Theory of Minimum Time Operation in Gas Chromatography J. CALVIN GlDDlNGS Department of Chemistry, University of Utah, Salt lake City, Utah
b The theory of minimum analysis time for packed and capillary columns is developed with particular emphasis on the role of the column pressure drop. Using a previously derived equation for the effect of pressure gradients on plate height, this analysis shows that the column outlet should b e held under vacuum for optimum performance. The column inlet should be maintained somewhat higher than the critical pressure, pc, the inlet pressure below which a separation can never b e obtained. Using optimum inlet and vacuum outlet pressure, the best carrier gas is found as that with the largest diffusivity to viscosity ratio. In the order of decreasing desirability one obtains Hz, He, Nz, COa, and Ar. In addition, the theory is used to obtain the optimum thickness of the liquid layer in capillary columns. It is found that the capacity factor, k, directly related to thickness, is variable, being sometimes less than unity.
C
ERTAIN CHROMATOGRAPHIC ANALYSES, particularly in connection
with process control, must be achieved very rapidly. For this purpose various attempts have been made to predict optimum column parameters for minimum time operation. I n general one fiinds that the flow velocity and column length must be increased beyond that value leading to minimum plate height. For this reason the pressure drop in the column increases beyond the normal value, and it becomes important to consider the effect of pressure gradients on plate height. If the usual restriction of operating a column outlet a t atmospheric pressure is removed, the role of pressure gradients is even more important as shown below. Because of this importance, the theory of minimum time operation in packed and capillary columns is developed here with full allowance made for column pressure drop. The approach is significantly different from that of PurnelI and Quinn (16)) Scott and Hazeldean (17), Desty and Goldup (3), and &ox (16). I n the latter cases, allowance was not made for pressure gradients. From a practical viewpoint, therefore, the gas has been considered as an incompressible 314
ANALMICAL CHEMISTRY
fluid with a constant flow velocity throughout the tube. The following equations describing the optimum parameters, even those not apparently related to flow, are directly dependent upon the consideration of pressure and flow velocity variations. Consequently the theory departs considerably from those cited above. The conclusions regarding optimum parameters are in some cases the same as, and in some cases different from, those previously obtained. Ayers, Loyd, and DeFord ( I ) have undertaken a careful study of the requirements necessary for minimum time analysis. Gas compressibility was considered in their treatment. Their results are applicable, as they point out, over a wide range within which an empirical plate height equation is valid. This range does not, unfortunately, include all the conditions determined here as being optimal, particularly the condition of zero outlet pressure. When alIowance is macle for pressure correction terms in the plate height equation, some rather startling results are obtained. Under any column conditions, a least time separation can be made only when the outlet is held under vacuum. The savings in time are often significant when compared with normal atmospheric outlet (8). Consequently the choice of optimum conditions with high inlet-outlet pressure ratios is important. Even if one operates the outlet a t atmospheric pressure, the high velocity and increased column length may lead to significant pressure ratios. Under these circumstances a true optimum cannot be determined without a consideration of pressure correction terms. The effect of pressure is seen also in the optimization of other variables such as the amount of stationary liquid, These matters will be fully explored below from a theoretical point of view for both packed and capillary columns. THEORY
The time, t, required for the passage through a column of the second component, 11, of a pair can be obtained
from the James and Martin equation ( I S ) as
where L is the column length, RrI the velocity of the second component divided by the gas velocity, v i the gas velocity a t the inlet, and P the inletThe outlet pressure ratio, pi/p,. column length may be expressed as the product of the apparent plate height, E?, and the number of plates, AT. The apparent plate height is the measured value given operationally as L?/t2, where T is the standard deviation in peak width. It depends on the pressure ratio, P, through the following relationship (IO) :
