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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
Prediction of Gas Chromatographic Retention Behavior with Mixed Liquid Phases Robert L. Pecsok' and James Apffel Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822
It is shown that the behavior of the Kovats' Retention Index on a stationary phase composed of a binary mixture of pure Stationary phases can be predicted by the equation
I ( 4 ) = M ( 4 ) log [+*lO(IA+NA)/MA+ c$slO('S+NS)/Ms]-N
0
where M and N are column-dependent constants which can be calculated from the partition coefficients of the n-paraffins. 4 A and 4 refer to the volume fraction of pure phases A and S, respectively. Analytical criteria for optimization of appiications of this type are given.
I n gas-liquid chromatography (GLC), the success of a separation of a complex mixture of solutes is dependent upon the solubility characteristics of the stationary liquid phase. Often the task of selecting the appropriate liquid phase can be monumental, especially since there are over 200 stationary phases currently in use. It has been shown (1) that nearly all separations can be accomplished on only 12 stationary phases, carefully chosen for a wide range of characteristics. The use of stationary phases composed of mixtures, selected from the basic 12, could reduce this figure even further. Purnell and co-workers (2-6) have developed both theoretical and analytical aspects of such systems, based on the infinite dilution partition coefficient, KR. In their "micropartition theory", they have shown that the partition coefficient for a multicomponent system follows the simple equation,
KR = @AKR(A)'+ @sKR(s)'
(1)
where 4 represents the volume fraction. KR(A1' and K R ( s I 0 represent the partition coefficient on pure A and S, respectively. They have also developed the analytical criteria for selecting the optimum mixture of two phases for any given system ( 4 , 5 ) . The equations developed, however, are all in terms of partition coefficients, or more recently (7), in terms of relative retention and capacity factor data. In recent years, most retention data have been reported as relative retention times and given as Kovats' Retention Indices. The retention index I for component x is related to the partition coefficient by the equation,
where z is an n-paraffin with is a n equation of the form,
2
carbon atoms. In general, this
I =MlOg K R -N (3) where M and N are column dependent constants for any given phase. It would be of great analytical utility to be able to use the retention index in place of the partition coefficient to describe the behavior of mixed solvent systems, since the bulk of data reported in the literature are given in terms of retention indices. THEORY Rearranging Equation 3 in terms of
K R
yields;
KR =
(4) Substituting this equation into Equation 1 gives a relation between the retention index and the volume fraction of the individual stationary phases, A and S. I ( $ ) = M ( 4 ) log [4A10('A+NA)/MA 4S10(1s+Ns)/Ms]- N (m5) where subscripts A and S refer to pure solvents A and S, respectively. In a binary mixture, @A = (1 - &). M and N are given by the following equations; M = 1oo/log [ K R ( z + l ) O / K R ( r ) o l (6) 10(I+M/M
+
N = 100 log K R ( z ) ' / l o g [ K R ( z + l ) " / K R ( z ) ' I - loot ( 7 ) The first factor, M ( 4 ) , can be determined by substituting Equation 1 into Equation 6;
M ( @ )= 100 log
[(@AKRk+l),A'
-k @ S K R ( z + l i , S o ) / ( ~ A K R z , A '
+ @SKRz,So)l (8)
N ( @ can ) be defined in a similar way, yielding,
N(@)=
100 log (+AKR~,A' +
log
[
$AKR(z+1),A' 4AKRz,Ao
~@R~,S')
+ &8R(z+l),S0 + @sKRz,sO
1
-
1002
(9)
