Applied Spectroscopy

Moss, P.; Salin, E. D.

doi: 10.1366/0003702914335210pmid: N/A

A flow injection (FI) preconcentration system has been coupled with a direct sample insertion (DSI) system for inductively coupled plasma atomic emission spectrometry (ICP-AES). The FI system provides a preconcentration factor of approximately 30. The DSI detection limit improvement, as compared to conventional nebulization detection limits, is element specific and ranges between 10 and 130 when a cup specifically designed for liquid work is used. The FI-DSI hybrid provides detection limit improvements ranging between 140 and 1200 for the elements tested—Cu, Pb, and Zn. Precision of the FI-DSI-ICP system averages 4% RSD for these three elements. FI processing time is 6.0 minutes with the use of 5-mL injection volumes. The system offers considerable potential for further improvement by increasing the injection volume and improving the interface to the DSI.

Compton, David A. C.; Compton, Senja V.

doi: 10.1366/0003702914335148pmid: N/A

The feasibility of obtaining FT-Raman spectra of consumer products inside clear polymeric packaging is described. It is possible to obtain high-quality spectra of the sample without removing it from the package, thus enabling quality-control checks to be performed on a final product. Examples are shown for three pharmaceutical preparations and one food product.

Lai, Edward P. C.; Harris, Joel M.

doi: 10.1366/0003702914335120pmid: N/A

The efficiency of the stimulated Raman gain scattering (SRGS) in liquids is sufficiently high to make the phenomenon useful as a means of chemical analysis. Quantitative and qualitative studies of stimulated Raman gain detected by photothermal probe beam deflection are presented with a major simplification in instrumentation for generating the Stokes probe beam. This technique is evaluated for the detection and quantitation of a model chlorinated hydrocarbon, 1,1,1-trichloroethane. With a very modest laser pulse energy of 1.5 mJ (average power of 1.5 mW) and a pathlength of 10 cm, the technique is useful for the noninvasive, real-time monitoring of SRGS-active compounds at the 10 mM level. Selectivity against 1,1,2-trichloroethane, 1,2-dichloroethane, trichloroethylene, ethylbenzene, and benzene is excellent. Direct identification and simultaneous determination of individual compounds in process and effluent streams could be attained without sampling, separations, or other pretreatment procedures.

Lacy, W. B.; Rowlen, K. L.; Harris, J. M.

doi: 10.1366/0003702914335373pmid: N/A

Changes in spectral band parameters (width, center frequency, intensity) which arise from charge-trapping artifacts in the Thomson TH 7882 charge-coupled-device (CCD) detector are reported. These parameters are measured for a Raman scattering band of carbon tetrachloride with respect to CCD geometry (parallel vs. serial binning), in the presence and absence of preflash, vs. changes in integration time (variation in detected light level). The dependence of the spectral parameters on detector temperature was also measured. The degree of charge trapping and the charge transfer efficiency were estimated from the change in peak width and intensity vs. integration time, respectively, and were found to vary with detector temperature according to an Arrhenius relationship for the serial-binning geometry; from these results, the energy barriers to charge trapping and loss in the serial register were estimated. Practical guidelines for acquisition of binned spectra with this detector are suggested.

Govil, Anurag; Pallister, David M.; Chen, Li-Heng; Morris, Michael D.

doi: 10.1366/0003702914335201pmid: N/A

We describe the use of Hadamard transform Raman microscopy to acquire optically sectioned images of crystals of benzoic acid. Nearest-neighbor deblurring is used to reject out-of-focus information and sharpen the Raman images obtained from the crystal.

Lal, Sundeep; Yappert, M. Cecilia

doi: 10.1366/0003702914335328pmid: N/A

A double-waveguide evanescent sensor has been developed and characterized. A fraction of the evanescent field (λe = 488 nm) generated at the core (nc = 1.48)/cladding (ncl = 1.45) interface of a glass optical fiber penetrates into the cladding and the polymeric jacket (nj = 1.54) coated onto the cladding. Fluorescence is excited within the jacket and, due to the higher refractive index of the jacket, the emission is totally internally reflected within it. The emission (λem = 605 nm) has been collected with the use of lateral and front-end geometries. The sensor has been used to determine iodine in gas and liquid phases using an indirect approach. Iodine reacts with unsaturated sites of the polymer and absorbs the fluorescence, causing a reduction in the fluorescence background. The rate of fluorescence reduction is proportional to the concentration or partial pressure of iodine in the sample.

