RUN I I
-
4 0 MINUTES
Figure 26. Separation of the four unresolved components of Figure 2a
consideration should be given compounds strongly adsorbed by silica, such as alcohols and amines, that require a high volatilization temperature. Since the solid inlet system of the RMU-6E mass spectrometer introduces the sample directly into the high vacuum of the ionization chamber, less heat is generally required to volatilize a sample introduced in this manner than in the liquid inlet system. Preliminary experiments indicate that introduction of the extruded column bands via the solid inlet gives results comparable to those reported here. When the solid inlet was opened to the ionization chamber, the sudden decrease in pressure often causes much of the silica to blow out of the sample dipper into the chamber. After the spectrum was obtained, the ionization chamber was heated to 150 “C for a short time and then the next sample could be run. During the course of these experiments, the slight accumulation of silica in the chamber had no noticeable adverse effects on subsequent spectra. The silica used in this work adsorbed water very rapidly from the atmosphere. Thus all spectra run on bands from an extruded column showed strong peaks at mje 18. A specially prepared 3-gram controlled moisture silica is now available for use in the Centri-Chromatography system (11). If the water content of the silica is kept small by minimizing contact with the air, the strong peak at mje 18 does not interfere with the interpretation of the spectra. The presence of too much water causes the separating efficiency of the Centri-Chromatograph to fall off drastically, and its presence should be minimized for that reason. ACKNOWLEDGMENT
the data obtained in these experiments indicated they produce reproducible spectra if the inlet system is held below 150 “C. The combination of centrifugal microparticulate bed chromatography and mass spectrometry provides a fast accurate method of separating and identifying components in a mixture without the necessity of isolation and purification after chromatography. No difficulty is encountered in running mass spectra of the compounds studied here in the presence of silica. Several of the compounds used in this study were found to be sensitive to heat and special care had to be taken when running mass spectra of these compounds. Special
We thank E. MacPherson for supplying the compounds used in this study and the Ivan Sorvall Company for making a prototype unit of the Centri-Chrom apparatus available for this work. RECEIVED for review August 9, 1971. Accepted January 13,
1972. (11) F. W. Karasek, personal communication, V. C. Rohde, Ivan Sorvall,Inc., Norwalk, Conn. 06856, September 1971.
Tetraphenylboric Acid Salts as Stationary Phases in Gas Chromatography George E. Baiulescu Department of Analytical Chemistry, University of Bucharest, Romania Vasile A. Ilie Institute of Chemical Researches, Bucharest, Romania SELECTIVE SEPARATION using metallic salts as liquid or solid stationary phases in gas chromatography is well known (1). Recently, bis(2-ethy1hexyl)phosphoric acid and its lithium, sodium, potassium, thorium, zirconium, uranium(VI), and [Co(en)J3+salts (2, 3) were studied as stationary phases, and --
(1) V. Ilie, Rev. Chim. (Bucharest),20,43 (1969). (2) V. Ilie, M. Oprea, and G. E. Baiulescu, ibid., p 761. ( 3 ) V. Ilie, M. Oprea, and G. E. Baiulescu, “Proceedings of the 3rd Analytical Conference,” Budapest, August 1970, Vol. I, p 291. 1490
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
lithium and sodium bis(2-ethylhexy1)phosphates were proposed as stationary phases for the rapid separation in an isothermal system of chlorinated derivatives of benzene (4). Until recently, adsorbants used in gas-solid chromatography have included coal, silica gel, alumina, and molecular sieves (5). Using silver nitrate as the adsorbant, the separa~~
(4) V. Ilk, M. Boroanc;, and G. E., Baiulescu, Chim. Aiialiticz (Bucharest), 1, 33 (1971). ( 5 ) A. B. Littlewood, “Gas Chromatography,” Academic Press, New York, N.Y., 1962, p 372.
