Ch27 Sample Preparation

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    Introduction

    A chemical analysis is meaningless unless you begin with a meaningful sample.

    To measure cholesterol in a dinosaur skeleton or herbicide in a truckload of

    oranges, you must have a strategy for selecting a representative samplefrom aheterogeneousmaterial.

    Figure 27-1shows that the concentration of nitrate

    in sediment beneath a lake drops by two orders of

    magnitude in the first 3 mm below the surface. If youwant to measure nitrate in sediment, it makes an

    enormous difference whether you select a core

    sample that is 1 m deep or skim the top 2 mm of

    sediment for analysis.

    Sampling is the process of collecting a

    representative sample for analysis.1 Real samples

    generally require some degree of sample

    preparation to remove substances that interfere in

    the analysis of the desired analyte and, perhaps, toconvert analyte into a form suitable for analysis.2 Figure 27-1

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    Introduction

    The terminology of sampling and sample preparation is shown in Figure 27-2. A

    lot is the total material (dinosaur skeleton or truckload of oranges) from which

    samples are taken. A bulk sample (also called a gross sample) is taken from thelot for analysis or archiving (storing for future reference).

    The bulk sample must be representative of the lot, and the choice of bulk

    sample is critical to producing a valid analysis. Box 0-1 gave a strategy for

    sampling heterogeneous material.

    Sampling is the process of selecting a

    representative bulk sample from the lot. Sample

    preparation is the process that converts a bulk

    sample into a homogeneous laboratory sample.

    Sample preparation also refers to steps that

    eliminate interfering species or that concentrate the

    analyte.

    Figure 27-2

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    Introduction

    From the representative bulk sample, a smaller, homogeneous laboratory

    sample is formed that must have the same composition as the bulk sample.

    For example, we might obtain a laboratory sample by grinding the entire solid

    bulk sample to fine powder, mixing thoroughly, and keeping one bottle of

    powder for testing.

    Besides choosing a sample judiciously, we must be

    careful about storing the sample. The composition may

    change with time after collection because of chemical

    changes, reaction with air, or interaction of the sample

    with its container.

    Figure 27-2

    Small portions (called aliquots) of the laboratory

    sample are used for individual analyses. Sample

    preparation is the series of steps that convert a

    representative bulk sample into a form suitable for

    analysis.

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    IntroductionGlass is a notorious ion exchanger that alters the concentrations of trace ions in

    solution.

    Therefore, plastic (especially Teflon) collection bottles are frequently employed.

    Even these materials can absorb trace levels of analytes.

    Figure 27-2

    For example, a 0.2-MHgCl2 solution lost 40-95

    % ofits concentration in 4 h in polyethylene bottles.

    A 2-MAg+ solution in a Teflon bottle lost 2% of its

    concentration in a day and 27% in a month.

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    IntroductionPlastic containers must be washed before use. Table 27-1 shows that

    manganese in blood serum samples increased by a factor of seven when

    stored in unwashed polyethylene containers prior to analysis.

    In the most demanding trace analysis of

    lead at 1 pg/g in polar ice cores,

    polyethylene containers contributed a

    measurable flux of 1 fg of lead per cm2

    per day even after they had been

    soaked in acid for seven months.

    Steel needles are an avoidable source

    of metal contamination in biochemical

    analysis.

    TABLE 27-1 Manganese concentration of

    serum stored in washed and unwashed

    polyethylene containers.

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    IntroductionA study of mercury in Lake Michigan found levels near 1.6 pM (1.6 x 1012 M),

    which is two orders of magnitude below concentrations observed in earlier

    studies.

    Previous investigators apparently unknowingly contaminated their samples. A

    study of handling techniques for the analysis of lead in rivers investigated

    variations in sample collection, sample containers, protection during

    transportation, filtration, preservatives, and preconcentration procedures.

    Each step that deviated from best practice doubled the apparent concentration

    of lead in stream water. Clean rooms with filtered air supplies are essential in

    trace analysis. Even with the best precautions, the precision of trace analysis

    becomes poorer as the concentration of analyte decreases (Box 5-2).

    Unless the complete history of any sample is known with certainty, the

    analyst is well advised not to spend his [or her] time in analyzing it.

    Your laboratory notebook should describe how a sample was collected and

    stored and exactly how it was handled, as well as stating how it was analyzed.

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    27-1 Statistics of Sampling

    For random errors, the overall variance,s20, is the sum of the variance of

    the analytical procedure, s2a, and the variance of the sampling operation,

    s2s:

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    27-1 Statistics of Sampling

    To understand the nature of the uncertainty in selecting a sample for analysis, consider a

    random mixture of two kinds of solid particles. The theory of probability allows us to

    state the possibility that a randomly drawn sample has the same composition as the bulk

    sample. It may surprise you to learn how large a sample is required for accurate

    sampling.

    Suppose that the mixture contains nA particles of type A and nB particles of type B. Theprobabilities of drawing A or B from the mixture are

    Origin of Sampling Variance

    If nparticles are drawn at

    random, the expected number of

    particles of type A is npand thestandard deviation of many

    drawings is known from the

    binomial distribution to be

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    27-1 Statistics of Sampling

    How much sample corresponds to 104particles? Suppose that the particles are 1-mm-

    diameter spheres.

