Anal. Chem. 1990, 62, 1389-1391
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Chlorodifluoromethane as the Mobile Phase in Supercritical Fluid Chromatography of Selected Phenols Chye Peng Ong, Hian Kee Lee, and Sam Fong Yau Li*
Department of Chemistry, National University of Singapore, Kent Ridge, Republic of Singapore 0511
The results on the use of chlorodlfluoromethane (Freon 22) as an alternatlve supercrltlcal fluid to carbon dioxide for the analysis of polar solutes are presented. Five phenols are analyzed by uslng a caplllary column with UV detection at a wavelength of 280 nm. A comparatlve study uslng pure carbon dloxlde and Freon 22 Is carried out. The effects of pressure and temperature on the capacity factors for the phenols are Investigated. The usefulness of Freon 22 in supercrltlcal fluid chromatography Is demonstrated.
INTRODUCTION Interest in the use of s-upercritical fluids (SFs) as solvents for extraction as well as for the mobile phase in chromatography has been increasing rapidly over the past few years (1). T o date, carbon dioxide is the most common S F used in supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC). Although supercritical CO, yields satisfactory results in most of these applications, it is relatively nonpolar and has limited solvating power. Consequently, the analysis of polar compounds using it is very difficult. Even though the solvating power of C 0 2 can be improved by the addition of polar modifiers (2), there are many problems associated with the use of such mobile-phase systems. Firstly, one needs to have a good mixing chamber than can ensure thorough mixing. This iaespecially crucial if the two solvents differ greatly in their densities. Secondly, when capillary columns are used, the modifier concentration needs to be of the order of 5-20 mol 70 in a mixture with C 0 2 to effect reasonable changes in the retention times (2). Since some of the common polar organic modifiers, such as methanol, usually have high critical temperatures and pressures (ca. 512 K and 8.1 MPa for methanol), the working pressure and temperature of the system has to be brought higher relative to the pure CO, case. Consequently, there will be narrower working ranges for temperature and pressure control in the actual analysis. This situation is further exacerbated if thermally labile compounds are being analyzed. Hence, there is an urgent need for an alternative S F to overcome the inherent problems presented by the mixed mobile-phase systems. The criteria in the selection of an appropriate SF can be summarized as follows: (1)it should have better solvating power than CO,; (2) it should possess a relatively low critical temperature and pressure so as to provide larger working temperature and pressure ranges; (3) it should be nontoxic; (4) it should be available cheaply at high purity; and (5) it should be compatible with a wide range of detectors. In our study of steroids (3,4),chlorodifluoromethane (Freon 22 or R22) was successfully employed as the extracting solvent in SFE of these compounds. To date, the use of R22 as a mobile phase for SFC analysis of phenols has not been reported. The aim of the present work is to explore the potential of R22 as a S F for SFC applications. In particular, the fea-
* Author to whom correspondence should be addressed.
Table I. Some Chemical and Physical Properties of Carbon Dioxide, Methanol, and Freon 22
crit density/kg m-3 crit pressure/MPa
crit temp/K dipole moment/D toxicity
C02
CH30H
Freon 22
470 7.3 304 0
272 8.1 512 1.7
no
no
525 4.9 369 1.4 no
sibility of using R22 for the analysis of polar compounds will be investigated. For this purpose, five highly polar substituted phenols are used in the study.
EXPERIMENTAL SECTION The experiments were performed on a Model SFC 3000 supercritical fluid chromatographic system (Carlo Erba Instruments). The detection of peaks was carried out on a micro UV-vis detector (Carlo Erba Instruments) with the wavelength set at 280 nm. The column employed was a RSL-300 fused silica capillary column (12.5 m X 0.1-mm i.d. X 0.2-pm coating thickness). A tapered restrictor fabricated in our laboratory with a calibrated flow rate of 8 mL/min was connected after the UV cell. The calibration of the flow was performed by using supercritical carbon dioxide at a pressure of 10 MPa. The end of the restrictor was maintained at a temperature of 320 "C. Injections were made with an airactuated Valco VICI injection valve equipped with a 1-pL loop. The injection time was 1 s. The chromatographic data were collected on a Linear Instruments Corporation Model 252A/MM chart recorder. The temperature at the injection port was set at 40 "C throughout the analysis. All chemicals were of analytical reagent grade or better. Standard solutions of the individual phenols were prepared in HPLC-grade methanol (J. T. Baker). A standard mixture containing 100 ppm of each phenol was also prepared. Carbon dioxide of 100% purity was purchased from the British Oxygen Co., and R22 of better than 99.8% purity was supplied by Atochem.
