The Kováts Retention Index System τ Ν THEIR FIRST PAPER on gas chro-
-*- matography James and Martin {17) mentioned that the retention time (or more correctly, the reten tion volume) of a pure substance on certain gas chromatographic column is a characteristic value which could be used for the identi fication of sample components. The same authors also described the basic relationships between the retention time and volume and the analytical conditions such as tem perature. Since the beginning of gas chro matography, numerous authors and committees have dealt with the way gas chromatographic retention data can be expressed and collected in order to make possible the general use of published results. The many suggestions can be summarized in two basic groups: (1) Calculation of the specific retention volumes. (2) Expression of the retention relative to some standard (s). The specific retention volume was introduced by Littlewood and coworkers {22) in order to make the retention data independent of the temperature, the amount of sta tionary phase present in the col umn, the carrier gas flow rate, and the pressure drop through the col umn. However, although theoreti cally it is an exact value, the spe cific retention volume is not used in practice, basically for three rea sons: (a) Its calculation requires the knowledge of values (e.g., the amount of stationary phase in the column) which are generally not known and which may even alter in use.
(b) It has no descriptive value. (c) Since it is always reduced to 0° C, there is no way to describe the temperature dependency of the retention. As a conclusion of this, the most wide-spread method used today is the use of relative retention data, where the retention behavior of various substances is compared to that of a standard substance. In the determination of the relative retention, both the sample compo nents and the reference substance are analyzed under identical condi tions (temperature, carrier gas flow rate)—in most of the cases, the reference substance is actually part of the sample. Therefore the rela tive retention of the component of
interest can be calculated directly from the chromatogram. The use of relative retention val ues is at present the most accepted method. Its basic shortcoming is, however, the fact that it is almost impossible to fix one standard; therefore, in many cases, published data cannot be used directly. In order to overcome this difficulty, Evans and Smith {5) introduced the theoretical nonane values: the retention is first determined relative to the closest rz-paraffin and consec utively, this value is transferred to a system in which n-nonane is the standard. Although this system helps in having one standard, the accuracy of the determination de pends on how far the normal paraf-
In gas chromatography, the proper expression of the retention data is one of the most discussed problems. In order to enable the general use of pub lished data, various systems were developed and described. One of the most logical systems is the expression of the retention data in the retention index scale first proposed by E. Kovats of the Federal Insti tute of Technology, Zurich, Switzerland, in 1958. This system has a wide acceptance in Europe but is virtually unknown in the U. S., probably because all the basic papers were published either in German or French. At the recent Second International Symposium on Advances in Gas Chromatography, at Houston, Texas, Dr. Kovats summarized in detail the pnnciples and advantages of the retention index system. In a subsequent lecture, Dr. A. I. M. Keulemans, professor at the Institute of Technology, Eindhoven, The Netherlands, discussed among other subjects, the error sources of this system and the possibilities for obtaining more accurate data. According to many practical gas chromatographers, the retention index system seems to be the most logical for the generalization of retention data. Thus it was felt that a condensation of the two papers as a Report for Analytical Chemists would contribute to better understanding in this field. This report was written by L. S. Ettre of the Perkin-Elmer Corporation, Norwalk, Conn.
VOL 36, NO. 8, JULY 1964
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REPORT FOR ANALYTICAL CHEMISTS
fin used as primary standard appears from n-nonane. Another problem in using relative retention data is that it is difficult to express the temperature dependency of the values.
