CONCLUSIONS
The method described is a n effective and rapid means for determining volatile hydrocarbons in water mixtures. In styrene analysis, the agreement obtained with the distillation-bromination method is good evidence that the techniques employed in the gas-chromatographic method are satisfactory and the vaporization of styrene from the polymer and its combustion are essentially quantitative. The capillary tube sdmpling device is believed to be an effective means of charging aqueous emulsions which are difficult t o handle with a hypodermic syringe. This application of the combustion technique illustrates one of its advantages, that of eliminating water interference. consequently, the column and conditions can be designed entirely for optimum and rapid separation of the hydrocarbons. Other advantages of the technique are that detection as carbon dioxide increases sensitivity, provides uniform thermal conductivity response, and allows room temperature operation of the detector.
I
0.1 hlV.
SAMPLE A 7.1
MC.
0.(3% STYREKE
The method is demonstrated here only with styrene in styrene-butadiene latex, but it has also been applied in this laboratory for volatile components in polyisoprene latexes. If one wishes to extend the method for the determination of several volatile compounds, longer columns may be employed. LITERATURE CITED
(1) Durrett, L. R., ANAL. CHEM. 31, 1825 (1959). (2) Eggertsen, F. T., Groennings, S., Horst, J. J., Zbid., 32, 904 (1960). (3) Haslam, J., Jeffs, A. R., Analyst 83, 455 (1958). (4) Hunter,' I. R., Ortegren, V. H., Pence, J. W., ANAL.CHEM.32, 682 (1960). (5) Porter, R. S., Johnson, J. F., Zbid., 31, 866 (1959). (6) U. S. Reconstruction Finance Corp., Office of Synthetic RubberllResearch
cm WATER, 10 MG.
I
0
I 5
___-
io
AII>LTES
Figure 3. Typical chromatograms of free styrene in latex Column temperature, 75' C.
and Development Division, Specifications for Government Synthetic Rubbers," rev. ed., Sect. E2, Oct. 1, 1962.
RECEIVEDfor review March 2, 1961. Accepted May 2, 1961. Division of Petroleum Chemistry, 139th Meeting, ACS, St. Louis, hto., March 1961.
I
Low I emperature Gas Chromatography ROGER S. PORTER and JULIAN F. JOHNSON California Research Corp., Richmond, Calif.
b Gas chromatography has been studied at -78' C., solid COZ temperature. Separations were performed over glass beads and crushed brick and with these supports coated with n-heptane, n-octane, and acetone. Column characteristics were studied at n-heptane concentrations on glass beads from 0.02 to 2.0 weight and from 0.5 to 28.0 weight on crushed brick. Components from Hz io isopentane were eluted in studies which evaluated.the naiure of separations, the imporiance of sample size, and possible analytical applications. With this simple equipment, for example, the major components in ambient air may be analyzed at -78" C. on a column of 28 weight % n-heptane on crushed brick.
70 70
T
evaluates the nature and analytical advantages of low temperature gas chromatography. Although gas-liquid is the most common type of vapor chromatography, this method has been virtually unreported at solid C o n temperatures, -78' C., and below. Exploration of this region is inviting because of the low molecular weight compounds that may be used as partitioning agents. Low temperatures also take advantage of differences HIS WORK
1152
ANALYTICAL CHEMISTRY
in heats of solution for the separation of close boiling components (3, 11). Gas and liquid diffusion phenomena, which reduce column efficiency, should be less a t low temperature (3). Finally, a number of practical analytical problems, such as air pollution, appear potentially amenable to low temperature gas chromatography. These mild analytical conditions can be important for systems which contain reactive species such as olefins and oxides of nitrogen. The chromatograph used was a custom-built thermal conductivity detector unit. The unique features were column temperature and partitioning agents. Separations were performed a t -78" C. over n-heptane, n-octane, and acetone, as well as on the support materials, crushed brick and glass beads. Studies were made at n-heptane concentrations from 0.5 to 28.0 weight yo on crushed brick and from 0.02 to 2.0 weight % on glass beads. Tests revealed several marked effects which lead to useful applications and to an understanding of the partitioning process. METHOD
