Table IV that both crudes have distributions which continue beyond Cw. However, the lack of resolution combined with the small concentration involved made measurement difficult above this carbon number, although, qualitatively, the Xigerian and Libyan distributions extended to and C58, respectively. The relative abundance of odd- over even-numbered paraffins is evident in the C d & range of both crudes. The
relative abundance of even- over oddnumbered Paraffins that Occurs above C, in the Libyan distribution, and is detectable in the chromatograms of the higher boiling Nigerian distillates, has, as far as is known, not Previously been reported. The significance of these data in conjunction with certain peculiarities in the distributions of the lower carbon ranges have already been discussed from the standpoint Of current thinking on the origin of petroleum ( 2 ) .
LITERATURE CITED
(1) Brenner, N., Coates, V. J., Nature 181, 1401 (1958). (2) Brunnock, J. v.9 Ibd.1 in Press.
‘ 3 { 2 ~ ~ ~ p ~ ; ” 9 7 ~ 9 6 6 ~ * K’J ’ (4) Van der Wid, A., Erdoel Kohle 18, 632 (1965). (5) Whittam, €3. T.7 Nature 182, 391 (1958).
J2i
~~~~~~$ ~ ~ ~ i ~ ~ ~ M ~ e r ~ ~ publish this paper has been given by The British Petroleum Co., Ltd.
Gas Chromatographic Separation of Polar Compounds Using Water Vapor in the Carrier Gas and Water as the Liquid Phase L. H.
PHIFER and H. K. PLUMMER, Jr.
FMC Corp., American Viscose Division, Marcus Hook, Pa.
Chromatographic separation of alcohols which appears to b e based primarily upon their polarity can be accomplished using water as a liquid support and a mixture of nitrogen and steam as the carrier gas. The necessary modifications in the flame ionization chromatograph, types of solid supports which are applicable, and operating variables are discussed. Examples of applications described include detection of 1.8-p.p.m. 2,3butanediol in 1,2-propanedioI, separation of 1,2-ethanedioI from 1,2-propanediol, and separation of C1 to Cg straight-chain alcohols in reverse order.
P
one of the most difficult problems encountered in gas chromatography is the separation of trace amounts of polar compounds. This is particularly true in water solutions. The limited amilability of very stable liquid phases and the presence of active sites throughout the system are the chief sources of these problems. Various methods have been used to minimize the effect of active sites, including direct “on column” injection, inert solid supports, coating of the column walls, etc. One of the most interesting and effective approaches has been to use xater vapor in the carrier gas stream. This functions by becoming partially associated with the active sites as well as displacing solute adsorbed on the active sites (3). Addition of water vapor to the gas stream in most of the conventionally used highly polar liquid substrates results in decomposition to varying deROBABLY
1652
0
ANALYTICAL CHEMISTRY
grees of the liquid phase and a resulting increase in the background noise and short column life. In practice, in our laboratories, no conventional polar phase has been found which is sufficiently stable in the presence of water vapor to operate a flame ionization detector at its maximum sensitivity or even near it. In reviewing the problems of separation of polar compounds, in particular glycols, it was felt that water itself would be an ideal liquid phase for the separations. With a flame ionization detector one at least would not be troubled with the effect of liquid phase bleeding off the column. If a chromatographic system composed of a column containing only the solid support and steam as a component in the carrier gas were used, it is evident that an equilibrium would be established in which any active sites would be associated with water, giving the effect of a thin layer of water as the liquid phase. As the water present as the liquid phase is lost by temperature and gas flow effects, it is immediately replaced by the water in the gas phase or, in other words, HzO (gas)eHzO (solid support). Assuming this equilibrium, one would expect the amounts of water present as the liquid phase to depend on the temperature of the column and the fraction of the gas stream which is water vapor. The effect of the water in the carrier gas phase on the actual separation would be difficult to predict, since displacement of solute from any free active sites is possible as well as displacement of the solute associated with the water phase. In addition,
there is the possibility of association with the solute in the gas phase. This report is devoted to a study of some of the characteristics of this system, with particular emphasis on the operating parameters necessary for the separation of diols. The use of water as the liquid phase for the separation of chloromethanes was described by Pollard and Hardy in a paper presented in 1955 (4). In their work no steam was used in the carrier gas and the original water substrate level was allowed to decrease through normal attrition caused by the carrier gas, although the temperature of the column was a t 24.2’ C. Separation of the chloromethanes was obtained, suggesting that the water was functioning as a highly polar liquid phase. They report that the water was lost from the column to the gas stream a t a rate of 0.23% of the weight of water per 100 ml. of nitrogen. When operating a t high column water levels, the column bleeding effect apparently functioned like steam addition to the gas phase and tailing was reduced. EXPERIMENTAL
Apparatus. An F and M Model 609 chromatograph was modified so that the nitrogen gas stream passed through a. 500-ml. stainless steel tank containing water in a constant temperature oil bath. The tank was equipped with the inlet line reaching to the bottom of the tank and the exit from the top. The portion of the gas line from the tank to the normal injection port was wrapped with heating tape and kept over looo C. using a Variac for control. As an aid in calculation of the flow
n-AMYL ALCOHOL
+BUTANOL
n- PROPANOL
