Table I.
Temp. 000 000
ooo ...
Column Description code 0002 000% 0002 . .~ 0002 0002 0002 0002 0002 0002 0002 ~
000
000 000 000
000 000 000
000
0002
0002 0002 0002 0002
000 000
000 000
Retention Vol./Gram 1.35 1.53 3.87 4.50 7.83
BP., so. 001 00 1 001 00 1 001 001 001 00 1 001 001 00 1 001 001 001 00 1 ~
~~
O
c.
-88 63 -103 71 -U 07 -78 5 -47. 70 -11.73 -0 50 9 50 -6 26 -6 90 27 85 0 88 3 72 36 07
8.27
13.3 13.8 22.7 24.7 30 4 30.9 37.1 42.0 57.5
-4 41
values ( 5 ) are listed in increasing order in the next column of five spaces. Use of a niechanical divider (S) could aid in rapid calculation of relative retention values. The standard used for the relative retention value is listed in the last column; the number refers to the number of carbons in the normal paraffin standard. Sormal paraffins were chosen as standards rather than the standards recommended a t the London meeting of the International Symposium on Gas Chro-
20% 2,4-Dimethylsulfolane
Partition
API Code
CoefT.
1.56 1.77 4.49 5.21 8.56 9.60 15.4 16.0 26.2 28.7 35.3 35.9 43 1
48 7 66 7
I
Compound
1J
2-
8 1 0
8 1 5 4
1J 1J 1J
8 23 1J 21 7 28 5 1J 8 4 1J 1 6 1J 11 3 1J 6
matography in 1956 ( 2 ) . These paraffins are readily available and cover a temperature range from -165" to 464" C. Use of a honiologous series makes it possible to transfer data from onr referrnce to another. LITERATURE CITED
(1) 4mbrose, D., Keulemans, A . I. AI., Purnell, J. H., Division of .Analyti-
cal Chemistrv, 132nd lleeting, ACS, S e x YoEk, S . T.,September 1957. 12) Desty, D. H., S a f w e 179, 241 (1957).
Ethane Ethylene Propane Carbon dioxide Propylene Isobutane n-Butane Seopentane 1-Butene Isobutene Isopentane trans-2-Butene cis-2-Butene n-Pentane 1,3-Butadiene
Relative Retention Value Stdr 0.101 4 4 0.115 0.290
0,338 0.559 0.662 1,000 1,034 1 703 1.858 2.284 2.324 2.790 3.15.5
4.318
4 4 4 4.
4 4
4 4. 4 4 4 4
4
(4) James, A . T., Martin, A. J. P., Biochem. J . 50, 679 (1952). ( 5 ) Littleivood, -4.B., Phillips, C. S. G., Price, D. T., J . Chem. SOC. 1955; 1480. (6) Phillips, C. S.G., International Symposium on Gas Chromatography, East Lansing, Rlich., Aug. 28, 1957. (7) Porter, P. E., Deal, C. H., Stross, F. H., J . dvz. Chem. SOC. 78, 2909 11956). RECEITED for review January 8, 1958. accepted February 28, 1958.
High Temperature Gas Chromatography Apparatus STEPHEN DAL NOGARE and
L. W.
SAFRANSKI
Experimental Station, Polychemicals Department,
b A high temperature gas chromatography apparatus was developed for the qualitative and quantitative resolution and estimation of high-boiling organic mixtures. The partition columns were operated in the range 150" to 350°C. for the resolution of hydrocarbon, ester, and glycol mixtures. The platinum filament thermal-conductivity detectors were operated at 10" to 100°C. higher than column temperature to avoid condensation of high-boiling components. Excellent resolution was obtained on relatively short columns containing silicone grease or linear polyethylene as the partition medium. Thermal degradation was minimized by the all-glass construction and short residence time in the columns.