The conventional design gas chromatograph was built especially for these tests. Samples were commonly injected with a gas Elample loop, nominal
volume 0.25 cc., connected to the chromatograph through a commercial O-ring injection valve. The columns consisted of copper refrigeration tubing, '/r-inch o.d., which were packed with either 42- to 60-mesh size JohnsManville C-22 insulating brick or glass beads, of which 85% were retained on No. 230 U. S. sieve but passed No. 140. These soda-lime-silica beads, average nominal diameter 0.10 mm., were obtained from the Minnesota Mining and Manufacturing Co. The brick support columns were 20, 50, and 100 feet in length. Glass bead columns were all 10 feet long. The columns were coiled and maintained during tests in a n 9 quart, metal Labline Dewar contaiting Y solid COz-acetone slurry. Phillips 99 mole % normal heptane was used as a partitioning liquid. Constants for pure normal heptane are: melting point, -90.5" C., boiling point, +98.4" C. Normal octane, 99 mole %, was also used: pure freezing point, -56.5' C. Acetone, Baker's analyzed reagent grade, was also tested as a partitioning liquid: pure melting point, -95" C., boiling point, $56.5" C. Column packings were prepared by adding partitioning liquid to dry brick or to glass beads in the desired weight ratio. The mixing was performed in a jar which was then tightly capped and vigorously agitated at room temperature for a prolonged period. The packing was then added to the column with little
v)
Y
'1
c 2o
z
~ R T I T I O N I N G*GENT + ~ W T X ~ - H E P T * N EON BRIO( 20 FT COLUMN AT 78.C HELIUM FLOW 35 ml/MIN
-
7
Y
0 z I
10I J N ~ E RETENTION TIMES
I-
5 w +
C
T
E
~
~
6H2 D2
a
4-
CORRECTED FOR M A 0 VOLUME I1
-ZM)
-240
-200
-im
NORMAL BOILING POINT,
-120
-ao
C'
Figure 1. Separations on n-heptane at -70" C.
exposure to air. The completed columns were capped with rubber policemen and stored at room temperature. The partitioning liquids likely become evenly distributed on the support by this procedure. This is ensured by the high vapor pressures of the liquids at room temperature and the fact that a small amount of partitioning liquid is observed on the walls of the storage jars. Absolute column compositions may be slightly in error because of liquid volatility. The detector system consisted of matched 8000-0hm nominal thermistors which were used with a 0-1 mv. k e d s & Northrup strip-chart recorder. The thermistors were thermostated a t 35" C. in a regulated oil bath. Helium was used as the carrier gas and mainly a t room temperature, at flow rates of 35 cc. per minute as indicated by a soap-bubble buret flowmeter. Flow rates were set by varying inlet pressures with the column exit a t atmospheric pressure. RESULTS
Figure 1 graphically describes the separation of many compounds and elements at -78" C. on a 20-foot column of 28% n-heptane on crushed brick. A comparable plot was obtained with results from a 50-foot column of the same composition. Figure 1 gives gas chromatographic retention times as a function of normal boiling point for eluted components. Column retention, corrected for dead volume, changes close to linearly on this semilog plot. This indicates that heptane a t high concentrations is essentially nonselective, Le., gives retention times corresponding approximately to vapor pressures. Hydrocarbons and other gases, e.g., Hz, Nz, Ar, Kr, Xe, and NzO, fit the same correlation. Deviations from the plot do occur for CzHe,
CHFI, and NO. Methane and ita tetradeutero-derivative represent one of the few systems tested which was not resolved on this column. Figure 1 mdicates the elution of carbon monoxide, nitrogen, and ethylene, all with the same molecular weight, in order of boiling point, with carbon monoxide and nitrogen being close together as expected. Likewise, argon and oxygen, which have different molecular weights, 40 and 32, emerge close together in accord with their similar boiling points. Difficulties in resolving this pair have been previously reported (8, 16). Figure 2 and Table I indicate the separation of hydrogen, oxygen, nitrogen, carbon monoxide, and argon on a 100-foot n-heptane column, 28 weight % on brick. Reducing the flow rate on shorter columns aids separation of this system but does not achieve the resolution obtained in Figure 2 on a 100-foot column. Table I indicates the variation in separation with column length. Data on longer columns and retention times of about 30 minutes are preferred for ease