the retention index (J) of a particular substance was calculated according to the following equation:
THE RETENTION INDEX SYSTEM
where VN = the net retention vol ume, n-Cz = η-paraffin with ζ carbon atoms, n-Cz+g = n-paraffin with ζ -f- 2 carbon atoms, and ζ = an even number ; by definition :
To overcome the difficulties of the previously mentioned expressions, Dr. E. Kovâts proposed in 1958 (18) the introduction of the socalled retention index system. There is a basic difference between the retention index and the previously mentioned expressions. In the latter, the retention of a substance is given either as an absolute value or is compared to one standard. The retention index, by contrast, expresses the retention behavior of the substance of interest in a uniform scale determined by a series of closely related standard substances. In this respect, it could be compared well to our common temperature scale where arbitrary numbers are assigned to the temperatures of two specific transitions, and the other temperatures are characterized with help of inter- or extrapolation using an arbitrary scale (e.g., 100 equal divisions between the two fixed points). In the original definition, the normal paraffins with even carbon atoms were used as fixed points, and
j _
™rv l o g VΝ (substance) — l o g log
FAT(„-C»+2)
—
log
VN(n-Cz) FiV(n-Cz)
+ 1002 (Eq. 1)
VN(TI-CZ)
^
V Ν (substance)
n-Cz+ 2)
(Eq. 2)
According to Equation 1, the re tention index of the even number normal paraffins will be 100 times the carbon number—i.e., 200, 400, for ethane, η-butane, etc. Since in practice, the normal par affin mixture (used as the fixed points) and the sample are ana lyzed under identical conditions or are even mixed prior to analysis, it is obvious that the net retention volumes in Equation 1 can be re placed by the adjusted retention times or the corresponding dis tances on the chromatogram. A logarithmic scale is used in the calculation of the retention index because it is known that the loga rithms of the net (or adjusted) re tention volumes (times) of normal paraffins increase linearly with the chain length. This fact makes the retention index scale linear, which
can be illustrated by the graphical determination of the index values. Figure 1 shows the calculation of the indices of toluene and cyclohexane. As given, the determina tion consists of three steps: (1) The sample and the mixture of the corresponding normal paraf fins (in this case n-hexane and η-octane) are analyzed and the ad justed retention volumes (or times) determined (Figure 1, A). (2) These adjusted retention vol umes (or times) are transferred to a logarithmic scale (Figure 1, B). In this scale, the retention values of n-parafnns will be equidistant from each other. (3) Finally, a new linear scale is drawn to this logarithmic scale on which the arbitrary number of 400, 600, 800, etc. are assigned to normal butane, hexane, octane, etc., and the distance between two such standards is divided into 200 equal units (Figure 1, C). As shown, the retention indices of toluene and cyclohexane on this particular column (and at given temperature) are 724.6 and 688.4, respectively. The even number normal paraf fins were originally selected as the fixed points because it was errone ously thought that there would be an oscillation in the chromato graphic properties of successive numbers of the complete n-paraffin
Dr. E. Kovats is in the Organic Chemistry Department of the Eidg. Technische Hochschule in Zurich, Switzerland. He was born in 1927 in Budapest, Hungary, where he attended primary and high school. After 4 years at the Techno logical University of Budapest, where he received the Diploma in 1949, he moved to Zurich, and studied at the Eidg. Tech nische Hochschule (ΕΤΗ). There he re ceived the "Dipl. Ing. Chem." in 1951 and the Doctorate in 1953. His first work at the ΕΤΗ was on catalysis in the gas phase, and for the last seven years he has worked on essential oils. In this latter area Dr. Kovats began work on pre parative and analytical gas chromatog raphy and later worked on the identifica tion of substances. He played a key role in developing a fully automatic prepara tive gas chromatograph.
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ANALYTICAL CHEMISTRY
Prof. A. I. M. Keulemans is Director < the Instrumental Analysis Division of tr Chemical Engineering Department in tr Technical University, Eindhoven, tr Netherlands. Prof. Keulemans is tfi author of the book "Gas Chromatoj raphy," and has contributed chapters t several other books on gas chromatoi raphy. He has also authored over 1 papers in this area. Born in 1908 i Rotterdam, Prof. Keulemans received th Degree in Mathematics from Leiden Un versify in 1931, and the Degree in Cherr ical Engineering in 1937 and the Doctoi ate Degree in 1942 from Delft Univei sity. From 1938 to 1958 Dr. Keuleman worked at the Royal Dutch Shell Oil Corr pany—first in fundamental research an then in process development and pile plant work. He joined the Technical Un versity in 1958.