I
I
I
I
I
I
450
200
250
300
350
400
Temperature
( 0 0
Figure 1. Thermogravimetric curves of the compounds (1) TIB(Cd-M4; (2) NaB(CsHd4; (3)KB(C6HJ4; (4) RbB(CBHd4; (5) C ~ ~ ( C G H S ) ~
tion of olefins was achieved (6) and the separation of saturated from unsaturated hydrocarbons is improved when the solid phase which covers alumina is changed in the series NaCINaBr-NaI (7). The existence of selective interactions is also proved in the case of some salts of heavy metals, such as copper complexes with ammonia, pyridine, phenanthroline, and dipyridyl(8,9). The behavior as adsorbants of phthalic acid isomers and its salts with potassium, as well as silver and copper tetraiodomercurate was studied (10, 11). Some copper amine complexes were proposed as selective stationary phases for the separation of pentanone and heptanone isomers (12). Except for the difficulties connected with low thermal stability, the use of these adsorbants is limited by their low capacity which demands a very small amount of sample, and by disorders caused by nonuniform filling of chromatographic columns. The last two disadvantages should be removed if the adsorbent can be dissolved in a common solvent and deposited on a solid support of the Chromosorb type (13). (6) J. J. Duffield and L. B. Rogers, ANAL.CHEM., 34, 1193 (1962). (7) C. G. Scott, in “Gas Chromatography,” M. Van Swaay, Ed., Butterworths, Washington, 1962, p 46. (8) A. G. Altenau and L. B. Rogers, ANAL.CHEM.,36,1726 (1964). (9) Zbid.,37, 1432 (1965). (10) J. E. Heveran and L. B. Rogers, J . Chromatogr., 25, 213 (1966). (11) B. T. Guran and L. B. Rogers, J . Gas Chromatogr., 5 , 574 (1967). (12) L. B. Rogers and A. G. Altenau, ANAL.CHEM.,35, 915 (1963). (13) A. G. Altenau and Ch. Merritt, J. Gas Chromatogr., 5, 30 (1967).
The present paper reports the behavior as stationary phases in gas chromatography of tetraphenylborates of sodium, potassium, rubidium, cesium, and thallium(1). EXPERIMENTAL
T o prepare these salts, we used the following reagents: sodium tetraphenylborate, Merck; potassium chloride, Union Chimique Belge; rubidium chloride, Riedel de Haen A.G. ; cesium chloride, B.D.H.; and thallium(1) sulfate, Riedel de Haen A.G. The synthesis of potassium, rubidium, cesium, and thallium([) tetraphenylborates constitutes a simple precipitation from concentrated solutions of the chloride (or sulfate) salt of each metal with a dilute solution of sodium tetraphenylborate followed by filtration and washing the obtained precipitate with distilled water until the reaction of the respective cation is negative. The precipitates were air-dried. The thermogravimetric curves of these compounds made using a Derivatograph model O.D.-102 show very good thermal stability up to at least 200 “C (Figure 1). Measurements were carried out with a Fractovap Model C gns chromatograph equipped with a hydrogen flame detector, using nitrogen as carrier gas. Chromatographic elution peaks were recorded on a Speedomax G 0-1 mV recorder (Leeds & Northrup, U.S.A.). For the introduction of various compounds, a Terumo 1-111 syringe was used, Then 0.1 pl of each compound was introduced separately. The substances used were of chromatographic purity. The chromatographic columns were made of thermc-resistant glass with an internal diameter of 3.6 mm and a length of 1.40 m. The column packing was prepared by mixing 60-80 mesh hexamethylsilanized Chromosorb W with a 15 solution of the various tetraphenylborates in acetone. The chroANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
1491
Table I. Relative Retentions (Referred to n-Heptane at 115 "C) and the Logarithms of the Capacity Ratios of Compounds Eluted from the Column with NaB(CBH& Compound a-Hexane +Heptane Octane Benzene Toluene Ethylbenzene Monochlorobenzene n-Propanol n-Butanol n-Amyl alcohol 2-Propanol 2-Butanol 2-Pentanol tert-Butyl alcohol tert-Amyl alcohol Propanone Butanone 2-Pentanone Methyl acetate Methyl butyrate Methyl chloride Chloroform Carbon tetrachloride
r log K 115 "C 75 "C 0.40 -1.387 1.00 -0.921 2.39 -0.597 0.43 -1.149 1.71 -0.735 3.54 -0.461 3.04 - 0.513 ... 69.14 40.57 ... 29.82 9.89 o:i95 10.86 0.317 17.79 0.597 3.75 -0.334 5.86 0.015 48.29 0.588 41.82 0.766 39.64 0.907 28.39 0.380 19.07 0.575 4.25 -0.308 0.61 -1.259
0.39
-1.387
log K 95 "C -1.657 -1.243 -0.885 -1.553 -1,032 -0.719 - 0.793
-0.198 -0,185 0.131 -0.757 - 0,463 0.368 0.401 0.432 0.127 0,143 -0.627 -1,509
log K 115 "C -1.958 -1.552 -1.173 -1.921 -1.319 -1.004 - 1.071 .0.713 0.055 -0.078 -0.558 -0.517 -0.303 -0.979 - 0.785 0.131 0.069 0.045 -0.099 -0,272 -0.924 -1.769
-1.678
-1.959
...
