D. Harvey - Modern Analytical Chemistry (794078), страница 58
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Typically 2–100 L of gas is sampled when collectingvolatile compounds, and 2–500 m3 when collecting semivolatile gases.* A variety ofinorganic, organic polymer and carbon sorbents have been used. Inorganic sorbents, such as silica gel, alumina, magnesium aluminum silicate, and molecularsieves, are efficient collectors for polar compounds. Their efficiency for collectingwater, however, limits their sorption capacity for many organic compounds.Organic polymeric sorbents are manufactured using polymeric resins of2,4-diphenyl-p-phenylene oxide or styrene-divinylbenzene for volatile compounds,or polyurethane foam for semivolatile compounds. These materials have a low affinity for water and are efficient collectors for all but the most highly volatile organiccompounds and some low-molecular-weight alcohols and ketones.
The adsorbingability of carbon sorbents is superior to that of organic polymer resins. Thus, carbonsorbents can be used to collect those highly volatile organic compounds that cannotbe collected by polymeric resins. The adsorbing ability of carbon sorbents may be adisadvantage, however, since the adsorbed compounds may be difficult to desorb.Nonvolatile compounds are normally present either as solid particulates orbound to solid particulates. Samples are collected by pulling large volumes of gasthrough a filtering unit where the particulates are collected on glass fiber filters.One of the most significant problems with sorbent sampling is the limited capacity of the sorbent to retain gaseous analytes.
If a sorbent’s capacity is exceededbefore sampling is complete then a portion of the analyte will pass through the canister without being retained, making an accurate determination of its concentrationimpossible. For this reason it is not uncommon to place a second sorbent canisterdownstream from the first. If the analyte is not detected in the second canister, thenit is safe to assume that the first canister’s capacity has not been exceeded. The volume of gas that can be sampled before exceeding the sorbent’s capacity is called thebreakthrough volume and is normally reported with units of m3/(gpack), where gpackis the grams of sorbent.The short-term exposure of humans, animals, and plants to gaseous pollutantsis more severe than that for pollutants in other matrices. Since the composition ofatmospheric gases can show a substantial variation over a time, the continuousmonitoring of atmospheric gases such as O3, CO, SO2, NH3, H2O2, and NO2 by insitu sampling is important.10Sample Preservation and Preparation After collecting the gross sample there isgenerally little need for sample preservation or preparation.
The chemical composition of a gas sample is usually stable when it is collected using a solid sorbent, a filter, or by cryogenic cooling. When using a solid sorbent, gaseous compounds maybe removed before analysis by thermal desorption or by extracting with a suitablesolvent. Alternatively, when the sorbent is selective for a single analyte, the increasein the sorbent’s mass can be used to determine the analyte’s concentration in thesample.7C.3 SolidsTypical examples of solid samples include large particulates, such as those found inores; smaller particulates, such as soils and sediments; tablets, pellets, and capsulesused in dispensing pharmaceutical products and animal feeds; sheet materials, suchas polymers and rolled metals; and tissue samples from biological specimens.*1 m3 is equivalent to 1000 L.1400-CH07 9/8/99 4:03 PM Page 197Chapter 7 Obtaining and Preparing Samples for Analysis197dCloseOpenSeaombottrslayetaryenimSedSample Collection Solids are usually heterogeneous, and samples must be collected carefully ifthey are to be representative of the target population.
As noted earlier, solids come in a variety offorms, each of which is sampled differently.Sediments from the bottom of streams, rivers,lakes, estuaries, and oceans are collected with a bottom grab sampler or with a corer. Grab samplers areequipped with a pair of “jaws” that close when theycontact the sediment, scooping up sediment in theprocess (Figure 7.5).
Their principal advantages areease of use and the ability to collect a large sample.Disadvantages include the tendency to lose finergrained sediment particles as water flows out of thesampler and the loss of spatial information, both laterally and with depth, due to mixing of the sample.An alternative method for sampling sedimentsuses a cylindrical coring device (Figure 7.6). Thecorer is dropped into the sediment, collecting a column of sediment and the water in contact with thesediment. With the possible exception of sedimentat the surface, which may experience mixing, samples collected with a corer maintain their verticalprofile. As a result, changes in the sediment’s composition with depth are preserved.
The main disadvantage to a corer is that only a small surface area issampled. For this reason sampling with a corer usually requires more samples.Soil samples collected at depths of up to 30 cm are easily collected withscoops or shovels, although the sampling variance is generally high. A bettermethod for obtaining soil samples near the surface is to use a soil punch. Thisthin-walled steel tube retains a core sample when it is pushed into the soil andremoved.
Soil samples collected at depths greater than 30 cm are obtained bydigging a trench and collecting lateral samples with a soil punch. Alternatively,Figure 7.5Schematic diagram of grab sampler. Whenthe sampler reaches the sediment, the jawsof the grab sampler are closed, collecting asample of the sediment.Figure 7.6Schematic diagram of a piston corer inoperation.
The weight of the corer issufficient to cause its penetration into thesediment, while the upward motion of thepiston allows water pressure to help forcethe sediment column into the barrel of thecorer.1400-CH07 9/8/99 4:03 PM Page 198198Modern Analytical ChemistryFigure 7.7Example of a four-unit riffle. A sampleadded through the top is divided into fourpiles, two on each of the riffle’s sides.an auger may be used to drill a hole to the desired depth and the sample collected with a soil punch.The sampling of particulate material is often determined by the size of the particles.
Large particulate solids, such as coals and ores, can be sampled by randomlycollecting samples with a shovel or by riffling. A riffle (Figure 7.7) is a trough containing an even number of compartments with adjacent compartments emptyingon opposite sides of the riffle. Particulate material dumped into a riffle is dividedinto two parts. By repeatedly passing half of the separated material back through theriffle, a sample of any desired size may be collected.
Smaller particulate materials,such as powders, are best collected with a sample thief, which allows material to becollected simultaneously from several locations (Figure 7.8). A typical sample thiefconsists of two tubes that are nestled together. Each tube has an identical set of slotsaligned down their length. Before the sample thief is inserted into the material beingsampled, the inner tube is rotated so that slots are closed. When the sample thief isin place, the inner tube is rotated to open the slots, allowing the powder to enter thesample thief through each slot. The inner tube is then rotated to the closed positionand the sample thief withdrawn.When sampling a metal, it usually is necessary to obtain material from both thesurface and the interior.
When the metal is in the form of a sheet, random samplescan be collected with a metal punch. Samples can be obtained from a metal wire byrandomly cutting off pieces of an appropriate length. Larger pieces of metal, such asbars or bricks, are best sampled by sawing through the metal at randomly selectedpoints and collecting the “sawdust” or by drilling through the metal and collectingthe shavings. A surface coating can be sampled in situ or by dissolving the coatingwith an appropriate solvent.Sampling of biological tissue is done by removing the entire organ, which isthen homogenized before smaller portions are taken for analysis.
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