li
=
Ho
9 (P4 - 1) (P2 - 1) g(P3 - 1)2
where C1 is the term for resistance to mass transfer in the stationary liquid and H , includes all plate height terms remaining constant throughout the column. This includes the resistance to mass transfer in the gas phase, longitudinal diffusion in the gas, and for packed columns, the eddy diffusion term to whatever extent it is present. Substitution of fi N for L in Equation l, using Equation 2, gives
The number of plates, N , is a quantity fixed by the degree of separability required, and by the characteristics of the pair being separated. It will remain constant for a pair except when changes are made in temperature or relative amounts of liquid loading. For these cases a more general criterion for the degree of separation such as the resolution or separation function (4), F , must be used. The latter is given by F = ( ~ I I - t 1 ) ~ / 1 6 ~ 2 .The quantity N is then written N
=
1
16F/R2 ( 6 ~ )
2
(4)
where N may vary but F is fixed a t or near a value of unity (4) depending on requirements. The difference in migra-
tion rates of the two zones is found in s ( l / R ) ,the increment in (1/R)values. For a given column operated a t constant temperature, the absolute minimum of time is N C ~ I R I I . This does not mean, however, that t can be made to go to zero when CI = 0. Part of the process of obtaining optimum values involves reducing the actual time as near as possible to NCl/RII (Le., reducing the first term in the brackets of Equation 3), and part involves reducing the value of NCl/RII itself. We will begin by assuming that NCL/RII is a fixed quantity, and attempt to reduce t accordingly. h definitive treatment of the above problem rests on the choice of the plate height term, H,. I n the case of capillary columns this may be taken as (11)
H , = B/pv
+ C, pv
(5)
For packed columns it is generally assumed that a n eddy diffusion term, A , independent of velocity, contributes to the plate height. Very careful experiments, however, indicate that this term is practically negligible under ordinary conditions ( I , 9, IC). Furthermore, there is no theoretical justification for such a term except a t very high velocities (6, 7‘). While the minimum-time velocities are somewhat larger than usual, it is doubtful if this term is important. This is even more so since the increased gaseous diffusion coefficient, accompanying reduced pressure, tends to eliminate the eddy diffusion effect (6). The term, H,, will consequently be assumed to have the same form (above) for packed as for capillary columns. The two will differ in the makeup of the B and C, (also Ct) terms, as will be discussed later, but for the moment this is of no concern. It should be noted that the validity of this treatment is based on the assumed capability of Equations 2 and 5 in approximately describing the performance of columns under a wide range of conditions. By allowing for the pressure drop through a column, as did James and Martin (IS), the following equation for column length, L, can be obtained.
measuring all other parameters in the equation for a given column. Equation 6, combined with the equation L = GN, gives
This equation relates pi, vi, and P when the column has exactly N plates. By substituting Equations 2 and 5 into 7 , vi is found to be vi =
(8)
where
p =
p i / p , and
p c = (9 $6qBN/S ~ , 2 ) 1 1 2
(9)
The significance of the critical pressure, p,, will be discussed later. When Equations 8 and 5 are substituted into the time expression, Equation 3, a rather complicated function of P results. This can be simplified by defining the dimensionless quantities s 1 =
3 (P4- 1) 4 p(p3 - 1)’
8 2
=
(11)
Inlet and Outlet Pressure. For a given column operating a t a constant temperature with a particular carrier gas only two variables, p i and P (appearing in p, SI,and SS),are subject to change. Since P = pi/p,, we may think of the inlet and outlet pressures as the variables. To optimize these variables, assume first that p i is fixed and that p , is free to vary. Let us postulate an infinitesimal decrease in p,. The terms p and s in Equation 11 will be unchanged. The pressure ratio P mill increase. Since
and 4 dSi -
where is the coefficient of viscosity of the carrier gas, and, for capillary columns, 6 = 16 and ro = tube radius, whereas for packed columns, 6 is the order of lo3 and is a function of the compactness of the packing, and ro may be taken as the mean particle radius, d,/2. A rigorous approach would relate ro to the hydraulic radius in packed columns, but 9 as defined in Equation 6 is more easily obtained than the hydraulic radius simply by
