918
Anal. Chem. 1982, 5 4 , 918-924
Ligand-Exchange Gas Chromatography of Lower Aliphatic Amines on Solid and Liquid Crystalline Stationary Phases Kazuml Fujlmura, * Masanobu Kltanaka, Hlroakl Takayanagi, and Teiichi Ando Department of Industrial Chemistry, Faculty of Engineerlng, Kyoto Universjty, Sakyo-ku, Kyoto 606, Japan
Ligand-exchange gas chromatographic separatlon of lower aliphatic amines has been investigated by uslng a column of copper( I I)stearate coated on Chromosorb G AWIDMCS. A retention mechanism of the amlnes in the present system has been proposed and the complex formation constants between the amines and copper( I I ) stearate In the gas phase In the presence of ammonia vapor have been obtained. From the comparison of the retention data of ailphatlc amines obtained on a column of copper(I1) stearate In the solid state with those in the liquid crystalline state, the use of the latter has been found to be more suitable for adjusting both elution order and analysis time, because they can be varied much more easlly by changing the concentratlon of the mobile phase ligands and/or the column temperature If the iiquld crystalline Stationary phase Is used.
Ligand-exchange chromatography is a useful technique for the separation of organic compounds having complex-forming ability, such as nitrogen-, sulfur-, oxygen- or phosphoruscontaining compounds. In ligand-exchange gas chromatography the retention of the sample compounds depends on their gas-phase basicity as well as on their steric crowdedness; i.e., the smaller the gas-phase basicity and the bulkier the substituents around the donating element, the shorter the retention time (1,2). This retention behavior is quite different from that observed in conventional partition or adsorption gas chromatography. Furthermore, as will be described in this paper, the selectivity in ligand-exchange chromatography is dependent not only on the temperature but also on the concentration of the mobile phase ligand. These characteristics strongly suggest the suitability of the ligand-exchange gas chromatographic method for the separation of homologues or isomers of organic compounds. In the previous work of this series, pyridine bases were separated successfully by use of zirconium phosphate microcrystalline gels, an inorganic cation exchanger in metallic ion form, as stationary phase and nitrogen gas containing ammonia and water vapor as mobile phase (2). It was difficult, however, to eliminate completely the peak tailing which was due to the adsorption on the matrices of zirconium phosphate and the slow rate of the ligand-exchange reaction; neither the use of a metallic ion form which was capable of forming labile complexes with sample compounds nor the application of a gradient elution in which the concentration of mobile phase ligands was increased continually with elution time could solve the problem. Most of the recent investigations on gas chromatographic separation or analysis of lower aliphatic amines have been performed by adsorption (3-8) and partition (9-19) chromatography modes. In either of these, however, it is necessary to deactivate the surface of the adsorbents or supports by treatment with alkali hydroxide (4, 9, 11, 13, 15, 16) or phosphate (17) in order to obtain sharp symmetrical peaks. Moreover, the use of the columns treated with alkali hydroxide is limited when water is contained in the sample, because of 0003-2700/82/0354-0918$01.25/0
the low stability of the column, which in turn results in poor reproducibility in retention time. In such cases, the derivatization of free amines (6,10,12,14,19) and, sometimes, the addition of aqueous ammonia to the sample solution of free amines (9, 13)have been reported to be effective to reduce the peak tailing, though the derivatization is troublesome. On the other hand, some metal stearates, such as stearates of calcium(II), nickel(II), zinc(II), and cadmium(II), were used as an additive to the stationary phase in the separation of free aliphatic amines under the conditions of partition chromatography, and the interaction between amines and metal stearates was discussed (20). This paper will describe the results obtained when copper(II) stearate coated on Chromosorb G AW/DMCS was used as column packings instead of zirconium phosphate microcrystalline gels to improve the peak shape and to reduce the peak tailing in the ligand-exchange gas chromatographic separation of free aliphatic amines. The retention behavior of free amines based on ligand exchange with copper(I1) stearate in its liquid crystalline state will also be described.
