"The defensive practice of analytical chemistry thus requires detecting and measuring as many sample constituents as possible, known or unknown. " hand, does not require a volatile sample, giving a greater universality of separation. But it is bottlenecked by the lower sensitivity and universality of its detection, a phenomenon which is probably not inherent. However, we must remember that LC, with volume concentrations of sample in its effluent similar to GC, has vastly lower mass concentrations, so that some of its apparent sensitivity loss is not real. Beyond this, LC, with the extra degree of freedom of its sample-solvent interaction, has a higher potential for selectivity. To have enough distinct resolvable elements to avoid overlap in complex mixtures, both techniques would benefit from multidimensional separation. This may be done by column concatenation in either technique, or by using two-dimensional techniques like TLC in place of LC. At present,
the output of GC is more compatible with further analytical methods, but this is only a temporary limit. Present circumstances put GC at a slight advantage, while ultimately the advantage will lie with LC. Perhaps the optimal tradeoff may occur with techniques intermediate between GC and LC, such as ultrahigh pressure GC (21). The matrix constituents can perturb the properties of the sample either chemically or physically. Chemically, we must consider alterations of dissociation caused by common ion effects and complexation effects with both the analyte and the reagent. Catalysis or catalytic poisoning of the analytical reaction, as well as any incompatibility with the reagents used, are other obvious sources of problems.
Physical effects can be considerably more subtle. Any micelle forming constituents in the medium will in effect turn the medium into a two-phase system, a fact which will not always be observable. If the dispersed phase has selective affinity for the analyte, the latter will have an inhomogeneous spatial distribution. This profoundly affects statistical error levels, reaction kinetics, and due to the change in solvation, the sample's physicochemical properties. If the matrix affects surface tension, the analyte's distribution between the bulk and the surface will change, contaminants may be displaced from the walls, and the presence of other constituents in the surface layers may affect transport-type sensors such as electrochemical. Furthermore, if the analyte is displaced to the surface, this will produce consider -
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976 · 27 A
