Ashing, trace analysis

Choice of an Internal Standard. One of the difficulties in the spec-trometric trace analysis of coal ash samples, in addition to choosing a suitable comparison standard matrix, is choosing an internal standard. The first choice in both analytical methods was indium, which was used as a constant internal standard added to the graphite powder diluent-buffer. The results obtained had poor reproducibility, as previously... 

Trace analysis has its special hazards for the unwary. The most important of these are loss of material in the analytical process and contamination by outside sources. Everyone realizes that trace constituents can be lost from samples, but few are aware of the many ways in which this can occur. For example, phosphate has been observed to disappear mysteriously from water samples in polyethylene bottles (10). Nitric acid, used to clean plastic vials, has been observed to convert these surfaces to ion exchangers, which readily take up as much as 10 12 moles per sq. cm. of trace metals (16). Lead nitrate solutions unless made distinctly acidic, plate out much of the lead on the walls of glass bottles. While everyone realizes that formation of a precipitate is liable to carry out trace constituents either by adsorption or occlusion, it is not as well-known that vanishingly small amounts of precipitates—amounts likely to be overlooked on casual observation—may also do this. The fly-ash and soot, which seem to be inescapable components of city air,... 

In the beginning of the nineteenth century, analytics of plant matter samples started with that of plant ashes. In addition, no methods were available then which could have enabled intact biological materials to be digested for complete, no-Ioss analyses without burning them before. Hence, volatile elements then could not be detected, let alone quantified in biomass. Elements then found in plant ashes (Fe, Na, K, Ca, etc.) were both abundant and had been discovered in other sources before. As, e.g., no spectroscopic methods whatsoever were at hand earlier than about 1860, technical prospects for trace analysis then were dim at best (there are very few instances of elements detected in environmental samples/spectra prior to their isolation on Earth helium (in 1868) and techne-tinm (in 1952) were found in stellar spectra before being isolated from or detected in terrestrial minerals... 

Combining the fusion technique with ICPES measurements gives a rapid and accurate method for the ash elemental analysis. The total analysis time needed is 20-25 minutes per sample. However, although the fusion procedure is excellent for the determination of all major elements, it is not suitable for the determination of trace elements, because the final solution (1 L) is too dilute for detection of trace elements. If the solution volume is kept small, extremely high concentrations of lithium and boron in the solution give an undesirable high background spectrum for trace element measurements. Hence, it is necessary to resort to a separate procedure where both trace and major elements can be simultaneously determined. 

As mentioned previously, many instrumental methods can handle a sample as it is. Sometimes processing is called for to comply with standardized procedures or to create conditions for the specific determination of a substance. In trace analysis, sample processing is performed to concentrate a trace constituent to a level at which it can be determined by a given instrumental technique or to eliminate interfering constituents, or both. Some of the techniques used are solvent extraction, evaporation, distillation, precipitation and ashing. 

La Fleur, P. D. and D. Von Lehmden. Material distributed at Symposium on Trace Analysis of Coal, Fly Ash, Fuel Oil and Gasoline, Research Triangle Park, N.C., May 16-17,... 

Mass spectrometry is one of the most powerful multielemental determination methods, allowing the estimation of non-pollution levels in a multielement run. In field-desorption MS, 10 pg Tl could be detected in 2, L of non-ashed sample solution (homogenized rat brain) by means of an isotope dilution technique (Schulten et al., 1978), offering possibilities of microlocal trace analysis. Ionization of CHCI3 extracts from plant cytosols with Ar" (secondary ion MS) enabled the detection of dimethylthallium-t- directly in the mass spectrum (Gunther and Umland, 1989). 

An AA spectrometer is also available with a graphite furnace and vapor generation accessories for the trace analysis of lead, antimony, arsenic, and mercury at parts-per-billion levels. AA is used for quantitative analysis of these metals in polymers as well as finished formulations. It has been used to determine the elemental composition of catalysts and plastic additives, polymer formulations, and composite materials. Samples may be rapidly acid digested prior to analysis using a microwave oven or similar techniques. Microwave furnaces are also available for dry ashing. 

Figure 10.146 Trace analysis of anions in 35% hydrogen peroxide solution after matrix elimination. Separator column lonPac ASH eluent (A) SOmmol/L NaOH and (B) water gradient 3% A for 3 min isocratic and then linearly to 80% A in 13 min flow rate ...

Fig. 9-54. Trace analysis of anions in presence of morpholine utilizing direct injection. -Separator column lonPac ASH (2-mm) eluant NaOH gradient 0.5 mmol/L for...

Fig. 9-68. Trace analysis of anions in high-purity boric acid. - Separator column lonPac ASH eluant 9 mmol/L sodium tetraborate flow rate 1 mL/min detection suppressed conductivity injection 500 pL boric acid...

