theoretical limits hold promise for an extremely versatile, sensitive, and selective analytical system. The Eimac illuminator, when used as a light source for excitation of fluorescence from molecules in the condensed phase, has been shown to give a signal-to-noise advantage over a standard Hanovia xenon arc lamp in an ellipsoidal condensing system, used under identical conditions. The cost of the Eimac system, including lamp, lamp housing, and power supply, i s approximately equal to that of the ellipsoidal condensing system (lamp and power supply are purchased separately at an additional cost). Also, the collimating mirror, being an integral part of the Eimac lamp, is replaced each time the lamp is changed. Replacement Eimac lamps are 1.5 times more expensive than standard xenon arc lamps, and lifetimes for both lamps are approximately equal (75% of peak radiant flux at 1000 hr). As with any more intense light source, the Eimac lamp will destroy photosensitive molecules more quickly than the Hanovia source, if both are used a t the same excitation monochromator slit settings. However, the photon flux of the Hanovia system can be equalled by the Eimac system at narrower slit widths, giving greater resolution and selectivity. If incorporated into a high-pressure liquid chromatographic detection system, in which the dwell-time of each molecule in the light beam is relatively short, photodecomposition becomes a very minor, or nonexistent, problem, and the full light flux can be used to decrease detection limits. The analysis of ergotaminine, presented in this study, shows that high-pressure liquid chromatography can be as sensitive as gas-liquid chromatography. Although there are more sophisticated means of increasing the sensitivity of fluorescence analysis, such as ratio recording, photon counting, and synchronizing amplification and excitation frequencies, the least expensive method is to increase the usable photon flux through the excitation monochromator. The Eimac illuminator does this by simply capturing more of the radiation which is available in the xenon arc. If addit,ional S/N enhancement is desired, the more sophisticated methods can again be applied.
LITERATURE CITED (1)P. Brazeau, in "The Pharmacological Basis of Therapeutics," 4th ed., L. S. Goodman and A Gilman. Ed.. The Macrnillan Co., New York, N.Y., 1970,pp 898-900. (2)J. R. Graham and H. G. Wolff, Arch Neurol. Psychiatry, 39, 737 (1938). (3)H. Ernmenegger and W. Meier-Ruge, Pharmacology, 1, 65 (1968). (4)T. G. Alexander, J. Assoc. Off. Agr. Chem., 43, 224 (1960). (5) J. L. McLaughlin, J. E. Goyan, and A. G. Paul, J. Pharm. Sci., 53, 306
(1964).