RESULTS AND DISCUSSION R22 from the environmental viewpoint is relatively safe to use (5). Among the common Freons, the ozone-depletion and global-warming potentials of R22 can be considered as relatively negligible (5). Furthermore, its critical temperature and pressure of 369 K and 4.9 MPa are both low enough to meet our selection criteria. Other factors listed in Table I also make R22 a suitable choice for our purposes. The five phenols selected for study are shown in Table 11. The basis for selecting these five phenols in the study is that they are sufficiently resolved from one another when neat R22 is used as the mobile phase. On the other hand, when all 11 priority phenols were analyzed, complete separation was not achieved. Thus, for the purpose of characterizing the effects of R22 as the mobile phase on solute retention, only these five phenols were chosen. In the course of this investigation, attempts were made to use the flame ionization detector (FID) as the detection system with R22 as the mobile phase. However, the results obtained to this point were not very satisfactory. The main problem encountered was the decomposition of R22 a t the FID. It is known that, even though there are many claims of R22 being
0 1990 American Chemical Society 0003-2700/90/0362-1389$02.50/0
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990
Table 11. Structures and Some Physical Constants for the Five Phenols Studied in This Work
compd
abbrev phenol
phenol
structure
6
RMM" bp/"C
6""
94
185.0
122
212.0
142
235.0
198
NA
OH
2,4-dimethylphenol
2,4-DMP
CH3 OH
4-chloro-3-methyl-
4,3-CMP
phenol
@ CH,
Cl
2-methyl-4,6-dinitro- DNOC phenol NO2
")$" OH
pentachlorophenol
PCP
266
CI
310.0
CI CI
a
Relative molecular mass.
Table 111. Comparison of the Retention Times of Some Phenols Obtained by Using Pure C 0 2 and R22 mobile phase
carbon dioxide
R22
retention time/min pressure/MPa phenol 4,3-CMP DNOC 10 15 20 25 30 5.0 7.5
10.0
20.1 12.3 7.0 4.5 2.0
34.0 21.0
3.3 2.3
2.0
50.5
13.0
17.0 12.0
9.0
9.0
4.5
8.5
3.4 2.4
3.4 2.4
2.1
2.1
nonflammable, under high temperatures and favorable conditions, it is capable of forming weakly combustible mixtures with air (6). The decomposition process is found to be aggravated by the presence of the metal parts at the FID. The decomposition process resulted in the formation of products that give large background signals as well as corrosion problems. Subsequently, the investigation was performed by using an alternative detection system. Since most phenols are UV active, UV detection was used for subsequent experiments. In spite of this limitation, two critical parameters of R22, its nontoxicity and its more polar nature, are attractive features to justify its consideration as a SF in this type of application. A typical SFC chromatogram obtained by using R22 for the analysis of phenols is illustrated in Figure 1. From the figure, it is observed that R22 is capable of eluting the polar phenols in reasonably short times even at a relatively low pressure of 5 MPa. To demonstrate the fact that R22 can elute polar compounds more rapidly than can COz, attempts were also made to elute these phenols by using pure COP. The results for three of the phenols, phenol, 4-chloro-3-methylphenol (4,3-CMP),and 2-methyl-4,6-dinitrophenol (DNOC), are given in Table 111. From the results, it was noted that, with pure COP a t 10 MPa, the peak for DNOC, which is one of the phenols that is eluted out last, is only observed after more than 50 min. A retention time of about 8 min for DNOC can only be achieved at a very high pressure of 30 MPa. On the
I
0
,&TUVIE/Min
Figure 1. Typical SFC chromatogram of the phenols using pure R22. A, phenol; B, 2,4-DMP; C, 4,3-CMP, D, DNOC; E, PCP. The abbreviations are defined in Table 11. 0
Flgure 2. Typical SFC chromatograms of the phenols using pure C02. The retention times are listed in Table 111. D, DNOC.
other hand, with the use of R22, shorter retention times for DNOC can be easily obtained at a very low pressure ( 5 MPa). Typical chromatograms for DNOC are illustrated in Figure 2. Similar trends were also observed for the rest of the phenols investigated. In all the cases, shorter retention times were obtained when R22 was used as the mobile phase. This observation strongly suggests that R22 exhibits higher solvating power than does COz, and therefore, it is able to elute the polar phenols more effectively. This is also supported by
ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990
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n
4
50
8 50
10 50
P r F S S U R E /MPa Figure 3. Varlatlon of the capacity factors of the phenols with pressue measured isothermally at 150 O C .
