The internal cyclization of a new phenylcliazene liquid crystal (called A), with an activated ... The Van't Hoff plot (of In reaction constant kagainst 1000/7).
HPLCStudy of the Intramolecular Cyclization of a New Nematogenic Molecule. The Gas Chromatographic Properties of the Resulting Liquid Crystal B. Sardat 1/ M. H. Guermouche 2. / C. Canlet 3 / p. Berdagu@ / J.- P.Bayl@ 1 Centre Universitaire, Laghouat, Alg6rie 2 Institut de Chimie, BP 32 El-Alia, Bab-Ezzouar, Alger, AIg6rie 3 Laboratoire de Chimie structurale, ICMO, Bt 410, Universit6 de Paris-Sud, 91405 Orsay-Cedex, France
KeyWords Column liquid chromatography Gas chromatography Nematic liquid crystal Intramolecular cyclization
Summary The internal cyclization of a new phenylcliazene liquid crystal (called A), with an activated methylene group in the ortho position to the cliazo linkage, has been studied. The kinetics of cyclization were studied at different temperatures and followed by HPLC.Separations were performed on a 30cm• silica column with n-heptane-tetrahyclrofuran-acetonitrile, 190:20:5 (v/v), as mobile phase. The Van't Hoff plot (of In reaction constant kagainst 1000/7) gives a mean activation energy of 101.3 • 2.1 kJ mol 1. The analytical properties of A and the final compound B during the decomposition were investigated by gas chromatography on home-made glass capillary columns coated with A. The retention times of the solutes tested became constant when the B/A ratio reached 5, which corresponds to 83% cyclization. The nematic phase of B has interesting properties enabling the separation of the isomers of clecalin, the positional isomers of cliethylbenzenes and phenols, and some polyaromatic hydrocarbons and their clerivatives.
Introduction Liquid crystals are anisotropic fluids [1]. Chemistry can be performed in such media and, because of the anisotropic packing of molecules, some energy modifications of the transition states are expected [2]. Unfortunately, the dissolution of molecules in liquid crystals leads to reduced mesophase stability or the disappearance of the liquid crystal phase. For this reason the reactions studied and reported in the literature are mainly monomolecular and
Chromatographia 2002, 55, January (No.
Original 0009-5893/00/02
the synthetic applications of the liquid crystal state are rather limited [3, 4]. These preliminary remarks suggest that if we wish to perform chemistry inside the liquid crystal phase it is obviously better to perform it on the liquid crystal molecules themselves. We have recently been interested in the internal cyclization of new phenyldiazene liquid crystals with an activated methylene group in the position ortho to the diazo linkage [5, 6]. The reaction involved in the cyclization is shown in Figure 1 and it
55- 07
$ 03.00/0
has been studied for a long time to obtain, in solution or in the melt, the 2H-indazole ring [7 9]. Because the starting material and the final mixture occur as the nematic phase, the cyclization can be obtained in the nematic phase or in the melt. With the laterally substituted mesogens described in Figure 1, we have followed the reaction kinetics in the melt only by observing the appearance of the nematic phase by use of polarizing microscopy. It was assumed that at any temperature the nematic phase starts to form when a fixed concentration of the 2H-indazole compound is produced. We therefore studied the kinetics of the reaction by tracking the time lap p for the first LC droplet to appear in the melt, at each temperature. It was supposed that cyclization of the compound obeyed first-order kinetics. A linear relationship was obtained between In(lapp) and 1/T and the activation energy was found to be E~ = 71.5• 116]. In this paper we describe the use of HPLC to follow the reaction kinetics in a related compound, with a different leaving group, in the solid, nematic, and isotropic phases (Figure 2). The gas chromatographic properties of the initial liquid crystal A during the decomposition were investigated. The final 2H-indazole compound B occurred as a nematic phase; its analytical properties were determined by use of gas chromatography.
1/2)
9 2002 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH
55
Clal'lzsO~
2
,0
H2C~'x
analytical properties of the final product B were performed by use of a third glass capillary column prepared in the same way and coated dynamically with a 10% solution of B in tetrahydrofuran.
A
~o,,,,,,,H-q + 112 C12H2sO
~
Figure 1. The intramolecular cyclizationreaction.