of identification and integration. Such determinations offer a gas-liquid chromatographic analogy to an efficient, low temperature, fractional distillation of air. In this way, normal components of air, as well as low molecular weight hydrocarbons, may be separated and analyzed in a single determination. Analysis of rare gases is difficult by certain other schemes because of their chemical inertness and similarlty of physical properties (7). Partial chromatographic resolution has been obtained on heptane-brick columns for the isotopes of both krypton and xenon. This is indicated by broad and skewed detector peaks of the order expected from natural isotopic ratios. Such observations are clearer in the case of gas-liquid rather than gas-solid chromatography with its potential of tailing effects due to strong adsorption. A previously reported separation of these rare gases indicated a symmetrical wave form for xenon (7). The number of theoretical plates for systems of n-heptane on brick was about 100 per foot. Such low values are apparently enhanced (see Figure 1) by other factors such as reduced temperature in achieving separation of similar components (6, 6). Low temperature tests were also performed using acetone as partitioning liquid. Table I1 indicates that acetone columns give generally poorer separa tions than heptane for comparable conditions. Oxygen and nitrogen are not resolved over acetone for even longer retention times than developed on h e p tane columns. Acetone does show, however, an interesting selectivity, as ethylene is eluted well after higher boiling ethane.
100 FT C W M N 28 WT. PER CENT n-HEPTANE 3N BRICK, 78'C, nr FLOW 35 m l / M I N
-
RETEN
1.
Figure Low temperature gas-liquid chromatography
Columns of uncoated brick offer little low temperature separatory power. Table I1 shows that air is not resolved and that hydrocarbon retention times are short on this gas-solid column (3). This illustrates the important role of partitioning liquid, particularly n-heptane, in facilitating separation. Plain brick and brick coated with high percentages of n-heptane exhibit a remarkable difference in retention times and separatory power. For example, the elution order of ethane, ethylene, and acetylene is completely reversed by varying liquid concentration. The
Table 1. Gas Separation Column conditions: 28 wt. yo normal heptane on brick at -78' C., helium flow, 35 cc./min. corn- Retention Time, M i n u t e s Donents 20-Foot 5O-Foot 100-Foot 'Eluted column column column Hz,D2 4.3 11.7 28.2
Nz
co 02
Ar
Table 11.
4.4 4.5 4.7 4.8
13.1 13.6 14.1 14.6
29.5 30.4 31.6 32.4
Air and Hydrocarbon Separation
Retention times in minutes on 20-foot columns at -78" C., helium flow, 35 cc./min. 28 Weight yo on ComBrick onent Uncoated Normal Eluted Brick Acetone heptane
VOL 33, NO. 9, AUGUST 1961
1153
transition between these conditions is illustrated in Figure 3. Tests were performed on a series of 20-foot columns which differed only in per cent of nheptane on brick. Systems with the highest percentages of n-heptane are approximately boiling point columns. Retention times in this region are governed principally by gas-solution properties. From 2 to 10% of heptane on brick, a transition region exists where other effects, gas-solid and perhaps gasliquid adsorption, are also involved. A third region is evident below 2% partitioning liquid. Here, gas-solid adsorption properties appear t o dominate retention properties. Liquid at l o x concentrations may be considered more as a deactivator for adsorption than a surface for gas solution. Air, CO, HP, and Ar, in contrast to most hydrocarbons. show a small but significant increase in retention times at lower liquid concentrations on brick. These increases are about 1 minute in going from 28 to 0% heptane; see Table I1 and Figure 3. This reveals the increased importance of adsorption effects at lower liquid concentrations. The prominent nonlinearity between rvtention times and partitioning liquid concentrations for certain compounds may be due to support forces “showing through” thin films of liquid or due to liquid adsorption. rldsorption on the surface of nonpolar liquids is uncommon, although it is more likely to occur a t these conditions of low temperature and low concentrations of liquids on the support (10). Paraffins, C1-C4.show regular changes in retention with column composition. Olefins. however. tend to have their greatest retention