Anal. Chem. 1991, 63,1874-1879
1874
4 does not depend on the ionic strength. According to eq All, I$depends on the one-fourth power of the ionic strength so that the phase ratio term in eq 10 will vary as In P/‘. In the range of practical salt concentrations this may be neglected in view of the much stronger dependence of the first term on the ionic strength. If, for example, the ionic strength dependence of the phase ratio is taken into account in evaluating the charge of the protein, the value differs by less than 5% from the charge obtained from eq 11 and this is likely to be less than the experimental error. LITERATURE CITED Monis, C. J. 0. R.: Morrl6, P. &patamMstkds k, m b y , 2 n d ed.;Wlby: New York. 1976 pp 86-87.
&”, N. K.: Patlrldge, S. M. &&em. J . 1955, 59, 543. Velayudhan, A.; HorvBIh, Cs. J . chrometog*. 1986, 367. 160. Melender, W. R.; El Rasd, 2. J.; Horv6tt1, Cs. J . chrometog*. 1989, 469, 3. Marsaroff, 1.: Varady. L.; Mouchawar, 0. A.; Regnier, F. E. J . Chrometog. 1990. 499, 63. Vold, R. D.; Vdd, M. J. chwdd and Interface Chemlsby; AddisonWesley Pubi. Co.: Rea-. MA. 1983. Israelachvili, J. N. Intemwk&r and Surface Forces; Academlc Press: London, 1985. Sdhl-9, J. J . -tog.. 1980, 356, 231. Sthlberg, J.; Furilnwn, A. Chromato@aph& 1987, 24, 78. St&hlkg, J.; HBg~lund,1. Anal. Chem. 1988, 60, 1956. Parseglan, V. A.; Gingel, D. Bbphys. J . 1972, 12, 1192.
i2li (22) (23) (24) (25) (26) (27) (26)
Qegory, J. J . CduU Intwfacs Scl. 1975, 51, 44. Sobec,H.A.hmW&ofBkdwwnlbtry. ~ D a t a f o r k k l e c u b r Bkkgy; The Chemical Rubbur Co.: OH, 1970. Altman, P. L.; DimMr, D. S. Eb@y Data Bodr; Fedemtbn of A& can Socletks for -tal Bkkgy: Washington, DC, 1964. Kenschlngton, A. W. In A Laboratory Man& of A n a W / Mstkds of Protdn chemlsby: Alexander, P.. Block. R. J., Eds.; Pergamon Press: Oxford. U.K., lB60. Tanford, C.; Wagner, M. L. J . Am. Chem. Soc. 1954, 76. 3331. Tanford, C.; Roxby, R. B h x h e d b y 1972, 1 1 , 2192. Parente, E. S.; Wetleufer, D. B. J . Chrwnatogr. 1986, 355, 29. Lehman, L. D.; Hanania. 0. I. H.; Gud, F. R. N. ”m. Bkphvs. Res. c4”un.1978, 81, 416. Tanford. C.: Hauenstein. J. D. J . Am. Chem. Soc. 1956. 78. 5267. Tanfird, C.i Swanson, S. A.; Shore, W. S. J . Am. Chem. Soc. 1955, 77, 6414. Cannan, R. K.; Palmer, A. H.; Klkick, A. C. J . Bld. Chem. 1942, 142. 803. Tanford, C. J . Am. C b m . Soc. 1950, 72. 441. Stout, R. W.; Shrakoff, S. I.; Ricker, R. D. J . chrometog. 1986, 353, 439. Heern. M. T. W.; Hodder, A. N.: Stanton, P. G.; AguHar. M. 1. Chromatopapt& 1987, 24, 769. Kopeciewicz, W.; Rounds, M. A,; Fausnaugh, J.; Regnler, F. E. J . chrometop. 1983, 266, 3. Haff, L. A.; Figerstam, L. 0.;Barry, A. R. J. Chfomeibgr. 1989.266, 409 * Fausnaugh, J. L.; Kennedy, L. A.; Regnler, F. E. J. Chromatog. 1984, 317, 141.
RECEIVED for review May 3,1991. Accepted May 17,1991.
