fere with the absorbance of zirconium. Table I1 summarizes the interferences caused by various species on the absorbance of 0.01M zirconium in the presence of (a) no added material, (b) O.lMNaF, (c) 0.1MNH4C1,(d) 0.1MNH4F. The zirconium solutions containing 0.1M NH4F are considerably less susceptible to interference than are the others. Presumably the enhancements caused by ammonia and fluoride are such that extra enhancement by other species does not occur unless very high concentrations of the potentially interfering species are present. Furthermore the very high stability in solution of the zirconium-fluoride complexes presents interference from both the weakly complexing nitrate and strongly complexing sulfate ions (11) which otherwise form complexes which decompose in the flame to give ZrOz. Analytical Use of Ammonium Fluoride. For determination of zirconium by the atomic absorption method, the presence
of 0.1M NH4F in all standard and unknown solutions is recommended. Such a procedure gives an improvement in sensitivity of about eight- to tenfold and a corresponding improvement in the limit of detection. Furthermore, many interferences are either removed or greatly suppressed in this manner. For the same reasons the addition of ammonium fluoride is also useful in the determination of hafnium, titanium, and tantalum. ACKNOWLEDGMENT
The author thanks J. B. Willis for his interest and assistance in the planning of this work and for many helpful discussions. RECEIVED for review February 11, 1970. Accepted April 15, 1970.
Preparation of Thermally Stable Gas-Chromatographic Packing Materials A. H. Al-Taiar, J. R. Lindsay Smith, and D. J. Waddington Department of Chemistry, University of York, Heslington, York, YO1 5DD, England
GAS-LIQUID CHROMATOGRAPHY, when used in conjunction with mass spectrometry, is generally only practicable up to about 250 "C. Above this temperature, the vapor pressure of the liquid phase is too high ( I ) . Although a trap may be inserted to absorb the vapor of the liquid phase (2), the lifetime of the column is reduced. Gas-solid chromatography does not have these drawbacks, but on solids such as silica gel and alumina, polar solutes are irreversibly adsorbed on the surface active groups. The attempts to modify silica gels by chemical deactivation with, for example, trimethylchlorosilane (3), with dimethyldichlorosilane (4), with a cyanoalkylchlorosilane (3, and with alcohols (6, 7), have not been particularly successful for the monolayers of organic material are not dense enough to prevent specific interactions between the solutes and bulk solids (3, 5). Stationary phases which are chemically-bonded to the solid support will be more thermally stable than conventional liquid materials. Moreover, in theory, they will give more reproducible results than the latter, for GLC separation of solutes occurs in solvent pools in the capillaries of the solid support, and separation is dependent, in part, on its structure (1) R. Teranishi, R. G. Buttery, W. H. McFadden, R. T. Mon, and J. Wasserman, ANAL.CHEM.,36, 1509 (1964). (2) R. L. Levy, H. Gesser, T. S. Herman, and F. W. Hougen, ibid., 41, 1480 (1969). (3) A. V. Kiselev, Yu.S.Nikitin, V. K. Chuikina, and K. D. Shcherbakova, Russ. J . Phys. Chem., 40,71 (1966). (4) N. V. Akshinskaya, A. V. Kiselev, Yu. S.Nikitin, R. S.Petrova, V. K. Chuikina, and K. D. Shcherbakova, ibid., 36, 597 (1962). (5) A. V. Kiselev, Discuss. Faraday SOC.,40, 205 (1965). (6) C. Rossi, S . Munari, L. Cengarle, and G. F. Tealdo, Chim. Ind. (Milan),42, 724 (1960). (7) I. V. Borisenko, N. I. Bryzgalova, T. B. Gavrilova, and A. V. Kiselev, Neftekhimiya, 5, 129 (1966).
