SAMPLE PREPARATION FOR METAL ANALYSIS (Essay Sample)

SAMPLE PREPARATION FOR METAL ANALYSIS Introduction Sample preparation is an important step of any analytical procedure because there is usually a delay between sample collection and sample analysis. Sample preparation ensures that the sample retains its physical and chemical characteristics so that the analysis truly represents the object under study (Mitra & Brukh, 2003). Once the sample is ready for analysis,sample preparation is the next step. Most samples are not ready for direct introduction into instruments. There might be several processes within sample preparation itself. The following table shows possible steps between sample collection and analysis. Homogenization, Size reduction Extraction Concentration Clean - up Analysis Figure1: Possible steps within sample collection and analysis The sample preparation depends on the analytical techniques to be employed and their capabilities. For instance, only a few microliters can be injected into a gaschromatograph (Mitra & Brukh, 2003). Sampling, sample preservation and sample preparation are all aimed at producing those few microliters that represent what is in the sample. It is of great importance to note that an error in the first three steps cannot be rectified by even the most sophisticated analytical instrument. So the importance of the prior steps, in particular the sample preparation, cannot be under stressed. The purpose of sample preparation can be one or a combination of the following; 1 To homogenize sample or remove moisture If the sample to be analyzed is heterogeneous or contain too much moisture, the sample will need to be air-dried, homogenized, ground and sieved. This will remove water moisture and ensure that the minor portion of the subsample is a true representative. 2 To increase or decrease analyte concentration Most analysis for environmental chemicals needs the concentration of the analyte increased. To increase the concentration, solvent evaporation has been a widely used technique. In other cases, dilution is carried out to decrease the concentration of highly contaminated sample. 3 To change sample phase Different instruments will require the sample to be in different phases. Very few instruments used for analysis can deal with solid samples. 4 To liberate analyte from sample matrix The analyte is required to be removed/liberated from the sample matrix so that the instrument used can respond. The digestion of soil sample for metal analysis serves this purpose. This also increases the instrument sensitivity. The following are some of the conventional solid sample preparation techniques; Size reduction The main purpose of reducing the size of the sample is to efficiently dissolve the solid in an appropriate solvent. If the solid is not broken down to small sizes, the solvent will only interact with the outer surface of the solid and not the whole solid. For this reason it is of great importance that the solid size is reduced to that of fine powder so that the solvent can completely interact with the solid as completely as possible. The most common ways of reducing the sample size are crushing, milling and grinding.If in some instances the solid cannot be reduced into fine powder, then the size reduction might be achieved by cutting, chopping or blending. In many analytical laboratories, crushers, ball mills, mortars and pestles, scissors and blenders are commonly used for sample size reduction. Homogenization and division The amount of solid sample needed for any analytical procedure may be less than the amount of the sample collected; hence a small portion from the larger sample is taken for analysis. Therefore obtaining a portion from the larger sample that is to be a true representative of the entire bulk will require the larger sample to be homogenized. In most cases, particle size reduction may not leave a sample homogeneous. Thus a mixing procedure is often employed which follows particle reduction. Finally, passing the solid through a sieve ensures that the solid particles are of uniform size. Solid- liquid extraction The metal analyte may be in one material phase (either solid or liquid). It is required to be separated from sample matrix phase and placed in another phase (a liquid). The metal analyte is extracted by dissolving the sample matrix in a liquid. The analyte dissolves in the liquid while the other matrix component which is insoluble will remain in the solid phase. If the sample is in a solid phase, the extraction is termed a solid- liquid extraction. This extraction depends greatly on the solubility of the analyte and the insolubility of the sample components. Solid- liquid extraction might be carried in two ways. In one, the solid and liquid are shaken together in the same containerthen filtered and the filtrate which will contain the analyte collected. The other method employs a