series. However, experimental results later indicated that this is not the case; therefore, at the Houston Meeting, Dr. Kovâts suggested the redefinition of the retention index concept and the use of all normal paraffins as the fixed points :
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Τ — 1()Γ) l o g V Ν (substance) ~ l o g FjVfo-Cs) l o g VN(n-Cz+l)
— log
VN(n-Cz)
+ 100 ζ (Eq. 3) where again: VN(TI-CZ)
i=
V.V(substance)
^
Vjf(n-Ct+1)
(Eq. 4) In Figure 1, the even number n-paraffins are indicated by dashed lines and the odd number n-paraffins by dotted lines. This redefinition of the retention index offers two distinct advan tages. First, by using all n-paraffins as the scale, a closer setting of the fixed points is possible which enables a more accurate evaluation. This is particularly true in case of programmed temperature analysis (see below). A further advantage results in the calculation of in dices in the region of 0 to 100, by taking hydrogen as "zero paraffin" (C0H2X0+2) ; thus practically all substances can now be recorded within the retention index system although it should be emphasized that the linearity in this range is poorer than between paraffins over 2 = 2.
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VOL. 36, NO. 8, JULY 1964
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33 A
REPORT FOR ANALYTICAL CHEMISTS
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34 A
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ANALYTICAL CHEMISTRY
The big advantage of the reten tion index system is that the values given in this form are descriptive. For example, if the retention index of a substance on a particular col umn is 930, this value immediately shows that it will emerge some where after n-nonane. Similar to the usual retention values, the retention indices are also dependent on the chemical na ture of the stationary phase and the temperature. Thus, for better un derstanding of this concept, the cor relations between the retention in dex and the temperature and sta tionary phase have to be investi gated.
The difference between two reten tion indices determined on one sta tionary phase is termed as u / ; this can express either the difference of the retention indices of one sub stance measured at two tempera tures (dT/dT) or the difference of the retention indices of two sub stances determined at the same tem perature. When the retention index of a particular substance is determined on two different stationary phases but at the same temperature, the difference between the two values is indicated as^I. TEMPERATURE DEPENDENCY ON THE RETENTION INDEX
NOMENCLATURE
The retention index is expressed by the symbol I. By convention, the temperature at which the meas urement was made is given in sub script with the stationary phase as superscript. Thus, for example, a retention index determined at 130° C. on squalane stationary phase is characterized as Γ*™ίαη\
It is known that the relationship between the logarithms of the net retention volumes or the adjusted retention times and the reciprocal of the absolute column temperature is linear. On the other hand, the retention index values are directly proportional to the column temper ature and in first approximation, this correlation is linear although—
Table I. Retention Index of Limonene on Apiezon L Stationary Phase at Various Temperatures
NEW FOR NEAR-IR SPECTROPHOTOMETRYonly the CARY 14 Rl combines these 6 benefits:
Temperature
°C.
/
150
1067
170
1074
190
1081
210
1089
dl/dT 1 1 8
as shown by C'hovin and Lebbe (1), and Hoigné et al. (14)—for a wide temperature range this function is hyperbolic. For example, the retention index of limonene determined on Apiezon L stationary phase in 20° C. intervals is given in Table I.
High-resolution spectrum of DCI gas, path length 100 cm, pressure 250 mm Hg. Obtained using high-intensity source accessory Spectrum of -OH stretching bands in LiF crystal. Path length 33 mm; total scan time less than 4 min.
