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Table 11. Comparison of Metal Values Obtained Using Soluene and Literature Values for Several Tissues Literature, ppm Amount Level detected, ppm Zn Cu Fe Zn cu Fe Tissue assayed 30.3 6.0 100 30-32 (8) 6.76-12.5 (IO, 11) 30-220 (8) Rat liver 300 mg 11.2 1.7 30 15 (7) 1.85 (11) 40-90 (8) Rat brain 300 mg Rat plasma 0 . 3 ml 0.9 0.5 2.5 1.07 (9) 0.81 (12) 3.0-5.0 (8) Table 111. Comparison of Soluene and Wet Ashing Values for Several Tissues Soluene range, ppm Wet ashing range, ppm Amount Tissue assayed Zn cu Fe Zn cu Fe Rat liver 100 mg 31.3-31 . 8 5.0-5.3 150.G152.0a 26.5-32.5 3.8-7.5 270.0-272.0 Rat brain 100 mg 15.0-15.5 3.9-4 .O 27.0-28.0 14.3-16.4 2.9-3,6 19.1-33 .O Rabbit plasma 0 . 1 ml 1.2-1.5 0.7-0.8 1.5-2.0 0.8-1.0 0.6-0.9 2.0-3.5 Rat muscle 100 mg 18.0-18.1 2.0-2.1 15.0-15.3 18.0-23 .O 1 . O-1 . 6 10.0-13.0 These values are lower than those obtained for wet ashing, but they are within the range of those cited in the literature.
solvents by the manufacturer (7). The unit is equipped with a recorder read out and Boling burner. RESULTS AND DISCUSSION
The applicability of the procedure for Zn, Cu, Fe, and M n was investigated but may not be limited to these elements. Data obtained using the method of additions followed Beer’s law in the concentration range of 0.05 to 0.50 pg/ml. The same tissue analyzed using standard solutions containing known concentrations of the metals in a Soluene matrix gave values in agreement with those obtained by the method of additions. Flame scattering at a non-absorbing wavelength was approximately 3-5 absorption. (7) “Analytical Methods for Atomic Absorption Spectrophotometry,” Perkin-Elmer, Norwalk, Conn., p 17. (8) W. S . Spector, “Handbook of Biological Data,” W. B. Saunders Company, Philadelphia, Pa., 1956, pp 50-72. (9) E. I. Dreosti, Shyy-Hwa Tao, and L. S . Hurley, Proc. SOC. Exptl. Biol. Med., 128, 169-174 (1968). (10) D. D. Grant and E. J. Underwood, Aust. J . Exptl. Biol., 36, 339-346 (1958). (11) S. La1 and T. L. Sourkes, Biochem. Med., 4, 260-276 (1970). (12) D. H. Cox and D. L. Harris, J . Nutr., 70,514-520 (1968).
Comparative sensitivity between the organic medium and aqueous standards are shown in Table I demonstrating the greater sensitivity of the Soluene method. Table I1 shows that our values are within the range of those reported in the literature. In a similar manner, Table I11 shows that values obtained using the Soluene method are comparable t o those obtained on the same tissue sample using the accepted wet ashing procedure. Precision as estimated by the coefficient of variation (13) was calculated to be 5.1 %. The method provides a quick, simple, and reproducible procedure for preparing a homogeneous and aspiratable tissue sample. This method is also capable of giving reproducible data with a sample size of 50-100 mg of tissue. Because of the conditions of sample preparation, the sample is likely to be less contaminated and, in addition, one obtains increased sensitivity because of the organic medium.
RECEIVED for review August 2, 1971. Accepted December 14,1971. (13) H. Kaiser and B. Meddings, A t . Absorption Newslert., 6 (2), 28 (1967).
Application of Multiple Internal Reflection Spectrometry to Aircraft Materials Evaluation T.T. Bartels Engineering Laboratories, McDonnell Aircraft Company, St. Louis, Mo. 63166 THE PROPERTIES of many polymeric materials make them suited for use in modern aircraft construction. Polyesters and silicones are used in electrical casting, encapsulating, and potting and sealing applications; urethanes are used in chemical foams t o render fuel tanks inert and as protective coatings; polycarbonates are used in internal stores containers and in canopy applications; polyimides and epoxies are used in resin matrix composites for structural hardware and in adhesives. These applications, of course, represent only a few of the many possible. The variety, complexity, and widespread application of current polymeric materials require rapid test techniques for their characterization and evaluation.
