D. Harvey - Modern Analytical Chemistry (794078), страница 67
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Chem. Educ. 1993, 70, A133–A140.This paper describes the construction and use of adiffusion tube for sampling NO2 from the atmosphere.Examples of its use include the determination of NO2concentrations at various heights above ground level in anurban environment and through a tree’s leaf canopy.—Continued1400-CH07 9/8/99 4:04 PM Page 226226ExperimentsModern Analytical ChemistryContinued from page 225The following experiments introduce students to the importance of sample preparation and methods for extracting analytes from theirmatrix. Each experiment includes a brief description of the sample and analyte, as well as the method of analysis used to measure theanalyte’s concentration.Dunn, J.
G.; Phillips, D. N.; von Bronswijk, W. “An Exerciseto Illustrate the Importance of Sample Preparation inChemical Analysis,” J. Chem. Educ. 1997, 74, 1188–1190.Ore samples are analyzed for %w/w Ni. A jaw crusher isused to break the original ore sample into smaller pieces thatare then sieved into 5 size fractions. A portion of eachfraction is reduced in size using a disk mill and samples takenfor analysis by coning and quartering. The effect of particlesize on the determination of %w/w Ni is evaluated.“Extract-Clean SPE Sample Preparation GuideVolume 1,” Bulletin No. 83, Alltech Associates, Inc.Deerfield, IL.This publication provides several examples of theuse of solid-phase extractions for separating analytesfrom their matrices. Some of the examples includedare caffeine from coffee, polyaromatic hydrocarbonsfrom water, parabens from cosmetics, chlorinatedpesticides from water, and steroids fromhydrocortisone creams.
Extracted analytes may bedetermined quantitatively by gas (GC) or liquidchromatography (LC).Freeman, R. G.; McCurdy, D. L. “Using Microwave SampleDecomposition in Undergraduate Analytical Chemistry,”J. Chem. Educ. 1998, 75, 1033–1032.Although experimental details are not included, theapplication of microwave digestion is discussed for thedetermination of Na in food samples by flame atomicemission spectroscopy, and the Kjeldahl analysis ofnitrogen is detailed.Snow, N.
H.; Dunn, M.; Patel, S. “Determination of CrudeFat in Food Products by Supercritical Fluid Extractionand Gravimetric Analysis,” J. Chem. Educ. 1997, 74,1108–1111.Supercritical CO2 is used to extract fat from candy bars.Samples are placed in an extraction vessel and its weightdetermined before and after extraction. The %w/w fatcontent is determined by difference. The volume of CO2needed to effect a complete extraction is determinedexperimentally. Variations in results for different samplesillustrate the importance of sampling.Yang, M. J.; Orton, M. L.; Pawliszyn, J. “QuantitativeDetermination of Caffeine in Beverages Using a CombinedSPME-GC/MS Method,” J.
Chem. Educ. 1997, 74,1130–1132.Caffeine is extracted from beverages by a solid-phasemicroextraction using an uncoated fused silica fiber. The fiberis suspended in the sample for 5 min and the sample stirred toassist the mass transfer of analyte to the fiber. Immediatelyafter removing the fiber from the sample it is transferred tothe gas chromatograph’s injection port where the analyte isthermally desorbed. Quantitation is accomplished by using a13C caffeine solution as an internal standard.37L PROBLEMS1.
Because of the risk of lead poisoning, the exposure of childrento lead-based paint is a significant public health concern. Thefirst step in the quantitative analysis of lead in dried paintchips is to dissolve the sample. Corl evaluated severaldissolution techniques.24 In this study, samples of paint werecollected and pulverized with a Pyrex mortar and pestle.Replicate portions of the powdered paint were then taken foranalysis. Results for an unknown paint sample and for astandard reference material, in which dissolution wasaccomplished by a 4–6-h digestion with HNO3 on a hot plate,are shown in the following table.Replicate%w/w Pbin Unknown%w/w Pbin Standard12345.096.296.644.6311.4811.6211.4711.86(a) Determine the overall variance, the variance due to themethod, and the variance due to sampling.
(b) Whatpercentage of the overall variance is due to sampling?How might the variance due to sampling be decreased?1400-CH07 9/8/99 4:04 PM Page 227Chapter 7 Obtaining and Preparing Samples for Analysis2. A shipment of 100 barrels of an organic solvent is to beevaluated by collecting and analyzing single samples from 10of the barrels. A random number table is used to determinethe barrels to be sampled. From which barrels should thesamples be drawn if the first barrel is given by the twelfthentry in the random number table in Appendix 1E, withsubsequent barrels given by every third entry?3. The concentration of dissolved O2 in a lake shows a daily cycledue to the effect of photosynthesis and a yearly cycle due toseasonal changes in temperature.
