Anal. Chem. 1990, 62, 968-971
968
Preparation and Characteristics of a Crown Ether Polysiloxane Stationary Phase for Capillary Gas Chromatography Cai-Ying Wu,* Cheng-Ming Wang, Zhao-Rui Zeng, and Xue-Ran Lu Department of Chemistry, W u h a n University, W u h a n 430072, People's Republic of China
A new crown ether, n-undecyloxymethyC18-crown-6 polysiloxane (PSO-11-18C6) is prepared and coated on a fused silica capillary column. Chromatographic characteristics, including column efficiency, allowable temperature range, thermal stability, polarlty, and selectivity, are studied. The new statlonary phase Is comparable to Carbowax-2OM In polarity and selectivlty and has an operational temperature range of 70-300 OC. Selectivity is superlor to Carbowax-2OM for n-alcohols and esters, and some aromatic compounds separate well on the crown ether column. The separation mechanism is also dlscussed.
CH, I
CH3 I
CH3 I
I
I CH3
I
CH3 I
. CH,-Si-O(-Si-O)m-(Si-O-)TSi CH3
-CH3
I
CH,
CH3
I
(CH2)IO I 0
$ 0 9
11 C o d Figure 1. Structure of the crown ether in this work.
INTRODUCTION Crown ethers are useful as chromatographic stationary phases because of the good selectivity resulting from the cavity structure and the strong electronegative effect of heteroatoms on the crown ether ring ( I ) . Although the synthesis of crown ethers is expensive, the amount of crown ether for making a capillary column is small,only 3 mg of it is sufficient to prepare a typical 20-m capillary column (Z),crown ethers are primarily used as a stationary phase for capillary gas chromatography. There have been a few articles concerning such use of crown ethers as stationary phases in capillary GC (3-6). The use of small crown ethers is limited because of coating difficultly, poor column efficiency, and column bleeding at high temperature. Polymerization of the crown ethers may alleviate some of these problems. In 1985, Fine ( 4 ) bonded vinyl crown ethers onto the inner wall of capillary column, but his results were not satisfactory. Lee ( 5 ) recently synthesized a crown ether substituted polysiloxane with a polymer spacing of three, which showed a unique selectivity for nitrogen-containing polycyclic aromatic compounds. In this work, w-undecyleneoxymethyl-18-crown-6is substituted onto a polysiloxane backbone t o yield a stationary phase for capillary GC (Figure 1). A polymer spacing of 11 is used to facitate the free movement of the crown ether ring as well as cross-linking (7) of the phase.
1
II
6
15
10
5
0
Retention (min)
EXPERIMENTAL SECTION Synthesis of PSO-11-18C-6. The crown ether polysiloxane
was prepared by a hydrosilylation technique (8). A mixture of 0.12 g of poly(methylhydrosi1oxane) (JiangXi Xinghuo Chemical Plant, People's Republic of China), 0.70 g of w-undecyleneoxymethyl-18-C-6 (obtained from the Department of Chemistry, Wuhan University, People's Republic of China), and 4 mL of pure benzene were stirred rapidly at 90 "C for 1 h under an argon atmosphere. Ten microliters of chloroplatinic acid solution (1% H2PtClgH20,1% ethanol, and 98% THF) as catalyst was added and the mixture was stirred a t 90 OC for 6 h under the same conditions as above. From IR spectra one can see that the Si-H bond still exists in the polymer, indicating that the reaction does not proceed completely. One milliliter of 1-n-decylene was added and the mixture was further stirred for 1 h. After the mixture was allowed to cool, the gummy polymer was taken out and dissolved in 4 mL of CH,Cl,, then the polymer solution was
Figure 2. Chromatogram of Grob test mixture. Temperature was programmed from 80 to 150 OC at 4 'C/min. Peaks identification is as folows: 1, ndecane; 2, n-undecane; 3, ndodecane; 4, 1,3-butanediol; 5, 1-octanol; 6, naphthalene; 7, 2,4dimethylphenylamine;8, 2,6dimethylphenol; 9, methylundecanoate; 10, methyl dodecanoate. The unlabeled peak represents the solvent.
washed 7 times with 14 mL of methanol aqueous solution (1:l) to remove the catalyst. The solvents were removed, and finally, the polymer was dried under vacuum. Capillary Column Preparation. Fused silica capillary tubing (0.22-0.24 mm i.d., Academy of Post and Telecommunication, Wuhan, People's Republic of China) was rinsed with 10 mL of methanol and purged with nitrogen gas a t 250 "C for 2 h. Capillaries were statically coated with a solution of 0.5% (w/v) PSO-11-18C6 in methylene chloride, following the coating procedure and flushing with nitrogen gas for 3 h, and then conditioned
0003-2700/90/0362-0968$02.50/00 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 9, MAY 1, 1990
2.0 1 0 3 ~(1K 1 )
2.3
2.6
969
2.9
1 0 3 ~( 1K 1 )
Figure 3. log K vs reciprocal absolute temperature for naphthalene on PSO-11-18C6 column: (A) without cross-linking; ( 6 ) cross-linked.
