John B. Phillips Southern Illinois University Department of Chemistry and Biochemistry Carbondale, 111. 62901
Classification of Analytical Sample
Separation Selective Signal
Scientists have a strong tendency to classify things. Classification is essential because it brings order to the large collection of facts in any field of study. As a field of study matures, people working in it develop a consensus of primary definitions to serve as a boundary. These definitions automatically filter out a large bulk of probably worthless information and ideas. No one is able to pay attention to everything that just might be of value. A problem arises, however, when the classification becomes too automatic. Creative thinking then suffers and the field of study becomes stale. Since scientists, by the nature of their work, become habitual classifiers, they must be especially careful about unconsciously accepting consensus definitions. Analytical chemistry tends to he divided into subfields based on analytical methodology. There are overlaps between methodologies and disagree0003-270O/81/A351-1463$01 .W/O @ 1981 American Chemical Sociely
ments among chemists as to their proper definitions hut, in general, boundaries supported hy consensus definitions divide the science into groups such as spectroscopic methods, electrochemical methods, and chromatographic methods. Certain common features of each group have been selected to define its members. Teachers of analytical chemistry must discuss as many individual methods as possible to produce broadly educated chemists, with the result that time devoted to fundamental principles is restricted, and much of what little time remains is used to ex-
Figure 1. An analytical determination starts with a sample and ends with some information about the sample. Separation is an intermediate step that selects a desired signal or signals out of the many originally present in the sample
plain the existing consensus definitions as a framework for the science. By providing a structure, these definitions allow more information to he presented, understood, and remembered. But they also appeal to our tendency to classify things and are very habit-forming. They are often mistak-
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enly given the status of fundamental principles, which makes challenging them very difficult. New analytical chemists start out with their tools, their analytical methods, all neatly classified. More often than not a chemist’s work reinforces both the tendency to classify and the original classification scheme learned in school. Researchers become so specialized within a single methodology that they miss connections between their work and the broader science. Practicing analysts pride themselves on their broad knowledge of many different methods, but they, too,unconsciously accept the neat boundaries drawn around each one and, in concentrating on solving practical problems, miss connections between them. Whether an idea is classifiable or not is often the most important factor in determining a chemist’s reaction to it. New ideas that fit into some personal classification scheme are readily accepted, while those that seem alien hecause they cross established boundaries are automatically rejected as being irrelevant. We need an occasional assault on the boundaries between methodologies. It is not necessary to destroy the boundaries; that would just lead to total confusion. A few well-placed holes are sufficient to weaken the consensus, and allow fresh ideas to get in and stimulate creative thinking.-The “analytical separations” boundary around chromatographic methods is a tempting target. Everyone knows that analytical chromatography is a separation method. But, should it really he bound by that consensus definition?
Analytical Separation The existing consensus definition of analytical separation goes something like this. Many of the best chemical measuring devices respond to a wide range of chemical substances; they are general purpose chemical transducers. The flame ionization detector, for example, is extremely sensitive to almost all sufficiently volatile organic compounds. We cannot directly use such a device to determine one specific substance in a mixture containing others of the same general category. But, by first doing a chemical separation. we can disperse the mixture in space or time and present the components individually to the general transducer. The preliminary chemical separation allows a general purpose transducer to he used for specific determination. By simultaneously doing many separations, as with a chromatographic column, we may sequentially determine many components of a mixture. A great many methods, including chromatographic methods, are generally
classified as analytical separations. In a recent REPORT ( I ) it was stated that it is somewhat misleading to apply the term “separation” to multiplex chromatographic methods because it is unlikely that in the course of an experiment any component of a mixture will ever emerge from the column physically separated from all other components. But multiplex chromatographic methods do give the same information (the chromatogram), depend on the same chemical process (differential migration), and use the same instrumentation (a chromatograph) as other chromatographic methods. Therefore, both multiplex and conventional chromatographic methods should he placed in the same class of analytical procedures. Since conventional chromatography is firmly placed within the separations class, multiplex chromatography should also be classified as separation despite the fact that physical dispersion does not occur. There is a problem here either in the definition of analytical separation or in the classification of chromatographic methods. A similar problem exists in the classification of frontal analysis chromatography. Multiplex chromatography may be discussed in terms directly analogous to conventional chromatography except that it is signals that are separated rather than the chemical comDo-
nents themselves. In all multiplex chromatographic techniques reported so far, a given chemical concentration signal is imposed on every component of the mixture by a valve that modulates the sample stream with a reference stream at the head of the column. The various components each travel through the column at their own migration rate, carrying the modulating signal. Many copies of the modulating signal arrive at the end of the column, dispersed in time by amounts characteristic of the rarrying signal; they then are summed by a general purpose transducer. Through the use of signal detection techniques such as crosscorrelation, the signal power at each delay time may he computed to give the equivalent of a chromatogram. The modulating signal is analogous to the chopper signal in a spectrophotometric instrument. It is imposed on the information-carrying signals, in this case on the mixture component concentrations, to make them more readily measurable.
