4) 5) 6) 7) 8)
-
0.03M FeC13-60% MeOH, 0.6 X 50 cm, 0.3 1.0 ml/ min. 3-Methy1(7-13-33) 5-Sulf0(50-81-140) 0.03M FeC13-60% MeOH, 0.6 X 50 cm, 0.3 1.0 ml/ min. 3-Nitro(7-12-28) 5-Sulf0(57-88-150) 0.03M FeC13-60% MeOH, 0.6 X 50 cm, 0.3 1.0 ml/ min. 4-Nitro(10-13-23) 5-Sulf0(50-65-92) 0.07M FeC13-60% MeOH, 0.6 X 50 cm, 0.3 0.5 ml/ min. 5-Nitro(17-26-44) 5-Sulf0(50-65-98) 1.0 ml/ 0.07M FeC13-60% MeOH, 0.6 X 50 cm, 0.3 min. 4-Hydroxy(16-22-31) 5-Sulf0(50-67-97) 0.07M FeC13-60% MeOH, 0.6 X 50 cm,0.3 1.0 ml/min.
-
-
-
-
-
-
-
-
-
-
9) Salicylic(27-47-111) 5-Chloro(120-175-240) 0.30M FeC13-H20,0.6 X 50 cm, 0.5 1.0 ml/min. 10) 3-Nitro(43-77-110) 5-Nitro(120-165-240) 0.30M FeC13-H20,0.6 X 50 cm, 0.5 1.0 ml/min. 11) Salicylic( 17-34-78) 5-Nitro(90-125-160) 0.30M FeCla-HzO, 0.6 X 45.5 cm, 1.0 f 0.1 ml/min.
-
-
ACKNOWLEDGMENT The authors thank J. S. Choi, C. S. Han, S. J. Kim, and K. S. Chung for their continued interest and helpful advice. It is a pleasure to acknowledge the financial support of T. S. Park, the President of Yonsei University. RECEIVEDfor review January 9, 1974. Accepted May 6, 1974.
Silica-Containing Cross-Linked Polymer as Highly Efficient and Thermally Stable Stationary Phase for Gas Chromatography
Wilhelm Bruening, 1. M. R. de Andrade Bruening, and A. L. Scofield Petroleo Brasileiro S A . , Centro de Pesquisas e Desenvolvimento (PETROBRAS/CENPES), llha do Fundio, Rio de Janeiro, Brazil
A new type of silica-containing polymer has been developed, consisting of submicroscopic silica particles trapped in a cross-linked polymer matrix. The silica/polymer ratio has considerable influence on the phase and column properties. A detailed study resulted that stationary phases with about 40 % polymer content show maximum performances in gas chromatography. The new phase was employed in different types of hydrocarbon analysis. Its major characteristics were high thermal stability, high selectivity, and high efficiency.
Recently (I), silica-containing polymers have been introduced as a new type of stationary phase for gas chromatography. Their major characteristics were high speed, high selectivity, and high efficiency for the analysis of polar and nonpolar substances. The synthesis of phases suitable for polar compounds involved a deactivation of silica surface hydroxyl groups and formation of chemical bondings between the organic and inorganic components. For nonpolar substances the synthesis was simpler, just requiring an intimate and homogeneous mixing of the silica with the polymer. Nevertheless, these phases presented a relatively low working temperature limit. After being used above 200 "C, their efficiencies decreased considerably. To overcome this problem, Bruening and de Andrade Bruening ( I ) suggested polymer cross-linking. This paper describes the synthesis and properties of a cross-linked silica-containing polymer, suitable for nonpolar compound analysis. The influence of polymer content on phase characteristics is evaluated, and its application in different types of hydrocarbon analysis is presented. (1) Wilhelm Bruening and I. M. R. de Andrade Bruening, Anal. Chem., 45,
1169(1973).
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* ANALYTICAL CHEMISTRY, VOL.
