1167
Anal. Chem. 1985, 57. 1107-1168
-'
;y, ,
,
,
,
,
100 200 300 400 500 600 700 800
m/z Figure 3. Intensity enhancement In OH- NCI spectrum of PFKacetone-diphenylmethaethane mixture with scanning pole bias as a function of mass.
scope screen, and the pole bias sweep potentiometer (R17) is adjusted to give the highest peak intensity with acceptable shape. If necessary, the resolution can be decreased a t this point to about unity by increasing slightly the dc imbalance of the poles (DC BALANCE control in the Biospect). In general, this procedure is adequate to obtain a well reproducible tuned state. For scans to high mass (mlz up to goo), the above procedure is repeated with an appropriate high mass ion displayed on the oscilloscope screen.
RESULTS In the well-tuned state, one can easily observe the important role of the pole bias scanner by setting R17 (the pole bias sweep potentiometer) to zero. While the peak intensities below m/z 100 do not show any change, those around m / z 300
decrease by about a factor of 2 and those for m / z 600 fall nearly to zero. An example of the spectra obtained with the different conditions is shown in Figure 2 parts A (pole bias scanner off) and 2B (pole bias scanner on). The enhancement in intensity at higher mlz values produced by the pole bias scanner is obvious. A quantitative measure of the enhancement is given in Figure 3, which shows the ratio of intensities with and without the pole bias scanner at selected mass ions in the OH- NCI spectrum of a PFK-acetone-diphenylmethane mixture. These intensities were measured with the oscilloscope of the mass spectrometer. The intensity enhancement varies from a factor of 1.5 a t m / z 231 to 6.5 a t m / z 849.
ACKNOWLEDGMENT We wish to thank Lawrence Eisenberg, Rockefeller University Electronics Laboratory, and Marvin Vestal, University of Houston, for helpful discussions and advice.
LITERATURE CITED (1) Dawson, P. H. In "QuadrupoleMass Spectrometry";Dawson, P. D., Ed.; Elsevier: Amsterdam, 1976; Chapter V. (2) Brubaker, W. M. Adw. Mass Spectrom. 1968, 4 , 293. (3) Carter, M. H. "Techniques for Optimizing a Quadrupole"; EPA-6001476-004, March 1976. (4) Malmstadt, H. V.; Enke, C. G.; Horlick, G. "Electronic Measurements for Scientists";W. A. Benjamin: Menlo Park, CA, 1974; p 94.
RECEIVED for review October 24, 1984. Accepted December 20,1984. This work was one of the activities of the Rockefeller University Extended Range Mass Spectrometric Research Resource, which is supported by the Division of Research Resource, NIH Grant RR00862.
Preparation of Unlform Films from Latex for Infrared Analysis G. C. N. Cheesman* and L. J. Gaskin Analytical Laboratory, Doverstrand, Ltd., Harlow, Essex, England The determination of the monomer ratio of copolymers is conveniently carried out by infrared analysis (I), but in the case of synthetic rubber latex there are difficulties due to the fact that the latex often contains a considerable amount of gel (2) polymer, i.e., polymer which is cross linked and insoluble in all solvents unless it is degraded. It is therefore not possible to use the conventional solution techniques for IR analysis. A technique commonly used is to spread the latex by hand direct onto a silver chloride window. Because the latex or polymer emulsion is nearly always pseudoplastic in its rheology, it is impossible to prepare a film of uniform thickness in this way because it will not flow out as a polymer solution would. This leads to inaccurate results.
EXPERIMENTAL SECTION More uniform films can be formed by spinning the window. A spinner made for preparing paint films was used (Sheen spinner, IC1 pattern, Sheen Instruments, Ltd., 15/16 Sheendale Road, Richmond, Surrey, England). The window was fixed about 4 cm from the axis, excess latex was poured on the window, and the window spun for 30 s at 650 rpm. Neither distance, time, nor speed is critical. The particular latex used was Revinex 98F, a carboxylated styrene/butadiene latex made by Doverstrand, Ltd., and used as a paper coating binder. The viscosity was 150 mPa.s (Brookfield LVT, Spindle 2, Speed 60). Latices with much higher viscosity need diluting with water and those with much lower viscosity may need thickening, but the exact viscosity is not critical. Infrared measurements were carried out on a Perkin-Elmer 681 ratio recording spectrophotometer connected to a 3600 Data 0003-2700/85/0357-1167$01.50/0
Station. For this sample, styrene was determined using the 765-cm-' band and butadiene using the 967-cm-' band of the trans isomer. In this case, the method is valid only if the trans/cis/vinyl butadiene ratios are constant, i.e., for polymers prepared at the same temperature. However, the method of film preparation is suitable for any latex, and any suitable internal reference band can be used.
RESULTS AND DISCUSSION Nonuniform films introduce both random and bias errors into infrared determinations. For example, consider a uniform film which has two measured infrared bands giving absorbances of 0.6 and 0.2, respectively, then the ratio of the bands is 3.0. For a film of the same mean thickness, but which has half the film a t 1.5 X the thickness, and half the film a t 0.5 X the thickness, then the calculated absorbances would be 0.504 and 0.189, respectively, giving a ratio of 2.67. Since this error will always make the ratio nearer to 1than the true value, no amount of replication will remove the bias error introduced by uneven films. Twelve determinations of the butadiene/ styrene absorbance ratio using windows prepared by hand spreading gave a mean of 1.22, standard deviation of 0.0606, and hence a relative standard deviation of 5.0%. Twenty-three determinations of the same absorbance ratio using windows prepared by spinning gave a mean of 1.43, standard deviation of 0.0198 and hence a relative standard deviation of 1.38%. Both the increase in the mean and the decrease in the standard deviation are highly significant, with a probability of
