Porous Glass as an Adsorption Medium f0 r Gas Chr o ma to g ra ph y H. L. MACDONELL, J.
M. NOONAN,’
and J. P. WILLIAMS
Glass Research and Development, Corning Glass Works, Corning, N.
b Porous glass, Code 7930,has been studied as an adsorption medium for gas chromatography. Marked improvement was obtained when this glass was pretreated with organic solvents; however, clxygen-containing compounds were still very strongly adsorbed. Possible explanation for chromatographic improvement is offered on the basis of earlier reaction mechanism investigations.
I
HAS BEEN demonstrated that porous glass (8;,Corning glass Code 7930, is capable of supporting and effecting both liquid chromatographic (5-7) and electrophoretic (4, 6) separations. Additional application using this g ass as an adsorption medium for gae chromatography has also been reported by Rogers and Spitzer (9). Modification of porous glass by pretreatment with methanol or fluoride has been proven effective in promoting migration in liquid chromatography ( 7 ) and is now reported to produce similar desirable changes when pretreated glass is used for gas chromatography. Untreated porous glass contains single or free silanol groups, siloxane groups, and double or adjaceit pairs of silanol groups which were dercribed by Sidorov (10) as perturbed by a hydrogen bond from one hydroxyl group to another. These silanol groupc; are responsible for the phenomenon of “irreversible adsorption” and some means was sought whereby their hydroxyl component could be “de ictivated,” either by removal or substitution. Such changes in the structL.re of porous glass have been reported by other investigators who describe reaztion mechanisms when certain organic solvents are used. Folman and Yates have suggested both hydrogen bonding (3) and methylation (8) to explain reactior s between methyl alcohol and porous g1,iss. Terenin and Sidorov acknowledge the actual reaction or chemisorpi,ion of methanol on free silanol groups (11). Hydrogen bonding on adjacent silanol groups is T
Prrserit address, Department of Chemistry, St. Bonaventure University, St.
Bonaventure, N. Y.
Y.
inferred by Sidorov (10) by extending his water adsorption mechanism to include methanol. The result is somewhat different from that proposed by Folman and Yates. Regardless of which hydrogen bonding structure is more nearly correct, i t is certain that either will be less stable than methylation, which is an actual chemical phenomenon. Because of these properties of porous glass, and considering earlier research using this material as a liquid chromatographic medium, the following investigation was made to determine the potential usefulness of pretreated porous glass in gas chromatography. EXPERIMENTAL
Apparatus. A Perkin-Elmer Vapor Fractometer, LModel 154-D, was used throughout the investigation. Standardization t o 125’ C. was adopted for comparison purposes, as all samples
COLUMN R (PERKIN-ELMER)
PRETREATED POROUS GLASS, 50 HR. REFLUX IN DIBUTYL ETHER
POROUS GLASS
6
RETENTION TIME -MIN.
Figure 1 . Comparison of column response to pentane S a m p l e volume: 4 PI.; Column temp.: 125’ C.; Flow rater 60 ml./minute; Atlenuationr 32
selected for study were well resolved a t this temperature. Chromatographic columns were prepared from l/g- x 72-inch copper tubing that had been cleaned with dilute nitric acid, rinsed several times with distilled water, and dried. Glass wool was used to plug each end of the filled columns. Procedure. Porous glass, Code 7930, was ground into a variety of mesh sizes and several ranges were evaluated t o obtain acceptable flow characteristics. It was decided t h a t 50- to 60-mesh should be used for all experimental work as this fraction permitted desirable flow rates. Mininium H E T P values mere obtained a t a flow rate of 50 t o 60 ml. per minute of helium under 7 . 