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Anal. Chem. 1980, 52, 1402-1407
initially obtain a null. The chromatogram shows four to five distinct negative-going peaks and one positive-going one. The interesting point is that except for the peak a t 8 min, none of the other peaks match up with those in the UV absorption detector. This speaks for the unique selectivity available in this detection scheme. We also found that fresh urine, as opposed to a 24-h void, contains many more features, just as one would predict from degradation and mutarotation properties. T h e same urine sample diluted 100-fold did not give any disturbance in the laser beam. As expected, a typical glucose peak was found in that case. Judging from the sizes and directions of the peaks in Figure 3, and from expected extents of retention and typical concentration in urine (8), we tentatively conclude that the peaks correspond to cystine, histidine, phenylalanine, and albumin. A more detailed study of components in urine and potential applications to clinical diagnosis will be presented in a future article (9). In conclusion, we have a working HPLC detector based on optical activity that is capable of determining submicrogram quantities of material. A similar design can be used as a micropolarimeter. Further improvements are foreseen but,
even a t this stage, the applications to clinical, environmental, and energy-related ( I O ) situations seem to be quite broad.
LITERATURE CITED (1) Griffiths, P. R. "Transform Techniques in Chemistry", Griffihs, P. R., Ed.; Plenum: New York, 1978; p 147. (2) Yeung. E. S. "Lasers and Chemical Analysis", Hieftje, G. M., Lytle, F. E., Travis, J. C., Eds.; Humana: Clifton, N.J., 1980; in press. (3) Lowman, D. W. J. Am. SOC. Sugar Beet Technol. 1979. 20. 233. (4) Rossi, P. Analyst (London) 1975, 100, 2 5 . (5) Baumann, A. Fresenius' Z . Anal. Chem. 1977, 284, 31. (6) Moeller, C. E.; Grieser, D. R. Appl. Opt. 1969, 8 , 206. ( 7 ) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1979, 57, 728. (8) Davidsohn, 1.; Henry, J. 6. "Clinical Diagnosis"; Saunders: Philadelphia. Pa., 1969; p 51. (9) Kuo, J. C.; Woodruff, S. D.; Yeung, E. S. To be published. (10) Anders, D. E.; Robinson, W. E. Geochim. Cosmochim. Acta 1977. 35, 661.
RECEIVED for review April 8, 1980. Accepted May 27, 1980. Ames Laboratory is operated for the U S . Department of Energy by Iowa State University under Contract No. W7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Science, WPAS-KC-03-02-03,
Utility of Mixed Packings in Gas-Liquid Chromatography C.-F. Chien and
R. J.
Laub'
Depaflment of Chemistry, The Ohio State University, Columbus, Ohio 432 10
M. M. Kopecni Chemical Dynamics Laboratory, Boris Kidric Institute of Nuclear Sciences-
The linearity of plots of solute iiquid/gas partition coefficients KR(,,,)against volume fraction q5 of binary stationary phases is examined in terms of the relation
for squalane (B) and dlnonyi phthalate (C) solvents over the temperature range 50-70 OC, where K&,) ( 1 = B or C ) represents a partition coefficient with a pure phase. Retentions with mechanical mixtures of pure-phase packings exhibit iinearky on average to better than f2%, in good agreement with a calculated minimum relative standard deviation of f1.41%. I n contrast, partition coefficients with intimately-blended solvents exhibit positive deviations from linearity of ca. 10 YO. The liquid loading of pure-phase packings employed in fabrication of mixed-bed columns appears to have little if any effect upon the linearity of the data; thus, 5 % w/w packings give values of KR(,, at 4 = 0.50 which are virtually identical to those comprising 12% w/w which, in addition, are mutually consistent with straight lines drawn between K&Bl and & ,). Le., in accordance with the above equation.