Preparation and Study of Two Benzo-Crown Ether Polysiloxane Stationary Phases for Capillary Gas Chromatography Cai-ying Wu,* Xi-chun Zhou, Zhao-rui Zeng, Xue-ran Lu, and Li-fong Zhang
Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
I n thb study, two new gas chromatography (GC) statlonary phases were presented by 8ubstHutlng 2,3-benz0-9-[(prOpenyloxy)methyl~l5-crown-5 and 2,3-benz0-1 l-l(propenyloxy)methyl&l8-aown-6 onto fkxlM. @ysHoxanebackones, udng a hybodyhtbn technique. Impolzant datIonclry-pha80 properties of these two new phases, Including efflchcy, polarity, and selectlvlty, have been examlned and compared wHh those of w-[(undecyloxy)mahyl&l&rown-6 (PSO-11l8C-S). Temperature stablllty from 75 to 305 O C has been found. The mechanism of rp.cifk selectivity for podtlon konnr compounds based on the crown ether rlng and solute molecular rlze and shape are d k c u d .
CH3 I CH3 I CH,-Si-0 (-Si-O)m-
I
I
CH3
CH3
YH3
CH3 I (Si-0-),-Si-CH,
I
YH2
I
CH,
(?42)2
I
O\
9 = 0, the production is PSOB-3-l5C-5 9 = 1, the produdion is PSOB-3-18C-6
INTRODUCTION It was well-known that crown ethers have wide applications in chemistry, especially in analytical chemistry, resulting from the cavity structure and strong electronegative effect of hetetroatoms on the crown ether ring (1). The use of crown ethers as gas chromatography (GC) stationary phases began in 1985 (2-5). There have been only a few articles that deal with the use of crown ethers with small molecular weights as stationary phases (2-4), the reasons are due to the difficulty 0003-2700/91/0363-1874502.50/0
Flgurr 1. Structure of benro-crown ether polyslloxanes.
of coating them on the capillary column wall, poor column efficiency, and serious column bleeding at high temperatures. This fact suggests that a crown ether molecule attached to a polymer may overcome these disadvantages. In 1988,Rouse et al. synthesized a crown ether substituted polysiloxane that showed good chromatographic charactentics and had a unique selectivity for nitrogen-containingpolycyclic aromatic com0 I991 American Chemlcai Soclety
ANALYTICAL CHEMISTRY, VOL. 63. NO. 17, SEPTEMBER 1, 1991 5700
+ 5100
C O T S
a L'
C
1875
1
t
,
t
//
q = 0 , the productionis 2,3-benzo-9-[(propenyloxy)methyl]-l5-crown-5 9 = 1, the productionis 2,3-benzo-1l-[(propenyloxy)methyI]-18-crown-6 Flgurr 2. Synthesis method of crown ethers.
column temperature ('c) Flgurr 5. Column efflclency vs temperatwe: (A) on PSOE3-15C-5;
( 8 )on PSOB-3-18C-6.
1
I
6
Colunlc t e m p e r a t u r e ('c)
Flgurr 6. Shlft of basetlne vs column temperature: (A) on PSOE 3-15C-5; (8)on PSOB-3-18C-6.
60
80
100
120
140 Temp(C)
0
5
10
15
io T i e ( m i n )
Flgu,3. C t " a t o g a m of Qob test mixtures on PSOB-3-15C-5. The temperatwe was programmed from 80 to 140 Cat 4 OC/mIn. Peaks: (1) ndecam (2.42 ng); (2) n-undecane (2.36 ng); (3) ndodecane (2.32 ng); (4) loctand (2.22 ng); (5) 1,3-butanedbl(3.80 ng); (6) naphthalene (2.50 ng); (7) methyl undecanoate (2.30 ng); (8) 2,g-dimethylphenol (1.94 ng); (9) (2,4dlmethylphenyl)amine (2.05 ng); (10) methyl dodecanoate (2.30 ng).
