Analysis of soft drink by three different techniques

Analysis of Soft Drinks: UV Spectrophotometry,
Liquid Chromatography, and Capillary Electrophoresis1

An experiment for the undergraduate instrumental analysis
laboratory should accomplish several instructional goals.
First, it must demonstrate the capabilities and limitations of
the method, as well as the proper procedures for data acquisition
and computation of results. Students must also be reminded
of the importance of the chemistry of the sample
and how this relates to the analysis. Application of the method
to a commercial product always helps stimulate student interest
and teaches the extra considerations necessary in a realworld
analysis. It is also instructive to analyze the same product
by more than one instrumental method.
Analyses of regular and diet soft drinks fulfill all these
objectives. The samples are common everyday products, and
they may be analyzed by a variety of means. Soft drink components
have been determined by HPLC with UV detection
(1–5) for a number of years, and methods utilizing capillary
electrophoresis to determine caffeine (6, 7) and other components
(8) have been developed. Although certainly not as
useful as LC or CE, multicomponent UV analysis can also
be used if the product does not contain too many absorbing
components.
This paper describes a series of undergraduate experiments
using these three instrumental methods for the analysis
of components of public interest in commercial soft drinks:
caffeine, a central nervous system stimulant; sodium benzoate
(determined as benzoic acid), which serves as a preservative;
and the artificial sweetener aspartame. In addition to teaching
the physical bases and practical applications of the three
instruments, the experiments stress the chemical nature of
the sample, especially the acid/base character of the three
compounds and the importance of pH in the design of LC and
CE separations. As part of the data acquisition and analysis,
students also determine the method detection limits (MDL)
for the three compounds by LC and CE. The concepts of MDL,
false positives, and false negatives are especially relevant,
considering the current interest in “natural” foods.