Table 111. Ultimate Sensitivity of Electron Drift-Velocity Detector to Various Gases
Compound Hz
s,
CH, CO COZ
CzHe 0,
Lower detectable limit P.p.m. Gram/sec. v0l.a 1 0 . 3 x 10-lo 1 x 10-10 0.2 1 3 x 10-10 3 x 10-10 0.6 4 x 10-10 0.6 1 5 X lo-'' 2 11 x 10-10 15 X IO-'' 6
HzO 35 X 50 He a Sensitivity of chromatographic analyzer with 10-ml. sampling volume and 30-second peak width on compound of interest. synthesis circulating gas, a longer column is required to resolve the CO peak from the nitrogen. -1chromatogram for 20 p.1i.m. of CO in air is shown in Figure 8. .2 3,/1,j-in~hColumn 5 feet in length, packed with 5.1 Molecular Sieve, was used in this analysis. Carbon Di0xid.e. -1 sample of 20 p.1i.m. of carbon dioxide in argon was used to establish t'he sensitivity of the detector to carbon dioxide. I n this measurement the column used was inch by 20 feet, packed with 207, silicon SF96 on 30- to 60-mesh firebrick. The column was run a t 25' C. This column separat,es nitrogen and oxygen from the CO, so that, possible air contamination in the sampling valve could not cause erroneous results. The response to carbon dioxide is about equivalent to t h a t for carbon monoxide. Hydrocarbons. Although the detector is primarily useful for the
permanent gases, it also responds t o hydrocarbon samples. lIeasurement,s were made on methane, ethane, etbylene, propane, and propylene using a 1/4-inch by 20-foot column packed with 20% silicone SF96 on 30- to 60mesh firebrick. Methane and ethane give normal responses and can he detected in the range 10-10 to 10-9 gram/second. However, the responses to propane and the olefins were anomalous; the propane produced a double, negative peak and the olefins appeared to act as electronegative gases causing complete desensit'ization of the cell for some period of time after their emergence. Further work will be required on these hydrocarbons to clarify the results and to determine whether the observed behavior is reproducible. Water Vapor. Onl!. one satisfactory experiment was done with water vapor. The chromatogram in Figure 9 shows the response obtained from the injection of 100 PI. of air with a relat'ive humidity of 40% at, 24" C. The column used for this experiinent' was 3/!6 inch X 40 inches, packed with 40% Poly G400 on Chromosorb W at' 75' C. From this experiment, it is estimated that the lower detectable limit for water is about 15 X 1O-lo gramlsecond. Linearity of Response. The response of the detector is linear only in the low parts per million range (less than about 100 p.p.ni.). Qualitative experiments have shown considerable nonlinearity above 100 or 200 p.p.m. of nitrogen or oxygen and total saturation a t a level of a few hundred p.1i.m. of either of these gases. Similar nonlinear response is to be expected with all gases, and the detector
Figure
9. Response to water vapor in air
is essentially useful only for trace analysis. ACKNOWLEDGMENT
The author$ gratefully acknowledge the assistance of F. 13. Rolfson in the design of the pulse generator circuit. LITERATURE CITED
(1) Healey, R. H., Reed, J. "The Behavior of Slow Electrons in Gases," Amalgamated Rireless, London, Iliff e, 1941. ( 2 )Lovelock, J. E., A4NAL. CHEM. 33, 162 (1961). (3) Lovelock, J. E., Suture 187, 49 (1960). (4) Shahin, &I. 3f.] Lipsky, S.R., ANAL. C ~ M35, . 467 (1963). 15) Smith. V. N.. hlerritt. E. J.. Zbid.. 34, 1476 (1962) RECEIVEDfor review March 11, 1964. .4ceepted May 14, 1964. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1964.
Carbon Skeleton Chromatography Using Hot-wire The rma I-Cond uctivity Detection MORTON BEROZA and RAFAEL SARMIENTO Entomology Research Division, Agricultural Research Service,
b The catalytic apparatus previously used with a flame ionization gas chromatograph to determine the chemical structure of minute amounts of compounds has been modified for use with gas chromatographs having hotwire thermal conductivity detectors. Chromatograms of different types of compounds are presented. Peaks are usually sharp and well resolved. Sample size i s between 0.05 and 2 pl. and the effect of varying sample size i s illustrated. The modified apparatus produces enough product for spectral analysis, from which identifications made on the basis of retention
-
1744
ANALYTICAL CHEMISTRY
U. S.
Department o f Agriculture, Beltsville, Md.
time may b e confirmed or denied. The effect of catalyst temperature on the yield of aromatics and cycloaliphatics from 6-membered carbocyclic structures was studied. With the catalyst temperature at 200" C. cycloaliphatics are favored; at 360' C. the aromatics predominate. Features of the apparatus design that contribute to maximum performance, advantages, and disadvantages of the flame ionization and katharometer catalytic units are discussed. The analysis i s rapid and is as broadly applicable as the one previously advanced for use with flame ionization.