EXPERIMENTAL SECTION Reagents and Column Preparation. All the aliphatic amines, listed in Table I, were of the highest quality available and were purchased from various suppliers. They were used as 1% v/v aqueous or hexane solutions without further purification. Some test mixtures were prepared so as to contain 1% of each amine. Commercially available copper(II), manganese(II), zinc(II), and sodium stearates were used as the stationary phase after purification by recrystallization from benzene or absolute methanol. For the determination of water in the mobile phase, Karl Fischer reagent “SS Mitsubishi” (titer: 3.0 mg H20/mL), anhydrous absorbing solvent “ME Mitsubishi” (a mixture of dried methanol and ethylene glycol) and a standard solution of water in methanol (titer: 2.0 mg H20/mL),purchased from Mitaubishi Chemical Industries Ltd. (Tokyo, Japan), were used as received. Glacial acetic acid which had to be added to the absorbing solvent to prevent the interferencefrom ammonia in Karl Fischer titration, was purified by distillation prior to use. All the other chemicals used were of reagent grade. Column packing materials were prepared by adding weighed Chromosorb G AW/DMCS, 80-100 mesh (Johns Manville, Denver CO), into a benzene solution of metal stearate of known concentration and then evaporating the solvent with a rotary vacuum evaporator; for sodium stearate, absolute methanol was used as the solvent. The amount of the stationary phase on each packing material was 3% or 5% by weight. After the dried packing materials had been resieved to 80-100 mesh fractions, they were equilibrated with ammonia vapor for 6 h to form metal-ammine complex by spreading them in a dish, which was placed in a desiccator containing concentrated aqueous ammonia at the bottom. Glass spiral columns of 4 mm i.d. and 3 m in length were prepared and were conditioned for at least 12 h by passing a constant flow (20 mL/min) of nitrogen containing ammonia and water vapor through the columns, at a temperature higher than the operating temperature by 5 “C or more. Apparatus and Procedure. A Hitachi gas chromatograph Model 0-23 (Hitachi Seisakusho Co. Ltd, Tokyo, Japan) equipped with a hydrogen flame ionization detector and an on-column sample injection system was used. The injection port was usually held at a temperature 10 “C higher than that of the column oven and a sample solution of 1.0 pL was injected using a microsyringe. 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
Table I. Liist of Aliphatic Amines Employed and Their Gas-Phase Basicities (GB) no. compound GB no. 1
2 3 4 5 6 I 8 9 10
11 12 13 14 15
ethylam ine n-propylamine isopropylamine n-butylamine isobutylamine sec-butylamine ter t-bu tylsunine n-amylamine isoamylamine meopent.ylamine tert-amylamine n-hexylamine n -hept ylamine 2-et hylhexylamine 1,1,3,3-tetramethylbutylamine
212.5 213.7 214.7 214.3 214.6 215.8 216.8 214.6
16 17 18 19 20 21 22 23 24 25 26 27 28 29
214.7 214.8
compound
GBa
dimethylamine diethylamine di-n -propylamine diisopropylamine N-ethyl-n -butylamine N-ethyl-tert-butylamine di-n-butylamine diisobutylamine di-n-amylamine diisoamylamine trimethylamine triethylamine tri.n-propylamine tri.n-butylamine
216.6 221.8 223.6 226.0
919
224.4
221.3 228.0 230.3
All values in kcal/mol (see ref 21). The detector was maintained at the same temperature an that of the colurnm oven. The column temperature, the flow rate of the nitrogen carrier gas, and the inlet pressure were 80" C,20.0 mL/min, and 0.96 kg/cm2, respectively, when the metal stearate was used in its solid state and chromatographic separation8 were performed in the gas-solid ligand-exchange mode. When copper(I1) stearate was used in its liquid crystalline state and the separation wm carried out in the gas-liquid ligand-exchange mode in mesophaeie, the column was kept at above 100 O C with the carrier gas flow rate of 12.0nnL/min and the inlet pressure of 0.85 kg/cm2. Ammonia and water vapor used as mobile phase ligands were fed into the column by passing, without bubbling, the carriler gas through a vessel, containing aqueous ammonia of appropriate concentratioin and placed in a thermostat, and sweeping away the generated vapor. The concentration of ammonia and/or vvater vapor in the carrier gas was adjusted by changing either the temperature of the thermostat or the concentration of the aqueous ammonia. The ammonia concentration in the mobile phase was determined by absorbing it ,at the column outlet in a standard hydrochloric acid solution during a fixed period and then titrating the excess of hydrochloric acid with a standard sodium hydrloxide solution. The concentration of water in the mobile phase was determined by the Karl Fischer method usnng a Model MK-13 moisture content titrator equipped with a dead-stop end-point detector (Kyoto Electronics Manufacturing Co. Ltd., Kyoto, (Japan);the mobile phase gas was absorbed for fixed time in the absorbing solvent containing dried glacial acetic acid and then the solution was titrated with the standard Karl Fischer reagent.