/=~
0-~,,,,,,0
Results and Discussion
N e m a t i c Mixture
A
Thermal Properties
B
HO--CIL
~
O--C6H13
O
Figure 2. The structures of the two mesogeniccompounds before and after cyclization. A
B
I
I
N
Cr
r
I
12
I1 9
Cr1
Cr 2
N
50
~ i 90
I
~O_.CHT
~__/
Cr
i
N 130
Cr
I 170
210
250
T PC
50
-~..~
o
N
9'0
' 130
I
' 170
2 0
250
T/~
Figure 3. DSC curves obtained for compounds A and B during the first cycleof heating and cooling (heating rate and cooling rate • 15o min 1 ).
Experimental
ChromatographicStudy
Reagents
HPLC was performed with a Waters chromatograph comprising a 600 pump, 7625 Rheodyne injector with 20-1xL sample loop, and Waters 900 diode-array detector. Compounds were separated on a 30 cm • 0.4 cm Microporasil column (Waters) with n-heptane-tetrahydrofuranacetonitrile, 190:20:5 (v/v), as mobile phase. The flow rate was fixed at 1 m L m i n 1. Data were collected with Millennium 32 software (Waters). Quantitation was performed at 250 nm. The void volume was determined as the retention volume of benzene. Gas chromatography was performed with a HP 5730A gas chromatograph equipped with dual FID and split/splitless injector. Helium of high purity was used as carrier gas. Three glass capillary columns (30 m • 0.25 mm i. d.) were prepared from borosilicate glass. After etching by the method of Rijks et al. [10] and deactivation with Carbowax 20M two capillaries were coated dynamically with a 10% solution of liquid crystal A in chloroform. To avoid decomposition of the liquid crystal the column was not conditioned. The
Solvents (chromatographic grade) were from Fluka (Switzerland); compounds studies as solutes were purchased from Aldrich (USA) and Meyreau-Boiveau (France).
KineticStudy The kinetics were studied by preparing a 1 mg mL 1 solution of the liquid crystal A in chloroform and transferring this solution to several 1-mL vials (0.25 mL in each) where the chloroform was evaporated. The residue was then heated at the appropriate temperature in a regulated oven. At selected times the vials were removed from the oven and frozen. The residues were then dissolved in T H F (1 mL) and analysed by HPLC. Titration of the mixture was performed by use of standard solutions containing an appropriate amount of the initial and final compounds A and B, respectively.
56
Chromatographia 2002, 55, January (No. 1/2)
The transition points were measured by differential scanning calorimetry (DSC; Mettler FP 52) at a heating rate of 15~ min 1. The DSC curves of A obtained during the first cycle are depicted in Figure 3A. Starting with the unmelted sample two endothermic transitions are encountered; these correspond to solid-nematic and nematic-liquid transitions. On further increasing the temperature a broad exothermic peak corresponding to the cyclization reaction is observed; its maximum is at 195 ~ During the cyclization, water is formed and leaves the medium at the reaction temperature, yielding the 2Hindazole compound. After standing at 250~ for 5 min the compound was cooled to room temperature. A nematic state occurs at 185 ~ and persists until 92 ~ The DSC curve during the second cooling is very similar to that obtained for the first cooling-although the exothermal peak was absent, because the sample now contained the compound B, with the result that the sample could no longer change chemically with temperature. The DSC curves obtained from pure B are presented in Figure 3B. Comparison of the transition temperatures for the reaction product and for the pure 2H-indazole gives an idea of the cleanness of the reaction. There is only a ten-degree decrease in the TIN temperature between the two DSC curves, showing that the yield of the reaction is quite high and without significant amounts of by-products.
KineticStudy The chromatograms obtained at 100 ~ shown, as an example, in Figure 4, characterize the evolution with time of liquid crystals A and B. Treatment of the data obtained shows that the cyclization reaction obeys first-order kinetics in the solid, nematic, or liquid A states. Table I shows the straight line equations obtained when the Naperian logarithm of the concentration of A is plotted against time for the different temperatures investigated. It is Original
A
A
B
Y=A+B*X
B
A=25.54457
B=-12.20424 R=-0.99691
0.15
]
4
i
-4
0.0 5.00
10.00 0.00
Minutes
220.00
80.00
-6.
nm
-8-
. . . .
