Volatilization interference

Figure 13 Differential Responses of TNT and DNT, and a volatile interferent, in a dual- channel Fido sensor based on AFP technology. Figure courtesy of ICxTechnologies.

In 1985, Ruzicka and Hansen established the principles behind flow injection optosensing [13-15], which has subsequently been used for making reaction-rate measurements [16], pH measurements by means of immobilized indicators [17,18], enzyme assays [19], solid-phase analyte preconcentration by sorbent extraction [20] and even anion determinations by catalysed reduction of a solid phase [21] —all these applications are discussed in Chapters 3 and 4. Incorporation of a gas-diffusion membrane in this type of sensor results in substantially improved sensitivity (through preconcentration) and selectivity (through removal of non-volatile interferents). The first model sensor of this type was developed for the determination of ammonium [13] and later refined by Hansen et al. [22,23] for successful application to clinical samples. 

Fig. 4.8. Custom-made set-up for the separation of potential semi-volatile interferences from volatile analytes. (Reproduced with permission of the American Chemical Society.)...

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. 

Aside from the extensively studied volatility interferences, it has been demonstrated that the conventional method of background correction which is based on the use of a continuum source (D2), is subject to spectral interferences from iron and for phosphate decomposition products (presumably PO and P2) (Saeed and Thomassen, 1981). Even though these spectral interferences are highly reduced by matrix modification with either nickel, a nickel/platinum or a nickel/palladium matrix modifier, the use of a Zeeman based instrument is highly recommended (Bauslaugh et al., 1984 Radziuk and Thomassen, 1992 Hoenig, 1991). 

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... 

In flame AAS, the nebulization system must be able to generate small droplets ( < 10 pm), as only such droplets can be transported and completely vaporized in the flame. This is a prime condition for the realization of high sensitivity and low volatilization interference. Nebulizers for AAS generate not only small droplets, but also larger ones (up to 25 pm or more). For fragmentation of the latter, the nebulizer should be positioned in a mixing chamber provided with impact surfaces. In pneumatic nebulization, the sampling efficiency... 

Many samples containing silicon also contain aluminum and iron. After dehydration, these metals are present as AI2O3 and Fe203. These oxides are potential interferents since they also are capable of forming volatile fluorides. 

Although chloroform is an analyte, it also can be interferent. Due to its volatility, chloroform present in the laboratory air may diffuse through the sample vial s Teflon septum, contaminating the samples. How can we determine whether samples have been contaminated in this manner ... 

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. 

EPA has also developed pretreatment standards for industrial faciHties that discharge directiy to pubHcly owned treatment works (POTWs). The three types of pollutants of principal concern are pollutants that interfere with the operation of the POTW, pollutants that contaminate the sludges produced in the POTW, and pollutants that pass through the POTW or that are otherwise incompatible. One particular concern is volatile contaminants that can be stripped into the air during conventional wastewater treatment and become air pollution problems. These pretreatment standards are included in the effluent guidelines for the different industries. 

Ammonia.. The most rehable results for ammonia are obtained from fresh samples. Storage of acidified samples at 4°C is the best way to minimi2e losses if prompt analysis is impossible. The sample acidity is neutrali2ed prior to analysis. Ammonia concentrations of 10 -0.5 M can be determined potentiometricaHy with the gas-sensing, ion-selective electrode. Volatile amines are the only known interferents. 

Polymerization-grade chloroprene is typically at least 99.5% pure, excluding inert solvents that may be present. It must be substantially free of peroxides, polymer [9010-98-4], and inhibitors. A low, controlled concentration of inhibitor is sometimes specified. It must also be free of impurities that are acidic or that will generate additional acidity during emulsion polymerization. Typical impurities are 1-chlorobutadiene [627-22-5] and traces of chlorobutenes (from dehydrochlorination of dichlorobutanes produced from butenes in butadiene [106-99-0]), 3,4-dichlorobutene [760-23-6], and dimers of both chloroprene and butadiene. Gas chromatography is used for analysis of volatile impurities. Dissolved polymer can be detected by turbidity after precipitation with alcohol or determined gravimetrically. Inhibitors and dimers can interfere with quantitative determination of polymer either by precipitation or evaporation if significant amounts are present. 

Method 25 applies to the measurement of volatile organic compounds (VOC) as nonmethane organics (TGNMO), reported as carbon. Organic particulate matter will interfere with the analysis, and, therefore, in some cases, an in-stack particulate filter will be required. The method requires an emission sample to be withdrawn at a con-... 

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. 

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