RESULTS AND DISCUSSION Effect of Ammonia Concentration in Mobile Phase on Retention and Resolution. In ligand-exchange chromatography a change in the concentration of ligands in the mobile phase exerts5 a pronounced effect on the retention of sample components. Table I1 lists the adjusted retention times of aliphatic amines obtained on a column of copper(I1) stearate at 80 "C under different sets of mobile phase conditions. It shows that, when the coated copper(I1) stearate is employed in its solid state, the retention times of all amines decrease with an increase in the concentration of ammonia in the mobile phase. However, the effect of ammonia concentration on retention of each amine is not even. It is most marked for primary amines, a little less for secondary amines, and the least for tertiary amines. This suggests that there is EL difference in sorption mechanism among primary, secondary, and tertiary amines. Table I1 also shows that the retention times of the amines having alkyl chains of a similar type such as normal- or iso-substituent increase with an increase in their chain length and those of the isomers decrease with an increase in the degree of branching in alkyl groups >whenthe concentration of aimmonia in the mobile phase is kept constant.
0
10
20
30
Retention Time.
40
50
60
70
minutes
Flgure 1. Effect of ammonia concentration in mobile phase on separation of mono-, di-, and tributylamines at 80 OC. Column: 5% copper(I1) stearate, 4 mm i.d. X 3 m. Flow rate: 20.0 mL/min. Ammonia concentration in mobile phase (pmol/mL): (a) 13.51, (b) 9.28, (c) 6.85, (d) 4.51, (e) 1.82. Water concentration in mobile phase (pmol/mL): (a) 0.97, (b) 0.90, (c) 0.84, (d) 0.80, (e) 0.77. Peak numbers refer to compounds listed in Table I .
I t is to be noted that not only the retention time of each amine but also the overall elution order and resolution are affected by the change of the concentration of ammonia in the mobile phase. These results differ significantly from those obtained on a column of zirconium phosphate of a metal-ion form, where neither the elution order nor the resolution could be changed by the change in the concentration of ligands in the mobile phase (2). The drastic effect of ammonia concentration on the separation of a mixture of mono-, di-, and tributylamines is illustrated in Figure 1. The selection of the most suitable ammonia concentration is, therefore, of crucial importance for obtaining the optimum analysis time and resolution by the present system. Effect of Metal Species in Stationary Phase on Retention and Resolution. Adjusted retention times of some typical primary, secondary, and tertiary amines measured on columns of manganese(I1) and zinc(I1) stearates in a similar manner as on a copper(I1) stearate column are shown in Figures 2 and 3, respectively, as a function of the concentration of ammonia in the mobile phase. With regard to the effect of ammonia on retention of amines, a similar tendency has been observed on both manganese(I1) and zinc(I1) stearate columns as was found on a copper(I1) stearate column, except for secondary amines with longer alkyl chains and all tertiary amines on a zinc(I1) stearate column at the concentration of ammonia below 7 yM/mL. The reason for this somewhat
920
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
Table 11. Effect of Ammonia Concentration in Mobile Phase on Adjusted Retention Times (tR')of Aliphatic Amines at 80 "C t R ' , m i q Qat
NH, concn in mobile phase (hmol/n&) of
no.
compound
1.04
2.79
4.68
6.51
8.60
10.80
13.21
15.39
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
ethylamine n-propylamine isopropylamine n-butylamine isobutylamine sec-butylamine tert-butylamine n-amylamine isoamylarnine neopentylamine tert-amylamine n-hexylamine n-heptylamine 2-ethylhexylai-nine 1,1,3,3-t etramethylbutylarnine dimethylamine diethylamine di-n-propylamine diisopropylamine N-ethyl-n-butylamine N-ethyl-tert-butylamine di-n-butylamine diisobutylamine di-n-amylamine diisoamylamine trimethylamine triethylamine tri-n-propylamine tri-n-butylamine
11.4 15.6 4.1 33.5 13.3 6.5 2.1 74.6 52.7 9.6 4.4
3.7 4.9 1.3 10.8 4.5 2.3 0.9 23.7 17.0 3.1 1.7 55.5 127.0 83.2 5.3 1.4 0.7 1.5 0.5 2.2 0.7 5.9 1.8 26.7 14.3 0.2 0.5 2.2 14.6