... ...
Table 11. Relative Retentions (Referred to +Heptane at 115 "C) and the Logarithms of the Capacity Ratios of Compounds Eluted from the Column with KB(CeH& r 115 "C Compound n-Hexane 0.50 n-Heptane 1 .oo 1.96 Octane 0.79 Benzene 1.90 To1uene Ethylbenzene 3.86 Monochlorobenzene 3.29 n-Propanol 12.92 13.25 n-Butanol 13.79 n-Amyl alcohol 4.63 2-Propanol 5.75 2-Butanol 2-Pentanol 8.29 tert-Butyl alcohol 2.46 3.79 tert-Amyl alcohol Propanone 12.38 10.38 Butanone 10.75 2-Pentanone 14.08 Methyl acetate 7.79 Methyl butyrate 0.49 Methylene chloride 0.38 Chloroform Carbon 0.33 tetrachloride
log K 75 "C -1.348 - 1.009 -0.662 -1.114 -0.684 -0.423 -0.463 0.315 0.562 0,592 -0.203 -0.124 0.265 -0,633 -0.363 0.036 0.106 0.235 -0.252 0.049 -1.131 -1.174
log K 95 "C - 1.638 - 1.309 - 1.027 -1.420 - 1.027 -0.735 -0.785 -0.152 -0.116 -0.092 -0.659 -0.602 -0.366 -0.966 -0,742 -0,112 -0.203 0.037 -0,334 -0.353 - 1.538 -1.601
log K 115 "C - 1.921 -1.619 - 1.347 - 1.721 - 1.357 -1.031 -1.102 -0.509 -0.498 -0.480 -0.955 -0.860 -0.701 - 1.229 - 1.041 -0,527 -0.604 -0.588 -0.471 -0.728 - 1.959 -2.046
-1.215
- 1,649
-2.097
matographic column was filled by tamping and conditioned in a stream of carrier gas for 12 hr a t 180 OC. RESULTS AND DISCUSSION
The relative retention calculated at 115 "C and the logarithms of the capacity ratios at temperatures of 75, 95, and 115 "Care reported in Tables I-V. The elution of alkanes, aromatic hydrocarbons, and of the monochlorobenzene on the columns containing as stationary 1492
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
Table 111. Relative Retentions (Referred to n-Heptane at 115 "C) and the Logarithms of the Capacity Ratios of Compounds Eluted from the Column with RbB(C6H& Compound n-Hexane n-Heptane Octane Benzene Toluene Ethylbenzene Monochlorobenzene n-Propanol n-Butanol n-Amyl alcohol 2-Propanol 2-Butanol 2-Pentanol tert-Butyl alcohol tert-Amyl alcohol Propanone Butanone 2-Pentanone Methyl acetate Methyl butyrate Methylene chloride Chloroform Carbon tetrachloride
r 115 "C 0.49 1.00 1.98 0.86 1.92 3'.16 2.59 20.14 17.81 15.95 5.38 5.51 8.41 2.38 3.89 44.41 15.27 12.68 14.16 9.51 0.59 0.54
logK 75 "C -1.180 -0.848 -0.510 -0,907 -0.514 -0.124 - 0.292 .., 0.789 0.816 0.001 0.158 0.572 -0.282 - 0.011 0,501 0.578 0.662 0.366 0.508 - 1.056 -1.092
logK 95 "C -1.408 -1.124 -0.857 -1.268 -0.845 -0.604 - 0.673 0.163 0.169 0.135 -0.402 -0.346 -0.114 -0.673 - 0,529 0,260 0.055 0.107 0.072 -0.126 - 1.357 -1.409
logK 115 "C -1.745 -1.432 -1,180 -1.495 -1.198 -0,932 - 1.018 -0.128 -0.181 -0,229 -0,701 -0,692 -0,507 -1,056 - 0,842 0.216 -0.248 -0,329 -0.281 -0,453 - 1 ,658 -1.699
0.43
-1.187
-1.495
-1.796
Table IV. Relative Retentions (Referred to +Heptane at 115 "C) and the Logarithms of the Capacity Ratios of Compounds Eluted from the Column with CsB(CgH& Compound n-Hexane n-Heptane Octane Benzene Toluene Ethylbenzene Monochlorobenzene n-Propanol n-Butanol n-Amyl alcohol 2-Propanol 2-Butanol 2-Pentanol tert-Butyl alcohol tert-Amyl alcohol Propanone Butanone 2-Pentanone Methyl acetate Methyl butyrate Methylene chloride Chloroform Carbon tetrachloride
