Q u a nti ta tive Determination of FIuo rinated Hydrocarbons by Gas Chromatography W. C. PERCIVAL Jackson laborafory, E. 1. du font de Nemours & Fluorinated hydrocarbon mixtures (Freons) may be analyzed accurately, rapidly, and conveniently by gas chromatography. Data showing the accuracy and reproducibility for several commercial mixturesof Freon compounds have been obtained. The 95% confidence limits for the average of duplicate determinations is approximately ~ t o . 5 7 0 . The method offers several advantages over the physical and instrumental methods commonly used a t present. The effects of variations in eluting gas flow rate and pressure, thermal conductivity cell current and ambient temperature, column temperature, length of adsorbent packing and warm-up period on the results have been quantitatively measured.
F
LUORINATED hydrocarbon mixtures
have been commonly analyzed by a number of methods including mass spectrometry, infrared spectrophotometry, liquid density measurements (6),and vapor pressure determinations (4). While results of analyses by the spectrometric instruments are rapid, accurate, and reproducible, the high equipment cost restricts their general use, The methods involving the measurement of vapor pressures and densities are nonspecific and may be subject to many interferences. Gas chromatography is particularly well suited for the analysis of mixtures of gases and low-boiling liquids (7-12). The equipment cost is low in comparison with that of the spectrometric methods, and the analysis is capablp of being run easily in a routine fashion. This information prompted the investigation of gas chromatography for the analysis of Freon fluorinated hydrocarbons (Du Pont). Janak (13) has recently reported on the qualitative separation of a number of fluorinated hydrocarbons. Although it is true that some Freon compounds are chlorofluoro derivatives, others contain fluorine and carbon exclusively. The Du Pont Co. uses the registered trademark Freon to apply to compounds possessing certain desirable properties, and the designation Freon fluorinated hydrocarbons to include fluorine-containing compounds which have these properties. This terminology does not exclude the presence of other elements such as chlorine or bromine in these derivatives. The published information on gas 20
ANALYTICAL CHEMISTRY
Co., Inc.,
Wilmington, Del.
chromatography does not define in sufficient detail the equipment or operational techniques necessary for high precision, because most of the work has been directed toward qualitative applications. Therefore, a systematic evaluation of the variables whose control is important for accurate, reproducible analyses was also carried out. APPARATUS
The equipment used in the present work was generally the same as that used by Ray (It?), Patton (17), and Lichtenfels (14). However, it appears to differ in two major aspects: specifically, in the control of the eluting gas pressure and the detector current. Figure 1 shows the apparatus as it was used in the analysis of the Freon compounds. A constant head of eluting gas is obtained by setting the diaphragm valve a t the cylinder. Needle valves and a n adjustable re$rictor (Foxboro Co., Foxboro, Mass.) are used to control the flow rate. The gas stream is split into two paths, one leading directly to one side of the thermal conductivity cell (the reference stream), and the other going first through the column and then to the thermal conductivity cell (the column stream). Thus, both sides of the thermal conductivity cell are a t atmospheric pressure. The importance of this fact is discussed below.
The sample is introduced into the column either with a syringe through a serum cap or from a sampling bulb in which various quantities of the sample gas may be added. This sampling apparatus (Figure 2) is operated as follows: The bulb is evacuated while the eluting gas is flowing through the bypass line. The pressure is read on the manometer and a quantity of gas is added from the sample cylinder. The final pressure is read, the bulb isolated from the sample by closing stopcock B , and the contents of the bulb are swept into the column by reversing stopcock A . The temperature of the column is fixed by the choice of a solvent having the proper boiling point. An air bath is used to control the temperature of the thermal conductivity cell. The flow rate is measured by timing the rise of a soap film through a known volume as described by James (9). The electrical circuit for the gas chromatography apparatus is shown in Figure 3. Constant voltage is obtained from a 6-volt storage battery fed continuously by a trickle charger. The thermal conductivity cell is Model RCT, Gow-Mac Instrument Co., with M / T filaments and employs the Naval Research Laboratory geometry ( 2 , 6, 15, 81). The bridge current is set a t a predetermined fixed value before each run by adjustment of a precision potentiometer in series with the bridge. The potential drop across a precision
TO HOOD
I
u Figure 1. A. B. C.
