LITERATURE CITED (1) T. S. Scott, "Carcinogenic and Chronic Toxic Hazards of Aromatic Amines", Elsevier Publishing Co., New York, 1962. (2) J. H. Stender, Fed. Regist., 38, 20074 (1973). (3) J. H. Stender, Fed. Regist., 39, 3756 (1974). (4) A. Ziatkis, H. A. Lichenstein. and A. Tishbee, Chromatographia, 6 , 67 (1973). (5)J. P. Mieure and M. W. Dietrich, J. Chromafogr. Sci., 11, 559 (1973). (6) A. Savitsky and S. Siggia, Anal. Chem., 46, 153 (1974). (7) T. A. Bellar and J. J. Lichtenberg, Am. Water Works Assoc. J., 739 (1974). (8) J. Janak, J. Ruzickova, and J. Novak, J. Chromatogr., 99, 689 (1974). (9) P. W. Jones, R . D. Giammar, P. E. Strup, and T. B. Stanford, 68th Meeting of the Air Pollution Control Association, Boston, Mass., June 15-20, 1975. (IO) J. S. Parsons and S. Mitzner, Environ. Sci. Techno/.,9, 1053 (1975). (11) S. L. Yasuda, J. Chromatogr. Sci., 104, 283 (1975).
(12) G. Ivan and R. Ciutacu, J. Chromatogr., 88, 391 (1974). (13) J. F. Dacher, J. P. Guenier, B. Herve-Bayin, and 0. Moulut, Chromatographia, 8, 228 (1975). (14) P. Haefelfinger, J. Chromatogr., 111, 323 (1975). (15) I. M. Jakovljevic, J. Zynger, and R . H. Bishara, Anal. Chem., 47, 2045 (1975). (16) J. S.Fok and E. A. Abrahamson, Chromatographia,7, 423 (1974). (17) C. C. Milionis, Polymer Intermediates Department, E. I. du Pont de Nemours and Co., Experimental Station, Wilmington, Del., unpublished work, 1975. (18) H. D. Deveraux, Plastics Products and Resins Department, E. I. du Pont de Nemours and Co., Experimental Station, Wilmington, Del., unpublished work, 1975.
RECEIVEDfor review March 30, 1976. Accepted June 17, 1976.
Programmed Thermal Field-Flow Fractionation J. Calvin Giddings,* LaRell K. Smith, and Marcus N. Myers Department of Chemistry, University of Utah, Salt Lake City, Utah 84 112
The role of programmed thermal field-flow fractionation in making tractable a greater range in sample molecular weights is established. The theory of retention is described, and retention plots are given. Experiments are described using nine polystyrene solutes ranging in mol wt from 4000 to 7 100 000. These nine components are resolved in a single run using either linear or parabolic programming. The effects of various experimental parameters are described.
Thermal field-flow fractionation (TFFF) (1-4), among all the techniques of field-flow fractionation (FFF) (5-8), has so far shown the greatest versatility for polymer separations. However, like all elution systems based on a positive retention (as opposed to exclusion, or negative retention), FFF must be modified to handle samples of a broad molecular weight range. Otherwise the early peaks are inadequately resolved and the late peaks are strung out over increasing increments of time. This general problem is usually handled through various kinds of programming systems in chromatographic work (9, I O ) . Earlier we described two programming systems for sedimentation FFF (SFFF) (11). These employed variations of the centrifugal field and variations of solvent density, respectively. These systems succeeded in speeding up the heaviest components in a mixture of polystyrene beads, improving peak spacing, and reducing analysis time. In that TFFF is applicable to a wider molecular weight range of macromolecules (down to mol wt = 500 ( 4 ) ) than SFFF, programming in this system should be especially useful. Here we report the development of a programmed TFFF system. Programming is achieved through the variation of the external field: in this case, the temperature increment, A T . Parameter A T controls retention; it can be varied externally and continuously according to any reasonable program, extending all the way to zero increment and thus zero retention. Here, as in SFFF, one can, in theory, program solvent parameters as well as external fields. However, with SFFF, the relevant solvent parameter (density) has a precisely understood role, whereas in TFFF, no single solvent parameter can be identified with predictable changes in retention. Hence rational solvent programs must await more fundamental developments in our understanding of thermal diffusion parameters.