5 - to 10p.s.i. pressure. -411 porous glass was initially heated to 500’ C. and held a t this temperature for 2 hours. Columns prepared from porous glass which received no additional treatment were unsatisfactory as they retained all samples most tenaceously, resulting in long retention times and low, wide peaks with considerable tailing. In fact, several compounds were apparently never eluted a t all and seemed t o “disappear” as noted by Rogers and Spitzer (9). For purposes of uniformity, and to keep the study within a reasonable scope, only very few representative samples were taken of the various class compounds: alcohol, ketone, ether, and glycol. These compounds were reacted with porous glass by simply soaking the glass in one of the solvents or by refluxing porous glass in the solvent for several hours. Following exposure to a solvent, porous glass was filtcred and dried a t 140’ C. for 4 hours, Evaluation of columns prepared for pretreated porous glass was limited to changes in retention time of five selected compounds. RESULTS
Retention times of five compounds were measured and compared between untreated porous glass columns and a commercial column of polypropylene glycol (Perkin-Elmer R) of the same length and flow characteristics. Further comparisons were also made between untreated and pretreated porous glass columns. Plate numbers increased from approximately 100 for iiritreatecl porous glass to 160 to 200 for pretreated porous g1:tss. Ii’igure 1 illustrates the improvement of peak VOL. 35,
NO. 9, A U G U S T 1963
1253
shape resulting from pretreatment. Table I illustrates the high adsorptive character of porous glass that had not been subjected to pretreatment (other than drying at 500" C. for 2 hours). The data in Table I indicate that considerable bonding of oxygen-containing species occurs; aromatic compounds elute more readily, although some bonding mechanism is indicated; aliphatic
Table I. Retention Time in Minutes of Various Samples on Untreated Porous Glass Column Compared to Column R
Sample Carbon tetrachloride Chloroform Pentane Hexane Heptane Octane Benzene Toluene Acetone Methanol 3-Heptanol Isopropanol Ethanol a
Untreated 7930
Column R
1 .i
3.9
1.9 5.1
4.5 1.3 2.0 3.2 6.0 4.8 8.6
7.0 11.3 15 .O 33 .O 43.0 Not observeda
Not observeda Not observeda Not observed. S o t observeda
2.8
2.9 3.6 4.5 4.8
Retention time of over 2 hours.
Table II.
compounds, particularly if halogenated, elute most easily with retention times of less than those obtained on column R. Considerable shortening of retention times as well as improvement in peak response was noted when porous glass was pretreated with organic compounds. Simply soaking the glass in a solvent reduced its adsorptive character considerably and refluxing for several hours reduced i t still further. This is due to the "blocking" or "deactivation" of silanol groups. Columns prepared from porous glass which had been pretreatcd were considerably siilierior to columns of untreated porous glass. Table I1 illustrates results obtained using pretreated porous glass. From these data it is evident that merely soaking porous glass in an alcohol, ketone, or ether will greatly improve its ability to function as a gas chromatographic medium. Chromatographic proprrties are further improved after porous glass is refluxed in these solvents for 24 hours. After refluxing for 50 hours, the ext,ent of experimentation, further improvement mas still noted. Columns were also prepared from porous glass which had been refluxed in heptanol, decyl alcohol: and trimethylchlorosilane. Finally, pretreat-
Retention Time in Minutes as a Function of Length of Pretreatment
Column Untreated (Code 7930) porous glass Isobutyl alcohol Soaked 1 hour Refluxed 24 hours Refluxed 50 hours Methyl isobutyl ketone Soaked 1 hour Refluxed 24 hours Refluxed 50 hours Dibutyl ether Soaked 1 hour Refluxed 24 hours Refluxed 50 hours Table 111.
Pentane
Heptane
Benzene
Octane
Toluene
5.1
11.3
15.0
33.0
43.0
2.8 2.4 1.i
11.1 9.4 5.9
13.6 9.s 9.6
22.7 19.2 11.4
29.0 23.9 22.6
3.2 2.5 2.2
12.6 9.0 8.3
15.2 12.4 12.2
26.2 17.8 17.6
37.F) 31.8 31.4
2.4 2.0 1.5
8.5 7.0 5.1
9.5 7.4 5.6
16.2 13.4 9.5
22.8 16.8 12.2
Retention Time in Minutes Following Various Pretreatment of Porous Glass
Column Pentane Heptane Untreated (Code i930) porous glass 5.1 11.3 Refluxed in heptanol, 2.2 s.s 24 hours Refluxed in trimethyl1.3 3.3 chlorosilane, 16 hours Impregnated with 5'30 high vacuum silicone grease" 1.2 3.6 Refluxed in decyl 2.8 1 .o alcohol, 24 hours Impregnated with 4'30 2.6 0.95 polypropylene glycolb a Original solvent was toluene. b Original solvent was isobutyl alcohol.