I
logk' 1.8' 1.4
4.0 0.6 " 2 -0.2 ~
-c . 6
4
6 Carbon
Flgurr 4. Plot of log k'vs reciprocal absolute temperature: (A) on PSOB-3-15C-5; (8)on PSOB-3-18C-6.
e
10
r\umber
Figurr 7. Logarithm of the adJusted retention t l m vs carbon number for l-elcohols: (A)on a FSOE3-15G5 column; (e) on a pso63-18C-6 column; (C) on a Carbowax-2OM column. The temperatwe was 110 OC.
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ANALYTICAL CHEMISTRY, VOL.
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Table I. Chromatographic Properties of PSOB-3-15C-5and PSOB-3-18C-6Columns stationary phase
column
PSOB-3-15C-5
column size I X id., m X mm
column efficiency,O plates/m
capacity factor k'
peak assymmetry
10.00 X 0.25 9.50 X 0.25 15.00 X 0.25 9.00 X 0.25 10.00 X 0.25 10.00 X 0.25
4870 4543 4578 4919 4580 4680
3.88 4.67 4.82 3.69 4.81 4.62
1.00 1.00 1.00 1.00 1.01 1.00
1 2
3
PSOB-3-18C-6
1 2
3
"Test compound is naphthalene; column temperature is 120 O C .
7
I
2
7 1
5
6
I
100
120
I I
60
80
I
130
60
80
Time( min)
100
120
nme(min
Figure 8. Chromatogram of alcohol mixtures: (A, left) on PSOB-315C-5 column 3 (15 m X 0.25 mm); (6, rlght) on Carbowax-POM column (30 m X 0.24 mm). The temperature was programmed from 60 O C (1 mln) to 130 O C at 4 OC/mIn. Peaks: (1) butanol; (2) pentanol; (3) hexanol; (4) heptanol; (5) octanol; (6) nonanol; (7) decanol.
Table 11. McReynold's Constants of PSOB-3-15C-5and PSOB-3-l8C-6" stationary phase
X'
Y'
Z'
U'
S'
PSO-ll-18C-6 304 229 141 252 218 PSOB-3-15C-5 149 380 383 456 456 PSOB-3-18C-6 210 381 452 461 465 Carbowax-2OM 332 536 368 572 510 "X' = benzene, Y' = butanol, Z' = 2-pentanone, U' DroDane. S' = uvridine.
mean 229 365 394 464 = nitro-
pounds (5). Recently, we have prepared a w-[(undecyloxy)methyl]-18-crown-6 (PSO-ll-18C-6) polysiloxane stationary phase that gives excellent characterizations (6),and a new method for preparing a crown ether polysiloxane stationary phase by directly cross-linking a crown ether with SE-54in different proportions was developed by our lab (7);those cross-linked columns also possess high column efficienciesand have good thermal stabilities. In this paper, a polysiloxane backone was substituted by 2,3-benzo-9-[(propenyloxy)methyl]-15-crown-5 and 2,3-
1 , 150
h
L
Figure 8. Separatlon of aromatic hydrocarbon compounds (on PSO6-3-15E5 column 3). The temperature was programmed from 120 to 240 OC at 4 OC/mln. Peaks: (1) naphthalene; (2) 2-methylnaphthalene; (3) dlphenylmethane; (4) 3-methylnaphthalene; (5) biphenyl; (6) dlphenylethane; (7) acenaphthene; (8) fluorene; (9) phenanthrene; (10) triphenylmethane.
benzo- 11-[ (propeny1oxy)methyll-18-crown-6, respectively, to yield two new stationary phases (Figure 1). Because the easily polarizable benzene ring exists in the polymers, they were expected to increase the thermal stabilities and have a strong influence in separating different aromatic compounds and their derivatives.
EXPERIMENTAL SECTION Synthesis. The two new crown ether polymers were prepared by a hydrosilylation techque (8). The synthesis method of 2,3benzo-9-[(propenyloxy)methyl]-15-crown-5and 2,3-benzo-ll[ (propenyloxy)methyl]-18-crown-6are shown in Figure 2. Poly(methylhydr0siioxane) was obtained from Xmghuo Chemical Plant, Jing Xi Province, PRC. The percentage of Si-H in the siloxane fluid is 52.8%. (1) Synthesis of PSOB-3-15C-5. A 0.1-g sample of poly(methylhydrosiloxane) and 0.38 g of 2,3-benzo-9-[(propenyl-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991
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50
0 14C
4
0
I I
I
10
20
I
30 t i m i ( a i t i l
Figm lo. Separation of nitrotoluene isomers. The temperature was programmed from 140 to 200 "C at 2 "C/min. Peaks: (1) o-MNT; (2) m-MNT (3) p-MNT; (4) 2,6-DNT; (5) 2,5-DNT; (6) 2,49NT; (7) 3,5-DNT; (8) 3,eDNT.