5700
1
/
51001
/
J
/
RESULTS AND DISCUSSION
3300
70
90
110
130
150
Column temperature ("C) Figure 4. Column efficiency vs temperature for naphthalene on the crown ether column.
Table I. Charateristics of PSO-18C6 Capillary Columns column no. 1
2 3 40 50 6O
column size L x i.d. m x mm 10.0 X 9.5 x 24.0 X 14.0 X 15.0 X 9.5 X
0.23 0.22 0.23 0.24 0.24 0.23
curing process, it was washed with 10 times column volume of purified methylene chloride and conditioned at 280 "C for 10 h before use. Column Evaluation. The apparatus used for column evaluation was A GC-7A gas chromatograph (Shimadzu, Japan) equipped with a capillary split injection system and a flame ionization detector. Nitrogen gas was the carrier gas. Its linear velocity was set at 12-16 cm/s and split flow at 80-100 mL/min. Peak measurements and intergations were performed by an CR-3A integrator and R-112 recorder. The characteristicsof the column, such as column efficiency, thermal stability, allowable operating temperature range, glass transition temperature, and selectivity for the capillary columns, were tested.
cross-linking efficiency, %
column efficiency, plates m-l
95.2 97.2 96.0
4722 5085 4365 4706 4926 5417
Cross-linked columns, others are coating columns: test compound, naphthalene; column temperature, 120 "C. (1
at 280 "C for 6 h. The coated columns were cross-linked with ATB (azo-tert-butane)vapors by bubbling nitrogen through ATB and purging the column for 2 h at room temperature. Both ends of the column were sealed and the column was heated from 40 to 200 "C at 8 "C min-' and held at 200 "C for 1.5 h. After the
PSO-11-18C6 for use as a stationary phase in this study has a nonpolar polysiloxane chain, long aliphatic spacer arm, and polar crown ether ring. It is therefore predicted to have good film forming ability, high thermal stability (9),and a unique selectivity. Table I shows that all of the columns have a column efficiency higher than 4300 plates m-l and a cross-linking efficiency of more than 95 % . Grob test mixture was used to evaluate the characteristics of a PSO-11-18C6 column. Figure 2 shows that the Grob test mixtures separate well and that each peak shape is symmetric, which demonstrates that PSO-11-18C6 has the ability to deactivate the inner surface of a chromatographic column. Because both 1-octanol and 1,3-butanediol eluted behind n-dodecane, it is obvious that PSO-11-18C6 has a strong hydrogen-bonding force to alcohols. Temperature is an important parameter for a stationary phase. The glass transition temperature of PSO-11-18C6 is much decreased after cross-linking. See Figure 3. I t is reasonable that the extended network, formed by cross-linking, could have changed both the thermodynamic and kinetic properties of PSO-11-18C6 (IO);the change of the slope between the two curves in Figure 3B is not apparent. This indicates that thermodynamic properties of PSO-11-18C6with cross-linking are very close at two states in the transition point of glass transition temperature. I t is also proven by the dependence of column efficiency on temperature as shown in Figure 4. Generally, column efficiency will fall off very fast below glass transition temperature, with temperature being decreased. Figure 4 shows, however, that the column efficiency of the PSO-ll-18C6 column with cross-linking still has 3380
970
ANALYTICAL CHEMISTRY, VOL. 62, NO. 9, MAY 1, 1990 2.9
2.5
2.1 U E
E?
i 0,
i
J
1.7
1.3
0.9 I 4
I
I
1
Carbon number
1
I
6
I
8
10
Carbon number
Figure 5. Logarithm of the adjusted retention time vs carbon number for homologous n-fatty acid methyl esters (A) and 1-alcohols (B): (A) PSO-11-18C6: (0)Carbowax-20M. Column temperature was 110 OC.
8
40
30
0
10
20 Retention (min)
15
10 5 Retention (min)
O
Figure 6. Separation of ethyl-substituted nitrobenzene isomeric mixture (on column 3). Temperature was programmed from 150 to 210 "C at 3 OC/min: Peaks are as follows: (1) nitrobenzene; (2) o-nitroethylbenzene; (3) m-nitroethylbenzene; (4) p-nitroethylbenzene; (5) 2,54iethyinitrobenzene; (6) 3,4diethylnitrobenzene; (7) 35diethylnitrobenzene. The unlabeled peaks represent the solvent.