1)4
Equation 11, describing the analysis time, changes to
The column length, obtained as L becomes L
The resulting expression is
3dp-3 ( P -
tive, increases. Consequently, the decrease in p , leads to a decrease in analysis time, t. This can be verified by a direct differentiation, dt/dp,. This conclusion is independent of the original value of p,, pi, the plate height coefficients, and the column and carrier gas characteristics. Consequently it is always applicable, subject perhaps, only to a questionable application in the Knudsen flow regime below 10-3 atmosphere. The optimum value of p,, near zero, is thus to be obtained with a vacuum outlet. The assumption that p , = 0 (or is near zero) greatly simplifies many of the preceding equations, since P may be taken as approaching infinity. Equation 8, giving the inlet velocity, becomes
- S ( P - 1 1 3 - G ( P - i)2 P ( P 3 - 1) < 0 (13)
the inequality sign holding for the physically meaningful requirement, P >1, the decrease in p , will lead to an increase in Sz and a decrease in &. The second term in the brackets of Equation 11, PSIIS,obviously decreases. The numerator of the first term decreases also. The denominator of the first term, which must always be posi-
=
‘V(9 H,/S
+ 3 Ci/2)
=N
H,
(16)
The expression for time, Equation 15, is still a function of inlet pressure, so that this remains to be optimized. First, however, an exploration will be made of the dependence of t on pi over the whole range of p , values. The variable pi appears, of course, in the dimensionless form, p = pi/p,. At very large p values, t is very large and increasing in proportion to p . As p is decreased from this, t becomes smaller until p begins to approach unity. In this general region the optimum p or pi will be found. As p approaches unity more closely, t goes to infinity. For p values slightly less than unity t approaches minus infinity, and as p decreases to zero, t remains negative but approaches zero. It is apparent that Equation 15 is meaningless when p 5 1 or pi 5 p,. Since t approaches infinity as p i approaches p , from above, we may regard p , as the critical inlet pressure below which a separation cannot be effected under any circumstances nor in any amount of time. The physical situation may be explained as follors: As p i is reduced beyond the optimum, the mean flow velocity decreases. For sufficient reductions this increases the plate height due to the axial diffusion ( B ) term, and a greater length L is needed to obtain the required plate number, N . The added length diminishes the mean velocity even more, requiring more length for separation. At an inlet pressure of p , the velocity approaches zero, as shown by Equation 14, and the desired separation cannot be accomplished. A partial separation, VOL. 34, NO. 3, MARCH 1962
315
Equations 17 and 18 can be combined to give
pc ( dynes /ern*)
lo5'
IO
dlnt dln s
40.
pz
-2 - 1
This ratio is always negative. Consequently a maximum value of s is desired. For packed and capillary columns, C, is inversely proportional to DU1,and B is proprtional to DU1,where the latter quantity is the gaseous diffusion coefficient at unit pressure. By employing Equations 9 and 10, s is found to be
3.0-
.-e 0
L
.-:2.0. c
I
I
N (PPICKED COLUMN) IO0
I.o
102
IOL
IO=
I0'N
IO'
IO'
lo'
10
Io'
(CAPILLARY COLUMN)
Figure 1. Ratio of minimum time obtainable with atmospheric outlet and vacuum outlet as function of p. and number, N, of plates required for separation in typical columns
corresponding to a reduced N , can be effected a t a pressure incapable of complete separation. This effect is seen in p,, Equation 9, which is proportional to the square root of the number N of required plates. If the outlet pressure is held a t some value other than zero, the inlet pressure must exceed p , since, in Equation 11, Si > SZ. The optimum value of p i or p can be found by differentiating Equation 15. The optimum p must, as shown by this process, be obtained froin the cubic equation. p 2 = 3 + -
8s 3 P
A single p us. s plot serves to identify p with s specified. If t h e resistance to mass transfer is located entirely in the gas phase, s = 0 , p = d z o r p , = d g p , , the least value for optimum inlet pressure. If the resistance is located entirely in the liquid, pi approaches infinity. The increase of p with s is not, however, very sudden; when s = 19.5, p = 4.0, and for larger s values, p varies in proportion to the cube root of s. Equation 15 can be greatly simplified by substituting the s of Equation 17 into it. The resulting equation is