T
HE wide acceptance of gas chromatography in organic analysis refleets the usefulness of this technique for the separation, detection, and estimation of small amounts of gases and liquids. In general, publications
894
a
ANALYTICAL CHEMISTRY
E. 1.
du Pant de Nemours & Co., Inc., Wilmingfon, Del.
have dealt with separation. performed a t moderate temperatures (room temperature to 150" C.),ifith only a few references indicating the potentialities of high temperature applications. Cropper and Heywood ( 2 , 3) described initial work on the separation of highboiling compounds a t reduced pressures on several column packing.. Dijkstra, Keppler, and Schols (5, S) reported separations a t temperaturesup to25O"C. a t column lvorking pressures. In a later paper, they ( 7 ) indicated that allglass thermal conductivity cells could be constructed to withstand temperatures up to 300" C., but their description of the construction and operation of such glass cells is meager. The recent work of Ashbury et al. ( 1 ) and Killiams (9) also points to the potentialities of high temperature gas rhromatography. This paper describes an apparatus n hich is relatively simple to construct and has given trouble-free performance for a year. The apparatus is sensitive
to sniall amounts of organic vapors and was successfully operated a t temperatures as high as 350" C. Mixtures of compounds boiling in the range 150" to 450" C. have been easily resolved and analyses by the peak area summation method have been moderately accurate. ¶tions of high-boiling hydrocarbon mixtures, in particular, were markedly successful. APPARATUS
The assembled apparatus consists of a carrier gas source and metering device, thermal-conductivity cells, colunin and heating jacket, and the necessary circuit for detecting and recording the detector cell signal. Carrier-Gas Control. A cylinder of helium fitted with a reducing valve supplies the carrier gas a t lG p.s.i. The gas was led through gum rubber tubing to a flowmeter (0 t o 150 cc. per minute range), then t o t h e carrier gas preheater in the heating jacket (Figure 1). The flon-meter 11-as calibrated for each new column, or as re-
quired, by measuring the displacement of a soap bubble in a calibrated cylinder connected through a short length of tubing to the exit end of the column. Thermal-Conductivity Cells. Details of t h e hot-nire thermal conductivity cells are shon-n in Figure 2 . T h e sensing elements are platinum filaments 0.002 inch in diameter. approuiniately 2.5 inches long, TT hich ai e gold-soldered or spot-welded under slight tension t o tungsten supports. Tlie supports are mounted in 8-mm. horosilicate glass tubing thi ough a cobalt glass seal. As shon-n in Figure 1. the cells are placed in an aluminum hlock, n hich serves as a heat sink and support, after which the column is attached through a glass-to-glass seal. Four openings in the block accommodate the carrier-gas preheater and the TKO thermal-conductivity cells. smaller ports are provided for measuring tlie temperature of the block and heating jacket by means of thermocouples. The block is wpported on the metal heating jacket by a flange. This type of mounting requires that the unit be in a vertical position, but permits easy access for replacing the column when required. Partition Columns. Coluinns and all connecting tubing were constructed of borosilicate glass to avoid catalytic decomposition which m a y occur on metal surfaces a t high temperatures. Short colurnns, about 2 feet long, of tubing 4 t o 9 mm. in internal diameter, are generally preferred because t h e small pressure drop and rapid throughput further reduce t h e hazard of thermal decomposition. The packed columns are sealed directly to the sample inlet and detector cell (Figure 1). A criss-cross arrangement of the carriergas preheater and the column is used to obtain symmetrical eyposure to the heater jacket walls. For most separations. the inert supporting material for the partition medium was acid-n ashed Celite 545, 30-80 mesh fraction (JohnsAIanville). Coarse Celite offers low resistance to gas f l o and ~ consequently fast elution. KO particularly adverse effect on resolution IFas observed ivith this coarse support. Untreated high vacuum silicone grease (Don.-Corning) and commercial linear polyethylene had the necessary thermal stability and low vapor pressure for use as partition media. The silicone grcase n a s the preferred partition medium for elevated temperatures. To prepare the column packing, the partition niediuni is dissolved in a suitable sol\ ent, such as niethj-lene chloride or toluene. The calculated amount of Celite is then addcd to provide a packing containing 20 to 30% of the partition medium. Tlie resulting slurry is heated to rcniove most ot the solvent and tlicn placed under high vacuuni until the mixture appearb dry. Final traces of solvent arc rcniorcd by a preliniiiiary conditioning with carrier-gas flow a t nioderatcly high temperature in the apparatus. The use of high temperatures was >urce'sful in removing volatiles from the untreated silicone grease. If the column n as accidentally contaminated n-ith
POST H E A T E R
DETECTOR C E L L A L U M I NUM BLOCK
REFERENCE-
I
f
i STAINLESS S T E E L 'HEATING JACKET TUBE
I
I '
2,
OMATOGRAPHIC COLUMN
1
Figure 3. Detector bridge circuit
u Figure 1.