D. E.
F. G. H. 1.
Gas chromatography apparatus
Eluting gas supply Diaphragm valve pressure regulator Needle valves Adjustable restrictor (Foxboro Co.) Manometer (0to 760 mm.) Sampling bulb (Figure 2 ) Serum cap Flasks for refluxing liquid Constant temperature jacket
Chromatograph column Thermostated chamber I . Thermal conductivity cell (Gow-Mac Instrument Co.) M. Electrical controls (Figure 3) N. Recording potentiometer (Leeds & Northrup Co.) 0. Flowmeter
J. K.
resistor is used to calculate this current. A 1-mv. Leeds & Northrup Speedomax recording potentiometer is used to plot detector signal us. time. The span of this instrument is adjustable to 200 mv. by means of a voltage divider (50,000ohm Helipot) ; the full-scale response time is 2 seconds.
SAMPLE CY LlNDER
n
GASCHROMATOGRAPH COLUMN
Figure 2.
Gas sampling apparatus
Theanalysisis carriedout by first introducing a k n o m volume of sample onto the chromatograph column. The sample is separated into its individual components, as each compound possesses a different affinity toward the column packing and therefore moves through the column a t a different rate. The chromatogram recording is obtained as a plot of the thermal conductivity cell detector signal against time. The time required for the apex of the peak to appear (retention time) indicates, from previous calibration, which compound is being eluted from the column; the intensity of the detector signal is proportional to the quantity of a compound which is present. All concentrations are given as per cent by volume.
rate of 50.0 ml. per minute. Samples of each compound are obtained with a purity greater than 99% for use as calibration standards. Calibration is carried out by introducing a t least five samples of differing volumes of each pure compound into the apparatus andobtaining the chromatogram recording. For each peak that is produced the area iscalculated by multiplying the peak height (estimated to within 1 0 . 2 mm.) by the width of the curve a t the midpoint of the altitude. The calibration is made convenient for calculations by dividing the area of the peak by the volume of the sample to give a term called the speci$c urea. The specific area is a constant for each individual Freon compound for a given set of operating conditions. -4 typical calibration is shown in Table I, nhich also illustrates the constancy which may be obtained over a period of several weeks. The sample mixture which has been equilibrated by thorough mixing is analyzed using the same instrumental conditions as were used for the calibration. The area of each peak from the unknown is measured and the volume of gas corresponding to the area is calculated from the specific area constant. The content of Freon-11 is calculated as follows:
Constancy of Calibration for Freon-1 1
Run
KO. Date 1 :/I 2 3
7/5
7 8
7j6 8/1 8/1
:S
Sample Volume, bI1. of Gas 2.6
i/1
xi
Specific Area, Sq. Mrn./Ml.
x
4.8 1.0 2.0 1.0 5.0
3.2
3.8
102 22.8 22.8 22.5 22.5
22.2
22.3 22.6 22.4
Freon-12. The chromatogram recording is given in Figure 4. The data given in the table, normalized to loo%, are from runs made on a single day, and illustrate the accuracy and reproducibility which may be obtained by calibrating and running the unknown on the same day. The reproducibility over a period of several days is approximately the same as that for a single day. This fact is illustrated in the analysis of Freon-12 and Freon-114. Blends of Freon-12 and Freon-114. The analysis of Freon-12 and Freon114 is carried out using the same general procedure as was described for the mixture of Freon-11 and Freon-12. The blend is separated on a 105-cm.
Area under peak for Freon-11 from unknown sample = 311. Freon-ll in sample specific area for Freon-11 ml. Freon-11 in sample ml. sample used
x 100 = % Freon-11
The concentration of Freon-12 is determined similarly. Table I1 gives the results of typical analyses on a mixture of Freon-11 and
EXPERIMENTAL
7
I IO
The chemical constitution of the various Freon compounds studied is R S follows:
VOLTS A.C.
column of 4-mm. inside diameter packed with alumina (pass 40 mesh b u t not 60 mesh) (Aluminum Co. of America). T h e column temperature is set a t 56" C. by refluxing acetone in the column jacket. The eluting gas is hydrogen a t a flow rate of 40.0 ml. per minute.
7 TRICKLE CHARGER
Boilinog Point, C. (760 Mm.)