The practical conclusions of this section are best made in reference to Figure 1. The ratio of the minimum time obtained with normal atniospheric outlet t o that obtained with vacuum outlet is plotted against p, and against 316
ANALYTICAL CHEMISTRY
the number A' of plates required in typical packed and capillary columns. The typical packed column is assumed to have particles of radius T~ = 6.85 X 10-3 em. (-100 mesh), viscosity 7 = 10-4 poise, diffusion parameter B = 1.5 X 105 dynes per second, and flow parameter 6 = lo3. The typical capillary column has T~ = loe2 cm., 9 = 10-4 poise, and B = 2 X 105 dynes per second. For columns significantly different from these, a p , value can be calculated and the time ratio obtained in Figure 1 by reference to the p, scale. The parameter g = Cl/lOB C, expresses the ratio of the liquid mass transfer term t o the gas mass transfer term a t one atmosphere pressure (more precisely, 108 dynes per sq. em.). The plot shows that vacuum outlet conditions are most beneficial when the gas phase mass transfer term tends to be dominant, and for easy separations rcquiring few plates. For the intermediate case, g = 1.0, a 15yoor larger time reduction is found for capillary columns requiring fewer than l o 5 plates and packed columns requiring fen er than l o 3 plates. Carrier Gas. The most satisfactory carrier gas can be determined by Equation 18 providing optimum inlet a n d outlet pressures are employed. T h e first term of Equation 18, NCiIRII, does not depend on the nature of the carrier gas. The reduced pressure, p, varies with the properties of the carrier gas by virtue of its dependence on s, which is a function of gas viscosity and solute diffusion coefficient. Considering s as the independent variable,
\There U is constant. This equation shows that the ratio of the diffusion to the viscosity coefficient is the important carrier gas parameter, and should be maximal. This agrees with the conclusions of Ayers, Loyd, and DeFord (I). Equations 19 and 20 show the extent of the dependence of t on this ratio. For a typical p value of 3, an 8% change in the ratio Da1/7 leads to a 1% variation in analysis time. This can be an important factor since the above ratio changes by an order of magnitude throughout a series of the common carrier gases. To make these results more specific, experimental values (18) hare been used to calculate ratio for the following Dgl X propane in c.g.s. units a t 0' C.: Hz (4.4), NH3 (1.7), CH, (1.3), C,H, (0.87), COz (0.56), Kz0 (0.55). The lvay in which these gases would affect analysis time depends on the values of p and s, which are determined by the relative rate of mass transfer in liquid and gas phases. If the liquid mass transfer is very s l o ~ Cl , > Cap, or s >> 1, the choice of gas is irrelevant. If the gas phase mass transfer is very slow, s > JP1 and u2 >> ul. Under these conditions the ratio in Equation 21 becomes proportional to Tu12,’X1, a quantity depending only on the carrier gas and independent of the solute propertiei. This limiting expression is probably adequate for the comparison of carrier gasps. Collision diameters, g , and molecular weights, M , are rvadily available (22). The ratio m2/JI1,n-it11 n1 in angstroms, for a series of carrier gases is shown as follows: H2 (4.2), He (1.7), CH, (0.91), CSHB(0.65), S2 (0.49), Ne (0.391, CO, t0.35), N2O (0.34), Ar (0.29). This rlosely parallels the series of D,ljq ratios shown earlier. Since that series applied only to propane, the series above provides a more general grouping of carrier gases according to desirability. It is qeen that H2 is by far the most satisfactory carrier, H e is second best, and argon is relatively unsatisfactory. Amount of Liquid Loading. T h e foregoing conclusions concerning inlet pressure, outlet pressure, a n d carrier gas are general, a n d apply t o both packed and capillary columns. T h e difference between these columns,
