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 . Am. 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 , unpublished results.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 CY 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,
P.0. Box 522, I100 1-Beograd,
Yugoslavia
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 CY 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
1408
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
combinations of B + C. In addition, of course, retentions with phases corresponding to intermediate selectivity can also be replicated. Laub and Purnell (18) thus claimed (as have others) t h a t a wide range of separations problems can hence be attacked with only a few stationary phases which requires only t h a t the latter be blended in the correct proportions. A particular class of stationary phases which would seem readily amenable to t h e above approach is that comprising the OV series of methylphenylsilicone fluids. Indeed, Mann a n d Preston (27) and Lynch, Palocsay, and Leary (28) have presented compelling evidence that retentions obtained with OV-3 (10% phenyl), OV-7 (20% phenyl), OV-61 (33% phenyl), OV-11 (35% phenyl), OV-17 (50% phenyl), or OV-22 (65% phenyl) can be reproduced by appropriate mixtures of OV-101 (0% phenyl) with OV-25 (75% phenyl, t h e last member of the series). However, Parcher, Hansbrough, and Koury (29) disagreed and presented plots of against weight percent phenyl which in fact passed through maxima (e.g., Figures 1 and 2 of Ref. 29). They also reported curvature in plots of -Msor A S , against percent phenyl in agreement with the work of Reinbold and Risby (30),Figgins, Reinbold, and Risby (31),Butler a n d Hawkes (32), and Kong and Hawkes (33). Further, even though the physical properties and in particular molecular weight of members of the OV series vary to a significant degree (34-36),Parcher and colleagues (29)found that redefinition of retention data consistent with volume-fraction, weight-fraction, or molal-fraction based activity coefficients (37-45) did not alter significantly the curvature found in the plots. Thus, the entire matter of replication of retentions with pure phases with mixtures of other “standard” solvents is called into question since if the concept proves not to be viable in what may well be regarded as the simplest case of the OV series, it most likely will not be successful for any other class of stationary phases. This study was therefore undertaken in order to explore further and to clarify the use of mixed phases in GLC and in particular the concept of formulation of a “standard” set of phases which can be used to replicate (hence, eliminate the need for) other pure solvents. Because of the volume of relevant data already extant in the literature, our studies were initiated with the OV methylphenylsilicone class of compounds.
EXPERIMENTAL The equipment, techniques, and procedures pertaining to high-precision GC measurements have been summarized elsewhere (15,46). Briefly, the laboratory-constructed chromatograph used for this work consisted of a Tamsen water-bath, a Gow-Mac thermal conductivity detector, a Hamilton heated injection port, a Linear recorder, and a Brooks dual-channel electronic flow controller with Negretti-Zambra pressure regulation. The column temperature was monitored with a Hewlett-Packard platinum resistance thermometer, the column inlet pressure was monitored with a US. Gauge pressure gauge, and the flow rate (helium carrier) was measured with a water-jacketed 50-mL soap-bubble burette. The gas chromatograph employed for the analytical work was a Varian Model 3700. The OV stationary phases (Alltech Associates) and the solutes (Chem Samples Co.) were used as received. Packings were prepared by rotary evaporation with 60/80-mesh or as noted 120,’ 140-mesh Chromosorb G (AW/DMCS-treated) and were packed into ’/&. or, for the analytical studies, 1/8-in.,0.d. stainless steel tubes by suction or by the procedure described by Laub and Purnell (47). Weight-percent liquid loadings were determined to *2% by exhaustive extraction of used materials. Specific retention volumes V“ were calculated in the usual way , (Ref. 15) from the solute adjusted retention time t ~ ’ the James-Martin corrected flow rate jF,,the column temperature T,and the mass m of stationary phase:
Table I. Solute Specific Retention Volumes with OV-101, OV-11, and OV-25 Stationary Phases at 6 0 to 80 “ C OV-101 Stationary Phase
Vi,cm3 g - ’ no. 1
2 3 4 5 6 7 8
9 10
11 1
2 3 4 5 6 7 8
9 10 11
solute n-hexane n-heptane n-octane n-nonane methylcyclohexane benzene toluene o-xylene m-xylene p-xylene chlorobenzene
60.0 “ C
n-hexane n-heptane n-octane n-nonane methylcyclohexane benzene toluene o-xylene m-xylene p-xylene chlorobenzene
44.31 105.2 243.8 563.3 153.1 130.0 308.1 895.0 709.7 706.5 655.0
61.95 142.8 323.7 727.5 172.9 95.25 222.2 596.6 510.7 510.9 406.1
70.0 “ C
80.0 “ C
45.63
34.19 71.48 147.9 304.4 87.73 52.42 111.4 271.0 222.6 230.0 190.2
100.0
216.3 464.6 121.9 70.05 155.8 397.5 333.1 338.8 274.9 OV-11 Stationary Phase 32.86 74.24 164.8 364.5 108.6 92.75 209.3 578.8 463.2 460.4 433.9
24.79 53.44 113.9 241.7 78.54 67.47 145.3 383.6 309.7 307.3 294.2
OV-25 Stationary Phase 1
2 3 4 5 6 7 8 9 10 11
n-hexane n-heptane n-octane n-nonane methylcyclohexane benzene
toluene o-xylene m-xylene p-xylene chlorobenzene
21.95 49.35 110.4 244.7 86.74 113.9 254.6 723.1 555.3 556.2 569.2
16.91 36.04 77.21 163.9 62.75 81.24 173.9 471.2 364.3 361.6 378.2
13.22 26.80 55.11 112.4 46.24 59.06 121.4 314.5 244.8 240.9 257.1
All peaks were symmetric and gave retentions which were independent of sample size.
RESULTS Table I lists the specific retention volumes of the solutes employed here with the phases OV-101, OV-11 (chosen as representative of a methylphenyl composition intermediate between those of OV-101 and OV-251, and OV-25 at three temperatures, which is a portion of the data determined in this work. T h e solutes include n-alkanes, alicyclic and aromatic hydrocarbons, and, for purposes of comparison, a chlorinated aromatic hydrocarbon; we found, however, t h a t solutes such as ethanol, acetone, acetonitrile, and nitromethane exhibited considerable peak asymmetry (indicative of gasliquid interfacial adsorption) and so were not included in the study. In Table I1 data from Table I are compared with those of Parcher and colleagues (29): t h e differences are as much as 11% (n-hexane and n-heptane with OV-11) which emphasizes the well-known difficulties encountered with reproducibility of absolute retentions with polymeric stationary phases. Batch-to-batch variations, different temperatures a t which the packings were conditioned, and so forth, undoubtedly contribute significantly t o the discrepancies shown. However, the errors associated with relative retentions (nhexane = 1.00), except for benzene with OV-11, are no larger than 1.9%. Indeed, the average discrepancy for all solutes is only fO.6%. Thus, while there is some disparity concerning absolute retentions, the relative retention data of this work
ANALYTICAL CHEMISTRY, VOL. 52, ~-
NO. 9,
AUGUST 1980
1409
~
Table 11. Comparison of Solute Specific Retention Volumes with OV Phases at 66 “C ov-101
ov-11
OV-25
vi,cm3 g-’ v:, cm3 g-’ ref. rel. ref. rel. ref. rel. this 26 A%a A%b solute this 26 A%’ A%b this 26 A%a A%b n-hexane 51.46 48.35 32.86 16.97 36.96 18.74 (0) 9.45 (0) 6.04 11.1 (0) n-heptane 115.0 8.24 -1.32 108.21 5.90 0 85.14 75.52 11.3 0 40.78 37.42 n-octane 253.5 5.18 -0.81 192.2 171.61 7.82 -1.90 240.37 10.7 -0.39 88.86 81.91 5.81 -1.21 87.40 benzene 79.05 75.16 4.92 -1.30 105.9 97.96 7.50 -3.83 92.79 a (This work - Ref. 26)lthis work x 100; V: data. (This work - Ref. 26)/this work x 100; a data relative to n-hexane. Vg, cm3 g-’
are consistent with those of Ref. 29.