In order to present the retention index values in tabulated form, Dr. Kovâts suggested that three values be given : (1) The retention index value at a selected temperature, possibly close to the middle of the temperature range. (2) The retention index increment per 10° C. (3) The temperature range in which the index had been measured. For example, the values for limonene as tabulated in Table I can be summarized as follows: jApÛMon L
=
For details on the Model 14RI, send for Data File A402-74
1 0 8 1
dIAPiezonL/10oCm
_
3.7
Temperature range — 150°210° C. The tabulation of such data allows interpolation in the given range and also certain extrapolation out of the range although only in a limited way since—as mentioned— the temperature dependency of the index over a wide range is in fact hyperbolic. CORRELATIONS BETWEEN RETENTION INDEX A N D MOLECULAR STRUCTURE
As shown above, the retention index values are characteristic of the substance on a given stationary phase and can be used for substance Circle No. 92 on Readers' Service Card
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REPORT FOR ANALYTICAL CHEMISTS
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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
REPORT FOR ANALYTICAL CHEMISTS
/ ^ loo x^ubstance>
- ^n'Cz)
+ loo ζ
X (n-Cz+ 1) — A (n-Cz)
(Eq. 6.)
Figure 2: Retention dispersion of Emulphor-0 liquid phase, at 130°C. R is a normal paraffin chain with six or more carbon atoms
the relative retention of substances with the same nonpolar group but having different functional groups. Figure 2, for example, gives the re tention dispersion for Emulphor-0 stationary phase. RETENTION INDEX FOR PROGRAMMED TEMPERATURE ANALYSIS
It was already described (12) that when using linear temperature programming, there is a more or less linear relationship between the elution temperature (the net retention volumes or the adjusted retention times) of a normal paraffin and its chain length. Therefore, for a tem perature programmed chromatogram, the peaks of normal paraffins (or other homologous series) will be closely, evenly spaced. As shown by van den Dool and Kratz (29), the retention indices can be calcu lated by replacing the logarithms of the net retention volumes (adjusted retention times) by the numerical values or retention temperatures.
where X is the retention tempera ture, the net retention volume, or the adjusted retention time of the given components. The reason why the approximate equality sign was used in Equation 6 is that the retention temperatures (or the other corresponding values) for the η-paraffins in linear pro gramming are not exactly linear. However, the use of Equation 6 still gives results close to those calcu lated during isothermal operation for substances for which the reten tion index increment for tempera ture change is small. In this re spect, the use of all η-paraffins in stead of only the even numbered members offers the advantage of closer graduation of the fixed points. Further considerations of the de termination of retention indices in programmed temperature operation can be found in two recent papers (11, 13). This question, however, still needs some further investiga tions.
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ACCURACY AND ERROR SOURCES In any system which intends to give tabulated data for identifica tion purposes, the most important questions are how accurately the values can be determined and whether the measured value unam biguously characterizes a particular substance. In the retention index system, we arbitrarily divided the range between two normal paraffins into 100 units. However, it is evi dent that generally more than 100 organic substances will fall in such a range. Let us suppose that a substance has a retention index of 1026. If the accuracy of the determination is three units, then we can exclude all substances which have an index be low 1024.5 and above 1027.5. If 1000 substances would have reten tion indices between 1000 and 1100, we could in this way exclude about 96%, i.e., 960, and have a choice of only 40. By measuring the index on a column of different polarity, the possibilities can be narrowed down presumably to a few choices,
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36, NO 8, JULY 1964
·
37
A
REPORT FOR ANALYTICAL CHEMISTS