THEORY OF RETENTION In the cited paper on programmed SFFF ( I I ) , general programmed FFF was divided into two categories: uniform programming and solvent programming. Both are theoretically applicable to TFFF. However, only the former concerns us here, for the reason just discussed. In uniform programming, variations occur as a function of time but not of position. This is achieved in TFFF by keeping the temperature increment equal throughout the column, but forcing it to vary in some predesigned manner with time. The time-program is therefore some function AT = A T ( t )
(1)
Inasmuch as retention parameter, R , depends on A T , R also undergoes systematic change with time, R = R ( t ) ,and programming is achieved. The general equation for peak retention time, t,, is ( 1 1 )
where L is column length and ( u ) is mean flow velocity. The particular form of the variation of R ( t ) must now be established. In FFF, solute is compressed into a layer of exponential form, and of characteristic thickness 1. The ratio of 1 to column width w is the dimensionless thickness, X = l/w.Studies of T F F F have shown that X is approximately inversely proportional to A T ( 2 ) .Explicitly ( 4 ) , X = d / A T MI12
(3)
where 4 is a solvent constant, different for each polymersolvent series, and M is molecular weight. For polystyrenes in ethyl benzene, q5 = 1420 "C (g/mol)l/*. For integration purposes, Equation 3 is written in the form,
X = A/AT
(4)
We must now relate R to X to make Equation 2 useful. This is done by the standard retention equation (7).
(5) which, with Equation 4, becomes
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER
1976
1587
10
.9
8 7 6
8'
5 4
3 2 I
0 001
0001 A0
Flgure 1. Plots of
0 vs. Xo for
different
A 0
T
Figure 2. Plots of
values
This plot specifies elution times in linear systems, as shown by Equation IO. Shaded area shows region in which Equation 10 is not valid, as expressed in Equation 11
R = (6A/AT)[coth( A T l 2 A ) - 2 A / A T ]
R = 6X = 6 A / A T
- 12X2 = 6AlAT - 12A2/(AT)2
(7)
(8)
These equations are valid to within about 10% up to R = 0.2 (Equation 7 ) and R = 0.7 (Equation 8). We now give the theoretical description of two programs developed for this work. Linear Program. This program is both simple and mathematically tractable. I t is described by A T ( t ) = AT0
- at
(12)
= t,[l - exp (-7/6Xo)]
Parabolic Program. This program was developed to allow a more gradual approach to AT = 0. For heavy macromolecules, which only undergo significant migration at low A T , the linear program does not provide a reasonable time for migration near zero A T . The parabolic program will prolong operation in the low AT range. 1588
The parabolic program is expressed by AT = (ATOItp2)( t P- t)' = ATo[l - (t/tp)'] (13) where t , is again the total program time, of such duration that AT reaches zero. The approach here is identical to that of the linear case, but the integration and subsequent reduction is more complicated. Equation 13 substituted in Equation 8 yields
This gives as the specific form of Equation 2 L=
dt
6AtP2( u ) AT0
(15) which, upon integration, becomes
L= 1 This equation rearranges to the cubic form
+
k ~ 0 ' ~h20"
+ k18' + K O = 0
(17)
where 0' is the reduced retention time, t,/t,. The coefficients are
- 6x0 4- 4 x 0 ~ k 2 = 37 + 12x0 - 12x0' k3
(11)
which corrects Equation 20 of reference ( I I ) , where 0.7 mistakenly appears in place of 0.2. When Equation 7 replaces Equation 8, an explicit equation results (11) .