Column and detectors
-COPPER ---SILVER
LEADS SOLDER JOINT
TUNGSTEN LEADS COBALT G L A S S SEAL
OD PYREX TUBING
-8mm
1 1 lbSPOT 1
GOLD SOLDER OR P L A T I N U M WELD
ca
ca
PLATINUM W I R E I / Z " LONG 4 n RESISTANCE
e
1
id I /-2mm l1
I
CAPILLARY
Figure 2. Thermal conductivity cell
high boilers, these were readily removed by raising the temperature for a time and purging before returning to the lower temperature required for a particular analysis. Heating Jacket. T h e heating jacket consists of a thin-walled steel tube 23/3 inches by 2*/2 feet, \trapped n i t h Electrothernial heating tape and insulated from the en\-ironment \\-ith Fiherglas insulation. Closer n rapping of the heating tape a t the upper end of the jacket allon-s the detector-block temperature t o remain 10" to 100" above the column temperature. I n this way the poPsibility of condensation in the detector cell is reduced. The jacket temperature can be adequately controlled by means of a variallle transformer. The time required for equilibrium after a temperature change \-arks with the extent of the change, about 3 hours for a 50" C. adjustment. Preheater, Sample Inlet, a n d Postheater. d resistance winding is wrapped around t h e section of tubing leading t o t h e injection poit to pie-
lieat t h e carrier gas and promote ins t a n t volatilization of t h e sample. A similar arrangement is used to provide a postheater which effectively prevents condensate from collecting in t h e outlet tube. T h e temperature of the preheater and postheater is adjusted to about 280" C. under tlie operating conditions, with the aid of a thermocouple probe. Liquid samples are injected into the column through the neoprene serum stopper by means of a syringe, such as the Agla micrometer syringe (Burroughs Kellcome. Ltd.). Small amounts of solids can also be injected by pressing or melting in a syringe needle and injecting the melted material by displacement nitli air or an inert gas. A more convenient procedure for solid samples is the injection of a solution in a suitable noninterfering solvent. The neoprene serum stoppers should be frequently checked for leakage. Deterioration of stoppers rvas retarded by a small disk of aluminum or copper foil just below the stopper. The disk acted as a cooling fin. Detector-Bridge Circuit. The bridge-circuit components are shown in Figure 3. -1stable current supply of from 300 t o 500 ma. can be obtained from two &volt storage batteries, connected in series. nhich must be maintained in well charged condition. For operation, t h e current is turned on and adjusted t o the desited level by varying t h e current-regulating resistor. The zero-control Helipot is set at its midpoint, and the bridge balanced by adjustment of the cell-balance resistor. Fine adjustment is then provided by the zero-control Helipot. Detector sensitivity can be altered by changing the bridge current or, alternatively, by potentionietrically selecting a fraction of tlie output signal. -2 recording potentiometer provides a convenient means of recording thP chromatogram. DlSCUSSlON
The performance of the platinumfilament detectors was measured over the temperature range 160" to 250" C. by deteriiiinjng the peak heights and areas obtained ii-ith a knonn equimolar VOL. 30, NO. 5 , MAY 1958
e
895
hydrocarbon mixture (Figure 4). Height and area rrere measured for each of the four peaks a t the indicated column temperatures. I n each case, 0.36 pl. of the mixture was injected (in 3 pl. of a hexane solution) and the helium flow was maintained a t 35 cc. per minute. The bridge current and recorder sensitivity were also maintained constant at 400 ma. and 8 mv., respectively. As expected, peak areas decreased and peak heights increased with increasing temperature. An interesting crossover is seen in the peak height us. temperature curves for the CI4,CI6,and CIShydrocarbons a t about 205" C. The crossover is absent in the area curves. Peak inversion, occasionally encountered when nitrogen is used as a carrier gas, was not observed in this work with helium. These data indicate the necessity of calibrating the detector response to known materials under the conditions of analysis when quantitative results are desired. A study of the factors influencing the response of this type of detector to organic vapors is under nay. The essentially complete resolution of an equal-parts mixture of glycols (mono-, di-, tri-, and tetraethylene glycols), generally classed as nonvolatile, is demonstrated in Figure 5, A . I n this case, essentially complete resolution was obtained on a 4 mm. X 24 inch column containing 25YGsilicone
I
I
I
I
I
I
I
c:
O'
I70
I90
I80 Figure 4.