In a recent series of papers (1-13), Laub, Purnell, and their colleagues have described a novel approach to the quantitative use of multicomponent solvents in gas-liquid chromatography (GLC), which is based upon a model of solutions wherein the components of the solution are said to be mutually immiscible (14-19) and which predicts t h a t 0003-2700/80/0352-1402$01.00/0
VINCA, P.O. Box 522, 7 100 I-Beograd, Yugoslavia
Le., that retentions with mixed phases can be calculated from those pertaining to each of the pure phases, where I(0R is a solute partition coefficient with pure phase B or C or with some combination of the two (M = B + C) and where 4;(i = B or C) is a volume fraction. In terms of analytical applications, Equation 1 has proved to be useful from the standpoint of quantitative assessment of relative retentions ( a values), viz., KR(M)2
cY2/1
= --
[@BKoR(B)
+
~BKoR(C)lsolute 2
KR(M)~ [ ~ B K ~+( B ~cK!$(c)lsoiute ) 1
( 2)
In addition, plots of (Y against 4c, called window diagrams, permit the prediction of optimum B C solvent compositions for resolution of a given mixture which, further, provides access to the a value of the most difficult solute pair. The number of plates, hence the column length required to effect the separation, may then be calculated in turn (20). The window diagram strategy has to date been applied with complete success to a wide variety of solutes (including samples of initially unknown content and complexity) with a remarkable range of stationary phases; it has recently been reviewed by Laub (10) and by Laub and Wellington (19). An important consequence of Equation 1 (or its equivalents in terms of specific retention volumes, capacity factors, or retention indices with solvent-component weight fractions) is that separations with mechanically-mixed packings, that is, (support + B) plus (support + C), are identical to those
+
1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52,
NO. 9,
AUGUST 1980
1403
__-__
Table I . Solute Partition Coefficients with Pure Squalane (B) and with Pure Dinonyl Phthalate (C) Solvents at 50-70 "C m ( c )
KOR(B)
solute n-hexane n-heptane n-octane benzene toluene cyclohexane methylcyclohexane tetrahydrofuran dichloromethane chloroform carbon tetrachloride thiophene a
50 "C 148.2 149.6O 401.4 404.4O 1076 214.2 214.3O 621.7 622.5O 276.3 519.4 129.8 44.19
6 0 "C 108.8
109.7O 278.4 279.7O 706.9 156.2 158.2O 427.8 429.6O 198.5
361.2 96.7 8 33.92
118.6
88.10
117.8O 229.5 229.6O 218.0 218.4'
88.60a 167.3 168.6O
159.0 l60.Oa
70 "C
50 "C
60 "C
70 " C
82.62 81.33O 198.3 198.7O 479.9
99.30 97.24O 254.4 256.6a 683.7 307.4 307.0a 847.5 840.9= 195.1 353.1 217.9 127.0 302.9 300.1a 243.7 243.5a 375.7 377.4a
73.66 72.69O 182.1 181.6O 460.4 219.3 217.Ba 577.7 572.6O 142.8 245.2 157.3 93.17 215.8 213.4a 182.1 176.8O 272.7 268.9O
56.14 55.47a 132.2 132.0a 313.6 162.2 161.0a 40l.2 399.2a 105.5 178.4 116.7 68.89 152.6 152.1a 133.6 130.2a 195.6 196.7O
118.8
117.6O 303.7 304.6a 146.8 259.9 73.30 27.38 68.44 67.42O 124.5 124.9O 120.2 119.Ba
Reference 34.