The behavior of M ( 4 )and N ( 4 )for the squalane/l-dodecanol system at 56 "C can be seen in Figures 1 and 2 . Although it is these factors, M and N, which give the retention index its column independence in most applications, in the case of mixed stationary phases, these same factors introduce a column dependency into Equation 5 . This presents an inherent impossibility of describing the behavior of mixed phase systems solely in terms of retention indices. This failure is due to the definition of the retention index, since this definition does not require the calculation of partition coefficients which are needed in M ( @ )and N ( 4 ) . For analytical purposes, however, Equation 5 is an improvement over Purnell's original equation. Through the use of Equation 5 , the behavior of a particular solute in a mixed solvent system can be predicted by knowing the retention index of that solute on each of the pure phases, and the partition coefficient of two n-paraffins which bracket the solute with respect to retention time, rather than the partition coefficients of all solutes and all the n-paraffins spanning the range of retention times involved. This simplifies the analytical problem considerably, since, in practice, the partition coefficients would have to be measured for each system, for each phase. Using Equation 5 , only the n-paraffins which span the retention times in the system must be measured. In fact, for practical purposes only two n-paraffins need be measured. The retention indices of all the other solutes can be obtained from the literature. Although the prediction of the retention indices of solutes in mixed stationary phase systems is of importance, a greater
0003-2700/79/0351-0594$01 .OO/O 0 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY
1979
595
400.--r. 3001
Figure 1. T h e 4-dependence of M in mixtures of (1) 1-dodecanol/ squalane at 56 "C, (2)di-n-nonyl phthalate/squalane at 100 "C,(3) diethyl maleate/quinoline at 35 "C. 4 refers to the first named
compound 200,
I
I
I
where the subscripts 1 and 2 again refer to different solutes 1 and 2, and the factor M is that described by Equation 6. Substituting into this the @-dependencyof I yields
I L.,
OOO
02
04
c
cb
c8
IS
Figure 2. T h e @dependenceof N in mixtures of (1) 1-dodecanol/ squalane at 56 "C, (2)di-n-nonyl phthalate/squalane at 100 OC, (3) diethyl maleate/quinoiine at 35 OC analytical problem involves finding the optimum mixture of two phases t o use for a given system. Purnell and Laub ( 4 , 5 ) have developed a method for doing this based.on the relative retention, cy, which is defined by the equation; CY
= KR~/KRz
(10)
where 1 and 2 refer to two solutes on the same stationary phase. By substituting Equation 1 into this equation, Purnell and Laub obtained a +-dependent equation for a. a is first plotted against for each pair of solutes in the system, including the n-paraffins, yielding a so-called "window diagram". In these plots, cy is always taken to be greater than or equal to 1; that is, if a is less than 1, the reciprocal of a is plotted. In production of window diagrams, several approximations can be made, such as plotting a with straight lines, and disregarding any solute pair with (Y greater than 1.4 for all @, since such separations are trivial. The diagrams are then analyzed. The "window" is a triangle formed by the intersection of the a's for two pairs of solutes. The vertex of the largest triangle containing no smaller triangle is a t the point where the optimum separation occurs. At this point, all relative retentions are a t least as large as that of the vertex of the largest triangle. For a more complete discussion of this method as developed by Purnell et al. in terms of partition coefficients, see references 2 through 8. By analogy to Purnell's method, somewhat similar to the preceding development of Equation 5, an equation for cy in terms of Z can be formulated. In general, for a pure stationary phase, a is described by
+
Figure 3. Dependence of Ion @M, in mixtures of 1dodecanoVsquahne for solutes listed in Table I at 56 "C. Plotted points represent experimental data. Solid lines are calculated from Equation 5. Dashed lines are calculated from Equation 13
where ZAlorefers to the retention index of solute 1 on pure phase A, and MA refers to the factor described by Equation 6 for pure phase A. N A is the factor described by Equation 7 for pure phase A. In Equation 12, it should be noted that the @-dependentM factor has cancelled out simplifying the calculation of cy. Hence, the window diagrams can be produced from the retention indices of the solutes and the partition coefficients of the n-paraffins over the range of retention times.
