Programmed Temperature Gas Chromatography Apparatus STEPHEN DAL NOGARE and J. C. HARDEN Polychemicals Departmenf, E. 1. du font de Nemours & Co.,Inc., Wilmingfon, Del.
b A programmed temperature gas chromatograph is described for the rapid separation of mixtures of wide boiling point ranges. Linear heating rates of 2.5" to 30" C. per minute were obtained with a relatively simple proportional controller and low heat capacity column and heater. Detector stability was obtained by incorporating a buffer block between the column and thermal conductivity detector to eliminate the effect of varying column temperature. A sensitivity of 1900 ml. X mv. per mg. was obtained with high-resistance thermistors. Linearity of the programmer and separations obtained with the apparatus are demonstrated.
C
separations by gas chromatography are effected a t constant column temperature. With mixtures of wide boiling point ranges, early peaks are likely to be sharp and closely spaced and late peaks diffuse and widely separated. Numerous workers have indicated the improvement in both peak shape and distribution which results from programming column temperature (1, 2, 6-14). A linear programmed temperature operation offers the following advantages: ONVENTIONAL
Relatively uniform resolution of peaks over the entire chromatogram even a t moderate heating rates. Applicability to mixtures of wide boiling point range with the upper temperature limit set by the column packing and apparatus. Speed and flexibility of operation. For a constant flow rate, a solute eluted in 10 minutes a t a heating rate of 10" C. per minute was eluted in about 5 minutes a t a heating rate of 30" C. per minute. The starting temperature may also be varied according to the analysis. The apparatus described below consists of an electrically heated column, a thermistorequipped thermal conductivity cell and bridge, a linear temperature programmer, and accessory equipment. The column heater and programmer were designed for heating rates of 2.5" to 30" C. per minute, although rates approaching 50" C. per minute were obtained with similar equipment. A detection sensitivity of
1900 ml. x mv. per mg. was obtained with the lO6-ohm thermistor-equipped thermal conductivity cell. The detector was made insensitive to column temperature changes by insertion of a large maas buffer block between the column and detector. Consequently, base line drift occurred only a t temperatures where the liquid phase was detectably eluted from the column. The apparatus has been used to separate polar and nonpolar mixtures containing compounds boiling in the range from 35" to 300" C. The apparatus could equally well be used for isothermal operation. Because of the iow column heat capacity and constant detector cell temperature, the column could be quickly adjusted to various temperatures. APPARATUS
A schematic diagram of the essential components of the programmed temperature apparatus is shown in Figure 1. The requirements for the apparatus differ in several respects from apparatus used for isothermal separations. T o approach temperature equilibrium at high heating and rapid cooling rates, the column and heater should have a low heat capacity. A temperature programmer is required for reproducible heating of various columns a t different rates from different initial temperatures. Accelerated chromatograms were obtained with linear voltage programming of the column heater, but linear column temperature programming resulted in the most uniform distribution of peaks along the time axis. With increasing column temperature the carrier gas flow rate supplied a t constant pressure decreases, primarily because of increasing gas viscosity. Consequently, a constant differential type of flow controller is easential for maintaining constant flow rate over a wide range of column temperature and packing characteristics. A device, similar to the commercial controller described in this paper, was reported by Guild and coworkers ( I d ) . The detector adapted to a programmed temperature apparatus should function with adequate sensitivity and stability a t temperatures high enough to prevent condensation of high-boiling solutes. A thermal conductivity cell utilizing high-resistance thermistors was used in this work, although other types of detectors should be applicable.
