Volatilization metal analysis

Figure 15-12 is a schematic illustration of a technique known as acid volatile sulfides/ simultaneously extracted metals analysis (AVS/SEM). Briefly, a strong acid is added to a sediment sample to release the sediment-associated sulfides, acid volatile sulfides, which are analyzed by a cold-acid purge-and-trap technique (e.g., Allen et ai, 1993). The assumption shown in Fig. 15-12 is that the sulfides are present in the sediments in the form of either FeS or MeS (a metal sulfide). In a parallel analysis, metals simultaneously released with the sulfides (the simultaneously extracted metals) are also quantified, for example, by graphite furnace atomic absorption spectrometry. Metals released during the acid attack are considered to be associated with the phases operationally defined as "exchangeable," "carbonate," "Fe and Mn oxides," "FeS," and "MeS."... 

Nearly every area of measurement science can boast of progress in measuring ever-smaller quantities of chemicals, but several stand out in their stunning trace-analysis capabilities. Trace-metal analysis has come to be dominated by methods that volatilize the sample and then either measure its spectroscopic emission or absorption, or measure the masses of the gaseous metal ions using mass spectrometry. Volatilization is accomplished by various thermal means that include flames, furnaces, and inductively coupled or microwave plasmas. The com-... 

Metals emissions, e.g., mercury emissions, in relation to EPA s hazardous waste combustion maximum achievable control technology (MACT) standards and a site-specific, risk-based analysis that is particularly focused on silver and other metals impacted by the formation of chlorinated and nitrated volatile metals. 

Volatile metal chelates are also useful in determining isotope ratios of geological interest, e.g., the Zr/Hf ratio (197). This method proved invaluable as a microtechnique for chromium isotope analysis of lunar samples from the Apollo program (198). 

Usually, samples are presented for analysis as liquids. Thus, solid samples must be dissolved. Analytical or ultra-high-purity grade reagents must be used for dissolution to prevent contamination at trace levels. Certain volatile metals (e.g. cadmium, lead and zinc) may be lost when dry ashing, and volatile chlorides (e.g. arsenic and chromium) lost upon wet digestion. It is particularly easy to lose mercury during sample preparation. Appropriate steps must be taken in the choice of method of dissolution, acids and conditions (e.g. whether to use reflux conditions) to prevent such losses. 

The RMBC assessed its regional public-health priorities and developed the following nine demonstration projects on the basis of the needs of the community possible correlation of exposure to arsenic in drinking water and type 2 diabetes, a spot blood metals-analysis feasibility study, health-clinic samples for chemical-terrorism baselines, of relationship between urine arsenic and metal concentrations and drinking-water exposure, assessment of exposure to VOCs from subsurface volatilization, cotinine concentrations associated with environmental tobacco smoke, assessment of exposure to mercury from ingestion of fish, analysis of radionuclides in urine, and biomonitoring of organophosphorus pesticides in urine (Utah Department of Health 2006). 

Common to nearly all analyses is preservation with refrigeration at 2-6°C, a practice, which minimizes the volatilization of organic compounds with low boiling points and the bacterial degradation of most organic compounds. That is why we must place samples on ice immediately after they have been collected, ship them in insulated coolers with ice, and keep them refrigerated until the time of analysis. Water samples collected for metal analysis and preserved with nitric acid are an exception to this rule as they may be stored at room temperature. The addition of methanol or sodium bisulfate solution to soil collected for VOC analysis is the only chemical preservation techniques ever applied to soil samples. 

Feldmann, J. 2003. Sample preparation for the analysis of volatile metal species. Compr. Anal. Chem. XLI 1211-1232. 

Dry ashing of cmde oils can cause serious loss of ash or elements through volatility of some metals, even in the presence of metal-retaining compounds. The methods using microwave acid digestion or bomb combustion are suitable for sample preparation for most trace metal analysis because they are retained in solution. This includes those that are volatile. Unfortunately, these methods are time-consuming and can be erroneous, and require experience skilled operators, but are necessary because they are precise, accurate and quantitative. 

There are a range of metal catalyst/activators used in phenolic and polyurethane adhesives that can be monitored using ICP-OES. In the metal analysis study of phenolic adhesives, the salts Ca(S03H)2 and Cu(S03H)2 were formulated as listed in Table 6.21, with and without fillers. Several metal salts are employed in polyurethane adhesives for the reasons listed in Table 6.23. As part of the sample preparation study of these products metal salts were formulated into typical polyurethane products consisting of volatile and non-volatile metal salts, listed in Table 6.24, showing the metals of interest and the concentrations expected. 

The simplest method of decomposing an organic sample prior to determining the cations it contains is to heat the sample over a flame in an open dish or crucible until all carbonaceous material has been oxidized to carbon dioxide. Red heat is often required to complete the oxidation. Analysis of the nonvolatile components follows dissolution of the residual solid. Unfortunately, there is always substantial uncertainty about the completeness of recovery of supposedly nonvolatile elements from a dry-ashed sample. Some losses probably result fiom the entrainment of finely divided particulate matter in the convection currents around the crucible. In addition, volatile metallic compounds may be lost during the ignition. For example, copper, iron, and vanadium are appreciably volatilized when samples containing porphyrin compounds are ashed. 

