Page 24 - Modern Analytical Chemistry

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Chapter 1 Introduction 7

As an exercise, let’s adapt this model of the analytical approach to a real prob-
lem. For our example, we will use the determination of the sources of airborne pol-
lutant particles. A description of the problem can be found in the following article:
“Tracing Aerosol Pollutants with Rare Earth Isotopes” by
Ondov, J. M.; Kelly, W. R. Anal. Chem. 1991, 63, 691A–697A.

Before continuing, take some time to read the article, locating the discussions per-
taining to each of the five steps outlined in Figure 1.3. In addition, consider the fol-
lowing questions:
1. What is the analytical problem?
2. What type of information is needed to solve the problem?
3. How will the solution to this problem be used?
4. What criteria were considered in designing the experimental procedure?
5. Were there any potential interferences that had to be eliminated? If so, how
were they treated?
6. Is there a plan for validating the experimental method?
7. How were the samples collected?
8. Is there evidence that steps 2, 3, and 4 of the analytical approach are repeated
more than once?
9. Was there a successful conclusion to the problem?
According to our model, the analytical approach begins with a problem. The
motivation for this research was to develop a method for monitoring the transport
of solid aerosol particulates following their release from a high-temperature com-
bustion source. Because these particulates contain significant concentrations of
toxic heavy metals and carcinogenic organic compounds, they represent a signifi-
cant environmental hazard.
An aerosol is a suspension of either a solid or a liquid in a gas. Fog, for exam-
ple, is a suspension of small liquid water droplets in air, and smoke is a suspension
of small solid particulates in combustion gases. In both cases the liquid or solid par-
ticulates must be small enough to remain suspended in the gas for an extended
time. Solid aerosol particulates, which are the focus of this problem, usually have
micrometer or submicrometer diameters. Over time, solid particulates settle out
from the gas, falling to the Earth’s surface as dry deposition.
Existing methods for monitoring the transport of gases were inadequate for
studying aerosols. To solve the problem, qualitative and quantitative information
were needed to determine the sources of pollutants and their net contribution to
the total dry deposition at a given location. Eventually the methods developed in
this study could be used to evaluate models that estimate the contributions of point
sources of pollution to the level of pollution at designated locations.
Following the movement of airborne pollutants requires a natural or artificial
tracer (a species specific to the source of the airborne pollutants) that can be exper-
imentally measured at sites distant from the source. Limitations placed on the
tracer, therefore, governed the design of the experimental procedure. These limita-
tions included cost, the need to detect small quantities of the tracer, and the ab-
sence of the tracer from other natural sources. In addition, aerosols are emitted
from high-temperature combustion sources that produce an abundance of very re-
active 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 selected
as tracers. The choice of tracer, in turn, dictated the analytical method (thermal
ionization mass spectrometry, or TIMS) for measuring the isotopic abundances of