    The volume of a 1-mm-diameter sphere is (4/3)p(0.5 mm)3= 0.524 L. The density of

    KCl is 1.984 g/mL and that of KNO3is 2.109 g/mL, so the average density of the

    mixture is (0.01)(1.984) + (0.99)(2.109) = 2.108 g/mL.

    The mass of mixture containing 104particles is (104)(0.524103mL)(2.108 g/mL) =

    11.0 g.

    If you take 11.0-g test portions from a larger laboratory sample, the expected sampling

    standard deviation for chloride is 9.9%.

    The sampling standard deviation for nitrate will be only 0.1%.

    Origin of Sampling Variance

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    27-1 Statistics of Sampling

    Origin of Sampling Variance

    27-2

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    27-1 Statistics of Sampling

    Origin of Sampling Variance

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    27-1 Statistics of Sampling

    Sampling uncertainty arises from the random nature of drawing particles from a

    mixture. If the mixture is a liquid and the particles are molecules, there are about 1022

    particles/mL.

    It will not require much volume of homogeneous liquid solution to reduce the sampling

    error to a negligible value. Solids, however, must be ground to very fine dimensions,

    and large quantities must be used to ensure a small sampling variance.

    Grinding invariably contaminates the sample with material from the grinding apparatus.

    Table 27-3illustrates another problem with heterogeneous materials. Nickel ore was

    crushed into small particles that were sieved and analyzed. Parts of the ore that are

    deficient in nickel are relatively resistant to fracture, so the larger particles do not have

    the same chemical composition as the smaller particles. It is necessary to grind the

    entire ore to a fine power to have any hope of obtaining a representative sample.

    Origin of Sampling Variance

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    27-1 Statistics of Sampling

    Table 27-3illustrates anotherproblem with heterogeneous

    materials. Nickel ore was crushed

    into small particles that were

    sieved and analyzed. Parts of the

    ore that are deficient in nickel arerelatively resistant to fracture, so

    the larger particles do not have the

    same chemical composition as the

    smaller particles. It is necessary to

    grind the entire ore to a fine power

    to have any hope of obtaining a

    representative sample.

    Origin of Sampling Variance

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    27-1 Statistics of Sampling

    A well-mixed powder containing KCl and KNO3is an example of a heterogeneous

    material in which the variation from place to place is random.

    How much of a random mixture should be analyzed to reduce the sampling variance for

    one analysis to a desired level?

    Choosing a Sample Size

    To answer this question, consider

    Figure 27-3, which shows results for

    sampling the radioisotope 24Na in

    human liver. The tissue was

    homogenized in a blender but was

    not truly homogeneous, because it

    was a suspension of small particles in

    water.

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    27-1 Statistics of Sampling

    The average number of

    radioactive counts per secondper gram of sample was about

    237. When the mass of

    sample for each analysis was

    about 0.09 g, the standard

    deviation (shown by the error

    bar at the left in the diagram)

    was31 counts per second

    per gram of homogenate,

    which is13.1% of the

    mean value (237).

    Choosing a Sample Size

    When the sample size was increased to about 1.3 g, the standard deviation decreased to

    13 counts/s/g, or5.5% of the mean. For a sample size near 5.8 g, the standard

    deviation was reduced to5.7 counts/s/g, or.2.4% of the mean.

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    27-1 Statistics of Sampling

    Equation 27-4told us that when nparticles are drawn from a mixture of two kinds of

    particles (such as liver tissue particles and droplets of water), the sampling standarddeviation will be n= , wherepand qare the fraction of each kind of particle

    present. The relative standard deviation is n/n= = .

    The relative variance, (n/n)2, is therefore

    Choosing a Sample Size

    Noting that the mass of sample drawn, m, is proportional to the number of particles

    drawn, we can rewrite Equation 27-5in the form

    in whichRis the relative standard deviation (expressed as a percentage) due to

    sampling andKsis called thesampling constant.Ksis the mass of sample producing a

    relative sampling standard deviation of 1%.

    (27-5)

    (27-4)

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    27-1 Statistics of Sampling

    Let's see if Equation 27-6describes Figure 27-3. Table 27-4shows that mR2is approximately

    constant for large samples, but agreement is poor for the smallest sample. Attributing the poor

    agreement at low mass to random sampling variation, we assignKs 36 g in Equation 27-6.

    This is the average from the 1.3- and 5.8-g samples in Table 27-4.

    Choosing a Sample Size

    (27-5)

    (27-6)

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    27-1 Statistics of SamplingChoosing a Sample Size

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    27-1 Statistics of SamplingChoosing a Sample Size

    (27-6)

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    27-1 Statistics of Sampling

    We just saw that a single 0.7-g sample is expected to give a sampling standard deviation

    of7%.How many 0.7-g samples must be analyzed to give 95% confidence that themean is known to within 4%?

    The meaning of 95% confidence is that there is only a 5% chance that the true mean lies

    more than4% away from the measured mean.

    The question we just asked refers only to sampling uncertainty and assumes that

    analytical uncertainty is much smaller than sampling uncertainty.