oxy)methyl]-15-crown-5 were mixed in 3 mL of pure benzene solvent, and the solution was stirred rapidly at 90 "C for 1h under an argon atmosphere. A 6-pL aliquot of chloroplatinic acid solution (1%H3tCbH2O,0.1% ethanol, and 98% THF)was added to catalyze the polymerizing reaction, and the mixture was stirred at 90 "C for 6 h. n-Decylene (1 mL) was then dropped in and the mixture was stirred for an additional 1 h to completely substitute the residual Si-H (monitored from spectra). After this, the gummy product was taken out and dissolved in 8 mL of methylene chloride and washed seven times with 30 mL of methanol/water (1/1) to remove the catalyst. The solvent was vaporized, and the gummy product was dried under reduced pressure. (2) Synthesis of PSOB-3-18C-6. The synthesis method of PSOB-3-18C-6 is similar to the preparation of PSOB-3-15C-5 above, except 0.08g of poly(methylhydrosi1oxane) and 0.3 g of 2,3-benzo-ll-[(propenyloxy)methyl]-18-crown-6were used. Column Preparation. Fused-silica capillarieswere obtained from Yong Ning Fused Capillary Plant (Huo Bei, PRC). The capillaries were rinsed with methanol and purged with nitrogen gas at 250 OC for 2 h to remove acidic impurities. Then the column was staticallycoated with a solution of 0.5% (w/v) PSOB-3-15C-5 or PSOB-3-18C-6 in dichloromethane. All columns were conditioned at 290 "C for 10 h under a slow nitrogen purge before use. Column Evaluation, Instruments, and Methods. The gas chromatograph used throughout was a SC-7 gas chromatograph apparatus (Sichuan Analytical Apparatus Plant, PRC.) equipped with a capillary column split-injection system and FID dedetector; nitrogen was the carrier gas, the linear velocity being 12-15 cm/s and flow being 60-80 mL/min. The columns were tested for polarity and efficiency by measuring the McReynold's constants
Temp ('c) 70 90
5 10 TimeCmin)
F l g m 11. separation of toluene and xylene isomer compounds. The temperatwe was programmed from 50 to 90 "C at 4 "Clmin. Peaks: (1) toluene; (2) mdhnethylbenrene; (3) pdlmethylbenrene; (4) odlmethylbenzene.
and number of plates per meter for naphthalene at 120 "C; the glass transition temperature was determined by slope change of the log k' versus 1/T plot for naphthalene. Some standard mixtures of alcohols and aromatic hydrocarbons as well as some polar position isomers were used to demonstrate the unique selectivity of the two stationary phases. The thermal stabilities of the stationary phases were tested by measuring column bleeding.
RESULTS AND DISCUSSION Table I s u m m a r k the chromakgraphic properties of these two benzo-crown ether polysiloxane columns; it indicates that the theoretical plate number is over 4500 plates/m for all of the columns and the reproducibility is good: the peak asymmetry factor for octanol on fused-silica columns explains the chemical inertness of the columns. It also indicates that benzo-crown ether polysiloxanes possess good film-forming ability, and the polarizable benzo-crown ether ring in the polymer does not affect this characteristic. Figure 3 presents the chromatogram of the Grob test mixture obtained from PSOB-3-15C-5. It shows the mixture as being well separated, and each peak shape is symmetric. An appreciable difference is that both 1-octanol and 1,3-butanediol eluted behind n-dodecane, in contrast to the case with the nonpolar columns. It is obvious that the phase has a strong hydrogen-bonding force to alcohols. Another noticeable difference is 2,6-dimethylphenol eluted before 2,4-dimethylaniline, which is in contrast to the result on the crown ether polysiloxane column of PSO-11-18C-6 (6). This difference is caused by the steric hindrance of the 2,6-DMP by the benzo-crown ether ring, even if the hydrogen-bonding force of 2,6-DMP is larger than that of 2,4-DMA. The selectivity and average polarity of those two new phases
1878
ANALYTICAL CHEMISTRY. VOL. 63, NO. 17, SEPTEMBER 1, 1991 1 upk ' 0,Y
0.b
0.3
1 0
3
-0.3
2.3
2.4
2.5
2.7
2 . 6
2.8
2.9
lO'/T(.K)
Flgure 13. Plot of log k'vs reciprocal absolute temperature on the P s o 8 3 1 5 C - 5 column: (1) mdlchlorobenzene; (2) pdWorobenzene; (3) odichiorobenzene.