obtained with intimately-blended solvents, which has been substantiated in all cases thus far examined to within the experimental errors of the analytical GC instrumentation employed (see, e.g., Ref. 10, 19). However, recent studies by Ashworth a n d Hooker @ I ) , Parcher and Westlake (22), Christian, Tucker, and Mitra (23,241, Acree and Bertrand (2.51, Tiley (26),and others have shown that the relation is in the several instances examined with intimately-blended solvents accurate to no better than 20%. Harbison, Laub, Martire, Purnell, and Williams (27) showed in fact that plots of K R ( M ) against & were indeed curved for a number of solutes with squalane a n d dinonyl phthalate phases a t 30 "C; deviations from linearity were in all cases positive and exhibited maxima of ca. 9%. [In contrast, similar studies (28,29) with blended n-alkane solvents demonstrated that retentions of n-alkane solutes forecast from Equation 1 were indistinguishable from those predicted via conventional solution theories (30-33).] T h e success of t h e window diagram procedure with intimately-blended solvents was, however, rationalized by Harbison and co-workers (27) on the basis that whatever curvature is extant with solvents B and C is, curiously, exhibited approximately to the same extent by all solutes irrespective of their chemical type. Thus, any deviations from Equation 1 are in effect divided out in Equation 2 and separations with intimately-blended phases are hence virtually indistinguishable from those found with mechanically-mixed packings. I n any event, as argued by Laub and Purnell(3), by Laub and Wellington (19),and later by Tiley (26), retentions with mixed-bed sorbents must conform to Equation 1, since the phases are thereby separated physically within the column. This feature is a major attraction of the window diagram procedure since mechanical mixtures of pure-phase packings are fabricated more easily than intimately-blended phases and since mechanically mixed packings can readily be provided in a laboratory on a routine basis. A wide range of separations problems can thereby be attacked with just a few stationary phases, the window strategy requiring only that the stationary phases be mixed in the correct proportions. Nevertheless, the linearity of plots of K R ( M , against 4~ for mixed-bed packings has yet to be tested comprehensively and with high-precision apparatus. Accordingly, and because much of the utility of the optimization strategy rests upon this proposition, the current study was undertaken; squalane (B) and dinonyl phthalate (C) phases were chosen since accurate KOR data are available for a number of solutes with these pure phases and
since there is no question of the appreciable curvature in Equation 1 with intimate blends of these solvents. EXPERIMENTAL The fabrication of high-precision instrumentation and related techniques and procedures have been described in detail elsewhere (34). The gas chromatograph employed in this work was constructed from a Tamson water bath, a Gow-Mac thermal conductivity detector, a Hamilton heated injection port, and a Linear recorder. The reference and sample carrier (helium) flows were controlled at the column inlet with a Brooks electronic dualchannel flow control module and Brooks pressure regulators. The flow rate was measured to 10.05 mL min-' with a 50-mL water-jacketed soap-bubble burette, the column inlet pressure to 10.05 psi with a U S . Gauge pressure gauge, and the column temperature to f0.02 "C with a Hewlett-Packard platinum resistance thermometer. The support material was 60/80-mesh Chromosorb G (AW/ DMCS-treated);the solvents, squalane (B) and dinonyl phthalate (C), were obtained from Applied Science Laboratories and were used as received. Stationary phases were fabricated by rotary evaporation with methylene chloride in the usual manner and packed by vacuum into lI4-in.0.d. nickel tubes. The amount of liquid phase in a pure-phase column was determined from the density of the phase as measured with a Mettler-Paar densitometer, the weight of packing in the column, and the stationary-phase weight percent (5%-7% w/w in this study except as noted), the latter being determined by replicate ashings of used materials. Mixed-phase columns were fabricated by mixing mechanically the pure-phase packings and the resultant volume fraction of C in the B + C mixture calculated from mCwC/PoC
"=
+ mcwc/pt
(3)
~BWB/POB
where m, is the packing mass, withe percent (w/w) liquid loading, and pp the density of pure phase i. RESULTS Table I lists the solute partition coefficients obtained in this study with pure squalane and pure dinonyl phthalate over the temperature range 50 to 70 "C. Also shown are data for several of the solutes which were obtained in a separate laboratory and with different apparatus; the average discrepancy between the two sets of data amounts to ca. 1YO which is taken as a reflection of the level of accuracy attainable with the apparatus used for this work. Figures 1-3 illustrate plots of KR(M1 against @C for all solutes of Table I at 50-70 "C; the error bars indicate f 2 % , while
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
'
n - Hexane
C
n-Heptane
I
401 C
I
02
04
L
06
08
d 10
I 3001
Figure 1. Plots of K,(,,
Benzene
@3NP against @ c for indicated solutes and temperatures.
I
550
n-octane
I
Methylcyclohexane
I
@9NP f2%
Error bars represent
Tetrahydrofuran
180.