210 220 TEMP., ' C .
200
230
250
'
inches) in less than 10 minutes. The high sensitivity of the thermal conductivity cells is indicated by the fact that only 2 pl. of the phthalate and glycol mixtures was used with a 5-mv. fullscale recorder for these chromatograms. A series of hydrocarbon separations
grease-Celite at a column temperature of 172' C. Figure 5, B, is a chromatogram of five phthalate esters (dimethyl, diethyl, diallyl, di-n-butyl, and diethoxyethyl) obtained a t a column temperature of 206" C. This separation \vas effected on a 23% silicone grease-Celite column (9 mm. X 20 2
5-
240
Detector response to hydrocarbons
I I
A
I
I
4-
A
u) 3L
B
2
cn
5 0
! i
I-1 I
2 -1
rl
0
i
1
I:I :I:I (MOL.) n - DECANE n- TETRADECANE
I
J 'h; I/' , 'i!
2-
5
0
2
Jl
, w /, , IO 15
20
25
26
C I-
I
=
0
5
IO 0 MINUTES
I
0
Figure 5. Separation of polyethylene glycols and phthalates A. 6.
896
285OC.
Mono-, di-, tri-, and tetraethylene glycol (in order of appearonce). Column temperature, 172' C.; detector temperature, 250' C.; helium corrier 2 0 cc./min. Dimethyl, diethyl, diallyl, di-n-butyl, and diethoxyethyl phthalater (in order of appearance). Column temperature, 2 0 6 " C.; detector temperature, 3 1 8" C.; helium carrier, 1 2 4 cc./min. ANALYTICAL CHEMISTRY
Figure 6. A. 6. C.
I
15 20 25 MINUTES Separation of hydrocarbon mixtures
5
IO
C14 to CPO. Column temperature, 1 7 5 " C.; detector temperature, 2 6 5 ' C.; helium carrier, 1 2 4 cc./min. CIZ to c26. Column temperature, 2 2 6 OC.; detector temperature, 3 0 2 " C.; helium carrier, 40 cc./min. C12 to Cas. Column temperature, 2 8 5 " C.; detector temperature, 3 4 0 ' C.; helium carrier, 56 cc./min.
a t increasing column temperatures is shown in the sequence of chromatograms in Figure 6. Chromatogram A is that obtained for 4 p1. of a liquid mixture of Clr to C20 paraffins and olefins (C16omitted) a t 175" C. Complete resolution of the odd and even members
is evident, although the 1-mono-olefins were not differentiated from the corresponding paraffins. By increasing the column temperature to 226" C. and reducing carrier gas flow, it was possible to obtain a n excellent resolution of the paraffin series CIZ to CZS as shown in
A
B
I
w 3
I 2
i
IC
L
I,
0
I
2
3
4
5
MINUTES Figure 7. medium A.
B.
