Quantitative measurements of ammonium, hydrogenophosphate and Cu(II) by diffuse reflectance spectrometry

Diffuse reflectance spectrometry is shown to be useful for the quantitative determination of small amounts of pollutants. The relation between sample concentration and reflectance is described by the Kubelka-Munk equation. The experiments were performed with a laboratory constructed diode array spectrophotometer. We can obtain the quantitative reflectance values of different precipitates like ammonium with Nessler's reagent, hydrogenophosphate with silver nitrate and a complex such as Cu(II) with dithiooxamide 'rubeanic acid' by forming a spot colour on filter paper. We have obtained for each reagent a calibration curve by plotting the relative intensity of reflectance versus the log of the mol (dm(3))(-1) concentration. The linearity was obtained for Cu(II) from 8x10(-4) to 2.5x10(-2) mol l(-1) with r(2)=0.9838 and from 10(-3) to 10(-1) mol l(-1) for polyphosphate with r(2)=0.9975 and from 5x10(-4) to 5x10(-2) mol l(-1) for ammonium with r(2)=0.9889. We can consider that for a direct measurement of the intensity of reflectance, it is possible to perform quantitative spot-test analysis.     

Intercomparison of industrial near-infrared diffuse reflectance measurements

An intercomparison of diffuse reflectance measurements in the near-infrared region was carried out. Fourteen participants from industry and academia measured a number of different reflectance standards, calibrated at the NPL, over the wavelength range from 800 to 2500 nm. Measurements were made in the integrating sphere geometries, i.e. specular included and specular excluded and in the 0/45 geometry. The results of the intercomparison are presented. The raw data results showed significant variations in the value of diffuse reflectance measured across industry, in excess of 80% difference from NPL reflectance values in some cases. Renormalisation of the data to the NPL near-infrared diffuse reflectance scale significantly improved the agreement in measured values between participants, most being within 5% of the NPL values. Integrating sphere errors were present in many of the instruments used.     

Rapid quality control of solid materials by diffuse reflectance FTIR spectrometry

This paper describes an approach to quantitative analysis using diffuse reflectance spectra of solid materials in conjunction with the infrared data processing technique CIRCOM, to calculate equations correlating the FTIR spectra with the constituents or properties for a calibration set of well-characterised samples. Unknown samples, similar to those in the calibration set, can then be analysed from their infrared spectra. An example from the minerals industry will be given.     

Examination of organosilicon impregnating mixtures by static SIMS and diffuse reflectance FT-IR

Results of an extensive examination concerning effectiveness and durability of two different commercial organosilicon impregnating mixtures applied to Obernkirchener sandstone are presented briefly. Hygric tests and contact angle measurements prior to spectroscopic surface investigations of a propyl/octylsilane mixture and a methyl/octylsiloxane mixture reveal different long-term stability of the protective organosilicon coatings. General surface characteristics of the coatings were elucidated by time-of-flight secondary ion mass spectrometry (TOF-SIMS). The measurements on freshly soaked stone slabs give evidence of spherical silsesquioxanes and their homoderivatives only of the precursor compound with the shorter alkyl chain. Investigations on the weathered coatings (8 years) show the presence of silsesquioxanes only in case of the silane system. Additional DRIFT (diffuse reflectance FT-IR) measurements on both freshly soaked and exposed material confirm the presence of the coating in case of the silane mixture, but not in case of the siloxane mixture.     

Quantitative diffuse reflectance and transmittance spectroscopy of loosely packed powders

A self-consistent discontinuum theory describing the dependence of the diffuse reflectance and diffuse transmittance of powders on their optical absorp     

Polyurethane foams as solid chromogenic reagents for diffuse reflectance spectroscopy

The chemical reactions of the functional groups in polyurethane foams (PUF) have been studied by use of diffuse reflectance spectroscopy and infrared spectroscopy. It was found that the functional groups are highly reactive towards diazotization by sodium nitrite, azo coupling with 4-nitrophenyldiazonium tetrafluoroborate, oxidation by active chlorine, and condensation with formaldehyde, resulting in the formation of intensely colored products. Heterogeneous chemical reactions of PUF with these compounds in aqueous solution proceed rapidly at room temperature and at low solute concentrations. PUF do not undergo degradation as a result of chemical interactions. The linear response of the Kubelka-Munk function to analyte concentration makes it possible to recommend PUF as solid chromogenic reagents for the determination of nitrite, nitrate, and 1- and 2-naphthols.     