although it has to be understood that while this information tells us something about the functional group, it is not likely to give much additional information on the skele ton structure of the substance ana lyzed. This example shows the impor tance of the accuracy in the meas urement of the retention index. Namely, if the accuracy would be not three but only one unit, then the first determination immediately would reduce the possibilities to only ten substances, and if the ac curacy is 0.5 unit, then we have a choice of only five substances. Thus, it is of great importance to eliminate all possible errors in the measurement of the retention in dices. The error sources can be divided into five main groups. (1) The first group consists of errors resulting from the partition process itself and concerns devia tions from the linear distribution isotherm and changes in the tem perature and gas flow during par tition. In order to avoid any change in the retention index values, very small samples have to be used, in which case the possible fluctuations are negligible. (2) The second group of error sources concerns the instrumenta tion itself. It is evident that in accuracies in the measurement and regulation of column temperature and carrier gas flow will affect the result obtained. (3) It is very important that the stationary phase used be unam biguously defined and that its characteristics should not change during use. Even slight changes in the chemical composition of the phase can significantly alter its re tention characteristics (the reten tion dispersion). Thus, retention indices measured at different times might differ. Oxidation due to traces of oxygen in the carrier gas is one of the most common reasons for changes in the nature of the sta tionary phase: e.g., even squalane (which is a truly nonpolar phase) can acquire a definite polar charac ter with time. Therefore, the re moval of even traces of oxygen from the carrier gas is strongly rec ommended. (4) If the peak of the substance
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ANALYTICAL CHEMISTRY
of interest is not completely re solved from other impurity peaks, this will result in a shift of the position of the peak maximum. Dr. Keulemans constructed an ingenius device which can be used to demonstrate and measure the in fluence of overlapping peaks on the position of the peak maximum. The shift in the peak maximum de pends on the relative area of the two peaks, the distance {At) of the two original peak maxima, and the standard deviation (σ) of the original peaks. For example, in case two overlapping peaks with equal height, we obtain only one maximum for values of Δί2σ, we obtain two maxima, and the shift decreases as the maxima move further apart. For overlap ping peaks with other peak area ratio, the magnitude of the shift in peak maximum can be computed. Figure 3 demonstrates the case where the height of the impurity peak is half that of the main peak and Δ t = σ; in this case, the shift in the peak maximum is 0.23σ. Of course, the next question to be answered is how the shift of the maximum affects the retention in dex. Obviously this is going to de pend on the resolving power of the column. For example, benzyl alco hol has a retention index of 1046 on Apiezon L, at 190° C. A straightforward calculation shows that for a 2500-plate column, an increase of 1046 to 1046.5 corre sponds to a Δί of 0.23σ. This means that about 30% of an im purity is going to change the reten tion index less than 0.5 unit. This might look small ; however, it has a significance if—as discussed above —we want to improve the accuracy of the retention index measure ment. This fact stresses the impor tance of the use of high performance columns for the determination, par ticularly in case of highly complex natural samples. (5) As a last but perhaps the most important source of error, ad sorption by the support must be mentioned. This is particularly harmful if polar samples are run on a nonpolar column, but it can also be observed with polar samples on polar substrates. For example
REPORT FOR ANALYTICAL CHEMISTS
ACCESSORIES INCREASE CAPABILITIES OF THE MODEL 350 SPECTROPHOTOMETER
Figure 3: Shift in peak maximum for overlapping peaks. Peak A: main peak. Peak B: impurity peak. Peak C: the resulting peak as it would appear in the chromatogram. In this example, the corresponding peak maxima are marked with Ma, Mb, and Mc. The distance of the maxima of peaks A and Β is equal to the standard deviation (σ) of the peaks, the shift in the maximum of the com posite peak will be 0.23 σ .