Infrared spectrometry is one of the most convenient single techniques available for determining molecular structure and for qualitative identification. Precise quantitative determinations also are possible, provided the components of the material being analyzed possess unique absorption bands separated from absorptions of other components. The difficulty of evaluating many of the previously mentioned polymeric materials by conventional transmission infrared spectrometry lies in the area of sample preparation. Cured materials are generally chemically resistant and thoroughly cross-linked; these properties make solution spectrometry and “mull” techniques tedious and time-consuming. MicroANALYTICAL CHEMISTRY, VOL. 44, NO. 6, M A Y 1972
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Figure 2. Infrared working curve toming can be effective but requires a skilled operator, and the spectra of pyrolyzates are generally not reproducible when fairly complex mixtures are pyrolyzed. Multiple internal reflectance (MIR) infrared spectrometry requires virtually no sample preparation (1,2). The material to be analyzed is placed in contact with the internal reflectance plate and sufficient pressure is applied to hold the sample in intimate contact with the plate. The spectra obtained by MIR spectrometry are similar to those obtained by transmission. Many applications of MIR spectrometry are concerned with the identification of major components in surface coatings, and Wilks (3) has discussed quantitative analysis aspects of this technique. This paper reports applications of MIR spectrometry to the quantitative determination of the mix ratio and chemical composition of selected polymeric (1) N. J. Harrick, “Internal Reflection Spectroscopy,” Interscience Publishers, New York, N.Y., 1967, pp 219-221. (2) J. Fahrenfort, Spectrochim. Acta, 17, 698 (1961). (3) P. A. Wilks, Appl. Spectrosc., 23, 63 (1969). 1066
ANALYTICAL CHEMISTRY, VOL. 44,
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Apparatus. Samples were analyzed on either a PerkinElmer Model 137 or a Beckman IR-9 infrared spectrophotometer using a Wilks Model 50 internal reflectance attachment (MIR-29 Sample Holder). The reflector plate used was KRS-5,52.5 X 20 X 2 mm, 45’ face angle. Procedure. POTTING COMPOUNDS.Frequently, it is desirable to determine the mix ratio of cured two-component potting compounds. When available, the individual components are analyzed separately, and analytical bands specific for each component being measured are identified. As it usually is not practical to obtain the absorptivities for the components of the mixture, a relative absorbance ratio is employed. A typical example of this type of procedure is illustrated by reviewing the required test development for a method for determining the accelerator content of EC 2273 potting compound, The infrared spectrograms of the accelerator and base resin were recorded and examined (Figure 1). The 6.6micrometer band appeared to be suitable for characterizing the accelerator content, and the 6.8-micrometer band was chosen to measure the base-resin content. A typical working curve, prepared by plotting the absorbance ratio of the analytical bands as a function of mix ratio for a number of known EC 2273 mixes is presented in Figure 2. (The working curve samples were prepared by mixing 5 , 10, 15, 20, 25, and 30 parts of the accelerator per 100 parts of base resin and then curing the mix in the laboratory.) Test samples were analyzed by removing small portions of EC 2273 cured material directly from connectors and relays, recording the spectrogram by MIR spectrometry, calculating the absorbance ratio of the 6.6and 6.8-micrometer bands, and reading the corresponding concentration from the working curve. FUELTANKFOAMS.Fuel-tank fire and explosion suppression systems generally employ polyurethane reticulated foam as the baffle material. Current military specifications ( 4 ) do (4) Military Specification MIL-B-83054 (USAF) 26 June 1968.
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not define the raw materials used in manufacturing the foam, though the composition is not to be changed without notification once preproduction test specimens have been evaluated. Basic differences in chemical compositions of the foams can be detected by infrared spectrometry, and the MIR technique offers a convenient method of recording the spectrum. A sample of foam is placed in contact with the reflector plate, and the spectral region of interest is scanned. SILICONES.Frequently, silicone sealants are submitted to the laboratory to ascertain whether the formulation of the sealant has changed between vendor lots. Again, as in the case of the polyester compound, samples were analyzed by removing small portions of the cured material directly from connectors and relays and recording the spectrum directly by MIR spectrometry. ADVANCED COMPOSITE MATERIALS.The infrared technique which has been used to study epoxy and polyimide resin composites consists of mounting a thin film of the resin on a salt crystal, exposing the coated crystal to the cure cycle under investigation, removing the crystal at various stages in the cure cycle, and scanning the selected spectral region (direct transmission). The degree of polymerization was monitored measuring an absorption band assignable to the imide ring (5.6 micrometers). Quantitative measurement of the intensity of this band indicated the degree of polymerization at any given stage in the cure cycle. Solvent retention characteristics were monitored by measurement of absorption bands assignable to the solvents in the resin system. RESULTS AND DISCUSSION
Potting Compounds. The described procedure was practical and effective for field specimens. Usually less than 10 minutes was required for the complete determination. Sample size limitations were not rigorously defined ; however, fully cured samples as small as 40 milligrams were removed from aircraft parts and analyzed. Samples this small did not require that the connector or relay be repotted. While the same procedure could presumably be worked out using a pellet technique (direct transmission), the laborious grinding of the sample has been eliminated as have other difficulties associated with the pressed-disk technique. Fuel-Tank Foams. Certain prolonged high-humidity/ temperature conditions cause foam degradation. A noticeable change in the infrared spectrogram of the foam occurs as