i 500
02
04
06
GDNP
08
10
%
'
Ol.2 '
01.4 '
d.6 '
6DNP Figure 2. Plots of KR(,.,)against 4 c for indicated solutes and temperatures. Error bars represent f2%
OI.8
'
Ib
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
1405
I
40 I
i
0
0
37or
260-
Chloroform
02
,
i
04
,
I
,
06
J IC:
i
08
Carbon tetrachloride
Table 11. Linear Regression Correlation Coefficients ( r ) and Comparisona of Calculated (Least Squares) with Experimental End-Point K & Values for Data of Figures 1-3 50 " C 60 "C 70°C solute
r
Ba
Ca
n -hexane n-heptane n-octane benzene toluene cyclohexane methylcyclohexane tetrahydrofuran dichloromethane chloroform carbon tetrachloride thiophene
0.997 0.997 0.998 0.996 0.996 0.998 0.995 0.999 0.999 0.999 0.991 0.999
-0.07 -0.42 -0.28 0.00 0.18 0.00 1.41 1.08 1.22 1.52 0.04 0.64
0.997
0.44
av. a
r
BO
Ca
r
B O
Ca
-0.42 -0.21 1.23 1.06 0.21 -0.34 0.14 0.71 0.29 0.12 0.57
0.998 0.992 0.996 0.999 0.998 0.998 0.996 0.997 0.999 0.999 0.992 0.999
-0.74 -0.90 -1.50 -0.70 -1.19 -0.76 -0.06 -1.77 -3.39 -1.25 0.42 0.63
-0.61 0.27 -1.80 -0.41 -0.55 -0.70 -1.63 0.38 -0.25 -0.37 -0.06 0.07
0.999 0.994 0.994 0.991 0.994 0.996 0.995 0.999 0.999 0.998 0.995 0.999
0.50 -0.50 -0.50 1.77 -1.45 -1.02 -0.58 -0.25 -2.45 -0.38 -0.16 0.17
0.65 -1.29 -0.64 -0.86 -0.90 -1.71 -2.13 -0.26 -0.87 -1.18 -0.37 -0.20
0.28
0.997
-0.93
-0.75
0.996
-0.40
-0.81
0.00
100[Kk(i)(exptl)- K~(i)(least-squares)] /Kk(i)(exptl);i = squalane ( B ) or dinonyl phthalate (C). -
solid lines have been drawn between the two end points (Le.,
E(OR(B) and K&)) for each solute without regard to points a t
intermediate compositions of B + C ( 4 =~ 0.107,0.218,0.455, 0.584, 0.714, and 0.853). The plots are clearly linear as indicated by the correlation coefficients and comparison of calculated with experimental end-point data given in Table
11. DISCUSSION On the basis (35)that errors contributing to retentions may be treated as independent from one another, the relative standard deviation to be expected on K R ( M ) (i.e., that with mixed packings) is the square root of the sum of the errors in determining the pure-phase liquid loadings. Since in the present instance the ashing procedures employed were found to be precise to 1%,the minimum relative standard deviation expected is 1.41%. Thus, deviations of 2% in Figures 1-3 from
-____
the linearity predicted by Equation 1are entirely reasonable. Clearly, however, when the uncertainty regarding packing fabrication is larger than that here, the deviations may well exceed those shown in the plots. For example, a popular class of stationary phases which cannot be ashed comprise the silicone gums. Procedures heretofore used (e.g., Ref. 18, Chapter 3) to assess w , for such materials include controlled evaporation and Soxhlet extraction which in favorable instances can be as precise as &2%; nevertheless, deviations from Equation 1of as much as 5% will result if the errors associated with these techniques approach 3.5%. Some care is therefore required in the measurement of pure-phase liquid loadings when packings are used in admixture for the purposes of Equation 2. E f f e c t s of Liquid Loading. It might be supposed that if liquid loadings are sufficiently high, interparticle stationary-phase migration may occur in a mixed-bed column to such
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
Table 111. Comparison of Partition Coefficients Obtained with 5% w/wa and 12% w / w b Pure-Phase Packings (Mixed Mechanically) with 1 2 %w/w Intimate BlendC of Phases a t 50-70 "C at @ C = 0.50 50 "C
solute
mech.a
mech.6
inLc
n-hexane n-heptane n-octane cyclohexane me th ylcyclohexane benzene toluene dichlorome thane chloroform carbon tetrachloride tetrahydrofuran thiophene
124.3 333.3 882.1 235.5 433.2 263.9 739.5 84.88 214.4 241.4 173.0 300.1
125.6 336.4 892.7 236.1 436.6 266.4 742.8 84.71 214.6 246.2 172.9 302.2
137.2 367.1 973.9 253.6 472.7 276.0 788.3 88.65 227.2 258.8 181.2 313.0
KR(M)( @ c= 0 . 5 0 ) 6 0 "C mech.a mech.* inLC
mech.a
mech.6
inLc
92.86 235.3 593.1 171.9 305.3 189.8 507.9 64.24 152.9 174.5 127.6 215.3
69.49 166.6 399.0 127.8 221.8 140.2 356.5 48.77 111.6 129.4 95.24 158.0
69.56 170.5 410.3 129.4 224.6 140.5 356.2 47.25 109.8 128.9 95.81 157.7
75.12 181.6 438.0 137.7 240.7 147.9 379.7 49.91 118.5 137.6 96.64 163.4
93.01 235.9 596.1 175.3 303.5 188.8 507.7 64.20 150.2 174.7 128.4 214.5
100.4 257.4 648.4 185.1 333.0 201.2 540.4 65.93 160.8 186.2 131.9 224.6
7 0 "C
a From Figures 1-3. Mechanical mixture of 12% w/w pure squalane packing + 12% w/w pure dinonyl phthalate packing. Intimate mixture of squalane + dinonyl phthalate; 12% w/w solution loading.