which varies from batch to batch (8, 9). Celite has been modified by treatment with hexadecyltrichlorosilane (10) and with octadecyltrichlorosilane (11) followed by hydrolysis, yielding materials which are chromatographically equivalent to Celite coated with a silicone elastomer. The surface hydroxyl groups on porous glass particles have been esterified with 3-hydroxypropionitrile to give a material which is highly efficient (12) and significantly more thermally stable than chromatographic materials using this nitrile as a liquid phase. In this paper, we report some of our exploratory studies on the preparation of materials suitable for high temperature gas chromatography. EXPERIMENTAL
Operating Conditions of the Gas Chromatograph. All results were obtained isothermally on a Pye 104 gas chromatograph equipped with a flame ionization detector coupled to an RE 511 (Goertz Servoscribe) recorder. Glass columns (1.5 m X 4 mm i.d.) and stainless steel columns (5.5 m X 1.2 mm i.d.) were used. In some experiments, the chromatograph was connected to an A.E.I. MS 12 mass spectrometer, at 70 eV electron energy, 100 pA trap current, 8 kV accelerating potential, with a source temperature of 165 "C. The carrier gas, nitrogen (B.O.C. "oxygen-free"), was deoxygenated and dried by passing it through successively a solution of chromium(I1) chloride in hydrochloric acid over zinc amalgam, concentrated sulfuric acid, potassium hydroxide pellets, silica gel, and molecular sieve 5A. (8) N. C. Sana and J. C. Giddings. ANAL.CHEM., 37,822 (1965). (9) M. Krejfi, Collect. Czech. Chem. Commun., 32, 1152 (1967). (10) E. W. Abel, F. H. Pollard, P. C. Uden, and G. Nickless, J . Chromatogr., 22, 23 (1966). (11) W. A. Aue and C. R. Hastings, ibid, 42, 319 (1969). (12) I. Haldsz and I. Sebastian, Angew. Chem., 81, 464 (1969). ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
935
3
I 0
3 TIME,min.
0
Figure 1. Separation of compounds on Column B q e ) (octadecyltrichlorosilane, silicon tetrachloride, 98:2% v/v) at 325 "C. Linear velocity of nitrogen, 2.7 cm sec-l 1. 3. 5. 7. 9.
n-Decane n-Tretradecane n-Octanol n-Dodecanol Di-n-propyl ketone
Table I. Column Packing Materials (Concentrations of liquids from which materials are made given as %v/v) Column Number TYPEA. (100) Vinyltrichlorosilane A1 Octadecyltrichlorosilane A2 Phenyltrichlorosilane A3 Methyl-10-trichlorosilylundecanoate A4 TYPEB. SILICON TETRACHLORIDE WITH B1 Methyltrichlorosilane, (a) 1 , (b) 5, and (c) 10 B2 Methylvinyldichlorosilane, 1 Dimethylvinylchlorosilane, 1 B3 B4 Octadecyltrichlorosilane, (a) 1, (b) 15, (c) 50, (d) 90, and (e) 98 B5 Phenyltrichlorosilane, 98 Methyl-10-trichlorosilylundecanoate, 26 B6 TETRACHLORIDE, 10, WITH TYPEC. SILICON c1
Octadecyltrichlorosilane, 85, and
(a) methyltrichlorosilane, (b) vinyltrichlorosilane, (c) phenyltrichlorosilane, c2
(d) dichlorophenyltrichlorosilane (e) methyl-10-trichlorosilylundecanoate Octadecyltrichlorosilane, 45, with phenyltrichlorosilane, 45
The detector ionization current was measured for columns packed with the new materials and with Celite-silicone elastomer E301, and compared with that of an empty column under identical conditions. The recorder was calibrated so that when set at 10 mV, with amplifier attenuation at 1 x 103, a current of 10-9 A gave a full scale deflection. The carrier gas was used at a linear velocity of 4.0 cm sec-1 at each temperature. 936
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
2. 4. 6. 8. 10.