(6) S.Keipert and R . Voigt, J. Chromatogr., 84, 327 (1972). (7)R. Fowler, P. J. Gomm, and D. A. Paterson, J. Chromatogr.. 72, 351 (1972). (8) S.Agurell and A. Ohlsson, J. Chromstogr.,81, 339 (1971). (9)J. Axelrod, R. 0. Brady, B. Witkop, and E. V. Evarts, Ann. N.Y. Acad. Sci., 86, 435 (1957). (IO) G. K . Aghajanian and 0. H. L. Bing, Clin. Pharmacol. Ther., 5 , 611 (1964). (11) D. G. Upshall and D. G. Waiiling. Clln. Chim. Acta., 38, 67 (1972). (12) A. Stoll and W. Schlientz, Helv. Chim. Acta, 38, 585 (1955). (13)A. Bowd, J. B. Hudson, and J. H. Turnbull, J. Chem. SOC.,Perkin Trans. 2,I O , 1312 (1973). (14)W. D. Hooper, J. M. Sutherland, M. J. Eadie, and J. H. Tryer, Anal. Chim. Acta, 89, 11 (1974). (15)I. Jane and B. B. Wheals, J. Chromatogr., 84, 181 (1973). (16)R. A. Heacock. K. R. Langille, J. D. MacNeil, and R . W. Frei, J. Chromatogr., 77,425 (1973). (17)J. D. Wittwer and J. H: Kluckhohn, J. Chromatogr. Sci., 11, 1 (1973). (18) R. D. Venn, Hanover, N.J., personal comrnunicatlon, August 1973. (19)P. T. Kissinger, C. Refshauge, R . Dreiling, and R. N. Adams, Anal. Lett.. 8,465 (1973). (20)P. T. Kissinger, C. Refshauge, R . Dreiling, and R. N. Adams, ACS Abstracts, 166th National Meeting, Chicago, Ill., August 26-31, 1973. (21)C. Refshauge, P. T. Kissinger, R. Dreiling, L. Blank, R. Freeman, and R. N. Adams. Life Sci., 14, 31 1 (1974). (22) P. T. Kissinger, L. J. Felice, R. M. Riggin, L. A. Pachia. and D. C. Wenke, Clin. Chem., 20,992 (1974). (23)E. D. Peilizzari and C. M. Sparachino, Anal. Chem., 45, 378 (1973). (24) D. D. Chilcote and J. E. Mrochek, Clin. Chem., 18, 778 (1972). (25)R. W. Frei, J. F. Lawrence, J. Hope, and R. M. Cassidy, J. Chromatogr. Sci., 12,40 (1974). (26) J. F. Lawrence and R. W. Frei, J. Chromatogr., 98, 253 (1974). (27)R. M. Cassidy, D. S. LeGay, and R. W. Frei. J. Chromatogr. Sci., 12, 85 (1974). (28)W. Dunges, G. Naundorf. and N. Seiler, J. Chromatogr. Sci., 12, 655 (1974). (29)R. W. Frei and J. F. Lawrence, J. Chromatogr., 83, 321 (1973). (30)R. M. Cassidy and R . W. Frei, J. Chromatogr., 72,293 (1972). (31)D. R. Baker, R. C. Williams, and J. C. Steichen. J. Chromatogr. Sci., 12, 499 (1974). (32)H. Hatano, Y. Yarnarnoto, M. Saito, E. Mochida, and S. Watanabe, J. Chromatogr., 83, 373 (1973). (33)J. C. Steichen, J. Chromatogr., 104, 39 (1975). (34)M. P. Bratzel, Jr.. R. M. Dagnall, and J. D. Winefordner, Anal. Chim. Acta, 52, 157 (1970). (35)R. L. Miller, L. M. Fraser, and J. D. Winefordner, Appl. Spectrosc., 25, 477 (1971). (36)F. W. Piankey, T. H. Glenn, L. P. Hart, and J. D. Winefordner, Anal. Chem., 46, 1000 (1974). (37) D. J. Johnson, F. W. Plankey, and J. D. Winefordner, Anal. Chem., 48, 1898 (1974). (38)L. H. Luthjens, Rev. Sci. hsfrum., 44, 1661 (1973). (39)T. C. O'Haver and J. D. Winefordner, J. Chem. Educ., 48, 241 (1969). (40)V. E. Dell'Ova, M. B. Denton, and M. F. Burke, Anal. Chem., 48, 1365 (1974). (41)W. H. Melhuish, J. Opt. SOC.Am., 52, 1256 (1962) (42)A. Mondino, G. Bongiovanni. S. Fumero, and L. Rossi, J. Chromatogr., 74,255 (1972). (43)P. A. St. John, W. J. McCarthy, and J. D. Winefordner, Anal. Chem., 39, 1495 (1967).
RECEIVEDfor review May 5, 1975. Accepted July 7 , 1975. This work was supported in part by the U.S. Public Health Service Grant No. GM-11373-11, and the Epilepsy Research Foundation of Florida, Inc.