—as given by Kovâts (20)—the retention index of furfurol on Emulphor-0 at 190° C. was found as 1287 and 1246, respectively, depending whether only acid washed or acid and base washed and calcinated Celite was used as support. Therefore, for accurate retention index determinations, columns with inert support materials have to be used. SIMILAR SYSTEMS
The retention index system is a natural scale for the expression of the relative retention of various substances. This is also underlined by the fact that some other authors derived certain systems closely related to the retention index system although, usually, they were unaware of it. Actually, these expressions can be termed as special applications of a more general system. Five such terms can be mentioned briefly. (1) Woodford and van Gent (33) used the methyl esters of the straight chain saturated fatty acids as reference materials for the esters of branched chain acids and defined a characteristic number—the so-called "carbon number"—anal-
ogous to the retention index. (2) Miwa et al. (23) described practically the same term but named it the "equivalent chain length." I t is calculated by logarithmic interpolation between the esters of two fatty acids as the chain length of a hypothetical fatty acid which should appear at the same place as the substance being characterized. (In the retention index system, 7/100 would be the number of carbon atoms of a hypothetical normal paraffin.) (3) Evans and Smith (6, 7) introduced the so-called "effective molecular weights." The same values could be obtained if the fixed points in the retention index system would be designated by their molecular weights and not as 1002. (4) The "steroid number" (16, 31) described by Vanden Heuvel and Horning is analogous to retention index numbers except that two hydrocarbons with steroidal structure (androstane and cholestane) and not the corresponding normal paraffins are used as fixed points and that the multiplication factor of 100 is omitted. They also demonstrated that the "steroid number" can be calculated by the
Highly versatile as a basic analytical instrument, the Perkin-Elmer Model 350 UV-VIS-NIR Spectrophotometer has even more impressive capabilities with accessories to perform extra functions. Here are some of the auxiliaries that are available: Controlled-Temperature Cell Mount (illustrated), for Perkin-Elmer Cylindrical Sample Cells, maintains any specified temperature from 0°C to 100°C within 0.5°C. Helps determine the kinetics of reactions at various temperatures. Standard Time Drive Accessory-records transmittance or absorbance against time at any of 11 different
speeds, for
precise
one-point
kinetic
studies. Repetitive Scan Accessory enables the operator to re-scan any selected segment of an absorption spectrum automatically at regular time intervals, to analyze general chemical
reaction
changes.
Linear Wavelength Accessories provide UV, VIS or NIR linear wavelength readout on the standard preprinted Model 350 charf, where this mode of recording is desired. Auxiliary Recorder Readout produces additional spectra for any desired purpose: enables operator to stretch or compress spectra, by chart speed variations. Useful with Linear Wavelength Accessories, above. Other accessories include Potassium Bromide Disc Mount, Short Fixed-Path Liquid Cell, and VariablePath Liquid Cell. For complete details on Model 350 accessories, write to Instrument Division, PerkinElmer Corporation, 7 5 1 Main Avenue, NorwaJk, Connecticut.
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3 6 , N O . 8, JULY 1 9 6 4
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39
A
REPORT
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ANALYTICAL CHEMISTRY
summation of values relating to the carbon content of the steroid skeleton and values characteristic of the functional groups of the steroid. This rule again is a repetition of the corresponding rules discussed originally by Wehrli and Kovâts. (5) The "methylene unit" number described by Vanden Heuvel et al. (30) in this issue is physically identical to the retention index value divided by 100. WORK TO BE DONE
Investigations by Kovâts, Wehrli, and a large number of European authors (1-3, 8-10, 14, 15, 19, 21, 24-27, 34-37), proved the exceptional usefulness of the retention index concept for the generalization of the retention data. The following steps could accelerate its wide acceptance: (1) Agreement on the stationary phases to be used for retention index determination. (2) Investigations in the elimination of error sources. (3) Determination of a large number of reliable retention index values on preselected stationary phase (s). (4) Further studies on the relationship between I, Al, and the chemical characteristics of various substances. (5) Further investigations on the extension of the retention index concept to programmed temperature analysis. The G.A.M.S.—the French Society of Analytical Chemists—has spent considerable time on the standardization of the retention index concept, and in the institute of Dr. Keulemans at Eindhoven, detailed studies are now under way on these problems. The results of these activities can contribute to the establishment of a universally accepted system for the presentation of retention data in gas chromatography. LITERATURE CITED
(1) Chovin, P., Lebbe, J., "Journées International d'Étude des Méthodes de Séparation Immédiate et de Chromatographie," J. Tranchant, éd., pp. 99103, G.A.M.S., Paris, 1961. (2) Dhont, J. H., Nature 198, 990 (1963). (3) / M . , 200, 882 (1963). (4) Evans, M. B., Smith. J. F.. ./. Chromatog. 5, 300 (1961).