CATALYST-CONTAIXIX assembly that attaches to the injection port of a flame ionization gas chromatograph helps determine the chemical structure of a aide variety of organic compounds in microgram amounts ( 2 , 4). Thiough the use of improved palladium catalysts the length of carbon chain that may be analyzed has been raised to at leait the C ~ O level ( 6 ) , these c*atalyats also make possible the analysis of amines (or ainine precursors). The advantages of temperature programming of the analytical column, thi. results with different catalysts, the operating paramHOT
eters, and the results with structures containing two rings have been discussed recently ( 5 ) . The apparatus and technique have now been modified for use with a hotwire thermal-conductivity detector (will be referred to as katharometer) and satisfactory results ha7;e been obtained up to the Czolevel. Principles of operation do not differ from those already described ( 2 , 6). With our equipment a single injection of 0.05 to 2 pl. of compound gave good results (the lower limit of compound that can be analyzed would depend, of course, on the sensitivity of the katharoineter employed). Identifications of products based on retention times are readily verified because a single injection of compound can yield sufficient product for a n ultraviolet or infrared spectrum. Simple apparatus for determining the spectra are described. With these spectra the products of benzene and toluene derivatives xere identified and determined at different catalyst temperatures. Chromatograms of a variety of compounds-both aliphatic and aromatic-are presented. The effect of sample size is illustrated. The advantages and disadvantages of utilizing flame ionization us. katharometer detection with this technique are discussed. The present analysis is rapid and requires no special skill to carry i t out. EXPERIMENTAL
Apparatus and Materials. CATAASSEMBLY.T h e assembly is a modification of apparatus B of reference (6). The aluminum catalyst tube is 3/8-inch i.d. X 3/4-in~ho.d., and 9l/, inches in length. The new design differs from the previous one in having its hydrogen inlet tube wrapped around the heated catalyst chamber to preheat the incoming carrier gas (tube is between heating jarket and the outer insulation) ; the exit fitting accommodates a short length of 17-gauge needle stock which pierces the septum of the gas chromatograph injection port. This exit arrangement makes it possible to connect the assembly rapidly to any gas chromatograph without special fittings. Removal is likewise rapid. The exposed section between the catalyst tube acd the injection port is wrapped with glass wool. An aluminum spacer, which occupies the volume between the septum of the injection port and the hydrogen inlet port but 11hich has a small hole through its center to permit the passage of the syringe needle, has been included to provide a minimum dead volume. The catalyst tube exiting arrangement, also of minimum volume, and the injection port of the gas chromatograph, are kept hot to prevent any hydrocarbon delay or peak spread. This feature is dspecially important with higher hydrocarbon products--i.e., those in the Cl4-C20 range. LYTIC
Figure 1 . Schematic arrangement of carbon skeleton determinotor on gas chromatograph (dotted lines represent confines of oven) A. B. C. D.
E. F.
G. H. 1.
Hydrogen source Pressure regulator Molecular sieve trap Hot-wire thermal-conductivity detector (reference side on left) Toggle valve Toggle valve Catalytic assembly Injection port of gas chromatograph Chromatographic column
GAS CHROMATOGRAPH. An ilerograph Model A-90-AC (Wilkens Instrument & Research, Walnut Creek, Calif.) equipped with a 1-mv. recorder was used in these studies. Any other gas Chromatograph having a hot-wire thermal-conductivity detector of sufficient sensitivity may be similarly employed. A slight modification of the chromatograph is required to accommodate the carbon skeleton determinator-Le., the carrier gas (hydrogen) is permitted to pass through the reference side of the detector and then diverted to the inlet tube of the catalyst assembly. The tube that normally leads to the injection port from the reference side of the detector is closed. The arrangement used by the authors is schematically illustrated in Figure 1. It permits the catalytic assembly to be cut in or out of operation quickly and without disassembly of any tubing. When the catalytic unit is used, valve E is closed and valve F is open. When not in use (in regular gas chromatographic runs) valve F is closed and valve E is open. In the latter instance the catalytic unit is disconnected from the injection port. Davies and Howard (8) and Cowan and Stirling ( 7 ) point out that a thermistor detector (semiconductor of fused metal oxides) will deteriorate (especially above 100" C.), even if glass-coated, when hydrogen is the carrier gas. The gas apparently attacks the metal oxides and thereby promotes breakdown of the thermistor. Because hydrogen is the obligatory carrier gas in the present process, gas chromatographs employing the present type of thermistor detection are not believed to be suitable for use with the catalytic unit. Because thermistors lose sensitivity at elevated temperatures, they would not be suitable for analyses of high molecular weight compounds (C14-C& even if they were to survive exposure to hydrogen.
