splitting in both the NOz symmetrical and asymmetrical stretching bands. For example, the dinitrate derivative of a-hexadecyl glyceryl ether, unlike the p isomer, showed these characteristic doublets (Figure 4). ‘The derivative of 1-monostearin, however, showed a single NOz symmetrical and a split asymmetrical stretching band. The absence of the split in the former band of this compound may be due to interference from the 1,3-isomer, which might have formed by isomerization. Some acyl migration would be espected to occur, for example, in the nitration of both mono- and diglycerides as a result of the acidity of the acetyl nitrate solution. The tendency of these compounds to rearrange under various conditions was recently demonstrated by Privett and coworkers (13). CONCLUSIONS
are valuable for structural determinations on milligram amounts of the parent hydroxy compounds and for the analysis of complex mixtures containing these substances. The techniques described are only fully exploited, however, when used in conjunction with established methods for the analysis of alcohols and other compounds containing hydroxy groups. ACKNOWLEDGMENT
The authors appreciate discussions on the infrared analyses with E. J. Gauglitz, Jr., of the Seattle Technological Laboratory. LITERATURE CITED
(1) Bellamy, L. J., “Infrared Spectra of Complex Molecules,” 1st ed., p. 249, Methuen and Co., London, 1956. (2) 77, . , Brown, J. F., J . Am. Chem. SOC. 6341 (1955). .
( 3 ) Hilditch. T. P.. “The Chemical Con\ - ,
Some chromatographic and spectral properties of a variety of fatty nitrates have been presented. These properties
~~
stjtution ’of Natural Fats,” 3rd ed., Wiley, New York, 1956. (4) Lindenmeyer, P. H., Harris, P. M., J . Chem. Phys. 21,408 (1953).
(5) Malins, D. C., Houle, C. R., J . Am. Oil Chemists’ SOC.40, 43 (1963). (6) Malins. D. C., Mangold, H. K., Ibid., 37, 576 (1960). ( 7 ) LLIalins, D. C., Wekell, J., C., ,Houle, C. R., Bureau of Commerclal Fisheries
Technological Laboratory, Seattle, Wash., unpublished data, 1963. (8) Malins, D. C., Wekell, J. C., Houle, C. R., J . Am. Oil Chemists’ SOC.41,
44 (1963). (9) Mangold, H. K., Fette, Seifen, Anstrichmittel 61, 877 (1959). (101 Maneold. H. K.. J . A m . OiZ Chemists’ ‘ f!oc. 38,-708 (1961). (11) Mangold, H. K., Kammereck, R., Malins, D. C., Microchem. J., Symposium, t’ol. 11, p. 697, Wiley, Xew York. 1962. (12) Mangold, H. K., Malins, D. C., J . Am. Oil Chemists’ SOC. 37, 383 (1960). (13) Privett, 0. S., Blank, M. L., Lundberg, W. O., Ibid., 38, 312 (1961). (14) Sgoutay, D., Kummerow, F. A., Ibid., 40, 138 (1963). (15) Subbarao, R., Roomi, M. W., Subbarao, 11.R., Achaya, K T., J . Chromatog. 9, 295 (1962). (16) Tioque, E., Holman, R. T., J . Am. Oil Chemists’ SOC.39, 63 (1962).
RECEIVED for review September 18, 1963. Accepted November 22, 1963.