In the inductively coupled argon plasma emission spectrometer method, nickel, iron, and vanadium content of gas oil samples in the range from 0.1 to 100 mg/kg. Thus a 10-g sample of gas oil is charred with sulfuric acid and subsequently combusted to leave the ash residue. The resulting sulfates are then converted to their corresponding chloride salts to ensure complete solubility. A barium internal standard is added to the sample before analysis. In addition, the use of the ICAP method for the analysis of nickel, vanadium, and iron present counteracts the two basic issues arising from metals analysis. The most serious issue is the fact that these metals are partly or totally in the form of volatile, chemically stable porphyrin complexes and extreme conditions are needed to destroy the complexes without losing the metal through volatilization of the complex. The... 

Excitation of the outer ns electron of the M atom occurs easily and emission spectra are readily observed. We have aheady described the use of the sodium D-line in the emission spectrum of atomic Na for specific rotation measurements (see Section 3.8). When the salt of an alkali metal is treated with concentrated HCl (giving a volatile metal chloride) and is heated strongly in the non-luminous Bunsen flame, a characteristic flame colour is observed (Li, crimson Na, yellow K, lilac Rb, red-violet Cs, blue) and this flame test is used in qualitative analysis to identify the M ion. In quantitative analysis, use is made of the characteristic atomic spectrum in flame photometry or atomic absorption spectroscopy. 

Thus the reaction of sodium chloride with titanium from the alloy, or rutile from the scale leads to the formation of TiCl2, Na2Ti03, HC1 and Cl2.This list of reactions is by no means exhaustive, and may also involve reaction with other alloying additions which may be substituted for the titanium metal. Particularly, the role of aluminium must be of importance. A thermodynamic analysis by Travkin et al. [32] confirms that complex oxide scales are formed when titanium and its alloys react with NaCl. Particularly that the formation of volatile metallic chlorides are thermodynamically favourable, especially for alloying additions Zr, Mo and Al. The subsequent pyrohydrolysis of these metal chlorides results in the formation of HC1 gas, particularly with Mod., and AlClj. Furthermore, such pyrohydrolysis of halide salts may be accelerated by the presence of alumina within the scales, which acts as a catalyst [25]. 

Derivatives of this type are formed in order to obtain volatile compounds of sufficient stability for gas chromatographic analysis. Chlorides and fluorides of volatile metals... 

Kotz etal. (1972, Decomposition of biological materials for the determination of extremely low contents of trace elements in limited amounts with nitric acid under pressure in a Teflon tube) Hartstein et al. (1973, Novel wet-digestion procedure for trace-metal analysis of coal by atomic absorption) Jackson etal. (1978), Automated digestion and extraction apparatus for use in the determination of trace metals in foodstuffs) Campos etal. (1990, Combustion and volatilization of solid samples for direct atomic absorption spectrometry using silica or nickel tube furnace atomizers) Erber et al. (1994, The Wickbold combustion method for the determination of mercury under statistical aspects) and Woit-tiez and Sloof (1994, Sampling and sample preparation). 

In inorganic chemistry, mixtures of metal ions in solution can be analyzed by electron-impact mass spectrometry. First the metal ions are complexed with an organic ligand (usually various substituted acetylacetonates) to form volatile metal chelates. If many metal ions are anticipated, the mixture is separated by GC and the separated fractions identified by mass spectrometry. Simple mixtures can be analyzed directly using the mass spectrometer. Because of the high sensitivity of mass spectrometry, trace analysis is possible. 

In this method the sample is vaporized in a micro-oven placed in the ion source or out of a Knudsen-cell. Many metals can be analyzed qualitatively and quantitatively by this technique as metal organic compounds (see Refs. ). The metal chelates have lower volatilities than the metals and in many cases the mass spectra reveal higher sensitivities for these compounds compared with the analysis using direct evaporation of the metal. The latter technique of direct metal analysis by EIMS is only applied if the ionization energies of the metals are too high for thermal ionization mass spectrometry . 

A pyrometallurgical process for the direct recovery of zinc from zinc concentrates and zinc/iron residues has been proposed and tested extensively by Noranda. The process consists of smelting bone-dry zinc containing materials (sulfide concentrates and secondary zinc/iron materials) in a molten iron oxysulfide bath to volatilize metallic zinc into a SOz-fiee ofTgas. Sulfiir contained in the feed materials is fixed as an iron oxysulfide matte for di sal. Thus, this process not only is capable of treating zinc sulfide concentrates and secondary zinc materials simultaneously, but also eliminates the need of sulfuric acid production. Detailed thermodynamic analysis and experimental test work are described in this paper. 

Laboratory-scale preparations of ammonia and volatile acid solutions intended for use in trace metal analysis have been successfully performed by isothermal distillation, usually at room temperamres. In this technique, two beakers, one containing the chemical being purified and the other holding pure water, are placed together in a sealed closed container for a few days. The volatile component becomes uniformly shared between the contents of the two vessels and nonvolatile impurities remain in the original beaker. 

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