in the solid the sensitivities obtained will not be more than 20 times worse. Using DisUsing- Hollow-Cathode charge Lamps Lamps 0 . 1 Mn 1 Na 0.1 0 . 5 Ni 5 K 0.1 2 Pb 50 Rb 2 Ca cs 10 Pd 2 co 5 Pt 10 Cd 2 Cr 50 Rh 2 T1 10 1 cu Sr 2 2 Fe Sn 500 Hi2 50 0.1 Mg 0 . 1 Zn
62”,
Unsuccessful attempts were made to determine d l , Ir, Mo, Ti, Ta, W, and V, using the direct methods already described. Zirconium probably belongs to this category. Some success in special circumstances might be obtained by using a n indirect method-e.g., by employing the depressing effect of aluminum on magnesium to determine aluminum. CONCLUSIONS
The absorption technique is advantageous under the following circumstances: when one element is to be
determined in the presence of a large amount of another one, which may give strong interference in emission flame photometry, not due to actual chemical combination; when an isotope is to be analyzed in the presence of another isotope of the same element. Clearly if the source is a single isotope source, only that isotope in the flame can absorb the line. Electrodes intended t o be used as standards for ordinary emission arc or spark techniques may be analyzed by this method, and so be able to function as “standard electrodes” because they can be analyzed with respect to AnalaR reagents. ACKNOWLEDGMENT
The author acknowledges helpful discussions with, and experimental assistance from, R. A. Lockyer and G. E. Hames, and help from Wanda Jordan, also of this laboratory, who devised the arrangement of two slits and one recorder for one element and the twochannel direct reader. He thanks J. A. L. Gidley and J. T. Jones of Imperial Chemical Industries for early communication of results of their investigations and permission to quote.
REFERENCES
(1) Allan, J. E., Analyst 83, 466 (1958). (2) Brewster, Sir D., Report of 2nd Meeting, British Association, p. 320, 1832. ( 3 ) Coheur, P., Hans, H., Rev. uniuerselle manes 92, 63 (1949). (4) Gidley, J. A. F., Jones, J. T., Analyst 85,249 (1960). (5) Gidley? J. A. F., Jones, J. T., private
communication.
(6) Huldt, L., dissertation, Uppsala, 1948. ( 7 ) Kirchhoff, G., Pogg. Ann. 109, 275 (1860). (8) Liveiy, G., Dewar, J., “Collected PaDers, Cambridge University Press, Ckmbridge, EnglaGd, 1915. (9) Lockyer, R., Hames, G. E., .Inalyst 85, 385 (1959) (10) hlargoshes, Marvin, Vallee, B. L., ANAL.CHEM.28, 180 (1956). (11) Menzies, A. C., Actm do Congress0
XV Internacional de Quimica Pura e Aplicada, Vol. 11, p. 2, Lisbon, 1958. (12) Menzies, A. C., Colloquium Spectroscopicum Internationale VI, Amsterdam, 1956, Pergamon Press, London. (13) hfitchell, A. C. G., Zenianeky, M. W., “Reson:nce Radiation and Excited Atoms, Cambridge University Press, Cambridze. Eneland. 1934. (14) Russe&’B. Shelton, J. P., TF7alsh, A., Spectrochim. Acta 8, 317 (1957). (15) Walsh, A., Zbid., 7, 108 (1955). (16) Wollaston, W.H., Phil. Trans. Roy SOC. London. Ser. A 92,365 (1802). RECEIVED for review February 29, 19GO. Accepted May 5, 1960.
x,
Analytical Distillation by Gas Chromatography Programmed Tempera tu re Operation F. T. EGGERTSEN, SIGURD GROENNINGS, and J. J. HOLST Shell Developmenf Co., Emeryville, Calif.
A gas chromatographic method is described for obtaining information on hydrocarbon samples analogous to that from an analytical distillation. The gas chromatographic distillation employs a simple technique for the temperature programming of a short nonselective column. Boiling points of the components are determined from times or temperatures of emergence according to a general relationship which is adequately valid for all hydrocarbons. The effluent hydrocarbons are oxidized and detected as carbon dioxide by thermal conductivity; this makes response factors unnecessary for interpretation of peak areas. The method, which is inherently general in nature, has been applied to petroleum distillates ranging from -40” to 400” C. The results agree well with those obtained by timeconsuming conventional precision distillations, and also give a better ac-
904 *
ANALYTICAL CHEMISTRY
counting of light and heavy ends as well as more detailed information, particularly of minor components or fractions. An important feature of the method is that a few milligrams of sample are sufficient to yield a good distillation curve.