Suggest an appropriatesystematic sampling plan for monitoring the daily changes indissolved O2. Suggest an appropriate systematic sampling planfor monitoring the yearly changes in dissolved O2.4. The following data were collected during a preliminary studyof the pH of an industrial wastewater streamTime(h)pHTime(h)pH0.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.54.44.85.25.25.65.45.44.44.84.84.24.23.84.04.03.94.79.09.510.010.511.011.512.012.513.013.514.014.515.015.516.016.517.05.75.56.56.05.86.05.65.65.44.95.24.44.04.54.05.05.0determine if the population is homogeneous orheterogeneous?7. Examine equation 7.6 for sampling a heterogeneouspopulation.
(a) Explain why the contribution of heterogeneityto the overall sampling variance can be minimized byincreasing the number of samples, but not the mass of theindividual samples. (b) Explain why the contribution ofhomogeneity to the overall sampling variance can beminimized by increasing both the number of samples and themass of individual samples.8. Show that equation 7.5 can be derived from equation 7.4.Assume that the particles are spherical with a radius of r and adensity of d.9. The sampling constant for the radioisotope 24Na in a sample ofhomogenized human liver has been reported as approximately35 g.25 (a) What is the expected relative standard deviation forsampling if 1.0-g samples are analyzed? (b) How many 1.0-gsamples need to be analyzed to obtain a maximum samplingerror of ±5% at the 95% confidence level?10. Engels and Ingamells reported the following results for the%w/w K2O in a mixture of amphibolite and orthoclase.260.2470.2476.
The best sampling plan for collecting samples dependson whether the population is homogeneous orheterogeneous. (a) Define homogeneous and heterogeneous.(b) If you collect and analyze a single sample, can you0.3000.2750.2360.2120.2580.3110.3040.2580.3300.187Each of the 12 samples had a nominal weight of 0.1 g.Determine the approximate value for Ks, and the mass ofsample needed to achieve a percent relative standarddeviation of 2%.11. The following data have been reported for the determinationof KH2PO4 in a mixture of KH2PO4 and NaCl.27Nominal Mass(g)Actual Mass(g)0.100.10390.10150.10120.10100.10600.09970.0851.0780.4131.2480.6540.5070.250.25150.24650.27700.24600.24850.25900.8470.5980.4310.8420.9641.1780.500.50840.49540.52860.52320.49650.49951.0090.9470.6180.7440.5720.709Construct a graph of pH as a function of time, and suggest anappropriate sampling frequency for a long-term monitoringprogram.5.
Suppose you have been asked to monitor the dailyfluctuations in atmospheric ozone levels in the downtownarea of a city to determine the relationship between dailytraffic patterns and ozone levels. (a) Describe the samplingplan you would choose (random, systematic, judgmental,systematic–judgmental, or stratified). (b) Would you chooseto collect and analyze a series of grab samples or form asingle composite sample? (c) How would your answers tothese questions change if the purpose of your work is todetermine if the average daily ozone level exceeds a thresholdvalue?227%w/w KH2PO41400-CH07 9/8/99 4:04 PM Page 228228Modern Analytical Chemistry1.001.0270.9870.9910.9980.9971.0010.6960.8430.5350.7500.7110.6392.502.4962.5042.4962.4962.5572.5090.7660.7690.6820.6090.5890.617(a) Prepare a graph of %w/w KH2PO4 versus actualsample mass, and discuss how this graph is consistentwith your understanding of factors affecting samplingvariance.
(b) For each nominal mass, calculate thepercent relative standard deviation for the analysis.The value of Ks for this analysis has been estimated as350. For each nominal mass, use Ks to determine thepercent relative standard deviation due to sampling.Considering these two calculations, what conclusioncan you make about the importance of indeterminatesampling errors for this analysis? (c) For eachnominal mass, convert the percent relative standarddeviation to an absolute standard deviation.
Plotpoints on your graph corresponding to ±1 absolutestandard deviation about the overall average %w/wKH2PO4. Draw smooth curves through these twosets of points. Considering these results, does thesample appear to be homogeneous on the scale atwhich it is sampled?12.
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