    Rearranging Student's tEquation 4-6allows us to answer the question:

    Choosing the Number of Replicate Analyses

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    27-1 Statistics of Sampling

    Rearranging Student's tEquation 4-6allows us to answer the question:

    Choosing the Number of Replicate Analyses

    in which is the true population mean, is the measured mean, nis the number ofsamples needed, is the variance of the sampling operation, and eis the sought-for

    uncertainty.

    Both the quantitiesssand emust be expressed as absolute uncertainties or both must be

    expressed as relative uncertainties.

    Student's tis taken from Table 4-2for 95% confidence at n-1 degrees of freedom.

    Because nis not yet known, the value of tfor n= can be used to estimate n.After a

    value of nis calculated, the process is repeated a few times until a constant value of nis

    found.

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    27-1 Statistics of Sampling

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    27-1 Statistics of Sampling

    i i S f i

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    27-2 Dissolving Samples for Analysis

    Once abulk sampleis selected, a laboratory samplemust be

    prepared for analysis (Figure 27-2). A coarse solid sample should

    be ground and mixed so that the laboratory sample has the samecomposition as the bulk sample. Solids are typically dried at

    110C at atmospheric pressure to remove adsorbed water prior

    to analysis. Temperature-sensitive samples may simply be stored

    in an environment that brings them to a constant, reproducible

    moisture level.

    The laboratory sample is usually dissolved for analysis. It is important to dissolve the

    entire sample, or else we cannot be sure that all of the analyte was dissolved.

    If the sample does not dissolve under mild conditions, acid digestionorfusionmay be

    used. Organic material may be destroyed by combustion(also called dry ashing) or wet

    ashing(oxidation with liquid reagents) to place inorganic elements in suitable form for

    analysis.

    27 2 Di l i S l f A l i

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    27-2 Dissolving Samples for Analysis

    Solids can be ground in a mortar and pestlelike those shown in Figure 27-5. The steel

    mortar (also called a percussion mortar or diamond mortar) is a hardened steel toolinto which the sleeve and pestle fit snugly. Materials such as ores and minerals can be

    crushed by striking the pestle lightly with a hammer.

    The agatemortar (or similar ones made of porcelain, mullite, or alumina) is designed

    for grinding small particles into a fine powder. Less expensive mortars tend to be moreporous and more easily scratched, which leads to contamination of the sample with

    mortar material or portions of previously ground samples.

    A ceramic mortar can be cleaned by wiping with a wet tissue and washing with distilled

    water. Difficult residues can be removed by grinding with 4 M HCl in the mortar or bygrinding an abrasive cleaner (such as Ajax), followed by washing with HCl and water.

    A boron carbidemortar and pestle is five times harder than agate and less prone to

    contaminate the sample.

    Grinding

    27 2 Di l i S l f A l i

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    27-2 Dissolving Samples for Analysis

    Grinding

    Figure 27-5 Steel, agate, and boron carbide mortars and pestles. The mortar is the base

    and the pestle is the grinding tool. In regard to boron carbide, the mortar is a

    hemispheric shell enclosed in a plastic or aluminum body. The pestle has a boron

    carbide button at the tip of a plastic handle.

    27 2 Di l i S l f A l i

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    27-2 Dissolving Samples for Analysis

    Aball millis a grinding device in which steel or ceramic balls are rotated inside a drum

    to crush the sample to a fine powder. Figure 27-6shows a Wig-L-Bug, which

    pulverizes a sample by shaking it in a vial with a ball that moves back and forth.

    For soft materials, plastic vials and balls are appropriate. For harder materials, steel,

    agate, and tungsten carbide are used.

    Grinding

    Figure 27-6 Wig-L-Bug sample shaker and polystyrene vial and ball for pulverizing

    soft materials. The arrow shows the direction of shaking.

    27 2 Di l i S l f A l i

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    27-2 Dissolving Samples for Analysis

    A Shatterbox laboratory mill spins a puck and ring inside a grinding container at 825

    revolutions per minute to pulverize up to 100 g of material (Figure 27-7). Tungsten

    carbide and zirconia containers are used for very hard samples.

    Grinding

    Figure 27-7 Shatterbox laboratory mill spins a puck and ring inside a container at high

    speed to grind up to 100 mL of sample to a fine powder.

    Spex Shatterbox w/

    tungsten carbide

    27 2 Di l i S l f A l i

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    27-2 Dissolving Samples for Analysis

    Table 27-5lists acids commonly used for dissolving inorganic materials. The

    nonoxidizing acids HCl, HBr, HF, H3

    PO4, dilute H2

    SO4

    , and dilute HClO4

    dissolve

    metals by the redox reaction:

    Dissolving Inorganic Materials with Acids

    27 2 Di l i S l f A l i

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    27-2 Dissolving Samples for Analysis

    Metals with negative reduction potentials should dissolve, although some, such as Al,

    form a protective oxide coat that inhibits dissolution.

    Volatile species formed by protonation of anions such as carbonate ( CO32- H2CO3

    CO2), sulfide (S2 H2S), phosphide (P

    3 PH3), fluoride (F HF), and borate

    (BO33- H3BO3) will be lost from hot acids in open vessels.