Retention Indices in Programmed Temperature Gas Chrolmatography SIR: The retention indices system described by Kovate uses isothermal retention data (4, 6). It is based on the linearity of the plot of the logarithm of retention volume us. carbon number for n-alkanes heavier than n-pentane. The retention index of a compound X is given by: Ij(X) =
1002 ( 1 )
where nP, is the n-alkane with z carbon atoms; z is an even number; compound X is eluted lietween nP, and nP,+2; the retention volumes V R are corrected for the gaseous volume of the column (they are neasured from air peak). It seems obvious that if X is isothermally eluted between nP, and nP,+% a t each temperature in the range between initial tempel-ature and elution temperature, in prog*ammed temperature gas chromatogrtphy, X will be eluted between nP, m d nP,+2; so if the retention index of X is constant in this temperature range a relationship
may be established between its retention index and its elution temperature. If the retention index varies with temperature, a correlation may still exist. In programmed temperature gas chromatography there is an approximate linear relationship between elution temperature of n-alkanes and their carbon number, provided that the initial temperature is low before the elution temperature and that only a limited range of carbon number is used (2, 3 ) . I n fact, the linearity of this plot has not the same theoretical background that the linearity of the plot of logarithm of isothermal retention volume with carbon number. However, the experimental data support it sufficiently to allow a programmed temperature retention index to be defined by interpolation of elution temperature: Z P ? ( X )=
The purpose of this paper is to show that there is a relationship between L ( X ) and Ipr(X).
EXPERIMENTAL
The experiments were made with a gas chromatograph “Microtek 2500 R,” using the flame ionization detector and the automatic temperature pyogramming. Program I was made with constant flow rate a t 5’ C./min., Program I1 with constant inlet pressure a t the same rate of 5’ C./min., and Program I11 a t constant flow rate and 10’ C./min. Starting temperature is always 70” C. The column was made of 10% poly(neopentylglyco1)sebacate on C22 crushed firebrick, packed in a copper tubing, 4 mm. i.d., 10 metesr long. The column was previously conditioned in a separate oven a t 240’ C. No deviation of base line a t the low sensitivity used was noticed up to 240’ C. The constant flow rate was of 150 cu. cm./min. argon; so was the flow rate a t 240’ C. in constant inlet pressure experiments. This flow rate is somewhat more important than the optimum flow rate which is about 50 cu. cm./min. So the efficiency of the column is only 7250 theoretical plates for n-pentvl acetate at 157’ C. (H = 0.14 cm.).’ This allows the measurements of retention indices with a precision of more than 5 units (at a confidence level of 95%). VOL. 36, NO. 3, MARCH 1964
661
Figure 1. Variation of retention index of one compound with temperature
Figure 2. Variation of elution temperature of n-alkanes for the three programs used
n-Pentyl acetate on poly(neopentylglycol)sebocate
Number of curves refers to Table II
RESULTS
Table 1.
Temperature ("C.) Methyl ethyl ketone Methyl butyrate n-Butyl acetate n-Pentyl acetate n-Hexyl acetate Methyl-2-propanol-1 Benzene Toluene Ethyl benzene
100 822 910 1008
120 753 828 919 1022
135 769 835 916 1011
157 764 822 911 1016
841 788 893 980
844 794 908 998
841 808 909 994
846 808 918 1012
Table II.
Compounds Methyl ethyl ketone Methyl butyrate n-Butyl acetate n-Pentyl acetate n-Hexyl acetate Methyl-2-propanol-1 Benzene Toluene Ethyl benzene Methyl ethyl ketone Methyl butyrate n-Butyl acetate n-Pentyl acetate n-Hexyl acetate Methylethylketone Methyl butyrate n-Butyl acetate n-Pentyl acet,ate n-Hexyl acetate
Isothermal Retention Indices
OC.
120 135 154 178 199 138
127.5 151 173
152 170 192 218 242
210 718 808 909 1023 1119
220 722 800 900
1012 1124
886 994
0.92 TR,~ "C. Iw
Ii
-
(TR (TR) 6 20) Ii
6
Program I 753 756 829 1833/ 910 911 1028 1023 1120 1123 835 843 799 800 898 1919/ 998 1003
744 -9 830 +10 914 -5 1020 +3 1126 +6 +8 842 +7 +1 790 -9 +21 907 +9 $5 1005 +7
66 85 109 134 157
Program I1 745 740 821 830 916 912 1016 1022 1108 1 11231
-5 $9 -4 +6 +15
118
135 155 179 201
ANALYTICAL CHEMISTRY
(0.92 TR)
6
+3
+13 +1
Program I11 766 767 $1 836 18241 -12 923 @/ -19 1025 1016 -9
1118 1118
log V R = a
2;
820 916
1019
11126
-1 0 +3 $18
-4 -6 -15
762 830
/908
1025
0 +2
0 1120
(O
+ Tb
I1
88 102 120 142 161 105 95 117 137
T' = 0.92 TR is a relation betweenoabsolutetemperature these temperatures are given here in C.
