Principles of High-speed Gas Chromatography with Packed Columns B. 0. AYERS and R. J. LOYD Process Development Division, Phillips Petroleum Co., Bartlerville, Okla. DONALD D. DeFORD Department o f Chemistry, Norfhwestern University, Evanston, 111. The problem of achieving gas chromatographic separations in minimum time with packed columns has been treated theoretically, and equations for calculating the optimum column length and carrier gas velocity have been derived. The quantitative relationship among analysis time, column parameters, and operating conditions is complex, but several generally applicable rules for minimizing the analysis time can be stated. The capacity factor should be between 2 and 3, and the separation factor should be large as possible. The '>masstransfer constant, C, of the van Deemter equation should be small; small values of C can be achieved by employing low percentages of liquid phase (of ihe order of 5 to lo%), relatively fine-mesh supports (of the order of 1 00-mesh), and carrier gases of high diffusivity. The carrier gas should have low viscosity, particularly if long columns are required. Only modest gains in speed can be realized by optimizing the carrier gas velocity for minimum time rather than using the carrier gas velocity which yields the minimum plate height.
S
investigators have recently addressed themselves to the problem of minimizing the time required for gas chromatographic separations. Desty and Goldup (3) and Scott and Haseldean (11) have discussed the factors involved in the time required for separations with capillary columns. Purnell (9) and PurneIl and Quinn (IO) have proposed a generalized method for minimizing the time required when packed columns are used; their method has serious shortcomings which will be discussed later. This study is an extension of that reported previously (8) and deals with the derivation of equations which permit one to calculate the minimum time required to achieve a given separation, as well as the optimum column length and optimum carrier gas velocity for carrying out this separation in the shortest possible time. The treatment requires a prior knowledge EVERAL
986
0
ANALYTICAL CHEMISTRY
of certain column parameters, but them parameters are easily measured and are independent of column length and carrier gas velocity, The optimization can be accomplished only with respect to a single pair of components in the sample mixture. If the sample mixture contains several components, it is impossible to select one set of conditione that is optimum for separating every component from its neighbom. In the case of multicomponent mixtures it ia usually best to optimize conditions, so that the desired resolution of the most difficultly separable pair of components is achieved in minimum time, but sometimes compromises are neceasary in ordcr to obtain resolution of all components.
locity. Equation 3 is identical in form with the van Deemter equation, except that the A term is missing and the outlet velocity rather than the average velocity is used. The time required for the elution of the ith component from the column is given by
where P = p c / p , and ratio defined by kc
(ti
Ict is the capacity
- t.)/t.
(6)
where t. is the elution time for the air peak. The resolution of two components may be defined by the equation
THEORY
The inlet pressure of a gas chromatographic column is given by
+
pia = pa* wU0p.L (1) where pd is the inlet pressure, p , is the outlet pressure, u. is the outlet velocity, L is the column length, and w is the column resistivity, which is directly proportional to the carrier gas viscosity and which depends also upon the nature and particle size of the packing. The length of the column may be expressed as L = NH (2)
where N is the number of theoretical plates required for the separation and H is the height equivalent to a theoretical plate. This equation assumes that H is independent of column length; this assumption will be justified later. For all columns which we have studied it has been found empirically that a plot of Hu. vs. uozis a straight line. One may then write
+
Hue B CU.' (3) where B and C are constants which depend on the carrier gas, the nature and particle size of the packing, the outlet presaure, and certain other operating conditions, but are independent of the column length and the outlet ve-
where tc and t i are the retention tima of the two components and the At's are the distances intercepted along the base line by tangents through the points of inflection of the peaks. [When defined in this way a resolution of 1.5 (separation of six standard deviations) corresponds to substantially complete separation of two symmetrical Gaussian peaks of equal height; a resolution of 1.0 (separation of four standard deviations) is normally adequate for satisfactory quantitative peak height measurements.] When the sep aration factor is close to unity, the number of theoretical plates required to achieve any desired resolution is given by
where 'ais the separation factor of the two components and is defined by a = (kilkr)
>1
(8)
Combination of the preceding equations yields
s\-
where
__.
/l
wBX
q=1+--1+ PO
16wB(ki pM(.