insofar as the previous parameters are concerned, is expressed in t h e constant s, Equation 10. If one wishes t o optimize other variables, such as tube a n d particle size, temperature and amount of liquid phase, it is necessary to haye an expression for s since the latter is not longer a constant. The variation in s depends partly on the F a y in 11-hich the liquid phase distributes itself on the support. In the case of capillary columns the liquid presumably deposits as a uniform film. The distribution on column packing is more uncertain, although C I has recently been expressed in terms of the pore size distribution of the packing material ( 5 ) . There is not sufficient certainty on this point, however, to justify the treatment of packed columns at the present time. Consequently the following material deals only with capillary columns, and is concerned with the optimum thickness of the liquid coating on the capillary wall. Some qualitative conclusions on packed columns will be drawn from these results. If the partition coefficient, K , is fixed for each of two components, the thickness of the liquid film, d, is best expressed in terms of the dimensionless capacity factor, k , by means of the expression d = r, k/2K. The ratio k is the amount of solute in the liquid phase divided by the amount in the gas phase at equilibrium. The time is best minimized by formulating the
6(1
+ l z ) (1 + 6 k + 11 k 3
(29)
where
Since the p of Equation 18 is a function of s only, Equations 28 and 29 can be used to express analysis time in terms of k and the other column parameters. The minimum analysis time, found as that for which atjbk = 0, is obtained when k is a solution of the following equation: (1
(I
++ 2k )kk )4 (4 (1 + 7 k + 17 k 2 + 11 k 3 ) n + 21 k + 34 k2 f m)-
This equation yields but one physically meaningful solution. The solution is a function only of the dimensionless parameter G. The value of G in a typical case can be obtained from the folloR-ing values, all in c.g.s. units: ro = 2 x 10-2, ~ K / K= 1 0 - 1 ~ 4 ~=7 5 X 104, K = 102, D l = 10-6. This gives G = lo3. Under different circumstances, G might vary from 1 to lo5. The values of k obtained from Equation 31 are 1.712 (log G = 0), 1.407 (log G = l), 0.973 (log G = a), 0.531 (log G = 3), 0.281 (log G = 4), 0.150 (log G = 5 ) . A plot of k US. log G is shown in Figure 2. The optimum IC value is variable (depending on the parameter G), a result similar to that found for the optimum corresponding to maximum resolution. Unusually low k values, down to 0.150, are predicted to be optimal. It is interesting to look a t some of the factors contributing to low k values (high G values). Slow VOL. 34,
NO. 3, MARCH 1962
317
~
0‘
02.5
a50
0.75
1.25
1.00
1.50
1.75
2.00
2.25
250
k-
Figure 3.
Dependence of analysis time on k in vicinity of optimum values
liquid diffusion, reflected in D I , shifts the optimum k to lower values (this is expected since the diffusion distance, d , must be reduced to compensate). E a d y separated compounds, with high 6 K / K values, are in the same category. Small molecules, or larger ones separated a t high temperatures, generally partition with low K values, and thus have a decreased k. The actual thickness of the liquid layer, d = r 0 k / 2 K , should usually be increased in the latter cases, howver, since K varies more strongly than k. To determine the increase in analysis time as one moves away from the optimum value of k , a plot of t/kin. us. k for each of the previously mentioned values of G is given in Figure 3. The minima are rather sharp, especially for lo^ k values, a fact which shows the importance of applying the correct amount of liquid t o the tube. S o attempt has been made here to select optimum temperature, liquid phase, etc. This work would require the simultaneous optimization of several parameters, and, in view of the numerical nature of the solutions for optimum k , would probably be highly complicated It is, however, an important problem Fvhich should be investigated further. It is rather difficult to generalize the previous results such that the optimum k for packed columns can be obtained. Azs stated, this depends on the distribution of the liquid phase. It has long been clear, however, that a packed column has a higher relative capacity than a capillary column. This has been theoretically explained in this laboratory by analyzing Baker’s (2) data on pore size distribution, a quantity related t o the plate height (5). The 318
ANALYTICAL CHEMISTRY
high capacity result s h o w that a packed column would, a t optimum, be run a t larger k values than a capillary. These values may be in the vicinity of 2 or 3 as predicted by Ayers, Loyd, and DeFord ( I ) . CONCLUSIONS