a n extent that noticeable curvature would result in plots of
KFo) against dc. Two columns were constructed to assess this effect, one with mechanically-mixed packings and the other for purposes of comparison with intimately-blended solvents, dCbeing in each case 0.50. The liquid loadings were 1 2 % w/w for each, which represents close to the maximum loading possible with Chromosorb G. T h e resultant solute partition coefficients are given in Table I11 as are those interpolated from the plots of Figures 1-3 ( 5 % w/w liquid loading). Comparison of the data obtained with the two columns constituting mixed beds shows virtually no difference from solute to solute, the discrepancies being well within experimental errors involved in fabrication of the packings as discussed above. A twofold increase in liquid loadings with the pure-phase packings employed here thus appears to have no effect upon the linearity of Equation 1. In marked contrast, t h e intimately-blended solvent system gives partition coefficients which are uniformly 10% higher than the former which from t h e standpoint of analytical applications of Equation 1 lends further support to the view that the use of mechanically-mixed packings offers significant advantages. Stationary-Phase Volatility. Several situations may arise as a result of stationary-phase volatility. In the first, it is conceivable that with time appreciable solvent migration could result if, for example, solvents are employed a t temperatures a t which their vapor pressures exceed ca. 0.01 torr. Because curvature in plots of KRm against dCis at least in the present instance positive, this would result in a n increase in raw retentions. However, since in all cases reported to date deviations from Equation 1 have been of about the same magnitude for any one solute with this or that pair of stationary phases, the relative separation observed will in all likelihood remain unchanged. It might alternatively be supposed that volatility of one or the other of the solvents could be such that a steady-state loss of material from the column could produce a change in @C which would result in a decrease with time of the separation for which the mixture was initially fabricated; retentions, on the other hand, might increase, remain unchanged, or decrease depending upon the slopes of plots of KR(M) with dCand the reduction of the total-column liquid volume. The same will also be true when both phases exhibit equally-high bleed rate: resultant blending of the solvents may yield longer or shorter elution times. Once again, however, t h e relatiue separation will in all probability remain unchanged. In any event, the simplest solution in each case is use of a lower column temperature which will, of course, entail extended elution times; if required, one would then
proceed to apply the well-known principles of minimum time analysis (36). In summary, the propostion t h a t mechanically-mixed packings yield plots of KRm which are linear in @chas, within experimental error, been verified for squalane and dinonyl phthalate solvents a t 50-70 "C. In addition, differences in partition coefficients with mixed-bed and intimately-blended materials are for this system considerable; as a result, interparticle phase migration could easily be detected when the former method of column fabrication is employed which can in general be expected t o be the case and for which lower operating temperature is a n appropriate remedy. Related problems associated with phase migration and/or disparate bleed rates are in any case far outweighed by the advantages (notably, quantitatiue control over column selectivity) gained from utilization of multicomponent stationary phases in conjunction with the window diagram strategy.