EXPERIMENTAL All chromatographs were run on a Bendix Series 2300 Lab Chromatograph. Eighth-inch 0.d. copper tubing was used for the di-n-nonyl phthalate/squalene system at 100 "C and eighth-inch o.d. stainless steel tubing was used for the 1-dodecanol/squalane system at 56 "C. The stationary phases; squalane, di-n-nonyl phthalate (DNNP), and 1-dodecanol were of reagent grade and used as received. The densities of these phases were obtained from data collected by Laub ( 4 ) . The packings for the DNNP/squalane system were prepared by the rotary evaporator method (IO). The packings for the squalane/dodecanol system were prepared by a fluidized drying method described by Parcher and Urone (9). All packings were made approximately 3% loadings on Gas Chrom Q 100/120 mesh support. The packings which were mixtures of two pure phases were prepared by mechanically mixing packings of the pure phases rather than preparing packings from mixed solvents. Purnell and co-workers have shown this to be justified ( 4 , 5 ) .
RESULTS AND DISCUSSION The Kovats' retention indices for a variety of solutes were determined experimentally for the systems, squalane/ 1dodecanol and di-n-nonyl phthalate/squalane, for five volume fractions each, and are given in Table I. The values follow those of the function predicted by Equation 5 within the experimental limits of error, which generally were of the order of 5 to 6 index units. Graphs of the retention index vs. the Correvolume fraction are given in Figures 3 and 4. spondingly, the graphs of (Y vs. @ can be predicted with Equation 12 and correspond closely to those predicted by Purnell and Laub ( 4 , 5 ) . The values predicted for optimum separation by this method are the same values as those computed by Purnell from partition coefficients (2, 4 ) . Once the applicability of Equations 5 and 12 was shown, a third system, diethyl maleate/quinoline at 35 "C, was
596
ANALYTICAL CHEMISTRY. VOL. 51, NO. 6, MAY 1979
9 ri
W 0 Q,
'9
0
9 ri
0 0
a?
u?
0
a ri Q,
7
0
ri
0
c-
8
m c-
8
ririririri
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
597
Table 11. Retention Data for Various Solutes on Diethyl Maleate/Quinoline Mixed Phases at 35 ‘C Calculated from Data in Reference 3 KR I ‘@DiethylMaleate
0.0
0.25
0.50
0.75
1.0
0.0
0.25
0.50
0.75
1.0
solute 1. pentane 2. hexane 3. heptane 4. 3-methyl pentane 5. cyclopentane 6. furan 7. cyclohexane
48.1 131 352 116 116 302 294
50.3 141 39 1 120 129 28 3 321
52.6 152
54.9 162 469 127 154 246 377
57.1 172 508 131 167 227 404
500 600 7 00 588 588 685 682
584 591 668 681
581 594 653 680
578 595 639 679
575 597 626 679
430 124 141 265 349
so,
I
,
I
I
I
I
?
40 -
30-
6001
00
02
06
04
I
08
IO
+DNNs
100 IiI,
Data plotted as in Figure 3 for mixtures of di-n-nonyl phthalate/squalane mixtures at 100 OC
Flgure 4.