E . . . .
5.00
i
-10
10.00
-
Liquid
M inutes
Nematic i
2,2
f
T=I30 ~
2,4
2,6
2,8
1000 / T
Figure 5. Van't Hoffplot (ofln k against 1000/ -1] ,
,
i
. . . .
5.00 Minutes
L
10.00
_7 4
I
0
100
"
260
time (rain)
5.00 M in ute s
10.00
Figure 4. HPLC chromatograms characterizing the evolution of liquid crystals A and B with time: (a) 30 min, (b) 60 min, (e) 120 min, and (d) 180 min of cyclization. The cyclization temperature was fixed at 130 ~ The UV spectra of A and B, obtained by use of the photodiode array detector, are also given (e). The plot of the Naperian logarithm of the concentration of A against time (f) is indicative of first-order cyclization of A obtained from HPLC data. Table I. The linear relationship between In(A) and time (min) at different temperatures. A State
T (~
Solid
72 77 82 90 93 103 113 120 130 145 150 156 160 165 170
Nematic
Liquid
In[A] 116 • 7283 • 621.7• 708.3 • 1698.8 • 465.4• 931.1• 573.4• 460.8 • 1428.3 • 402.1 • 376.8 • 987.7 • 1594.6• 464.3 •
interesting to note t h a t 90% of c o m p o u n d A is cyclized after 43692 m i n (30 days) at 70 ~ in the solid phase. This a m o u n t of cyclization takes 585 m i n (9.75 h) in the
Original
Correlation 10 4 6.99 • 10 4 9.94 • 4 1.046 • 10 4 2.83 • 10 4 3.28 • 10 4 11.1 • 4 33.6 • 10 4 38.4 • 10 4 114.4 • 10 4 270 • 10 4 343.7 • 10 4 445.7 • 10 4 736.2 • 10 4 911.9 • 10 4 1359.1 •
10 5t 10 5t 4t 10 4t 10 4t 10 4t 4t 10 4t 10 4t 10 4t 10 4t 10 4t 10 4t 10 4t 10 4t
0.971 0.960 0.962 0.998 0.983 0.995 0.996 0.999 0.995 0.993 0.995 0.994 0.999 0.992 0.997
nematic phase at 120 ~ a n d only 16 m i n at 170 ~ in the isotropic melt. The V a n ' t H o f f plot (of In reaction constant k against 1000/7) is given in Figure 5.
C h r o m a t o g r a p h i a 2002, 55, J a n u a r y (No. 1/2)
The m e a n activation energy over the whole range of t e m p e r a t u r e s is 101.3 • 2.1 k J m o l 1. It is a p p a r e n t t h a t the activ a t i o n energy increases slightly f r o m the solid phase (91.5 k J mol 1), to the nematic phase (103 k J m o l 1) a n d then to the isotropic melt (109 k J m o l 1). This m i g h t be related to the m o t i o n of the C H 2 O H group, which hinders nucleophilic attack of the lone pair of the n i t r o g e n of the diazo group. It can be emphasized t h a t this effect should be m o r e i m p o r t a n t with a larger lateral substituent, because the m o t i o n of this substituent will be restricted inside the liquid-crystal phase c o m p a r e d with t h a t in the isotropic melt. The activation energy values are higher t h a n t h a t f o u n d (E~ = 7 1 . 5 • 1) in the previous series, for which the leaving molecule was a para-substituted benzoic acid. The carboxylate is a better leaving g r o u p t h a n hydroxyl. This explains the different activation energies.
Analytical Properties of Liquid CrystalAduring the Decomposition Glass capillary columns coated with comp o u n d A were h e a t e d at 120 ~ at which A is present as the nematic phase a n d B as the solid phase, or at 130 ~ at which b o t h A a n d B are present as the nematic phases. Kinetic study shows t h a t the 95% of A decomposes after 750 m i n (approximately 13 h) at 120 ~ The same a m o u n t of t r a n s f o r m a t i o n was reached after
57
~
16
9 9 9
14
citronellal
9 bromonaphthalena 9 dimethylnaphthalene 9 methylnaphthatene
cJlronellol
9
ct-pinene
naphthalene tetrarnethylnaphthalene
12 10 06
i
"%
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i
9
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,
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6
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9
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o,~o 9
9
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~
9 9 9
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2,3-dirnethylphenol 3.4-dimethylphenol 3,5-dimethylphenol
9
~.