Definitions The above discussion of multiplex chromatography suggests an alternative definition of analytical separation based on the separation of signals. Figure 1 illustrates how such a definition might fit into the general scheme of an analytical determination. The
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determination begins with a sample and ends with some desired information about the sample. Separation is any means of adding selectivity to the determination. Selectivity is a fundamental concept in analytical chemistry comparable in importance to sensitivity, accuracy, precision, and reliability. The classical chemical separation separates information-carrying signals by dispersing different chemical substances. The signal in this case is carried by the quantity or concentration of a separated substance. Selectivity can be added to a determination by means other than chemical separations-for example, by a monochromator dispersing light according to wavelength. Here the signal is carried by the intensity of light at a particular wavelength. The signal separator or separators in a determination are not always easy to find. In some methods the separator and transducer are combined in a single device-a specific detector. This has the advantage of simplifying the determination but also makes it more difficult to vary or optimize the selectivity. In other methods, several separators may be applied sequentially to gradually refine the signal. This approach is usually necessary in the determination of trace components, because at the trace level the number of possible components that could be mistaken for the desired component greatly increases. Chromatographic methods have been so widely applied because of their broad generality and the ease with which their selectivity can be varied independently of the transducer. Other general methods, especially spectrophotometric ones, also have independent separators and transducers and are also widely applied. Hyphenated (2) or two-dimensional analytical methods such as G C m S have two independent general separators preceding the transducer and can be made extremely selective. Preparative and production-scale chemical separation does not fit into a definition of separation based on signals because actual physical separation of the mixture components is required. But these are not analytical separations because their purpose is not the addition of selectivity for the production of information. Defining analytical separation as “any means of adding selectivity to a determination by separating signals” includes all techniques commonly understood to be analytical separations, but it also includes other techniques not usually classified as separations. Spectroscopic methods through the selection of wavelength. electrochemical methods through the selection of potential, mass spectrometry through
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the selection of mass-to-charge ratio, and other methods add selectivity to a determination hy separating signals. From the analytical chemistry point of view, all of these, including chemical separation, are related and can he considered simply as different manifeatations of the same fundamental operation as illustrated in F m e 1. Redefining analytical separation in such a broad fashion is not desirable because of the confusion that would follow. It would he better to adjust the definition slightly so that it still matches the existing consensus on what is and what is not an analytical separation, while correcting the multiplex chromatography inconsistency. Such an adjustment appears to be a difficult problem. For example, the use of a fiber optic waveguide to separate a pulse of light into ita frequency components has recently been reported (3).This is a differential migration method for electromagnetic radiation and is directly analogous to chromatography. Restricting the definition to include only those signals carried by the transport of chemical substances appears to work. Even this definition is likely to fail in the future, as more variations of chromatography are developed. There is no totally acceptable definition of analytical separation. “Any means of adding selectivity to a determination by separating signals carried by the transport of chemical substancea” is an accurate defmition. It is also useful in thinking ahout and teaching the concept of analytid separation. But it is not an aesthetically pleasing definition. There is not a neat division between what is included and what is excluded, nor is it as simple as the classical defmition.
Conclusion In practice, highly instrumental analytical separation methods such as gas chromatography have at least as much in common with other instrumental methods of achieving analytical selectivity as they do with physical separation methods such as solvent extraction or precipitation. By trying to draw an “analytical separation” boundary around chromatography we obscure this commonality. And, since definitions do have a strong influence on the direction of creative thinking, such an artificial boundary can limit the future development of chromatographic methods. Is analytical separation a worthwhile concept at all or should it he discarded in favor of some other classification scheme? The classical definition is still appealing and remains a useful way of thinking ahout some methodologies. An alternative definition based on the separation of chemi-
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cal signals is more general and is probably better for thinking about highly instrumental chromatographic methods. Discarding analytical separation in favor of the selectivity concept is also appealing because selectivity is a very good way of thinking about the fundamental purpose of separation. But, the selectivity concept applies in one way or another to all determinations and is, therefore, too general to serve as a classification scheme. I t is best to be conversant with all of these ways of thinking and use whichever is most appropriate in a given situation. Boundaries between analytical methodologies are required to provide a framework for the science of analytical chemistry and help guide the thinking of analytical chemists in ways that have generally proved productive. But we must remember that they are artificial boundaries arising out of our need to classify things and are not an inherent part of the science. Alternative classifications are also possible and for some purposes may be preferable to the consensus. I t is always possible to change from one classification scheme to another or even to use more than one a t the same time. Being aware of one's own classification schemes and always watching for alternatives is an important part of being an analytical chemist. References (1) Phillips,John B. A n d . Chern. 1980,52, 468-78 A. (2) Hirschfeld, Tomas. Anal. Chern. 1980, 52,291-312 A. (3) Whitten, W. B.;Ross, H. H. Anal. Chern. 1979,51,411-19.
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John R.Philiips is assistant profmsor in the Department of Chemistry and Biochemistry at Southern Illinois University, Carbondale. His research interests include analytical instrumentation, chromatographic processes, and applications of laboratory computers. CIRCLE 2 ON READER SERVICE CARD
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