Page 320 - Instrumentation Reference Book 3E

304 Chemical analysis: introduction

environment of the chemical plant and be capable coined the term chromatography for the method.
of coping with temperature changes and plant vari- It was then not used for twenty years; then
ables without loss of accuracy. Such instruments the method was rediscovered and used for the
have to be constructed to work for long continuous separation of carotenes, highly unsaturated hydro-
periods without exhibiting untoward drift or being carbons to which various animal and plant sub-
adversely affected by the materials in the plant stances (e.g., butter and carrots) owe their color.
stream being monitored. Chromatography is thus a separating proced-
Laboratory instruments, on the other hand, can ure with the actual measurement of the separated
be much simpler. Here the essential is a robust, substance made by another method, such as
easy-to-use instrument for a unique determination. ultraviolet absorption or thermal conductivity,
Temperature compensation can be made by man- but as it is such a powerful analytical tool it will
ual adjustment of controls at the time of making a be dealt with here as an analytical method.
determination, and the instrument span can be set All chromatographic techniques depend on
by use of standards each time the instrument is the differing distributions of individual com-
used. Thus, there is no problem with drift. Labora- pounds in a mixture between two immiscible
tory instruments in general-purpose laboratories, phases as one phase (the mobile phase) passes
however, can be as complex and costly as on- through or over the other (the stationary phase).
stream instruments but with different require- In practice the mixture of compounds is added
ments. Here flexibility to carry out several deter- to one end of a discrete amount of stationary
minatioiis on a wide variety of samples is of prime phase (a tubeful) and the mobile phase is then
importance. but again temperature compensation introduced at the same end and allowed to pass
and span adjustment can be carried out manually along the stationary phase. The mixture of com-
each time a determination is made. More expen- pounds is eluted, the compound appearing first
sive instruments use microprocessors to do such at the other end of the stationary phase being
things automatically, and these are becoming com- that which has the smallest distribution into the
mon in modern laboratories. Finally, although the stationary phase. As the separated compounds
cost of an analytical instrument depends on its appear at the end of the stationary phase they
complexity and degree of automation, there are are detected either by means of unique detectors
other costs, which should not be forgotten. Instru- or by general-purpose detectors which sense the
ment maintenance charges can be appreciable, and compound only as an impurity in the mobile
there is also the cost of running an instrument. The phase.
latter can range from almost nothing in the case of The apparatus used varies according to the
visible and ultraviolet spectrometers to several nature of the two phases. In gas chromatography,
thousand pounds a year for argon supplies to the mobile phase is a gas with the stationary
inductively coupled plasma spectrometers. Many phase either a solid or a liquid. This is described
automatic analytical instruments require the pre- in detail in Chapter 18. Liquid chromatography
paration of reagent solutions and this, too, can covers all techniques using liquid as a mobile
involve an appreciable manpower requirement, phase-these are column chromatography (liquid/
also something which should be costed. liquid or liquidlsolid), paper chromatography
More detailed analysis of the factors affecting and thin layer chromatography.
the costing of analytical chemistry techniques and
instrumentation is beyond the scope of this chap-
ter, but other chapters in this reference book give 15.2.2 Paper chromatography and thin layer
details and comparisons of analytical instrumenta- chromatography
tion for many applications. It is arranged with
large chapters on electrochemical and spectrochem- In paper chromatography the separation is car-
ical techniques and further chapters on the applica- ried out on paper, formerly on ordinary filter
tions of gas analysis and humidity measuring papers but more recently on papers specially
instruments. For completeness, the remainder of manufactured for the purpose. These are made
this chapter contains brief descriptions of chroma- free from metallic impurities and have reproducible
tography, thermal analysis and polarography. thickness, porosity, and arrangement of cellulose
fibers.
The paper used (which must not have been
15.2 Chromatography dried) contains adsorbed water and so paper
chromatography can be regarded as an absorp-
15.2.1 General chromatography tion process. However, the characteristics of the
paper can be changed by applying specific liquids
Around 1900 M.S. Tswett used the adsorbing to it. Silicone oils, paraffin oil. petroleum jelly,
power of solids to separate plant pigments and and rubber latex can be used to give a paper with