2148
Anal. Chem. 1983, 55,2148-2151
Support-Deactivating Fluorocarbon Stationary Phase for Gas Chromatography Subhash C. Dhanesar and Colin F. Poole* Department of Chemistry, Wayne State University, Detroit, Michigan 48202
The nonionlc fluorocarbon surfactant Flourad FC-430 is a useful medlum-polar stationary phase with a usable temperature range from -40 OC to 250 OC. It has good coating characteristics on diatomaceous supports and Imparts an unusually hlgh degree of support deactivation enabllng phenols, amines, and carboxylic aclds to be analyzed at low concentratlons. Compared to nonfluorinated phases of slmllar polarity the retentlon of organic compounds is generally a factor of - 2 to 6 times lower on Flourad FC-430.
Sporadic accounts of the use of fluorocarbon compounds as stationary phases have appeared almost from the inception of gas-liquid chromatography. Early reports dealt with the use of perfluoroalkanes, Kel-F oils, and fluoroalkyl esters for the analysis of corrosive and reactive chemicals (e.g., UFs, C102F, SF&l, etc.), which could not be separated on conventional phases (1-4). Attempts to use these phases for more general applications were less successful due to a combination of poor coating characteristics, low column efficiencies, and low upper operating temperature limits. At about the same time purified high molecular weight polysiloxane and polyester phases became available and virtually eliminated further interest in fluorocarbon phases. This was perhaps premature in the light of the unique physical characteristics of fluorocarbon liquids but was understandable on account of their very poor column characteristics. Intermolecular forces are very weak in perfluorocarbon liquids which also explains their poor column coating characteristics. However, these same weak intermolecular forces give rise to diminished retention in gas chromatography permitting the possibility of extending the technique to the separation of substances of higher-molecular-weight than is presently possible with conventional nonfluorinated phases. Recent research in the use of perfluorocarbon phases in gas chromatography has been directed toward their use for separation of fluorocarbon compounds (7,8), for the separation of involatile compounds at moderate temperatures (8, 9), and for the separation of polar organic compounds in aqueous solution (IO). Several features in the above work are significant to this paper. The fluorocarbon surfactant, Fluorad FC-431, produced column packings that were highly deactivated, allowing the direct analysis of polar compounds at low aqueous concentrations (IO). The poly(perfluoroalkyl) ether, Fomblin YR, was found to be suitable for the separation of a wide range of nonpolar and moderately polar organic compounds with average retention values better than an order of magnitude less than those obtained on nonfluorinated phases (8, 9). In this paper we will compare the chromatographic properties of a new nonionic fluorocarbon surfactant, Fluorad FC-430, with that of Fomblin YR and the published properties of Fluorad FC-431. Fluorad FC-430 and FC-431 are commercial products manufactured by the 3M Co. Their chemical structures are propriotory information. They share a similar application as wetting, leveling, and flow control agents for
a variety of organic polymer coating systems but are otherwise different materials (11).
EXPERIMENTAL SECTION Unless otherwise stated, all chemicals and solventswere general laboratory or analytical grade in the highest purity available. Test compounds were available as Thetakits (Anspec,Ann Arbor, MI). Fomblin YR was obtained from Montedison (New York, NY) and Fluorad FC-430 from 3M Co. (St. Paul, MN). Squalane and OV-225 were obtained from Anspec (Ann Arbor, MI). Column packings containing from 3 to 15% (w/w) of Fluorad FC-430 on Chromosorb P-AW or Chromosorb W-AW (100-120 mesh) were prepared by using the rotary evaporator technique and acetonitrile as solvent. The air-dried packings were sieved before use and packed into glass columns of 1to 3 m length with the aid of vacuum suction and gentle vibration. Fomblin YR required the use of special procedures to prepare efficient packings; details are given elsewhere (8). For gas chromatographya Varian 3700 gas chromatograph with an on-column heated injector,temperature programmer, and flame ionization detector was used. Separation conditions are given in the figure captions. The maximum allowable operating temperature for Fluorad FC-430 was established as the highest isothermal temperature that the column could be held at for 24 h without changing retention of a series of test probes measured under the conditions indicated in the captions to Figures 1 and 2. Columns have routinely been temperature programmed up to 250 "C over several months without noticeable deterioration of column performance. The asymmetry factors for the carboxylic acids were measured at 10% of the peak height.