I
N CHARACTERIZING hydrocarbon mixtures for specification or other purposes, a precise analytical distillationfor example, 20 t o 50 plates-is sometimes needed. Such a distillation requires several hours and large samples. Gas chromatography can be employed to obtain essentially a boiling point analysis, although, with the separating column at a constant temperature, the analysis is restricted to a rather narrow boiling range. Lighter components emerge too soon and tend to overlap, while heavy components emerge
very late, producing relatively wide bands or remaining in the column. The new technique of temperature programming of the separating column (1-4, 6-9) makes a wide-range, singlestage analysis possible. hloreover, it appears t o be well suited for obtaining the gas chromatographic equivalent of a conventional analytical distillation. By using a column packing nliich separates according to boiling point, and by precise programming of the column temperature, the boiling range for various peaks can be determined from times or temperatures of emergence. In such a gas chromatographic (GC) distillation, detailed separations are of secondary concern, and only a short separating column is required. The method described here was designed to yield in about hour of operating time the information equivalent to that obtained in an approximately 20-plate conventional distil-
lation. In addition, it provides a better accounting of minor components or fractions and of material in the light and heavy ends. Only a few milligrams of sample are required, which is of great advantage in small-scale research.
evolution, which begins to affect the thermal conductivity response a t about
700 O C.
EXPERIMENTAL
Apparatus. A schematic diagram of the apparatus is given in Figure 1. It consists essentially of a special column equipped with electrical heaters, a conibustion attachment for converting the hydrocarbons to carbon dioxide prior t o detection, and a conventional thermal conductivity detector unit. The essential parts and other features are discussed below.
SEPARATING CoLvnrx. The separating column is part of a unit consisting of a cylindrical brass block, 2 inches in diameter and 6 inches long, having drilled channels inch in diameter to acconmoclate the column packing, and additional channels for two 100-watt cartridge heaters and a thermocouple n-ell. To make the block water-tight for rapid cooling in an ice bath, copper tubing extensions were sealed to the drilled openings. S t the sample inlet is a silicone rubber disk held in place by a s c r w fitting. The vaporizing chamber just beneath the inlet is n short length of 1/4-inch stainless steel tubing packed 1Tith glass wool. The packing consists of silicone oil (General Electric Co. SF-96) (IOOO), supported on 30- to 60-mesh C-22 insulating brick in a weight ratio of 20 to 100. The bed length is about 13 inches (11 cc. of packing) and extends t o inch from the top of the block, the last half inch being filled with glass wool. This column has an efficiency of 200 theoretical gas chromatography plates (for n-decane a t 100' C. and 50 ml. per minute flow rate), which corresponds to a minimum of about 20 plates in a conventional batch distillation, according to van Deemter (12).