LITERATURE CITED Laub, R. J.; Purnell, J. H. J. Chromatogr. 1975, 112, 71. Laub. R. J.; Purnell, J. H. Anal. Chem. 1878. 48, 799. Laub, R. J.; Purnell, J. H. AM/. Chem. 1978, 48, 1720. Laub, R. J.; Purnell, J. H.; Williams, P. S. J. Chromafogr. 1977, 134, 249. (5) Laub, R. J.: Purnell, J. H.; Williams, P. S. Anal. Chim. Acta 1977, 9 5 , 135. (6) Laub. R. J.; Purnell, J. H.; Wllliams, P. S. J. Chromatogr. 1978, 155, 1. (7) Laub, R. J.; Purnell. J. H. J. Chromatogr. 1978, 161, 49. (8) Laub, R. J.; Purnell, J. H. J. Chromatogr. 1978, 161, 59. (9) Laub. R. J. Anal. Chem.. in press. 10) Laub, R. J. I n "Physical Methods of Modern Chemical Analysis", Vol. 3; Kuwana, T., Ed.; Academic Press: New York, N.Y., in press. 11) ACThamir, W. K.; Laub, R. J.; Purnell, J. H. J. Chromafogr. 1877, 142, 3. 12) ACThamir. W. K.; Laub. R. J.; Purnell. J. H. J. Chromafogr. 1979, 176, 232. 13) ACThamir, W. K.; Laub, R. J.; Purnell. J. H. J. Chromatogr. 1880, 788, 79. 14) Purnell, J. H.; Vargas de Andrade, J. M. J . Am. Chem. Soc.1875. 97, 3585. 15) Purnell, J. H.; Vargas de Andrade, J. M. J . Am. Chem. Soc. 1975, 97, 3590. 16) Laub. R. J.; Purnell, J. H. J. Am. Chem. SOC. 1978, 98, 30. 17) Laub. R. J.; Purnell, J. H. J. Am. Chem. SOC. 1978, 9 8 , 35. 18) Laub, R. J.; Pecsok, R. L. "Physlcochemlcal Applications of Gas Chromatography"; Wiley-Interscience: New York, N.Y., 1978; Chapter 6. (19) Laub. R. J.; Wellington, C. A. I n "Molecular Association", Foster, R., Ed.; Academic Press: London, 1979; Vol. 2, Chapter 3. (20) Purnell, J. H. J. Chem. SOC. 1980, 1268. (21) Ashworth. A. J.; Hooker, D.M. J. Chromafogr. 1877, 131, 399. (22) Parcher, J. F.; Westlake, T. N. J. Phys. Chem. 1877, 81, 307. (23) Christian, S.D.; Tucker, E. E.; Mitra. A. J. Chern. Soc.. Faraday Trans. 11977, 73, 537. (24) Tucker, E. E.; Christian, S. D. J. Am. Chem. SOC.1878, 700, 1418. (25) Acree, W. E., Jr.; Bertrand, G. L. J. Phys. Chem. 1879, 83, 2355. (26) Tiby, P. F. J. Chromatogr. 1979. 179, 247. (27) Harbison, M. W. P.; Laub, R. J.; Martire, D. E.; Purnell, J. H.; Williams. P. S. J. Phys. Chem. 1979, 83. 1262. (1) (2) (3) (4)
Anal. Chem. 1980, 52, 1407-1411 (28) Laub, R. J.; Martire, D. E.; Purnell, J. H. J . Chem. Soc., Faraday Trans. 1, 1977, 73, 1685. (29) h u b , R. J.; Marlire. D. E.; Purnell, J. H. J . Chem. Soc., Faraday Trans. 2 1978, 7 4 , 213. (30) Martire, D. E. Anal. Chem. 1974, 46, 1712. (31) Martire, D. E. Anal. Chem. 1976, 48, 398. (32) Janini, G.M.; King, J. W.; Martire, D. E. J . A m . Chem. SOC.1974, 96. =.QfiR -"--.
(33) Martire, D. E.; Sheridan, J. P.; King, J. W.; O'Donnell, S. E. J . A m . Chem. SOC. 1976, 98, 3101. (34) Harbison, M. W. P.; Laub, R. J.; Martire, D. E.: Purnell, J. H.; Williams, P. S . , unpublished results.
1407
(35) Harbison, M. W. P.; Laub, R. J.; Martire, D. E.; Purnell, J. H.; Williams, P. S. J . Chromatogr. 1978, 155, 233. (36) Purnell, J. H.; Quinn. C. P. I n "Gas Chromatography 1960". Scott, R. P. W., Ed.; Butterworths: London, 1960; p 184.