+- 1)'Rij' 1)'
and whew
z
=
WCA; u
a =
Po
1 6 ~ Cki(
+
1)'&la poki*(a - 1)'
uol
(11)
It is apparent from Equation 10 that the value of q is determined solely by the parameters of the column selected for the separation and by the desired resolution. All of the column parameters can be readily evaluated by appropriate measurements on a column of the type to be used for the separation. Once a value for q has been obtained in this manner, it can be shown that the separation can be achieved in minimum time if the outlet velocity is adjusted so that the value of 2 is given by
-
,z,
q [l
- 2 COB (;
+ 24091
(12)
where 4, which lies between 0' and 180°, is defined by
The optimum velocity thus depends not only on intrinsic column parameters, but also on the resolution (or the number of theoretical plates) desired and, hence, on the column length. The column inlet-outlet pressure ratio, outlet velocity, and length are given by the equations
P
= P { / P . = (p
+
($1)1'1] L
=i
[(+1)"'+
2)1/*
(14)
(RC)l/'N (16)
c
(q
0.02
0.05
0.20
0.10
+ 2)CN(ki + 1)
Equations 14, 15, and 16 are valid for
0.50
I
2
5
IO
q-1
Figure 1.
Values of functions of q and Z when Z = Zopt
any value of 2, whether optimum or not. Values of the significant functions of q and 2, where 2 = Z,t are shown graphically in Figure 1. The use of graphs of this type greatly reduces the labor involved in determining optimum conditions. Procedure for Selection of Optimum Conditions. The procedure for selecting the optimum column length and carrier gas velocity then involves the following steps:
If this is done, the dimensions and magnitudes of w, B, and C will be
dzerent, but they are self-consistent, and no error is introduced, provided the cross-sectional area of the column is kept constant.
Special Cases When (q - 1) 1. When(q - 1)isvery small (1), the value of Zoptapproaches 24 and Equation 17 reduces to
tude as that required for the desired separation. Operate the column at the same temperature and with the same carrier gas that will be used in the actual separation. 2. Measure the column inlet pressure, and also the plate height for the most strongly retained component of the pair to be separated, over a range of carrier gas velocities. Determine constant 20 from a plot of the square of the inlet pressure us. outlet gas velocity (Equation 1) and constants B and C from the intercept and slope, respectively, of a plot of Hu,us. uOo(Equation 3) * 3. Measure the elution times for both components and for air and calculate ki, k,, and a (Equations 5 and 8). 4. Using the quantities obtained above, together with the value of the resolution desired, Ri,, calculate the value of (q - 1) (Equation 10). 5. From Figure 1 determine the value [(q - 1)/2l1/* E/(P 1)11/*1 corresponding to the calculated value of (q - 11, and compute the length of the column required from Equation 16s. Similarly determine the value of (q 2)"' and compute the optimum inlet pressure from E uation 14. 6. If desired, %e carrier gas flow rate, F., measured at the column outlet, may be used instead of the carrier gas velocity, u,, in all plots and calculations.
(-3/']
Thus it is possible to calculate not only the optimum carrier gas velocity for a given separation but also the optimum column length and the pressure ratio required to operate this column a t the optimum gas velocity. When the value of 2 is adjusted to its optimum value, but only when 2 is so adjusted, Equation 9 may be written in the somewhat simpler form h
I 0.01
+
-
+
Little error is involved in using Equation 19 instead of Equation 17 when (q - 1) is greater than 10. Unfortunately, the value of (q - 1) usually lies between the limits where either Equation 18 or 19 is valid, and the more complex Equation 17 must then be used. EXPERIMENTAL
The apparatus and experimental techniques used in this study were identical with those used previously (8). Columns were fabricated from stainless steel tubing of 0.02inch wall thickness packed with Eastman Kodak bis (%(%methoxyethoxy) ethyl] ether on Johns-Manville 100- to 140-mesh Chromosorb. The percentage of liquid substrate on the packing is expressed as centigrams of liquid per gram of support. In order to relate measured peak widths exclusively to spreding in the column itself, i t is essential that the VOL. 33, NO. 8, JULY 1961
987
contribution of the sampling system and the detector to the total peak width be negligible. Of all the methods and devices tried, only a sampling valve which introduces a slug gas sample directly onto the column, used with a hydrogen flame detector, met this requirement. The linearity of the detector response was verified for the range of concentrations encountered in this study. RESULTS AND DISCUSSION