The foregoing theory suggests specific parameters t o be used in obtaining minimum time analysis. It does not, however, compare the performance of packed and capillary columns or suggest methods for the further decrease of analysis time. These matters will be briefly considered below. While a precise comparison of packed and capillary columns cannot be made without better knowledge of the former, some of the factors involved can be estimated. Packed columns are superior because of their higher capacity and capillary columns excel because of the low resistance to flow. If the plate height terms in the two columns were identical while the optimum capacity factor of the packed column is assumed to be five times as large as that for the capillary (say 2.5 and 0.5 for k ) , then the NCI/RII term of Equation 18, proportional to (1 k ) 3 / k 2 , would favor the packed column by a factor of 2.0, since 4.6 times more plates are needed for the capillary column (the capacity factor does not favor the packed column more than this because of the R in the denominator of Equation 18; under some circumstances it would actually favor the capillary column). The p 2 / ( p z - 3) term of Equation 18 favors the capillary column because of its larger s and p, and smaller p , . The s and p , terms differ
+
by a factor of approximately 5 between the two columns due to differences in the flow resistance term 6 and the number of plates hr. The rate of change of analysis time with respect to p is given by Equation 19. At high p values the factor of 5 would influence analysis time very little, while for small p values the analysis time would be reduced significantly for capillary columns. Thus one case may favor packed columns and another favor capillary columns. The direct dependence of analysis time on C I , Equation 18, suggests that more efficient columns with a reduced C I are needed. This applies, of course, to both packed and capillary columns. If Ci were reduced drastically (but, of course, not to zero), the term p 2 / ( p 2 - 3) would become highly important. Not many alternatives exist for reducing this term. One possibility would involve the use of pumps with very low volume a t intervals along the column. If these could be constructed so that no mixing m-as caused, the analysis time, t , would equal the number of stages, n, multiplied by the time required in each stage, t,. Since only 12’, = Ar/n plates would have to be generated in each stage, the p , value for each stage could be made arbitrarily small. Thus the term ~ , ~ / ( p , *- 3) could be reduced to unity. The analysis time would then equal
the minimum possible time with a fixed value of Cl. NOMENCLATURE = =
= =
= =
=
Coefficient in axial diffusion term of plate height equation Gas phase mass transfer term in plate height equation Liquid phase mass transfer term in plate height equation Thickness of liquid layer in capillary tube Mean particle diameter Gas phase diffusion coefficient at unit pressure Separation function, F = ( t r r t ~ ) ~ / T~ 16
= = = = = = =
= = = = = =
=
Dimensionless column parameter, Equation 30 Local plate height Plate height terms remaining constant throughout column ilpparent or measured plate height Capacity factor Partition coefficient Column length Molecular weight of carrier gas Molecular weight of solute Number of required plates Pressure at a given point in the column Inlet pressure Outlet pressure Critical pressure, minimum inlet pressure for separation, Equstion 9
P r.
=
R RII
=
s
=
=
=
SI, S P = t
=
T G
=
v
=
=
v,
=
7
=
p
u,
= =
m
=
T
=
Compression ratio, p i / p , Capillary tube radius or mean particle radius Ratio of zone to gas velocity R value of component I1 or last component Important dimensionless parameter, Equation 10 Dimensionless parameters, Equation 10 Analysis time Temperature Constant in Equation 20 Gas flow velocity a t a given point in the column Flow velocity at column inlet Viscosity coefficient for carrier Reduced inlet pressure, p i / p , Collision diameter of carrier gas molecules Collision diameter of solute molecules Standard deviation in time of peak width
@
=
n
=
Column constant characterizing the resistance to flow Integral expressions, Equation 21 LITERATURE CITED
(1) Ayers, B. O., Loyd, R. J., DeFord, D. D., ANAL.CHEM.33,986 (1961). (2) Baker, W. J., Second Symposium on
Gas Chromatography, East Lansing, Mich., 1959. (3) Desty, D. H., Goldup, A., “Gas Chromatography, 1960,” p. 162, R. P. W. Scott,. ed.,. Butterworths, Washington. (4)Giddings, J. C., ANAL.CHEM.32, 1707
(1960). ( 5 j lbid., 33, 962 (1961). (6) Giddings, J. C., J . Chromatog. 5 , 46, 61 (1961). (7) Giddings, J. C., Nature 184, 357 (1959); 187, 1023 (1960). (8) Ibid., 191, 1291 (1961). (9) Giddings, J. C., Robison, R. A., un-
published data. J. C., Seager, S. L., Stucki, L. R., Stevart, G. H., ANAL.
(10) Giddings,
CHEM.31, 1738 (1959); 32, 867 (1960). (11) Golay, M. J. E., “Gas Chromatography,” p. 36, D. H. Desty, ed., Butterworths, London, 1958. (12) Hirschfelder, J. O., Curtiss, C. F., Bird, R. B., “Molecular Theory of Gases and Liquids,”Chap. 8, Wiley, New
York, 1954.