on brick. This region of increased retention (Figure 3) is also characterized by the typically skewed adsorption peak, sharp front, and long tail ( I ) . Low temperature tests were also made using glass beads as both support and partitioning agent. Concentrations of partitioning liquidq on beads are equivalent to much higher percentages on brick. This is because of combined changes in surface area and adsorption effects. The results of glass bead tests a r e given in Figure 4. Over a broad range of higher heptane conrentrations, the systems in Figure 4 behave as gas-liquid columns. This is indicated by a nearly first power increase of retention times with heptane concentration and by elution times i n general order of boiling point. The desirability of choosing the proper concentration for performing sepsrations is strongly noted. For the highest concentrations of heptane on beads, 3 s on brick, surprisingly good separations of gases are achieved. Excellent srparations of CI-C3hydrocarbons were obtained on 0.5y0 columns. For hy1154
ANALYTICAL CHEMISTRY
RETENTION T I M E , MINUTES
Figure 3. position
Retention time changes with column com-
Temperature, -78‘
drocarbons into the C,’s, thc. most propitious heptane concentration is a power of ten less. The elution of cis-2butene for example, on a 0.0270 column gave a symmetrical peak, suggesting neither complications of strong adsorption nor column overloading. Even isopentane was also eluted from this column in less than 15 minutes. Elution was, thus, easily attained a t over 100’ C. below normal boiling points. This corresponds to elution a t nominal vapor pressures down to 1 mm. of Hg. Such successful separations a t low vapor pressures are accomplished, provided sample size is kept low enough for normal behavior. The maximum sample size for symmetrical peaks varies with eluted molecular weight, in accord with other work (4),and for systems tested here is generally below 10-5 mole. At higher molecular weights, e.g., C,H,, the maximum sample size drops to below 10- mole. Sample overloading of gliiss bead columns is evidenced by peaks with broad sloping fronts and sharp tails; see Figure 5. The true retention time occurs near the initial response with the peak maximum and average occurring at longer times for larger samples. This is different behavior than that obtained by varying sample size on lean brick support columns where solid adsorption determines behavior ( I ) . Indeed, adsorption on glass beads never becomes dominant for these test conditions as even elution times on uncoated beads are only slightly different than for lean heptane columns. The peak dissymmetry observed here may thus be due to sample sizes which eyceed saturation vapor pressures. In accord with this, the data of Hishta on glass bead columns indicate the type of peak skewing observed here and its greater prominence for higher molecular weight elutants (4). Heavier species have lower vapor pressures and, consequently, reach saturation a t lower concentrations. Overloading of glass bead columns may, therefore,
C., He now, 35 cc./rnin.
> c”
IO
20 30 40 RETENTION T I M E ,
c4u8
50
60
3
MINUTES
Figure 4. Retention time changes with column composition
mean that the sample itself is contributing to partitioning properties. A crucial test on the partitioning process was performed with columns composed of n-octane on glass beads. n-Octane, unlike n-heptane, is a solid a t -78” C. A column of 0.5% noctane on beads showed no retention beyond air for hydrocarbons through C2’s. Likewise, corrected retention times for propane. propene, and cis-2butene were only one tenth those shown in Figure 4 for heptane columns of the same concentration. Thus, it is the solution properties of the liquid that dominate retention characteristics at 2 0.5% heptane concentration on glass beads. Columns of n-octane and n-heptane were also compared a t -78’ C. a t a lower, 0.1% by weight. concentration on glass beads. The retention characteristics of these two columns were virtually identical. This indicates that partitioning agents at low concentration may behave as neither true liquid
I
Y I, 2
'1 1
-7.9~ ELUTIONS Or CIS-BUTENE-2 I O FOOT COLUMN, 0 3 W T PER CENT "-HEPTANE ON GLASS BEADS ne F L O W 3 5 m l l M I N '
I
Figure 5. Change in retention characteristics with sample size
nor solid in the nominal sense but may indeed serve as a useful surface for analytical partitioning (10). DISCUSSION