Quantitative investigation of matrices for diffuse reflectance infrared Fourier transform spectrometry

Five materials were investigated for use as diluent matrices for diffuse reflectance infrared Fourier transform (DRIFT) spectrometry of powdered samples. DRIFT spectra of powdered carbazole dispersed in germanium, silicon, diamond, potassium chloride and a chalcogenide glass were compared. Three particle size ranges were investigated for five concentrations of carbazole ranging from 0.1-10%. The effects of particle size and refractive index of the matrix are discussed.     

Quantitative analysis of biopolymer mixtures by pyrolysis-mass spectrometry

In this model study, a series of mixtures of dextran, albumin and lignosulphonic acid were analysed by pyrolysis-mass spectrometry. As a result of intermolecular reactions during the pyrolysis of these mixtures, non-linear single-component responses were observed. Despite this complication, quantitative analysis of the mixtures was achieved through application of discriminant analysis as a data evaluation method.     

Flow injection visible diffuse reflectance quantitative analysis of nickel

Flow injection (FI) methodology, using diffuse reflectance in the visible region of the spectrum, for the analysis of nickel, precipitated in the form of dimethylglyoximate, is presented. A reflectance cell, constructed in polytetrafluoroethylene, using a LED (light emitting diode) as light source and a LDR (light dependent resistor) as detector, is described. The analytical signal (S) correlates with nickel concentration (C) between 1.6 × 10⁻⁴ and 6.6 × 10⁻⁴ mol L⁻¹. This correlation is described by the equation S = −1.108 + 3.314 × 10⁴C − 2.081 × 10⁷C² (r = 0.9996). The experimentally observed limit of detection is about 1.3 × 10⁻⁴ mol L⁻¹, as in lower concentrations the formation of precipitate is not observed. The experimental quantitation limit is about 1.6 × 10⁻⁴ mol L⁻¹. The mean R.S.D. (relative standard deviation) is about 2.7%. Samples containing nickel were analyzed and the results obtained in this method were compared with those of other methods using the statistical Student's t-test.     

Coal analysis by diffuse reflectance near-infrared spectroscopy: Hierarchical cluster and linear discriminant analysis

An extensive study was carried out in coal samples coming from several origins trying to establish a relationship between nine coal properties (moisture (%), ash (%), volatile matter (%), fixed carbon (%), heating value (kcal/kg), carbon (%), hydrogen (%), nitrogen (%) and sulphur (%)) and the corresponding near-infrared spectral data. This research was developed by applying both quantitative (partial least squares regression, PLS) and qualitative multivariate analysis techniques (hierarchical cluster analysis, HCA; linear discriminant analysis, LDA), to determine a methodology able to estimate property values for a new coal sample. For that, it was necessary to define homogeneous clusters, whose calibration equations could be obtained with accuracy and precision levels comparable to those provided by commercial online analysers and, study the discrimination level between these groups of samples attending only to the instrumental variables. These two steps were performed in three different situations depending on the variables used for the pattern recognition: property values, spectral data (principal component analysis, PCA) or a combination of both. The results indicated that it was the last situation what offered the best results in both two steps previously described, with the added benefit of outlier detection and removal.     

Extraction of diffuse reflection spectrum on reflectance spectroscopy for solid or powder surfaces

Reflectance spectroscopy usually produces distorted spectra because it is arbitrarily composed of two different types of reflectance, specular reflectance and diffuse reflectance. That is why this has been an annoying problem for reflectance spectroscopists. In this report, we studied the extraction of only the diffuse reflectance spectra from several reflectance spectra using redundant multiple spectral information, non-negative transformation of the recorded spectra and information processing as the multi-curve resolution. As a result, we found that we could estimate just the diffuse reflectance spectral profile.     