Quantitative Analysis of Cation-Exchange Resins by Infrared Spectrophotometry and Pyrolysis-Gas Chromatography J. R. Parrish Department of Chemistry, Rhodes University, Grahamsto wn, South Africa
Two series of sulfonic acid resins, one with varying capacity and one with varying cross-linking, have been synthesized. These were used to develop methods for the determination of capacity as well as cross-linking by pyrolysis-gas chromatography and infrared spectrophotometry. The relative amount of styrene produced by pyrolysis of a resin is inverseiy related to its capacity, and capacity can be deter-
mined over a small range with a standard deviation of 0.1 mequiv g-'. Capacity can be determlned over a wider range with a standard deviation of 0.2 mequiv g-' by measurement of the IR absorbance at 1008 cm-'. Cross-linking is more difficult to estimate, and variations of published methods are compared. The standard deviations lie in the range 0.4-296 dlvlnylbenzene.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
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Table I. Resin Capacities Theoretical
01
m 0 P
capacity,
DVB,w l w
mequiv g - 1
mequiv g - 1
2.02 4.03 6.13 8.08 10.12
5.36 5.30 5.24 5.17 5.10
5.36 5.29 5.25 5.17 5.06
EXPERIMENTAL
u)
0 LI
I
I
I
10
15
20
Minutes Flgure 1. Xylene isomers in pyrograms of a cation-exchange resin with 10% cross-linking (A), and of the anion-exchange resin AG 1 X8, (B). Peaks are (1) ethylbenzene, (2) pxylene, (3) mxylene, and
o-xylene
In a previous paper ( I ) , it was shown that pyrolysis-gas chromatography could be used to estimate the degree of cross-linking of ion-exchange resins, and it was suggested that the yield of styrene from the pyrolysis of sulfonic acid resins was inversely related to the ion-exchange capacity. In an independent study using a capillary column for the separation of the pyrolysis products, Blasius et al. ( 2 ) showed that the degree of cross-linking of sulfonic acid resins could be estimated from the amount of divinylbenzene or of ethylvinylbenzene produced by pyrolysis. These authors found that some batches of commercial resin gave anomalous results, which they attributed to differences in the original particle size, although the resins were powdered before analysis. No estimates of precision were published. Measurements of infrared absorbance have also been used for the quantitative analysis of cation-exchange resins. The capacity of carboxylic acid resins could be estimated from the absorbance a t 1710 cm-l, but the precision was poor ( 3 ) .The published figures indicate that the standard deviation in capacity from a single absorbance measurement was 0.9 mequiv g-l. The cross-linking of sulfonated polystyrene resins has been estimated from the absorbance a t 1605 cm-' ( 4 ) ,but Whittington and Millar ( 5 ) have suggested that this absorbance correlates with the decreased extent of sulfonation of commercial resins as the cross-linking increases, rather than with the percentage of divinylbenzene (DVB) used in manufacture. They showed that the absorbance at 1092 cm-l, characteristic of a 1,2,4-substituted aromatic sulfonic acid, could be used to estimate the amount of DVB in highly cross-linked sulfonic acid resins, but they stated that "quantitative work is necessarily of somewhat low accuracy." In this laboratory, sulfonated polystyrene resins were specially prepared for examination by pyrolysis-gas chromatography and by IR spectrophotometry. These techniques have been compared for the estimation of capacity and of cross-linking. 2000
capacity,
?
