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Modern Analytical Chemistry

Experiments The following experiments introduce students to the importance of sample preparation and methods for extracting analytes from their

matrix. Each experiment includes a brief description of the sample and analyte, as well as the method of analysis used to measure the
analyte’s concentration.
determination of Na in food samples by flame atomic
Dunn, J. G.; Phillips, D. N.; von Bronswijk, W. “An Exercise
to Illustrate the Importance of Sample Preparation in
emission spectroscopy, and the Kjeldahl analysis of
Chemical Analysis,” J. Chem. Educ. 1997, 74, 1188–1190.
nitrogen is detailed.
Ore samples are analyzed for %w/w Ni. A jaw crusher is
Snow, N. H.; Dunn, M.; Patel, S. “Determination of Crude
used to break the original ore sample into smaller pieces that
Fat in Food Products by Supercritical Fluid Extraction
are then sieved into 5 size fractions. A portion of each
and Gravimetric Analysis,” J. Chem. Educ. 1997, 74,
fraction is reduced in size using a disk mill and samples taken
1108–1111.
for analysis by coning and quartering. The effect of particle
size on the determination of %w/w Ni is evaluated. Supercritical CO 2 is used to extract fat from candy bars.
Samples are placed in an extraction vessel and its weight
“Extract-Clean SPE Sample Preparation Guide determined before and after extraction. The %w/w fat
Volume 1,” Bulletin No. 83, Alltech Associates, Inc. content is determined by difference. The volume of CO 2
Deerfield, IL. needed to effect a complete extraction is determined
This publication provides several examples of the experimentally. Variations in results for different samples
use of solid-phase extractions for separating analytes illustrate the importance of sampling.
from their matrices. Some of the examples included
Yang, M. J.; Orton, M. L.; Pawliszyn, J. “Quantitative
are caffeine from coffee, polyaromatic hydrocarbons
Determination of Caffeine in Beverages Using a Combined
from water, parabens from cosmetics, chlorinated
SPME-GC/MS Method,” J. Chem. Educ. 1997, 74,
pesticides from water, and steroids from
1130–1132.
hydrocortisone creams. Extracted analytes may be
determined quantitatively by gas (GC) or liquid Caffeine is extracted from beverages by a solid-phase
chromatography (LC). microextraction using an uncoated fused silica fiber. The fiber
is suspended in the sample for 5 min and the sample stirred to
Freeman, R. G.; McCurdy, D. L. “Using Microwave Sample assist the mass transfer of analyte to the fiber. Immediately
Decomposition in Undergraduate Analytical Chemistry,” after removing the fiber from the sample it is transferred to
J. Chem. Educ. 1998, 75, 1033–1032. the gas chromatograph’s injection port where the analyte is
Although experimental details are not included, the thermally desorbed. Quantitation is accomplished by using a
application of microwave digestion is discussed for the 13 C 3 caffeine solution as an internal standard.

7L PROBLEMS

1. Because of the risk of lead poisoning, the exposure of children %w/w Pb %w/w Pb
to lead-based paint is a significant public health concern. The Replicate in Unknown in Standard
first step in the quantitative analysis of lead in dried paint
1 5.09 11.48
chips is to dissolve the sample. Corl evaluated several
24
dissolution techniques. In this study, samples of paint were 2 6.29 11.62
collected and pulverized with a Pyrex mortar and pestle. 3 6.64 11.47
Replicate portions of the powdered paint were then taken for 4 4.63 11.86
analysis. Results for an unknown paint sample and for a (a) Determine the overall variance, the variance due to the
standard reference material, in which dissolution was method, and the variance due to sampling. (b) What
accomplished by a 4–6-h digestion with HNO 3 on a hot plate, percentage of the overall variance is due to sampling?
are shown in the following table. How might the variance due to sampling be decreased?