r 115 "C 0.48 1.oo 2.09 0.92 2.02 3.83 3.54 10.21 10.46 10.67 4.08 4.88 6.96 2.08 3.13 10.63 8.92 9.38 9.29 8.75 0.54 0.33
log K 75 "C - 1,357 -0.921 -0.597 -0.996 -0.613 -0.296 -0,417 0.419 0.494 0.552 -0.107 -0,092 0.232 -0,476 -0.288 0.472 0.366 0.466 0,101 0.125 -1.149 -1.180
log K 95 "C -1.678 -1.243 -0.958 - 1.310 -0.951 -0.717 -0.796 -0.144 -0.116 -0.085 -0.544 -0.516 -0.359 -0.882 -0.747 -0.102 -0.237 -0.101 -0.283 -0.413 - 1.522 - 1.638
log K 115 "C -1.959 -1.553 - 1.309 - 1.658 -1.360 - 1.036 - 1.071 -0.611 -0.600 -0,592 - 1,009 -0.932 -0.777 -1,301 -1.125 -0.593 -0,669 -0.648 -0,652 -0,678 - 1.886 -2.097
0.17
- 1.229
- 1.795
-2.398
phases the tetraphenylborates of sodium, potassium, rubidium, and cesium follows the order of their boiling points, providing the lack of a noticeable interaction between the aromatic hydrocarbons and those stationary phases. The strong retention of the aromatic hydrocarbons compared with those aliphatics on the column containing thallium(1) tetraphenylborate is due t o the existence of thallium (I) ions having low-lying d-orbitals which may interact with r-orbitals of the aromatic hydrocarbons. The methyl and ethyl groups in the toluene and ethylbenzene molecules which
t0.8-
-0.4-
t0,6-
-0,6
t 0.4 -
-0.8 -
+0.2-
-1.0
-
-1.2
-
0 -0.2
-
-0.4
-
Y
5 -4.4 -
-0.6 -
@ -0.8IC, -f.U
-
-f.2
-
-f.4
-
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-
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-
-2.0
C
-C6
-
-1.8
-
50
70
90
110
t30
IS0
Boiling point
(OC>
Figure 3. Elution from KB(C6H& column at 115 "C Table V. Relative Retentions (Referred to n-Heptane at 115 "C) and the Logarithms of the Capacity Ratios of Compounds Eluted from the Column with TlB(C&)d
I
I
I
I
I
SO
70
90
{IO
t30
I
f50
Figure 2. Elution from NaB(CeH5)a column at 115 "C have a repulsive inductive effect, increase the reactivity of the aromatic nucleus and lead t o the increase of the relative retention of those compounds. The chlorine atom of the chlorobenzene molecule has the reverse effect. Like silver nitrate, thallium nitrate selectively separates saturated from unsaturated hydrocarbons (14, 13, because of its thermal stability (130 "C). It is t o be expected that thallium(1) tetraphenylborate, which is thermally stable above 200 "C, has the same properties. The alcohols are better retained on columns containing alkali metal tetraphenylborates compared with thallium(1) tetraphenylborate. The alcohol retention diminishes from the primary to the tertiary alcohols. It is interesting that elution is in the reverse order of the boiling points of the primary alcohols and ketones on the (14) B. T. Guran and L. B. Rogers, J. Gas Chromatogr., 3, 269
(1965). (15) D. V . Ranthorpe, C. Gatford, and B. R. Hollebone, ibid., 6 ,
61 (1968).
