Method to improve linearity of diffuse reflection mid-infrared spectroscopy.
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Letter↗
Biomedical subjects
Peter R Griffiths
Publications and source records attributed to Peter R Griffiths.
Quantitative vapor-phase infrared spectrometry of ammonia.
Reference spectra of ammonia from four sources are compared. Low-resolution spectra (i.e., spectra for which the spectrometer resolution is numerically greater than the full-width at half-height of the rotational lines) were obtained from the Environmental Protection Agency (EPA) web site and from Infrared Analysis, Inc. High-resolution (0.12 cm-1) spectra were obtained from the National Institute for Standards and Technology (NIST) and Pacific Northwest National Laboratory (PNNL). Two protocols were used to validate the EPA and Infrared Analysis spectra, with one requiring that Beer's law be obeyed by the low-resolution spectrum and the other that Beer's law be obeyed by the high-resolution spectrum. In all cases, the second protocol gave a significantly better spectral match. It is shown that the path-integrated concentrations for the low-resolution reference spectra were in error by as little as 4% to as much as an order of magnitude, presumably because of the effect of adsorption on the cell walls. Measured absorptivities of the NIST and PNNL spectra were different by approximately 2.6% and it is believed that the effect of adsorption on these spectra is small. When the same protocols were used to test the reference spectrum of methane, the calculated path-integrated concentration was only approximately 2% different from the one that was calculated from the NIST reference spectrum, suggesting that the data processing protocols provide accurate data.
Ammonia↗
Reduction of detection limits of the direct deposition GC/FT-IR interface by surface-enhanced infrared absorption.
Even though the interface of gas chromatography (GC) and mass spectrometry (MS) is by far the most popular way of identifying molecules eluting from a GC in real time, the identification of compositional isomers by MS is equivocal at best. Much better results would be found by infrared spectrometry (IR) if the sensitivity of the GC/IR interface could be improved. In this paper, we show how the smallest quantity of molecules injected into a GC for which an identifiable infrared spectrum can be measured on-line has been reduced by a factor of 10 below the detection limit of the most sensitive current technique. A commercial direct deposition interface between a GC and a Fourier transform infrared spectrometer was modified by vapor-depositing an island film of silver on the surface of the zinc selenide substrate. Band intensities in the spectra of molecules located within approximately 4 nm of the surface of the silver islands were increased by at least 1 order of magnitude through surface-enhanced infrared absorption (SEIRA). The effectiveness of this approach was illustrated by comparing the limits of identification of butylbenzene isomers measured with and without the silver film. Comparison with the spectra of the same molecules measured by mass spectrometry showed the increased sensitivity and specificity of the GC/SEIRA interface.
Journal Article↗
Kinetic modeling of aldehyde adsorption rates on bare and aminopropylsilyl-modified silica gels by ultra-rapid-scanning fourier transform infrared spectrometry.
Enhancements of a model capable of precisely defining rates of adsorption in terms of a Langmuir isotherm and determined from infrared and pressure data collected in the millisecond time regime are described in detail. Rates of adsorption are determined for formaldehyde and acetaldehyde exposed to bare and aminopropylsilyl-derivatized silica gel. The model fits the collected data by varying the number of adsorption sites and the adsorption and desorption rate constants and determines the best fit based on the measured IR absorbance, partial pressure, exposure backpressure, number of adsorption sites, and time. The rate of adsorption onto irregular-shaped silica gel was faster than the rate onto the corresponding spherical silica gel. Formaldehyde was adsorbed more efficiently than acetaldehyde on APS silica gel than on bare silica gel, whereas acetaldehyde was adsorbed more efficiently than formaldehyde on underivatized silica, indicating the possibility that formaldehyde is adsorbed through the formation of a Schiff base.
Journal Article↗
Kinetic modeling of the adsorption rate of a gaseous adsorbate on a granular adsorbent by ultra-rapid-scanning Fourier transform infrared spectrometry.