Caution. The hydrogen carrier gas must be burned a t the exit of the chromatograph or suitably vent'ed. The major difficulty in operat'ing this apparatus is the avoidance of leakage. A pressure-drop leak detector, described elsewhere, makes certain that no leaks are present (3). MOLECULAR SIEVETRAP.The carrier gas is purified by passing it through a dual pass trap containing 60 ml. of 5A molecular sieve (Linde Co.). The all metal trap assures a stable base line at high sensitivities (especially necessary with t,he flame ionization detector) and appears to extend the life of the catalyst. The molecular sieve may be regenerated without removal by placing the trap in a muffle furnace set at 300" C. and allowing a slow stream of nitroger, to flow into the exit tube. Caution: Both inlet and exit tubes must extend outside of the furnace to avoid releasing into the furnace any hydrogen that may be retained by the molecular sieve. CHROMATOGRAPHIC COLUMNS.Three were employed and all were 8-foot, inch 0.d. copper columns. One contained 570 of silicone gum SE-30 on 60- to 80-mesh acid washed Chromosorb W (Johns Manville, New York, N. Y.). The second contained 10% of Dow Corning silicone oil 710 on the same support. The third column contained a homogenized 1 : l physical mixture of the two packings. CATALYSTS.The neutral palladium cat,alyst (ly0Pd as the metal) on 60- to 80-mesh Gas Chrom P (Applied Science Laboratories, State College, Pa.) was used (6). The catalyst was activated in a stream of hydrogen (ca. 20 ml./per minute) as previously described for 30 minutesat 125"C.,30minutes a t 200'C., and 20 minutes a t the test temperature. The activity of the cat,alyst gradually deteriorates with age after activation; however, good analytical results have been obtained as much as 10 to 12 days after activation. Loss in catalyst activity is accelerated by operating at 360" C. and by analyzing too many large sized samples. In these cases activity was followed by iioting the extent to which 0.5 pl. of benzene was converted to cyclohexane a t a catalyst temperature of 280" C. This test, of course, provides a measure of hydrogenation rather than hydrogenolytic activity. SYRINGE. A 1- and IO-p1. syringe (Hamilton Co., Whittier, Calif.) was used as appropriate. Procedure. T h e general procedure is the same as t h a t described in reference ( 2 ) . The catalyst t'ube was maintained a t 280' to 285' C. unless otherwise specified. The hydrogen flow rate was 85 ml. per minute (in some experiments the flow rate was 100 ml. per minut'e). Although the amount of sample injected was 0.05 t'o 2 pl., good results were obtained with 0.1 to 0.2 pl. of compounds below the Cio level and 0.5 pl. of compound at the Cio to C20 level. Carboxylic acids or the acid portion of esters and amides require larger size samples and the response is poor. VOL. 36, NO. 9, AUGUST 1964
0
1745
RESULTS A N D DISCUSSION
Compounds having as many as 20 connected carbon atoms produced the hydrocarbons expected on the basis of previous work with the flame ionization detector. [See Table I1 of reference (2) for listing of expected hydrocarbons.] Figure 2 shows typical elution patterns of both smaller (A through I ) and larger (J through &) compounds of various types; the compounds are identified in Table I. Peaks are generally sharp and well defined. Many of the chemicals available to us mere straight chain compounds. -4ccordingly, two mixtures of straight chain, alpha olefins (Cs-Cs and Clo-C,o) , which were on hand, were used as standards to determine retention times of these compounds. Their chromatograms, shown in Figure 3, will give a n idea of resolution that may be obtained. Alpha olefins hare recently assumed considerable commercial importance (21). The present procedure can be
used to determine the branched chain isomer content of these compounds. EFFECTOF SAMPLESIZE. With the flame ionization unit the capacity of the catalyst for conversion of compounds was not readily determined because a reliable means of injecting known amounts of undiluted compound a t the 1- to 20pg. level was not available. The present apparatus can accommodate a sample size that is measurable with reasonable accuracy, usually with a 1-pl. syringe. Figure 4 shows the results of analyzing different classes of compounds at several sample size levels. To facilitate comparison of the chromatograms, the attenuation was doubled when the sample size was doubled, quadrupled when the sample was quadrupled, etc. By this procedure the chromatograms should remain the same if conversion of compound is complete and the analytical column can accommodate the quantity of product. With 3-hexenol (Figure 4A,
w v,
Z 0
a v, W
OL
a!
w n
OL 0 0 W
tLi
M I N U T E S Figure 2.