q R Programmer
Column
r-
Recorder
Figure 1. Schematic diagram of programmed temperature apparatus
Column and Heater. U-shaped, thin-walled, stainless steel tubes, 4foot x *//lrinch internal diameter, were used as columns. These were uniformly wrapped to within 11/2 inches of the ends with two parallel strands of insulated resistance heating wire. With a total parallel resistance of 20 ohms, 0 to 600 watts could be supplied to the column. Six thermocouples, forming the thermopile for actuating the tcmpwature programmer, were placed between the windings and column a t uniform intervals. I t was necessary to insulate the thermopile junctions with small pieces of glass tape to prevent shorting through the column. An additional thermocouple of the required resistance was also placed on the column to provide visual temperature indication on a panel mounted galvanometer. The column was attached to the buffer block in the dctcctor assrmbly with compression fittings modified to accept silicone rubber O-rings. These connections permitted convenient column changes and gave gas-tight performance over a temperature range of 250" C. Aluminum or lead O-rings severely constricted the metal columns at high temperat u m . Columns were returned to room temperature by cooling with air supplied to a thin-walled metal cylinder surrounding the column. Column Packings. The maximum temperature to which the columns were programmed was determined by the thermal stability and volatility of the liquid phase. Base line drift due to elution of the liquid phase appeared at about 280" C. for high viscosity silicone oils and silicone rubber gum. Apiezon L and polyglycol adipate were eluted a t about 225" C. With maximum detector sensitivity, drift became apparent a t lower temperatures. VOL. 31, NO. 11, NOVEMBER 1959
0
1829
In general, the silicone materials proved effective for the separation of most homologous mixtures investigated. The liquid phase was retained on carefully graded Celite or C-22 firebrick. Mesh ranges of 100-120 and 40-60-mesh C-22 firebrick with 15% DC-200 silicone oil C200 viscosity grade) gave, with carcful packing, efficiencies as high as 1900 and 1200 theoretical plates, respectively, for n-octane a t 75" C. using helium as carrier as. Detector. deasurement of solute peaks was obtained from a thermal conductivity cell employing 10s-ohm matched, mounted thermistors (Fenwal G-128). The detecting thermistor was placed in a through-flow cavity and the reference thermistor isolated in a helium atmosphere in a 4 x 4 inch brass block (Figure 2). Open-center screws and O-rings of Teflon TFE-fluorocarbon resin were used to obtain gas-tight seals. The detector block was coupled to a similar buffer block fitted with an injection port, column connections, and carrier gas inlet, Both buffer and detector block were held a t a constant temperature in an insulated still air oven heated by a 250watt probe-type heater supplied from a thermistor-actuated temperature controller (Model 53290, Fenwsl Electronics, Inc., Framingham, Mass.). Air temperature within the oven was maintained to *I" C. a t a typical cell temperature of 225" C. The relatively large air temperature fluctuations did not significantly &ect the detector except a t maximum sensitivity because of the large detector block maas. The inner walls of the insulated oven were constructed of '/,-inch compressed magnesia sheet to minimize conduction t o the metal outer shell. The detector temperature was maintained not more than 50" C. below the boiling point of the least volatile component in a sample. A temperature difference significantly greater than this results in peak broadening, probably due to condensation. All openings in the metal blocks and all connecting tubing were '/Isinch internal diameter to obtain a small free volume in the system. The total dead volume of the apparatus was 8 cc. The column couplings extended a minimum distance from the oven to climinate cool zones and peak broadening resulting from condensation of highboiling solutes. The injection port was packed with glass wool and independently heated to obtain rapid vaporization of the samples. Detector and injection port temperatures were determined with thermocouples. The reference and detector thermistors were incorporated into a typical Wheatstone bridge supplied from a 12-volt storage battery. Provision was made for reading the voltage on the bridge and current through the thermistors for reproducible operation. Stepping resistors were used in the fixed arms of the bridge so that nearly equalarm operation could be obtained for a range of thermistor bead temperatures. A step attenuator covering the range 1 1830
ANALYTICAL CHEMISTRY
COLUMN
1
t Reference
Carrier Gas
Deleclor
Figure 2. Thermal conductivity cell with isolated reference thermistor and buffer block
-
CLUTCH ~
?I-
I1
I
~
, y-
AMPLIFIER
\
TIMING
MOTOR
u - - - - - - J
\
SOLENOIO VALVE
POWERSTAT
Figure 3. Schematic diagram of closed loop proportional temperature controller
to 1/512 of full sensitivity was placed across the bridge output to the recorder. Carrier
Gas and Flow Control.