Relationship of Plate Height to Carrier Gas Velocity and Column Length. The treatment outlined previously for the selection of the column length and carrier gas velocity which will yield a desired separation in minimum time demands t h a t the plate height be related to the carrier gas velocity a t the column outlet by the relationship given in Equation 3 and t h a t the plate height be independe n t of column length. There are many theoretical objections to any equation, such as Equation 3, in which only two independent constants are involved in describing the relationship between plate height and carrier gas velocity (4,7). Despite these objections we have used plots of Hu, us. u.) to represent data for a large number of packed columns operated under a wide variety of conditions, and in no case have we encountered a set of data for which a linear relationship was not valid within the limits of the experimental error of the measurements involved. Our studies have included columns ranging from 4 to 12 feet in length operated a t outlet pressures between 1 and 4 atm. and having inlet-outlet pressure ratios up to 5. Hydrogen, helium, and nitrogen have been used as carrier gases with outlet velocities varying from 2.5 to 65 cm. pcr second. Both C-22 firebrick and Chromosorb s u p ports in mesh sizes from 30-42 to 120150 loaded with 5 to 30% liquid have been employed. hlost of our studies have been confined to C, hydrocarbons at 25' C., in which the capacity ratio lay between 0.4 and 8.0, but a few studies have been made with higher molecular weight hydrocarbons (up to n-nonane) at tcrnperatures up to 100" C. Samples other than hydrocarbons have not yet been studied systematically. Although our data, as well as those of others (4),iii(1icate that Equation 3 is applicable under a wide variety of operating conditions, this relationship breaks down in some extreme situations. For example, the equation is inadequate when the column outlet is operated under vacuum (4). Although we are not aware of any situation in which this equation fails when the operating conditions are in the vicinity of those re988
ANALYTICAL CHEMISTRY
COLUMNS CARRIER
./A
0'
0
Figure 2.
I
5
I
I
10 I5 uE,CM? X IOm2
I
20
Hu, as a function of uo2for various columns
quired for minimum time separations, the validity of the relationship should be checked before applying the quantitative treatment outlined in this paper. On the other hand, the qualitative generaliaations which have been drawn with respect to selecting conditions for minimum time operation are relativcly insensitive to the exact form of the plate height-gas velocity relationship, and these generalizations should be applicable even when Equation 3 is not strictly valid. A few typical examples of Hu, us. uo*plots are shown in Figure 2. These data are for butadiene samples where kr was 2.65 for the 10% columns and 8.79 for the 3oY0columns. Bohemen and Purnell (1) earlier employed plots of Hii us. 2, where ii is the average gas velocity, to represent the relationship of plate height to carrier gas velocity. While plots of this type are satisfactory for short columns having low pressure drops (ti z U J , marked deviations from linearity occur if the column pressure drop is high. Even when reasonably linear plots are obtained, the slopes and intercepts of the lines depend on column length. Plate height is independent of column length for all of the systems we have studied. This point is illustrated in Figure 2. The data for the three 10% columns operated with hydrogen carrier gas all fall on a single straight line. The data for the three 3oY0 columns do not agree as well, but again it is clear that the slopes and intercepts of the lines do not depend on column length, since the data for the 12-foot column lie on a line between those for the 4-foot and 8-foot columns. The differences between these three columns can be ascribed entirely to the lack of perfect reproducibility in packing the columns.
In Figure 3 the more conventional plot of plate height ua. carrier gaa velocity is shown for the three 30% columns. The solid curve is the calculated curve for B = 0.47 sq. cm. per second and C = 5.8 X 10-4 second, which are the values obtained from the intercept and slope of the line for the 12-foot column in Figure 2. The maximum deviation in plate hcight of any single point from the solid curve in this figure is only 0.005 cm. and the average deviation of all points is only 0.002 cm. ,or about ~ k 3 7 ~These . deviations are well within the limits of experimental error and indicate that there are no significant differences in the values of B and C for the three columns. The literature is replete with examples in which investigators have attempted to determine the value of constant C by graphical estimation of the asymptote of an H us. u, plot, somewhat similar to the dotted line in Figure 3. Such estimates almost invariably lead to erroneous values for C, and the estimated asymptotes often intercept the ordinate at some value other than zero; these intercepts are then often incorrectly equated to the A term of the van Deemter equation. The true asymptote for the curve in Figure 3 is indicated by the dashed line. I t is clear that any graphical method of estimating the position of the correct asymptote is virtually impossible, and this practice should be abandoned. Many investigators employ plots of plate height us. average carrier gaa velocity, rather than outlet gas velocity, to depict the relationship between the plate height and the gas velocity. Plots of this type for the three 30% columns are ahown in Figure 4. It is evident that the relationship between H and ti depends on the column length; hence, plots of the type shown in Figure 3, which are independent of column
