D. Harvey - Modern Analytical Chemistry (794078), страница 59
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Alternatively, several small portions of tissue may be combined to form a composite sample. Thecomposite sample is then homogenized and analyzed.Sample Preservation Without preservation, many solid samples are subject tochanges in chemical composition due to the loss of volatile material, biodegradation, and chemical reactivity (particularly redox reactions).
Samples stored at reduced temperatures are less prone to biodegradation and the loss of volatile material, but fracturing and phase separations may present problems. The loss of volatilematerial is minimized by ensuring that the sample completely fills its containerwithout leaving a headspace where gases can collect. Samples collected from materials that have not been exposed to O2 are particularly susceptible to oxidation reactions. For example, the contact of air with anaerobic sediments must be prevented.Figure 7.8Schematic diagram of a sample thief.Rotating the inner cylinder opens and closesthe openings along the outer cylinder’sshaft.Sample Preparation Unlike gases and liquids, which generally require little samplepreparation, solid samples usually need some processing before analysis.
There are tworeasons for this. First, as discussed in Section 7B.3, sampling variance is a function ofthe number of particles sampled, not their combined mass. For extremely heterogeneous populations consisting of large particulates, the gross sample may be too large toanalyze.
For example, a boxcar containing a load of a Ni-bearing ore with an averageparticle size of 5 mm may require a sample weighing one ton to obtain a reasonablesampling variance. Reducing the sample’s average particle size allows the same numberof particles to be sampled with a smaller, more manageable combined mass.Second, the majority of analytical techniques, particularly those used for aquantitative analysis, require that the analyte be in solution.
Solid samples, or atleast the analytes in a solid sample, must be brought into solution.1400-CH07 9/8/99 4:03 PM Page 199Chapter 7 Obtaining and Preparing Samples for Analysis(a)199(b)Figure 7.9(c)(d)Reducing Particle Size A reduction in particle size is accomplished by a combination of crushing and grinding the gross sample. The resulting particulates are thenthoroughly mixed and divided into samples of smaller mass containing the appropriate number of particles.
The process seldom occurs in a single step. Instead, samples are cycled through the process several times until a laboratory sample of desired mass is obtained.Crushing and grinding uses mechanical force to break larger particles intosmaller ones. A variety of tools are used depending on the particle’s size and hardness. Large particles are crushed using jaw crushers capable of reducing particles todiameters of a few millimeters. Ball mills, disk mills, and mortars and pestles areused to further reduce particle size.Significant changes in composition may occur during crushing and grinding.Decreasing particle size increases available surface area.
With more surface areathere is a greater risk of losing volatile components, a problem made worse by thefrictional heat accompanying the crushing and grinding. An increase in surface areaalso means that portions of the sample are freshly exposed to the atmosphere whereoxidation may alter the sample’s composition. Other problems include contamination from the mechanical abrasion of the materials used to crush and grind thesample, and differences in the ease with which particles are reduced in size. Softerparticles are reduced in size more easily and may be lost as dust before the rest ofthe sample has been processed.
This is a problem since the analyte’s distributionmay not be uniform between particles of different size.To ensure that all particles are reduced to a uniform size, the sample is intermittently passed through a sieve. Processing of those particles not passing through thesieve continues until the entire sample is of uniform size. The sample is then mixedthoroughly to ensure homogeneity, and a secondary sample obtained with a riffle orby coning and quartering. The latter approach is outlined in Figure 7.9. The grosssample is piled into a cone, flattened, divided into four quarters, and two diagonallyIllustration showing the method of coningand quartering as a means of reducing agross sample for subsampling.
(a) The grosssample is first piled into a cone and(b) flattened. Looking down from above,(c) the cone is divided into four quarters,(d) which are then separated.laboratory sampleSample taken into the lab for analysisafter processing the gross sample.coning and quarteringA process for reducing the size of a grosssample.1400-CH07 9/8/99 4:03 PM Page 200200Modern Analytical ChemistryTable 7.2Solution(≈ %w/w)HCl (37%)HNO3 (70%)H2SO4 (98%)HF (50%)HClO4 (70%)HCl:HNO3 (3:1 v/v)NaOHPressure relief valveTemperature probePressureprobeCapVessel bodyFigure 7.10Schematic diagram of a microwave digestionvessel.Acids and Bases Used for Sample DigestionUses and Properties• dissolves metals more easily reduced than H2 (Eo < 0)• dissolves insoluble carbonates, sulfides, phosphates,fluorides, sulfates, and many oxides• strong oxidizing agent• dissolves most common metals except Al and Cr• decomposes organics and biological samples (wet ashing)• dissolves many metals and alloys• decomposes organics by oxidation and dehydration• dissolves silicates forming volatile SiF4• hot, concentrated solutions are strong oxidizing agents• dissolves many metals and alloys• decomposes organics (reactions with organics are oftenexplosive, use only in specially equipped hoods with a blastshield and after prior decomposition with HNO3)• also known as aqua regia• dissolves Au and Pt• dissolves Al and amphoteric oxides of Sn, Pb, Zn, and Cropposed quarters are discarded.
The remaining material is cycled through theprocess of coning and quartering until the desired amount of sample remains.Bringing Solid Samples into Solution If you are fortunate, the sample with whichyou are working will easily dissolve in a suitable solvent, requiring no more effortthan gentle swirling and heating. Distilled water is usually the solvent of choice forinorganic salts, but an organic solvent, such as methanol, chloroform, or toluene, isused for organic materials. More often, one or more of the sample’s components resist simple dissolution.With samples that are difficult to dissolve, the first approach is usually to try digesting the sample with an acid or base. Table 7.2 lists the most commonly usedacids and bases and summarizes their use. Digestion is commonly carried out in anopen container, such as a beaker, using a hot plate as a source of heat.
The chief advantage of this approach is its low cost as it requires no special equipment. Volatilereaction products, however, are lost, leading to a determinate error if analyte is included among the volatile substances.Many digestions are now carried out in closed containers using microwave radiation as a source of energy for heating the solution.
Vessels for microwave digestion are manufactured using Teflon (or some other fluoropolymer) or fused silica.Both materials are thermally stable, chemically resistant, transparent to microwaveradiation, and capable of withstanding elevated pressures. A typical microwave digestion vessel is shown in Figure 7.10 and consists of the vessel body and cap, a temperature probe, and a pressure relief valve. Vessels are placed in a microwave oven(typically 6–12 vessels can be accommodated), and microwave energy is controlledby monitoring the temperature or pressure within the vessels. A microwave digestion has several important advantages over an open container digestion, includinghigher temperatures (200–300 °C) and pressures (40–100 bar).
As a result, digestions requiring several hours in an open container may be accomplished in less than1400-CH07 9/8/99 4:03 PM Page 201Chapter 7 Obtaining and Preparing Samples for AnalysisTable 7.3FluxNa2CO3Li2B4O7LiBO2NaOHKOHNa2O2K2S2O7B2O3Common Fluxes for Decomposing Inorganic SamplesMeltingTemperature (°C)851930845318380—300577CrucibleTypical SamplesPtsilicates, oxides, phosphates, sulfidesPt, graphitealuminosilicates, carbonatesAu, Agsilicates, silicon carbideNiPt, porcelainPtsilicates, chromium steel, Pt alloysoxidessilicates, oxides30 min using microwave digestion.
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