RESULTS AND DISCUSSION Fluorad FC-430 packings having 1300-1500 plates per meter were prepared by standard procedures and without any particular difficulties. Although the plate count is not particularly high, the values quoted reflect the mechanical stability of the support rather than undesirable coating characteristics of the phase (12,13). Similar values were obtained for the squalane and OV-225 columns prepared with the same batch of support and coated and packed in an identical manner to the Fluorad FC-430 column. Typical test chromatograms for efficiency, Figure 1, and support deactivation properties, Figure 2, illustrate the favorable chromatographic characteristics of Fluorad FC-430. The usable temperature range for Fluorad FC-430 was established as -40 "C to 250 "C. It is noteworthy that Fluorad FC-430 produces a stable film a t all temperatures unlike Fomblin YR for which phase contraction a t elevated temperatures establishes the upper operating temperature limit. This we attribute to the superior support wetting characteristics of Fluorad FC-430 compared to Fomblin YR. The McReynolds' constants values for Fluorad FC-430 are given in Table I. On the basis of these values Fluorad FC-430 could be described as a moderately polar phase possessing some additional Lewis acid character. I t is more polar than Fomblin YR (9) and has different selectivity to Fluorad FC-431 (IO). As the fluorocarbon chains are not likely to contribute to the polar character of the phase, we must assume that this is due to the presence of polar functional groups in
0003-2700/83/0355-2148$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
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Table I. McReynolds' Constants for Fluorocarbon Phases and OV-225
benzene butanol 2-pentanone nitropropane pyridine 2-methyl2-pentanol iodobutane 2-octyne 1,4-dioxane cis-hydrindan
-.5
0'-
15
10
20
25
MIN
Figure 1. Test chromatogram illustrating column efficiency. Test mixture C8-Cl2 hydrocarbons separated on a 3 m X 2 mm i.d. column of 10% (wlw) Fluorad FC-430 on Chromosorb W-AW (100-120 mesh) at 55 OC with a nitrogen carrier gas flow rate of 30 mL min-'.
2;
max allowable operating temp, "C
Fluorad FC-431
Fluorad FC-430
Fomblin YR
ov-
281 423 29 7 509 360 326
178 466 381 462 460 399
-15 138 88 141 51 66
228 369 338 492 386 282
223 183 294 78 1870 200
117 94 349 -42 1947 250
-15
225
150 122 403 255
1863 265
7-
C (FC-430
B (FC-430)
1 -+o
0.5
A (OV-225)
1.0
15
2 .o
SAMPLE AMOUNT (ng)
Flgure 3. Comparison of the deactivation properties of Fluorad FC-430 and OV-225. All measurements were made with 3 m X 2 mm i.d. columns containing 10 % (w/w) stationary phase on Chromasorb W-AW. Test probe identification is as follows: (Fluorad FC-430) (A) undecanol, (6) di-n-hexylamine, (C) 3,4-xylenol, (D) hexanonic acid; (OV-225) (A) hexanol, (6) di-n-butylamine, (C) phenol.
6
5
10
1'5
MIN Figure 2. Test chromatogram illustrating column deactivation by Fluorad FC-430: (1) decane; (2) undecane; (3) dodecane; (4) 5-110nanone; (5) trldeoane; (6) 1-octanol; (7) nalphthalene; ( 8 ) 2,6-dimethylanlline; (9) 2,6dimethylphenoL Same column as Figure 1 and temperature programmed from 50 to 180 OC at 15 OC mln-'.
