Metal analysis digestion methods

Advantages High analysis rate 3-4 elements per hour Applicable to many more metals than voltammetric methods Superior to voltammetry for mercury and arsenic particularly in ultratrace range Disadvantages Nonspecific absorption Spectral interferences Element losses by molecular distillation before atomisation Limited dynamic range Contamination sensitivity Element specific (or one element per run) Not suitable for speciation studies in seawater Prior separation of sea salts from metals required Suspended particulates need prior digestion About three times as expensive as voltammetric equipment Inferior to voltammetry for cobalt and nickel... 

Elemental composition Ti 59.95%, O 40.05%. The oxide may be identified by its physical properties and by x-ray methods. Titanium content may be measured by AA or ICP. The compound is digested in nitric acid or aqua regia, solubdized, and diluted sufficiently for metal analysis. 

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

Flame atomic absorption spectrometry can be used to determine trace levels of analyte in a wide range of sample types, with the proviso that the sample is first brought into solution. The methods described in Section 1.6 are all applicable to FAAS. Chemical interferences and ionization suppression cause the greatest problems, and steps must be taken to reduce these (e.g. the analysis of sea-water, refractory geological samples or metals). The analysis of oils and organic solvents is relatively easy since these samples actually provide fuel for the flame however, build-up of carbon in the burner slot must be avoided. Most biological samples can be analysed with ease provided that an appropriate digestion method is used which avoids analyte losses. 

Solid samples are digested for total metal analysis with hot nitric acid and hydrogen peroxide according to EPA Method 3050. The addition of hydrochloric acid is optional. There are two different digestion protocols within this method, one for FLAA and ICP-AES analyses and the other for GFAA analysis. These two procedures are not interchangeable, and for this reason samples prepared for FLAA or ICP-AES analysis must not be analyzed with GFAA methods. 

Microwave-assisted digestion procedures are used for total metal analysis in aqueous samples (EPA Method 3015) and for solid or oily samples (EPA Method 3051). These procedures allow for a rapid sample digestion with nitric acid under high pressure and temperature conditions the addition of hydrochloric acid is optional. Samples... 

The common methods used for dissolving samples for metals analysis are digestion in an open flask, digestion in a pressurized, sealed container, and microwave assisted decomposition. Some common solvents used are listed in Table 5.1. 

SW-846 EPA Method 3031 Acid Digestion of Oils for Metals, Analysis by FLAA or ICP Spectroscopy , in Test Methods for Evaluating Solid Waste, 3rd edit, 3rd update, US Environmental Protection Agency, Washington, DC (1995). 

The first step in analysing plastics for metals content in polymers by ICP-AES technique is that they must be prepared in solutions that are suitable for nebulization. There are four general methods applicable for sample preparation for metal analysis by ICP-AES and they are solvent dissolution of some plastics dry ashing using a muffle furnace acid digestion using a microwave oven and oxygen bomb combustion. 

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. 

Microwave acid digestion of the tissue, blood, serum, etc., can be used to prepare samples for metal analysis. The ICP-OES method is useful for monitoring the distribution of platinum compounds in the body but the information alone is not sufficient to support rigorous pharmacokinetic studies required to fully understand the total functionality as a cancer killing drug. 

Figure 5.27 Typical results obtained for the total metal analysis of soil using flame atomic absorption spectroscopy. Digestion procedures , aqua regia 0, US EPA Method 3050B , US EPA Method 3050B (optional) [32] (cf. DQ 5.9).

Example 5.2 Total Metal Analysis of Soil Using X-Ray Fluorescence Spectroscopy - Comparison with Acid Digestion (Method 3050B), followed by Flame Atomic Absorption Spectroscopy... 

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). 

Adrian, W.J. (1973). A comparison of wet pressure digestion method with other commonly used wet and dry ashing methods. Analyst. 98, 213-216 Alcock, N.W. (1987). A hydrogen peroxide digestion system for tissue trace metal analysis, Biol.Trace Bern. Res. 13, 363-370... 

The CS variable, chemical concentration in soil, is a site-specific value measured by performing U.S. Environmental Protection Agency (USEPA) SW-846 Method 3050 (10) on site soil samples for metal analysis. This is a destructive method involving a hot acid digestion with nitric acid (HNO3) and water, and results in a total metals analysis rather than a determination of a specific species or soil fraction of metal. The underlying assumption, in quantifying metal intake by the above formula, is that all of the As measured by the total metal analysis is quantified as the absorbed dose. 

---