2.3 2.9 0.9 6.4 2.7 1.5 0.6 14.3 10.3 2.0 1.1 32.7 74.8 48.2 3.9 0.9 0.6 1.2 0.5 1.7 0.6 4.7 1.7 20.7 11.2 0.2 0.5 2.2 14.4
1.6 2.2 0.7 4.7 2.0 1.2 0.5 10.5 7.7 1.5 0.9 24.1 54.7 35.9 3.3 0.6 0.5 1.1 0.4 1.4 0.6 4.3 1.6 18.4 10.0 0.1 0.5 2.2 14.4
1.2 1.7 0.6 3.7 1.6 1.0 0.4 8.4 6.1 1.2 0.8 18.8 43.2 29.0 3.0 0.5 0.5 1.0 0.4 1.3 0.5 4.0 1.6 17.0 9.3 0.1 0.5 2.2 14.3
1.0 1.5 0.5 3.2 1.4 0.8 0.4 7.1 5.1 1.1 0.7 16.7 37.7 24.8 2.7 0.5 0.4 1.0 0.4 1.2 0.5 3.8 1.5 16.0 8.8 0.1 0.5 2.2 14.0
0.8 1.2 0.4 2.7 1.1 0.7 0.3 6.1 4.4 0.9 0.6 14.2 32.6 21.5 2.6 0.4 0.4 0.9 0.4 1.1 0.5 3.6 1.5 15.4 8.5 0.1 0.4 2.1 14.0
0.7 1.o 0.4 2.4 0.9 0.6 0.3 5.5 4.2 0.8 0.5 12.4 28.5 19.1 2.4 0.3 0.3 0.9 0.3 1.0 0.4 3.5 1.5 15.4 8.3 0.1 0.4 2.1 13.9
16.0 5.2 1.7 2.8 0.7 4.6 0.9 11.7 2.5 58.8 30.0 0.4 0.7 2.3 15.0
a Retention time of methane was used as t o . Column: 5% copper(I1) stearate coated on Chromosorb G AW/DMCS (80-100 mesh), 4 mm i.d. Y, 3 m. Flow rate: 20.0 mL/min. Carrier gas inlet pressure: 0.96 kg/cm2. H,O concentration
in mobile phase: 0.75-1.12 pmol/mL.
2.0
-
15-
'E
10-
c
0.5
-
O t
L 50
10 0
150
NH3 concn , pMlrnL
O t
L >
L
50
I
10 0
P
_
15 0
NH3 concn , p M l r n L
pounds listed in Table I.
Figure 3. EffectOf ammonia concentration in mobile phase on log t,' of aliphatic amines at 80 O C . Column: 5% zinc(I1)stearate, 4 mm i.d. X 3 m. Flow rate: 20.0 mL/min. Water concentration in mobile Phase: 0.54-0.76 kml/mL. Curve numbers refer to compounds listed in Table I.
abnormal retention behavior is not clear a t present, though it may be ascribed in part to the impurities such as zinc(I1) palmitate or zinc oxide which could not be removed by recrystallization of zinc(I1) stearate, and in part to the difference in sorption mechanism between primary and secondary or tertiary amines as will be discussed later. Concerning the
effect of the difference of the metal species on retention, it is clear from Figures 2 and 3 as well as from Table I1 that the retentive properties of these three stationary phases for primary amines decrease in the order zinc > manganese > copper stearates. The same order is observed for secondary amines if the concentration of ammonia in the mobile phase
Figure 2. Effect of ammonia concentration in mobile phase on log t; of aliphatic amines at 80 O C . Column: 5% manganese(I1) stearate, in 4 mm ;,d. x 3 m. ~i~~ rate: 20.0 mL/min. Water mobile phase: 0.72-0.82 pmol/mL. Curve numbers refer to corn-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
is higher than 7 wM/mL. It, changes, however, to zinc > copper > manganese stearates for tertiary amines under the same conditions. These facts show that zinc(I1) stearate interacts more strongly with aliphatic amines than manganese( [I) or copper(I1)stearates do. Probably, the difference in t$ values of the metal species is correlated with the difference in their complex-forming abilities. It may well be added that the elution order of all amines, and hence the resolution, are also varied by metal species, even if the Concentration of ammonia in the mobile phase is kept constant. Sorption Mechanism. During the process of conditioning of a column of copper(I1) stearate coated on Chromosorb G AWJDMCS by passing the nitrogen carrier gas containing ammonia and water vapor through it, the following equilibria must be established in the column: StZCu(NH3)4 HzO + S ~ ~ C U ( N H ~ ) ~ ( H ZNH3 O) (1)
+
/
9
0
6
/
921
/
i
: p /
+
+
S ~ ~ C U ( N H ~ ) ~ ( H ZHzO O) S t z C u ( ~ H 3 ) z ( W ) z+ 3" StzCu(NH3)2(Hz0)2
+ HzO + St&u(NH,)(H,0)3
+
S ~ ~ C U ( N H ~ ) ( H , OHzO ) ~ + StzCu(H2(3)4
+ NH,
+ NH3
(2) I / ( "3
(3)
(4)
where the symbol St indicates the stearate moiety. It is very probable, however, that these reactions do not proceed virtually because the concentration of water in the mobile phase is sufficiently low and the complex-forming ability of water is much lowler than that of ammonia. Therefore, the St,Cu(NH3)4species can be considered approximately to be the only species that exists in the column. This indicates that the analytical concentration of' copper(II), Ccu, in packing materials in a unit column volume is nearly equal to [StCu(NH,),] and can be regarded as a constant for a given column, i.e.