tr = (AToIa) [ l - exp ( - a L / G A ( u ) ) ]
Shaded area shows region in which Equation 17 is not valid
(10)
where XO is the initial X value; 8 = AT,/ATo, where A T , is the temperature increment at the moment of peak-center elution; 7 = L a / ( u ) ATo, which, physically, is the ratio of void-peak elution time, t o = L / ( u ) ,to total program time, t , = AToIa. With XO and 7 established by experimental parameters, Equation 10 yields numerical solutions for 8. These are shown in Figure 1 with parameters appropriate to TFFF. The treatment fails for R > 0.7 ( h > 0.2) because of the limited range of Equation 8. This condition becomes 0 1 Xo10.2
for different T values with parabolic pro-
(9)
The integration of this in Equation 2 , using Equation 8 , is straightforward. The equivalent case from SFFF (11)reduces to In ( l l 0 ) - (2Xo/0) = (7/6X0)- 2X0
0' vs. Xo
gramming, Equation 17
(6)
Approximations to Equation 6 are sometimes helpful because of its difficult form. These include two limiting forms valid a t high retention (12)
R = 6X
01
=
-T
k l = -37
- 6x0 + 12x0' ko =
T
(18a) (18b) (184 (184
Each coefficient is a function only of 7 and &, where 7 , as before is the ratio of void peak elution time to program time, L l ( v)tp. We conclude, therefore, that retention parameter 8', much like 8, depends only on XO and 7 . Plots for 8' are shown in Figure 2. If we use the simpler approximation, R = 6X, Equation 7, then we retain only the first term in the brackets of Equation 15. Upon integration, this yields an equation whose direct solution is
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
t, = t,
- AToL6t p+2 (6ut),A( ~ ) A
After rearrangement, this also takes a dimensionless form t, = t, (1 -
r/6X0
d(t,/to) --AT --
+1
dM
which parallels the simplified equation of the linear program (Equation 12). Time Lag between Injection and Programming. In the above derivations, the program is begun a t injection, t = 0. In some cases, however, it is advantageous to operate at constant A T for a period of time to maximize separation of the early components. Following this, a programmed decrease in AT is initiated to speed up the more retained components. The retention time in this case is the sum of the time lag and the programming time needed to complete elution. This can be obtained from Equation 2 by breaking the latter into its two component parts
where t, is the lag time, the time of steady field strength. The equation is useful only when t , 2 t,, that is, when the last term is 2 0 . Since R is constant up to t,, Equation 21 reduces to
L - Ro(u)t, = ( u ) I s i r R ( t ) d t
(22)
a form for which a simple theoretical interpretation becomes possible. The second term on the left is the distance travelled by the component at constant field strength. This subtracted from column length L gives the remaining length to be covered after the start of programming. The effective length, L Ro( u ) t s, simply replaces L in Equation 2. Flow Programming. For theoretical completeness, we include here the equation for a different category, flow programming, in which ( u ) varies instead of R. With AT and R constant, Equation 2 becomes
L =R
Jtr
(u)dt
(23)
in which ( u ) is an arbitrary function of time. With a linear program, ( u ) = ( u ) ~ Pt. Equation 23 integrates to
+
L = R(u)ot,
+ RPtr2/2
(24)
a quadratic expression from which retention time t, can be extracted. Flow programming is theoretically capable of solving the problem of increased spacing between peaks, but i t does not mitigate against peak dilution among the most retarded species. With adequate detector sensitivity, the method could be highly useful.
