Quantitative Analysis of Menthol Stereoisomers by Gas-Liquid Chromatography SIR: We wish to report a new gas chromatographic procedure for the analysis of the four stereoisomeric menthols (I to IV). C A R B O W A X 6'
L
A
H
I Neomenthol
4 0 H
A
Y
I1 Neoisomenthol
!iP
111 Menthol
O l2
I!(
IV Isomenthol
Previous investigations of suitable stationary phases for this analysis amply demonstrate the difficulties. Smith, Bartlet, and Levi (4) utilized sucrose acetate isobutyrate as substrate for the separation of peppermint oil, reporting retention volumes for Z-menthol and d-neomenthol. Petrowitz, Nerdel, and Oliloff (a) found di-ndecyl phthalate preferable t o diethylhexyl sebacate, silicone oil, or polyethylene glycol, but separations of neomenthol-menthol and nienthol-isomenthol proved incomplete and of limited quantitative applicability. Similarly two further investigations (9, 6) employing Carbowax as the stationary phase have emphasized the difficulty of separating neoisomenthol and menthol. We have found that use of Hyprose SP-80, octakis (2-hydroxypropy1)sucrose ( I ) , (The Dow Chemical Co.), as the stationary phase affords significantly better resolution than Carbowax 1540 (Union Carbide Chemicals Co.) or Reoplex 400 (Wilkens Instrument Co.) and further, that in a 12foot column the degree of resolution permits accurate quantitative estimation of all four stereoisomers at all levels of concentration. Illustrations and relative retention times are presented in Table I for a sample of the menthols prepared b y the catalytic hydrogenation of thymol with a nickel-on-kicsclguhr catalyst and examined on three stationary phases: Carbowax, Reoplex, and Hyprose.
I,. I
I
8
O8
O
HYPROSE 12'
-
RETENTION TIME, MIN.
Figure 1. Analysis of menthol mixture on different stationary phases a t 150" C.
Packings were 2097, by weight and were prepared by evaporative deposition of the stationary phase from chloroform solution onto Chromosorb JV (60- to 80-mesh). The columns of quarter inch O.D. copper tubing were conditioned under helium flow a t 150" C. for several hours after packing. Analyses were conducted at 150" C., 15 pounds inlet pressure of helium, and
a flow rate of 66 ml. per min. in a
Perkin. Elmer Vapor Fractometer Model 154-D. ACKNOWLEDGMENT
The authors thank E. D. Wesch for his assistance with the experimental portion of this work. LITERATURE CITED
Table
1.
Retention
Times
Relative Retention Time Carbo- Reo- g y p - Hyp-
wax
Dlex
rose
rose
(6
(6
(6
(12
feet) feet) feet) feet) Neomenthol ( 1) 1.00 1.00 1.00 1.00 Neoisomenthol (11)
Cartoni. G. P.. Liberti. A.. J. Chromatog. 3,'121 (19CO). (2) Huckel, W., Feltkamp, H., Geiger, S., Ann. 637, l(1960). (3) Petrowite, H. J., Nerdel, F., Ohloff, G., J. Chromatog. 3, 351 (1960). (4) Smith, D. M., Bartlet, J. C., Levi, Leo, ANAL. CHEM.32,568(1980). (5) Tagaki, W.,hlitsui, T., Bull. Agr. Chem. SOC.Japan 24, KO.2, 217 (1960).
I 1)
Menthol
1.10 1.09 1.21 1.15 1.32 1.32 D
Menthol(II1) Isomenthol ( IV) 1.37 1.30 1.52 1.55 a Eluted with menthol.
\
,
I
.
DONALD R. MOORE AAROND. K o s s o ~ The Trubek Laboratories, Inc. East Rutherford, N. J. RECEIVEDfor review April 24, 1961. Accepted July 11, 1961. VOL. 33, NO. 10, SEPTEMBER 1961
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