EXPERIMENTAL Reagents. The following reagents were used in the synthesis: Silica (Cab-o-Sil), grade M5, from Cabot do Brasil, SBo Paulo; diethylenetriamine, technical grade from Eastman Kodak Company, Rochester, N.Y., and DDI-1410 diisocyanate ('236 dimer diisocyanate) from General Mills, Minneapolis, Minn. Standards. The light hydrocarbon standards were from Phillips Petroleum Company, Bartlesville, Okla., and the olefins from Polyscience Corporation, Evanston, Ill. The polycyclic compounds and the terpenes were graciously provided by Naegeli S.A. IndGstrias Quimicas, Rio de Janeiro, and Givaudan Companhia Brasileira, Sgo Paulo, respectively. Apparatus. The chromatograph used was a Perkin-Elmer 900, equipped with a flame ionization detector using nitrogen as carrier gas. The instrument was connected to a 1-mV Leeds and Northrup Speedomax G recorder. The carrier. and detector gases were dried in molecular sieve and silica gel filters. The thermal analysis was carried out with a Du Pont Model 990 Thermogravimetric Analyzer, and the polymer contents were determined gravimetrically in a high temperature furnace a t 900 "C. Column Preparation. The phase after being crushed and sieved was easily packed into one meter, ?kin. 0.d. X %,j-in. i.d. copper tubing, using the normal procedure with vibration and slight tapping. Synthesis. Ten grams of silica (Cab-o-Sil, M5) were dried overnight at 500 "C, transferred into a round-bottomed flask and dispersed in about 400 ml of dried xylene. Then 7.20 g of DDI-1410 diisocyanate were added all at once, followed by the dropwise addition under magnetic stirring of 0.80 g of diethylenetriamine dissolved in 100 ml of dried xylene. The flask was equipped with a reflux condenser plus CaC12 drying tube. After three hours of refluxing under magnetic stirring, the product was filtered off, suspended in about 500 ml of chloroform, and filtered again. This suspension/filtration procedure was repeated twice more. After the last filtration, the resulting gel was dried in air, crushed, and sieved.
RESULTS AND DISCUSSION General Considerations. The silica-containing polymer developed originally ("Phase B") ( I ) , consisted of a linear polyurea and submicroscopic silica. The polymer was ob-
46. NO. 13, NOVEMBER 1974
Table I. Typical Phase Properties at Different Polymer Contents Polymer content,
smface area,
surfnce **a,
Packed denriiy,
%
m*/9
m2I-3
g/m3
Mechanical Stabiliiy
21.08 30.03 39.59 49.15
102 76 52 22
0.23 0.29 0.38 0.47
Poor Satisfactory Good Very good
23 22 20 10
tained through homopolymerization of DDI-1410 diisocyanate, performed in a silica suspension to achieve a unique combination. The final product was a solid white porous material. Although the two components were not interconnected through chemical covalent hondings, the material showed good mechanical stability (at proper silica to polymer ratios) due to hydrogen bridge-bondings between the polymer urea groups and the silica surface hydroxyl ones. However, this type of bonding was affected above the maximum column operating temperature of 200 "C. Although the polymer did not suffer from thermal degradation ( I ) , the porous structure of the material changed, resulting in decreased efficiency. T o improve the temperature limit, it was decided to reinforce the silica-polymer combination through polymer cross-linking. Again, DDI-1410 diisocyanate was used as the principal monomer because its structural characteristics have proved to be an excellent base for stationary phases (I). Diethylenetriamine was chosen a s the second monomer because of its relatively low equivalent weight (about ten times less than DDI-1410), so the characteristics of the diisocyanate would be the predcominate ones in the final product. O B " " . & " " & . Effect of Polymer Content on Cot,.,-".I v y II. IIIc L n I. D. -. tics. The polymer/silica ratio affects strongly the physical properties of the phase and, consequently, also the column parameters. Therefore, it was necessary to investigate this function in detail. Table I presents typical physical properties of several phases a t different polymer contents, indicating that the mechanical stabilities and packed densities increase with the polymer content while the surface areas decrease. Overall, the polymer content of 39.59% presents the best compromise, because the surface area (m2/cm3) is still relatively high and its mechanical stability is good. As can be seen in Figure 1, a photomicra#graphobtained from a 100/150 mesh batch, this phase is pr actically free of fines, as a result of its mechanical stability. Its thermal stability is evidenced by the tnermogravimetric plot depicted in Figure 2. The curve shows a weight loss on a 100-mg sample of phase as a function of temperature. The material is essentially stable to about 370 "C, then begins to degrade slowly a t higher temperatures. The influence of polymer content on the plate height linear gas velocity relationship is presented in Figure 3. Being a nonadsorhed species under the conditions used, methane was utilized to measure the linear gas velocity. The curves show that the polymer affects the van Deemter curves differently than stationary liquid phases in partition chromatography. There, generallv, the H E T P values and the done of the van Deemter ,cnrves increase with the amount of liquid phase, while the optimum gas velocity values decrease. Here up to 39.59% polymer content, the tendency is just the opposite. Afterw ards.i.t inverts strongly. For the most successful application of this phase in hydrocarbon analysis, it was important to investigate the influence of polymer content on hydrocarbon-type selectivi-
..