    Volatile metal halides such as SnCl4and HgCl2and some molecular oxides such asOsO4and RuO4 also can be lost.

    Hot hydrofluoric acid is especially useful for dissolving silicates. Glass or platinum

    vessels can be used for HCl, HBr, H2SO4, H3PO4, and HClO4.

    HF should be used in Teflon, polyethylene, silver, or platinum vessels. The highest-

    quality acids must be used to minimize contamination by the concentrated reagent.

    Dissolving Inorganic Materials with Acids

    27 2 Di l i S l f A l i

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    27-2 Dissolving Samples for Analysis

    Substances that do not dissolve in nonoxidizing acids may dissolve in the oxidizing

    acids HNO3

    , hot, concentrated H2

    SO4

    , or hot, concentrated HClO4

    .

    Nitric acid attacks most metals, but not Au and Pt, which dissolve in the 3:1 (vol/vol)

    mixture of HCl:HNO3called aqua regia.

    Strong oxidants such as Cl2or HClO4in HCl dissolve difficult materials such as Ir at

    elevated temperature. A mixture of HNO3and HF attacks the refractory carbides,nitrides, and borides of Ti, Zr, Ta, and W.

    A powerful oxidizing solution known as piranha solution is a 1:1 (vol/vol) mixture of

    30 wt% H2O2plus 98 wt% H2SO4.

    Hot, concentrated HClO4(described later for organic substances) is a dangerous,

    powerful oxidant whose oxidizing power is increased by adding concentrated H2SO4

    and catalysts such as V2O5or CrO3.

    Dissolving Inorganic Materials with Acids

    27 2 Dissolving Samples for Analysis

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    27-2 Dissolving Samples for Analysis

    Dissolving Inorganic Materials with Acids

    HF causes excruciating burns.

    Exposure of just 2% of your body to concentrated (48 wt%)

    HF can kill you.

    Flood the affected area with water for 5 min and then coat the

    skin with 2.5% calcium gluconate gel kept in the lab for this

    purpose, and seek medical help.

    If the gel is not available, use whatever calcium salt is handy.

    HF damage can continue to develop days after exposure.

    27 2 Dissolving Samples for Analysis

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    27-2 Dissolving Samples for Analysis

    Digestion is conveniently carried out in a Teflon-lined bomb(a sealed vessel) heated in

    a microwave oven.The vessel in Figure 27-8has a volume of 23 mL and digests up to

    1 g of inorganic material in up to 15 mL of concentrated acid or digests 0.1 g of organic

    material, which releases a great deal of CO2(g).

    Dissolving Inorganic Materials with Acids

    Figure 27-8 Microwave

    digestion bomb lined with

    Teflon. The outer container

    retains strength to 150C

    but rarely reaches 50C.

    27 2 Dissolving Samples for Analysis

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    27-2 Dissolving Samples for Analysis

    Microwave energy heats the contents to 200C in a minute. To prevent explosions, the

    lid releases gas from the vessel if the internal pressure exceeds 8 MPa (80 bar).

    The bomb cannot be made of metal, which absorbs microwaves. An advantage of a

    bomb is that it is cooled prior to opening, thus preventing loss of volatile products.

    Dissolving Inorganic Materials with Acids

    Figure 27-8 Microwave

    digestion bomb lined with

    Teflon. The outer container

    retains strength to 150Cbut rarely reaches 50C.

    27 2 Dissolving Samples for Analysis

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    27-2 Dissolving Samples for Analysis

    Substances that will not dissolve in acid can usually be dissolved by a hot, molten

    inorganic flux(Table 27-6). Finely powdered unknown is mixed with 2 to 20 times its

    mass of solid flux, and fusion(melting) is carried out in a platinumgold alloy crucibleat 300to 1 200C in a furnace or over a burner.

    Dissolving Inorganic Materials by Fusion

    27 2 Dissolving Samples for Analysis

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    27-2 Dissolving Samples for Analysis

    The apparatus in Figure 27-9fuses three samples at once over propane burners with

    mechanical agitation of the crucibles.

    When the samples are homogeneous, the molten flux is poured into beakers containing

    10 wt% aqueous HNO3to dissolve the product.

    Dissolving Inorganic Materials by Fusion

    Figure 27-9Automated apparatusthat fuses three samples at once over

    propane burners. It also provides

    mechanical agitation of the Pt/Au

    crucibles.

    Crucibles are shown in the tippedposition used to pour contents out

    after fusion is complete.

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    27-2 Dissolving Samples for Analysis

    Most fusions use lithium tetraborate (Li2B4O7, m.p. 930oC), lithium metaborate

    (LiBO2

    , m.p. 845C), or a mixture of the two.

    A nonwetting agent such as KI can be added to prevent the flux from sticking to the

    crucible. For example, 0.2 g of cement might be fused with 2 g of Li2B4O7and 30 mg

    of KI.

    A disadvantage of a flux is that impurities are introduced by the large mass of solidreagent. If part of the unknown can be dissolved with acid prior to fusion, it should be

    dissolved. Then the insoluble component is dissolved with flux and the two portions are

    combined for analysis.