NONPROGRAMMED ELUTION SPECTRUM The above theory provides a means for calculating the elution spectrum of peaks in programmed TFFF. To this point, only qualitative arguments have suggested that the elution spectrum of nonprogrammed TFFF is unsatisfactory. We will now address this question in quantitative terms. Retention time, t,, in TFFF is described by the same general equation operative for all FFF and chromatographic systems t,lto = 1/R
The differentiation of this equation leads to
(25)
where t o is the void-peak time and retention ratio R is the function of X described by Equation 5. Parameter A, in turn, depends on molecular weight as indicated in Equation 3: h = 4/AT In the high retention limit, in which R approaches 6X (Equation 7), Equation 25 acquires the ideal form
1 124 M1/2
which shows that the peak spacing between homologues decreases with increasing molecular weight. In normal chromatography (not the exclusion form), by contrast, the peak spacing increases with each fixed increment in molecular weight, roughly according to ( 1 3 ) t,ho = (V,/V,) exp (aM) d(t,lto)/dM = (aV,/V,) exp (aM)
(29)
The exponential form makes broad-spectrum analyses in chromatography (not counting exclusion chromatography) almost impossible without programming. Equation 27 shows that TFFF is not as extreme as normal chromatography. It is, in effect, semiprogrammed by its very nature. But, as we shall demonstrate experimentally later on, programming is still highly desirable. The utility of programming in TFFF can be explained by noting that one does not try to separate successive homologues in polymer mixtures, but instead tries to isolate fractions having components within a certain percentage of the average molecular weight, for instance, those with a fixed M,IM, ratio. In this case, the relevant spacing is described by d(t,lto) = AT M1/2 d(t,/to) = -
(30) d l n M 124 Viewed in this way, spacing does increase with molecular weight. This, of course, can lead to awkward increases in the elution time of high molecular weight components, and often an associated dilution and detectability problem with the peaks. dMIM
EXPERIMENTAL The system employed here was essentially identical to that described earlier (21, employing a channel of dimensions 0.0254 cm X 2.54 cm X 39 cm. Only the temperature control and programming systems were new. Two completely different arrangements were employed for the two distinct types of programming. For linear programming, an electronic controller was designed and constructed by the departmental electronics shop. This device utilized two Yellow Springs Instruments thermistors t o measure the temperatures of the hot and cold walls. Provision was made to control either the hot wall temperature,or the actual AT of the column. The initial temperature increment, ATo, and the rate of change of A T , could both be individually set. The temperature cycled f0.75 "C around the set program. The two heaters at opposite ends of the upper bar were balanced for equal temperatures. In contrast with the elaborate electronics needed for linear programming, the parabolic controller was a purely mechanical device with no feedback. Over a limited range, AT will vary in direct proportion to power input P , where P is a function of the square of the voltage, P = V2/R.A linear voltage program will thus result in a parabolic power and temperature program. A variable speed motor (a modified G. K. Heller Model T-2) was connected through suitable gearing to the shaft of a large variable transformer. Two smaller variable transformers under the control of the large variable transformer were used to set the initial temperature and balance the heaters. Since the voltage output of a variable transformer is linear with respect to control rotation, turning the control shaft at a constant rate will drive the transformer linearly from maximum output to zero. The programming rate is varied simply by varying the speed of the motor. There was a slight time lag in AT due to thermal inertia in the parabolic system but, after taking this into account, the time vs. temperature curve followed the theoretical relationship with less than 3% error. Polystyrene samples with molecular weights of 1.8 X lo6 and below were obtained from Mann Research Laboratories and Pressure
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
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.
i. 75 iiotirs
Figure 3. Fractogram of polystyrene mixture at constant AT, 70 O C The 860 000; 1 800 000; and 7 100 000 mol wt components are not detectable. (v) = 0.023 cm/s
Figure 6 . Linear programmed run of polystyrene mixture without time lag tp = 5.75 h, ( v ) = 0.023 cm/s, ATo = 70
I+ +I* 1 hr
5.75 h r s
OC
.