Fluorad FC-430. In keeping with observations made with Fomblin YR the absolute retention of organic compounds is considerably lower than for a nonfluorinated stationary phase having similar polarity, 'Table 11. In terms of their McReynolds' constants the cyanopropylmethylphenylsilicone OV-225 is closest among generally available stationary phases nn matching the properties of Fluorad FC-430, although differences exist; see 'Table I. Under matched column operating conditions the capacity factor values for the test probes on Fluorad FC-430 are reduced by a factor of 1.8 to 6.3 with an average value of 3.4 compared to OV-225. The exception is pyridine which is retained longer on Fluorad FC-430 than
OV-225 emphasizing the greater acid character of the former. It can also be seen from Table I1 that the difference in retention is not as dramatic as in the case of Fomblin YR when it is compared to squalane, its polarity matched phase. In all cases the absolute retention of the test probes is diminished by more than an order of magnitude on the fluorocarbon phase. This is simply explained, as for the nonpolar phases the dominant interactions are dispersive forces, whereas for the polar phases dispersive, dipole, and hydrogen bonding interactions are likely. Retention on the Fluorad FC-430 phase is therefore a combination of selective interactions between the polar functional groups of the solute and phase which is largely independent of the presence of fluorine, except perhaps for some modification due to the inductive effect of fluorine, and diminished retention due to the weak dispersive interactions occurring between the solute and the perfluorocarbon portion of the phase. Of note from Figure 2 is the symmetrical peak shape of both 2,6-dimethylphenol and 2,6-dimethylaniline used to evaluate the acid/base character of the Fluorad FC-430 column. Perhaps the most useful property of Fluorad FC-430 is its ability to deactivate to a high degree the adsorptive properties of diatomaceous supports. Fluorad FC-430 and OV-225 are compared in Figure 3 for their ability to separate without
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
r 0
5
fo
'
1'5
25
10
MIN
Figure 4. Separation of fuel oll number 1 using a 3 m X 2 m i.d. column of 10% (w/w) Fluorad FC-430 on Chromosorb P-AW (100-120 mesh). Temperature programmed from 55 to 195 OC at 7 OC min-' with nitrogen carrier gas flow rate 30 mL min-'.
b
3
lb
MIN Flgure 6. Separation of phenols: (1) phenol, (2) m-ethylphenol, (3) mcresol, (4) 3,4-xylenol, (5) 2,4,Wlmethylphenol. Column is the same as described in Figure 1, temperature programmed 130 to 170 OC at 4 'C min-', and nitrogen carrier gas flow rate 30 mL min-'.
6
3
Ib
15
20
YIN Flgure 5. Separation of C3 to CI2 n-alkylbromides on a 3 m X 2 mm i.d. column of 10% (w/w) Fluorad FC-430 on Chromosorb W-AW (100-120 mesh). Temperature programmed from 45 to 170 OC at 8 'C min-I with nitrogen carrier gas flow rate 30 mL mln-I.
adsorption low concentrations of underivatized polar compounds. On the Fluorad FC-430 column sample sizes of 0.1 to 1.0 ng of 3,4-xylenol, di-n-hexylamine, and undecanol can be chromatographed without adsorption. Hexanoic acid is not adsorbed a t sample sizes above 1.0 ng, whereas on the
OV-225 column, it does not chromatograph at all with reasonable peak shape and adsorption by the column in the l to 25 ng sample size range is substantial. Adsorption of alcohols and amines is also exhibited by the OV-225 column in the 0.1- to 0.5-ng sample size range. We have used the Fluorad FC-430 packing for the separation of many different sample types. Figure 4 shows a separation of a complex mixture of hydrocarbons and Figure 5 a separation of a mixture of homologous 1-bromoalkanes. Fluorad FC-430 packings are particularly useful for the separation of underivatized polar compounds such as alcohols, aldehydes, amines, phenols, and carboxylic acids. Figure 6 is a separation of a mixture of phenols and Figure 7 a separation of carboxylic acids which illustrate this point. The separation of underivatized carboxylic acids is a particularly severe test of the support deactivating properties of this phase. Under isothermal conditions some peak asymmetry is observable; values of 1.4 to 1.6 are common for carboxylic acids having capacity factor values of 20 to 30. For an equivalently retained hydrocarbon the asymmetry factor value was approximately 1.2. The carboxylic acid asymmetry factor values do not change significantly with sample size until very low sample amounts are injected. This was discussed earlier with respect to Figure 3. As a final example, Figure 8 shows a separation of a mixture of monosubstituted benzene derivatives containing different functional groups illustrating the ability of Fluorad FC-430 to simultaneously separate substances of different polarity and acid/base character. Our work using fluorocarbon phases has led to several generalities which may be of use to other workers in predicting the usability of perfluorocarbon liquids as stationary phases. First of all, because of their weak intermolecular forces,
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
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--
~.~
Table 11. Capacity Factor Values for Test ProbesU test probe
squalane 20.7 12.2 13.9 20.7 35.0 23.4
benzene butanol 2-pentanone nitropropane pyridine 2-methyl2-pentanol iodobutane 2-octyne 1,4-dioxane cis-hydrindan heptane octane nonane
1210
188 21.9 > 300
Fomblin Fluorad YR FC-430 0.8 1.3 1.2 2.1 1.5 1.5 2.4 3.3 1.2 6.9
1.50
OV225
7.0
10.0
18.1
7.5 14.0 21.0 9.7
16.1 23.1 16.3 21.'7
3.8 4.4 7.0 6.6 0.7
24.0
1.5
3.2
17.8 31.1 2.4 5.6 12.4
I
a Column 3 m x 2 mrn i.d.; 10% (w/w) on Chromosorb P-AW (100-120 mesh); temperature, 451"C; flow rate, 30
m L min".