A novel method for determining the rates of adsorption of gaseous adsorbates on granular adsorbents in the millisecond time regime is described. Mixtures of the adsorbate and nitrogen are passed through a solenoid valve and 100 mg of the adsorbent for periods of (typically) 100 ms. The concentration of the adsorbate passing into a low-volume, long path length gas cell is measured with an ultra-rapid-scanning Fourier transform infrared spectrometer capable of gathering 200 mid-infrared spectra per second with 6-cm(-1) resolution. The pressure of the gas entering the cell is measured simultaneously with a capacitance manometer. A dynamic mathematical model was developed to analyze and describe the results in terms of a Langmuir isotherm. The success of this approach is demonstrated by the estimation of the rate of adsorption of vapor-phase acetaldehyde on aminopropylsilylated granular silica gel.
Journal Article↗
Effect of hydration on band intensities in diffuse infrared spectra of samples in alkali halide matrices.
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Alkalies↗
Off-line direct deposition gas chromatography/surface-enhanced Raman scattering and the ramifications for on-line measurements.
Surface-enhanced Raman spectra (SERS) of molecules separated by gas chromatography (GC) were measured off-line by condensing the analyte on a moving, liquid-nitrogen-cooled ZnSe window on which a 5 nm layer of silver had been formed by physical vapor deposition. After the components that eluted from the chromatograph had been deposited, the substrate was allowed to warm up to room temperature and transferred to the focus of a Raman microspectrometer where the spectrum of each component was measured. Band intensities in the spectrum of 3 ng of caffeine prepared in this way were approximately the same as in the spectrum of bulk caffeine. By making some logical assumptions, it was shown that identifiable GC/SERS spectra of 30 pg of many molecules could be measured over a 300 cm(-1) region in real-time and that if an optimized substrate were used the minimum identifiable quantity could be reduced to 1 pg or less.
Caffeine↗
Anti-Stokes Raman spectrometry with 1064-nm excitation: an effective instrumental approach for field detection of explosives.
Anti-Stokes Raman spectra of 28 explosive materials were obtained with 1064-nm excitation using fiber-optic sampling and a dispersive spectrograph equipped with a charge-coupled device (CCD) array detector. By using a silicon CCD detector, anti-Stokes features could clearly be observed for the majority of samples from -250 to -1650 cm(-1). Using the fiber-optic probe, spectra were routinely obtained from samples positioned up to twelve meters from the spectrograph within 240 s. The utility of an anti-Stokes correction routine is demonstrated, which routine allowed anti-Stokes spectra measured with 1064-nm excitation to be successfully searched and identified against libraries of Stokes spectra obtained using a Fourier transform (FT) Raman system equipped with a 1064-nm Nd:YAG laser.
Journal Article↗
Discrimination of normal and malignant gastric tissues with FTIR spectroscopy and principal component analysis.
In this paper, the identification of normal and malignant gastric tissues, including 11 cases of cancerous tissues and 10 cases of normal tissues, was investigated using mid-IR spectroscopy and principal componentanalysis (PCA). The results indicated that the difference between cancerous and normal tissues was found in the first principal component. The IR detection and PCA results are in agreement with the biopsy results. The combination of these two methods might provide a new opportunity for clinical application.
Algorithms↗
A comparative study of mid-infrared diffuse reflection (DR) and attenuated total reflection (ATR) spectroscopy for the detection of fungal infection on RWA2-corn.
An investigation into the rapid detection of mycotoxin-producing fungi on corn by two mid-infrared spectroscopic techniques was undertaken. Corn samples from a single genotype (RWA2, blanks, and contaminated with Fusarium graminearum) were ground, sieved and, after appropriate sample preparation, subjected to mid-infrared spectroscopy using two different accessories (diffuse reflection and attenuated total reflection). The measured spectra were evaluated with principal component analysis (PCA) and the blank and contaminated samples were classified by cluster analysis. Reference data for fungal metabolites were obtained with conventional methods. After extraction and clean-up, each sample was analyzed for the toxin deoxynivalenol (DON) by gas chromatography with electron capture detection (GC-ECD) and ergosterol (a parameter for the total fungal biomass) by high-performance liquid chromatography with diode array detection (HPLC-DAD). The concentration ranges for contaminated samples were 880-3600 microg/kg for ergosterol and 300-2600 microg/kg for DON. Classification efficiency was 100% for ATR spectra. DR spectra did not show as obvious a clustering of contaminated and blank samples. Results and trends were also observed in single spectra plots. Quantification using a PLS1 regression algorithm showed good correlation with DON reference data, but a rather high standard error of prediction (SEP) with 600 microg/kg (DR) and 490 microg/kg (ATR), respectively, for ergosterol. Comparing measurement procedures and results showed advantages for the ATR technique, mainly owing to its ease of use and the easier interpretation of results that were better with respect to classification and quantification.