Helium gas supplied to the apparatus from a cylinder was passed through a scrubber tube packed with Molecular Sieves or silica gel to remove traces of water. A constant flow rate was maintained with a Moore constant Merentia1 type flow controller (Model MSU, Moore Products Co., Philadelphia, Pa.) located upstream from the column and operated at a minimum 10p.8.i. differential. Moderate to high flow rates were controlled to about 1%, even a t a 30" per minute heating rate and correspondingly rapid column pressure changes. Flow rates were measured with a calibrated soap bubble flowmeter. Temperature Programmer. The temperature programmer was based on the closed loop proportional controller principle shown scherriatically in Figure 3. Voltage to the Wheatstone bridge was supplied from a 3volt dry cell in series with a fixed resistance which determined the bridge voltage and, therefore, the millivolt bridge output for a displacement in the 10-turn 1000-ohm Helipot, P-1. Helipot, P-1 was coupled through the magnetic clutch and lihearly driven by a synchronous 3-r.p.h. timing motor. The clutch was actuated by the reset switch. The unbalance signal from the bridge was amplified (Brown amplifier 3604952) and the output applied to the phase sensitive balancing motor, which was
coupled through a gear train to the balancing Helipot, P-2, and the Powerstat. As the temperature of the column increased, the millivolt output of the thermopile T-1in series with the bridge output, increased and was applied a8 a negative feedback to the bridge. An overshoot in column temperature resulted in increased negative feedback from T-1, and a compensating input signal to the amplifier from the bridge was supplied to the amplifier by repositioning P-2 and the coupled Powerstat. -4 reduction in voltage to the heater resulted proportional to the overshoot of T-1. Column temperatures were measured by the thermocouple T-2 and indicated on a galvanometer adapted from a Honeywell-BrownPyr-0-Vane temperature controller. A light source and photocell were arranged on the set-point arm so that the metal vane of the galvanometer needle interrupted the light beam when the needle and set-point arm coincided. This action activated the reset relay which disengaged the magnetic clutch, interrupting the programmer, and allowing the column to cool. A solenoid valve was simultaneously actuated to supply an air blast to the column to accelerate cooling. A pilot light indicated the condition of the reset switch. Helipot P-l was spring-loaded and automatically returned to zero. DISCUSSION
An indication of the linearity of the
2M
I80
Figure 4. Column temp e r a t u r e - t i m e plots showing linearity of temperature controller at various heating rates
160
"'I40 0-
w
rn ,320
a
rn Y a ZIOO
w
c
Figure 5. Unitized programmed temperature apparatus
80
60
40
0
I
temperature programmer is given in Figure 4, which shows the time-temperature plots for heating rates of 4.9", 10.3', 20.9", and 29.4" C. per minute. These rates were obtained by insertion of appropriate dropping resistances in series with the bridge voltage supply shown in Figure 3. Temperatures were read from the 0" to 4 W C. scale of the galvanometer and simultaneously indicated on a %inch per minute recorder Chart. The plots for the 5" and 10' C. per minute rates were linear within the precision of reading the galvanometer (+lo C.). A slight temperature lag became apparent at lower temperatures for the faster heating rates, possibly due to inertia in the column and heater. The indicated linearity was obtained with a k u p l e thermopile as negative feedhack in the =NO loop. A single thermocouple with amplification would have given similar results hut without the advantage of