0126
9 --
9
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~
~i 15-
i
l'i.
E l
. . . . . .
[B]/[A]
[BI/[A]
Figure 6. Variation of the capacity factors for selected solutes with the ratio [B]/[A]when the capillary column coated with A was heated at 120 ~ (A is in the nematic phase and B is in the solid phase)9
0.45 0,40
9 9
u-pinene cLtronellal
9
citronellol
035
9 :
="~'='~-,. ~
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9
9
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9
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trans-decalin
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9 \k
9 9 9
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~ ?
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2,3-dimethylphenoi 3.4-dimethylphenol 3,5-dimethylphenol
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a'o
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82
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&
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9
~'s " ='o
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9
5
tO
15
20
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l
25
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IB]/[A]
Figure 7. Variation of the capacity factors for selected solutes with the ratio [B]/[A]when the capillary column coated with A was heated at 130 ~ (A and B are in the nematic phase)9
260 min at 130 ~ Two capillary columns were coated with A. Four mixtures were injected on to the first after 750 min at 120 ~ The same mixtures were eluted from the second column after 260 min at 130~ Plots of the capacity factors 58
Figure 8 shows the separation of three volatile aroma compounds (~-pinene, citronellal, and citronellol) during the decomposition of A at 120 ~
Analytical Properties of Liquid Crystal B
2 ~-
~
14
3,5-
\
o2e -
12
ClS
9
0'34
lO
[B/1/A]
[BI/tA]
04Z o 40
;, .~,, .... 8
the ratio of the B/A concentrations reach 5 (A decomposition = 83 %). Retention times were longer on nematic A than on solid (Figure 6) or nematic B (Figure 7) The shape selectivity of liquid crystalline stationary phases A and B was still efficient during the decomposition, even when B is solid or nematic. The straight trans-decalin isomer elutes after the cis isomer.
against B/A ratio are given in Figures 6 (120 ~ and 7 (130 ~ Several observations were made. For the four mixtures it seems that the retention times become constant when Chromatographia 2002, 55, January (No. 1/2)
The use of liquid crystals as stationary phases in gas-liquid chromatography was first reported by Kelker [11, 12] and Dewar et al. [13]. Liquid crystalline stationary phases in capillary columns are advantageous because of the combination of the shape-selectivity of the stationary phase with the efficiency of the capillary column [14]. We have previously reported several successful separations of different types of solute on liquid crystal stationary phases [15 21]. It was noted that analytical performance was essentially dependent on the mesogenic core. C o m p o u n d B contains the new 2H-indazole core which is fairly rigid, and we can expect some interesting analytical behaviour. This behaviour was tested for the solid, nematic, and liquid states. The results are presented in Table II. Several observations were made. Positional isomers (xylenes and diethylbenzenes) and geometric isomers (decalins) are not totally separated on the solid phase of B, and cis isomers are more retained than trans. trans-stilbene elutes before cis-stilbene on liquid B. The characteristic separative properties of liquid crystals are observed for the nematic phase of B, which enables the separation of: the positional isomers of diethylbenzenes, with the para isomer being more retained (a known property of liquid crystals, because of the rode-like shape and ordered arrangement of their molecules); geometric isomers (decalins and stilbenes), with cis isomers always eluting before trans in the nematic state, indiOriginal
L_
L... 6
t-to+
e3
"~"
1
co
k.___
6 f.q
Cq eq
l
......