Chemical Trichloro23.77 fluoromethane Freon-12 Dichlorodi-29.80 fluoromethane Freon-114 1,2-Dichlorotetra3.55 fluoroethune
Table I.
Freon-I1
Blends of Freon-11 and Freon-12. Mixtures of Freon-1 1 and Freon-12 are resolved on a 160-cm. column of 5-mm. inside diameter containing a stationary phase of Celite 545 (2 parts by Feight) (Johns-Manville Co.) and di-n-octyl phthalate (1 part by weight) (Eastman Kodak Co., Practical Grade). T h e column temperature is set a t 56" C. by refluxing acetone in the column jacket. T h e eluting gas is hydrogen set a t a flow
25
-
HELIPOT
RUBICON PORTABLE POTENTIOMETER
0-1.61 V.
L a N SPEEDOMAX RECORDER
Figure 3.
Electrical circuit for gas chromatography apparatus VOL. 29, N O . 1, JANUARY 1957
21
Table I11 gives the results of runs made over a 3-day period on a mixture of these Freon compounds, using the calibration made on the first day. The data were obtained by three different operators. The chromatogram recording is given in Figure 5 .
Table II. Analysis of Blend of Freon-1 1 and Freon-1 2
Vol. % VOl. % Run Freon-12 Freon-11 KO. (CCLFg) (CC13F) 1 2
3
53.1 53 2
46.9 46 8 46 5 46 6
%
Recovery 99 5 99
4
53 5 98 9 4 53 4 99 7 -4v. 53.3 46.7 99.4 Theory 53.2 46.8 100.0 Std. dev. = 0.22 95% confidence limits for mean of dnplicates for either component = zt0.4%
Air in Freon Compounds. The sample for the analysis for air in Freon compounds is obtained as follows: A short length of Tygon tubing is used t o connect the cylinder of Freon t o a mineral oil bubbler. A small quantity of the gas is used t o purge the air from the tubing. A hypodermic syringe is injected into the tubing, all the air expelled from the needle and barrel by flushing several times, and a sample in excess of the size required is taken. Immediately before the sample is injected onto the column, the excess is expelled. The recorder is set on its lowest span (1 mv.) for samples with air concentration loiver than 0.5%. All samples were taken from the vapor phase in the cylinder after equilibration. Air is separated from any of the Freon compounds used in this work on a 160cm. column, 4-mm. inside diameter, packed with alumina (pass 40 mesh but not 60 mesh). The column temperature is set a t 56" C. by refluxing acetone in the jacket. The eluting gas is hydrogen a t a flow rate of 40.0 nil. per minute. B hypodermic syringe, rather than the gas bulb, is used for sampling in order to minimize dilution of the sample with eluting gas. I n this manner sharper peaks and better resolution are obtained. Furthermore, the lack of contamination (in this case by air) that can be obtained with the proper use of a syringe is also illustrated. I n cases where the volume of the sample introduced into the column is critical, however, it is recommended that the gas sampling bulb technique be used. The calibration is carried out by introducing from 0.10- to 0.50-ml. volumes of air from a 0.5-nil. syringe with the recorder set at a 20-mv. span and by plotting the peak height US. the volume of the air to give the calibra-
22
ANALYTICAL CHEMISTRY
tion curve. Peak height measurements instead of area are generally used because the width of the air peak is too small to be measured with any accuracy. hll. air in sample ml. sample used X recorder span for run recorder span for calibration X 100 = % air
eluted by argon gas. The response is given in terms of the area in square millimeters under the oxygen peak of the thermal conductivity cell signaltime recording. Eluting Gas Flow Rate. The thermal conductivity cell used in this work employs the Kava1 Research Laboratory geometry-Le., the cell is a diffusion type wherein the gas diffuses into the compartment containing the hot element rather than passing directly over the wire. Table V shows the magnitude of the variation in response from flow rate changes. From these data it is estimated that the eluting gas flow in the reference side of the cell may be varied by at least &25% without giving significant error, but that the column stream flow rate must be controlled to 02% by frequent
Table IV gives typical results of the determination of air in mixtures of Freon compounds as compared with the standard freezing method (16). I n some cases i t has been possible to detect air in concentrations as low as 50 p.p.m., although reproducibility is not extremely good a t this level. EVALUATION OF INSTRUMENTAL AND OPERATIONAL VARIABLES
A number of variables in the apparatus and in the operational technique were evaluated to determine which were of importance for quantitative work. These included: (u) eluting gas flow rate (column stream and reference stream); ( b ) eluting gas pressure in the thermal conductivity cell; ( c ) current through the thermal conductivity cell elements; (d) ambient temperature of the thermal conductivity cell; ( e ) chromatograph column temperature; (f) column length; and ( 9 ) warm-up period. The evaluation of the importance of each variable was carried out by fixing the conditions of the operation as exactly as possible and then varying one parameter a t a time. The effect of each variation is shown by obtaining the response from accurately measured volumes of oxygen introduced on a charcoal column and
Yol.
