Charles K. Mann

In situ Monitoring of Emulsion Polymerization using Fiber-Optic Raman Spectroscopy

The feasibility of a direct quantitative analysis of styrene by Raman spectroscopy during an emulsion polymerization is examined. A dispersive spectrometer fitted with an intensified diode array detector is used with excitation from the 514.5-nm line of an Ar-ion laser. The limits of detection are 0.26 weight percent styrene in the reaction mixture. Raman measurements are compared with those obtained by UV absorption measurements on extracted samples and found to give essentially the same results.

Raman spectroscopy with a fiber-optic probe

Abstract Considerations affecting the use of fiber-optic components in Raman measurements for quantitative analysis are described. Bidirectional single-fiber probe systems and a unidirectional multi-fiber probe are compared. Results are given for a system employing a 1 × 6 bifurcated probe. In situ monitoring of a reaction for formation of a conducting polymer, polypyrrole, is described. A spatial averaging advantage for the fiber-optic system in measurements on solid samples is described, and results for some purine and pyrimidine mixtures are reported.

Direct assay and shelf-life monitoring of aspirin tablets using Raman spectroscopy

A comparison was made between Raman and high-performance liquid chromatography (HPLC) analysis of aspirin tablets. The basis was an assay of aspirin content and the determination of salicylic acid produced by decomposition. Raman observations were performed directly on both intact and powdered tablet material. The limit of detection of HPLC with an ultraviolet detector is lower than that of the Raman measurement, but both are adequate for this application. The reproducibility of the Raman measurement is somewhat better than that of the HPLC measurement. Both methods were used in a degradation study in which samples were stored in a humid atmosphere for a maximum period of 8 weeks. Aside from somewhat higher salicylic acid responses from the HPLC method, which were attributed to hydrolysis during chromatography, results from the two methods were comparable. Direct Raman measurements are faster and do not require the use of solvents.

Curve Fitting and Linearity: Data Processing in Raman Spectroscopy

A study has been made of the use of polynomial curve fitting for removal of nonlinear background and high-spatial-frequency noise components from Raman spectra. Two variations on polynomial curve fitting through a least-squares calculation are used. One, involving fitting data x values to corresponding y values, was used to approximate background functions, which are subtracted from the original data. For smoothing, a reference matrix of six vectors that contains a unity d.c. level, a ramp made up of x values, a quadratic made up of x2 values, etc., is fitted to a section of data. The reference vectors are scaled by the fit values and added to give the smoothed estimate of a spectral peak. It is demonstrated, with factor analysis as a test procedure, that the background removal procedure does remove nonlinearities that were present in the original data. The smoothing procedure rejects high-spatial-frequency noise without introducing detectable nonlinearities.

Use of Water as an Internal Standard in the Direct Monitoring of Emulsion Polymerization by Fiber-Optic Raman Spectroscopy

Direct analysis of the organic phase of an emulsion polymerization is described. Quantification uses the bending mode peak of water, which makes up the bulk of the reaction medium, as an internal standard. Although demonstrated with methyl methacrylate, the process is generally applicable to emulsion polymerizations. It does not require the introduction of an extraneous internal standard component into the reaction mixture.

Wavelength Calibration of a Multichannel Spectrometer

A readily automated procedure for wavelength calibration of multichannel spectrometers is described. Once applied, line positions can be read from the multichannel display with good accuracy and precision for any spectrometer setting. The procedure uses Ne atomic lines as wavelength standards. A novel apodization procedure is used for accurate measurement of the pixel positions of the neon lines. Raman line positions can be determined with an average error of less than 0.2 cm−1.

Effect of Powder Properties on the Intensity of Raman Scattering by Crystalline Solids

The effect of particle size on Raman intensity has been measured for a number of crystalline solids with a fiber-optic-based Raman spectrometer. Particle sizes ranged from 76 to 605 μm. Materials examined were sodium nitrate, sodium chlorate, sodium bromate, potassium ferrocyanide, potassium ferricyanide, and copper(II) sulfate. Raman intensity was found to decrease with increasing particle size. The factors responsible for this trend are discussed. We conclude that the major factor is diffuse reflectance that enhances the overlap between the excitation and collection beams. The depth of sample contributing to the Raman signal has been examined for both powders and tablets as a function of powder particle size. Materials examined in this study were sodium nitrate, sodium sulfate, sodium chlorate, sodium bromate, potassium ferrocyanide, potassium ferricyanide, and copper(II) sulfate. For nonabsorbing powders, the depth of sample contributing to the signal exceeded 15 mm. The effect of the pressure used in forming tablets on the Raman signal strength and reproducibility has been examined for sodium nitrate. The Raman intensity was found to decrease with increasing pressure until a tablet of constant density was formed. The effect of particle size and particle size mismatch on the sodium nitrate Raman signal in binary mixtures with potassium chloride, potassium bromide, and potassium iodide has been examined. Good reproducibility was found to require matching of component particle sizes.

Quantitative and qualitative analysis of some inorganic compounds by Raman spectroscopy

Raman spectra have been measured for a number of nitrates, nitrites, sulfates, ferrocyanides, and ferricyanides, both in the solid phase and in aqueous solution. Accurate locations of peak maxima are given. Limits of detection for some of the compounds are given for solutions and for solid mixtures in NaNO3. Preliminary measurements have been made on core material recovered from the storage tanks on the Hanford site in Richland, Washington. Representative spectra are presented, showing that it is possible to observe responses of individual components from measurements made directly on untreated cores, with the use of a fiberoptic sampling probe.

The direct analysis of fermentation products by Raman spectroscopy

The increasing importance of chemical synthesis by fermentation has led to interest in monitoring this type of reaction by direct analysis for either the compounds which are being synthesized or the by-products of the synthesis. Special conditions must be met in this type of analysis. The reaction medium is aqueous, and it must be protected from bacterial invasion from the environment. Often the reaction products are nonvolatile compounds.

Spectrophotometric Analysis by Cross-Correlation

An examination of the use of cross-correlation in quantitative analysis under conditions that isolated peaks are not obtained is reported. Examples are taken from infrared absorption spectroscopy. The properties of correlation that are relevant to this application are described. A comparison between cross-correlation and peak measurement is made with emphasis on the effects of sample background and instrument noise. Results are presented to show that correlation allows substantial improvement of performance when spectra with complex backgrounds are involved. It affords a significant improvement when noisy signals must be handled.