c
(4)
Experimental
Percentage
Apparatus. The filament pyrolyzer and gas chromatograph have been described previously ( I ) . The column used to separate the xylene isomers was 180 cm long, 5-mm inside diameter, and contained Gas-Chrom P (70-80 mesh) loaded with 5% Bentone 34 and 5% dinonylphthalate. The temperature was 100 "C, and the rate of flow of nitrogen was 60 ml min-'. Retention times are shown in Figure 1. For most of the work, a 125-cm column, 3-mm inside diameter, of 10% polyphenyl ether (5 rings) on Celite was used under the same conditions, since it gave shorter retention times ( I ) . Approximately 0.5-mg samples of resin were taken for pyrolysis. IR spectra in the range 2200-500 cm-' were recorded on a Perkin-Elmer Model 180 infrared spectrophotometer, used in the linear absorbance mode. A Wilks Mini-press was used to press the KBr discs, and a Grindex mill (Research and Industrial Instruments Co.) was used to grind the samples. Reagents. Cross-linked polystyrene resins were made from freshly distilled styrene and commercial 54% divinylbenzene (Koch-Light). The latter was not distilled, but was extracted with dilute sodium hydroxide solution to remove inhibitor, and was then washed and dried. This procedure was not expected to alter the ratio of m- to p-divinylbenzene, which is about 7:3 in the commercial product. All resins were made from the same batch of divinylbenzene, so that the relative amount of cross-linking was exactly known. The resins contained 2, 4, 6, 8, and 10% divinylbenzene (DVB) by volume, and the percentage by weight in each sample was determined by direct weighing of the monomers. All resins were prepared in bead form, mostly by Pepper's technique ( 6 ) ,except for the 8% DVB resin. This was made in a larger amount (200 ml) by suspension polymerization as described by Millar (7). The first series of ion-exchange resins made from the hydrocarbon copolymers consisted of fully sulfonated resins with different cross-linking. Full sulfonation was obtained as follows. The polystyrene resin (5 g) was allowed to swell in 1,2-dichloroethane (1020 ml). Then 7% oleum (25 ml) was added. After standing for 1 hr a t 20 "C, the mixture was heated for 20 min on a boiling water bath, and was then poured onto crushed ice. The sulfonated resins contained negligible amounts of sulfone groups as shown by the very small absorbance at 1350 cm-', and the uncross-linked benzene rings had all been sulfonated as shown by the excellent agreement between experimental and theoretical ion-exchange capacities (Table I). A second series of ion-exchange resins was made by partial sulfonation of 8% cross-linked polystyrene. Capacities of 3-5 mequiv g-' were obtained by treatment of the swollen beads with oleum a t 24-45 "C. For capacities of 0.2-2.5 mequiv g-l, the swelling in 1,2dichloroethane was omitted, the dry beads being heated with sulfuric acid or oleum for various times. All resins were converted into the sodium form, sieved, converted into the hydrogen form, washed and dried at 110 O C overnight. Capacities were determined by titration of a weighed aliquot of the dry hydrogen form with standard 0.1M sodium hydroxide in the presence of sodium chloride (18). The hydrogen form of the resins was used for pyrolysis-gas chromatography, but for IR spectrophotometry the potassium form was used to avoid any possibility of ion-exchange during the pressing of KBr discs (8). Resins in the hydrogen form were treated with 1 M KC1 solution until the pH of the effluent rose above 4, were washed with water until no chloride ion could be detected, and were dried a t 110 "C overnight. Procedure. Pyrolysis-gas chromatography was carried out on unweighed samples of resin as previously described (11. For quantitative IR spectrophotometry, an internal standard of potassium thiocyanate was used as described by Wiberley et al. (9). A single batch of KBr containing 0.2% KSCN was used to make all the discs, and each resin was diluted with this mixture in two stages.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