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ANALYTICAL CHEMISTRY
Benzene
Octane
Toluene
15.0
38.0
43.0
10.0
15.0
27.0
2.6
5.4
4.5
4.2
6.5
8.8
2.2
5.0
4.2
2.1
4.8
4.3
ment was achieved by impregnating the glass with silicone grease and polypropylene glycol (column R material). The results obtained using these columns are listed in Table 111. I n addition to the five compounds listed in Tables I through 111, several other samples were injected into many of the columns. Although the retention times of the aliphatic compounds were shortened with almost every treatment, oxygen-containing species aere still very strongly adsorbed. I n fact, mixtures of alkanes and alcohols could be completely separated hy elimination of alcohols. When only 1% of heptane was present in heptanol, the heptane response was normal and all heptanol was completely adsorbed. This property suggests an excellent method for the purification of solvents by removal of oxygen-containing impurities. Porous glass that had neTer been heated over 125" C. was used to prepare one column. The response of this column was not significantly different from columns containing porous glass that had been heated to 500' C. DISCUSSION
Hydrogen bonding does not adequately explain the reaction mechanisms between porous glass and oxygen containing organic solvents. A11 columns prepared from porous glass that had been refluxed in solvents were later heated to 140' C. for 2 hours. This procedure should remove all hydrogen bonded organic compounds and return the modified porous glass to its original silanol structure ( 1 ) . This would explain the ability of pretreated porous glass columns to permit adsorption of compounds containing carbonyl groups, hydroxyl groups, or ether linkages. Unfortunately, it does not offer a satisfactory explanation for the reduced retention times which are, in fact, observed. It is doubtful that a partial loss of hydrogen bonded species is responsible. Most probably, all hydrogen bonded compounds are actually removed a t 140" C., leaving silanol sites available for future bonding of injected solvent samples. The column temperature of 125' C. permits temporary or intermittent bonding between oxygen-containing compounds and porous glass and what was described as a disappearance is actually not the case at all. The apparent irreversible adsorption actually results from extremely long retention times. When oxygen-containing compounds are eventually eluted from these column>, their effect on a detector cell is negligible and an increase in base line is not detected.
Therefore, the chroniatographic improvement in pretreatcd porous glass must be explained without consideration of fugitive hyd:.ogen bonding. Methylation is suggested as one possible reaction of a more permanent nature. Silanol groups become deactivated by reaction oi their hydroxyl component with an alcohol molecule on a one to one ratio. The ratio remains the same if e t h m are used as the reflux medium, but ketones present restrictions as to selec.;ion of reaction sites. Whereas alcohol:; and ethers may react with any random silanol hydroxide, ketones require adjacent' groups before reaction is pos:;ible. Evidence of this thesis may be found in the fact t h a t ketone modification did not reduce the adsorption charactr:ristic of porous glass nearly as rapidlj. or completely as did alcohol and ethars, as reported in Table 11. A second reaction hat, been suggested by Chapman ( 1 ) wherein siloxane groups are ruptured c.uring reflux in met,hanol. This reaction results in permanent bonding of a methoxy group on one silicon atom a i d simultaneous conversion of the secoid silicon atom
to a silanol group. This second group may then react either by methylation or hydrogen bonding becoming either permanently or temporarily deactivated, respectively. Reaction between siloxane groups and ketoncs is possible, producing a much larger and more complex structure than is obtained with alcohols or ethers. It is doubtful t h a t this reaction would proceed as easily as the less complicated reactions between ~iloxanegroups and alcohols or ethers. From the above limitrd (lata, and in consideration of the proposcd react'ion mechanisms, i t may be coiicluded that improvement of porous g1a.s for chromatographic use is a rcsult of a rcactiori or reactions which permanently destroy the hydroxyl component of some silanol groups. I n addition, all silanol groups are not permanently deactivated during refluxing due t o hydrogen bonding during or prior to refluxing which blocks the more permanmt reaction a t some sites; subsequent heating reverts these temporarily blocked groups to active silanol groups:. Finally, the ability of modified or p r ~ t r e a t c dporous glass to retain oxygen-containing 01'-