1 2
3 4 5
6
Date 6/28 6/28 6/29 6j29 6/30 6/30
Av. Theory
Std. dev.
. -
0
2 T I M E IN MINUTES
Figure 4.
Chromatograph recording
of mixture of Freon-1 1 and Freon-1 2 Stationary phase, di-n-octyl phthalate on Celite Temperature, 56' C. Hydrogen flow rate, 50 ml./minute Column, 160 cm. long, 5-mm. inside diameter 2-ml. sample
Analysis of Blend of Freon-12 and Freon-1 14
Table 111.
Run so.
FREON-I2
m
9
Freon-12 (CCLF2) 49.1
1-01. %
Fieon-114 (CClFJ-CClF9) 50.9
48.8 48.7
19.4
51.2 51.3 50.6
49.2 49.1
50.8
50.9
49.0
51.0
100 4
49 0
51 0
100.0
Sample KO.
I
I1
101 3
= 0.26
9570 confidence limits for mean of duplicates for either component
Table IV.
c
Recovei y 98 1 100 7 100 2 99 8 102 2
=
&0.5yc
Determination of Air in Mixtures of Freon-1 1 and Freon-1 2
Run so.
Sample Size, 111. of Gas
1 2 1 2
5 10 5 5
Vol. % Air Gas Freezing chromatography method (16) 0.015 0.011 0,010 0.030 0.025 0.030
the thermal conductivity cell temperature), the ambient temperature of the cell can be altered over a limited range with no effect upon the response. In actual practice the cell is thermostated to avoid frequent current adjustment a t a temperature sufficiently high to prevent condensation of the sample in the cell. Column Temperature and Length. T h e temperature at m-hich the column is operated affects the retention time of a compound and the resolution t h a t can be obtained between t n o coiiipounds. The d a t a for this variable in Table V demonstrate t h a t column temperature changes have little effect upon the accuracy when the area under the curve-Le., the detector signal-time plot-is used as the quantitative measure. Therefore, the retention time of a particular compound and the shape of its peak may he changed with little, if any, variation in the area under the curve. This fact is demonstrated even more clearly in Runs 16 and 19, where the length of the column packing is cut from 210 cm. to 99 cm. with only a minor change in the response. These results indicate that a column can be replaced with a near duplicate, and that the packing can be replaced in kind, TTith no shift of the calibration for a particular system, provided the current and flow rate are closely controlled. Warm-up Period. Another variable of importance whose definition is sonieii hat more vague than the conditions listed previously is the warmup period. T h e strict reproducibility of a calibration over a period of time is obtained only when the cell current and eluting gas have been flowing for a period of a t least 12 hours. Variations of a n obscure nature in the wires of the thermal conductivity cell evi-
Table V. Effect of Variables on Thermal Conductivity Cell Response Flow Rate of Eluti side side 1 30 0 30.0 228 ... 2 33.3 30 0 191 589 3 27.3 30 0 258 75; 60 0 229 665 4 30.0 219 639 r3 30.0 15 0 Eluting Gas Pressure, hIm. Gage Reference Column side side G 0 0 21; 663 7 0 50 163 493 8 0 100 141 425 9 50 0 224 675 10 100 0 230 680 . Cell Cell current, Ma. 11 40 102 228 666 12 40 9i 182 545 13 40 109 290 890 14 2i 102 21i 663 15 63 102 21; 666 Column Packing Tzmp., Length, C. Cm. 16 40 210 217 663 17 2i 210 218 660 18 97 210 216 635 19 40 99 221 636 " Given in terms of area under peak from designated quantities of oxygen. A-0. 1