04
d
5
io
1'5
2'0
Figure 8. Separation of ct-pinene (1), citronellal
(2), and citronellol (3) during the decomposition of A at 120~ (a) [B]/[A] = 0.14; (b) [B]/ [A] = 0.9; (c) [BI/[A] = 3.19; (d) [BI/[A] = 11.65; (e) [BI/[A] = 15.65. Original
2
4
6
8
I0
12
14
Figure 9. Separation of some phenols on the
Figure 10. Separation of some polyaromatic
nematic stationary phase. The experimental conditions are given in Table II.
hydrocarbons on the nematic stationary phase and on liquid B. The experimental conditions are given in Table II.
cating that the stretched form of the molecule is more retained, another characteristic of liquid crystal behaviour; the positional isomers of phenols and some derivatives (Figure 9), with the more stretched dimethylphenol (the 3,5 isomer) or trimethylphenol (the 2,3,5 isomer) again having the greater retention time; polyaromatic hydrocarbons and their derivatives (Figure 10 shows the separation of some polyaromatic hydrocarbons on the nematic and liquid forms of B); and volatile aroma compounds.
completely separate positional or geometric isomers. In the nematic state B had a characteristic separating properties of liquid crystals, e. g. the separation of decalin and stilbene isomers with the cis isomer eluting first, the separation of positional isomers of phenols and diethylbenzenes with the more stretched compound having the greatest retention time, and the separation of polyaromatic hydrocarbons and derivatives.
Conclusion
r
0
The cyclization kinetics of a new phenyldiazene liquid crystal with an activated methylene group in the position ortho to the diazo linkage has been studied by normal HPLC. Treatment of the data obtained show that the cyclization reaction follows first-order kinetics in the solid, nematic, or liquid state of compound A. The activation energy was found to be 101.4 kJ mol 1. During the decomposition it seems that the retention times becomes constant when the B/A ratio reaches 5 (decomposition o f A = 83 %). In contrast with the behaviour of polyaromatic hydrocarbons and their derivatives, retention of volatile aroma compounds, cis and trans isomers, and phenols on liquid crystal A was greater than on liquid crystal B. The primary results of the analytical study were that the solid state of B did not Chromatographia 2002, 55, January (No. 1/2)
References [1] Gray, G.W. Liquid Crystals and Plastic Crystals, Ellis Horwood, Chichester, 1974. [2] Leigh, J.W. Liquid Crystals, Applications and Uses, Vol. 2, Bahadur, B., Ed., World Scientific, 1991, p. 357. [3] Weiss, R.G. Tetrahedron 1988, 44, 3413. [4] Ramamurthy, V. Tetrahedron 1986 42, 5753. [5] Canlet, C.; Judeinstein, P.; Bayle, J.P.; Roussel, F.; Fung, B.M. Liq. Crystals 1999, 26, 281. [6] Canlet, C.; Khan, M.A.; Judeinstein, P.; Bayle, J.P.; Roussel, F.; Fung, B.M. New J. Chem. 1999,23, 1223. [7] Cava, M.P.; Noguchi, I.; Buck, K.T.J. Org. Chem. 1973, 38, 2394. [8] Fusco, R.; Marchesini, A.; Sannicolo, F. J. Heterocyclic Chem. 1987, 24, 773. [9] Bartsch, R.; Yang, I.-W. Y. Heterocyclic Chem. 1984, 21, 1063. [10] Franken, J.J.; Rutten, G.A.F.M.; Rijks, J.A.Y. Chromatogr. 1976, 126, 117. [11] Kelker, H. Ber. Bunsenger Phys. Chem. 1963, 67, 698. [12] Kelker, H. Z. Anal. Chem. 1963,198, 254. [13] Dewar, M.J.S.; Shr6eder, J.P.J. Am. Chem. Soc. 1964, 86, 5235. [14] Zhou, W.; Fu, R.; Dai, R.; Huang, Z.; Chen, Y. J. High Resol. Chromatogr. 1994, 17, 719. [15] Berdagu6, P.; Bayle, J.P.; Perez, F.; Courtieu, J.; Abdelhadi, O.; Guermouche, S.; 59