The final concentration of resin in the discs was about 1%,except when 4% resin was used to increase the small absorbance at 1092 cm-'. Times of grinding were carefully controlled to ensure uniformity, but since the efficiency of grinding depends also on the design of the mill and the mass of the sample, the particle size after the final grinding was checked microscopically. The particle diameter was below 20 fim as recommended by Strasheim and Buijs (10). The liquid-grinding technique used by these authors was found to be unnecessary, provided that the resins were finely ground before mixing with KBr. Discs were pressed for 2 min, and the spectra were recorded immediately after pressing. RESULTS AND DISCUSSION Estimation of Cross-Linking by Pyrolysis-Gas Chromatography. I t was shown previously (I) that the crosslinking of sulfonated polystyrene resins could be estimated from the relative height of the ethylbenzene peak in the pyrogram. Since this peak represented 0-,m-, and p-xylene as well as ethylbenzene, it was thought that separation of these isomers might lead to a more accurate estimation. When a column designed to separate these isomers was used, the main product was found to be ethylbenzene, with little o-xylene and very little m- and p-xylene. In contrast, the pyrolysis of anion-exchange resins gave mainly p -xylene, with a little ethylbenzene and very little 0 - and m-xylene (Figure 1). Presumably, the o-xylene from cation-exchange resins resulted from a rearrangement. The height of the o-xylene peak relative to benzene was too small (0.02) to be used as a measure of cross-linking, although it did increase with increasing cross-linking. The relative height of the ethylbenzene peak was measured for a series of four commercial resins (AG 50W-X2-AG 50W-X12), and a linear plot was obtained vs. the nominal percentage of DVB. The correlation coefficient, r = 0.995, was significant ( P < 0.01), and the standard deviation in percentage DVB obtained from a single measurement of relative peak height was estimated to be 0.7% for a resin with 7% DVB. However, when determinations of DVB in the same batch of a commercial resin (nominally 8% DVB) were done repeatedly, the measured standard deviation was 1.8%. This deterioration in precision was apparently caused by changing pyrolysis conditions, since the geometry and electrical resistance of the filament changed during use. For better precision, standard resin samples could be pyrolyzed before and after the unknown sample. When a general purpose column was used, ethylbenzene and the xylenes were not separated, but the plot of the height of the ethylbenzene peak (relative to benzene) was linear over the range 2-8% DVB. The estimated standard deviation in percentage DVB as obtained from a single measurement of relative peak height was 0.75%. Again the accuracy deteriorated if determinations were repeated without recalibration with standard resins. For this experiment, the series of laboratory-prepared fully sulfonated resins was used, and repeated determinations of the percentage DVB in one of these showed a standard deviation of 2.2%. This corresponded to a relative standard deviation of 28 pph, and was no better than the relative standard deviation of 24 pph in DVB content (equivalent to 12 pph in relative peak height) which was reported previously ( I ) for a commercial resin. This showed that the poor precision was not the result of the heterogeneity of commercial resin samples. When the height of the ethylbenzene peak was measured relative to toluene, instead of to benzene, the estimated standard deviation in percentage DVB was slightly better (0.6%). The difficulty in the determination of DVB by this method is the lack of sensitivity. Extrapolation to zero cross-linking showed that only half of the ethylbenzene produced by pyrolysis of an 8%cross-linked resin can be attributed to the cross-linking. Although slightly better pre-
n
0
1
3
Capacity
5
meq/g
Flgure 2. Plot of ion-exchange capacity vs. relative height of the styrene peak in pyrograms of cation-exchange resins
Table 11. P e a k Heights of Styrene Relative to Benzene in the Pyrograms Resin capacity, mequiv g-'
5.17 4.57 3.43
Styrenelbenzene
0.13
0.72 3.00
cision was obtained by separating ethylbenzene from the xylenes, at least twice the time (16 min instead of 7-8 min) was required for each determination. Capacity by Pyrolysis-Gas Chromatography. Since pyrolysis of polystyrene gives a high yield of styrene, whereas pyrolysis of sulfonated polystyrene resins gives benzene as the highest peak in the pyrogram ( I ) , the height of the styrene peak relative to benzene should be indicative of the proportion of unsulfonated benzene rings in a partially sulfonated resin, Le., it should vary inversely with the capacity of the resin. This suggestion could not be proved from the data previously published (I), because the crosslinking as well as the capacity varied in the commercial resins used. It was necessary to use a series of resins varying in capacity but constant in cross-linking, made by partial sulfonation of samples of the same batch of 8% cross-linked resin. Figure 2 shows the results obtained by pyrolysis. The relative amount of styrene