    Basic fluxes in Table 27-6(LiBO2, Na2CO3, NaOH, KOH, and Na2O2) are best used todissolve acidic oxides of Si and P. Acidic fluxes (Li2B4O7, Na2B4O7, K2S2O7, and B2O3)

    are most suitable for basic oxides (including cements and ores) of the alkali metals,

    alkaline earths, lanthanides, and Al.

    Dissolving Inorganic Materials by Fusion

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    27-2 Dissolving Samples for Analysis

    Dissolving Inorganic Materials by Fusion

    TABLE 27-6

    Fluxes for sample dissolution

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    27-2 Dissolving Samples for Analysis

    Dissolving Inorganic Materials by Fusion

    TABLE 27-6

    Fluxes for sample dissolution

    27-2 Dissolving Samples for Analysis

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    27-2 Dissolving Samples for Analysis

    Dissolving Inorganic Materials by Fusion

    TABLE 27-6

    Fluxes for sample dissolution

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    27-2 Dissolving Samples for Analysis

    Digestion of organic material is classified as either dry ashing,when the procedure

    does not include liquid, or wet ashing,when liquid is used.

    Occasionally, fusion with Na2O2(called Parr oxidation) or alkali metals may be carried

    out in a sealed bomb. Section 27-4discussed combustion analysis,in which C, H, N, S,

    and halogens are measured.

    Convenient wet-ashingprocedures include microwave digestion with acid in a Teflonbomb (Figure 27-8). For example, 0.25 g of animal tissue can be digested for metal

    analysis by placing the sample in a 60-mL Teflon vessel containing 1.5 mL of high-

    purity 70% HNO3plus 1.5 mL of high-purity 96% H2SO4and heating it in a 700-W

    kitchen microwave oven for 1 min.

    Teflon bombs fitted with temperature and pressure sensors allow safe, programmable

    control of digestion conditions. An important wet-ashing process isKjeldahl digestion

    with H2SO4for nitrogen analysis.

    Decomposition of Organic Substances

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    27 2 Dissolving Samples for Analysis

    In the Carius method,digestion is performed with fuming HNO3(which contains

    excess dissolved NO2) in a sealed, heavy-walled glass tube at 200300C.

    For safety, the glass Carius tube should be contained in a steel vessel pressurized to

    approximately the same pressure expected inside the glass tube.

    For trace analysis, sample should be placed inside a fused silica tube inside the glass

    tube. Silica provides as little as 110% as much extractable metal as glass.

    Decomposition of Organic Substances

    27-2 Dissolving Samples for Analysis

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    27 2 Dissolving Samples for Analysis

    Figure 27-10shows microwave wet-ashing apparatus. Sulfuric acid or a mixture of

    H2SO4and HNO3(15 mL of acid per gram of unknown) is added to an organic

    substance in a glass digestion tube fitted with the reflux cap.

    Decomposition of Organic Substances

    Figure 27- 10

    Microwave

    apparatus for

    digesting

    organic

    materials by wet

    ashing.

    27-2 Dissolving Samples for Analysis

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    27 2 Dissolving Samples for Analysis

    In the first step, the sample is carbonizedfor

    10 to 20 min at gentle reflux until all particles

    have dissolved and the solution has a uniform

    black appearance.

    Power is turned off and the sample is allowed

    to cool for l2 min. Next, oxidationis carried

    out by adding H2O2or HNO3 through thereflux cap until the color disappears or the

    solution is just barely tinted.

    Decomposition of Organic Substances

    If the solution is not homogeneous, the power is turned up and the sample is heated to

    bring all solids into solution. Repeated cycles of oxidation and solubilization may berequired.

    Once conditions for a particular type of material are worked out, the procedure is

    automated, with power levels and reagent delivery (by the peristaltic pump)

    programmed into the controller.

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    27 2 Dissolving Samples for Analysis

    The high-pressure asher in Figure 27-11uses a resistive heating element inside a

    sealed chamber for digestion at

    temperatures up to 270C under a

    pressure up to 140 bar.

    High pressure allows acids to be heated

    to high temperature without boiling.

    Decomposition of Organic Substances

    At high temperature, HNO3oxidizes organic matter without assistance from H2SO4,

    which is not as pure as HNO3and is therefore less suitable for trace analysis.

    Figure 27-11 High-pressure autoclaveallows digestion up to 270C without

    H2SO4in open vessels inside autoclave.

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    Decomposition of Organic Substances

    Silica or fluoropolymer vessels inside the

    sealed chamber are loosely sealed byTeflon caps that permit evolved gases to

    escape.

    The bottom of the vessel is filled with 5

    vol% H2O2in H2O. Hydrogen peroxide

    reduces nitrogen oxides generated by

    digestion of organic matter.

    As an example, a 1-g sample of animal tissue could be digested in a 50-mL fused-silica

    vessel containing 5 mL of high-purity 70 vol% HNO3plus 0.2 mL of high-purity 37vol% HCl.

    Metallic elements in the digestion solution could be measured at part per billion to part

    per million levels by inductively coupled plasmaatomic emission.

    27 2 Dissolving Samples for Analysis

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    Decomposition of Organic Substances

    Add fresh HNO3and repeat the evaporation several times. After the sample cools to

    room temperature, add HClO4and heat again.