soxhlet extraction where a fresh liquid is continuously passed through the solid in a continuous evaporation and condensation process. Total dissolution Total dissolution, or digestion, is usually employed when the extraction is not feasible. This involves a proper choice of a medium that will dissolve, or else lead to the total breakdown of the surrounding matrix. In most cases, a total dissolution is achieved by the use water or acid – water mixtures and concentrated acid solutions, but other techniques also exist. It is key to note that the dissolution (or, similarly, the digestion) of samples remains a very integral and extremely important part in the analyses of metal from a wide array of matrices (Sneddon et al., 2007). The present study seeks to outline the various methods used to achieve dissolution of samples prior to metal analyses, and the follow in the next section. Methods used prior to metal analyses * Acid digestion The purpose of acid digestion is to dissolve metals from sample matrix so that the metal can be in a measurable form (Zhang, 2007). The components of the matrix are broken down into simple chemical forms. The digestion can occur with the introduction of energy by using a chemical reagent, such as an acid. The most commonly used reagents for acid digestion are mineral and oxidizing acids. Acid digestion has the advantage of being effective for both inorganic and organic materials. It often destroys or removes the sample matrix hence reducing or removing some interference (Sigma-Aldrich 2012). Sensitive trace analysis requires extremely pure reagents. These digestive reagents cannot contain metal ions or other impurities. In addition to this, a complete decomposition of the sample is required to achieve reproducible and accurate elemental results by the various instrument analytical methods. Acid digestion is carried out for the following reasons; 1 Complete solution of the elements 2 Complete decomposition of the matrix 3 Avoiding loses and contaminants and, 4 Reduction of handling and process time The following are the acids used in acid digestion; * Nitric acid Nitric acid (HNO3) is the most frequently utilized sample dissolution medium. It oxidizes metals not dissolved by hydrochloric acid (HCl) and other non-oxidizing acids. Au, Pt metals (expect Pd), Nb, Ta, and Zr are not dissolved. Al and Cr are passivated. Sn, Sb, and Wgive insoluble hydrous oxides. It dissolves most sulfides (except HgS). Unfortunately, the carbon contained in organic materials is only partly converted to CO2 by HNO3at temperatures up to 200°C. Nitric acid should never be used for the digestion of highly aromatic compounds because of the potential for the formation of highly explosive compounds. In the case of alcohols, the samples should be pretreated with sulfuric acid. * Hydrochloric acid Hydrochloric acid (HCl) is used for many salts of weak acids, e.g., carbonates, phosphates, some oxides, and some sulfides. Though widely used, it should be avoided in Graphite furnace – AAS (GFAAS) due to the interfering effect of the chloride (Sneddonet al., 2007). A widely used acid mixture is the aqua regia (consisting of 3 parts HCl and 1 part HNO3) but again, the interference effect of the chloride present in the mixture should always be considered when GFAAS is to be used. * Sulfuric acid Sulfuric acid (H2SO4) is used when its high boiling point (300°C) is an advantage, as in expelling a volatile product or increasing the reaction rate. It provides dehydrating and oxidizing properties at high temperatures. The Achilles heel in the usage of sulfuric acid is that this high boiling temperature increases the decomposition of some material within (Sneddonet al., 2007). * Perchloric acid Perchloric acid (HClO4) is a very powerful oxidizing agent at fuming temperatures (boiling point 203°C). It is usually mixed with HNO3 to oxidize easily attacked organic matter that might otherwise react violently with HClO4. Sulfuric acid (dehydrating agent) increases oxidizing power. Though a good solvent for stainless steel, it should be used with extreme care due to its very hazardous nature. * Hydrofluoric acid Hydrofluoric acid (HF) is used for digestion of siliceous samples and as an auxiliary reagent to HNO3 or HClO4 to eliminatefluoride. With HNO3, HF dissolves Ti, W, Nb, and Zr (and their carbides, nitrides, and borides) as a result of formation of complex fluorides. Certain refractory silicates and other minerals are not decomposed; these must be dissolved by fusion. * Hydrogen peroxide Hydrogen peroxide (H2O2) is a very popular oxidizing reagent as it is converted to water and oxygen during the oxidation of biological material. Additional advantages are that there is no acid corrosion of the digestion vessel PTFE walls, no formation of insoluble salts with an acid anion, and no change of the sample matrix by an acid. Because of its strong oxidation po...