to benzene decreased as the capacity increased, and could be used as a sensitive measure of capacity (Table 11).The standard deviation in capacity was 0.09 mequiv g-' for the resin of capacity 4.57%mequiv g-l. This precision was obtained in repeated determinations with the same pyrolyzer filament, and with approximately the same size of unweighed sample. When the filament was changed, the amount of styrene produced changed, and recalibration was necessary. This method for the determination of capacity could be used to check resins for full sulfonation during manufacture, since it is most precise over the range of capacities of interest, and over a very short range the calibration curve could be assumed to be approximately linear. In the resins tested, sulfonic acid groups were the only substituents present in measurable amounts in the polystyrene chains, but if a resin contained appreciable amounts of other groups, such as sulfone, nitrile, or carboxyl groups,
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
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il
Table 111. Absorbances of Resins at 1600 cm-1 and at 1410 cm-1
I
m
1
Percentage
Capacity,
DVB, w / w
A at 1600 c m - l
A a t 1410 c m - I
mequiv 9-1
A at 2065 cm-’
A a t 2065 c m “
2.02 4.03
5.36 5.29
0.118 0.128
0.227 0.215
8.08 8.08 8.08 a. 08 8.08
4.57 3.4 3 2.41 1.72 1.37
0.135 0.149 0.184 0.185 0.200
0.194 0.154 0.091 0.082 0.067
u
Wave number crn-l
Flgure 3. IR spectrum of 8 % cross-linked cation-exchange resin with internal standard
incorrect values might be obtained for the capacity, since the method actually measures the concentration of unsubstituted polystyrene. The presence of sulfone, nitrile, or carboxyl groups can be detected by IR spectrometry. IR Spectrophotometry. The use of potassium thiocyanate as an internal standard in potassium bromide discs has been criticized by Dechant (11)on the grounds that the intensity and structure of the nitrile band depended on the duration of the milling. He claimed that the absorbance varied by 5-10% under “approximately constant” conditions. However, when the convenient “Mini-press” was used, a constant thickness of disc could not be ensured, and an internal standard was necessary. Potassium thiocyanate used as described in the literature (9) was found to be satisfactory. The absorbance of the band at 2065 cm-l was measured. A sharper band of comparable intensity a t 2078 cm-l was present in all the spectra (Figure 3). The spectrum of potassium thiocyanate has been studied by Jones (12) who reported that “samples prepared in KBr discs showed anomalous absorption bands,” but further studies of the absorption spectra of thiocyanate ions in alkali halide discs and crystals have been made recently (13-15).In the present work, the reliability of the internal standard was tested by preparing a series of discs from the same sample of resin. The relative standard deviation of the absorbance of the resin band a t 1008 cm-l relative to the absorbance of the thiocyanate band at 2065 cm-l (the resin absorbance being normalized to 1%concentration of resin), was 4 pph. Estimation of Cross-Linking by I R Spectrophotometry. Although Kvedar ( 4 ) has used the absorbance at 1605 cm-l to estimate the percentage of DVB in ion-exchange resins, the figures in Table I11 show that there is no significant correlation ( r = 0.469) with the DVB content of fully sulfonated resins. The absorbance of this peak, which occurred at 1600 cm-’ in the present spectra, showed an inverse linear relationship ( r = -0.986, P < 0.01) to the capacity of the 8% cross-linked resins, thus confirming the suggestion ( 5 ) that the correlation previously reported is with the decreased extent of sulfonation of commercial resins as the cross-linking is increased. An analytical method for cross-linking which depends on uniformly poor sulfonation of the commercial products i s undesirable, if not unreliable. Strasheim and Buijs (10) tentatively assigned the band a t 1416 cm-l (1410 cm-l in the present spectra) to crosslinks, since it was found in the spectra of both cation- and anion-exchange resins, but not in the spectrum of linear polystyrene. The results in Table I11 prove that this assignment was incorrect. There is no significant correlation with the DVB content ( r = -0.673), but the absorbance a t 1410 cm-l is linearly related to capacity (r = 0.994, P < 0.001). This peak was absent from the spectrum of 8%cross-linked polystyrene, so it must have arisen from p-sulfonated ben2002