averaging the temperature along the column. In separate experiments, the temperatures within the column packing were measured by a thermocouple fitted with spacers to prevent contact with the walk. slopes of the time-temperature plots obtained from a point 6 inches from the column inlet were identical with the corresponding external column k?mpeAtUre plots. Ideally, the entire length of a column should be at the same temperature. Prsetically, this condition was not possible, as it was necessary to use short eonnections between the column and the high-temperature detector to prevent coodeusation of high-boiling compounds. For this mason column con-
2
3
4
3 6 7 8 T I M E . Minutes
9
1 0 1 1
I2
nectors from the buITer block, Figure 2, were extended a minimum distance from the detector oven. With stainless steel columns at 40" C. and the detector compartment at 230" C., a temperatwr gradient extended a b u t 2 inches along both ends of the column. Columns of copper tubing exhibited gradients extending about 8 inches; glsss columns were considered too fragile for this application. The performance of the constant dBerential flow controller was determined over a wide range of heating rates and flowrates for a 4-foot by %S inch packed column. Table I lists the flow rates measured at m m temperature during the c o u m of 30' and 10" C. per minute heating cycles for two practical flow rates. Column inlet pressures are also listed. Helium carrier gas was used in all experiments in this work and flowrates were determined to within +1% by using a calibrated soap bubble flowmeter. From these data it is evident that the flow rate remained essentially constant for both slow and fast flow rates at heating rates of 10' C. per minute or less. Higher heating rates prcduced a measurable decrease in flow a t low
flow rates and a negligible change :it high flow rates. Satisfactory operation of the flow controller was obtained with a differential of at least 10 p.s.i. between the carrier gas supply pressure aud the column inlet pressure required to overcome the resistance of the spring-loaded diaphragm. An evaluation of a similar controller under static conditions was reported by Guild and coworkers (It). The detector assembly was designed to provide a constant temperature environment for the thermistor elements with only nominal temperature regulation. Large mass braas blocks as thermal sinks which averaged out the + l o C. fluctuation in the detector oven. With the heater dissipation adjusted to provide an on& ratio of 5 to 1 for a %minute cycle, a I h c k temperature of 240" C. was maintained a.ithout affecting the base line at :t sensitivity of 4 mv. full scale. The buffer block was interposed between the column and detector block to eliminate conduction to the detector at high flow rates and heating rates. With the bufferblock shown in Figure 2, base line drift occurred only when the liquid phase was eluted from the column. Approximately 20 hours were required to equilibrate the detector to 240" C . from room temperature. Small changes in the cell temperature, such as a reduction to obtain higher sensitivity, were more rapidly obtained. A maximum cell temperature of 250" C. and a maximum thermistor temperature of
Table I. Performance of Moore Flow Controller 10" C. per Minute 30' C. per Minute Column Temp.,
Flow,
cc./
*C.
minute
50 100 150 200
1W 108 108 108
Flow,
Flow,
P-ure, p.s.1.g.
16.1 18.6 21.8 25.3
Flow,
cc./ Pressure, ee./ Pressure, ce./ Pressure, minute p.s.i.g. minute p.s.i.g. minute p.e.i.g. 27.5 27.0
26.0 26.0
5.0
6.2 7.5 8.5
105 105 104 104
15.9 18.7 21.8 24.9
24.5 24.0 24.0 24.0
VOL 31. NO. 11. NOVEMBER 1959
4.8 5.7 7.0 8.2
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Table II.
