion of CO2 evolution, dry the saccharide by evaporation under vacuum. Removal of ammonium carbonate is essential, as the ammonium ion will interfere with any subsequent conjugations attempted with the glycosylamine derivative. 

Figure 4.1 depicts the difference between these two modes. Since there is no longer any partitioning of the analyte between the condensed phase and the vapour phase, compounds of limited volatility may yield a higher vapour pressure in this mode than by the conventional headspace approach, where they predominantly remain in solution. InvolatUe materials (such as API usually) do not vaporise, but instead condense onto the inside of the vial. The headspace vial effectively becomes a disposable injector liner. Care must be taken to operate at an incubation temperature that will not cause degradation of API into volatile components, which might interfere with the analysis. 

Siliceous materials—Si, Al, Fe, Ti, Ca, Mg, Na, K, Mn, Ni, Ba, Ag, Au, Ca, Cr, Cu, Ga, In, Mo, Sb and Zn—may be analyzed by a lithium tetraborate fusionr-acid dissolution technique using atomic absorption spectroscopy. Mercury, tin, and lead volatilize by this technique, and gold and silver in concentrations above 0.5 wt% cannot be held in solution. Coal ash is preconcentrated prior to analysis, and there is possible silica interference. Analytical results, where possible, are compared statistically with other reported values. 

The irradiated sample, diluted with Alundum in a porcelain boat containing mercuric nitrate carrier, is combusted very slowly in a slow oxygen stream in a 96% silica combustion tube. The volatile products are collected in two consecutive traps, both containing a solution of acetic acid-sodium acetate buffer, bromine, and mercuric nitrate hold-back carrier. The collection solutions in 2N HC1 are loaded onto Dowex 2, and radioactive interferences are eluted with aliquots of water and 2N HC1. The resin, in a small vial, is counted for the 0.077 MeV photopeak from 197Hg. 

---