Tadayyoni, M. Azar; Dando, Neal R.

doi: 10.1366/0003702914335166pmid: N/A

Raman spectroscopy is rapidly developing as a nondestructive technique for the surface characterization of carbon materials. An advantage of Raman analyses is that the substrate surface may be both spatially localized (determined by the position and diameter of the probe beam) and effectively depth profiled (comparing normal and surface-enhanced Raman data) without the need to employ surface-disruptive sample preparation techniques. The efficacy of normal and surface-enhanced Raman spectroscopy for investigating carbon surfaces is demonstrated by comparing Raman and SERS spectra of monolithic graphitic carbon and 7-μm graphite fiber. The data obtained indicate the existence of more disordered carbon at surfaces as compared to the bulk, exemplify the need for empirically determining innocuous silver deposition conditions (for SERS), and demonstrate the utility of Raman spectroscopy for investigating carbon materials.

Brill, Jeffrey H.; Narayanan, Bikshandarkoil A.; McCormick, John P.

doi: 10.1366/0003702914335193pmid: N/A

A highly selective, sensitive method for determination of carboxylic acids in water has been developed that is based on fluorine-selective gas chromatography (GC) detection of pentafluorobenzyl ester derivatives. The atmospheric-pressure helium microwave-induced plasma detector is well suited for fluorine-specific detection of these derivatives, which were well resolved by capillary GC and displayed a characteristic fragmentation behavior when examined by GC-mass selective detection. For demonstration purposes, a number of carboxylic acids, ranging from C1 to C20, were examined and found to be derivatized with yields greater than 90%, and a sample of creek water was examined for its carboxylic acid content.

Lo, Su-Chin; Brown, Chris W.

doi: 10.1366/0003702914335256pmid: N/A

A new algorithm is presented for searching medium-size infrared spectral libraries for the components in spectra of mixtures. The algorithm treats the spectra in the library as an m-component quantitative analysis problem in which each of the library spectra represents a standard mixture having a concentration of 1.0 for that component. Principal component regression (PCR) is used to reduce the dimensionality of the problem and to provide the regression coefficients for determining pseudo-concentrations or composition indices (CI) in mixtures. The PCR analysis is followed by the application of an adaptive filter to remove all similarity of the first target component from the mixture and from a selected subgroup of the library. This is followed by a second PCR analysis on the modified spectral data to identify the next target compound. If the correct target components are selected with successive applications of the adaptive filter, the residuals will approach zero. All components in five two- and three-component mixtures were correctly identified by this new Mix-Match algorithm, whereas only two of the five mixtures were completely identified by a typical dot-product search routine.

Lo, Su-Chin; Brown, Chris W.

doi: 10.1366/0003702914335111pmid: N/A

A routine for searching large spectral libraries with spectra of mixtures is presented. The dimensionality of a 3169-compound library is reduced to 12% of its original size by using Fourier transform compression and principal component analysis. A principal component regression is performed and used as a prefilter in selecting spectra having features (and chemical groups) similar to those of the unknown mixture. A dot-product metric is then used to identify a target component from the subgroup formed by the prefilter. This is followed by the application of an adaptive filter to remove the similarity of the target component from the subgroup and from the unknown mixture; the search is repeated on the modified data. Successive applications of the adaptive filter will produce minimum residuals if the correct identifications are made. Once the residuals are minimized, a similarity index is calculated to determine the closeness of the unknown mixture spectrum to a spectrum reconstructed from the library spectra. Four out of five two- and three-component spectra were correctly identified. One of the two components in the fifth mixture was correctly identified, and the residual values flagged the improper identification of the second component. After the adaptive filter was applied to the entire library, the second component was correctly identified. Results for this new algorithm are compared to those from four more traditional search routines, which were only completely successful on one of the unknown mixtures.