Figure 1. Photomicrograph of silica-containing cross-linked polymer (X 40) 1
6
0 100
1
:
io0
bOO
700
800
I E U P E R I T U I I E , .C
Figure 2. polymer Operating
ietric curve of silica-containing cross-linked IV..U.L.UII-.
I
.-..ing ate. 10 'Clmin; flow ate. 50 mllmin; carrier
gas, nitrogen
8.74
0.8
Figure 3. Effect of polymer content on the plate heightllinear gas velocity relationship Sample: Gas mixture of methane and 2.2dimelhylbutane. Column: Im. 'kin. '/r& i.d., copper tubing. 1001150 mesh. Column temperature: ambient. Carrier gas: nitrogen 0.d. X
ty. A common method (2.3) is to express the selectivity for naphthenes and paraffins ( t N f t p ) as the ratio of the reten(2)F. T . Eggettstsen and H. S. Knight, Anal Chem.30, 15 (1958). (3)R. D. Schwan.?.D. J. Brass,eaux, and R G. Mathews, Anal. Chem., 38, 303 (1966).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
1909
t
'"1
0 0 N y/
06
I I
I
x
I
I
~
I
I
25
I
I
I
20
15
IO
i f l 0
MINUTES ,
150 ISOTHERMbL-i.TEMPERbTURE
100
40
3
2
20
4
20
FROGRAMMEOAT~O'C/mn~fl
Figure 6. Separation of terpenes Operating conditions: Column: 1-m. '/B-in. 0.d. X '/ja-in. i.d., copper tubing, 100/150 mesh. Carrier gas: nitrogen at 15 mi/min, 26 psig. Temperatures: column: programmed: injector: 280 OC; detector: 310 O C
I
35
30
___
190
25
20 15 MINUTES
IO
5
0
90
65
40
1
165 TEHFERATURE
140
115
P R O G R A M M E D AT
U
S'CImin
Figure 5. Separation of olefins Operating conditions: Column: 1-m, %-in. 0.d. X '&in. i.d.. copper tubing, 100/150 mesh. Carrier gas: nitrogen at 11 ml/min, 20 psig. Temperatures: column: programmed: injector: 280 OC: detector: 295 O C
larly, the aromatidparaffinic selectivity ( t A / t p ) is evaluated through benzene and 2,4-dimethylpentane. Figure 4 presents the effect of polymer content on the retention ratios t A / t p and t N / t p . Generally, partition columns yield t N / t p values greater than 1, and adsorption columns less than 1. Both type of values occur in Figure 4, indicating the presence of both separation mechanisms (partition and adsorption). Though the polymer component of the packing does not have a melting point, it still shows "liquid" characteristics and, consequently, partition occurs. Similar behavior has been observed in polyaromatic polymer beads ( 4 ) . As it could be anticipated, the partition influence increases strongly with the polymer content for (4) 0. L. Hollis, Anal. Chern., 38, 309 (1966).