plates m-l at 70 "C. Therefore the allowable temperature range of PSO-11-18C6 is wider after cross-linking. The thermal stability of the cross-linked PSO-ll-18C6 was determined by measuring column bleed. The experimental results show that it begins to bleed at 245 "C and the base line drifts 1.2 X A at 300 "C; that is, it has a higher thermal stability. The polarity of PSO-11-18C6 is expressed by McReynolds
Figure 7. Separation of ethyl-substituted diphenyl ketone isomeric mixture (on column 3). Temperature was programmed from 180 to 260 "C at 2 OC/min. Peaks are as follows: (1) diphenyl ketone; (2) o-ethykliphenyi ketone: (3) m-ethyldiphenylketone; (4) p-ethyldiphenyl keones; (5, 7, 8, 13) diethylbenzophenone, (6, 9, 10, 11, 12) triethyldiphenyl ketone. The unlabeled peaks represent the solvent. ~~~~~
Table 11. McReynolds Constants of the New Phase4 stationary phase
X'
Y'
Z'
U'
S'
av polarity
PSO-11-18C6 Carbowax-2OM SE-30
304 332 15
229 536 44
141 368 53
252 572 64
218 510 41
229 462 43
"Key: X', benzene; Y', 1-butanol; Z , 2-pentanone; U', nitropropane: s', wridine.
constants; it was measured at 120 "C (Table 11). It is found that PSO-11-18C6 exhibits a moderate polarity, its polarity is between that of Carbowax-2OM and SE-30 but much lower
ANALYTICAL CHEMISTRY, VOL. 62, NO. 9, MAY 1, 1990
pounds and their derivatives, two typical examples are given in Figure 6 and Figure 7 . Interaction with the crown ether cavity is shown in Figure 8. The kovats indices of a series of aromatic hydrocarbon compounds on PSO-11-18C6 are plotted against indices on Carbowax-2OM. There is a linear relationship for the index values of most of the aromatic hydrocarbons. However, fluorene, phenanthrene, and triphenylmethane show positive index decrement, which implies that PSO-ll-18C6 has a ring cavity effect because the molecules of those compounds, e.g. phenanthrene, 9.04 8, X 6.16 A (without containing the radius of hydrogen), are too large to fit well in the cavity of the crown ether ring (18-C-6, 2.6-3.2 8, i.d. of ring (11)) and so the interaction between them is diminished.
2400
/ 2200 -
-
c 8
-
2000
-
c
8CL P
971
1800 -
CONCLUSION 1600 -
1400v 1600
I
1800
1
I
2000
1
1
2200
I
I
2400
KI (Carbowax-2OM)
Figure 8. Kovats indices of a series of aromatic hydrocarbons on Carbowax-20M vs those on the crown ether column. Column temperature was 170 OC. Peak identifications are as follows: (1) naphthalene; (2) 2-methylnaphthalene; (3)bmethylnaphthalene;(4) biphenyl; (5) diphenylmethane; (6) bibenzyl; (7) acenaphthylene; (8) fluorene; (9) phenthrene; (10) triphenylmethane.
than that of Carbowax-2OM. The straight lines shown in Figure 5 give the relationship between log tR' and the number of carbon atoms of n-alcohols or methyl esters of n-fatty acid when either Carbowax-ZOM or PSO-11-18C6 was used as a stationary phase; the slopes of the straight lines of PSO-1118C6 are greater than those of Carbowax-2OM, especially when the separated compounds are homologous of n-alcohols. Therefore, it can be concluded that PSO-11-18C1 has a higher selectivity for both alcohols and esters, because of hydrogen bonding forces and dipole-dipole forces, respectively. PSO-11-18C6 also has unique selectivity for aromatic com-
PSO-11-18C6 is a moderately polar stationary phase. The capillary columns coated with it have high column efficiency, a wide range of allowable temperature, good thermal stability and selectivity that depends on hydrogen bonding, dipoledipole interaction, and the size of the cavity of crown ether ring.
LITERATURE CITED (1) Li, Rushuen Sepu 1986, 4, 304. (2) Blomberg, L. G. TrAC, Trends Anal. Chem. 1987, 6(2), 41. (3) Jin, Yong-Hao; Fu, Ruo-Nong: Huang, Zai-Fu J . Chromatogr. 1989, 469, 153. (4) Fine, D. D.; Gearhart, H. L.; Mottola, H. A. Talanta 1985, 32, 751. (5) Rouse, C. A.; Fintinson, A. C.; Tarbet, B. J.; Pixton, C.; Djordjevie, N. M.; Markides, K. E.; Bradshaw, J. S.; Lee, M. L. A n d . Chem. 1988, 60, 901. (6) Bayona, J. M.; Tarbet. B. J.; et ai. I n t . J . Environ. Anal. Chem. 1987, 28. 279. (7) Kuei, J. C.; Tarbet, B. J.; et al. Chromatographla 1985, 20, 25. (8) Jones, B. A.; Bradshaw, J. S.; et al. J. Org. Chem. 1984, 4 9 , 4947-4951. (9) Blomberg, L. HRC CC, J . High Resolut. Chromatogr. Chromatogr. Commun. 1982, 5 , 520-533. (IO) Stark, T. J.; Larson, P. A. J. Chromatogr. S d . 1982, 20, 341. ( I 1) Huang, Shu; Xie, Ming-Gui HuaXue TongSao 1979, 2 , 44.
RECEIVED for review August 18, 1989. Accepted January 23, 1990. This work was supported by the National Science Foundation.
