D. Harvey - Modern Analytical Chemistry (794078), страница 6
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Identify the problem5. Propose a solutionDetermine type of information needed(qualitative, quantitative,characterization, or fundamental)Conduct external evaluationIdentify context of the problem2. Design the experimental procedureEstablish design criteria (accuracy, precision,scale of operation, sensitivity, selectivity,cost, speed)4. Analyze the experimental dataReduce or transform dataIdentify interferentsAnalyze statisticsSelect methodVerify resultsEstablish validation criteriaInterpret resultsFeedbackloopEstablish sampling strategy3. Conduct an experimentCalibrate instruments and equipmentStandardize reagentsFigure 1.3Flow diagram for the analytical approach tosolving problems; modified after Atkinson.7cGather dataanalysis.
Finding an appropriate balance between these parameters is frequentlycomplicated by their interdependence. For example, improving the precision of ananalysis may require a larger sample. Consideration is also given to collecting, storing, and preparing samples, and to whether chemical or physical interferences willaffect the analysis. Finally, a good experimental procedure may still yield useless information if there is no method for validating the results.The most visible part of the analytical approach occurs in the laboratory. Aspart of the validation process, appropriate chemical or physical standards are usedto calibrate any equipment being used and any solutions whose concentrationsmust be known.
The selected samples are then analyzed and the raw data recorded.The raw data collected during the experiment are then analyzed. Frequently thedata must be reduced or transformed to a more readily analyzable form. A statisticaltreatment of the data is used to evaluate the accuracy and precision of the analysisand to validate the procedure. These results are compared with the criteria established during the design of the experiment, and then the design is reconsidered, additional experimental trials are run, or a solution to the problem is proposed.
Whena solution is proposed, the results are subject to an external evaluation that may result in a new problem and the beginning of a new analytical cycle.1400-CH01 9/9/99 2:20 PM Page 7Chapter 1 IntroductionAs an exercise, let’s adapt this model of the analytical approach to a real problem. For our example, we will use the determination of the sources of airborne pollutant particles. A description of the problem can be found in the following article:“Tracing Aerosol Pollutants with Rare Earth Isotopes” byOndov, J. M.; Kelly, W.
R. Anal. Chem. 1991, 63, 691A–697A.Before continuing, take some time to read the article, locating the discussions pertaining to each of the five steps outlined in Figure 1.3. In addition, consider the following questions:1.2.3.4.5.6.7.8.9.What is the analytical problem?What type of information is needed to solve the problem?How will the solution to this problem be used?What criteria were considered in designing the experimental procedure?Were there any potential interferences that had to be eliminated? If so, howwere they treated?Is there a plan for validating the experimental method?How were the samples collected?Is there evidence that steps 2, 3, and 4 of the analytical approach are repeatedmore than once?Was there a successful conclusion to the problem?According to our model, the analytical approach begins with a problem.
Themotivation for this research was to develop a method for monitoring the transportof solid aerosol particulates following their release from a high-temperature combustion source. Because these particulates contain significant concentrations oftoxic heavy metals and carcinogenic organic compounds, they represent a significant environmental hazard.An aerosol is a suspension of either a solid or a liquid in a gas. Fog, for example, is a suspension of small liquid water droplets in air, and smoke is a suspensionof small solid particulates in combustion gases. In both cases the liquid or solid particulates must be small enough to remain suspended in the gas for an extendedtime. Solid aerosol particulates, which are the focus of this problem, usually havemicrometer or submicrometer diameters. Over time, solid particulates settle outfrom the gas, falling to the Earth’s surface as dry deposition.Existing methods for monitoring the transport of gases were inadequate forstudying aerosols.
To solve the problem, qualitative and quantitative informationwere needed to determine the sources of pollutants and their net contribution tothe total dry deposition at a given location. Eventually the methods developed inthis study could be used to evaluate models that estimate the contributions of pointsources of pollution to the level of pollution at designated locations.Following the movement of airborne pollutants requires a natural or artificialtracer (a species specific to the source of the airborne pollutants) that can be experimentally measured at sites distant from the source.
Limitations placed on thetracer, therefore, governed the design of the experimental procedure. These limitations included cost, the need to detect small quantities of the tracer, and the absence of the tracer from other natural sources. In addition, aerosols are emittedfrom high-temperature combustion sources that produce an abundance of very reactive species. The tracer, therefore, had to be both thermally and chemically stable.On the basis of these criteria, rare earth isotopes, such as those of Nd, were selectedas tracers.
The choice of tracer, in turn, dictated the analytical method (thermalionization mass spectrometry, or TIMS) for measuring the isotopic abundances of71400-CH01 9/9/99 2:20 PM Page 88Modern Analytical ChemistryNd in samples. Unfortunately, mass spectrometry is not a selective technique. Amass spectrum provides information about the abundance of ions with a givenmass. It cannot distinguish, however, between different ions with the same mass.Consequently, the choice of TIMS required developing a procedure for separatingthe tracer from the aerosol particulates.Validating the final experimental protocol was accomplished by running amodel study in which 148Nd was released into the atmosphere from a 100-MW coalutility boiler.
Samples were collected at 13 locations, all of which were 20 km fromthe source. Experimental results were compared with predictions determined by therate at which the tracer was released and the known dispersion of the emissions.Finally, the development of this procedure did not occur in a single, linear passthrough the analytical approach. As research progressed, problems were encountered and modifications made, representing a cycle through steps 2, 3, and 4 of theanalytical approach.Others have pointed out, with justification, that the analytical approach outlined here is not unique to analytical chemistry, but is common to any aspect of science involving analysis.8 Here, again, it helps to distinguish between a chemicalanalysis and analytical chemistry.
For other analytically oriented scientists, such asphysical chemists and physical organic chemists, the primary emphasis is on theproblem, with the results of an analysis supporting larger research goals involvingfundamental studies of chemical or physical processes. The essence of analyticalchemistry, however, is in the second, third, and fourth steps of the analytical approach. Besides supporting broader research goals by developing and validating analytical methods, these methods also define the type and quality of informationavailable to other research scientists.
In some cases, the success of an analyticalmethod may even suggest new research problems.1C Common Analytical Problemsqualitative analysisAn analysis in which we determine theidentity of the constituent species in asample.In Section 1A we indicated that analytical chemistry is more than a collection ofqualitative and quantitative methods of analysis. Nevertheless, many problems onwhich analytical chemists work ultimately involve either a qualitative or quantitative measurement. Other problems may involve characterizing a sample’s chemicalor physical properties.
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