Koster R, Wangner T, Delay M, Frimmel FH. Release of contaminants from bottom ashes-coUoid facilitated transport and colloid trace analysis by means of laser-induced breakdown detection (LIBD). In Frimmel FH, von der Kammer F, Flemming H-C (Eds.), Colloidal Transport in Porous Media. Springer, Berlin,... 

Traditional analyses of fuels include proximate, ultimate, and ash elemental analysis along with calorific value and, increasingly, trace metal concentrations. These values are presented below however fuel characterization increasingly needs to focus attention on additional measures of fuel structure, fuel volatility, and fates of certain elen nts such as fuel nitrogen. 

Acetic acid is a reasonable solvent which doesn t cause interferences. Its solvent strength for PAH is comparable to the low boiling chlorinated hydrocarborts. An important disadvantage of acetic acid is the formation of salts from the mineral part of the fly ash. After evaporation of the solvent a salt residue remains, which has to be isolated from the organic part by water extraction. Since it is unsuitable to have too many steps of sample preparation in trace analysis, extraction with acetic acid is not recommended. It was fotmd that water is contaminated with PAH in a way that it might affect the determination without further puriftcation by preextraction. 

Differential pulse polarography and stripping voltammetry have been applied to the analysis of trace metals in airborne particulates, incinerator fly ash, rocks. 

Analytical Procedures. Standard methods for analysis of food-grade adipic acid are described ia the Food Chemicals Codex (see Refs, ia Table 8). Classical methods are used for assay (titration), trace metals (As, heavy metals as Pb), and total ash. Water is determined by Kad-Fisher titration of a methanol solution of the acid. Determination of color ia methanol solution (APHA, Hazen equivalent, max. 10), as well as iron and other metals, are also described elsewhere (175). Other analyses frequendy are required for resia-grade acid. For example, hydrolyzable nitrogen (NH, amides, nitriles, etc) is determined by distillation of ammonia from an alkaline solution. Reducible nitrogen (nitrates and nitroorganics) may then be determined by adding DeVarda s alloy and continuing the distillation. Hydrocarbon oil contaminants may be determined by ir analysis of halocarbon extracts of alkaline solutions of the acid. 

Chemical Composition. Chemical compositional data iaclude proximate and ultimate analyses, measures of aromaticity and reactivity, elemental composition of ash, and trace metal compositions of fuel and ash. All of these characteristics impact the combustion processes associated with wastes as fuels. Table 4 presents an analysis of a variety of wood-waste fuels these energy sources have modest energy contents. 

The combination of oxidi2ing effect, acidic strength, and high solubiHty of salts makes perchloric acid a valuable analytical reagent. It is often employed in studies where the absence of complex ions must be ensured. The value of wet ashing techniques, in which perchloric acid is used to destroy organics prior to elemental analysis for the determination of trace metals in organics, has been well estabHshed (see Trace and residue analysis). 

Composition. Molasses composition depends on several factors, eg, locality, variety, sod, climate, and processing. Cane molasses is generally at pH 5.5—6.5 and contains 30—40 wt % sucrose and 15—20 wt % reducing sugars. Beet molasses is ca 7.5—8.6 pH, and contains ca 50—60 wt % sucrose, a trace of reducing sugars, and 0.5—2.0 wt % raffinose. Cane molasses contains less ash, less nitrogenous material, but considerably more vitamins than beet molasses. Composition of selected molasses products is Hsted in Table 7. Procedures for molasses analysis are avadable (59). 

The ash analysis receives special attention because of certain trace metals in the ash that cause corrosion. Elements of prime concern are vanadium, sodium, potassium, lead, and calcium. The first four are restricted because of their contribution to corrosion at elevated temperatures however, all these elements may leave deposits on the blading. 

Raw foods were freeze-dried and analyzed for carbon isotopes using mass spectrometry. Cooked foods were prepared following historic recipes, then were freeze-dried prior to analysis. For the trace element analysis, foods (both raw and cooked) were wet ashed using nitric acid in Teflon lined pressure vessels and digested in a CEM Microwave oven. Analysis of Sr, Zn, Fe, Ca and Mg was performed using Atomic Absorption Spectrometry in the Department of Geology, University of Calgary. 

Nineteen bone samples were prepared for analysis of the trace elements strontium (Sr), rubidium (Rb), and zinc (Zn). The outer surface of each bone was removed with an aluminum oxide sanding wheel attached to a Dremel tool and the bone was soaked overnight in a weak acetic acid solution (Krueger and Sullivan 1984, Price et al. 1992). After rinsing to neutrality, the bone was dried then crushed in a mill. Bone powder was dry ashed in a muffle furnace at 700°C for 18 hours. Bone ash was pressed into pellets for analysis by x-ray fluorescence spectrometry. Analyses were carried out in the Department of Geology, University of Calgary. 

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