zene rings. It could be used to determine capacity, but other peaks are better for the purpose. When spectra of the usual intensity (with 1%resin in potassium bromide) were recorded, the only measurable significant correlation with percentage DVB was a negative correlation ( r = -0.821, P < 0.01) with the absorbance of the strong band at 1127 cm-l. This band is believed to arise from p-substituted aromatic rings (5). Since the incorporated divinylbenzene residues were apparently not sulfonated, as shown by the capacities in Table I, this correlation is understandable, and so is its low sensitivity to changes in DVB content. The standard deviation in percentage DVB determined in this way was estimated to be 1.8%.The absorbance a t 1127 cm-l was about six times the absorbance at 1410 cm-l, and could be more accurately measured. This may account for the lack of a significant correlation of percentage DVB with absorbance a t 1410 cm-l, and it suggests that a negative correlation with this absorbance might be found if much more intense spectra were measured. Whittington and Millar ( 5 ) used 2% resin in potassium bromide to show that the absorbance a t 1092 cm-l, characteristic of a 1,2,4-substituted sulfonic acid, correlated with the percentage DVB in highly cross-linked resins. They found similar correlations with the even weaker absorbances at 890 cm-l and 795 cm-l. Unfortunately the present study has shown that when 1%of resin was used, and when the resin contained less than 8% DVB, the absorbance at 1092 cm-l appeared only as a shoulder, too small for accurate measurement. Potassium bromide discs containing 4% of resin were made, and found to be sufficiently translucent. The relative absorbance a t 1092 cm-l was measured for resins with 4-10% DVB, but could not be measured reliably for the resin with 2% DVB. The plot of relative absorbance against percentage DVB was linear ( r = 0.996, P < 0.01), and the standard deviation in percentage DVB from a single measurement of relative absorbance was estimated to be 0.35% for a 7% cross-linked resin. Of the methods considered, this is the most precise for the estimation of cross-linking, but it is still indirect, since the measured absorbance arises from sulfonated polymerized ethylvinylbenzene residues from the commercial DVB, and the concentration of these may not always correlate with the true DVB content ( 5 ) . Capacity by IR Spectrophotometry. The absorbance of any of the sulfonate bands gave a linear plot vs. the capacity of the resins, and the best results were obtained by using the relative absorbance at 1008 cm-l. Here r = 0.997, P < 0.001, and the standard deviation in capacity from a single absorbance measurement was estimated to be 0.2 mequiv g-l. This value was confirmed by repeated determinations of the capacity of resins with 6% and 8% DVB.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
1
The precision is better than that obtained for the estimation of the capacity of carboxylic acid resins ( 3 ) . The capacity of sulfonic acid resins could be estimated more rapidly by measuring the absorbance at 1008 cm-l relative to the absorbance of the resin a t 1450 cm-l. In this way, the necessity for accurate weighing and the use of an internal standard of potassium thiocyanate could be eliminated. However, the calibration curve (Figure 4) was no longer linear. For resins of the same cross-linking, the band a t 1127 cm-l, indicative of p-substitution, could be used to determine capacity. The absorbance was measured relative to that of the internal standard a t 2065 cm-l, and a linear relationship to capacity was found (7 = 0.996, P < 0.001). The standard deviation in capacity was estimated to be 0.2 mequiv g-l, and repeated determinations using a resin with 8%DVB gave a figure of 0.15 mequiv 8-l.
CONCLUSIONS Both the capacity and cross-linking of sulfonic acid resins could be measured by IR spectrophotometry with a relative standard deviation of 5 pph under the best conditions. Pyrolysis-gas chromatography was more precise (relative standard deviation 2 pph) for the measurement of capacity in the optimum range, but was less precise (relative standard deviation 10-28 pph) for the estimation of crosslinking. I t was quicker than IR spectrophotometry, since no sample preparation was required, but frequent calibration was necessary, and the calibration curve for the determination of capacity was not linear. IR spectrophotometry required more time for the preparation of potassium bromide discs containing an internal standard, but linear calibration graphs were obtained which could be used repeatedly. LITERATURE CITED (1) J. R. Parrish, Anal. Chem., 45, 1659 (1973). (2) E. Blasius, H. Lohde, and H. Hausler, 2. Anal. Chem., 264, 278, 290 (1973).