M i N U T E S
Figure 3. Chromatograms of hydrocarbons from straight chain, alpha olefins used as standards A.
CrCs
Clo-Czo (only even no. peaks labelled) Column SE-30. Column temperature, A 58' C.; B 208' C.
E.
product is pentane) there is a progressive loss of response with increased amounts of sample. The catalyst capacity appears to be exceeded and the proportionate response thereby decreased. This effect also appears to be operating with heptaldehyde (Figure 4B, products are hexane and a little heptane), but not with 1-ethylpentyl hexanoate (Figure 4C), which appears to have suffered practically no loss in response up to 1.6 plS3a t which level the peak broadens a little. (Product of 1-ethylpentyl is heptane; pentane expected from the hexanoate is not discernible.) With toluene (Figure 4 0 ) methyl cyclohexane (first peak) and toluene (second peak) are obtained. Sample size markedly alters the proportion of products obtained, the conversion of toluene to methylcyclohexane being proportionately decreased as sample amount grows; additionally, response diminishes. With methylcyclohexane (Figure 4E) products are the same as with toluene. Response diminishes and the proportion of toluene
Elution patterns of aliphatic compounds
Compounds identifled in Table I. I through Q, ca. 205' C.
Column SE-30.
Column temperature, A through I, ca.
60' C.j
-1x-
-1xTable 1.
Pattern A
Type
B C D E F G H I J
Acid Alcohol, primary Alcohol, secondary Aldehyde Ester Ether Halide Ketone Sulfide Alcohol (diol)
K
Amine
L
Epoxide Ester Ester Ether Halide Thiol
M
N
0 P
Q
1746
ANALYTICAL CHEMISTRY
-2x-
-4x-
-*
-2x-
-4x-
-ax-
I
Elution Patterns of Figure 2
Compound Hexanoic 3-Hexen-1-01 2-Octanol Nonyl aldehyde 1-Ethylpentyl pentanoate Hexyl ether Octyl bromide 3-Heptanone Pentyl disulfide Ricinoleyl alcohol (9-Octadecen-l,l2-diol) A',A7-Dimethyl-9-octadecenylamine 1,2-Epoxyhexadecane Methyl oleate Ricinoleyl diacetate Allyl hexadecyl ether Octadecyl bromide Dodecyl mercaptan
Microliter injected 1.o
1.0
0.5 0.5
1.0
0.5 0.5 0.1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
Attenuation 1x
8X 1x
1x 1x 1x 1x 4x 1x 1x
-1IX-
-2x-
-4x-
-8X-
2x
1x
1x 1x 1x 1x 1x
M I N U T E S Figure 4.
Effect of sample size
Microliters per analysis of A, 3-hexen-1 -01; E, heptaldehyde; C, 1 -ethylpentyl hexanoate; D, toluene; E, methylcyclohexane. Attenuation shown above curve and time in minutes below each chromatogram. Column SE-30. Column temperature A, 4 3 " ; B, 5 4 " ; C, 51 '; D, 56'; E, 53' C. Flow rate 100 ml. per minute
G
a F
Figure 5. Apparatus for collecting effluent for spectral analysis A. B. C.
D. E.
F. G.
Exitport Silicone septum Nut that holds septum in place Hollow rod terminatina in male Luer fltting is silver soldered to nut C Bent hypodermic needle Silica cell for ultraviolet spectra analysis Tube for collecting effluent far infrared spectra
increases slightly with increasing sample amount. The foregoing data show that samples in the 0.1- to 1.6-p1. range (1 p1. = ca. 1 mg.) may be readily analyzed as a single injection with the modified catalytic unit. Yields from larger amounts of material, however, are proportionately not imually as great as those from smaller sainples. Identification of Peaks by Ultraviolet and Infrared Analysis. Unlike the flame ionization detector, the katharometer does not destroy the sample and sufficient product can be produced for spectral analysis. Thus, the identification of aromatic compounds, which are formed from 6membered carbocyclic structures (see chromatograms of Figures 4 0 and 4E),may be facilitated or confirmed by determining their ultraviolet absorption spectra. 'The very simple arrangement, shown in Figure 5 , was used to obtain such 3pectra. The exit port of the chromatograph was fitted with a male Luer adaptor to which could be fastened a lOO-mm., 18-gauge hypodermic. needle (Becton, Dickinson & Co., Rutherford, N. J.) bent as shown in Figure 5 . When the desired peak started to elute (as indicated by recorder trace), the needle was attached to the male h e r fitting and the effluent emerging from the needle was bubbled directly into 2.5 ml. of isooctane contained in a 1-cm. square silica cell ;Is soon as the peak was
eluted, the needle (hot) mas removed and an additional 0.5 ml. of iso-octane was pushed through the needle into the cell. The injection of a 0.5-p1. sample usually provided sufficient product for the spectrum and sometimes more than enough (dilutions as great as 6-fold Fvere required in some cases t o obtain a spectrum on scale). Benzene. toluene, naphthalene, and tetralin spectra were readily identified. An increase in sensitivity that is a t least 10-fold was effected by bubbling the effluent of a peak into a small volume of iso-octane (or other suitable solvent) cooled in ice water (as in infrared procedure which follows) and the spectrum of the solution determined in a microcell. For example we have obtained a strong ultraviolet spectrum from 20 pg. of benzene that was injected into the flame ionization unit by diverting the effluent into cold iso-octane as the peak started to emerge. A 0.5-p1. sample usually supplied sufficient product for a very weak spectrum of saturated aliphatic hydrocarbons in the infrared from which it was usually possible to pick out the positions of the strongest bands and thereby confirm or deny identifications made on the basis of retention time. The procedures used follow.