DISCUSSION In addition to providing a quantitative basis for selection of appropriate mixtures of phases (or other system variables) for a given separation, the Laub-Purnell optimization strategy also yields criteria with which, in lieu of the commonly-perceived need to have to hand all 400 or so commerciallyavailable materials, only a few “standard” solvents need in most situations be employed. Broadly speaking, each liquid phase must, according to the window diagram procedure, give some separation of the sample components, yield reasonable retention times and column efficiencies and, most importantly, exhibit selectivity which differs significantly from the others such that success of a mixture of intermediate composition of any two of the phases is likely. [Several other criteria of a more practical nature (such as cost, availability, and so forth) may also require consideration a t some later stage.] A convenient method of assessment of pairs for the above purposes is representation of retentions as plots against column composition. In terms of specific retention volumes with phases B and C, the appropriate relation is (20,48-52):
where w is a weight fraction of the respective component in the binary (B C) stationary phase mixture. Figure 1 shows, for example, plots of against weight fraction of the solutes (except m-xylene) of Table I with (a) OV-11 with OV-101, (b) OV-25 with OV-11, and (c) OV-25 with OV-101 a t 60 “C, where only the end-point (i.e., pure-phase) data were employed in construction of the straight lines in accordance with Equation 2. We note first that the three pairs of phases offer in each case a range of selectivity which produces a number of reversals in retention order which may be somewhat surprising in view of the similar chemical nature of these solvents. Thus, for example, Figure l a shows that solute number 4 is the last to elute with OV-101 but is retained less strongly than three other solutes with OV-11. Several inversions are also seen in Figure l b which emphasizes the point that even subtle differences in solvent selectivity can be utilized to advantage in altering retention order and thereby can effect a separation [which parenthetically has undoubtedly added weight to the contention that a large variety of phases are required in GLC even within a particular class of solvents (e.g., the Apiezon greases)]. The alterations in retentions in Figure l a , b are, however, overshadowed by the variations evident in Figure IC,OV-101 with OV-25: solute number 4, for example, crosses four other solutes, number 7 crosses two others, and number 6 three others on passing from w ~ = 0~to 1.. ~ ~ We note a t this point that retentions in the first two figures across the range OV-101 to OV-25 pass through maxima with OV-11, thus verifying the findings of Parcher and co-workers (29). In addition and upon inspection of Figure IC, it does not appear possible to replicate retentions with OV-11 with any combination of OV-101 with OV-25.
+
e
. ‘. _
’0V
Flgure 1. Plots of
- 25
\p, against
w, constructed from the end-point data of the solutes (except rn-xylene) of Table I at 60 OC with the stationary-phase pairs: (a) O V - l l with OV-101, (b) OV-25 with O V - l l , and (c) OV-25 with OV-101
In contrast, in analytical separations relative retention data, Le., a values, are of primary importance, more so than absolute retentions. In terms of Equation 2 , for example, for solutes m and n with phases B and C,
i.e., a values for all pairs of solutes with combinations of the phases may be calculated in advance from retention data derived solely from the pure phases. Since Equation 2 is obeyed to within experimental error of the GLC method when B C compositions comprise mechanical mixtures of purephase packings (52),Equation 3 can be employed with considerable confidence for assessment of appropriate combinations of solvents (fabricated from mixed beds) for a given separation. Figure 2 illustrates plots (17) of a against w ifor the three combinations of liquid phases of Figure 1. Since only the lower boundary of a is important, regions of‘ difficulty less than the minimum found a t each column composition are shaded in. Several windows of separation are seen in each figure: in Figure 2a, for example, that labeled A indicates that the most difficult CY which must be dealt with at wov.ll = 0.043 is 1.172, that a t B provides an a of 1.103 a t U I O V . ~ = ~ 0.430, and t h a t a t C yields an a of 1.090 a t w o ~ . l l = 0.904. Figure 2b shows, however, windows which are considerably smaller (offer poorer a ) than those in Figure 2a. Thus, solely on the basis of these two figures, the optimum column composition for the separation is predicted to be 4.3% of OV-11 in an OV-101 OV-11