1910
ANALYTICAL CHEMISTRY, VOL. 46,
NO. 13,
both selectivities t A / t p and t N / t p , while the aromatic us. naphthene selectivity ( tA / t N ) is practically independent of polymer content, because the slopes of the two curves in Figure 4 are practically the same. The possibility of being able to vary the selectivity according to the needs of the analyst is an important advantage. In practice, this means that for carbon number separations, a polymer content of less than 30% should be chosen. However, for general applications, it can be concluded that the optimum polymer content is about 40%. Such packing has good mechanical stability and is superior in terms of efficiency and resolution to all significantly different polymer contents. Therefore, a silica/polymer ratio of about 1.5 was used for packings in the analytical applications. Analytical Applications. For analytical evaluation, the present phase was employed in different types of hydrocarbon analysis. The chromatograms presented in Figures 5 to 8 attempt to give some idea about the separations which can be performed. It should be emphasized that all the analyses were made in the same column of 1-meter length, without any matched column for base-line compensation during the temperature-programmed runs. The thermal properties suggested applications at higher temperatures. However, the phase also proved to be very useful a t ambient temperature, exhibiting high selectivity and efficiency (e.g., the separation of C1 to c6 saturated hydrocarbon isomers was easily accomplished at room temperature. In comparison with the previous packing ( I ) , the resolution among the c5/c6 isomers even improved). Of particular note is the separation depicted in Figure 8 which shows an analysis of polycyclic compounds normally accomplished using high temperature silicones. The chromatograms illustrate that, despite the small column length, the phase was able to perform relatively difficult separations, resulting from its high selectivity and efficiency. A great advantage is represented by the fact that the phase can maintain all these basic properties over a wide temperature range.
NOVEMBER 1974
I
50
45
40
35
I
I
1
30
25
20
15
1 0
5
IO
MINUTES
__
-
300 ISOTHERMAL
170
235 TEMPERATURE
PROGRAMMED
AT
6,5'C/m
1
I
1
105
40
q-
Figure 7. Analysis of straight run distillation cut, boiling range 30-344 O C Operating conditions: Column: 1-m. %-in. 0.d. X %s-in. i.d., copper tubing, 100/150 mesh. Carrier gas: nitrogen at 9 ml/min. 18 psig. Temperatures: column: programmed; injector: 380 OC; detector: 350 OC
CONCLUSIONS The thermal stability of silica-containing polymeric stationary phases can be improved through polymer crosslinking. A phase able to withstand up to 300 "C provided high performance in the gas chromatographic separation of nonpolar compounds. Although the packing presented previously ( I ) showed a similar TGA thermogram, its maximum column operating temperature was about 100 "C lower. This improvement can be explained through the reinforced porous structure, accomplished mainly by polymer cross-linking, and, to a minor extent, by the increased relative amount of hydrogen bridges between polymer urea groups and silica. Under the effect of temperature, this structure remained perfectly rigid, while the structure of the previous packing was not sufficiently supported by its linear polymer component. A parameter of significant influence on the column behavior was the polymer content which can be adjusted to achieve the desired selectivities. A 40% polymer content provided the best results for different hydrocarbon analysis. The major advantage of the present phase is its ability to keep over a wide temperature range the top characteristics of the silica-containing polymer stationary phases. Very good peak shapes and high efficiencies have been obtained with remarkable reproducibility for several types of hydrocarbons a t different temperature levels.
32
28
24
20
16
12
8
4
0
MINUTES
300
260
-s O r r E R M l , ~ T E N . ? E a r T j R E
220 PROGRAMMED b- 8 * C l m i +
180
I80 S7rhERMAL+
Figure 8. Separation of polycyclic compounds Operating conditions: Column: 1-m, l/e-in. 0.d. X i.d., copper tubing, 100/150 mesh. Carrier gas: nitrogen at 22 ml/min, 30 psig. Temperatures: column: programmed: injector: 350 OC; detector: 350 OC
RECEIVEDfor review February 26,1974. Accepted June 18, 1974. The authors thank PetrBleo Brasileiro S.A.-Petrobrds for permission to publish this work.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
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