7001
,
,
,
,
,
,
,
,
00
02
04 +DIflH”L
06
08
IO
MALEATE
Figure 5. Data plotted as in Figure 3 for mixtures of diethyl maleatdquinoline at 35 OC (3)
computed from data given by Purnell and Laub (2). The data as computed are given in Table 11, and the pertinent graphs are given in Figure 5. Comparison of the plots of I vs. 4 for these three systems indicates that the error involved in approximating the behavior of Z by a simple weighted sum similar to the case for K R in Equation 1, such as
I = 4*I*O
300
Figure 6. Maximum error resulting from the use of Equation 13 rather than Equation 5, in retention index units
,
- - - - _ __ _
500
200
error, the differences between the retention indices can be as large as 125 index units. Two important factors should be noted about this approximation, however. First, the average error for any particular solute would be smaller than that shown, since this is a maximum error and, even if this is relatively large, the logarithmic function of Equation 5 may closely approximate Equation 13 in other values of 4. Second, for this approximation to be of full use, all the solutes in the system must satisfy whatever error limit is desired. I t is, unfortunately, not of much use in calculating a , since when using the actual equation for a , Equation 12, the factor M(+) cancels, whereas in using the approximation, Equation 13, this factor does not cancel out. Purnell and co-workers have stated ( 4 ) that it is not feasible to substitute I for KRin describing systems of this type because of the large amount of arithmetic revision required. However, granted Equation 5 is mathematically more complex than but also the relative Equation 1, calculations of not only I(c#J), retention “window diagrams” are amenable to computer applications similar to those described by Purnell and coworkers in a recent paper (6). It should be noted, that although the present data have been collected for binary systems, the method can easily be extended to larger systems. In general, Equation 5 then becomes:
+ &Is”
(13) may be justified. When the difference of the retention indices of a particular solute on the separate pure phases is small, this seems to be a fair approximation. This is as one might expect since in the limit as I A goes to Is, Equation 5 becomes Equation 13. For each of the systems studied, the maximum error in this approximation, that is, the largest difference of the values of Equation 5 and Equation 13 over the range of 4, was determined and plotted against the absolute value of the (IAo - I s D ) factor of the pure phases. This is given in Figure 6. For an error of 5 index units, which is of the order of experimental
Similar sums can be applied to equations for M(+),N($),and a ( @ ) although, , obviously, for more than two phases the simplicity of the use of the “window diagrams” is lost. The analgous method based on partition coefficients has been developed by Laub and Purnell ( 4 ) . Although not an equation solely in terms of the Kovats’ Retention Index, Equation 5 is felt to be a definite improvement over Equation 1 for analytical purposes. One main advantage is that most of the data needed can be obtained
598
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
from the literature. Further, this equation requires no further adoption of a new system of reporting data in terms of partition coefficients which would be necessary for Purnell's system.
LITERATURE CITED (1) S.T. Preston, J . Chromatogr. Sci., 11, 201 (1973). (2) J. H. Purnell and J. M. Vargas Andrade, J. Am. Chem. Soc., 97,3585 (1975). (3) 6.J. 'Laub and J. H. Purnell, J . Am. m e m . soc., 98, 30 (1976). (4) R . J. Laub and J. H. Purnell, Anal. Chem., 48, 799, 1720 (1976). (5) R . J. Laub and J. H. Purnell, J. Chromatogr., 112, 71 (1975).
(6) R. J. Laub, J. H. Purnell, and P. S. Williams, J . Chromatogr., 134,249 (1977). (7) R. J. Laub, J. H. Purnell, D. M. Summers, and P. S. Williams, J. Chromatog., 155, 1 (1978). (8) R. J. Laub and R. L. Pecsok, "Physiochemlcal Appllcatlons of Gas Chromatography", John Wlley and Sons, New York, 1978. (9) J. F. Parcher and P. Urone, J . Gas Chromatogr., June 1984. (10) W. R. Supina, "The Packed Column in Gas Chromatography", Supelco Inc., Bellefonte, Pa., 1974. RECEIVED
for review September 11, 1978. Accepted January
25, 1979.
SimuIta neous Determination of 4- ChI oro-2- methy Iphenoxyacetic Acid and Its Metabolites in Soil by Gas Chromatography Mohammad A. Sattar and Jaakko Paasivirta" Department of Chemistry, University of Jyvaskyla, SF 40 100 Jyvaskyla 10, Finland
Analysis of MCPA (4-chloro-2-methylphenoxyacetlc acid) and two of Its main metabolites 4 chloro-o-cresol and J-chloro3-methylcatechol have been studied by gas chromatography of their pentafluorobenzyl derlvatives simultaneously in four different soil materials. After derivatlration of the residue extract, three dlfferent clean-up procedures were tried. The best recoveries of compounds from soils were obtained when the extraction was performed by shaking with ether-acetone-heptane-hexane (2:l:l:l) from acidified soil and when the cleanup was done by TLC.