    If possible, HNO3should be present during the HClO4treatment. A large excess of

    HNO3should be present when oxidizing organic materials.

    27 2 Dissolving Samples for AnalysisFigure 27-

    12 Reflux cap for

    wet ashing in an

    Erlenmeyer flask.

    The hole allowsvapor to escape, and

    the spout is curved

    to contact the inside

    of the flask.

    Wet ashing with refluxing HNO3-HClO4

    (Figure 27-12) is a widely applicable, buthazardous, procedure.

    Perchloric acid has caused numerous

    explosions.

    Use a good blast shield in a metal-lined fume

    hood designed for HClO4. First, heat the sample

    slowly to boiling with HNO3but withoutHClO4.

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    Decomposition of Organic Substances

    Bottles of HClO4should not be stored onwooden shelves, because acid spilled on

    wood can form explosive cellulose

    perchlorate esters. Perchloric acid also

    should not be stored near organic reagents

    or reducing agents.

    7 sso v g Sa p es o a ys sFigure 27-

    12 Reflux cap for

    wet ashing in an

    Erlenmeyer flask.

    The hole allowsvapor to escape, and

    the spout is curved

    to contact the inside

    of the flask.

    The combination of Fe2+and H2O2, calledFenton's reagent,oxidizes organic material

    in dilute aqueous solutions. For example, organic components of urine could be

    destroyed in 30 min at 50C to release traces of mercury for analysis.

    To do so, a 50-mL sample was adjusted to pH 34 with 0.5 M H2SO4. Then 50 L of

    saturated aqueous ferrous ammonium sulfate, Fe(NH4)2(SO4)2, were added, followed by

    100 L of 30% H2O2.

    Fenton's reagent generates OH radical and,

    possibly, FeIIOOH as the active species.

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    Sample preparationis the series of steps required to transform a sample so that it is

    suitable for analysis. Sample preparation may include

    dissolving the sample,

    extracting analyte from a complex matrix,concentrating a dilute analyte to a level that can be measured,

    chemically converting analyte into a detectable form, and

    removing or masking interfering species.

    p p q

    Liquid Extraction Techniques

    Solid-Phase Extraction

    Preconcentration

    Derivatization

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    In extraction,analyte is dissolved in a

    solvent that does not necessarily dissolve

    the entire sample and does notdecompose the analyte.

    In a typical microwave-assisted

    extractionof pesticides from soil, a

    mixture of soil plus acetone and hexane

    is placed in a Teflon-lined bomb (Figure27-8and Figure 27-13) and heated by

    microwaves to 150C.This temperature

    is 50to 100higher than the boiling

    points of solvents at atmospheric

    pressure. Pesticides dissolve, but the soil

    remains behind. The liquid is then

    analyzed by chromatography.

    p p q

    Liquid Extraction Techniques

    Figure 27-13 Extraction vessels in a microwave oven that processes up to 12 samples in under

    30 min. Each 100-mL vessel has a vent tube that releases vapor if the pressure exceeds 14 bar.

    Vapors from the chamber are ultimately vented to a fume hood. The temperature inside each

    vessel can be monitored and used to control the microwave power.

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    Acetone absorbs microwaves, so it

    can be heated in a microwave oven.Hexane does not absorb

    microwaves.

    To perform an extraction with pure

    hexane, the liquid is placed in afluoropolymer insert inside the

    Teflon vessel in Figure 27-8.

    The walls of the insert contain

    carbon black, which absorbs

    microwaves and heats the solvent.

    p p q

    Liquid Extraction Techniques

    Figure 27-13 Extraction vessels in a microwave oven that processes up to 12 samples in under

    30 min. Each 100-mL vessel has a vent tube that releases vapor if the pressure exceeds 14 bar.

    Vapors from the chamber are ultimately vented to a fume hood. The temperature inside each

    vessel can be monitored and used to control the microwave power.

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    Supercritical fluid extractionuses asupercritical fluid(Box 25-2) as the extractionsolvent.

    CO2is the most common supercritical fluid because it is inexpensive and it eliminates

    the need for costly disposal of waste organic solvents. Addition of a second solvent

    such as methanol increases the solubility of polar analytes.

    Nonpolar substances, such as petroleum hydrocarbons, can be extracted with

    supercritical argon. The extraction process can be monitored by infrared spectroscopy

    because Ar has no infrared absorption.

    p p q

    Supercritical fluid extraction

    27-3 Sample Preparation Techniques

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    Figure 27-14ashows how a supercritical

    fluid extraction can be carried out.

    Pressurized fluid is pumped through a heated

    extraction vessel.

    Fluid can be left in contact with the sample

    for some time or it can be pumped through

    continuously.

    At the outlet of the extraction vessel, the

    fluid flows through a capillary tube to release

    pressure.

    Exiting CO2evaporates, leaving extracted

    analyte in the collection vessel.

    Alternatively, the CO2can be bubbled

    through a solvent in the collection vessel to

    leave a solution of analyte.

    p p q

    Supercritical fluid extraction

    Figure 27-14 (a) Apparatus

    for supercritical fluid

    extraction.