-
r Compound 115 "C n-Hexane 0.42 n-Heptane 1.00 Octane 1.93 Benzene 1.56 Toluene 3.49 Ethylbenzene 6.02 Monochlorobenzene 2.88 n-Propanol 3.96 n-Butanol 14.71 n-Amyl alcohol 26.82 2-Propanol 1.24 2-Butanol 2.46 2-Pentanol 6.39 tert-Butyl alcohol 0.89 terr-Amyl alcohol 1.83 Propanone 0.98 Butanone 1.73 2-Pentanone 2.97 Methyl acetate 0.60 Methyl butyrate 2.15 Methylene chloride 0.43 Chloroform 0.53 Carbon tetrachloride 0.35
logK 75 "C -0,488 -0,093 0,297 0.209 0.764 1,049 0.561 0.623
0,339 -0.030 0.278 0.626 -0,309 0.409 -0.511 -0,372
logK 95 "C -0.786 -0.405 -0.074 -0.141 0.258 0.489 0.146 0.263 0.743 1.062 -0.297 0.034 0.439 -0.384 -0,061 -0.362 -0.137 0.166 -0.659 -0.037 -0.758 -0.678
1ogK 115 "C -1,060 -0,680 -0.394 -0.487 -0.137 0.100 - 0.220 -0.082 0.488 0.749 -0,585 -0.288 0.125 -0.728 -0,418 -0.690 -0,442 -0.207 -0.899 -0.348 -1.046 -0,959
-0.524
-0.854
-1.131
...
...
0.066
0.466 0.959 -0,051
columns containing sodium and rubidium tetraphenylborates. This effect is less pronounced in the case of the cesium and potassium tetraphenylborates columns and inexistent when thallium(1) tetraphenylborate is used as stationary phase. The elution esters in the order methyl-butyrate-methylacetate is a general phenomenon when using alkali metal tetraphenylborates as stationary phases. The esters are ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
1493
Table VI. The Adsorption Heats-AH (Kcal/mol) Cornpound +Hexane n-Heptane Octane Toluene Monochlorobenzene n-Propanol 2-Propanol rert-Butyl alcohol 2-Pentanone Methyl acetate Methylene chloride Chloroform Carbon tetrachloride
8.64 9.32 9.96 9.00 8.59
RbB(C&h
KB(CaHd4
NaB(CeHd4 f 0.32 i 0.41 f 0.64 i:0.64 f 0.41
...