XRD and UV-Vis diffuse reflectance analysis of CeO2-ZrO2 solid solutions synthesized by combustion method

A series of ceria-incorporated zirconia (Ce_1−xZr_xO₂,x = 0 to 1) solid solutions were prepared by employing the solution combustion synthesis route. The products were characterized by XRD and UV-Vis-NIR diffuse reflectance spectroscopy. The materials are crystalline in nature and the lattice parameters of the solid solution series follow Vegard’s law. Diffuse reflectance spectra of the solid solutions in the UV region show two intense bands at 250 and 297 nm which are assigned respectively to Ce³⁺ ← O²⁻and Ce⁴⁺ ← O²⁻ charge transfer transitions. The two vibrational bands in 6960 cm⁻¹ and 5168 cm⁻¹ in the NIR region indicate the presence of surface hydroxyl groups on these materials.     

Quantitative analysis of binary antioxidant mixtures

Quantitative analysis of individual inhibitors and their binary mixtures has been carried out. It has been established that the effect of binary mixtures (additive, synergistic or antagonistic) depends on the structure, concentration ratios and activity of their components. The conditions of the synergistic effect have been determined.

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Quantitative thermogravimetric analysis of binary mixtures

Thermogravimetric analysis is used to determine the amounts of Mg(OH)₂ and Mg(CH₃COO)₂in a mixture thereof. The application and suitability of different analysis methods are discussed. In the first method the mass losses in the temperature ranges as indicated by the decomposition of the pure compounds were used. Results obtained using these temperature ranges were unusable. The percentage mass losses due to the decomposition of Mg(OH)₂ and Mg(CH₃COO)₂ were then determined in a second method using the minimum in the derivative mass vs. temperature curves. The results obtained by this method compared well with the actual values for mixtures containing more than 15% magnesium acetate. The third method employed the total experimental mass loss of both decomposition reactions. The results obtained using this method compared well to the actual values, giving a R ² value of more than 0.99. This method of using the total mass losses can however only be used for binary mixtures that consist only of magnesium hydroxide and magnesium acetate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Quantitative analysis of immobilized proteins and protein mixtures by amino acid analysis

Biomolecular surface engineering of materials often requires precise, versatile and efficient quantification of immobilized proteins at solid surfaces. Acidic hydrolysis of surface-bound proteins and subsequent HPLC analysis of fluorescence-derivatized amino acids were adapted and critically evaluated for that purpose. Contaminations and concentration-dependent amino acid retrieval during HPLC were found to influence the accuracy of the method. In addition to the choice of adequate conditions for hydrolysis, derivatization and chromatographic separation extensions of the data evaluation were suggested to improve the accuracy of the approach when applied to single protein systems: comparing the experimentally obtained amino acid ratio to the protein constitution enabled to identify the properly separated and detected amino acids. Those amino acids were selected for a more precise calculation of the amount of immobilized protein. To further increase the accuracy of the method, the retrieval of amino acids corresponding to protein amounts in the range between 0.5 and 4.0 microg was analyzed for a variety of proteins of interest to derive protein-specific correction factors. The evaluation of amino acid data was furthermore applied to quantify binary protein mixtures at similar settings. This method was proven useful to detect the composition of protein mixtures throughout a wide range of absolute and relative concentrations.     

Determination of copper in urine by diffuse reflectance spectroscopy

A possibility of determining copper(II) after adsorption on polyacrylonitrile fiber filled with a KU-2 cation exchanger with immobilized 1-(2-pyridylazo)-2-naphthol (PANF–KU-2–PAN) is studied. The diffuse reflectance spectra of the carrier discs before and after the adsorption of copper in the batch mode are recorded. The dependences of the analytical signals of the copper complex on PANF–KU-2–PAN on adsorption conditions are studied. The conditions for determining 0.05–0.40 μg/mL of copper(II) by measuring diffuse reflection coefficient at 640 nm or visually by a color scale after the adsorption of copper from 20 mL of a solution containing 0.01 M of HCl are found. The determination of 0.1 μg/mL of copper does not interfered with equal amounts of Co, Zn, and Pb and double amounts of Ag, Fe(III), Cd, Mn(II), Bi(III), and Cr(III). The procedure is used for urine analysis. The following conditions of the preparation of urine samples for the determination of copper were selected: boiling with hydrogen peroxide followed by acidification to pH 2. A procedure for determining copper in urine with a limit of detection of 0.03 μg/mL was developed; the relative standard deviation does not exceed 25%; the duration of analysis of 5–6 samples is 30–35 min.     