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    Figure 27-14bshows the extraction of

    organic compounds from dust collected

    with a vacuum cleaner from door mats

    at the chemistry building of Ohio StateUniversity.

    p p q

    Supercritical fluid extraction

    Figure 27-14b) Vessel for extracting

    house dust at 50C with 20 mol%

    methanol/80 mol% CO2at 24.0 MPa

    (240 bar).

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    The chromatogram of theextract in Figure 27-14c

    exhibits myriad organic

    compounds that you and I

    inhale in every breath.

    p p q

    Supercritical fluid extraction

    Figure 27-14 (c) Gas chromatogram of CH2Cl2solution of extract using

    a 30 m0.25 mm diphenyl0.05dimethyl0.95siloxane column (1 m

    film thickness) with a temperature ranging from 40to 270C and

    flame ionization detection.

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    Figure 27-15shows glassware for

    continuous liquid-liquid extractionof

    a nonvolatile analyte.

    In Figure 27-15a, the extracting

    solvent is denser than the liquid being

    extracted. Solvent boils from the

    flask and condenses into the

    extraction vessel.

    Dense droplets of solvent falling

    through the liquid column extract the

    analyte.

    Supercritical fluid extraction

    Figure 27-15 Continuous liquid-liquid

    extraction apparatus used when extraction

    solvent is (a) denser than the liquid being

    extracted or (b) lighter than the liquid

    being extracted.

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    When the liquid level is high enough,

    extraction solvent is pushed through

    the return tube to the solvent

    reservoir.

    By this means, analyte is slowly

    transferred from the light liquid at the

    left into the dense liquid in the

    reservoir.

    Figure 27-15bshows the procedure

    when the extraction solvent is less

    dense than the liquid being extracted.

    Supercritical fluid extraction

    Figure 27-15 Continuous liquid-liquid

    extraction apparatus used when extraction

    solvent is (a) denser than the liquid being

    extracted or (b) lighter than the liquid

    being extracted.

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    Solid-phase extractionuses a small

    volume of a chromatographicstationary phase or molecularly

    imprinted polymer (Box 26-2) to

    isolate desired analytes from a

    sample.

    The extraction removes much of the

    sample matrix to simplify the

    analysis.

    Solid-Phase Extraction

    Figure 27-16 Steps in solid-phase extraction.

    27-3 Sample Preparation Techniques

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    Figure 27-16shows steps in the solid-

    phase extraction of 10 ng/mL of

    steroids from urine.First, a syringe containing 1 mL of

    C18-silica is conditioned with 2 mL of

    methanol to remove adsorbed organic

    material (Figure 27-16a).

    Then the column is washed with 2 mL

    of water. When the 10-mL urine

    sample is applied, nonpolar

    components adhere to the C18-silica,

    and polar components pass through

    (Figure 27-16b).

    Solid-Phase Extraction

    Figure 27-16 Steps in solid-phase extraction.

    The column is then rinsed with 4 mL of 25 mM borate buffer at pH 8 to remove polar

    substances (Figure 27-16c). Then rinses with 4 mL of 40 vol% methanol/60% water

    and 4 mL of 20% acetone/80% water remove less polar substances (Figure 27-16d).

    Finally, elution with two 0.5-mL aliquots of 73% methanol/27% water washes the

    steroids from the column (Figure 27-16e).

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    Figure 27-17compares chromatograms of the

    drug naproxen in blood serum with or withoutsample cleanup by solid-phase extraction.

    Without cleanup, serum proteins overlap and

    obscure the signal from naproxen. Solid-phase

    extraction removes most of the protein.

    Solid-phase extractions can reduce solvent

    consumption in analytical chemistry.

    For example, a standard procedure approved by

    the U.S. Environmental Protection Agency forthe analysis of pesticides in wastewater requires

    200 mL of dichloromethane for the liquid-liquid

    extraction of 1 L of water.

    Solid-Phase Extraction

    Figure 27-17 HPLC of naproxen in

    blood serum with no cleanup (upper

    trace) or with prior sample cleanup

    (lower trace) by solid-phase extraction on

    C8-silica.

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    The same analytes can be isolated by solid-

    phase extraction on C18-silica disks.

    The pesticides are recovered from the disks by

    supercritical fluid extraction with CO2that is

    finally vented into a small volume of hexane.

    This one kind of analysis can save 105 kg of

    CH2Cl

    2per year.

    Solid-Phase Extraction

    Figure 27-17 HPLC of naproxen in

    blood serum with no cleanup (upper

    trace) or with prior sample cleanup

    (lower trace) by solid-phase extraction

    on C8-silica.

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    The solid-phase extraction

    cartridge used to preconcentrate

    cocaine from river water is themixed-mode cation-exchange

    reversed-phase sorbent at the

    upper left in Figure 27-18.

    This is one of a family of resins

    whose backbone contains

    lipophilic benzene rings and

    hydrophilic pyrrolidone rings.

    The resins are wettable by water

    and have affinity for both polar

    and nonpolar substances.

    Cocaine purification and extraction

    FIGURE 27-18 Water-wettable,

    hydrophobic ion-exchange Oasis

    polymer sorbents for solid-phase

    extraction. [From Waters Corporation,

    Milford, MA.]