1
Figure 4. Relationship betwee k'of some primary amines and inverse of ammonia concentration in mobile phase at 80 O C . Column: 5 % copper(I1)stearate, 4 mm i.d. X 3 m. Flow rate: 20.0 mL/min. Water
concentration In mobile phase: 0.75-1.12 pmolirnL. Line numbers refer to compounds listed in table I.
6
I t
1,-
Ccu
(5) When a s,ample of an aliphatic amine, represented by A, [ S ~ ~ C U ( N H=~ coqst )~]
is introduced into the column, the following ligand-exchange reaction takes place with the corresponding equilibrium constant, P StZCu(NH3)d
+A
St&u(NH,):,A
+ NH3
k'
-
(6)
(7)
Ll
23
-il-L-p-b-b--L-
Since the 'distribution constant, Kd,of the aliphatic amine is given by
~o-o-o-o-~-o---
=w-~-~-~-n-
O O
0 1
-_
1
0 2
i E
18 17
27
L--i---
0.3
0.4
X103
I/("3]
the capacity factor, k', in this case can be expressed by the following equation using the phase ratio of the column, $
k'=
P -
[NH3J ccurc.
(9)
Equation 9 suggests that i fthe retention off aliphatic amines is based only on ligand-exchange mechanism, a plot of k'vs. inverse of arnmonia concentration in the mobile phase will give a straight line passing through the origin and the slope of the line indicate the equilibrium constant, P, of the ligand-exchange reaction. If that is not the case, i.e., if the line has an intercept and/or is not straight, it means that another sorption mechanism should be considered. On the basis of these theoretical considerations, k ' values of some typical amines were plotted against 1/[NH3], and the results are shown in Figures 4 and 5. It is found from Figure 4 that primary amines with shorter alkyl chains are retained by ligand-exchangesorption mechanism. However, the liner3 for
Flgure 5. Relationship between k' of some secondary and teritary amines and inverse of ammonia concentration in mobile phase at 80 O C . Cocditions are as in Figure 4. Line numbers refer to compounds listed in Table I.
primary amines with longer alkyl chains and those for all secondary and tertiary amines have an intercept, implying the deviation from ligand-exchange sorption mechanism. Concerning the retention mechanism of tertiary amines, it is considered that the hydrophobic interaction between the hydrocarbon parts of the sample amine and the stearate moiety plays rather a main role for the sorption, because the nitrogen atom in tertiary amines is surrounded by alkyl groups to such an extent that the complex formation with the metal in the stationary phase in hindered. This is supported by the fact that their k'values are little affected by the change of the concentratioin of ammonia in the mobile phase, and hence the plots of k' vs. 1/[NH3] result in straight lines almost parallel to the abssissa as shown in Figure 5. On the other hand, the plots for secondary amines show a similar tendency to those for primary amines, but the slopes are small and the
922
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
Table III. Net Retention Volumes (V,) of Aliphatic Amines Obtained on Ligand-Exchange, Hydrophobic Interaction, and Partition Chromatography Modes at 80 "C VN,
Table IV. Values of Slope and P for Some Aliphatic Amines compound ethylamine n-propylamine isopropylamine n-butylamine isobutylamine sec-butylamine tert-butylamine n-amylamine isoamylamine tert-amylamine n-hexylamine diethylamine di-n-propylamine di-n-butylamine diisobutylamine di-n-amylamine diisoamylamine
a'
copper( 11) sodium stearate stearate PEG 20M column column column [",Ib compound [H,0le ethylamine n-propylamine isopropylamine n-butylamine isobutylamine sec-butylamine tert-butylamine n-amylamine isoamylamine tert-amylamine n-hexylamine diethylamine di -n-propy lam ine diisopropylamine di-n-butylamine diisobutylamine di-n-amylamine diisoamylamine trimethylamine triethylamine tri-n-propylamine tri-n-butylamine