COLUMN TO0 NITROOEN- 60 ml/MIN. STEAM- 40 ml/MIN. I pl. AQUEOUS SOWTION CONTAINING 2 2 RRM. ALCOHOL
ETHANOL
METHANOL
rate, a pressure gauge was installed in the line just before the water tank. Columns were generally prepared from 1/4-inch stainless steel tubing. Various lengths from 2 to 10 feet were used. The column supports which appear to be most useful are Gas Chrom CLZ, 80-100 mesh (Applied Science Laboratories, Inc., State College, Pa.) and Chromosorb W, aw-DMCS, 60-70 mesh (Johns-Manville Products Corp., Celite Division, New York, N. Y.). Essentially all of the commercial diatomaceous earth supports were examined. In addition, some work was done with silica gel and glass beads. Procedure. The tank in the oil bath was filled with approximately 250 nil. of distilled water. The oil bath temperature was maintained in the temperature range of 70"-95" C. (constant to =t0.1" C.). The column oven was adjusted to the temperature in the experiment. The hydrogen and air were turned on and the flame lit. The nitrogen flow through the system was adjusted as specified in the particular experiment (generally 30-60 ml.jmin.). The air flow and hydrogen rate were adjusted for a minimum noise level and maximum sensitivity. For most experiments the injection port was maintained a t a temperature approximately 50°-100" C. over the boiling point of the highest boiling component being studied. The detector block was generally a t 250" C. The system was allowed to operate several hours to allow the water to reach equilibrium. In most cases from 1- to 5-p1. aqueous solutions of the components being studied were used. Results were calculated either from peak heights or peak areas, depending on whether the latter appeared necessary.
RESULTS AND DISCUSSION
Choice of Solid Supports. Since the separation of 2,3-butanediol and 1,2-propanediol is one of the most difficult under normal circumstances, an aqueous mixture of these glycols was used for the study. The chromatographic system was operated over a range of temperatures and steam flow compositions in an attempt to attain the best possible separation. From the results, it was apparent that for this particular separation, dimethyl dichlorosilane-treated supports were best. The particular lots of Gas Chrom CLZ and Chromosorb W, awDMCS gave the best separation of all the supports studied. Since what is apparently needed is some active sites and the manufacturing procedures are generally directed toward complete deactivation of these sites, it would appear that only a trial experiment would indicate whether the particular lot was useful for this technique. Untreated supports and silica gel generally resulted in total loss of the sample on the column. Other experiments which will be described later would appear to indicate that this is due to very high water sorption levels, rather than direct adsorption of the glycols on the solid supports. Separation was attained using glass beads but the capacity of the system was very low, and further work with this system was abandoned. Water and the Solid Support. The presumption can be made that the system being used is HzO (vapor) e HzO (solid support). The water associated with the solid support functions
as a highly polar liquid phase-much higher, in fact, than any of the normally used liquid phases. Water in the gas phase would tend to displace less polar materials absorbed in the water in the liquid phase. It follows, then, that separation should be much more dependent on the relative polarity of the materials being separated than on the boiling points. This is rather adequately demonstrated if a mixture of C1 to CS alcohols is chromatographed. Figure 1 shows the results of this separation. Even Cloalcohols will come off before methanol. X suggested use for this particular separation is for determination of alcohols resulting from saponification of esters in methanolic KOH solutions. Liquid Support Levels. The amount of water present as the liquid phase was studied using a short column loaded with a known amount of predried Gas Chrom CLZ. The column was weighed after allowing the system to reach equilibrium with various HzO vapor ratios and column temperatures. The results, which are compiled in Figure 2 indicate that the associated water level is essentially constant on the column as long as the column is operated at a temperature above the condensation point for the water concentration in the carrier gas. When the column temperature is below the condensation point, the equilibrium amounts of water in the liquid phase increase rapidly as the column temperature is decreased. I t appears reasonable to assume that the essentially constant water levels represent water associated with the active sites in the system. When operating at conditions where condensation is occurring on the column, the water may be associated with weak active sites in the system, the water already associated with active sites, or may be present as free mater. A study was made of the separation of alcohols as it related to column temperature and the corresponding liquid support levels. An injection of C1 to CS alcohols made at a column temperature of 93' C. and 59.3y0 water vapor in the carrier gas resulted in almost a flash through of alcohols with only a slight separation of methanol from the other alcohols. The column was above the condensation point of the water vapor and the water on the column was approximately o.4y0. As the temperature was lowered, separation improved until complete separation was attained a t about 87' C. (the water level on the column a t 87" C. was approximately 2.0%). Lowering the temperature of the column further had very little effect on the efficiency of the separation, but did increase the retention times of all of the components. At 78" C. the retention time for methanol was approximately 2 hours with a VOL. 38, NO. 12, NOVEMBER 1966
1653
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%WATER IN GAS PHASE
------z
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26.8
*La
-
59.3 %
i
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0.'