The same needle and adaptor arrangement shown in Figure 5 was used as described above except that' the effluent of the desired peak was bubbled into 50 pl. of carbon disulfide contained in a thin-walled glass tube (90-mm. ..length, 2-mm. i d . ) whicah is cooled with dry ice. The solution was transferred to a 3-mm. path length microcell (Perkin-Elmer) and the infrared spectrum was determined. A scaleexpansion attachment on the infrared spectrophotometer was most helpful. If enough sample is available, the desired peaks from several injections may be collected, or a larger amount of sample may be injected. If dry ice is not available, tetrachloroethylene cooled in ice water has given good results, but less of irs spectrum is usable. We have also trapped compounds and obtained infrared spectra in the gaseous phase essentially as described by Anderson ( I ) and Haslam, Jeffs, and Willis (12) by transferring the trapped product to a commercially available gas cell having a i.5-cm. path length. This technique is more elaborate hut gives better results. I t is suitable for compounds boiling below 160" C. and compounds too volatile to trap by the procedure of the previous paragraph.
2x7
4 xO 7" r-4X
Q
r4x
M I N U T E S Figure 6. Effect of catalyst temperature on production of cyclohexane and benzene from cyclohexane and benzene Lower graphs: 0.5 pl. o f benzene injected a t indicated catalyst temperatures. pl. o f cyclohexane injected a t indicated catolyst temperatures. Column DC 71 0. ca. 64' C.. Attenuation IX except a i noted
Upper grophs: 0.5 Column temperature
VOL. 36, NO. 9, AUGUST 1964
1747
0
b
A
I1
I,
4
0
I1
A - ,
" I BLJI , , ,
,
/
,
a
,
M I N U T E S Figure
7.
Effect of catalyst temperature on products obtained from:
A. B. C. D.
Methylcyclohexone Toluene Benzyl alcohol Benzaldehyde E. Benzonitrile Sample size 0.2 4. Column SE-30-DC tenuation 1 X except as noted
710 combination.
Aromatic and Cycloaliphatics from Ring Compounds. This effect was studied with benzene and cyclohexane and the results are summarized in Figure 6. The amount of conversion of benzene to cyclohexane decreases as the catalyst temperature is increased from 200' to 360" C. The conversion of cyclohexane to benzene is increased as the temperature is raised between 200" and 360' C. Otherwise expressed, saturated structures are favored at the lower temperatures, aromatic ones a t the higher temperatures. The data are consistent with the heat of hydrogenation being exothermic:
49.8 kcal. per mole
Thermochemical data show that the heat of formation of an aromatic system is greater than the sum of the heats of formation of the individual bonds. The difference has been ascribed to an increase in molecular stability conferred by the delocalization of the electrons of the aromatic structure (23). This energy is emitted as heat on hydrogenation, which provides a measure of 1748
ANALYTICAL CHEMISTRY
Column temperature ca.
64'
C.
At-
mesomeric energy of aromatic systems. Ingold ( I S ) gives an approximate mesomeric energy of 40 kcal. per mole for every 6-membered aromatic ring of a polycyclic structure. Accordingly, we can expect aromatic products to be favored a t the higher catalyst temperatures and cycloalkanes a t the lower catalyst temperatures. I n our work with the present unit a t catalyst, temperatures between 200' and 360' C., which is admittedly limited, we have found no exceptions to this generalization. RIethylcyclohexane, toluene, benzyl alcohol, benzaldehyde, and benzonitrile-which represent several classes of compounds-were subjected to analysis with the catalyst a t 200°, 280", and 360" C. The results are shown in Figure 7 . Even cursory inspection of the data shows that the results fall in line with the foregoing reasoning. the product With the catalyst a t 200" C., is methylcyclohesane in each case. Raising the catalyst temperature to 280" C.causes the aromatic structures to appear, and in the cases of benzyl alcohol, benzaldehyde, and benzonitrile, the next lower homologs (aromatic and cycloaliphatic) , as expected. R h e n the catalyst temperature is elevated to 360' C., only aromatics are produced.
