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Anal. Chem. 1086, 58,2570-2571
temperature evaporation is quite different from the morphologies of as-received and solution-processed PFSI (8). Apparently, low-temperature evaporation does not impart sufficient thermal energy to the polymer chains to allow them to assume the morphology present in the as-received polymer. We will report the results of these studies soon.
Registry No. Nafion, 39464-59-0. LITERATURE CITED Martin, C. R.; Rhoades. T. A.; Ferguson, J. A. Anal. Chem. 1982, 54, 1639. Martin, C. R.; DoHard, K. A. J. Electroanal. Chem. 1983, 759, 127. Szentirmay, M. N.; Martin, C. R. Anal. Chem. 1984. 56, 1898. Szentirmay. M. N.; Campbell, L. F.; Martin, C. R. Anal. Chem. 1988, 58,661. Buttry, D. A.; Anson, F. C.J. Am. Chem. SOC. 1983, 105. 685.
(6) Rubinstein, I.; Rishpon, J.; Gottesfeld, S.J . Nectrochem. SOC.1986, 733,729. (7) U S . Patent 4417969. (8) Moore. R. B.; Martin, C. R., unpublished results, Texas A&M University, February 1986
Robert B. Moore, I11 Charles R. Martin* Department of Chemistry Texas A&M University College Station, Texas 77843
RECEIVED for review May 9,1986. Accepted July 1,1986. This work was supported by the Dow Chemical Co. and the Office of Naval Research.
AIDS FOR ANALYTICAL CHEMISTS Variable Path Length Flow-Through Cell for Spectrophotometry Thomas Choat, Johannes J. Cruywagen,* and J. Bernard B. Heyns Department of Chemistry, University of Stellenbosch, Stellenbosch, 7600 South Africa A convenient experimental setup for spectrophotometric titrations is to circulate the reaction mixture through a flowthrough cell by means of a peristaltic pump. A similar titration procedure can be followed when absorbance data for equilibrium studies are desired. Because the change in absorption of the reaction mixture with the change in concentrations of the components is monitored continuously, conditions can be controlled to obtain the most suitable set of data points. This method of data collection is advantageous because it affords greater accuracy and is less tedious than a procedure whereby a series of solutions is prepared and the absorbance of each solution is measured separately. Flow-through cells of fixed path lengths in the range 1-10 mm are commercially available. However, when a cell of fixed path length is used, it is often not possible to vary the concentration of a strongly absorbing reagent over a wide range without exceeding the optimum absorption limits of the spectrophotometer. For instance, cells with path length varying from 0.1 to 10 mm are commonly used when condensation reactions (1-3) are investigated and in such cases a titration procedure cannot be followed since switching of cells is impractical. The above considerations have led us to design an inexpensive flow-through cell of which the path length can be varied from 0 to 10 mm at precisely graduated increments of 0.01 mm.
CELL CONSTRUCTION The cylindrical outer casing of the cell was constructed from nickel-plated brass, and fitted with a PTFE inlet and outlet (see Figure 1). A fine pitch thread cut along part of the casing inner wall allows for the use of a graduated brass ring (henceforth, the drum) for adjustment of the optical path length. The quartz windows are mounted in two separate cylindrical inner Perspex housings, one of which is let into the drum, the other being fixed. Perspex to quartz joints are sealed with an epoxy adhesive. To maintain a liquid seal during use, the outer brass casing has an inner Perspex sleeve
along which can slide a tight-fitting O-ring, recessed into the movable inner housing. The drum is graduated into 100 divisions, and since a full rotation adjusts the optical path by 1 mm, the path length can be very precisely set. When set at its designed maximum optical path of 10 mm, our cell has a total length of 75 mm; the brass casing has an outer diameter of 37 mm. The cell is mounted in the spectrophotometer beam by means of a clamp bracket that can slide over the front end of the cell. The clamp bracket has a base plate that can be screwed down onto the floor of the cell compartment. APPLICATION In typical equilibrium study experiments in this laboratory a thermostated reaction vessel is connected to the cell and charged first with a solution of the titrand at constant ionic medium. Circulation is then started, and the Tygon lines and the cell are freed of air bubbles and the path length set at a suitable value. After temperature equilibration the titration is carried out, using titrant(s) of the same ionic medium and spectra are measured against air at appropriate stages of the titration. The path length can be adjusted in the course of the titration if required. The reaction vessel and cell are afterward cleaned by flushing the whole system beveral times with deionized water and blowing dry with air. To obtain the correct blank absorbances, a solution of the ionic medium is then circulated and its spectrum recorded (again with air as reference) at the appropriate path length(s). If all the measurements for a titration can be made at a single path length, the following alternative procedure is perhaps more convenient: a known volume of the blank (ionic medium in this case) is first circulated and its spectrum recorded. A suitable volume of the titrand (in the same medium) is then added to the blank in the reaction vessel after which the spectrophotometric titration can be conducted in the usual way. CALIBRATION Precision machining methods allow the path length graduations to be given to very fine tolerances. However, the cell
0003-2700/86/035&2570$01.50/0 0 1986 American Chemical Society
Anal. Chem. 1986, 58, 2571-2576
Brass
2571
Table I. Absorbance Values at Various Path Lengths Used to Calibrate the Cell drum (L)/mm
A (220 nm)
A (230 nm)
PTFE
0.05
rtz
0.10 0.15 0.20
0.3489 0.5840 0.8212 1.053
0.2351 0.3926 0.5492 0.7080
6 = 0.0243 mm
6 = 0.0246 inm
Perspex
(luartz Windows IQ15x21 Calibration RI
\Inlet
Figure 1. A diagram of variable path length flow-through spectrophotometer cell.