E E
29.3 ml/MIN. NITROGEN 3 I .7 ml/MIN. STEAM (52KSTEAM)
40.0 %
8 $
22.0 ml/MH. NITROGEN 39.0 ml/MN. STEAM (64KSTLAM'
36.0mVMIN. NITROGEN 25.0ml/MIN. STEAM (41*STEAM'
0.1
3
0.1 IN4
Figure 3. rate
0.;
I
ro
1
1
80
90
6
I2
9
TIME, MINUTES IRATE 3MIN./HCH)
'ION
Separation of glycols as affected
by steam
One microliter of aqueous solution containing 2 2 pap.m. of each glycol was used. Order of peak appearance was 2,3-butanedlol, 1,2-propanediol and 1,2-ethanedioI. Column was ot 94' C.
COLUMN TEMPERATURE
Figure 2.
Water levels on column
chromatographic peak that was still syiunietrieal. Steam in the Carrier Gas. When steam is present in the carrier gas, two distinctly different phenomena can be postulated. The displacement of adsorbed solute, associated either with the water. phase or active sites on the solid support, is to be expected. That this occurs was demonstrated by setting the column temperature a t 94' C. (where the water content. associated with the solid support is reasonably constant regardless of the water i-itpor level in the gas stream), and injecting aqueous solutions of 2,3butanediol. As the water vapor level is increased, t'ailing of the peak decreases. At ol,tiinum flow levels, a perfectly symmctrical peak is obtained. This observation is consistent wibh effects of steam in the carrier gas which have been observed by Knight (3) and others ( 1 , d , 5 ) . A second possibility is association in the gas phase between the water molecules and active groups on the solut,e. That this type of possibility exists can be shown if one repeats the previously cited experiment using an aqueous solution containing 2,3-butanediol, 1,2propanediol and 1,kethanediol (Figure 3). A t low water levels in the carrier 1654
ANALYTICAL CHEMISTRY
COLUMN 94.C. NITROGEN 51 rnl/MIN. STEAM 37 ml/MIN. I pl. AQUEOUS SOLUTION CONTAINING 22 RBM. OF EACH GLYCOL
Figure 4. Separation of glycols.
I
I
I
1
1
I
s
18
27
36
45
54
TIME,MINUTES (RATE, 9 MIN./INCH)
L
63
4 Figure 5. Estimation of 2,3-butanediol in commercial 1,2-propanediol. Sample contains 3 6 p.p.m. of 2,3-butanediol
COulMN 94%.
NITROGEN 51ml/MlN.
bath a t 83' C., and comparison of these conditions with amounts of water associated with the column and influence of the steam on the separation, reveal that relatively large fluctuations in these controllable variables can be tolerated. (-4t 83" C., a change of 0.2' C. causes a change in the vapor pressure of water of approximately 3 mm. or a change in the order of 0.3y0 in the water vapor content in the carrier gas stream. This order of fluctuation is not detectable in the measurement.) For separation of less polar materials such as mono alcohols or ketones, it is necessary t o have an excess of water on the column. This situation is attained by operation of the oil bath a t a temperature greater than or equal to that of the column. Under these conditions, slight changes in temperature of either the column or oil bath result in drastic changes in amounts of water liquid phase. Since the alcohol and ketone peaks are virtually symmetrical and there is no evidence of any loss of the components within the system, using peak areas as a means of calculation gives reproducible results even with TIME,MINUTES ( R B E , 3 MINANCH)
gas, no separation of the glycol, occurs. -4s the water level is increased, the retention time for the 2,3-butanediol changes very slightly toward a shorter time, 1,2propanediol increases somewhat, and the effect is most pronounced on the 1,2-ethanediol. Since the primary hydroxyl group is much more polar and therefore much more likely to associate with water, a reasonable explanation would appear to be that the water is associating with the primary hydroxyl and that the associated molecules are moving down the column as entities. The degree that retention time is increased would depend on the degree of association of the water with the solute in the gas phase. Precision and Sensitivity of Chromatographic Separations. Since it has been demonstrated that column temperature and water vapor levels in the carrier gas h a w an extreme effect on the separations, the question arose as to what fluctuations in these controllable functions are permissible if a reasonable level of precision in quantitative measure is to be attained. An examination of the conditions necessary for the separation of glycols, for example, column a t 93' C. and oil
COLUMN 9IoC. 9IoC, NITROGEN 51 ml/MIN. STEAM 46ml/MIN. 2 . 5 ~ 1 SAMPLE .
