REPORT FOR ANALYTICAL CHEMISTS
WILL
identification. At the same time, certain rules allow the prediction of the retention indices with fairly good accuracy. D r . K o v â t s elaborated seven rules which help in the prediction of retention indices. Their detailed discussion can be found in the references. T h e first four rules deal with the retention indices determined on one stationary phase, while the last three rules refer to retention index values of a given substance determined on different stationary phases.
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(1) In any homologous series, the retention index of the higher members increases by 100 per CH 2 -group introduced (18, 20, 32). The only exception reported until now are the esters of some dibasic acids where the increment amounted to only 90-95 (35, 36). (2) On a nonpolar stationary phase, the difference in the retention indices (dl) of two isomers can be calculated from the difference of their boiling points (dtb) with help of the following equation (18,20,32): àl ^
5 àtb
(Eq. 5)
In this respect, a nonpolar stationary phase is defined as a pure paraffin or mixture of pure paraffins. (3) The retention index of an asymmetrically substituted compound can be calculated from the retention indices of the corresponding symmetrically substituted substances (Jj). (4) Similar substitution in similarly constructed compounds increases the retention indices by the same amount (28). (5) The retention indices of nonpolar substances (paraffins) remain almost constant for any kind of stationary phase (15,18, 20). (6) The retention indices of any substance determined on various nonpolar stationary phases are identical or very close to each other (18,20,32). (7) If the retention index of a substance is determined on a polar and nonpolar stationary phase, the difference in the retention indices (AI) is characteristic of the structure of the substance and can be predicted by adding up the individual increments pertaining to — Circle No. 30 on Readers' Service Card
various adhering zones in the molecule (18, 20, 32). With the help of such a calculation, unknown substances can be identified by comparing the experimentally determined Δ7 value with values cal culated for the possible structure.
C H A R A C T E R I Z A T I O N OF STATIONARY PHASES
Stationary phases are usually characterized by their "polarity." Although one understands well enough w h a t is implied by the terms, nonpolar, weakly polar, medium polar, and highly polar; these terms only describe gross ef fects and cannot be used for more exact characterization of the indi vidual stationary phases, nor could the different grade of polarity be expressed in form of numerical parameters. The retention index system en ables us to express the separation character of the various stationary phases in more exact form. Since the retardation of a substance on the column depends on interactions between the functional groups, one could characterize the stationary phase by the change in retardation as compared to a nonpolar sta tionary phase in which the r e t a r d a tion is mainly a function of the boil ing point of the sample components. This change in retardation can be expressed numerically by the Δ7 values—i.e., the differences of the retention indices of selected sub stances measured on the stationary phase of interest and on a nonpolar stationary phase. Wehrli and K o v â t s (32) suggested the following method for this purpose. One should measure the Δ 7 values for substances of the R - X general structure where R is a η-paraffin chain with six or more carbon atoms and X is the func tional group. For X = H , (i.e., for the η-paraffin), the value of Δ7 will usually be zero or a very small number (see the fifth rule above). Thus, we can plot the Δ7 values on a numerical scale starting with zero. This scale is called the retention dispersion of a particular station ary phase and it characterizes the phase in exact form. At the same time, it also gives information on