REPORT (5) /bid., 6, 293 (1961). (6) Ibid., 8,303 (1962). (7) Evans, M . B., Smith, J. F , Ibid., 190,905 (1961). (8) Felix, D., Mêlera, G , Seibl, J , Kovâts, E., Helv. Chim. Acta 46, 1513 (1963). (9) Felix, D., Ohloff, G , Kovâts, E., Liebigs Ann. Chem. 652, 126 (1962). (10) Ferrand, R., "Journées International d'Étude des Méthodes de Séparation Immédiate et de Chromatographie," J. Tranchant, éd., pp. 132-140, G. A. M . S., Paris, 1961. (11) Guiochon,
G.,
ANAL.
CHEM.
no need
to
36,
661 (1964). (12) Habgood, H . W., Harris, W. E., ANAL. C H E M . 32, 450 (1960).
(13) Habgood, H . W., Harris, W. E ,
stick
ANAL. C H E M . 36, 663 (1964).
(14) Hoigné, J., Widmer, H., Gâumann, T., J. Chromatog. 11, 459 (1963). (15). Huguet, M., "Journées International d'Étude des Méthodes de Séparation Immédiate et de Chromatographie," J. Tranchant, éd., p p . 69-84, G.A.M.S., Paris, 1961. (16) Horning, E . C , Vanden Heuvel, W. J. Α., Creech, B. G., "Method of Biochemical Analysis," Vol. X I , D . Glick, éd., p p . 69-147, Interscience, New York, 1963. (17) James, A. T., Martin, A. J. P., Biochem. J. 50, 679 (1952). (18) Kovâts, E., Helv. Chim. Acta 41, 1915 (1958). (19) Kovâts, E., Helv. Chim. Acta 46, 2705 (1963). (20) Kovâts, Ε., Ζ. Anal. Chem. 181, 351 (1961). (21) Kugler, E., Kovâts, E., Ibid., 46, 1480 (1963). (22) Littlewood, A. B., Phillips, C.S.G., Price, D . T., J. Chem. Soc. (London) 1955, 1480. (23) Miwa, T . K., Mikolajczak, K. L ,
out your neck
Earle, F . R., Wolff, Ι. Α., ANAL. C H E M .
32, 1739 (1960). (24) Schomburg, G., Z. Anal. Chem. 200, 360 (1964). (25) Stadler, P . Α., Oberhànsli, P., Helv. Chim. Acta 46, 1480 (1963). (26) Strieker, H., Kovâts, E., J. Chromatog. 8, 289 (1962). (27) Struppe, H . G , "Gas Chromatographie 1963," H . P . Angelé and H . G. Struppe, eds., pp. 378-401, Akademie Verlag, Berlin, 1963. (28) Swoboda, P . A. T., "Gas Chromatography 1962," M . van Swaay, éd., pp. 273-291, Buttcrworths, Washington, D. C., 1962. (29) Van den Dool, H., Kratz, P . D., J. Chromatog. 11, 463 (1963). (30) Vanden Heuvel, W. J. Α., Gardiner, W. L., Horning, E. C , ANAL. C H E M . 36,
1550 (1964). (31) Vanden Heuvel, W. J. Α., Horning, E. C , Biochim. Biophys. Acta 64, 416 (1962). (32) Wehrli, Α., Kovâts, E , Helv. Chim. Acta 42, 2709 (1959). (33) Woodford, F . P., van Gent, C M . , J. Lipid Res. 1, 188 (1960). (34) Zulaica, J., Guiochon, G., ANAL. C H E M . 35, 1724 (1963).
(35) Zulaica, J., Guiochon, G., Bull. Soc. Chim. France 1963, 1242. (36) Zulaica, J., Guiochon, G., C. r. hebd. Séances Acad. Sci. 255, 524 (1962). (37) Zulaica, J., Landault, C , Guiochon, G., Bull. Soc. Chim. France 1962, 1294. END
•
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