Figure 4. Fractogram of polystyrene mixture using parabolic programming with a 1-h time lag between injection and the start of program AT, = 70 ' C , (v) = 0.023 cm/s, f,= 1.0 h, t, = 6.0 h. First spike is void peak
Figure 5. Parabolic programmed run of polystyrene mixture without time lag at start fp = 5.0 h, (v) = 0.023 cm/s, AT, = 70 O C
Chemical Co. The 7.1 X 106 molecular weight sample was supplied by Duke Standards.Ethyl benzene supplied by Aldrich Chemical Co. was used as solvent in these studies. RESULTS AND DISCUSSION Field-strength programming in thermal FFF was studied using a nine-component mixture of polystyrenes ranging in molecular weight from 4000 to 7 100 000-a weight ratio of nearly 1800:l. Without programming, it has not been possible to adequately resolve more than half of these components in a single run. The reasons are the expected: high AT's fail with high mol wt components, and low AT's fail a t low mol wt. Neither can span the entire range necessary. A typical non1590
Figure 7. Linear programmed run of polystyrene mixture with a time lag of 1 h between injection and start of the program fp = 5.75 h, total time = 6.75 h, ( v ) = 0.023 cm/s, AT0 = 70 OC programmed run, showing the loss by dilution of the higher mol wt species, is shown in Figure 3. The effect of programming on the resolution of this mixture is shown in Figure 4; all of the components are now resolved in a reasonable length of time. Resolution and detectability are improved and peak spacing is more uniform. The fractogram is therefore more tractable to qualitative and quantitative analysis. Our first experimental study of programming parameters compares linear and parabolic programs. Figures 5 and 6 show the separation achieved by the two types. Although the parabolic program has a slightly longer duration (6 h vs. 5.75 h for the linear program), the faster initial AT drop causes all components to elute sooner and therefore gives the low mol wt components a somewhat poorer resolution. The higher mol wt components, however, have somewhat better resolution in the parabolic case. The rationale for this was explained in the theory section: the parabolic program provides a more gradual approach to AT = 0 and thus elutes the highly retained species less abruptly. In fact, differentiation of Equation 13 shows that (dAT/dt) approaches zero as t approaches the end, t,, of the parabolic program. In Table I, the measured elution times are compared to the theoretical values calculated from Equations 10 and 17. The agreement is quite satisfactory considering the assumptions used: the approximate R expression of Equation 8; the neglect of the slight deviations from Equation 4 due to nonconstant thermal conductivity and viscosity; the assumption that molecular weight trends valid to 160 000 mol wt extrapolated to 7 000 000 mol wt. In order to mitigate the latter problem,
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
I
-1-1
Table I. Comparison between Measured and Calculated Elution Times of Polystyrene Polymer for Two Types of Programming in Thermal FFFn Linear program Parabolic program
3 hrs-4
Exptl, Calcd, Exptl, Calcd, Sample h h Error,% h h Error, % 4000 0.51 ... 0.51 ... f2.9 0.72 0.76 +5.1 20 400 0.73 0.75 -1.3 1.03 1.08 +5.3 1.07 51 000 1.08 -5.1 1.34 1.42 +6.0 97 200 1.51 1.43 1.78 1.92 f7.7 -2.9 200 000 2.05 1.99 -10.3 2.60 2.54 -2.3 411 000 3.03 2.72 3.62 -15.2 3.53 3.27 -7.4 860 000 4.28 -13.6 4.36 4.02 -7.8 1800 000 5.23 4.52 5.63 -4.5 5.34 5.31 -1.0 7 100000 5.90 a Parameters are A T 0 = 70 "C, ( u ) = 0.023 cm/s, t , = 6.0 h (parabolic) and 5.75 h (linear). I
Figure 8. High-speed run of polystyrene mixture with parabolic program Total time = 3.75 h, tp = 3.0h, fs = 0.75h. ( v ) = 0.45 crn/s, AT, = 70 O C
Table 11. Comparison between the Experimental and Calculated Elution Times for the Two Types of Programming in Thermal FFFn Linear program Parabolic program tr
-
3 hrs
*
tr