Reproducibility of
Chro-
matograms"
I\
Retention Peak Time, Min. Heightb c, 2.8 f O . O O 81.5 f 0 . 2 Ct 4.15 f 0.05 71.2 f 0.7 Cd 5.6 f O . 1 0 73.5 f 0 . 8 C, 7.2 f O . O O 73.5 f 0 . 2 Cs 8.7 f 0 . 0 5 70.7 f 0 . 3 a 10 pl. equal volumes mixture, 100 ml. helium/minute, heating rate 10" C./ minute, starting temp. 45" C. b Chart divisions. nAlcohol
300" C. are recommended by the manufacturer. When higher temperatures are required, hot wire elements with appropriate bridge modifications may be substituted. The sensitivity of the 106-ohm thermistor detector measured for benzene at a cell temperature of 240" C. using Dimbat's equation (4) was 1900 ml. X mv. per nig. Experimentally, 20 p.p.m. benzene was detected in 20 PI. of n-nonane. The components of the apparatus were mounted in a double compartment cabinet as shown in Figure 5. Carrier gas supply, flow controls, detector, and column were mounted in the compartment on the right. The injection port is seen in the lower right. The temperature programmer is located in the lower left compdment beneath the detector bridge. An integrator of the voltage integrating motor type (3) is also located on the bridge panel for measuring peak areas. Heating rate is selected with the switch located to the left of the temperature-indicating galvanometer. The reset, programmer, heater, and master power control switches are located below the recorder. The reproducibility of chromatograms using this apparatus is shown in Table I1 which lists the retention time and peak height variations observed for a five-alcohol mixture run in triplicate. Such results, of course, include variations in initial temperature setting, syringe errors, and speed of injection. I n general, both retention time and peak height can be reproduced to better than 2% with careful manipulation. The performance of the apparatus is illustrated in Figure 6 for the separation of 1 pl. of a Cs to Clo mixture of nparaffins on a 4-fOOt X 3/le-inch column containing 15% DC-200 silicone oil (200 viscosity grade) on 100-120 mesh
1832
ANALYTICAL CHEMISTRY
60 I
2
Figure 6. A. 8.
C.
A
4
Separation of a
6
Cs-Clo n-paraffin mixture
Heating rate 30" C. per minute, flow rate 100 ml. per minute, starting temperature Same conditions except heating rate 5 " C. per minute Isothermal chromatogram, temperature 75' C., flaw rate 100 ml. per minute
C-22 firebrick. Chromatogram A in Figure 6 was obtained at a heating rate of 30" C. per minute and a helium flow rate of 100 ml. per minute. A 10' C. per minute heating rate gave the An isothermal chromatogram B. chromatograph of the same sample a t 75" C. and a flow rate of 100 ml. per minute is shown as C. Chromatograms A , B, and C were completed in about 5, 10, and 28 minutes, respectively. The column isothermal efficiency calculated at optimum conditions was 1900 theoretical plates (noctane, 75" C.). Equivalent performance was obtained with esters, ketones, halides, alcohols, and aromatic hydrocarbons. ACKNOWLEDGMENT
The authors acknowledge the capable assistance of C. R. Talley and Valdis Ivansons in the construction and evaluation of this apparatus.
10
40" C.
(2) Dal Nogare, S., Bennett, C. Is., ANAL.CHEM.30, 1157 (1958). (3) Dal Nogare, S., Bennett, C. E;: Harden, J. C., "Gas Chromatography, p. 117, Academic Press, New York, 1958. (4) Dimbat, M.,Porter, P. E., Stross. F. H., ANAL.CHEM.28,290 (1956). (51,Drew, C. M., McNesby, J. , , R . , Vapour Phase Chromatography, p. 213, Butterworths, London, 1957. (6) Evans, J. B., Willard, J. E., J . A m . Chen. SOC.78,2908 (1956). (7) Gordus, A. A., Willard, J. E., Zbid.. 79,4609 (1957). ( 8 ) Greene, S. A. Moberg, M. L., Wilson, E. M., ANAL.(?HEM. 28,1369(1956). (9)Greene, S. A., Pust, H., Zbid., 29, 1055 (1957). (10)Greene, S. A., Pust, H., J . Phys. Chem. 62,55 (1958). (11)Griffiths, J. H., James, D. H., Phillips, C. S. G., Analyst 77, 897 (1952). (12) Guild, L., Bingham, S., A d , F., "Gas Chromatography 1958," p. 230, Academic Press, New York, 1958. (13) Harrison, G. J., Knight, P., Kelly R. P., Heath, M. T., Zbid., p. 230. (14)Ryce, S. A., Bryce, W. A., ANAL CHEM.29,925(1957).
LITERATURE CITED
(1) Berridge, N. J., Watts, J. C., J . Sci. Food Agr. 5,417 (1954).
.RECEIVEDfor review June 24, 1959. Accepted September 2,1959.