Several gas chromatographic s e p arations reported for components in Figure 1 require two or more columns, temperatures, and/or carrier gases (2, 3, 7, 8, 11, 12). Previous studies have generally been limited to gas-solid chromatography and to tests a t room temperature and above. Improved separations at low temperature may replace the more complex gas-liquid or gas-solid columns previously required to give comparable analyses. Low temperature separations may also find advantage for ultrafast analyses in process control (9). Carbon monoxide, methane, ethylene, and other light hydrocarbons, which are prominent in industrial atmospheres, may be easily analyzed. This potentiality may be enhanced by maintaining the thermistor detectors a t the same low temperature as the column which ill increase sensitivity. With detectors a t 25' C., amounts and concentrations below 10" mole and near 5 p.p.m. are easily detected. Irreversible adsorption effects have been observed in this and other laboratories with gas-solid chromatographic columns (8). These effects become more prominent and limit the use of gas-
solid chromatography a t reduced temperatures. On the other hand, gasliquid columns generally give quantitative, reproducible results without the tailing effects found in gas-solid chromatography. Peak tailing may be reduced by deactivating the substrate (1). This process, however, tends to make gas-solid data harder to interpret and reproduce than the established behavior of gas-liquid columns. Separations performed on concentrated n-heptane columns represent an approximate low temperature extension of boiling point type packings such as hexadecane, squalane, and silicone. For these conditions, elution times are nearly proportional to the concentration of partitioning liquid on support (1). However, many of the columns tested here showed some selectivity. Even with 28y0normal heptane on brick, several compounds, e.g., CzH2, gave retention times which deviate from the boiling point plot in Figure 1. Inert gases and hydrocarbons which fall on the same curve have similar solubility functions while others, e.g., CHF, and CClF,, may have lower solubilities. I t has been suggested previously that adsorption sites for different types of solutes differ in some way (1). At low concentrations of partitioning liquid, gas-solid and/or gas-liquid adsorption effects become dominant. This is indicated clearly by comparative tests with columns
of n-heptane and n-octane a t two concentrations on glass beads. It has recently been reported that gas-liquid chromatographic separations may be performed on systems as much as 250' C. below their normal boiling points (5, 6). This is accomplished on columns of glass beads coated with small percentages of partitioning liquid. Extension of this general technique to tests a t -78" C. has led here to useful information as to the nature and types of separations that can be achieved. It appears that, with proper conditions, any .system with a significant vapor pressure may be gas chromatographed. Low temperature gasliquid chromatography us. gas-solid with its strong adsorptive effects, may also be of value in separating and analyzing reactive and labile species. ACKNOWLEDGMENT
The authors are indebted to K. E. Thompson for construction and operation of the chromatograph.
LITERATURE CITED
J., Langer, S. H., Perrett, R. H., Purnell, J. H., J. Chem. SOC. 1960, 2444. (2) . , Greene, S. A,. ANAL. CHEM.31. 480 (1959). (3) Hardy, C. J., Pollard, F. H., J. C h r m l o g . 2, 10 (1959). (4) Hishta, C., Messerly, J. P., Reschke, R. F., ANAL.CHEM.32, 1730 (1960). (5) Hishta, C., Messerly, J. P., Reschke, R. F.. 137th National Meeting. ACS. Cleveiand, Ohio, April 1960; Kbstraci (1) Bohemen,
p. 29-B.
(6) Hishta, C., Messerly, J. P., Reschke, R. F.,,Fredericks, D. H., Cooke, U'. D., ANAL.CHEM.32,880 (1960). (7) Koch, R. C., Grandy, G. L., Nucleonics 18. 76 (1960). (8) Lard; E.'W., Horn, R. C., ANAL. CHEM.32,878 ( 1960). (9) Loyd, R. J., ilyers, B. O., Karasek, F.W.. Ibid.. 32. 698 (1960). (10) Martin. 8. L..Ibid.. 33. 347 (1961). (11) Porter,' R. S.,'Johnson,' J. F'., Znd. Eng. Chem. 52,691 (1960). (12) Vizard, G. S.,Wynne, rl., Chem. & Ind. (Ladon)1959, 196. RECEIVEDfor review April 19, 1961. Accepted June 13, 1961. Presented in part, Division of Petroleum Chemistry, 139th Meeting, ACS, St. Louis, 110.. March 1961.
END OF SYMPOSIUM
VOL. 33, NO. 9, AUGUST 1961
1155