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the material is exposed to high-humidity/temperature. The relative intensities of absorption bands located at 1600 and 1650 cm-1 were approximately equal in unused foam specimens, but after the foam had been exposed to a high-humidity/temperature environment, the relative intensities of the absorption bands changed with the higher wavenumber band increasing in intensity (Figure 3). Investigation revealed that temperature exposure alone would not produce a change in the relative intensities of the absorption bands cited. Consequently, infrared spectral data can be employed to detect humidity/temperature damage to the foam. Water is a common contaminant in jet fuels, and relatively high temperatures can be expected in certain geographical locations. Thus, the potential for this type of deterioration of foam may be fairly high at certain sites. Silicones. The infrared spectrograms of two such samples from different lots are presented in Figure 4. Casual examination of the spectrograms failed to reveal significant differences; however, more detailed review employing relative absorbance ratios disclosed significant differences in the formulation of the specimens. Some calculated band ratios are presented in Table I. The clear difference in the 7.1/5.8 micrometer band ratio indicated a difference in formulation of the specimens; the fact that the 5.8-micrometer band was of low intensity and not readily assignable to any functional group usually associated with silicones suggested a possible additive. The nearly identical values of the other band ratios calculated and the general similarity of the spectrograms indicated that the basic molecular structure (excluding additives) of both samples were identical. ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
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Table 11. Solvent Retention Absorbance at 7 . 1 micrometers Absorbance at Cure schedule (N-methyl 9 , 4 micrometers (cumulative) pyrrolidinone) (ethyl alcohol) As received 0.095 0.225 1 Hour at 200 OF (No vacuum“B” Stage) 0.065 0.218 1 Hour at 200 OF (Full vacuum) 0.065 0.218 i Hour at 275 “F (Full vacuum) 0.067 0.170 21/2Hours at 340 “F (Full vacuum) Trace Trace 2 Hours of post cure at 400 “F Trace Trace 2 Hours of post cure at 500 “F Not detected Not detected
Advanced Composite Materials. Data relating to the degree of polymerization are presented in Figure 5. Examination of the data points revealed that the bulk of polymerization occurred in the 21/2-hour interval at 340 O F . In addition, polymerization continued at a nearly constant rate during the post-cure period (400-600 O F ) . The fact that a constant absorbance value was never reached indicated that longer post-cure periods would be required to achieve maximum polymerization. The initial absorbance value can be used to calculate the per cent advancement of as-received resin. In the illustrated example, the initial absorbance value was 0.075 and the maximum value obtained 0.565 (at 600 O F ) . The per cent advancement of the as-received resin consequently was 0.075/0.565 X 100 = 13.3 %; i.e., 13.3 % of the maximum possible polymerization obtainable with this cure schedule had occurred prior to heat treatment. In this case the solvents were present initially (prior to cure) and were volatilized during the cure cycle. Consequently, the absorption bands assignable to the solvents were at maximum values in the as-received material and diminished during the cure cycle. Table I1 shows absorbance values (of the absorption bands assignable t o the solvents) as a function of cure time. From the data it can be seen that all traces of detectable solvents were not removed until after the 500 O F interval was completed in the post-cure cycle. The most rapid rate of solvent removal occurred during the 2l/?hour/340 O F portion of the post-cure cycle. There has been a justifiable reluctance, however, to apply results obtained on thin resin films directly to thicker laminates, The optimum sample thickness for transmission infrared spectrometry is severalfold thinner than the typical
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laminate; thus, it is not possible to pass radiation directly through a typical laminate. A MIR technique which was evaluated consisted of mounting a half-thickness section of laminate (prepreg) directly on a MIR crystal, (the center of the laminate in contact with the crystal), exposing the coated crystal to the cure cycle under investigation, removing the crystal at various stages in the cure cycle, and scanning the selected spectral region. This mode of analysis yielded unrealistic data, probably because of heat transfer from the crystal t o the surface of the resin being analyzed. The MIR sampling mode could be optimized for characterization of the chemistry of the resin in the center of a laminate by burying a n MIR crystal in the center of a laminate, with one end exposed which could extend outside the curing oven. In situ measurements could then be performed at any stage in the cure cycle without interruption of the cure cycle. An instrument known as an infrared probe ( 5 , 6 ) has recently been introduced and may be suited for laminate curing evaluation. This unit is essentially a n MIR infrared sensing head, detector, and preamplifier assembly which is separated from the main unit. The sensor head (optical element) can be buried or dipped into a sample, with the optical element shielded except for the portion embedded in the sample. Other potential applications include determination of the rate of moisture or solvent penetration into the laminate, detection of degradation due to aging of the resin matrix, and solvent retention characteristics. The mode of analysis would minimize heat transfer into the center of the laminate and would offer the convenience of in situ measurements.
RECEIVED for review September 29, 1971. Accepted December 14,1971. (5) N. J. Harrick, ANAL.CHEM.,36, 188 (1964). (6) Zbid., 43, 1533 (1971).
Correction Indirect Spectrophotometric Determination of Nanomole Quantities of Oxiranes In this paper by H. Edward Mishmash and Clifton E. Meloan [ANAL.CHEM., 44, 835 (1972)l on page 836, column 1, line 14 should read “A range of 0 to 5 pmoles can be determined t o 1 2 0 nmoles.”