I
Separations with
polyethylene partition
Cio, cl4, C16, and Cle hydrocarbons. Column temperoture, 240' C.; helium carrier, 20 cc./min. Di-n-butyl dibutoxyethyl, and diethylhexyl odipater. Column temperature, 250' C.; detector temperoture, 268' C.; helium carrier, 70 cc./min.
c
MINUTES Figure 8.
Chromatogram
of a silicone oil
Dow Corning 200 fluid, viscosity grade 100. Column temperature, 350' C.; detector temperature, 450' C.; helium carrier, 50 cc./min
Figure 6, B. The large initial peak is due to air (1 cc.) which was used to displace a small amount of this solid sample from the syringe needle into the column. The same injection technique was used to obtain the chromatogram of a Cl2 to CB6 hydrocarbon mixture (Figure 6, C) with a column temperature of 285' C. A significantly slower flow rate improved the resolution between lower members of this mixture, but reduced the later peaks below the detection limit (broadening effect). Another suitable partition medium, b-hich has the requisite low vapor pressure and fair thermal stability, is linear polyethylene. Several chromatograms obtained on 20% polyethyleneCelite columns (4 mm. X 24 inches) are shown in Figure 7. Figure 7, A , illustrates the resolution of the C!O, C14, c16, and CI8 hydrocarbons in less than 4 minutes a t 240" C. Two microliters of a mixture of three adipate esters were resolved in about the same length of time (Figure 7, B ) . Peaks obtained on the polyethylene columns shorn pronounced tailing. I n addition to qualitative separations, the high temperature technique was also utilized for semiquantitative analyses. The compositions of a number of highboiling mixtures were estimated by the area summation method, where the percentage of a component is obtained by the ratio of its area (peak height X width a t half height) to the total area of the chromatogram (6). When this method was applied to several highboiling mixtures, best agreement was obtained between area and weight per cent rather than mole per cent. This is evident from Table I, which shows data obtained on 1-p1. samples of the ester mixture. Results with an average relative error within 5% met the needs in the analysis of mixtures of highboiling esters. The maximum temperature a t which the column mas operated was 350" C. At this temperature, a silicone oil showed a large number of peaks (Figure 8) apparently corresponding to successive homologs in the oil. The appearance of this chromatogram is similar to those in Figure 6, showing a series of hydrocarbons. The chromatogram in Figure 8 showed a high background, which is probably due to the formation of volatile degradation products a t a rate proportional to the amount of sample remaining in the column. Some initial degradation of silicone oils and greases had previously been observed in this laboratory and elsewhere (4). The chromatogram in Figure 8 was obtained with DC 200 silicone fluid sample (viscosity grade 100) a t a column temperature of 350' C. and detector temperature of 450' C. Work is under way to extend the technique to higher temperatures. VOL. 30, NO. 5, MAY
iwa
a97
Table I.
Analysis of Adipate Ester Mixture
Known Component Mol? % w t . yo Unknown Unknown Dibutyl adipate .35 9 28 7 Dibutoxyethyl adipate 31 3 33 4 Diethylhexyl adipate 32 7 37 4 Column temperature, 2-13' C., detector temperature, 310" cc./min. * Column temperature, 259" C.. detector temperature, 320' cc./min. Operation in the region of 4.50' C. or higher may be possible, if thermal decomposition or rearrangement is not limiting. At these temperatures. gas chromatography may prove competitive mith molecular distillation for the resolution of natural products.
by Area Method Run 1" Run 2b 0 8
0 29 31 38 C.;
2
0 7
0 1 26 8 1 33 2 2 39 3 helium carrier, 63
6
C.; helium carrier, 63
LITERATURE CITED
(1) .Ashbury, G. K., Davles, -1.J., Drinkwater, J. W.,.4bstracts, p. 16B, 129th lIeeting, .ICs, Ilallas, Tex., April 1956. ( 2 ) cropper, F. R.,~ ~ ~.I, .\-uture ~ , 172, 1101 (1953).