E b
Figure 6. Estimation of lI2-ethanediol in commercial 1,2-propanedioI. Sample contains approximately 2 0 p.p.m. of 1,2-ethanediol
91
J J 1
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1
0
6
12
18
24
30
36
TIME,MIWTES (RATE, 6 MIN./INCH)
VOL 38, NO. 12, NOVEMBER 1966
1655
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I COLUW 85.C. N, 20.7 ml/MIN. STEAM 3l.Oml/MIN.
-
f
1
P
J
P
ki
9 kl
TIME, MINUTES (RATE, 4MINANCH)
Figure 7. Estimation of menthol extracted from cigarette filters using ethanol. Sample contains 41 p.p.m. of menthol
appreciable fluctuation of the retention times. The apparatus can be operated a t its maximum sensitivity with a noise level essentially equivalent to the electrometer noise level. On the particular instrument which has been used, this is less than 2y0 of the full recorder scale. This has permitted separations and measurements a t concentration levels as low as 0.01-ppm alcohols, ketones or aldehydes in water solutions. At these concentrations, the chromatographic peaks are essentially the same shape as those obtained at higher concentrations. APPLICATIONS OF STEAM CHROMATOGRAPHY
Separation of Glycols. An aqueous solution containing 22 p.p.m. each of 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 2,3butanediol, and 1,4-butanediol was
1656
ANALYTICAL CHEMISTRY
injected, operating the system where the liquid phase level is in the order of 0.4%. The results which are shown in Figure 4 show a complete separation of all of the components. When an effort is made to determine trace amounts of one of these glycols in another glycol, slight changes in the conditions can be used to produce a sharper front or tail. Figure 5 is a chromatograph of 36 p.p.m. of 2,3butanediol in 1,2-propanediol. The large amount of material in the first portion of the chromatographic curve indicates that relatively large concentrations of less polar contaminants are present. Since the separation is polarity dependent, and the glycols are quite polar, the possibility of interference in this measurement is very remote. The detection limit of the measurement is 1.8 p.p.m., with the limit being set by the tendency of
the less polar contaminants to tail into the 2,3-butanediol rather than by either instrument restrictions such as noise or the amounts of I ,Zpropanediol present. Although 1,2-ethanediol comes off the column after 1,2-propanediol, similar optimization of conditions gives a separation such as shown in Figure 6. A detection limit in the order of 10 p.p.m. is obtainable. Estimation of Trace Extractables with Alcohols. A problem frequently encountered is the removal and measurement of trace components from solid samples. Ethanol or methanol in many cases are excellent solvents but are infrequently used for extraction if the measurement is to be made chromatographically because of the tailing tendency of the alcohols. Since less polar materials generally come off the steam chromatographic system before ethanol or methanol and it is possible to operate the equipment at its maximum sensitivity, the system is ideal for a number of measurements of this type. An example of this is given in Figure 7 . Menthol was extracted from the filters in mentholated cigarettes and chromatographed. Menthol boils a t 215’ C., yet it comes off before the ethanol. -2 practical detection limit of less than 0.1 p.p.m. is possible, since the extraction appears to be quite selective. ACKNOW LEDGMENl
The authors thank Charles M.Rosser and William B. Swann for their many helpful suggestions. LITERATURE CITED
( I ) Abma, Charles B., “Steam as a
Carrier Gas,” Gulf Coast Spectroscopic Group 33rd Meeting, Oct. 12, 1962, Corpus Christi, Texas. (2) Davis, A., Roaldi, A., Tufts, L. E., J. Gas Chrom. 2, 306 (1964). (3) Knight, H. S., ANAL.CHEM.30, 2030 (1958). (4)Pollard, F. H., Hardy, J., “Papour Phase Chromatography, Desty, D. H., ed., Paper 10, p. 115, Academic Press, 1957. ( 5 ) Wilkins Instruments and Research, Inc., Aerograph Research Notes, Fall (1961). RECEIVEDfor review April 5, 1966. Accepted August 4, 1966. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1966.
2.