n-Dodecane n-Hexadecane n-Decanol Diethyl ketone Di-n-butyl ketone
Materials. Silicon tetrachloride (Fisons Ltd.) and the chlorosilanes (Hopkin and Williams Ltd.) were purified by distillation. Silicone elastomer E301 (Imperial Chemical Industries Ltd.) was used without further purification. Methyl-10-trichlorosilylundecanoate(1 13-115 "C, 1 mm) was prepared by reaction of methyl-10-undecenoate (Kodak Ltd.) with trichlorosilane and diacetyl peroxide (Laporte Industries Ltd.), using the procedure of Pietrusza, Sommer, and Whitmore (13). The chromatographic materials were prepared by the hydrolysis of a trichlorosilane (type A), or cohydrolysis of one (type B) or two (type C) chlorosilanes with silicon tetrachloride. Some samples were subsequently treated with dimethyldichlorosilane (14). A typical preparation is described below, and a list of these materials is given in Table I. Preparation of Material C l(c) (octadecyltrichlorosilane: phenyltrichlorosilane: silicon tetrachloride, 85: 5: 10% v/v). The mixture of octadecyltrichlorosilane (25.5 ml), phenyltrichlorosilane (1.5 ml) and silicon tetrachloride (3 ml) was added dropwise, with continuous stirring, into ion-free water (150 ml). After hydrolysis, ion-free water (150 ml) was added and the soft white precipitate was allowed to stand for 6 hours, filtered, washed thoroughly with water, dried at 150 "C.for 24 hours and finally heated at 370 "C under nitrogen (flow-rate ca. 45 ml min-1) for 60 hours. The residual solid was ground and sieved (30-60 mesh). RESULTS AND DISCUSSION The materials of type A prepared from trichlorosilanes only (Table I) were of little use. The silicones from octadecylboth trichlorosilane and methyl-10-trichlorosilylundecanoate melted at low temperatures and the other two materials, al(13) E. W. Pietrusza, L. H. Sommer, and F. C . Whitmore, J . Amer. Chem. Soc., 70, 484 (1948), (14) D. M. Ottenstein,J . Gas Chromafogr.,6 , 129 (1968).
10
(C) 1
.
5'0
1'00
1'50
2'50
2'00
Ne Figure 2. Mass spectra of n-hexadecane direct insertion (b) 0.08 p1 n-hexadecane obtained after passage through column B4(e) at 325 "C (c) Background of column under conditions of ( 6 ) (a) By
though thermally stable above 350 "C, were chromatographically unsatisfactory. The type B materials prepared by cohydrolysis of silicon tetrachloride and a trichlorosilane, and which contained 50 or less of the organic material, were mixtures of amorphous and crystalline particles and, although they separated alkanes, they adsorbed irreversibly more polar compounds. However, materials containing a greater percentage of trichlorosilane have more promising properties, in particular the two solids prepared from octadecyltrichlorosilane and silicon tetrachloride in the proportions (90: 10) and (98 :2)% v/v [materials B4(d) and (e)]. A wide variety of organic compounds of different polarities (for example, alkanes, alkenes, aromatic hydrocarbons, alcohols, alkyl halides, ethers, ketones, and amines) are eluted from columns packed with these materials (Figure 1). On the (90:lO)x v/v material, the peaks of the more polar compounds were slightly distorted, and secondary alkyl halides were dehydrohalogenated at 200 "C, indicating the presence of some surface hydroxyl groups. Following reaction with dimethyldichlorosilane, the modified solid behaved like the (98 :2) v/v material and could be used up to 350 "C for the separation of alkanes and n-alkylamines. Primary, secondary, and tertiary alkyl halides were eluted without dehydrohalogenation up to 300, 250, and 200 "C, respectively. The thermal stability of the solids was examined in two ways, as a function of the detector ionization current and by mass spectrometry. The amount of column bleed, as measured by the detector ionization current, was compared with that of Celite coated with 2 0 x w/w silicone elastomer E 301, The silicone elastomer column chosen was one which had been used for three years previously and which, before the comparative studies were made, had been heated at 100 OC for 72 hours, at 200 "C for 48 hours and finally at 240 "C for 2 hours. The results for the elastomer (Table 11) are similar
x
Table 11. Flame Ionization Currents ( X 1Olo A)
Temperature "C 100 150 200
224 250
275 300 320 342 360
Celite (AW)silicone elastomer E301 (20% w/w) 0.2
Octadecyltrichlorosilane : silicon tetrachloride (98:2) % v/v
...
0.5
0.1 0.5
1.4 3.1 9.0 23.8
... ...
... ...
... 1.5 ...