2.50 0.35 82.6 92.9 13.7 188.2 92.9 35.6 6.9 408.6 298.4 19.5 929.5 5.7 16.1 4.6 101.0 20.7 441.8 219.2 1.1 4.6 24.1 163.0
2.49 0.38
2.50 0.53
36.7 39.3 18.4 81.3 35.4 24.9 13.1 171.7 128.5 28.8 397.2 19.7 38.0 13.1 173.0 66.9 901.7 464.0 3.9 17.0 104.9 977.9
82.2 90.0 41.7 183.9 71.7 65.2 31.3 327.3 237.2 50.9 596.0 52.2 82.3 28.7 279.1 80.9 989.3 530.8 7.8 27.4 75.6 366.4
V , values were calculated from VN = ~ V R 'where , j is pressure drop correction factor and VR' is adjusted retention volume. Column: 5% of each stationary phase coated on Chromosorb G AW/DMCS (80-100 mesh), 4 mm i.d. x 3 m, flow rate: 20 I0.1 mL/min. NH, concentration in the mobile phase (pmol/mL). H,O concentration in the mobile phase (pmol/mL). a
intercepts are large, compared with those of primary amines. Therefore, it is reasonable to assume that secondary amines are retained by a mixed sorption mechanism based on ligand-exchange and hydrophobic interaction. The contribution of hydrophobic interaction for the sorption of primary amines was also recognized from the retention data obtained on a sodium stearate coated column where the retention by ligand-exchange cannot occur. As can be seen from Table 111, however, the sorptivity of primary amines based on hydrophobic interaction is about 2.2 to 2.6 times smaller than that based on the ligand-exchange mechanism, and besides it differs clearly from that based on the gas-liquid partition mechanism on a PEG 20M column. Therefore, it may be concluded that primary amines are retained mainly by the ligand-exchange sorption mechanism, except for the samples with a bulkly substituent such as isopropylamine, secand tert-butylamines, and tert-amylamine. The values of the slopes determined by the least-squares method and 0's for some amines obtained from Figures 4 and 5 and eq 9 are listed in Table IV. The larger V , values for secondary and tertiary amines on a sodium stearate column than on a copper(I1) stearate column may be explained as being due to the larger steric hindrance for both complex formation and hydrophobic interaction with a copper(I1) stearate molecule than with a less bulky sodium stearate molecule. Effect of Water Vapor in Mobile Phase on Retention. Table V lists the adjusted retention times of some primary, secondary, and tertiary amines measured on a column of copper(I1) stearate in the presence and absence of water vapor in the mobile phase: only primary amines show a significant increase in tR' values if water is not present in the mobile
slopea 3.05 x 3.91 x 8.21 x 8.82 x 3.62 x 1.67 x 5.47 x 1.90 x 1.30 X 1.10x 3.92 X 4.69 x 6.79 x 2.44 x 3.26 x 1.13 X 6.47 x
Measured from Figures 4 and 5. eq 9 using r?i = 0.03 and Ccu = 45.9
Pb
10-3 10-3
10-4 10-3 10-3 10-3
10-4 10-2
10.' 10-3
10-4
10-3 10-4
lo-'
10-3
2.14 2.7 5 0.58 6.21 2.55 1.18 0.39 13.38 9.15 0.77 27.61 0.33 0.48 1.72 0.23 7.96 4.56
Calculated from
a
X
M.
Table V. Effect of Water Vapor in Mobile Phase on Adjusted Retention Times ( t R ' )of Some Aliphatic Amines at 80 "C compound et hylamine n-propylamine n-bu tylamine isobutylamine sec-butylamine tert-butylamine n-amylamine n-hexylamine n-heptylamine di-n-propylamine di-n-butylamine di-n-amylamine triethylamine tr i-n -propy lam ine tri-n-butylamine
tR', min,a at [H,O]of 0 0.80 3.8 3.9 8.8 3.6
1.7 0.9 19.2 43.6 101.3 1.2 4.6 20.3 0.4 2.3 15.1
2.0 2.8 6.0 2.5 1.4 0.6 13.5 30.2 67.6 1.2 4.6 19.5 0.5 2.2 14.5
a Column: 5% copper(I1) stearate coated on Chromosorb G AW/DMCS (80-100 mesh), 4 mm i.d. X 3 m. Flow rate: 20.0 mL/min. NH, concentration in mobile phase: 5.03 pmol/mL. H,O concentration in mobile phase (Mmol/mL).