Figure 7 is a good illustration of the fact that higher temperatures favor hydrogenolysis (or fragmentation) of end carbons holding oxygen and nitrogen substituents ( 2 ) . Analyses a t the 360'-catalyst temperature may be used to determine the carbon skeleton of polycyclic structures, which should aromatize and give a good ultraviolet spectrum with but little sample. Advantages of this technique over zinc and selenium dehydrogenation procedures are the small amount of sample required and the ease and speed of carrying out the reaction. The neutral catalyst is well suited for this reaction because it does not tend to cleave C-C bonds unless oxygen is attached to an end carbon ( 2 ) . I n a complex molecule the presence of such carbon atoms could be sought for in an ShIR spectrum. The ratio of products formed (see Figure 7C, D , E a t 280' and 360" C. catalyst temperatures and Figure 8) will vary with the ring substituents. The benzene and cyclohexane derivatives shown in Figure 8 were run with the catalyst at 280" C.because at this temperature aromatic and saturated structures are obtained and the effect of different substituents on the amount of each is readily observed. The patterns are not quantitatively sufficiently reproducible probably because of variation in catalyst activity. However, the amounts produced can be related to those given by a reference compound such as benzene. When this defect is overcome and the rules governing the formation of these products are understood, ratios of products may be another means of facilitating identifications. The dragging of peaks, observable in Figure 8A, F , G, J , K , indicates that the substituent is polar. Another facet relating to the amount of aromatic and cycloaliphatic product that is obtained has already been discussed-Le., sample size. Thus the injection of 0.5 pl. of benzene on catalyst a t 200" C. produced about 60% cyclohexane. Immediately after this run 0.1 pl. of benzene was injected under identical conditions; the product was solely cyclohexane. Sample size is therefore an important consideration. I n summary, the extent to which 6-membered carbocyclic compounds are saturated or aromatized in the present apparatus depends on catalyst temperature and activity, size of sample, whether aromatic or saturated originally, and substituents. Comparison of Catalytic Units Used with Katharometer and Flame Ionization Gas Chromatographs. The flame ionization and katharometer units each have their advantages and disadvttntnges. If the amount of sample avai1al)le is severely limited, the flame
ionization unit will be preferred and one must usually be content with a n identification made on the basis of retention time on one or more columns [except if the product has absorption in the ultraviolet or a mass spectrometer is available (f8)].With enough sample on hand, the katharometer unit may be preferred because enough product is readily obtained for identification by an spectral independent means-Le. , analysis. Retention times and resolution of peaks with the flame unit are superior to those obtained with the katharometer unit for several reasons. The smaller bore permits less peak spread than the larger bore. The response of the flame detector is much( more rapid than that of the katharometer and gives consequently a much more accurate record of the reaction. The thermal pattern in the smaller bore tube is more uniform. Smaller samples, such as are handled by the flame unit, give better resolution of peaks (less overlapping of peaks) and avoid overloading the catalyst (which decreases catalyst activity). We could not, for example, get a recognizable Ci6 peak from palmitic acid with our katharometer unit but did get one with the smaller flame unit. This objection would be overcome, at least partly, if a very sensitive katharometer were to be used. Some very sensitive katharometers, recently devised, can undoubtedly operate satisfactorily with the small bore catalytic assembly.
The flame unit does not respond to inorganic substances-e.g., COZ, CO, H20, NHs, HCl, HBr, HI. I n the hydrogenolysis of compounds such inorganic substances are formed. Oxygenated compounds may give water, sulfides HzS, amines KH3, etc. Compounds giving the next lower homologe.g., acids, aldehydes, primary alcohols-may produce CO or Con. (Identity of these'products has not yet been determined.) These products appear on the chromatograms of the katharometer unit (see first peaks of Figure 7 C , D ,E , a t 360' C.) but not on those of the flame unit. With our katharometer unit, water does not cause appreciable interference because it does not form a peak. Thus, injection of water a t a column temperature of 65' C. caused only a slight rise in base line, which line gradually returned to zero. The estimation of water may be possible-e.g., through collection on an adsorbent and weighing, or by reaction with a reagent such as calcium carbide (16) or Karl Fischer reagent. Analyses of other inorganic products, which need not be chromatographic, may likewise be possible. However, if the action of HCl is indicative, desorption of such polar compounds from the catalyst is slow. GENERAL COMMENTS
Gas chromatography has enabled the chemist to isolate minute amounts of compounds in pure form. Techniques are now needed to help identify the
M I N UTES Figure 8. Elution patterns of benzene and cyclohexane compounds. hexane peak a t 2.4 minutes, benzene at 3.2 minutes
Cyclo-
A. 6.
Aniline Benzenethiol C. Bromobenzene D . Chlorobenzene E. Chlorocyclohexone F. Cyclohexonol G. Cyclohexanone H . Diphenyl ether 1. lodobenzene 1. Nitrobenzene K. Phenol 1. Phenetole M. Phenyl acetate Sample injected 0.5 pl. Column D C 71 0. noted. Catalyst temperature 280' C.