II
s i
u co
0 0
Capacity
meqlg
Figure 4. Plot of ion-exchange capacity vs. ratio of absorbance at
1008 cm-' to absorbance at 1450 cm-' (3) J. Klaban and V . Radl, Chem. Prum., 21, 445 (1971); Chem. Abstr., 76, 73106t (1972). (4) H. Kvedar, Kem. lnd., 19, 141 (1970); Chem. Abstr., 73, 99526t (1970). (5) D. Whittington and J. R. Millar, J. Appl. Chem. (London). 18, 122 (1968). (6) K. W. Pepper, J. Appl. Chem. (London), 1, 124 (1951). (7) J. R. Millar, J. Chem. SOC., 1960, 1311. (8)V. W. Meloche and G. E. Kalhus, J. lnorg. Nucl. Chem., 6, 104 (1958). (9) S. E. Wiberley, J. W. Sprague, and J. E. Campbell, Anal. Chem., 29, 210 (1957). (10) A. Strasheim and K. Buijs, Spectrochim. Acta, 17, 388 (1961). (11) J.Dechant,Z. Chem.,5, 114(1965). (12) L. H. Jones, J. Chem. Phys., 25, 1069 (1956); 28, 1234 (1958). (13) M.V. Belyi, G.I. Ermolenko, and I. Ya. Kushnirenko, Ukr. Fiz. Zh. (Russ. Ed.), 19, 489 (1974); Chem. Abstr., 81, 18844k (1974). (14) G. I. Ermoienko, I. Ya, Kushnirenko, and Kh. K. Maksimovich. 2h. Prikl. Spektrosk., 20, 460 (1974); Chem. Abstr., 81, 18673u (1974). (15) D. J. Gordon and D. Foss Smith, Spectrochlm. Acta, Part A, 30, 1953 (1974).
RECEIVEDfor review March 26, 1975. Accepted June 12, 1975.
Flameless Atomic Absorption Spectrometry Employing a Wire Loop Atomizer M. P. Newton' and D. G. Davis2 Department of Chemistry, University of New Orleans, New Orleans, La. 70 122
The use of a versatile flameless atomlzer for atomic absorption spectrometry, which employs an electrically heated tungsten alloy wire loop, Is reported In this study. Several sampllng methods were investigated Including electrolytic deposition. The relative standard deviation was usually between l-3.5%. The detection limits of 19 elements have been compared for the various sampling methods.
ers, however, are expensive, require water cooling, large power supplies, and are not always sufficiently versatile to eliminate special sample preparation. The purpose of this work was to develop an atomizer which would overcome these disadvantages. Sampling methods were also developed to minimize interferences and background absorption while allowing maximum sensitivity, reproducibility, and versatility.
Atomic absorption has become one of the most widely used analytical techniques. The weak link in atomic absorption is the flame atomizer ( I ) . Flameless atomizers have been developed which eliminate the disadvantages of the flame atomizers (2-8). Some of these flameless atomiz-
Reagents. Reagent grade chemicals and doubly distilled deionized water were used in all studies. Solutions were stored in polyethylene or polypropylene bottles. Dilute solutions were remade before each set of experiments. These solutions were made by adding five or more microliters of a concentrated stock solution, with a disposable plastic pipet to 100 ml of the doubly distilled deionized water. Apparatus. All work reported here was performed on a Varian
EXPERIMENTAL
Present address, Merck Co., Inc., Rahway, N.J. Author to whom correspondence should be addressed.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
2003