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    Cocaine purification and extraction

    FIGURE 27-18 Water-wettable,

    hydrophobic ion-exchange Oasis

    polymer sorbents for solid-phase

    extraction. [From Waters Corporation,

    Milford, MA.]

    The four ion-exchangederivatives are useful

    for retaining and then

    releasing different

    types of analytes when

    conditions such as pH,

    solvent, and ionic

    strength are changed.

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    Cocaine purification and extraction

    The conventional plate at the upper

    right has eight rows of 12 syringe-like wells, each of which can

    contain 5 to 60 mg of resin.

    The Elution plate at the upper

    left has 96 Pasteur-pipet-like wells

    with a small volume, which can be

    eluted by 25-50 L of solvent.

    FIGURE 27-19 96-Well Plate and 96-well mElution

    Plate for solid-phase extraction. [Courtesy Waters

    Corporation, Milford, MA.]

    Preconcentration of cocaine from 500 mL of river water was done with just 60 mg of

    resin in a single syringe. For multiple samples or exploratory research, a 96-well plate

    such as those shown in Figure 27-19 can be used.

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    Solid-phase extraction was used to preconcentrate and partially purify traces of cocaine

    and benzoylecgonine at the opening of this chapter.

    A 500-mL volume of river water was filtered, spiked with 10 ng of internal standard,

    and acidified to pH 2.0 with HCl.

    A solid-phase cation-exchange extraction cartridge containing 60 mg of resin was

    conditioned before use by washing with 6 mL of CH3OH, 3 mL of deionized H2O, and

    3 mL of H2O acidified to pH 2.0 with HCl.

    River water was sucked through the cartridge at 20 mL/min. Liquid was removed from

    the cartridge by suction for 5 min.

    Analytes were then eluted from the cartridge with 2mL of CH3OH followed by 2 mL of2% ammonia solution in CH3OH.

    This process preconcentrates the sample by a factor of 500 mL/4 mL = 125.

    Solid-Phase Extraction

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    Solid-phase extractions reduce solvent consumption in analytical chemistry.

    One standard procedure for the analysis of pesticides in wastewater requires 200 mL ofdichloromethane for the liquid-liquid extraction of 1 L of water.

    The same analytes can be isolated by solid-phase extraction on C18-silica disks.

    The pesticides are recovered from the disks by supercritical fluid extraction with CO2that is finally vented into a small volume of hexane.

    This one kind of analysis can save 105kg of CH2Cl2 per year.

    Solid-Phase Extraction

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    Ion-exchange resins can capture basic or acidic gases.

    Carbonate liberated as CO2from (ZrO)2CO3(OH)2xH2O

    used in nuclear fuel reprocessing can be measured byplacing a known amount of powdered solid in the test tube

    in Figure 27-19and adding 3 M HNO3.

    When the solution is purged with N2, CO2is captured

    quantitatively by moist anion-exchange resin in the sidearm:

    Preconcentration

    Figure 27-19 Apparatus for trapping basic or acidic gases

    by ion exchange.

    Carbonate is eluted from the resin with 1 M NaNO3andmeasured by titration with acid. Table 27-7gives other

    applications of this technique.

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    Preconcentration

    TABLE 27-7 Use of ion-exchange resin for trapping gases .

    Carbonate is eluted from the resin with 1 M NaNO3and measured by titration with

    acid. Table 27-7gives other applications of this technique.

    27-3 Sample Preparation Techniques

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    Trace analysis often requires preconcentrationof analyte to bring it to a higher

    concentration prior to analysis. Metal ions in natural waters can be preconcentrated with

    cation-exchange resin.

    For example, a 500-mL volume of seawater adjusted to pH 6.5 with ammonium acetate

    and ammonia was passed through 2 g of Chelex-100 in the Mg2+form to trap all the

    trace-metal ions.

    Washing with 2 M HNO3 eluted the metals in a total volume of 10 mL, thereby giving a

    concentration increase of 500/10 = 50. Metals in the HNO3solution were then analyzed

    by graphite furnace atomic absorption, with a typical detection limit for Pb being 15pg/mL.

    The detection limit for Pb in the seawater is therefore 50 times lower, or 0.3 pg/mL.

    Preconcentration

    27-3 Sample Preparation Techniques

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    Figure 27-18shows the effect of pH on the recovery of metals from seawater. At low

    pH, H+competes with the metal ions for ion-exchange sites and prevents complete

    recovery.

    Preconcentration

    Figure 27-18 The pH dependence of the recovery of trace metals from seawater by Chelex-100.

    The graph shows the pH of the seawater when it was passed through the column.

    27-3 Sample Preparation Techniques

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    Derivatizationis a procedure in which analyte is chemically modified to make it easier

    to detect or separate. For example, formaldehyde and other aldehydes and ketones in

    air, breath, or cigarette smoke can be trapped and derivatized by passing air through atiny cartridge containing 0.35 g of silica coated with 0.3 wt% 2,4-

    dinitrophenylhydrazine.

    Derivatization

    Carbonyls are converted into the 2,4-

    dinitrophenylhydrazone derivative, which is

    eluted with 5 mL of acetonitrile and analyzed by

    HPLC.

    The products are readily detected by their strongultraviolet absorbance near 360 nm.

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