chemical structure of these minute amounts of substances, especially if they are new or unknown. Classically, the determination of an unknown structure has been accomplished through the chemical degradation of the molecule into identifiable fragments. If the molecules can be degraded or modified practically instantaneously (or the products held up for sudden release), the reaction and gas chroma tographic analysis can be united into a single operation. This technique, which has come to be known as reaction gas chromatography, includes work on the cracking, hydrogenation, dehydrogenation, elemental analysis, and pyrolysis of compounds and mixtures (9-11, 14, 16, f7, 19, 20, 22, 24); but prior to the present technique reaction gas chromatography as a structure determining tool, particularly a t the submilligram level, has not seen much use. One of the major advantages of the present technique is its applicability to a great variety of compounds. The procedure has been refined to give maximum resolution of peaks and therefore analytically useful data, but much more work is needed to realize its full potential (which encompasses more than structure determination). The reactions of the process are mainly vapor phase hydrogenation, dehydrogenation, and hydrogenolysis. Additional information on these reactions can be amassed quickly with the new apparatus and may help us answer our need for determining chemical structure of microgram amounts of compound recovered from gas chromatographic runs. The new apparatus extends the technique previously advanced ( 2 , 46) to the hot-wire katharonieterequipped gas chromatograph. Theie are presently a t least several times as many katharometer-equipped gas chromatographs as there are gas chroniatographs utilizing flame ionization detection. Other detectors, each providing their own special advantages and disadvantages may be adapted to utilize and explore the present catalytic process and apparatus. Toward this end it is pertinent to note that only minor modifications are required to adapt the apparatus to almost any gas chromatograph. The apparatus is being manufactured by Kational Instruments Laboratories, Inc., Rockville, h l d . LITERATURE CITED
Column temperature 65'.
Attenuation 4X except where
(1) Anderson, D. 11. W , Analyst 84, 50 ( 1959). (2) Beroza, Morton, A ~ A L CHEM . 34, 1801 11962). ( 3 ) Beroza, Morton, J . Gas Chrom. 2 , 138 11964). (4j Beroza, llorton, '\-atwe 196, i 6 8 (1962). ( 5 ) Beroza, Morton, Acree, F , Jr., J Assoc. O#c. Agr. Chemzsts 47, 1 (1964). VOL. 36, N O . 9, AUGUST 1964
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(6) Beroza, Morton, Sarmiento, Rafael, ANAL.CHEM.35, 1353 (1963). (7) Cowan, C. B., Stirling, P. H., “Gas Chromatography,” 2’. J. Coates, et al., eds., p. 165>Academic Press, Sew York, 1958. (8) navies, A. D., Homrd, G. A,, J . A p p l . Chem. 8, 183 (1958). 19) Ilrawert. F.. Feleenhauer. R. Anoew. Chem. 72,555 (19Gi). (10) Drnaert, F , Reuther, K. H., Chem. Ber. 93, 3068 11960). (11) Hall, FV. K Emmett, P. H., J . A m . Chem. SOC.79, 2091 (1957). (12) Haslam, J., Jeffs, A. R., Rillis, H. A,, Analyst 86, 1018 (1961). ~
(13) ,Ingold, C. K., “Structure and Mechanism in Organic Chemistry,” p. 182, Cornel1 Cniv. Press, Ithaca, S . Y., 1953. (14) Janak, J., Xature 185, 684 (1960). (15) Keulemans, A. I. AT., 1-oge, H. H., J . A m . Chem. SOC. 63, 476 (1959). (16) Knight, H. S., Weiss, F. T., A I ~ A L . CHEM.,34,749 (1962). (17) Kokes, R . J., Tobin, H., Jr., Emmett, P. H., J . Am. Chem. SOC. 77, 5860 (1955). (18) Miller, D. O., Ari.4~.CHEM.35, 2033 (1963). (19) Mourgues, L. de, Chim. ,4naI. 45 (3), 103 (1963).