662
200 731 801 897 1008 1115
Programmed Temperature Retention Data
TR,"
95 116 142 169 194
180 744 826 918 1023 1128
The isothermal retention indices were measured a t different temperatures between 100" and 220' C. for a number of different solutes. These data are given in Table I. A plot of these retention indices us. temperature allows the determination of the indices a t other temperatures. As may be seen in Figure 1 the variation of indices with temperature is not linear but hyperbolic, as results from substitution in Equation 1 of each log VR using the relation:
823 919 1019 1130 841
$3
i: ++610
(7861 - 13
905 1003
$7
817 913 1017
-4 -3
+5
+1
- 13 834 918 1023 1116
-2 -5 -2
-2
K.). For convenience
Table I1 gives the results obtained with the three programs used-Le., programmed temperature retention indices, and elution temperature. The computed isothermal retention indices at elution temperature and a t a temperature 20' C. below it, are also given. It may be seen that there is a fair correspondence between programmed temperature retention indices and isothermal indices at a temperature of 20' C. below the elution temperature. Figure 2 shows the plot of elution temperature us. carbon number for n-alkanes. Because of lack of reproducibility in temperature programming and injection temperature, the standard deviation of the programmed temperature indices is estimated to 5 units. In Table I1 the computed isot,hermal indices which differ from the measured programmed temperature indices of more than 10 units are squared. For 19 values they are 2 for Z i ( T R - 20) and 5 for Z,(TR). Table I11 gives the mean of the absolute differences d = Zr - Z i and the variance 1 -
4%. The correlation between n-1 I,, and Z i ( T ~- 20) appears to be quite
good, and the discrepancies may be attributed to the inaccuracy of measurements. However the data presented here are too few to ascertain definitively that the programmed temperature retention index is equal to the isothermal index a t a temperature 20" C. below the elution temperature. The choice of 20" C. is somewhat arbitrary; since the temperature of the column during the time the peak travels through it is always below the elution temperature, the correction which must bc made to account for the variations of inllices with temperature is the easiest by computing the isothermal index a t some temperature below the elution tempersture. A temperature of 20" C. seems to give the best results, but the exact value may depend on the program rate. A more theoretically sound way of correcting for the variation of retention indices with temperakure would take advantage of the theoretical work of Giddings (1) which showed that a programmed temperature chromatographic separation is very alike to an isothermal one a t a temperature T' given by 0.92 TR to a good approximation. Table I1 gives the corresponding value T' = 0.92 TR and the isothermal retention indice a t this temperature (temperatures are given in " C., although the relation between 1" and TR holds in " K.). The data given in Table I11 compare the two methods of correction. The
last one appears to be slightly but definitely better, The correlation data were computed either with the whole data available or with only those for which all isothermal retention indices were available. The standard deviation obtained is about twice that which may be obtained by isothermal measurements. Better results would probably have been obtained if a lower starting temperature had been used, especially with methyl ethyl ketone. If this relationship is really true within less than 10 units, this result is important because it provides an easy and fast way to use isothermal retention data in the form of retention indices and increments of retention indices in programmed temperature gas chromatography. The lack of such a way to compute easily programmed temperature retention data from isothermal data was until now one of the most important drawbacks of this very useful technique. Since submission of this paper a work of Van den Do01 and Kratz (6) has appeared, describing substantially the same results, but with no correction for temperature dependence of retention indices. ACKNOWLEDGMENT
The authors are indebted to E. Kovats and C. Landault for fruitful discussions. We thank J. Bargain for
Table 111.