L 3
-0
10
15
MIN Flgure 8. Separation of substituted benzene derivatives: (1) benzene, (2) toluene, (3) chlorobenzene, (4) rn- and p-xylene, ( 5 ) o-xylene, (6) bromobenzene, (7) p -dichlorobenzene, (8) iodobenzene, (9) benzaldehyde, (10) benzyl alcohol, (11) nitrobenzene. Column is the same as described in Figure 5, temperature programmed 48 to 165 OC: at 8 OC min-', nitrogen carrier gas flow rate 30 mL min-l.
giving inefficient packings after conditioning at a temperature below which column bleed is observed to occur. There remains a need for fluorocarbon phases that can be used at high temperatures (-350 "C or above) to exploit the unique ability of fluorocarbons to allow the separation of higher-molecular-weight compounds than is presently possible employing conventional phases. Registry No. Fluorad FC-430, 11114-17-3.
LITERATURE CITED (1) Baiulescu, G. E.; \
b
s
io
-
15
MIN Flgure 7. Separation of (2, to Clo carboxylic acids and :!-methylpropionic acid (peak 3). Column is the same as described in Figure 1, temperature programmed 110 to 210 O C at 10 OC min-', and nitrogen ciirrier gas flow rate 30 mL min-'.
perfluorocarbori liquids do not form stable films on diatomaceous upp pork or open tubular glass capillary column wdls. Those phases giving efficient columns clontained an "anchor center" (moderately polar functional group) which presumably stabilizes the stationary phase film. The coating characteristics of fluorocarbon phases can change with temperature
(2)
(3) (4) (5) (6) (7) (8) (9)
(Id) (1 1)
Ilie, V. A. "Stationary Phases In (Gas Chromatography"; Pergamon Press: Oxford, 1975. Ellis, J. F.; Forrest, C. W.; Allen, P. A. Anal. Chim. Acta 1960, 22, 27. Campbell, R. H.; Gudzlnowicz, 8. J. Anal. Chem. 1961, 33, 842. Lyzyt, I.; Nawton, P. R. Anal. Chem. 1963, 35, 90. Juvet, R. S.;Fisher, R. L. Anal. Chem. 1968, 38, 1860. Pappas, W. S.; Mllllon, J. G. Anal. Chem. 1966, 4 0 , 2176. Muller, U.; Dietrich, P.; Prescher, D. J . Chromatogr. 1983, 259, 243. Dhanesar, S . C.; Poole, C. F. J . Chromatogr. 1983, 267, 388. Dhanesar, S . C.; Poole, C. F. Anal. Chem. 1983, 55, 1462. Blaser, W. W.; Kracht, W. R. J . Chromatogr. Sci 1978, 16, 111. Fluorad FluorochemicalSurfactants, Product Information Bulletin Nos. Y-ISIC (92.5) BPH, Sept 1982, 3M Chemical Co., St. Paul, MN
55144. (12) Conder, J. R.; Young, C. L. "Physicochemical Measurement by Gas Chromatography"; Wiley: New York, 1979; p 108. (13) Spark, A. A. HRC CC, J . High Resoiut. Chromatogr. Chromafogr. Commun. 1972, 2 , 577.
RECEIVED for review June 30,1983. Accepted August 12,1983.