8.82 i 0.55 9.51 f 0.69 10.56 f 0.46 10.05 f 0.46 9.83 f 0.59 12.70 i 0.73 11.61 f 1.23 9.19 f 0.69
13.33 i 0.41 7.36 f 0.37 9.32 f 0.09 7.86 f 0 . 3 2 8.77 f 0.55
7.91'; 0.69 12.75 f 0.69 13.44 i:0.64 13.62 =t0.78
1 1 . 5 6 2 0.55
8.77 9.00 10.33 9.78 10.56
f 0.82
i 0.55 i 0.32 f 0.18 f 0.82
2
10.83 0.78 10.05 f 0.37 15.26 f 0.73 10.19 f 0.82 9.28 f 0.37 9.37 f 0.37 9.37 f 0.37
CsB(CeHd4
TIB(CeHd4
9.28 f 0.37 9.69 i:0.69 0.64 10.83 =I= 9.96 f 0.46 10.10 f 0.69 15.86 f 0.87 13.89 f 0.78 12.70 f 0.69 17.14 0.82 11.61 i 0.37 11.33 zt 0.69 11.12 i 0.69 17.96 f 0.91
8.82 f 0.23 9.05 f 0.37 10.65 f 0.50 13.89 f 0.87 12.02 f 0.69 10.22 f 0.73 10.05 f 0.41 10.42 f 0.59 12.80 i 0.59 9.09 f 0.78 8.23 f 0.46 9.05 f 0.32 9.32 =!= 0.69
-0.5 -0.7
-0.9 -A!
-0.41 -0.6
-
-Q8
-
-t.3
y
3
c3
- l.5
- i.0 -
- 1.7
-
419
-1.4-
- 2.1 - 2.3
-1.2
-!.6
-
-4.8
1
I
I
I
50
70
90
I
110
I
130
I
150
Boiling point P C )
p%q % I
I
1 %
I
50
70
90
if0
I
I
130 150
Bolling point C°C) Figure 5. Elution from CSB(C&)~ column at 115 "C
Figure 4. Elution from RbR(C6H& column at 115 OC
similarily eluated from the thallium(1) tetraphenylborate column in the order of their boiling points. The interaction between the alkali metal tetraphenylborates and alcohols, ketones, and esters at 115 "Cis strongly manifested with the first members of the homologous series used. In some cases this interaction overcomes the effect of the boiling points of a given series. 1494
ANALYTICAL CHEMISTRY, VOL. 44, N O . 8, JULY 1972
The relative retention of chloromethanes does not vary considerably from one column to another, except for methylene chloride which is retained longer on sodium tetraphenylborate. The plotting of capacity factor logarithms against the boiling points of the components in the homologous series at 115 "C gives generally straight lines. Exceptions are chloromethane on columns of sodium tetraphenylborate and thallium tetraphenylborate, and the primary alcohols on a column of sodium tetraphenylborate or the secondary
alcohols and ketones on a column of rubidium tetraphenylborate (Figures 2 to 6). The straight lines obtained for the hydrocarbon homologs are almost parallel. Similar plots are obtained for ketones and secondary alcohols OR alkali metal tetraphenylborates, but the primary alcohols and the chloromethanes give straight lines generally differing in their slopes from one column to another. The slopes of the straight lines from plots of log K us. 1/T X l o 4 allow the estimation of adsorption heats ( A H ) of different components on the stationary phases. The slopes have been calculated by making use of the least squares procedure, the standard error of the slope being determined from the regression analysis. Table VI shows these adsorption heats. For the aliphatic hydrocarbons, the quantity AH does not noticeably differ from one column to another. Its value increases with increase of the molecular weight in the homologous series. For alcohols, the adsorption heat values diminished generally in the order: primary, secondary, and tertiary. The highest values were obtained on a column of cesium tetraphenylborate. For toluene, the highest value is reached on thallium tetraphenylborate. Pentanone-2 and methyl acetate have AH values that increase going from sodium tetraphenylborate to cesium. Except for methylene chloride on a column of sodium tetraphenylborate, the adsorption heats for chloromethanes increase from methylene chloride to carbon tetrachloride, the highest values being obtained on a column of cesium tetraphenylborate.
-
tOS8
+0.6 tu.4
-
t0.2
-
0 -
a
-0.2
-
9 -0.4-0.6
-
-0,8
-
-f.O
-
-1.2
I
I
I
I
I
I
I
50
70
90
if0
f30
150
I
Boiling
point (OC) Figure 6. Elution from T1B(C6H& column at 115 "C
RECEIVED for review November 2, 1971. Accepted January
27, 1972.
Gas Chromatographic Determination of Residual Methanol in Food Additives M. A. Litchman and R. P. Upton Pfizer Inc., Groton, Conn. 06340 QUANTITATIVE DETERMINATION of