(3) Zbid., 174, 1063 (1954). (4) Cropper, F. R., Heywood, -\., in "Vapoiir Phase Chromatography," D. H. Desty, ed., p. 316, Academic Press, S e w York, 1957. (5) Dijkstra, G., Keppler, J. G., Schole, J. .I.,Rec. trazi. chzm. 74, 805 (1955). (6) Dimbat, ll., Porter, P. E., Stross, F. H., , \ S A L . CHEJI. 28, 290 (1956). ( 7 ) Iieppler, J. G., Dijketra, G., Schols, J. in "1-apour Phase Chromatographr," D. H. Destv, ed.. p. 222, Academic Press, Sew Torl;, 1957. (8) Keppler, J. G., Schols, J. -I., Dijkstra, G., Rec. truz,. c k m i . 75, 965 (1956). (9) Killiams, E. F., hbstracts, p, l5B, 129th Meeting, ACS. Dallas, Teu., -\priI, 1956.
RECEIVED for revien- Sovember 30, 1956. Accepted January 6, 1958. South\\-ide ~ Chemical ~ d Conference, , 18-4-dCS,Memphis. Tenn., Ilecember 6, 1956.
Trace Analyses by Gas Chromatography C. EUGENE BENNETT, STEPHEN DAL NOGARE, L. W. SAFRANSKI, and C. D. LEWIS Polychemicals Deparfmenf, Du Ponf Experimenfal Sfafion, E. 1. du Ponf de Nemours and Co., Inc., Wilmingfon, Del.
1
A gas chromatography apparatus suitable for analyses of impurities a t the parts per million level utilizes an amplifier to increase the signal from a thermistor detector. Techniques necessary for obtaining low noise and drift levels are described. Several applications of the apparatus to trace analyses are discussed.
T
determination of trace coniponents (1 to 200 p.p.ni.) in organic mixtures usually requires the development of a specific method. The successful application of gas chromatography to the study of numerous complex mixtures suggested that this technique could be extended to trace analyses. Two logical approaches to the problem were to find a more sensitive detector or to amplify the signal obtained from an available, stable detector. After a survey of the characteristics of existing detectors [radiation (1-3), gas density balance (?), hydrogen flame ( 8 ) ,gas-flojT- impedance ( d ) , surface potential (4), infrared] thermal conductivity, etc.], the combination of a thermal conductivity detector utilizing thermistors as sensing elements and an amplifier was selected as the best practical combination. This choice was based primarily on the high signal-noise ratio of thermistors and the availability of a suitable amplifier. The thermistors used were inexpensive, readily available, and usable a t temperatures u p to 150" C. HE
898
ANALYTICAL CHEMISTRY
The amplification of the bridge signal and the conditions necessary for stable operation of a chromatographic apparatus are discussed. Several analyses are presented to demonstrate the potential of this technique in organic analysis. APPARATUS
The assemdled apparatus consisted of a controlled gas source, sample inlet, thermal conductivity cells, column, flowmeter, vapor jacket, and the necessary circuitry for detwting, amplifying, and recording the detector signal. General Operation. The carrier gas,, continuously supplied from a helium cylinder, f l o w d through a preheater, reference thermistor chamber, sample inlet, U-shaped column, over the detector, and through a flo~vmeter to the atmosphere (Figure 1). The two thermistors were part of a Kheatstone bridge (Figure 2 ) , in which
the unbalance signal was constantly amplified and measured on a recording potentiometer. K i t h the carrier gas flon-ing and the bridge a t balance, the sample was injected by means of n syringe int'o the gas stream. The vaporized sample passed onto the column and the components of the mixture were carried through the column a t different rates. They arrived separately a t the exit of the column and the detecting cell. The appearance of sample vapor in the det'ecting cell produced a change in thermistor temperature and a corresponding resistance change proportional to the concentration of solute vapor in the carrier gas. The unbalance in the Wheatstone bridge resulting from the resistance change was recorded as the chromatogram. Flow Control. A cylinder of helium fitted with diaphragm-type reducing valves supplied the carrier gas a t the desired flow rate. The gas from t h e cylinder flowed through a preheater
He
1 He
Figure 1.
Schematic diagram of detector block, column, and vapor jacket