+
+
1410
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980 3
Figure 2. Window diagrams ( 7 7 ) for the solutes of Figure 1 with the three pairs of phases: (a)OV-11 with OV-101, (b) OV-25 with OV-11, and (c)OV-25 with OV-101 at 60 O C , as calculated from the end-point data of Table I with Equation 3
(mixed-bed) stationary phase. Shown in Figure 2c is the window diagram obtained with OV-101 + OV-25 which appears remarkably similar to what would be a composite of Figure 2a,b. Three significant windows are found as before, that at A' (cu = 1.173 a t wov-25 = 0.040), at B' ( a = 1.099 a t u;ov-25= 0.3101, and at C' ( a = 1.082 - ~ where ~ in each case the a's are virtually the a t u ; ~ =~0.553), same as those for A, B, and C. Thus, even though the straight-line plots of Figure 1 seem to indicate that relative retentions (hence separations) differ considerably from one pair of phases to the next, the window diagrams demonstrate that mixtures of OV-101 with OV-25 can indeed be used to replicate those obtained with OV-11 and by extension, with other OV phases of intermediate phenyl content. I t is also worth noting that the number of plates required for separation Nw, as calculated from the relation first derived by Purnell (53)
Figure 3. Chromatograms at 60 O C of solutes 1-8 and 10 and 11 of Table I with (a)OV-11 -I- OV-101 (wov.,, = 0.043) and (b) OV-25 iOV-101 (w,,,, = 0.040) corresponding respectively to windows A and A' of Figure 2. Column length: 5 f l ('3)
6
7
Ii
10
24
28
4
Time, min
(b)
6 I2
1
(4) is in this instance and upon assumption of a capacity factor exceeding 10 for the first-eluting solute, 1700 plates for windows A and A', 4100 plates for windows B and B', and 5300 plates for windows C and C'. Assuming further that an efficiency of 350 plates per foot can be achieved with these phases, the column lengths required in each case are 5, 12, and 16 ft. (We of course recognize that the best window offers little improvement in the separation over that expected with pure OV-101. However, the point of this study is to provide illustration of the feasibility of replication of phases and so the relative merits of this or that pure phase f o r separation of t h e solutes employed here is of no concern.) Experimental verification of the equivalence of the windows is given in Figures 3 and 4 where the chromatograms of solutes 1-8 and 10 and 11 of Table I are presented at 60 "Cwith the two best combinations of OV-101 with OV-11 and of OV-101 with OV-25 corresponding, respectively, to windows A and A' (Figure 3) and B and B' (Figure 4). Virtual base-line separation is, as predicted, achieved in each case, even though the respective times of analysis, particularly those in Figure 4,are not identical. (The nonequivalence of retention times is in fact expected on the basis of the plots shown in Figure 1.) Thus, the chromatograms offer compelling evidence as does Figure 2 that while absolute retentions cannot be duplicated, mixtures of OV-101 with OV-25 can indeed be employed to replicate relative retentions (hence separations) with other
3
0
4
12
8
16 20 Time, min
32
Figure 4. Chromatograms at 60 O C of solutes 1-8 and 10 and 11 of Table I with (a) OV-11 OV-101 (wov.,, = 0.430) and (b) OV-25 -t OV-101 ( wov.25= 0.310) corresponding, respectively, to windows B and B' of Figure 2. Column length: 12 ft
+
OV phases. There appears, therefore, to be little advantage gained in utilizing pure phases of selectivity intermediate between those of OV-101 and OV-25 since separations with the former can be mimicked with mixtures of packings of the latter. The time and effort saved by this procedure as opposed to detailed examination of each of the eight OV phases is obviously considerable. We hope soon to report on extension of these studies with other classes of stationary phases and on the utility of the concept of mixed solvents with open-tubular column systems.
LITERATURE CITED (1) Pecsok, R . L.; Appfel, J. Anal. Chem. 1979. 57. 594. (2) Preston, S.T., Jr. J . Chromatogr. Sci. 1973, I I , 201. (3) Leary, J. J.; Justice, J. 8 . ; Tsuoe. S.;Lowry, S. R.; Isenhour. T. L. J . Chromatogr. Sci. 1973, 7 1 , 201. (4) Haken, J. K J . Chromatogr. Sci. 1975, 73, 430. 15) Snvder. L. R. J . Chromatoar. Sa. 1970. 76.223 (6) Supina, W. R.; Rose, L. P.-J Chromatogr. Sci 1970, 8. 214 (7) McReynolds, W. 0. J . Chromatogr. Sci. 1970, 8 , 685.