Determination of furosemide in pharmaceutical formulations by diffuse reflectance spectroscopy

In this report an analytical method to determine furosemide by using diffuse reflectance spectroscopy is presented. This study shows that this technique can give quantitative results using spot test analysis, particularly in the case of pharmaceuticals containing furosemide. The color spot test could be obtained by reaction between furosemide with p-dimethylaminocinnamaldehyde, in acid medium. This reaction produced a stable complex on filter paper after heating to 80 °C for 5 min. All reflectance measurements were carried out at 585 nm and the linear range was from 7.56×10⁻³ to 6.05×10⁻² mol l⁻¹, with a correlation coefficient of 0.999. The limit of detection was estimated to be 2.49×10⁻³ mol l⁻¹ (R.S.D.=1.7%) and the effect of common excipients on the reflectance measurements was evaluated. The method was applied to determine furosemide in commercial brands of pharmaceuticals. The results obtained by the proposed method were favorably compared with those of the official method, showing for the first time ever that quantitative spot test analysis by diffuse reflectance could be successfully used to determine furosemide in tablets.     

Characterization of estuarine sediments by near infrared diffuse reflectance spectroscopy

It has been developed a partial least squares near infrared (PLS-NIR) method for the determination of estuarine sediment physicochemical parameters. The method was based on the chemometric treatment of first order derivative reflectance spectra obtained from samples previously lyophilized and sieved through a lower than 63 μm grid. Spectra were scanned from 833 to 2976 nm, averaging 36 scans per spectrum at a resolution of 8 cm⁻¹, using chromatographic glass vials of 9.5 mm internal diameter as measurement cells. Models were built using reference data of 31 samples selected through the use of a hierarchical cluster analysis of NIR spectra of sediments obtained from the Ria de Arousa estuary and prediction parameters were established from a validation set of 50 samples of the same area. pH, redox potential (Eh), carbon (C), nitrogen (N) and hydrogen (H) content together with Sn, Pb, Cd, As, Sb and total Cr and also acid soluble, reducible and oxidable Cr fractions were employed as characteristic parameters of the studied sediments. Standard error of prediction values for C and N content were of the order of 4 and 1.3 mg g⁻¹ for H. Prediction errors for pH and Eh were 0.15 units and 37 mV, respectively, thus indicating the good prediction capabilities of the method. Regarding trace metal concentrations PLS-NIR provided prediction error levels for unknown samples around 20% for Sn, Pb, As and Sb and root mean square errors of prediction around 40% for concentration levels of 400 ng g⁻¹ Cd and 100 μg g⁻¹ Cr. For the different extractable fractions of Cr the residual prediction deviation varied from 1.3 to 1.7 but relative errors found for samples of the validation set were only useful for screening purposes.     

Detection of propranolol in pharmaceutical formulations by diffuse reflectance spectroscopy

This paper describes an analytical reflectometric method that has an objective not only the industrial quality control but also to detect possible falsifications and/or adulterations of propranolol in pharmaceutical formulations. The method is based on the diffuse reflectance measurements of the colored product (III) of the spot test reaction between propranolol hydrochloride (I) and 2,6-dichloroquinone-4-chloroimide (II) using filter paper as solid support. Spot test conditions have been investigated using experimental design in order to identify and optimize the critical factors. The factors evaluated were DCQ concentration, propranolol solvent and DCQ solvent. The best reaction conditions were achieved with the addition of 30 microL of propranolol solution in ethanol 35% (v/v) and 30 microL of DCQ solution at 70 mg mL(-1) in acetone, in this order. All reflectance measurements were carried out at 500 nm and the linear range was from 8.45x10(-4) to 8.45x10(-2)mol L(-1) (r=0.998). The limit of detection was 1.01x10(-4)mol L(-1). No interference was observed from the assessed excipients and drugs. The method was applied to determine propranolol in commercial brands of pharmaceuticals. The results obtained by the proposed method were favorably compared with those given by the British Pharmacopoeia procedure.