ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
was found over a range of injection frequencies varying from one every 0.75 min to one every 7.5 min. It was also found that there was no significant effect on precision when a given analysis time was achieved under different combinations of flow rate and temperature in the range 225 to 265 "C.
MAINTENANCE OF PRECISION This paper illustrates the high precision with which the p value for the sucrose analysis may be determined if suitable precautions are taken. In addition to the factors that have already been discussed, it is essential that the chromatographic system be leak-free and t h a t the column used be in good condition. The septum used should be changed after about 50 injections, even though precision can sometimes be maintained for a greater number of injections. A double septum was used against the possibility of leaks at high pressure even though this is probably not essential. The detector and split restrictor should be cleaned regularly, as the condition of these is also critical to precision. This may be effected by submersing in acetone in an ultrasonic bath. Blocking of the split restrictor is largely eliminated by leaving the splitter permanently in the vent position.
CONCLUSION A precise measurement of /3 may be made over a wide range of operating conditions. The actual value for p varies with inlet and column temperature, with carrier flow rate, and with volume of sample injected. In spite of these possible sources of error, highly reproducible results are obtainable if proper care is taken as illustrated in our preliminary note (9) where it was shown t h a t five derivatives prepared from the same aqueous sugar solution all had values within about 0.1 % of each other.
857
Under optimum conditions, the precision as measured by the relative standard deviation is good and varies between 0.01% and 0.1% in a random fashion. The use of a capillary column results in a faster analysis than is attainable with a packed column and thus allows more replicate determinations to be performed in a given time. The overall precision is a function of the standard error of the mean, u/&, where u is the standard deviation and n is the number of replicate determinations. Thus, in a given time, a better overall precision is attainable with a capillary column than with a packed column even when the standard deviation of p for both techniques is the same.
ACKNOWLEDGMENT The authors thank B. T. M. Clayton for technical assistance, J. G. Halley for calculating relative standard deviations, and L. Troskie of the Department of Mathmatical Statistics a t the University of Natal for helpful discussions.
LITERATURE CITED L. Messineo and E. Musarra. Int. J . Biochem., 3, 691 (1972). J. Cerning-Beroard, Cereal Chem., 52, 431 (1975). J. C. Linden and C. L. Lawhead, J . Chromatogr., 105, 125 (1975). V. C. Mahoney and P. C . Lucas, Int. Sugar J . , 73, 291 (1971). K. J. Schaffler and C. Loker, Proc. Int. Soc. Sugar-Cane Techno/. (Durban), 15, 1380 (1974). K . Zurcher, H. Hadorn, and C. Strack, Mitt. Geb. Lebensmiftelunters. Hyg,, 66, 92 (1975). K. J. Schaffler, R o c . S . A h . Sugar Tech. ASSOC.(Durban), 50, 220
(1976). D. Nurok and T. J. Reardon, Proc. S. Afr. Sugar Tech. Assoc. (Durban), 49 94 - 119751 \ - -3 - - 7
D. Nurok and T. J. Reardon, Carbohydr. Res., 56, 165 (1977). D. W. Grant and A. Clarke, J . Chromatogr., 92, 257 (1974). M. Goedert and G. Guiochon, J . Chromatogr. Sci., 7, 323 (1969).
RECEIVED for review October 7,1977. Resubmitted December 23, 1977. Accepted March 20, 1978.
Compositional Determination of Styrene-Methacrylate Copolymers by Pyrolysis Gas Chromatography, Proton-Nuclear Magnetic Resonance Spectrometry, and Carbon Analysis Donald L. Evans, Judith L. Weaver, Ani1 K. Mukherji," and Charles L. Beatty Xerox Corporation, Webster, New York 14580
Compositional analyses of styreneimethyl methacrylate and styrenein-butyl methacrylate copolymers have been accomplished by pyrolysis gas chromatography and compared to both proton-NMR and carbon analysis. Agreement between the three independent methods is excellent. The comparison stresses the complementary nature of all three methods. Pyrolysis gas chromatography uniquely possesses advantages such as simplicity and rapidity. Percent standard deviation for pyrolysis gas chromatography is 1-2 YO whereas it is less than 1% for NMR.
Pyrolysis gas chromatography has been used for both qualitative and quantitative analysis, for differentiating copolymers and homopolymer blends of the same composition and for the analysis of stereo-specific polymers (I). Conditions 0003-2700/78/0350-0857$01.OO/O
essential to quantitative reproducibility in pyrolysis gas chromatography have been studied by various workers (2-10). Effect of pyrolysis temperature rise time, sample size, sample surface area, and sample thickness have also been investigated (11-14). Recently, the American Society for Testing Materials (ASTM) published a round-robin study of interlaboratory reproducibility of pyrolysis gas chromatography for some selected polymers (15). It was concluded that reproducibility can be achieved provided carefully prescribed parameters for gas chromatography and pyrolysis are followed (e.g., temperature, column and eluent flow conditions in GC and temperature rise time, sample size and thickness for pyrolysis). T h e pyrolysis GC offers many advantages over other techniques such as infrared and NMR. These advantages are small sample size, little sample preparation, and rapid turnaround time. The equipment is relatively inexpensive, easily operable, and can be automated. :C 1978 American Chemical Society
858
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
METHYL METHACRYLATE
i
STYRENE
10
9
8
7
6
5
4
i
3
OPPmlBl
I
Figure 1. NMR spectra. Styreneln-butylmethacrylate, 10 % (wlv) in CDCI,
STYRENE
I bl
1 n-BuTYL
1
1 .
P
D
"
-
d
C
1
8
TIME
IO
MINJTES
Figure 3. Pyrolysis gas chromatograms of styrenelmethylmethacrylate and styreneln-butylmethacrylate copolymers
L . , I , , , 9
I3
Figure 2. NMR CDCI,
l , , , , , , , , , ! , , , , l , , - l , , 8 7 6 5 4
,
3
1
z
,
,
,
I
,
Cpp*S
spectra. Styrenelmethylmethacrylate, 10% (wlv) in
The present study was undertaken to evaluate the precision
and accuracy of pyrolysis gas chromatography, with two other independent methods, proton-NMR spectrometry and carbon analysis in determining percent composition of copolymers.
EXPERIMENTAL Synthesis of Styrene/Methyl Methacrylate and Styreneln-Butyl Methacrylate Copolymers. All samples are random copolymers prepared by suspension polymerization. In samples 26, 27, and 28, the reaction was stopped after -10% conversion to ensure completely random monomer addition. In other cases however, copolymers were polymerized to essentially 100% conversion. Their number average molecular weights range from 14000 to 600000 with an average dispersity (M,/M,) of 2.3 as determined by gel permeation chromatography. Carbon Analysis. The carbon content in polymers was determined by Galbraith Laboratories, Knoxville, Tenn. Percent styrene, methyl methacrylate and n-butyl methacrylate were calculated using simultaneous equations. Proton-NMR. The proton-NMR spectra were obtained using a Jeol C-60H, 60 MHz spectrometer. Sweep width of 50 Hz and sweep time of 2.5 min were employed. For integration, sweep time was increased to 1.25 min. Deuterated chloroform was used as a solvent and trimethylsilane as an internal standard. The spectra were run a t ambient temperature using 10-15 wt0'9 solutions. Small variations in room temperature do not affect the precision of spectra as demonstrated by