Anal. Chem. 1980, 52, 1411-1415 (8) Rohrschneider, L. Fresenius' 2. Anal. Chem. 1959, 170, 256. (9) Kovats, E. sz. Helv. Chim. Acta 1958, 41, 1915. (10) Supina, W. R. I n "Modern Practice of Gas Chromatography", Grob, R. L.. Ed.; Wiley-Interscience: New Ywk. 1977; Chapter 3. (1 1) Okamura, J. P.; Sawyer, T. I n "Physical Methods in Modern Chemical Analysis", Vol. 1, Kuwana, T., Ed.; Academic Press: New York, 1978; Chapter 1. (12) Bertsch, W. J. High Resolut. Chromatogr., Chromatogr. Commun 1978, 1 , 187, 289. (13) Bertsch, W. J . High Resolut. Chromatogr., Chromatogr. Commun. 1979, 2, 85. (14) Laub, R . J.; Wellington, C. A. I n "Molecular Association", Vol. 2, Foster, R.. Ed.; Academic Press: London; Chapter 3. (15) Laub, R. J.; Pecsok, R. L. "Physicochemical Applications of Gas Chromatography"; Wiley-Interscience: New York. 1978; Chapter 6. (18) h u b , R. J. I n "Physical Methods of Modern Chemical Analysis", Vol. 3, Kuwana, T., Ed.; Academic Press: New York, in press. (17) Laub. R. J.; Purnell, J. H. J. Chromatogr. 1975, 112. 71. (18) Laub, R. J.; Purnell, J. H. Anal. Chem. 1976, 48, 799, 1720. (19) Laub. R. J.; Purnell, J. H.; Williams P. S. J. Chromatogr. 1977, 134, 249. (20) Laub. R. J.; Purnell, J. H.; Summers, D. M.; Williams, P. S.J. Chromatogr. 1978, 155, 1. (21) Laub, R . J.; Purnell, J. H. J . Chromatogr. 1976, 161, 49, 59. (22) Laub, R. J.; Pelter, A.; Purnell, J. H. Anal. Chem. 1979, 57, 1878. (23) AI-Thamir, W. K.; Laub, R. J.; Purnell, J. H. J. Chromatogr. 1977, 142, 3. (24) AI-Thamir. W. K.; Laub, R. J.; Purnell, J. H. J . Chromarogr. 1979, 176, 232. (25) AI-Thamir, W. K.; Laub, R. J.; Purnell, J. H. J. Chromatogr. 1980, 180, 79. (26) Laub, R. J.; Purnell, J. H.; Williams, P. S. Anal. Chim. Acta 1977, 95, 135. (27) Mann, J. R.; Preston, S. T., Jr. J. Chromatogr. Sci. 1973, 1 7 , 216. (28) Lynch, D. F.; Palocsay, F. A.; Leary, J. J. J . Chromatogr. Sci. 1975, 13, 533. (29) Parcher, J. F.; Hansbrough. J. R.; Koury, A. M. J. Chromatogr. Sci. 1978, 16, 183.
141 1
(30) Reinbold, 9. L.; Risby, T. H. J . Chromatogr. Sci. 1975, 73, 372. (31) Figgins, C. E.; Reinbold, B. L.; Risby, T. H J. Chromatogr. Sci. 1977, 15, 208. (32) Butler, L.; Hawkes, S . J. J . Chromatogr. Sci. 1972, 10, 518. (33) Kong, J. M.; Hawkes, S . J. J . Chromatogr. Sci. 1976, 1 4 , 279. (34) Rotzsche, H. Plaste Kautsch. 1968, 15. 477. (35) Trash, C. R. J. Chromatogr. Sci. 1973, 11, 196. (36) Coleman, A. E. J. Chromatogr. Sci. 1973, 1 1 , 198. (37) Smidrod, 0.; Guillet, J. E. Macromolecules 1971, 4 , 356. (38) Lavoie. A.; Guillet. J. E. Macromolecules 1969, 2, 443. (39) Patterson, D.; Tewari, V. B.; Schreiber, H. P.; Guillet, J. E. Macromolecules 1971, 4 , 356. (40) Guillet, J. E.; Stein, S.N. Macromolecules 1970, 3 , 102. (41) Meen. D. L.; Morris. F.; Purnell, J. H.; Srivastava, 0.P. J. Chem. Soc., Faraday Trans. 11973, 69, 2080. (42) Roberts, G. L.; Hawkes, S. J. J. Chromatogr. Sci. 1973, 1 1 , 16. (43) Huber, G. A.; Kovats, E. sz. Anal. Chem. 197:,, 45, 1155. (44) Fritz, D. F.; Kovats, E. sz. Anal. Chem. 1973, 45, 1175. (45) Martire, D. E. Anal. Chem. 1974, 46, 626. (46) Laub, R. J.; Purnell, J. H.; Williams, P. S.; Harbison, M. W. P.: Martire, D. E. J . Chromatogr. 1978, 155. 233. (47) Laub. R. J.; Purnell. J. H. J. High Resolut. Chromatogr., Chromatogr. Commun. 1980, 3 , 195. (48) Primavesi. G. R. Nature (London) 1959, 184, 210. (49) Hildebrand, G. P.; Reilley, C. N. Anal. Chem. 1964, 36, 47. (50) Keller, R. A.; Stewart, G. H. Anal. Chem. 1964, 36, 1184. (51) Klein, J.; Widdecke, H. J. Chromatogr. 1978, 147, 384. (52) Chien, C.-F.; Kopecni, M. M.; Laub, R. J. Anal. Chem., preceding paper in this issue. (53) Purnell, J. H. J . Chem. SOC. 1960, 1268.