The column unit was designed to permit immersion in ice water or other coolants and, after removal from the bath, to give reproducible and essentially linear heating. Excess liquid is wiped off the block prior to placing in a close-fitting Dewar vessel to minimize heat losses during the heating cycle. Before use the column is well purged by back-flushing with helium while heating the column a t 290' C. Only about 5 minutes are required to cool the column block in an ice bath from 250" to the starting temperature, 2' C., for the next analysis. COMBUSTIONATTACHMENT. This unit, similar to that employed in previous work (6),contains hot copper oxide to convert the effluent hydrocarbons to carbon dioxide, followed by calcium sulfate (Drierite) to remove water of combustion (Figure 1). The use of combustion in chromatography, which was introduced by Martin and Smart (IS), has several possible advantages: simplified detec-
Figure 1 . paratus
Schematic diagram of ap-
tion of high boiling materials; unambiguous interpretation of peak areas; operation of the detector at room temperature which simplifies temperature control and allows more general use of thermistors, if desired; cleanliness of detector and connecting lines, sensitivity increased about threefold for C4 and higher hydrocarbons; and elimination of interference from water, if present. The combustion tubes are 8-inch lengths of I/rinch diameter stainless steel tubing containing a 41/2-in~hbed, about 2 cc., of 14- to 48-mesh copper oxide, Both Baker's wire-form copper oxide and a precipitated copper oxide containing 1% ferric oxide as a catalyst (14) have been employed. At an operating temperature of 650" to 'iOO'C., these catalysts will yield essentially complete conversion of C3 and higher hydrocarbons; the precipitated oxide is slightly more active and needs less frequent regeneration. The wire-form copper oxide requires initial activation by reduction with pentane (about 30 pl.), followed by reoxidation with air. After use with about 20 mg. of hydrocarbon, the catalyst is regenerated with air a t the usual operating temperature. The activity is checked periodically with 1pl. of pentane. By inserting a small tube of Ascarite ahead of the Drierite tube, carbon dioxide from the pentane is adsorbed and any unconverted pentane is registered by the thermal conductivity detector. Conversion of n-pentane, initially over 99%, slowly declines nith use, and the copper oxide eventually sinters and impedes the flow; therefore it needs to be replaced after 50 to 100 analyses. Methane and ethane are only partially oxidized under the conditions employed, and therefore are not determined quantitatively. With either copper ovide a t 675" C., the conversions of methane, ethane, and propane were 15 to 30, 60 to 75) and 95 to 99%, respectively, in sweral tests with used catalysts. A 96y0 conversion of methane was achieved with the precipitated catalyst at 800" C., but such high temperature intensifies deactivation and sintering. Furthermore, base-line trouble niay arise because of oxygen
The indicating Drierite, 10- to 20mesh, was packed in 6 inches of */Ir inch inner diameter Tygon tubing. Fresh Drierite absorbed a little carbon dioxide, but this difficulty was eliminated by passing air, conveniently that used for regeneration of the copper oxide, through the freshly packed tube for a few minutes. It is advantageous to use two copper oxide tubes, and these are connected, via tubes containing indicating Drierite, to a four-way valve used for reversing the direction of flow of the helium carrier gas. By employing two copper oxide tubes in this way the effluent hydrocarbons from both forward and reverse flow (back-flush) are converted to carbon dioxide before they pass through the valve. This permits operation of the valve a t room temperature. Previously, only one copper oxide tube was employed and the hydrocarbon vapors during reverse flow passed through the valve before heing oxidized. The valve then had to be maintained a t a high temperature t o avoid condensation of high boilers, and this resulted in leak problems. The flowreversing valve facilitates the elution of heavy ends, which are determined as a single band, and allows back-purging the column before the next test. The four-way Teflon Dlug valve, available from the Republic Manufacturing Co., Cleveland, Ohio (30. 310-6-1/8 D), is suitable. The combustion unit caused no significant broadening of the peaks. The effective detector volume, obtained at a helium flow of 50 ml. per minute by measuring the width of a noncolumn air peak (IO), was 5 ml. with and 3 ml. without the combustion unit. GAS CHROhIATOGRAPHY DETECTION UNIT. Detection is by thermal conductivity using two Gow-Mac elements of the flowthrough type, and a total current of 150 ma. The detector cell unit need not be heated, as only carbon dioxide is observed, but is merely enclosed in a water bath a t ambient temperature. The apparatus has a 1mv. recorder equipped with a pipping integrator (Hallilrainen Instrument CO., Berkeley, Calif.). HELIUMFLOWSCHEME. Guild (8) points out the necessity of purifying the helium to avoid interference from certain impurities which accumulate in the cold column and which are eluted upon heating. With the scheme used here, such interference mas not observed because the helium is purified by the combustion train. Water in the inlet helium is removed by passage through Drierite, and essentially all hydrocarbon impurities that could interfere with the determination of Ca and higher hydrocarbons are converted to earbon dioxide, nhich does not accumulate in the column a t ice temperature. To ensure complete removal of water, the helium from the cylinder is passed through 2 feet of */4-inch copper tubing packed with Drierite. To obtain a constant flow rate VOL. 32, NO.