4.6 7.0 12.0 17.0
to those obtained by Kiselev (15). Second, the mass spectra of materials, following gas chromatographic separation between 325-350 "C, were compared with the mass spectra of the materials obtained by direct insertion (Figure 2). Both studies show that the type B materials can be used up to at least 350 "C, in conjunction with a mass spectrometer. In contrast, the background obtained for the silicone elastomer column, under conditions similar to those used in Figure 2(c), was such that the identification of the solutes became difficult even at 250 "C. The thermal stability of the type B materials must be due to the framework provided, on hydrolysis, by the small amount of silicon tetrachloride, for the material obtained on hydrolysis of pure octadecyltrichlorosilane melts at 150 "C. Solids were prepared from mixtures of octadecyltrichlorosilane with a second organic chlorosilane and silicon tetra(15) A. V. Kiselev, Adtian. Chromatogr., 4, 113 (1967). ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
937
1
-
3
U, cm
5
sec-1
Figure 3. Relation of HETP and linear velocity of nitrogen for n-dodecane on column Cl(a) (octadecyltrichlorosilane, silicon tetrachloride, methyltrichlorosilane, 85:10:5% v/v) at 325 "C
Column number C W Cl(b) Cl(C) We)
Table 111. Retention Times of Some Compounds Relative to That of Dodecane at 325 "C Linear Velocity of Nitrogen, 2.7 cm sec-l (a) Column: ClsH3~SiC13,SiCld, RSiC13 (85: 1 0 : 5 z v/v) Di-nAnalysis Tri-nbutyl Dimethyl R carbon Decane butylamine ketone phthalate 1.0 0.7 2.2 56.2 0.6 CH3 1.o 0.8 2.2 CHp=CH 54.6 0.6 1.o 0.7 2.4 57.4 0.6 C6H5 1 .o 0.7 2.2 (CH302c)Cl0H2o 65.1 0.6
x
Benzyl benzoate 6.0 6.6 6.2 5.9
(b) Column: ClsH3,SiCl3,SiCla, RSiC13 (45: 10:45% v/v)
c2
C6H5
46.3
Table IV. Retention Times of Some Compounds Relative to That of Dodecane at 325 "C Linear Velocity of Nitrogen, 2.7 cm sec-l Column C2: ClaHaiSiCla,SiCla, C8H5SiCI3 (45 :10:45x v/v) Column B4(d): C18H3iSiC13, SiCI, (90:lOz v/v) Relative Relative Relative t r c2 tr on tr on Relative Compound Column C2 Column B4(d) tr Bqd) ti-Dodecane 1 .oo 1 .oo 1.oo Tetralin 1.61 1.61 1 .oo Naphthalene 1.96 1.90 1.03 Aniline 0.94 0.84 1.12 Biphenyl 3.13 2.74 1.14 Tri-/?-butylamine 1.11 0.97 1.15 Anthracene 12.10 10.10 1.20 Acetophenone 1.18 0.93 1.27 Di benzyl 4.74 3.70 1.28 Diphenylmethane 3.74 2.92 1.28 1,8-Diaminooctane 1.91 1.45 1.32 Anthraquinone 20.45 13.81 1.48 Benzophenone 7.16 4.39 1.63 Benzyl benzoate 9.92 5.87 1.69 Dimethylphthalate 4.10 2.06 1.99
chloride (type C), followed by treatment with dimethyldichlorosilane. They allow elution of both nonpolar and polar compounds, but the polarity of the materials is not affected if the concentration of the second trichlorosilane is 5 % or below (Table Ma), but a material containing equal proportions, by volume, of octadecyltrichlorosilane and phenyltrichlorosilane is significantly more polar (Tables IIIb and IV). 938
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
0.6
1.2
0.8
4.2
9.9
Several of the solids described above were examined to find the conditions of maximum efficiency. The HETP decreases markedly with low flow rates of the carrier gas, the minimum occurring at a lower linear velocity than for many conventional GLC materials. Moreover, the HETP at this minimum is larger than often obtained (Figure 3). Both these findings indicate that the effective loading of the column is too high-Le., the materials, although thermally stable, resemble, chromatographically, solids coated with a high proportion of liquid phase and, consequently, the retention times for even simple molecules are long. However, present studies, which include using thinner columns, particles of smaller diameter, and lower effective loading, indicate that these materials can combine efficiency with high thermal stability. Moreover, a wide range of different organic compounds can be separated using phases prepared from organic chlorosilanes selected for their different polarities.
ACKNOWLEDGMENT We thank C. B. Thomas and Miss M. A. Warriss for their assistance with the mass spectrometric analyses.
RECEIVED for review January 12, 1970. Accepted March 9, 1970. This work was presented at the Symposium on Chromatography, Liverpool, England, May 1969, and one of us (A.H.A.-T'.) thanks the Calouste Gulbenkian Foundation for a research scholarship and the Universities of Baghdad and Basrah for leave of absence.