RECEIVED for review March 31, 1980. Accepted May 21, 1980. We gratefully acknowledge support from the National Science Foundation, grant no. CHE-7820477, ;and from the Graduate School of The Ohio State University.
Determination of BHT, Irganox 1076, and Irganox 1010 Antioxidant Additives in Polyethylene by High Performance Liquid Chromatography J. F. Schabron" and L. E. Fenska Phillips Petroleum Company Research Center, Bartlesville, Oklahoma
A method was developed for the rapid extraction of the three most common antioxidant additives, BHT, Irganox 1076, and Irganox 1010, from polyethylene pellets. The pellets were dissolved In decalin at 110 OC followed by coding to precipitate the polymer. The concentrations of the additives present were determined by normal-phase high performance liquid chromatography (HPLC) of a portion of flttered extract. The HPLC stationary phase was pPorasli and the mobile phase was a heptane to methylene chloride gradient. The relative standard deviatlons were 1.2% for BHT, 1.3% for Irganox 1076, and 2.0% for Irganox 1010. The limits of detection were 0.0006% for BHT, 0.002% for Irganox 1076, and 0.004% for Irganox 1010 in polyethylene,
T o ensure that additives or combinations of additives have been added properly to polyolefin batches following synthesis, reliable and rapid analytical methods are needed. The methods would be used for both quality control and lot certification analyses. In some limited cases, rapid additive analysis can be carried out without extensive pretreatment steps such as extraction. Direct spectroscopic methods such as ultraviolet absorption, infrared, fluorescence or phosphorescence ( I ) , and X-ray fluorescence (2) have been re0003-2700/80/0352-1411$01.00/0
74004
ported. In some cases these methods we useful, but generally they suffer disadvantages from interferences due to nonspecificity ( I ) . A more desirable approach for most additive packages is the separation and determination of each additive in the polymer. This requires extraction of the additives followed by chromatographic separation. Wheeler ( I ) reviewed several polyolefin extraction procedures. Most involved lengthy treatment of ground polyolefin with a volatile organic solvent under heat. Crompton (3) extracted polyolefins in various volatile organic solvents from 6-24 h prior to chromatographic analyses of the extracts. Wims and Swarin ( 4 ) extracted 8-mesh polypropylene pellets for 24 h using tetrahydrofuran. The extracts were separated by size exclusion chromatography (SEC) or normal-phase high performance liquid chromatography (HPLC). Lichtenthaler and Ranfelt (5) extracted ground polyethylene with chloroform in Soxhlet apparatus for 6 h. The extracts were concentrated by evaporation prior to separation by HPLC. The separations were on a 5-pm silica column using mobile phase gradients with heptane and methylene chloride. Another procedure involved a 2-h extraction of thin polyolefin slabs with refluxing dichloromethane (6). This was followed by concentration of the extract and separation by reversed-phase HPLC on p-Bondapak CIS The British Standard Method involves dissolving ground 1980 American Chemical Society