checking during any series of analyses t o keep the error introduced by flow rate variation below 0.5%. Eluting G a s Pressure. The description of equipment previously given in the literature for gas chromatography (14. 17, 18) s h o w the eluting gas passing first through the reference side of the thermal conductivity cell, through the column, and then through the second side of the cell. Thus, the reference side is a t a pressure highei t h a n the column side because of the back pressure generated by the column packing. Small changes in the flon rate and in the packing density vaiy the back pressure and, therefore, vai 1the pressure difference between the t n o sides of the cell. The effect of these pressure changes on response is shown in Table T'. The greater effect upon response is obtained by increasing the pressure in the column side of the cell; the effect of pressure changes in the reference side on response is of a loiver magnitude. However, in order to minimize the effect of pressure fluctuations on base line stability ( 1 , 19))the split eluting gas stream was employed, whereby both sides of the cell are open to the atmosphere. Similar arrangements have also been used by Van de Craats (20) and Drew ( 3 ) . Cell Current and Temperature. The current supplied t o the cell is 'one of the principal factors determin-
924 1255 1125 11-10
1124 796 696 1145 1150
1130 896 1496 1124 Ill5
1124 1113 1100 1060
ing the response for a given amount of gas; thus, t h e current must be maintained a t a constant value. The effect of current variation on thermal conductivity cell response is given in Table T'. d current variation of 5% gives appioxiniately a 25% error in response. Also s h o n n is t h e effect of variation of the temperature a t which t h e cell is maintained. As long as the current is closely controlled (the curlent will change with variations in
40r
I
l
FREON-I2
n
I
0
I
I
2 TIME
I
4 6 IN MINUTES
I
8
I IO
I
12
Figure 5. Chromatograph recording of mixture of Freon- 1 2 and Freon- 1 1 4 Stationary phase, 60-mesh alumina Temperature, 5 6 ' C. Hydrogen flaw rate, 40 ml./minute Column, 105 cm. long, 4-mm. inside diameter 4-ml. sample
VOL. 2 9 , NO. 1, JANUARY 1957
e
23
dently make this Farm-up period necessary. CONCLUSIONS
Mixtures of Freon fluorinated hydrocarbons may be analyzed rapidly, accurately, and conveniently by gas chromatography. The analyses of two mixtures which are available commercially have been studied in some detail; these mixtures are (a) Freon-11 and Freon-12 and ( b ) Freon-12 and Freon114. The studies show that the analyses have 95% confidence limits for the mean of duplicates of about 3~0.5%a t the 50% level for any component. The accuracy is within the reproducibility limits. Approximately 0.5 hour is required to run and to calculate the results of duplicate determinations. I n addition, an analysis for air (noncondensable gases) in Freon compounds and their mixtures has been developed. This method illustrates the applicability of gas chromatography to the determination of components present in low concentrations-e.g., < 0.5’%. Of the operating variables evaluated,
the eluting gas flow rate, the eluting gas pressure, and the current supplied to the thermal conductivity cell xere found to require precise control to obtain reproducible results. ACKNOWLEDGMENT
The author wishes to acknowledge the valuable technical suggestions offered by G. H. Patterson and S. S. Lord, Jr. Most of the data reported were obtained with the assistance of W. 0. Augustin. LITERATURE CITED
Callear, A. B., Cvetanovic, R. J., Can. J. Chem. 33, 1256 (1955). Coull, J., IND.ENG.CHEM.,ANAL. ED. 14,459 (1942). Drew, C. M., McNesby, J. R., Smith, S.R., Gordon, A. S., ANAL.CHEY. 2 8 , 979 (1956). Du Pont de Kemours & Co., Inc., E. I., Organic Chemicals Dept., Wilmington, Del., “Kinetic” Tech. Memo. Nos. 8. 11.
Ibid., KO. 19.
Gow-Mac Instrument Co., Bulletin TC-953.
Griffiths, J., James, D., Phillips, C., Analyst 7 7 , 897 (1952).
