D. Harvey - Modern Analytical Chemistry (794078), страница 57
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Since sampling exposes the target population to potentialcontamination, the sampling device must be inert and clean.Once a sample is withdrawn from a target population, there is a danger that itmay undergo a chemical or physical change. This is a serious problem since theproperties of the sample will no longer be representative of the target population.For this reason, samples are often preserved before transporting them to the laboratory for analysis. Even when samples are analyzed in the field, preservation may stillbe necessary.The initial sample is called the primary, or gross sample and may be a singleincrement drawn from the target population, or a composite of several increments.In many cases the gross sample cannot be analyzed without further treatment.
Processing the gross sample may be used to reduce the sample’s particle size, to transferthe sample into a more readily analyzable form, or to improve its homogeneity.In the sections that follow, these three steps are considered for the sampling ofliquids (including solutions), gases, and solids.7C.1 SolutionsTypical examples of liquid samples include those drawn from containers of commercial solvents; beverages, such as milk or fruit juice; natural waters, includingfrom lakes, streams, seawater, and rain; bodily fluids, such as blood and urine; and,suspensions, such as those found in many oral medications.Sample Collection Homogeneous solutions are easily sampled by siphoning, decanting, or by using a pipet or syringe. Unfortunately, few solutions are truly homogeneous. When the material to be sampled is of manageable size, manual shaking isoften sufficient to ensure homogeneity.
Samples may then be collected with a pipet,a syringe, or a bottle. The majority of solutions, however, cannot be sampled in thismanner. To minimize the effect of heterogeneity, the method for collecting thegross sample must be adapted to the material being sampled.The environmental sampling of waters and wastewaters provides a good illustration of many of the methods used to sample solutions.
The chemical composition of surface waters, such as streams, rivers, lakes, estuaries, and oceans, is influenced by flow rate and depth. Rapidly flowing shallow streams and rivers, andshallow (<5 m) lakes are usually well mixed and show little stratification withgross sampleThe initial sample, collected from thetarget population without anyprocessing.1931400-CH07 9/8/99 4:03 PM Page 194194Modern Analytical Chemistrydepth. Grab samples are conveniently collected by submerging a capped bottlebelow the surface and removing the cap.
The air–water interface, which may be enriched with heavy metals9 or contaminated with oil, is avoided when collecting thesample. After the sample bottle is filled, the cap is replaced and the bottle removed.Slowly moving streams and rivers, lakes deeper than 5 m, estuaries, and oceans mayshow substantial stratification. Grab samples from near the surface can be collectedas described earlier, whereas samples at greater depths are collected with a weightedsample bottle that is lowered to the desired depth. Once it has reached the desireddepth, the sample bottle is opened, allowed to fill, and closed before retrieving.Grab samples can be analyzed individually, giving information about changes in theanalyte’s concentration with depth. Alternatively, the grab samples may be pooledto form a composite sample.Wells used for collecting groundwater samples must be purged before the sample is collected, since the chemical composition of water in the well-casing and inthe adjacent matrix may be significantly different from that of the surroundinggroundwater.
These differences may result from contaminants introduced whendrilling the well, or differences in the groundwater’s redox potential when exposedto atmospheric oxygen. In general, wells are purged by pumping out a volume ofwater equivalent to several well-casing volumes, or until the water’s temperature,pH, or specific conductance are constant. Samples collected from municipal watersupplies must also be purged since the chemical composition of water left standingin pipes may differ significantly from the treated water supply.
Samples are collectedat faucets after flushing the pipes for 2–3 min.Samples from municipal wastewater treatment plants and samples of industrialdischarges often are collected as 24-h composites. Samples are obtained using anautomatic sampler that periodically removes individual grab samples. The volumeof each sample increment and the frequency of sampling may be constant or mayvary in response to changes in flow rate.Sample containers for collecting solutions are made from glass or plastic.
Containers made from Kimax or Pyrex brand borosilicate glass have the advantage ofbeing sterilizable, easy to clean, and inert to all solutions except those that arestrongly alkaline. The disadvantages of glass containers are cost, weight, and thelikelihood of breakage. Plastic containers are made from a variety of polymers, including polyethylene, polypropylene, polycarbonate, polyvinyl chloride, and Teflon(polytetrafluoroethylene).