60
80
100 Temo(C1
Flgwo 12. Separatbn of the dichlorobenzene isomers. The temperature was programmed from 60 to 100 "C at 4 "Clmin. Peaks: (1) mdichkrobenzene; (2) p-dlchkrobenzene; (3) odlchlorobenzene.
are presented by the McReynold's constants in Table 11. It shows that the benzo-crown ether polysiloxane phases have a medium polarity that is higher than that of PSO-11-18C-6 (6)and a little lower than that of Carbowax-2OM. The operating temperature range is determined by the glass transition temperature and column bleed. Figure 4 shows the log k' vs 1/T plot for naphthalene on PSOB-3-15C-5 and PSOB-3-18C-6 columns. Changes in the slope at 137 "C for PSOB-3-15C-5 and 140 "C for PSOB-3-18C-6, respectively, corresponded to phase changes in the columns. These results also corresponded to the glass transition temperature determined by differential scanning calorimetry (DSC) of theae two polymers. The change of the slope between transition points is not apparent compared to the plot of log k' vs 1/T of PSO-11-18C-6 columns (6),and the glass transition point temperature is 25 or 22 OC below that of PSO-11-18C-6 (6). This fact indicates that the benzo group in the crown ether ring has changed the thermodynamic properties of the stationary phases and implies that the two phases have a wide allowable temperature range. This conclusion is also proved by the column efficiency vs temperature plot shown in Figure 5. Generally, column efficiency will fall off very fast below the glass transition temperature with decreasing temperature but the increase of mass-transfer resistance (Figure 5) however, shows that PSOB-3-15C-5 and PSOB-3-18C-6 columns still have 3400 plates/m at 75 "C and the rate of decreasing is very slow with decreasing temperature. The column bleed curve measured by the column being heated from 180 to 305 "C was shown in Figure 6. The column began to bleed at 230 OC, and the shift of baseline was 4.5 X A for PSOB-15C-5 and 6.0 X A for PSOB-3-18C-6 at 305 "C. From above, we can see that the benm-crown ether polysiloxane extends the operating temperature
from 75 to 305 "C, which is a considerable improvement over the crown ether polysiloxane stationary phase of 120-300 "C reported by Rouse et al. (5). Figure 7 gives the relationship of log tR' versus the carbon number of alcohol homologoes; the slopes of the straight lines of PSOB-3-15C-5 and PSOB-18C-6are a little higher than that of Carbowax-OOM, the slopes of the PSOB-3-15C-5 and PSOB-3-18C-6 being 1.27 and 1.24, respectively. and the slope of Carbowax-2OM being 1.05 at the same condition. Therefore, the benzo-crown ether polysiloxane phases have higher selectivity for alcohols than Carbowax-2OM. The selective separation chromatographs are shown in Figure 8. Figure 9 gives excellent separation of aromatic hydrocarbons. These compounds eluted in the order of their increasing dispersion force as they are nonpolar or polarizable compounds. Figures 10-12 illustrated the separation of nitrotoluene isomers and dimethylbenzene isomers as well as dichlorobenzene isomers. It is interesting to note that the solutes of Figures 10 and 11 were not eluted only depending on the dipole-dipole interaction. For example, the sequence of the dipole-dipole force of nitrotoluene isomers is H2,bDNT< H ~ , ~ - D