Food Analysis↗
Accurate wavelength measurements of a putative standard for near-infrared diffuse reflection spectrometry.
The diffuse reflection (DR) spectrum of a sample consisting of a mixture of rare earth oxides and talc was measured at 2 cm-1 resolution, using five different accessories installed on five different Fourier transform near-infrared (FT-NIR) spectrometers from four manufacturers. Peak positions for 37 peaks were determined using two peak-picking algorithms: center-of-mass and polynomial fitting. The wavenumber of the band center reported by either of these techniques was sensitive to the slope of the baseline, and so the baseline of the spectra was corrected using either a polynomial fit or conversion to the second derivative. Significantly different results were obtained with one combination of spectrometer and accessory than the others. Apparently, the beam path through the interferometer and DR accessory was different for this accessory than for any of the other measurements, causing a severe degradation of the resolution. Spectra measured on this instrument were removed as outliers. For measurements made on FT-NIR spectrometers, it is shown that it is important to check the resolution at which the spectrum has been measured using lines in the vibration-rotation spectrum of atmospheric water vapor and to specify the peak-picking and baseline-correction algorithms that are used to process the measured spectra. The variance between the results given by the four different methods of peak-picking and baseline correction was substantially larger than the variance between the remaining five measurements. Certain bands were found to be more suitable than others for use as wavelength standards. A band at 5943.13 cm-1 (1682.62 nm) was found to be the most stable band between the four methods and the six measurements. A band at 5177.04 cm-1 (1931.61 nm) has the highest precision between different measurements when polynomial baseline correction and polynomial peak-picking algorithms are used.
Calibration↗
Determination of the resolution of a multichannel Raman spectrometer using Fourier transform Raman spectra.
The resolution of a grating polychromator for Raman spectroscopy has been simulated by measuring spectra on a Fourier transform (FT) Raman spectrometer and selecting the FT of the apodization function so that the instrument line shape function mimics the triangular spectral slit function of the polychromator. To this end, FT-Raman spectra measured with a nominal resolution of 0.5 cm-1 were modified through the application of sinc2 apodization functions of various widths to simulate spectra measured on a polychromator at lower resolution. The success of this approach was validated using the 1085 cm-1 band of calcite. When the modified FT-Raman spectra were compared with spectra measured on a grating polychromator equipped with slits of widths 100 and 150 microns, the resolution of the polychromator was estimated to be 6.3 and 7.8 cm-1, respectively. This conclusion was verified experimentally by measuring the separation of two bands in the Raman spectrum of BaSO4 at approximately 460 cm-1.
Algorithms↗
Design and performance considerations of cat's-eye retroreflectors for use in open-path Fourier-transform-infrared spectrometry.
A ray-tracing analysis of cat's-eye retroreflectors for use in active open-path Fourier-transform-infrared (OP/FT-IR) spectrometry and the results of testing f/0.5 and f/1.75 cat's-eye retroreflectors built in our laboratory with a commercial active OP/FT-IR spectrometer are presented. The ray-tracing model is based on the optical characteristics of a commercial single-telescope monostatic OP/FT-IR spectrometer and explores trends in cat's-eye behavior in practical but rigorous field conditions encountered during transportable outdoor use. All mirrors modeled are paraboloids for which the focal ratios of the primary mirror are f/0.5, f/1.75, and f/3. The effect of the focal ratio of the primary mirror, the focal length of the secondary mirror, and the off-axis alignment of the primary and the secondary mirror have been evaluated as a function of path length, including variable input-beam divergence, between the spectrometer and the cat's-eye. The paraboloidal mirrors comprising the primary and secondary of the cat's-eye retroreflectors tested were made in our laboratory by spin casting liquid epoxy-graphite composite mixtures followed by in situ polymerization with no postpolishing.
Journal Article↗