length, are to be preferred. The method employed by Purnell and Quinn (IO) for the estimation of optimum conditions for high speed separations is based upon the assumption that H is a unique function of ii, independent of the length of the column. This asdumption is not corrwt for the conditions we have described. Hence the method which they have proposed ia not generally valid. Effect of Capacity Ratio on Minimum Time. If a11 quantities other than the capacity ratio, k,, are assumed to be constant, i t can be shown that any given resolution can be accomplished in minimum time when k, = 2 if (q - 1) 1 (Equation 19). The optimum value lies somewhere between 2 and 3 for any column, the exact value dcpending on the magnitude of the quantitics involved in the definition of q (Equation 10). Relative times for a fixed rcsolution as a function of k, are shown in Figure 5 for the two extreme valucs of (q - 1); the curve for any actual column is rather difficult to calculate, but it will always lie someBhcre between these two extremes. It is evident that the analysis time is rclativcly insensitive to the magnitude of k, with any column, so long as k, is in the vicinity of 2 to 3. The time required increases very rapidly, however, as k, falls below about unity or rises above about 8. It is frrquently difficult to adjust column parameters and operating conditions to bring the value of k, to the optimum without simultaneously changing other quantitirs, particularly C and OL, which also inflricnce the analysis time. At prrscnt there is no generally applicable tlicory which will predict the total effcct of changes in the pcrcentage or type of liquid phase or changes in the temperature, all of which might be uscd to adjust the value of k, to its optimum. However, several rulcs of limitcd applicability aid in the selection of optimum conditions. The values of I(, C, a,and w are almost indepcndcnt of thc percentage of liquid substrate on Chromosorb supports, providrd this pcrccntage is of the order of 10% or less and the percentage of liquid phase is not reduced to such a low value that adsorption effects become significant. Particularly when one is dealing with separations of nonpolar compounds, which show little tendency to be adsorbed even a t low liquid loadings, there is a fairly wide range over which liquid loadings may be varied in order to bring k, to the optimum range. Although nearly all of the quantities mentioned above are influenced by changes in temperature, the temperature coefficients of both B and C are small for columns having low liquid loading
CARRIER GAS VELOCITY AT COLUMN OUTLET, u,, CM/SEC.
Figure 3. outlet
Plate height as a function of carrier gas velocity at column 30%
ether columrn Hydrogm carrier gas
0.10 I
0,09-
3
- 0.08-
I
k '3
W
I
w
2 -I
0.07 -
a
0.06
0.05
t'
A I I
I
I
20
IO 15 AVERAGE CARRIER GAS VELOCITY, 8. C d S E C .
5
Figure 4.
Plate height as a function of average carrier gas velocity 30% ether columns Hydrogen carrlcr gas
I
0.2
0.4
Figure 5.
0.6
I
I
1
1
2 4 6 CAPACITY RATIO, ki
I
IO
I
20
I
40
Effect of capacity ratio on analysis time VOL. 33, NO. 8, JULY 1961
989
(-10% liquid), while the value of ki is sensitive to temperature changes ( 8 ) . The temperature coefficients of w and a are also generally small. On the basis of the limited data available, it appears that changes in temperature up to about 60' C. can be used to optimize kc without drastically affecting other quantities involved in the minimum separation time ( 8 ) . Further studies on the effeot of temperature are planned. Effect of Separation Factor on Analysis Time. It is apparent from Equations 18 and 19 that analysis time is inversely proportional to (a 1)' when (q - 1) is !aero, inversely proportional to (a - 1)' when (q - 1) is infinite, and intermediate between these two extremes for all finite values of (q -7 1). Since the analysis time is so cntically dependent upon a, the selection of a liquid substrate having the highest possible degree of selectivity for the components to be resolved is obviously highly desirable. Since the value of a normally increases with decreasing temperature, operation of the column at the lowest possible temperature is indicated. To keep kr in the optimum range a t lower operating temperatures, the amount of liquid substrate on the column must be reduced. Fortunately, the value of C, to which the time is directly proportional, is a minimum at low liquid loadings. The efficacy of this general approach of reducing analysis time by simultaneous reduction of temperature and liquid loading has been demonstrated by Hishta and his associates
-