(20) Okamoto, T., Tadamasa, O., Chem. Pharm. Bull. 1 1 , 1086 (1963). (21) Poe, R. W.,Kaelble, E. F., J . Am. 022 Chemists’ SOC.40, 347 (1963). (22) Radell, E. A , , Strutz, H. C., ANAL. CHEM.183, 1671 (1959). (23) Sykes, P., “4 Guidebook t o Mechanism in Organic Chemistry,” p. 10, Wiley, New York, 1961. (24) Zlatkis, A,, Oro, J. F., Kimball, A. P., A N A L . CHEM. 32, 162 (1960). R E C E ~ V EforD review February 25, 196.1. Accepted May 5 , 1964. Mention.of a proprietary product does not constitute an endorsement by the U. S.Department of Agriculture.
Study of Solid Support and Partition Liquid Interactions In Gas Chromatographic Separation of Ethanol-Methanol Mixtures EDGAR D. SMITH, JUNIOR L. JOHNSON,’ and J. M. OATHOUT* University o f Arkansas, Graduate Institute o f Technology, Little Rock, Ark. The use of independently determined solid support and partition liquid selectivities has been studied in connection with the separation of ethanol and methanol. Superior columns for this separation were readily achieved, with either alcohol being selectively retained through the proper choice of the solid support and partition liquid. The separations reported should be of value in analyzing trace quantities of either alcohol in a preponderance of the other, though problems still remain from residual gas-solid adsorption effects. Typical sensitivity limits obtained in this study were 10 p.p.m. for ethanol in methanol, and 100 p.p.m. for methanol in ethanol. Use of a more sensitive detector and improvements in instrument design would undoubtedly permit these limits to be lowered.
I
K AX E.4RLIER ARTICLE, methods were
described for the independent determination of relative values of substrate and partition liquid selectivities ( 7 ) . I t was shown that superior columns for the separation of 2- and 3-pentanone could be prepared by choosing a solid support, and partition liquid which acted in unison t,o bring about the desired separation. The order of elution of these close boiling pentanone isomers could be reversed with practically base line separation by either sequence. The present work was undertaken to 1 Present address, Dowsmith, Inc., Little Rock, .4rk. 2 Present address, Hendrix College, Convay, Ark.
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ANALYTICAL CHEMISTRY
determine whether similar results could be achieved using ethanol and methanol as model compounds. Since ethanol boils about 14’ C. higher than methanol, it is, of course, a simple matter to develop packings capable of separating these alcohols in order of their boiling point. Many such packings have been reported; inert types of substrates generally are used to avoid the excessive tailing that is usually noted with more active solid supports (3, 4,9 ) . Since this work was completed, Rombaugh and Thomason have reported a separation of these alcohols in the reverse order of elution ( 1 ) . d highly selective liquid partition phase was employed along with an especially acetylated grade of Chromosorb IV. Tailing of the alcohols was practically eliminated on this acetylated support, thus allowing the detection of parts-per-million quantities of ethanol in methanol. Since this article did not report relative retention data for the alcohols on the bare solid supports, it is not possible to sag whether their inherent selectivity characterihtics were also affected by this treatment. In the present work, the selectivity characteristics of both the solid supports and the partition phases were evaluated so that the interaction of these two important variables could be assessed. EXPERIMENTAL
-1 Perkin-Elmer Model 154-L vapor fractometer having a thermistor-type detector mas used with a Leeds & Sorthrup Speedomax H recorder. The vapor fractoineter was modified by connecting a small ponerstat in the heating circuit to control independently
the injection block temperature. Helium was used as the carrier gas. One-fourth-inch copper tubing, 2 meters in length, was used for all screening columns with the exception of some of the bare solid support columns, where intense adsorption made the use of 1meter lengths necessary. The solid supports investigated were commercial materials sold under the names of firebrick, Chromosorb P, Chromosorb IT-> and Gas-Chrom Z. The first three were 60- to 80-mesh material purchased from Wilkens Instrument and Research, Inc., while the’last named was 80- to 100-mesh material purchased from Applied Science Laboratories, Inc. Partition liquids were obtained from various commercial sources and used without purification. Fisher reagent grade methanol and anhydrous USP ethanol (undenatured) were used as solutes. d Hamilton 701-SCH microsyringe equipped with a Chaney adapter was used to inject these solutes. The conditioning and screening procedures used in this work were the same as those described in reference 7 . RESULTS A N D DISCUSSION
Table I summarizes the relative selectivity characteristics of the various treated and untreated solid supports. These supports are listed in the order of decreasing alpha values (corrected retention time ratios) of the untreated support. In all cases, injections \yere reported a t exact 10-minute intervals until reproducible value3 of retention times were obtained. Column lengths and temperatures were varied as necessary to obtain retention times which could be measured with reasonable precision (see reference 7 ) . These data