phase. This fact reveals that for the sorption of primary amines, water vapor acts as an additional mobile phase ligand so long as its concentration is relatively high. On the contrary, none of the sample amines could be eluted in a reasonable time if the mobile phase contains no ammonia, even at high concentration of water. Separation Based on Gas-Solid Ligand-Exchange Mode. In Figure 6 is shown a chromatogram of a mixture of primary amines of C5-C8 on a copper(I1) stearate column at 80 "C. Since the copper(I1) stearate is still solid at this column temperature, the chromatographic separation is based on the gas-solid ligand-exchange mode. The elution order of sample components in ligand-exchange gas chromatography is generally associated with their gas-phase basicity (21); i.e., the larger the basicity in the gas phase, the longer the retention time. However, the steric hindrance for complex formation between sample components and the metal in stationary phase also plays an important role in determining the elution order. As can be seen from Figures 1and 6 and Table 11, the elution order of the isomers agrees with the order of decrease in the branching of their alkyl groups, although the gas-phase basicity
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
92:3
I
/
al
-
c
...
Q 0
al [I
L
L
-cn
..
0
al
E
29
'-P
U
aJ
E
/-------
18
AI.
0
40
20
60
2
103
80
Flgure 6. Separation of a mixture of Cs-C8 primary amines by gassolid ligand-exchange mode at 80 "C. Column: 5 % copper(I1) stearate, 4 mm i.d. X 3 m. Flow rate: 20.0 mL/min. Ammonia concentration in mobile phase: 4.68 pmol/mL. Water concentration in mobile phase: 0.80 pmol/mL. Peak numbera refer to compounds
listed in Table I.
I
120
130
Column Temperature ,
"C
110
Retention Time, m i n u t e s
Flgure 8. Effect of column temperature on retention. Column: 3%) copper(I1) stearate, 4 mm i.d. X 1 m, flow rate: 20.0 mL/min. Ammonia concentration in mobile phase: 12.00 KmollrnL. Water concentration in mobile phase: 0.95 pmol/mL. Line numbers refer to
compounds listed in Table I.
I
0
-
L - - L - . - I - - -
0
10
Retention Time.
20 minutes
_I
30
Flgure 7. Separation of mixtures of (a) Secondary and (b) tertiary amines by gas-solid ligand-exchange mode at 80 "C. Column: 5% copper(I1) stearate, 4 mm i d . X 3 m. Flow rate: 20.0 mllmin. Ammonia coricentration in mobile phase &mol/mL): (a) 9.28, (b) 6.85. Water concen??ationin mobile phase (pmol/mL): (a) 0.90, (bl 0.84.
Peak numbers refer to compounds llsted in Table I. increases with an increacie in branching. Therefore, it is reasonable to conclude tlhat the elution order of aliphatic primary amines is determined by the balance of their basicity effect in gas phase and steric hindrance for complex formation, because primary amines are retained mainly by ligand-exchange mechanism as discussed above. It is to be noted, however, that although the basicity of the homologue increases generally with an increase of the alkyl chain length, thie hydrophobic interaction between alkyl groups of the sample and the stearate moiety also increases a t the same time, so it is difficult to distinguish between these two effects. The separation of a mixture of secondary or tertiary amines was also aclhieved successfully as shown in Figure 7. The
30
60 Retention Time, minutes
ll
I
"
90
Flgure 9, Dependence of elution order and of peak resolution on column temperature. (a) 80 O C , (b) 100 "C, (c) 115 "C. Column: 3%) copper(I1) stearate, 4 mm i.d. X 3 m. Flow rate: 12.0 mL/min. Ammonia concentration in mobile phase: 7.50 pmol/mL. Wateir concentratlon in mobile phase: 0.82 Kmol/mL. Peak numbers refeir to compounds listed in Table I.