Exptl, Calcd, Exptl, Calcd, Sample h h Error,% h h Error,% 4000 0.52 .. . 0.52 ... -1.3 +5.8 0.75 0.79 20 400 0.78 0.77 f0.9 1.15 1.19 +3.5 51 000 1.14 1.15 -4.8 1.62 1.62 97 200 1.66 1.58 0 200 000 2.22 2.24 t0.9 2.20 2.25 +2.2 -10.4 3.15 3.01 -4.5 411 000 3.47 3.11 -21.1 4.18 3.87 -7.3 860000 5.32 4.18 5.14 4.72 -8.2 5.16 -17.2 1800000 6.22 -4.1 6.19 6.08 -1.7 7 100 000 6.84 6.56 The program start in each case was delayed for 1 h after sample injection. AT0 = 70 "C, ( v ) = 0.023 cm/s, t , = 6.0 h (parabolic) and 5.75 h (linear). however, we have used an improved equation for the trend in the thermal diffusion factor derived from our studies of thermal diffusion (14).Using A = T/(thermal diffusion factor), we get
(4.47
1 A = T / M0.6 --
I
Figure 9. High-speed run of polystyrene mixture with linear program tp = 3 h, ( v )
__
= 0.045 cm/s, ATo = 70 OC
2000
This equation was used for all calculated tr values in this paper. Next, a series of experiments was conducted on the effect of the time lag between injection and the initiation of the programmed decrease AT. Figure 7 shows the results of delaying the linear program for 1 h after injection. Figure 4 shows the corresponding outcome in the parabolic case. Time lags from 15 min to 2 h were tried. The optimum appeared to be about 1h for the parabolic program. A comparison of Figures 4 and 5 shows that the 1-h time lag improves the resolution of the low mol wt components with no loss of resolution for the others. The linear program (Figure 7 ) ,by contrast, shows a marked peak spreading for the middle components (411 000 and 860 000 mol wt) with a 1-h time lag. In fact, all of the time lags attempted had negative effects with the linear program. It appears generally that linear programming works best if the
program starts at injection, while parabolic programming profits by a time lag. The length of the optimum time lag will depend, of course, on the mol wt distribution of the solute. A comparison of the actual calculated elution times with time lag is shown in Table 11. The results, again, are good. Finally, we investigated the possibility of speeding up the program by doubling the flow rate and halving the program time. Elution times were cut in half. Various time lags were also tried. Again, any time lag in the linear case broadened the middle peaks. The parabolic runs were improved, as expected, by a moderate time lag; 45 min appeared to be about optimum. With this, all components were reasonably separated in a time of 3 h and 45 min. Figures 8 and 9 show the best of these separations achieved for each type of program. Indeed, some resolution has been lost, but the separations are still satisfactory, particularly in the parabolic case, for these high speed runs.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
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The programmed system has not been pursued to its theoretical optimum because the latter is not clearly defined. Nonetheless, programming has succeeded here in extending the molecular weight range of a single run by a very large factor. It has also yielded favorable peak capacities and reasonably fast elution. The promise is clearly such that the method merits further study.
LITERATURE CITED
(7) E. Grushka, K. D. Caldwell, M. N. Myers, and J. C. Giddings, in “Separation and PurificationMethods”, Vol. 2, E. S. Perry, C. J. Van Oss, and E. Grushka, Ed., Marcel Dekker, New York, 1974. (8) J. C. Giddings, F. J. F. Yang, and M. N. Myers, Anal. Chem., 46, 1917(1974). (9) W. E. Harris and H. W. Habgood, “Programmed Temperature Gas Chromatography”, Wiley, New York, 1966. (10) L. R. Snyder and J. J. Kirkiand, “Modern Chromatography”, Wiley, New York, 1974. (11) F. J. F. Yang, M. N. Myers, and J. C. Giddings, Anal. Chem., 46, 1924 (1974). (12) J. C. Giddings, J. Chem. Educ., 50, 667 (1973). (13) J. C. Giddings, J. Gas Chromatogr., 5, 143 (1987). (14) J. C. Giddings, K. D. Caldwell, and M. N. Myers, Macromolecules, 9, 106 (1976).
(1) G. H. Thompson, M. N. Myers, and J. C. Giddings, Anal. Chem., 41, 1219 (2) (3) (4) (5) (6)
(1969). M. N. Myers, K. D. Caldwell, and J. C. Giddings, Sep. Scl., 9, 47 (1974). J. C. Giddings, Y. H. Yoon, and M. N. Myers, Anal. Chem., 47, 126 (1975). J. C. Giddings, L. K. Smith, and M. N. Myers, Anal. Chem., 47,2389 (1975). J. C. Giddings, Sep. Sci., 1, 123 (1966). K. D. Caldwell, L. F. Kesner, M. N. Myers, and J. C. Giddings, Science, 176, 296 (1972).
RECEIVEDfor review January 28, 1976. Accepted May 10, 1976. This investigation was supported by National Science Foundation Grant MPS74-05260 A03.