TR) l i ( T ~- 20) Ii(0.92 TR) Ii(
Ii(TR) li(TR - 20) l i ( 0 . 9 2 TR)
Correlation Data
Variance Mean of absolute of absolute differences differences For the Whole Data f2.7 9.75 +0.78 8.3 -0.75 7.2 Excluding Methyl Ethyl Ketone and n-Hexyl Acetate $1.1 11 0 7.7 -0.5 6.5
the loan of a Microtek 2500 R by SociBtB Techmation, Paris, France. LITERATURE CITED
(1) Giddings, J.
C., "Gas Chromatography," K. Brenner, J. E. Callen, M. D. Weiss, ed., p. 57. Academic Press, New York, 1962. (2) Giddings, J. C., J. Chromatog. 4, 11 I\----,. 1 RfiO\
(3) Habgood, H. W., Harris, W. E., ANAL. CHEM.32, 450 (1960). (4) Kovats, E., Helv. Chim. Acta 41, 1915 (1958). (5) Van den Dool, M., Kratz, P. D. C., J. Chromatog. 1 1 , 463 (1963). (6) Wehrli, A., Kovats, E., Helv. Chim. Acta 42, 2726 (1959).
GEORGE GUIOCHON Laboratoire du Professeur L. Jacque Ecole Polytechnique 17, Rue Descartes, Paris 5e, France RECEIVED for review September 16, 1963. Accepted December 2, 1963.
Retention Indices in Programmed Temperature Gas Chrolmatography SIR: As Guiochon (4) and van den Do01 and Kratz (6) have pointed out, retention indices sho d d be applicable in programmed temperature gas chromatography (PTGC) with the modification that log VR in the isothermal expression for retention index would be replaced by I&,where lTE is the net isothermal retention volume and TR is the retention temperature in PTGC. Thus, for esample, a compciund X eluted between n-octane (C,) and n-decane (Clo) would have retention indices for isothermal and programmed elution, respectively, given by the following expressions: I , ( X ) = 800
IdX)
+
= 800
+
Guiochon has also noted that the linearity with carbon number of TR is
less general and less complete than the linearity of log Vg but still sufficient that I,, should, in many cases, be identical with I,. While van den Do01 and Kratz have presented data to show that the increment in TR between successive homologs is "remarkably constant," examination of these data shows significant variations with both carbon number and program. Guiochon has also pointed out that if I , varies with temperature then agreement would be expected of I %with , I, for a temperature somewhat below the retention temperature. It is important to establish the relationship between I,, and IC if advantage is to be taken of the extensive available tribulations of I,. (It is somewhat unfortunate that the standard polar stationary phase recommended by Kovats, viz., Emulphor 0, a polyethylene glycol of molecular weight 500, has a relatively low upper temperature limit so that i t is not too satisfactory for PTGC.) We feel that the experi-
mental comparisons of I,, and Ii which have been presented (4, 6) might advantageously be supplemented by some calculritions of typical behavior. First of all it is worth distinguishing, as shown schematically in Figure 1, the two possible situations: an isothermal retention index invariant with temperature and one which changes with temperature. On the standard plot of log VR against reciprocal of the absolute temperature, the reference normal alkanes are shown as a family of straight lines. Compounds with temperature dependent and temperature independent indices are represented by the dashed lines A and B , respectively. Line B is uniformly one quarter of the distance between the lines for octane and decane so that it corresponds to a retention index of 850 while line A , over its length as drawn, corresponds to indices ranging from 870 to 980. Wehrli and Kovats (7) describe the temperature variation of Ii by a simple linear coefficient dIi/dT based on a 60" VOL. 36, NO. 3, MARCH 1964
663