is fabricated so that the quartz windows just about touch when the drum is set to reading zero, resulting in a very small difference between the drum reading and the actual path length. Calibration is therefore necessary for measurements at very short path lengths and, depending on the degree of accuracy required, also at longer path lengths. One calibration method is to measure absorbances of an aqueous solution (e.g., KNOBor K2Cr04)of suitable concentration against air at a number of short path settings and at any convenient wavelength. The stray light error of the spectrophotometer should of course be negligible at the chosen wavelength (4). The procedure is then repeated with distilled water to obtain the correct absorbance of the solution by subtraction of the blank values. An illustrative set of absorbance values measured at two wavelengths for each of four different path length settings is given in Table I. These values were used to calibrate the cell for a titration where a path length of 0.2 mm was required. The true path length, b, is deduced as follows from the data: If the reading on the drum is denoted by L, then the path length is given by
b=L+6
(1)
Values for cc and ec6 can be readily obtained as the slope and intercept of the straight line plot of A against L, or by fitting eq 3 to the data by least-squares analysis. The quotient of tc6/cc then gives the path length correction. The agreement between the two 6 values, 0.0246 and 0.0243 mm, calculated from the two sets of independent measurements is satisfactory and implies an uncertainty of less than 0.1%at path length 0.2 mm. The path length to be used and the accuracy required should dictate the range and number of path length settings chosen for calibration purposes. Since aging of the O-ring can have a measurable effect on the value of 6, occasional calibration is recommended. In fact, calibration can be most conveniently carried out as part of a titration experiment by using the reaction mixture and its blank as described above.
CONCLUSION The variable short path capability of the cell offers advantages in cases where a titration procedure can be utilized to vary conditions of a reaction mixture to be investigated by spectrophotometry (e.g., ref 5). Examples are, end-point determinations in volumetric analysis, the determination of complex stoichiometry by the slope ratio or molar ratio methods, and especially equilibrium investigations in which the concentration of a strongly absorbing component has to be varied over a wide range. LITERATURE CITED
where 6 is the path length correction. From Beer’s law A = ecb one obtains
A = EC(L+ 6 ) = tcL ec6
+
(1) Llnge, H. G.; Jones, A. L. Aust. J . Chem. 1988, 21, 1445. (2) Lyhamn, L.; Pettersson, L. Chem. Scr. 1980, 16, 52. (3) Cruywagen, J. J.; Heyns, J. B. 8.; Rohwer, E. F. C. H. J . Inorg. Nucl. Chem. 1978, 4 0 , 51. (4) Edisbury, J. R. fractlcal Hints on Absorption Spectrometry; Hllger and Watts: London, 1966; Chapter 12. (5) Cruywagen, J. J.; Heyns, J. B. B. Taknta 1983, 30, 197.
RECEIVED for review February 25,1986. Accepted May 8,1986.
Rejection of Spike Nolse from Size Exclusion Chromatography/Low-Angle Laser Light Scattering Experiments Steven A. Berkowitz* Celanese Research Company, 86 Morris Avenue, Summit, New Jersey 07901 The direct coupling of a low-angle laser light scattering detector, LALLS, with a high-performance size exclusion chromatography, SEC, unit ( I , 2) has provided a unique and attractive technique for obtaining absolute molecular weight information and other physicochemical information about macromolecular systems (3-7). The feasibility of this coupling has been made possible by the use of a laser light source that *Present address: J. T. Baker Chemical Co., 222 Red School Ln., Phillipsburg, NJ 08865. 0003-2700/86/0358-2571$01.50/0
permits the scattering volume to be greatly reduced without degrading the signal to noise ratio (this is due to the laser’s high-power density). This reduction in the scattering volume along with the spatial coherent nature of the laser source and novel instrumentation design (8-1 0) has permitted intensity light scattering measurements to be made at very low angles (3-7O). Hence, intraparticle interference effects are negligible for most studies allowing molecular weights to be obtained without the need of extrapolation procedures to Oo (1.2). In addition, the high level to which particulate material (e.g., 0 1988 Amerlcan Chemical Society