Separation and Concentration of Azo Compounds with Crosslinked Poly(vinylpyrro1idone) Thomas Mourey, Alan P. Carpenter, Jr., Sidney Siggia,* and Alan Lane Department of Chemistry, University of Massachusetts, Amherst, Mass. 0 1002
The interaction of several azo compounds In aqueous solution with water insoluble, cross-linked poly(vinylpyrro1idone) was examined. Studies were performed with one plate batch equilibrations and percent uptake was determined spectrophotometrically. Results showed that removal from solution of azo compounds containing polar groups is favorable when the compounds are in their nonionic form. It is believed that the binding of these polar aromatics to poly(vinylpyrrol1done) is a function of azo solubility and degree of aromaticity. Several azo compounds containing basic and/or acidic groups were studied to better elucidate the selectivity of the poly(viny1lactam) resin. The influence of hydroxyl, amino, carboxylic, sulfonic groups on the ring is to enhance the take-up of the dye. Removal of azo compounds with one plate equillbratlons was favorable enough to use poly(vinylpyrrol1done) for the concentration of aqueous solutions. Removal by columns was extended to the part-per-billion region, and the azos were effectively eluted with ethanol and quantitated using ultraviolet and visible absorption spectrophotometry.
The interaction of poly(vinylpyrro1idone) (PVP) with azo dyes has been utilized extensively over the past several years, particularly by the textile and plastic industries. The strong affinity of many azo compounds for polymers with the poly(vinyllactam) backbone has been used to improve the dye receptivity of several fibrous and molded polymers which are normally unreceptive to azo dyes in conventional dyeing processes. Dye affinity for polyesters ( I ) , and blends of cotton and viscose rayon with nitrile alloy fibers ( 2 ) ,is increased upon addition of 5-40% by weight of poly(vinylpyrro1idone) or its copolymers. Graft copolymers in which PVP is the linear side chain, such as in Nylon 6 or 66 have improved dyeability ( 3 ) , and graft copolymers of PVP with acrylonitriles produce dye receptive, stabilized fibers (4-6).Surface grafting or “ring dyeing” (7), where the dye resistant polymer is passed through a bath of PVP before wet spinning or molding, is also a common method of producing a thin dye receptive coating of 1592
*
“ring” poly(vinyl1actam). Uniform dyeing of rubber coated fabrics (8) as well as poly(viny1 chloride) (9) is obtained by modification of natural rubber, neoprene, or styrenehutadiene with 1-6% PVP, and polymerization of vinyl chloride with less than 200h poly(vinylpyrro1idone).Water soluble PVP is used as a solubilizing agent to keep azo compounds in solution and maintain fluidity in the preparation of dye baths ( l o ) ,as well as in fugitive tinting (11),in which aqueous solutions of the dye and PVP are impregnated in the fiber simultaneously to ensure even tinting. Poly(vinylpyrro1idone) alone is used extensively as a dye stripping agent for cellulose fibers dyed with sulfur, vat, and direct dyes (12,13). Industrial applications of poly(vinylpyrro1idone) extend far beyond the few mentioned above. Yet the selective binding of aromatic organics such as azo compounds as a function of polar substituents, solubility, number of aromatic rings, and hydrogen ion concentration is not completely understood. Cross-linked poly(vinylpyrro1idone) has been used in the chromatographic separation of flavanoids (14), anthocyanidinglucosides (15),anthocyanins (16,17),and aromatic acids, aromatic aldehydes, and phenols (18) in aqueous media. I n all cases, binding of the solutes to PVP was attributed mainly to hydrogen bonding between proton donating groups and the carbonyl oxygen on the lactam ring. The effective separation of polycyclic aromatic hydrocarbons in isopropanol using cross-linked PVP has most recently come to our attention (19). Obviously hydrogen bonding cannot be the binding mechanism, indicating that a t least two, and possibly three or more interactions between the solutes and poly(viny1pyrrolidone) are responsible for sorption. This apparently complex binding mechanism has made a complete characterization of the resin’s selectivity for certain aromatics difficult. As a result its full potential as an analytical tool has yet to be realized. The present study investigates the removal of azo compounds from aqueous solution by water insoluble, cross-linked poly(vinylpyrro1idone). One plate batch equilibrations of several azo compounds in the 10 ppm range, limited to con-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