RECEIVED for review March 31.1980. Accepted May 23,1980. The authors gratefully acknowledge S U P P O ~from the National Science Foundation, grant no. CHE-'i820477~and the Graduate School of The Ohio State University.
Replication of Gas-Liquid Chromatographic Retentions with Silicone Stationary Phases C.-F. Chien and R. J. Laub' Department of Chemistry, The Ohio State University, Columbus, Ohio 432 10
M. M. Kopecni Chemical Dynamics Laboratory, Boris Kidric Institute of Nuclear Sciences-
The concept of replication of retentions obtained with pure stationary phases with those arising from binary mixed packings is examined with the OV series of methyiphenyisilicone fluids. I t is shown that absolute retentions with mixtures of OV-101 ( 0 % phenyl) with OV-25 (75% phenyl) do not correspond to those of OV-11 (35% phenyl). However, relative retentions (Le., separations) with the latter phase appear to be reproduced almost exactly with mixtures of the former two, appropriate combinations of OV-101 with OV-25 being deduced from plots of a against column composition (window diagrams) constructed solely from retention data with each of the pure phases. Thus, it is argued that when a particular sample is for whatever reason thought to be separable with a methyiphenyisiiicone, the entire spectrum of selectivity attainable with this class of liquid phases can readily be examined wlth just two chromatographic runs of the mixture with, respectively, pure OV-101 and pure OV-25.
Since the inception of gas-liquid chromatography (GLC) almost 30 years ago, a problem of major concern has been the selection of a solvent which is appropriate for a given separation. As a result of a paucity of quantitative methodologies designed to facilitate the selection of GC solvents, there are now over 400 stationary phases currently available commercially. However, a number of workers have over the years (1-4) argued t h a t most separations commonly encountered in a chromatographic laboratory require only a few phases. Snyder ( 5 ) , for example, has evolved a scheme whereby most GC phases can be grouped into eight classes. Notable efforts to this end also include the work of Supina and Rose (6) and McReynolds (7) and conventions for reporting retention data with various stationary liquids are now commonplace (8, 9). Nevertheless, the similis similibus solvantur approach (like dissolves like) continues to be advocated with regard to the selection of a phase if the separation a t hand has not previously been documented in the literature or encountered within the experience of the analyst ( I O , 11). This view is often cited, furthermore, in conjunction with the evolution of techniques 0003-2700/80/0352-1407$01 .OO/O
VINCA,
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of column fabrication (notably open-tubular capillary systems) wherein hundreds of thousands (or even millions) of theoretical plates N are said to be attainable. It has alternatively been suggested that mixed multicomponent phases and/or one or another variants of multiplecolumn systems provide significant advantages from both practical and theoretical points of view in dealing with the separation of complex samples. Several approaches based upon such systems have in fact been reported; these include multiple-column and/or multiple-temperature switching and the utilization of mixed stationary beds or multicomponent mobile phases. The subject has been reviewed and discussed recently by Bertsch (12,13),Laub and Wellington (14),Laub and Pecsok (15),and Laub (16). Unfortunately and with few exceptions, use of any one or more of these methodologies has been hindered because of imprecise criteria available for optimization of the relevant (multivariant) system parameter(s). Recently, however, Laub, Purnell, and their colleagues (17-26) have introduced a new graphical method of representation of retention data wherein separations are optimized quantitatively with respect, e.g., to binary stationary-phase composition, to column temperature, to binary mobile-phase partial pressure, or to any other property of the chromatographic system which can be utilized to alter elution behavior. In essence, their procedure requires only that retentions (i.e., partition coefficients, specific retention volumes, capacity factors, a values, retention indices, etc.) be described by a function which relates the change in retention t o variation of the system parameter (the latter being cast as the independent variable). Plots of relative retention (cy) are next constructed for all pairs of solutes on a common graph (window diagram) against the parameter of interest from which the optimum value of the parameter can then be deduced. For example, plots of a against composition of a binary stationary phase will prescribe the optimum mixture of the solvents for separation of the solutes from which the data were derived. Further, and with regard to solvents, employment of a selective phase (B) in admixture with a second solvent (C) of different selectivity will result in a spectrum of solute-solvent interactions, intermediate regions being produced with appropriate 1980 American Chemical Society