(13) James, A. T., Martin, A. J. P., Biochem. J . 50, 679 (1952). (14) Kieselbach, R., ANAL.CHEW33, 806 (1961). (15) Knox, J. H., J . Chem. SOC.1961,433. (16) Purnell, J. H., Quinn, C. P., “GaS Chromatography, 1960,” R. P. W.
Scott, ed., Butterworths, Washington,
n. 184. r .
(17) .Scott, R. P. W., Hazeldean, G. S. F., Ibzd., p. 144. (18) Trautz, M., Muller, W.,Ann,. Physik 22, 353 (1938).
RECEIVEDfor review July 31, 1961’ Accepted December 26, 1961. Work supported by the U. S. Atomic Energy Commission under Contract AT-( 11-1)748.
Determination of Sulfur in Organic Compounds by Gas Chromatography DONALD R. BEUERMAN’ and CLIFTON E. MELOAN Department of Chemistry, Kansas State University, Manhatfan, Kan.
b Sulfur in organic compounds can be determined by combusting the compounds at 850” C. in a stream of oxygen using a platinum catalyst. The water of combustion i s removed with calcium sulfate, and the sulfur dioxide and carbon dioxide are trapped at liquid nitrogen temperatures along with some oxygen. The sulfur dioxide i s separated from the carbon dioxide and oxygen by a dinonylphthalate column using helium as a carrier gas. The complete analysis requires about 20 minutes, and a relative error of less than 1% can be expected. The method has been satisfactory with sulfoxides, sulfones, thiones, sulfides, disulfides, and thioethers, but sulfates were not converted to sulfur dioxide quantitatively under these conditions. Compounds containing fluorine, chlorine, nitrogen, and oxygen, in addition to carbon and hydrogen, were tested and found not to interfere.
T
HERE ARE, at present, two predominant methods for determining the sulfur content of organic compounds-the Carius and Pregl methods with their many variations (10). The Carius method consists of combusting the organic compound in a sealed tube in the presence of nitric acid and an
alkali salt other than a sulfate. The sulfate formed is then determined either gravimetrically or titrimetrically. The Pregl method consists of combusting the compound in a combustion tube at 650” C. using platinum as a catalyst. The resulting oxides of sulfur are then converted into sulfuric acid by their reaction with hydrogen peroxide. If nitrogen or halogens are present, the sulfate must be precipitated as barium sulfate and determined by gravimetric techniques. Both methods require several hours for completion. Other methods have been described in the literature, but most of them are adapted to specific types of analysis. Huffman (6) adapted Pregl’s method t o the simultaneous determination of carbon, hydrogen, and sulfur. H e trapped the sulfur on silver and subjected t h e silver sulfate formed to electrolysis. The sulfur in organic compounds has been hydrogenated to hydrogen sulfide and determined by various techniques (9). Sulfur has been determined manometrically by Hoagland (4). The Schoniger oxygen flask (7) can be used for the combustion of sulfur compound. The above methods either require considerable time for completion or else are limited in application by interfering substances. A fast, widely applicable, reasonably accurate method for sulfur t h a t could be used over a
wide range of sample sizes, particularly small samples, was desired. The possibility of gas-liquid partition chromatography was examined because it should provide a means of separating interfering materials. Four questions had to be answered before this approach would be feasible. Could sulfur be combusted to SO, without forming SO3? Would mater react with the SO,; and if so, could this be eliminated? Could the SO, be efTectively trapped for a concentrating step? Could SOz be separated from the other possible gases in a reasonable time? Duswalt and Brandt (3) and Sundberg and Maresh (11) devcloped a method for determining carbon and hydrogen in organic compounds by utilizing the principles of gas chromatography, enabling a carbon-hydrogen determination to be completed in 20 minutes. Scott et al. ( 8 ) modified this technique for the determination of oxygen and nitrogen. hIaresh (6) adapted i t to the simultaneous determination of carbon, hydrogen, and nitrogen. This paper describes development of techniques using gas chromatography for the determination of sulfur in organic compounds.
1
Present address, Monsanto Chemical
Co., St. Louis, Mo.
VOL. 34, NO. 3, MARCH 1962
319