a,
JULY 1960
905
-during the heating cycle, a Moore diaphragm-type flow controller was employed (Moore Products Co., Philadelphia, Pa., KO.63-BUL): using a pressure of 50 p.s.i.g. from the cylinder regulator. Although not necessary for very short columns, such a flow controller is desirable with long columns, as pointed out by Guild (8). A helium slip stream is provided (Figure 1) to prevent condensation of high-boiling hydrocarbons under the silicone rubber disk during reverse flow operation. The flow rate of this stream is about 25% of total during forward operation and about 5% during back-flushing. Without this feature the disk would have t o be kept hotter, a t the risk of overheating it. The additional helium inlet is also useful for adding gaseous or trapped samples. Conditions of Operation. The conditions selected as standard for the method were: Helium flow rate
50 ml. per minute
Sample size Heating rate of
5
column
Inlet pressure
906
0
(soap film meter)
kl.
From 2" to 275' C. in 30 minutes 100 mm. (2" C.) t o
130 mm. (275" C . )
ANALYTICAL CHEMISTRY
The standard heating rate was achieved by applying exactly 74 volts to the column heaters by a potentiometer connected to a constant voltage transformer (115 volts, 250 watts). The heating rate was nearly linear in the 50" to 200" C. range, as shown by the curve marked column temperature in Figure 2, and it was easily duplicated within about 2%. Outline of Procedure. T o make a n analysis, the column is cooled t o 2" C. in ice water, and the sample is discharged from a hypodermic syringe into the vaporizer. K h e n t h e air peak is observed, about 20 seconds from the injection point, the column heaters are immediately turned on. The ice bath is quickly removed, and the column block niped free of water and placed in an empty Dewar (Figure 1). A pad of insulating material placed around the top of the Dewar minimizes heat losses. At suitable intervals column temperatures are recorded on the chromatogram to serve as references in establishing the boiling points of the peaks observed. Khen the sample has been substantially eluted, the flow-reversing valve is turned to the back-flush position to recover residual material, if any, and the heating
continued until the column is sufficiently purged for the next analysis. Analysis of Synthetic Blends. Calibration Curves. For correlating peak position in the chromatogram n i t h boiling point a synthetic blend of nparaffins was used as t h e primary standard. When this mixture was tested under t h e conditions described above, t h e chromatogram v-as as s h o m a t t h e top of Figure 3. T h e peak width of the individual paraffins is nearly uniform and corresponds to about 10" C. on the boiling point scale. The separation of adjacent n-paraffins decreases uniformly with increasing carbon number, as the boiling point intervals also decrease in that order. For any particular set of conditions each component can be characterized either by the emergence time or by the column temperature corresponding t o the peak. The latter is termed emergence temperature. The emergence times from the air peak and emergence temperatures of hydrocarbons of various types were determined under the standard operating conditions, and are plotted against boiling points in Figure
2. The plots show that the silicone SF-96 separates the hydrocarbons essentially by boiling point, only slight type selectivity being observed. The lines drawn through the points for the n-paraffins serve as calibration curves for determining boiling points of components, or boiling range of fractions, from emergence times or emergence temperatures. The chromatogram for the n-paraffin blend in Figure 3 shows a base-line rise a t colunin temperatures above about 250' C., which is due to volatility of the silicone packing. Therefore, with this liquid phase the method is limited to coniponents boiling up to 350" C. unless a base-line correction is made, in which case it can be extended to about 400' C. The emergence times, emergence temperatures, and resolution of the n-paraffins were not affected significantly by varying the sample size of the blend, a t least not over the range 2 to 10 pl. The accuracy of the method was determined by analyzing five synthetic blends, each containing five to eight pure C5 to C f l hydrocarbons in k n o m percentages. Paraffins, naphthenes, mono-olefins, diolefins, and mono- and dicyclic aromatics \\-ere all represented in the blends. Weight percentages were calculated from peak areas which had first been corrected for differences in carbon content by dividing peak area for each component by per cent weight of carbon. Data are given in Table I for t n o of the synthetic blends; these results are representative of the reFeatability and accuracy obtained. The average accuracy was about k370 of the amount present for 34 values in the range 5 to 25 ii-eight 70. Thus the method appears to be reliable.
emergence time and emergence teniperature. The emergence temperature is less sensitive to changes in heating rate than is the emergence time, particularly with n-paraffins above C7. ?\'either emergence time nor temperature is affected appreciably by a moderate change in helium flow rate.
fl-PARAFFIN
*-
-