Table II. Relative retention times, r, o f some solutes on the capillary column coated with the final c o m p o u n d B. Compound Benzene derivatives Toluene o-Xylene m-Xylene p-Xylene Ethylbenzene Cumene Isobutylbenzene 1,2-Diethylbenzene 1,3-Diethylbenzene 1,4-Diethylbenzene Hexamethylbenzene Hexylbenzene Phenols and derivatives Phenol 2,3 -Dimethylphenol 2,4-Dimethylphenol 2,5-Dimethylphenol 2,6-Dimethylphenol 3,5-Dimethylphenol 3.4-Dimethylphenol 4-Ethylphenol 2,4,6-Trimethylphenol 2,3,5-Trimethylphenol 2,4,5-Trimethylphenol 2,4-Dinitrophenol o-Nitrophenol 4-Chlorophenol 2,4,6-Trichlorophenol 4-Chloro-3-methylphenol 4-Bromophenol Pentachlorophenol Volatile aroma compounds c~-pinene Eucalyptol Limonene Fenchone Camphor Citronellol Borneol Menthol Isopinocampheol Citronellal Carvone Carvacrol Bromocamphor Vanillin Polyaromatic compounds Naphthalene Perhydroanthracene 1-Methylnaphthalene Biphenyl 1,6-Dimethylnaphthalene Acenaphthylene Fluorene Phenanthrene Anthracene 9-Methylanthracene 1,3-Benzofluorene Ketones 2 '-Chloroacetophenone 2-Chloroacetophenone 1-Acetonaphthone 2-Acetonaphthone Benzophenone cis and trans isomers trans-Decalin cis-Decalin trans-Decalin cis-Decalin trans-Stilbene cis-Stilbene trans-Stilbene cis-Stilbene
60
T(~
B state
60 ~
Solid
r 1 1.06 1.06 1.06
1.21 1.21 1.27
1.36 1.37 1.37 1.81 2.2 135 ~
Nematic
1
2.54 1.79 1.94
1.15 2.88 3.48 2.54 1.74
4.12 3.93 140 ~ for 5 m i n then 10 ~ min 1 to 190 ~
Nematic
1 1.30
2.76 2.88 3.48 4.02 10.58 120 ~ for 3 min then 2 ~ min 1 to 180 ~
Solid then nematic
1 1.04
1.10 1.14 1.18 1.18 1.23 1.28
1.31 1.38 1.59 1.59 2.53 5.66 180 ~ for 4 min then 2 ~ min 1 to 240 ~
Nematic then liquid
1
1.10 1.23 1.34 1.43
1.61 2.17 4.39 4.63 5.99 6.99 140 ~ for 5 min then 4 ~ min 1 to 180 ~
Nematic
1 1.34
3.27 3.82 3.20 120 ~
Solid
160 ~
Nematic
1 1.01 1 0.75 1
205 ~
Liquid
0.45 1 1.02
C h r o m a t o g r a p h i a 2002, 55, J a n u a r y (No. 1/2)
Original
Guermouche, M.H.J. High Resol. Chromatogr. 1995, 18,304. [16] Berdagu6, P.; Bayle, J.P.; Perez, F.; Courtieu, J.; Abdelhadi, O.; Guermouche, S.; Guermouche, M.H. Chromatographia 1995, 40, 581. [17] Berdagu6, P.; Bayle, J.P.; Perez, F.; Courtieu, J.; Boudah, S.; Sebih, S.; Guermouche M.H. Bull. Soc. Chim. Ft. 1996, 133, 427.
Original
[18] Perez, F.; Berdagu6, P.; Courtieu, J.; Bayle, J.P.; Boudah, S.; Guermouche, M.H.J. Chromatogr. A 1996, 746, 247. [19] Perez, F.; Berdagu6, P.; Courtieu, J.; Bayle, J.P.; Boudah, S.; Guermouche, M.H.J. High Resol. Chromatogr. 1997, 20, 379. [20] Ammar-Khodja, F.; Guermouche, S.; Guermouche, M.H.; Berdague, P.; Bayle J.P. Chromatographia 1999, 50, 338.
Chromatographia 2002, 55, January (No. 1/2)
[21] Judeinstein, P.; Berdagu6, P.; Bayle, J.P.; Rogalska, E.; Rogalski, M.; Petit-Jean, D.; Guermouche, M.H.J. Chromatogr. A 1999, 859, 59. Received: May 17, 2001 Revised manuscript received: Aug 24, 2001 Accepted: Sep 3, 2001
61