(8) James, A , , Ibid., 77, 915 (1952). (9) James, A., Martin, A,, Biochena. J. (London) 50, 679 (1952). (10) James, A., Martin, A., Smith, G., Ibid., 52,238 (1952). (11) James, D., Phillips, C., J . Chena. SOC. 1953, 1600. (12) James, D., Phillips, C., Ibid., 1954, 1066. (13) Janak, J., Mikrochim. Acta 1956, 1038. Lichtenfels, D., Fleck, S., Burow, F., ANAL.CHEM.27, 1510 (1955). Minter, C. C., Burdy, L. Jf. J., Ibid., 23, 143 (1951). Parmelee, H., Refrig. Eng. 59, 573 (1951). Patton, H., Lewis, J., Kaye, W., ANAL.CHEM.2 7 , 170 (1955). Ray, N., J. Appl. Chem. (London) 4, 21, 82 (1954). Sullivan, L. J., Lotz, J. R., Willingham, c. B.. ANAL.CHEhi. 2 8 , 495 (1956). (20) Van de Craats, F., Anal. Chzm. Acta 14, 136 (1956). (21) Webb, G. 9., Black, G. S., IND.ENG. CHEM.,ANAL.ED. 16, 719 (1944). RECEIVED for review March 13, 1956. Accepted September 4, 1956. Contribution S o . 203, Jackson Laboratory. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February-March 1956; Delaware Chemical Symposium, Newark, Del., February 18, 1956.
Interpretation of Areas Used for Quantitative Analysis in Gas-Liquid Partition Chromatography L. C. BROWNING and J. 0. WATTS Research and Development Department,
b Gas-liquid partition chromatography is applied to the quantitative analysis of water-ethyl alcohol-diethyl ether, carbon tetrachloride-acetone, ethyl alcohol-chloroform, and carbon tetrachloride-chloroform solutions. In solutions in which the difference in thermal conductivity of the components is small, areas under the curves obtained may b e used to determine weight per cent directly. In cases where the thermal conductivities differ considerably, percentages calculated directly from areas do not give correct weight per cent values. In these cases, however, division of the individual areas by the thermal conductivity of the compound allows calculation of the weight per cent from these “reduced” areas with fair accuracy. This empirical correction is applied to the systems studied, and to one previously reported.
I
partition chromatography for the quantitative analysis of liquid and gas solutions using thermal conductivity cells as the N THE USE OF GAS-LIQUID
1 Present address, American Instrument Co., Silver Spring, Md.
24
ANALYTICAL CHEMISTRY
U. S.
Naval Powder Factory, Indian Head, Md.
sensing elements, it is generally assumed (5) that the recorded response for the individual components is closely proportional to the mole per cent of the components, provided certain conditions are fulfilled: that the thermal conductivity of the carrier gas differ greatly from those of the components, and the thermal conductivities of the component be not too different. I n systems in which helium or hydrogen is the carrier gas, and the components are similar in nature, these conditions are met to a close approximation. I n quantitatively analyzing waterethyl alcohol-ether solutions it was found that the areas under the curves could be used directly to obtain weight per cent rather than mole per cent. Since this work was started, two excellent articles have appeared on the application of gas-liquid partition chromatography to the determination of hydrocarbons (1, 9). The areas under the recorded curves in these cases were also found to be proportional to the weight per cent of the components, although this fact was not emphasized. I n view of the importance of these results, the analyses have been extended
to four additional two-camponent sy6tems having components of varying thermal conductivities and molecular weights. The results reported here are for solutions of water-ethyl alcohol-diethyl ether, water-ethyl alcohol, carbon tetrachloride-acetone, ethyl alcohol-chloroform, and carbon tetrachloride-chloroform. Results from Dimbat, Porter, and Stross (1) have been included. EXPERIMENTAL
Thermal Conductivity Cells. Two types of thermal conductivity cells were used. The first was a Victory Engineering Corp. M142 thermistor cell, modified by boring a hole through one side of the cell block, so t h a t the sample and carrier gas passed directly over the thermistor. The reference side of the cell was not modified, and in this case the carrier gas passed through a T in the cell block. The second cell was constructed here (Figure 1). The block is stainless steel, and has two passages drilled through it for passage of the carrier gas and carrier gas plus sample, respectively. Thermistors (Victory Engineering Corp. A22) were inserted in the block a t right angles