(6). Effect of Column Resistivity. It is apparent from Equations 18 and 19 that column resistivity, w, has no effect on the analysis time when (q - 1) is very small, but that its effect becomes increasingly significant as (q - 1) increases. When (q - 1) is very large, the time is proportional to the square root of the column resistivity. Since the resistivity is directly proportional to the viscosity of the carrier gas, the use of a carrier gas of low viscosity is desirable especially when (q - 1) is large. Hydrogen, which has a viscosity roughly half that of helium or nitrogen, is the best of the commonly used gases from this standpoint. Keulemans (6) has shown that the column resistivity is inversely proportional to the square of the average particle diameter of the solid support used for the column packing; we have verified this relationship. Unfortunately, the use of coarse packings to reduce column resistivity results in large values of C; the over-all effect is one which results in longer analysis times when coarse packings are used. The relationship between the value of 990
ANALYTICAL CHEMISTRY
Figure 6. Comparison of minimum time operation with minimum plate height operation
C and the average particle diameter of the support is very complex, and there is some evidence that there is an optimum particle size for minimum time. This optimum appear0 to be in the general vicinity of lW-mesh, but further studies, which are now in progress, are needed to establish this point conclusively. Influence of Constants B and C. Since the analysis time is directly proportional to C for all columns, the use of conditions which make C as small as possible are highly desirable. B has no effect on analysis time when (q - 1) is very small, but the time becomes proportional to the square root of B when (q - 1) is large. B is directly proportional to the diffusion coefficient of the solute in the carrier gas, meaaured under the conditions of temperature and pressure prevailing at the column outlet, and is substantially independent of all other column parameters and operating conditions. This relationship would suggest the use of carrier gases having low diffusivities, especially when ( q - 1) is large, but the value of C increases as the carrier gas diffusivity is reduced. For all cases which we have studied, the combination of these two effects is such that analysis times are always lowest with carrier gases having the highest diffusivity. The value of C is influenced by nearly all column parameters and operating conditions, including the type and particle size of the solid support, the type and quantity of the liquid substrate, the column outlet pressure, and the diffusion coefficients of the solutes in the carrier gas. Surprisingly, for the systems we have investigated, the value of C, contrary to the predictions
of current theoria, is almoat inde-
pendent of the capacity ratio, h, per Be. Theae effects are now being studied further and will be reported shortly. Comparison between Operation of Columna under Minimum Time and under Mfnfmum Plate Height Conditions. If a oolumn is operated with an outlet velocity such t h a t the plate height is a minimum, the value of Z is equal to (q 1). The time, inletoutlet pressure ratio, and column length required for any desired resolution when operating under these conditions may be calculated from Equations Qa, 14, and 16a, respectively, by substituting (q 1) for Z. The ratio of the time required for a given resolution under minimum time conditions to the time required for the same resolution under minimum plate height conditions is plotted as a function of ( q - 1) in Figure 6. The ratios of the outlet velocity of the carrier gas, the column length, and the inlet pressure for the two seta of conditions are also shown. The time required under minimum time conditions is one half that required under minimum plate height conditions when (q 1) is zero. The relative improvement in analysis time which can be achieved under the former condidions gradually decreases as (q - 1) increases; when (q - 1) is very large, the time required under minimum time conditions is 91% of that required under minimum plate height conditions. It is clear that one can achieve only a modest improvement in analysis time at any value of (q - 1) by operating under minimum time conditions rather than under minimum plate height conditions. Such improvement as can be
-
-
achieved may be rather costly in terms of the increased operating difficulties arising from the higher inlet pressures required, increased carrier gas consumption, and the greater amount of labor involved in measuring and calculating the quantities required to design a system for minimum time operation. The quantitics w, B, C, ki, and a affect the analysis time under minimum plate height conditions in substantially the same fashion as under minimum time conditions. The conclusions drawn in the preceding sections regarding the influence of these quantities on analysis
time apply equally to either type of operation. LITERATURE CITED
(1) Bohemen, J. Pimell, J. H., in “Gaa Chromatograph D. H. Desty, ed., pp. 12-13, Acagmic Press, New York, 1968. (2) DeFord, D. D., Lyndrup, Ma, unublished results. (3rDeaty, D. H., ,$hldup, A., in “Gaa Chromatography, D. H. Desty, ed., p. 162-83, Butterworths, London, 1960. (4P Giddinga, J. C., Seager, S. L., Stucki, L. R., Stewart, G. H., ANAL.CHEM.32, 867 (1960). (5) Hishta, C., Messerly, J. P., Reschke, R. F., Fredericks, D. H., Cooke, W. D., Zbid., 32,880(1960).