Phenoxyalkanoic acid herbicides prevent direct gas chromatographic determination because of the high polarity or low volatility of the compounds and must be converted to their more volatile derivatives. The sensitivity of the EC detector toward alkyl esters of MCPA (4-chloro-2-methylphenoxy acetic acid (I)), MCPB (4-chloro-2-methylphenoxy butyric acid) etc. is very poor. The methyl ester of MCPA was 100 times less sensitive to electron affinity detection than 2,4-D methyl ester ( I ) . Chau and Terry ( 2 ) reported the formation of pentafluorobenzyl derivatives of 10 herbicidal acids including MCPA (I). They found that 5 h was an optimum reaction time at room temperature with pentafluorobenzyl bromide in the presence of potassium carbonate solution. Agemian and Chau (3) studied the residue analysis of MCPA (I) and MCPB from water samples by making the pentafluorobenzyl derivatives. Bromination ( 4 ) , nitrification (5), and esterification with halogenated alcohol ( 1 ) have also been used to study the residue analysis of MCPA (I) and MCPB. Recently pentafluorobenzyl derivatives of phenols and carboxylic acids (6, 7 ) were prepared for detection by ECD a t very low levels. Pentafluorobenzyl bromide has also been used for the analytical determination of organophosphorus pesticides (8). MCPA (I) is one of the most effective hormone herbicides. 4-Chloro-o-cresol (11),5-chloro-3-methyl catechol (111) and cis,cis-4-chloro-a-methyl muconic acid (IV) were first identified as metabolites of MCPA (I) by Gaunt and Evans (9). Several other investigators have also studied the microbial degradation of MCPA (10-14). The technical product of 0003-2700/79/0351-0598$01 .OO/O
Table I. Physical and Chemical Characteristics of the Finnish (i and ii, Kemira Co.)and Bangladesh (iii and iv) Soils sandy
clay
sandy
loam,
loam, iii
clay,
soil types
clay, i
moisture, % field capacity, % ignition loss, %
13.9 28.1 33.2
14.8
2.0 12.1 2.2 6.6 0.6 1.1 5.1 20.7 58.5 15.7
2.3 15.5 2.0 7.5 0.7 1.2
PH
organic carbon, % organicmatter, % coarse sand, % fine sand, % silt, % clay, %
4.6 10.3 17.8 19.7 31.1 30.1 18.7
ii
34.0 36.0 4.8 12.2 21.1 10.8 24.2 25.3 39.5
iv
4.9 32.7 18.9
42.4
MCPA in Finland contains approximately 4% of I1 as a n impurity (15). MCPA (I) is a widely used pesticide in Finland especially against herbs in the grain field (16). This is due to the confirmation of MCPA (I) as a safe compound (17, 18). Analysis of MCPA (I) from soil material has not been described in detail in the literature. We met with difficulties in efforts t o apply bromination (19)to detect the substance by ECD. Our determinations of MCPA methyl ester using FID were also not readily applicable to soil samples because of numerous interfering compounds. Thus we developed a n improved analysis method using the pentafluorobenzyl derivative of MCPA (20). In the present study, derivatization with pentafluorobenzyl bromide and clean-up processes are extended to the residue model compounds of I1 and I11 with four typical soils. The formulas of I-IV and the pentafluorobenzyl derivatives VI-VI1 are illustrated in Figure 1. The structures of the derivatives VI-VI11 were also verified by us with NMR spectra.
EXPERIMENTAL Two Finnish (Kemira Co., Helsinki) and two typical Bangladesh (Soil Science Department, Agriculture University, Bangladesh) soils were used for the experiment. The soils were dried at room temperature, ground to pass through a 2-mm sieve and stored in plastic containers. The physical and chemical characteristics 0 1979 American Chemical Society