Flgure 4. Variation of the capacity factors of the phenols with temperature measured isobaricaly at 10 MPa.
the higher dipole moment as shown in Table I and the higher extraction efficiencies obtained in SFC of polar compounds in our previous studies ( 3 , 4 ) . Furthermore, Carraud et al. (7)in their investigation of phenols also found that the phenols could only be eluted out satisfactorily by using methanol as a modifier together with supercritical carbon dioxide. As observed in our previous study on steroids (3, 4 ) , a general trend observed was that higher signal-to-noise ratios were obtained with R22 than with COz. The sensitivity enhancement can be attributed to the lower adsorption of UV by R22. The peaks are also much sharper with negligible tailing in the case of R22. Bearing in mind the possibilities to achieve faster analysis and better sensitivity, it can be concluded that R22 is a very useful SF for analyzing polar solutes in SFC. Effect of Pressure. In Figure 3, the results obtained by using R22 a t different pressures under isothermal conditions are shown. The results show that, with an increase in pressure from 5 to 10 MPa, the capacity factors for all the phenols were reduced. Such an observation can be attributed to an increase in density, which in turn increases the solvent strength of the mobile phase. Effect of Temperature. The effect of the temperature on solute retention a t constant pressure can be observed in Figure 4. The retention profiles of the five phenols obtained by using R22 as the mobile phase show decreasing trends and are similar to those observed in the analysis of other solutes employing different mobile phases (8-13). The results obtained clearly demonstrate the usefulness of R22 as a SF. Its low critical pressure and temperature make it easy to work with. In view of the fact that R22 exhibits a much higher solvating power than does COz, it can be used as an alternative mobile phase for rapid analysis of polar compounds. In some cases, it can be used as an alternative
modifier for SFC applications if the solvating power of COz is not high enough to effectively elute the compounds, but that of neat R22 is too high to effectively separate mixtures with existing stationary phases. Similarly, there is great potential for R22 to be employed as a SF extracting solvent for SFE applications. R22, like COz, can easily be vaporized when depressurized. Therefore, when used as a SF extraction solvent, R22 can be readily removed from the extract, leaving the extract free from R22. This is, of course, an important factor to consider in the extraction applications of food samples. Registry No. 2,4-DMP, 105-67-9;4,3-CMP, 59-50-7; DNOC, 534-52-1; PCP, 87-86-5; Freon 22, 75-45-6; phenol, 108-95-2.
LITERATURE CITED Fields, S. M.; Markides, K. E.; Lee, M. L. HRC & CC. J . High Resolut. Cbromatogr. Ctwomatogr. Commun. 1988, 11, 25. Yonker, C. R.; Smith, R. D. J . Cbromatogr. 1988, 355. 367. Li, S. F. Y.; Lee, H. K.; Lee, M. L.; Ong, C. P. Supercritical Fluid Chromatography And Extraction Of Steroids Using Freon-22. Presented at CIS'89, Tokyo, Japan, Oct 17-20, 1989. Li, S. F. Y.; Lee, H. K.; Lee, M. L.; Ong, C. P. J . Cbrometogr., in press. Zurer, P. S. Cbem. Eng. News 1989, 67, 7. Sand, J. R.; Andrjeski, D. L. Asbrae J . 1982, 24 (9, 38. Carraud, P.; Thiebaut, D.; Caude. M.;Rosset. R.; Lafosse. M.; Dreux, M. J . Cbromatogr. Scl. 1987, 25(9), 395. Lauer, H. H.; McManigili, D. C.; Board, R. D. Anal. Cbem. 1983, 55, 1370. Schimtz, F. P.; Leyendecker, D.; Kiesper, E. Ber. Bunsen-Ges. H y s . Cbem. 1984, 88, 912. Jinno, K.; Kuwajima, M. Cbromatograpbia 1987, 23, 631. Takeuchi, T.; Niwa, T.; Ishii, D. Ctwomatograpbia 1887, 23, 929. Chester, T. L.; Innis, D. P. HRC & CC, J . Higb Resolut. Cbromatogr. Cbrmatogr. Commun. 1985, 8 , 561. Jinno, K.; Niimi, S. J . Cbromatogr. 1989, 455, 29.
RECEIVED for review November
27, 1989. Accepted March 21,1990. We thank the National University of Singapore for financial support.