(6) Keulemana,, A. I. M., “Gaa .Chromatography, pp. 13C-8, Remhold, New York, 1957. (7) Kieselbach, R., ANAL. CHEM. 32, 880 (1960). (8) Lo d R. J., Ayers, B. O., Karaaek, F. tbzd., 32,698 (1900). (9) Purnell, J. H., Nalurs 184, 2009 (1969). (10) Purnell, J. H., Quinn, C. P.,in “Gas Chromatography,” D. H. Desty, ed., pp. 184-98, Butterworths, London, 1960. (11) Scott, R.P. W., Hazeldean, C.S. F., Ibid., pp. 144-61.
d,
RECEIVEDfor review November 21, 1960. Accepted March 30, 1961. Presented in part before the Division of Anal ical Chemistry, 137th Meeting, ACS, &veland, Ohio, April 1960.
Automotive Exhaust Gas Analysis by Gas-Liquid Chromatography Using Flame Ionization Detection Determination of
C1
to
c 6
Hydrocarbons
RAYMOND FEINLAND, A. 1. ANDREATCH, and D. P. COTRUPE’ Central Research Division, American Cyanamid Co., Stamford, Conn.
b Gas chromatographic techniques using the flame ionization detector have been applied to the determination of automotive exhaust gas hydrocarbons including C1 to Cg compounds and several Ca compounds. Because of the high sensitivity of the detector, it is possible to analyze a 1-ml. gas sample without a concentration step. Two columns are used to resolve the components. The minimum detectability using a 1 -ml. sample is estimated to be 0.01 p.p.m. for n-butane. The standard deviation for n-butane standards at the 10- to 100-p.p.m. level is 4%. A comparable level of reproducibility may be expected for various components in exhaust gas samples which are not subject to peak overlap.
T
HE DETERMINATION of automotive exhaust gas hydrocarbons is an important problem in air pollution studies. Most of the previous work employing gas chromatography has been done with the conventional thermal conductivity detector which necessitated a concentration of the trace hydrocarbons (3, 8, 9). Heaton and Wentworth (6) analyzed the hydrocarbons directly by oxidizing them to
Present address, Chemical Research Laboratories, Amcrican Machine & Foundry, Springdale, Conn.
COZ and passing the latter into an infrared analyzer. This technique requires the use of expensive equipment and, moreover, the sensitivity leaves something t o be desired. Previous workers have used dimethylsulfolane a t 0” C. for determining C1 to CS hydrocarbons (3, 4). The resolution of components was good but the retention time was excessively long. Moreover the presence of Cs saturated hydrocarbons introduces overlapping peaks. The use of a flame ionization detector for the direct determination of exhaust gas hydrocarbons including CI to Cg compounds and several Ca saturated compounds is described. The detector is more sensitive than any previously applied to this problem. Two columns at 25’ C. are employed to give adequate resolution in a reasonable period of time, EXPERIMENTAL
Flame Ionization Detector. An inexpensive rugged detector was designed in our laboratories employing a simple direct current amplifier ( 1 ) . Its sensitivity measured on the basis of the S factor as defined by Dimbat, Porter, and Stross (9)is of the order of 108 mv. ml. per mg. with a noise level of 10 pv. This sensitivity is several thousand times greater than that of the thermal conductivity detector.
Apparatus and Procedure. T o determine the C1 to Cs exhaust gas hydrocarbons, two column packings were used: 20% by weight of dimethylsulfolane on 60- to 80-mesh Chromosorb solid support and 10% by weight of diisodecylphthalate on 60- to SO-mesh Chromosorb W solid support. Each material was packed into 4 meters of l/d-inch copper tubing which was coiled and inserted into a water bath thermostated a t 25.0’ C. The columns were connected in parallel. Hydrogen and nitrogen were mixed in equimolar amounts and passed throug!i the columns a t a total flow rate in each column of 50 cc. per minute. Capillary restrictors (thermometer tubing) were placed in the hydrogen and nitrogen inlet lines to help regulate the gas flow rates and to eliminate back pressure. Thermometer tubing was also used as a restrictor following the column t o regulate the relative flow rate of each. An inlet port was ins+illed a t the entrance of each column for sample injection. Each column was connected to a flame ionization detector and the response of each detector measured by the same IO-mv. recorder, having a chart speed of 48 inches per hour. The dimethylsulfolane substrate has sufficient vapor pressure to cause an extremely high background signal. This signal may be offset by electrical means, but a high noise level would still be present. A trap immersed in a dry ice-acetone slurry was inserted between this column and the detector to condense the dimethylsulfolane vapor. VOL. 33, NO. 8, JULY 1961
0
991