elution order of the isomers of secondary amines again agreeFi with the order of the decrease in branching in alkyl groups just as in primary amines. Since the ligand-exchange sorptiori mechanism is still effective for the sorption of secondary amines on a copper(I1) stearate coated column, the selectivities between the isomers are better than those obtained on a sodium stearate coated column, as predicted from the retention data listed in Table 111. On the contrary, the selectivities of tertiary amines were better on a sodium stearate coated1 column than on a copper(I1) stearate coated column. Retention Behavior on Liquid Crystalline Phase. Copper(I1) stearate is known to behave as a liquid crystal in the temperature range from 112 to 230 "C (22). In order to examine the ligand-exchange retention behavior and selectivity of aliphatic amines on a liquid crystalline stationary phase, the retention times of some typical amines were measured on a copper(I1) stearate coated column a t various temperatures between 110" and 135 "C. The results are shown as the plots of log tR' vs. column temperature in Figure 8. The tR' value, first increases markedly with an increase of column temper-
924
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
The effect of the column temperature on retention and selectivity is manifested evidently in a chromatogram shown in Figure 9. Since the retention time, and hence the elution order, are varied not only by the concentration of the mobile phase ligand but also by the column temperature, the ligand-exchange gas chromatographic separation on a liquid crystalline phase possesses greater advantages over those on a solid phase. A successful separation of butylamine isomers was achieved by means of a temperature programming method, from 115 to 100 O C , as shown in Figure 10. I
LITERATURE CITED
I 0
13
30
20
Retenltcn Time
mlnutes
Flgure 10. Separation of mono-, di-, and tributylamines using liquid
crystalline stationary phase under temperature programming from 115 to 100 O C . Conditions are as In Figure 9. Peak numbers refer to compounds as listed in Table I. ature from 110 O C , reaches its maximum at around 115 "C, and then decreases gradually. The temperature of 115 "C corresponds to the phase transition point of copper(I1) stearate, i.e., the temperature at which the solid melts to form the smectic phase. A similar phenomenon has been observed in partition chromatography on conventional liquid crystalline stationary phases. It is also to be noted here that not only the retention time but also the elution order vary with the increase of column temperature from 110 to 115 "C. The magitude of the increase in the retention time is greater for primary and secondary amines carrying linear alkyl chains than for tertiary amines. This fact is explained as follows: since the copper(I1) stearate molecules are arranged regularly in its liquid crystalline state, sample amines having higher length-to-breadth ratios can enter the liquid crystalline phase and can form complexes more easily than those having lower length-to-breadth ratios. The possibili'ty that the separation on a liquid crystalline stationary phase is based on partition rather than on ligand exchange can be ruled out because the retention times of the amines become much longer if there is no mobile phase ligand such as ammonia in the carrier gas.
(1) Fujimura, K.; Ando, T. J. Chromatogr. 1975, 174, 15-21. (2) Fujimura, K.; Ando, T. Anal. Chem. 1977, 49, 1179-1182. (3) Schomburg, G.; Husmann, H.; Behiau, H. Chromatographia 1980, 13, 321-333. (4) Kuwata, K.; Yamazaki, Y.; Uebori, M. Anal. Chem. 1980, 5 2 , 1980-1982. (5) Sugii, A,; Harada, K. J . Chromafogr. 1979, 778,71-78. (6) Hoshika, Y. Anal. Chem. 1078, 4 8 , 1716-1717. (7) Andre, C. E.;Mosier, A. R. Anal. Chem. 1973, 45, 1971-1973. (8) Dave, S. B. J. Chromatogr. Sci. 1989, 7 , 389-399. (9) Oaiene, M.; Mathiasson, L.; Jonsson, J. A. J. Chromatogr. 1981, 207, 37-46 -.
(10) Hamano, T.; Hasegawa, A.; Tanaka, K.; Matsuki, Y. J. Chromatogr. 1979, 179, 346-350. (11) Corcia, A. D.; Samperi, R.; Severini, C. J. Chromatogr. 1979, 170, 325-329. (12) Hoshika, Y. Anal. Chem. 1977, 4 9 , 541-543. (13) Dum, S. R.; Simenhoff, M. L.; Wesson. L. G., Jr. Ana/. Chem. 1978, 48, 41-44. (14) Hoshika, Y. J. Chromatogr. 1975, 775,596-601. (15) Corcia, A. D.; Liberti. A.; Samperi, R. J. Chromatogr. Sci. 1974, 12, 7 10-714. (16) Corcia, A. D.; Samperi, R. Anal. Chem. 1974, 4 6 , 977-981. (17) Goiovnya, R. V.; Zhuravieva, I. L. Chromatographla 1973, 6 , 508-513. (18) Metcaife, L. D.; Martin, R. J. Anal. Chem. 1972, 4 4 , 403-405. (19) Irvine, W. J.; Saxby, M. J. J. Chromafogr. 1989, 43, 129-131. (20) Castelis, R. C.; Catoggio, J. A. Anal. Chem. 1070, 4 2 , 1268-1271. (21) Aue, D. H.; Webb, H. M.; Bowers, M. T. J. Am. Chem. Soc. 1972, 94. 4726-4728. . -- . -(22) Takekoshi, M.; Watanabe, N.; Tamamushi, 8. Colloid Polym. Sci. 1078, 256,588-590.
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RECEIVED for review November 10, 1981. Accepted January 18, 1982.
