160
Anal. Chem. 1981, 53, 160-163
Liquid Chromatographic Separation of A Fluorescence and Ultraviolet Detection
phenols with
Kenneth Ogan" and Elena Katz The Perkin-Elmer Corporation, Main A venue, Norwalk, Connecticut 06856
The liquld chromatography of 19 alkylphenols is described. Condltlons were chosen to separate as many of these compounds as possible, resulting in better separatlon than has been previously reported. The detection of these compounds by UV and fluorescence was compared. Fluorescence detectlon gave detectlon limlts lower than those from UV detection at 274 nm and detection limlts'comparabiewith those from UV detectlon at 215 nm. Chromatographlc analysis of a coal liquefaction sample demonstrated the beneflts of the spectral selectivity offered by fluorescence detection.
Table I. Alkylphenol Peak Identification peak no.a 1
2 3 4 5 6 7 8 9
10 Phenols serve as reactants, intermediates, and products in many chemical processes. They are used in the manufacture of many pharmaceuticals, dyes, and pesticides and are generated as byproducts in manufacturing processes such as coke production, paper pulp processing, and coal liquefaction. Because phenols are so widely used, there is great interest in analytical methods for individual phenols. Modern liquid chromatography (LC) is a natural choice for the separation of individual members of this class for polar compounds. Bhatia (1) was the first to evaluate the us9 of high-pressure liquid chromatography in the separation of phenols. Recent studies have evaluated the chromatography of 33 alkylphenols on cyano and C18bonded-phase columns (2), of 13 alkylphenoh on silica columns (3),and of 35 alkylphenols on cyano, amine, and Cls bonded-phase columns and on a silica column (4). These studies were directed toward elucidating the structure-retention relationships for these compounds. We have selected chromatographic conditions with the object of separating as many of these compounds as possible. We also found that a CI8 bonded-phase support different from that used in these earlier papers exhibited a different selectivity. This permitted the better separation of several alkylphenols. The net result of our work is a better analytical separation of a large group of alkylphenols. In all of these previous studies, the phenols were detected by their UV adsorption at 254 nm. Schabron, Hurtubise, and Silver (5) used UV detection at two wavelengths, 254 and 280 nm. These authors used an extensive chromatographic sample preparation procedure for the analysis of alkylphenols in coal-derived liquids. The absorbance ratios a t these two wavelengths for fractions from real samples were compared with those of standards for further confirmation of peak identity in the chromatograms of the real samples. These authors also used fluorescence measurements on the collected fractions for additional characterization of the sample peaks. We have compared the detection of phenol and 18 alkylphenols at 274 nm, corresponding to a characteristic alkylphenol peak, and at a low wavelength, 215 nm, where these compounds absorb even more strongly. Since phenol and its alkyl and amine derivatives also fluoresce, these compounds could be detected with a on-line fluorescence detector, which offers greater sensitivity and selectivity than does UV detection. In a preliminary study evaluating fluorescence detection coupled with LC for the determination of phenol (6),
a
name phenol 3-cresol 4-cresol 2-cresol 3,4-xylenol 3,5-xylenol 2,3-xylenol 2,4-xylenol 2,5-xylenol 2,6-xylenol
peak no.a
name
3-ethylphenol 4-ethylphenol 2-ethylphenol 3,4,54rimethylphenol 2,3,5-trimethylphenol 2,4,6-trimethylphenol 2-isopropylphenol 18 2-propylphenol 11
12 13 14 15 16 17 19
2,3,5,64etramethylphenol
In Figures 1-4.
we found a detection limit of 25 pg for phenol itself, corresponding to a minimal detectable concentration (in the injected sample) of 3 ng/mL (or 3 ppb). In the present study, we have compared on-line fluorescence detection with UV detection for the alkylphenols. Finally, both UV and fluoreacence detection were used in the chromatographic analysis of a coal liquefaction sample in order to provide a qualitative demonstration of the superior spectral selectivity of the fluorescence detector.
EXPERIMENTAL SECTION Reagents and Samples. Acetonitrile was LC grade purchased
from Fisher Scientific (Fair Lawn, NJ). Water was filtered, deionized, and passed through a carbon bed in a system from Continental Water Conditioning Corp. (El Paso, TX). All solvents were degmed with a Branson Model 185sonicator (Danbury, CT). The phenols used in this study are listed in Table I. The phenol standards were obtained from Analabs, Inc. (North Haven, CT), Kit No. GCR-082. The phenol mixture was made up in acetonitrile. The concentration of each phenol in the test mixture was 0.5 Mg/mL except for tetramethylphenol which was 1.0 pg/mL. A coal liquefaction sample was obtained from a source within the industry. No further information as to its source or treatment was available. About 2 mL of this coal liquid was added to 30 mL of dichloromethane,and this mixture was extracted with three 30-mL portions of basic water (adjusted to pH 11with NaOH). The aqueous portions were combined, acidified (pH