ganic compounds is based upon the presence of both residual and reverted silanol groups. This investigation suggests that additional research is necessary before a more complete understanding of the mechanisms between porous glass and organic compounds may be achieved. LITERATURE CITED
(1) Chapman, I. D.. Corning Glass Works, private communication, October 1962. ( 2 ) Folman, &I., Trans. Faraday Sac. 57, 2000 (1961). (3) Folman, PI.,Yates, D. J. C., Ibid.,
54. 1684 (1958). (4) &lacDonell, H. L., A N ~ L CHEM. . 33, 1554 (1961). ( 5 ) MacDonell, H. L., J . Criminal Law, Crzmznol Polzce Scz 53, 507 (1962). 16) . . MacDonell, H. L., Nature 189. 302 (1961). ( 7 ) RlacDonell, H. I>.)Williams, J. P., ANAL.CHEM.33, 1552 (1961). (8) Nordberg, Martin E., J. Am. Ceram. Soc. 27, 299 (1944). (9) Rogers, L. B., Spitzer, J. C., ANAL. CHEM.33, 1959 (1961). (10) Sidorov, A. N., Opt. Spectry. USSR E n g h h T r a m / . 9, 493 (1960). (11) Terenin, A. X., Sidorov, A. K.,Opt.Mekh. Prom. 1, 1 (1959). RECEIVETI for review December 13, 1962. Accepted May 23, 1963.
Separation of Titanium by Cation Exchange and its Spectrophotometric Determination with Disodium-'I ,2-di hydroxybenzene-3,5-disuIfonate B. T. KENNA and
F. J.
(CONRAD
Organization 7 122, Sandia Corp., Sandia Base, Albuquerque,
j+ Titanium samples are dissolved in dilute sulfuric acid, and the cations in solution are absorbed on a cationexchange column. Titanium i s preferentially eluted b y a small volume of sulfuric acid-amnionium fluoride solution. The spectrophotometric procedure of Yoe and Armstrong i s employed to determine the titanium content of the effluents. Using NBS standards, this procedlJre yielded results within the rang(% of the NBS values. The average error i s k0.270 of the titanium content. The combination of the cation-exchange procedure with the spectrophotometric determination enables one to determine titanium without interference from a t least 17 elements.
S
procedure3 are available for the spectrophotomi+ic determination of titanium, the hydrogen peroxide method being the simplest and most EVERAL
N. M.
common (6). Although the production of a color by the addition of hydrogen peroxide or certain organic compounds to a titanium solution has been used to determine titanium with good results ( 6 ) , Yoe and -1rmstrong ( 8 ) have reported a n extremely sensitive colorimetric procedure using Tiron (disodium-l,2-dihydroxybenzene3 ,5 - d i s u l fonate). Apparently this method is applicablc to smaller quantities of titanium than those detected by hydrogen peroxide. However, as in most spectrophotometric methods, the procedure employing Tiron suffers from a diverse ion effect which invludes iron and 20 other ions. The quantitative separation of titanium from these ions is often difficult and tedious. Much of this difficulty could be eliminated if a valid ionexchange procedure were available. However, because of the lack of knomledge concerning the ion-exchange he-
havior of titanium, there are f e x specific procedures for this element. Brown and Rieman ( I ) employed a n HC1-citric acid eluent system to separate titanium from zirconium and thorium on a cation column. Because the eluent p H has to be controlled and effluent volumes of between 1 and 3 liters are necessary, use of this procedure has been limited. Other investigators have utilized anion-exchange techniques ( 3 ) . The eluent \vas a n HC1-HF mixture. This method xould appear to be somewhat tedious, because different HC1-HF mixtures are required to elute each ion and a relatively large volume of 250 ml. is needed for the complete elution of titanium. Yoshino and Kojiina (9) have reported a separation of iron and tihnium by a combination of complex formation, precipitation, and cation-exchange techniques. Kim ( 6 ) has report,ed that, when a VOL. 35,
NO. 9, AUGUST 1963
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