Plastic containers are lightweight, durable, and, exceptfor those manufactured from Teflon, inexpensive. In most cases glass or plastic bottles may be used, although polyethylene bottles are generally preferred because oftheir lower cost. Glass containers are always used when collecting samples for theanalysis of pesticides, oil and grease, and organics because these species often interact with plastic surfaces. Since glass surfaces easily adsorb metal ions, plastic bottlesare preferred when collecting samples for the analysis of trace metals.In most cases the sample bottle has a wide mouth, making it easy to fill and remove the sample.
A narrow-mouth sample bottle is used when exposing the sampleto the container cap or to the outside environment is undesirable. Unless exposureto plastic is a problem, caps for sample bottles are manufactured from polyethylene.When polyethylene must be avoided, the container cap includes an inert interiorliner of neoprene or Teflon.Sample Preservation Once removed from its target population, a liquid sample’schemical composition may change as a result of chemical, biological, or physicalprocesses. Following its collection, samples are preserved by controlling the solu-1400-CH07 9/8/99 4:03 PM Page 195Chapter 7 Obtaining and Preparing Samples for AnalysisTable 7.1Preservation Methods and Maximum Holding Timesfor Selected Water and Wastewater ParametersParameterPreservationMaximum Holding Timeammoniachloridemetals—Cr(VI)metals—Hgmetals—all othersnitrateorganochlorine pesticidescool to 4 °C; H2SO4 to pH < 2none requiredcool to 4 °CHNO3 to pH < 2HNO3 to pH < 2none required1 mL 10 mg/mL HgCl2;or addition of extractingsolventnone required28 days28 days24 h28 days6 months48 h7 days without extraction40 days with extractionpHanalyze immediatelytion’s pH and temperature, limiting its exposure to light or to the atmosphere, or byadding a chemical preservative.
After preserving, samples may be safely stored forlater analysis. The maximum holding time between preservation and analysis depends on the analyte’s stability and the effectiveness of sample preservation. Table 7.1provides a list of sample preservation methods and maximum holding times for several analytes of importance in the analysis of water and wastewater.Sample Preparation Most analytical methods can be applied to analytes in a liquidor solution state.
For this reason a gross sample of a liquid or solution does notneed additional processing to bring it into a more suitable form for analysis.7C.2 GasesTypical examples of gaseous samples include automobile exhaust, emissions fromindustrial smokestacks, atmospheric gases, and compressed gases. Also includedwith gaseous samples are solid aerosol particulates.Sample Collection The simplest approach for collecting a gas sample is to fill acontainer, such as a stainless steel canister or a Tedlar/Teflon bag, with a portion ofthe gas.
A pump is used to pull the gas into the container, and, after flushing thecontainer for a predetermined time, the container is sealed. This method has the advantage of collecting a more representative sample of the gas than other collectiontechniques. Disadvantages include the tendency for some gases to adsorb to thecontainer’s walls, the presence of analytes at concentrations too low to detect withaccuracy and precision, and the presence of reactive gases, such as ozone and nitrogen oxides, that may change the sample’s chemical composition with time, or reactwith the container. When using a stainless steel canister many of these disadvantages can be overcome with cryogenic cooling, which changes the sample from agaseous to a liquid state.Due to the difficulty of storing gases, most gas samples are collected using either a trap containing a solid sorbent or by filtering.
Solid sorbents are used to collect volatile gases (vapor pressures more than approximately 10–6 atm) and semivolatile gases (vapor pressures between approximately 10–6 atm and 10–12 atm), andfiltration is used to collect nonvolatile gases.1951400-CH07 9/8/99 4:03 PM Page 196196Modern Analytical Chemistrybreakthrough volumeThe volume of sample that can be passedthrough a solid sorbent before theanalytes are no longer retained.Solid sorbent sampling is accomplished by passing the gas through a canisterpacked with sorbent particles.
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