Calibration in non-linear NIR spectroscopy using principal component artificial neural networks

Near-infrared (NIR) spectroscopy was used in simultaneous, non-destructive analysis of antipyriine and caffeine citrate tablets. Principal component artificial neural networks (PC-ANNs) were used to construct models for the analytes, using the testing set for external validation. Four pretreated spectra, namely, first-derivative, second-derivative, standard normal variate (SNV) and multiplicative scatter correction (MSC) spectra led to simplified and more robust models than conventional spectra. In PC-ANNs models, the spectra data were analyzed by principal component analysis (PCA) firstly. Then the scores of the principal compounds (PCs) were chosen as input nodes for input layer instead of the spectra data. The artificial neural networks (ANNs) models using the spectra data as input nodes were also established, which were compared with the PC-ANNs models. The result shows the SNV model of PC-ANNs multivariate calibration has the lowest training error and predicting error. The concept of the degree of approximation was introduced and performed as the selective criterion of the optimum network parameters.     

一种有机酰腙类配体及其金属凝胶的结构及性能

设计合成了一种带有长链烷基和配位基团的新型有机配体2-吡啶甲醛-4'-十二烷氧基苯甲酰腙(简称L),并采用核磁(1H-NMR)、红外(FTIR)、电喷雾质谱(ESI-MS)和元素分析确认其结构.在乙醇和水的混合溶剂中,配体L可使溶剂凝胶化,并可与多种金属离子(Co(Ⅱ),Ni(Ⅱ),Zn(Ⅱ)和Cu(Ⅱ)作用,形成金属凝胶.采用扫描电镜分析表明,L自身形成的凝胶及金属凝胶的微观形貌均为相互搭接的纤维状结构.采用紫外光谱分析表明在乙醇溶液中L分子形成了聚集体.采用平板流变仪分析表明引入金属离子可提高凝胶的强度.进一步合成了L与Cu(Ⅱ)的配合物,通过对比配合物与金属凝胶的红外光谱和紫外光谱,证明金属凝胶中配体L与金属离子间形成了配位作用.     

Discrimination of poly(vinyl chloride) samples with different plasticizers and prediction of plasticizer contents in poly(vinyl chloride) using near-infrared spectroscopy and neural-network analysis.

In the recycling of poly(vinyl chloride) (PVC), it is required to discriminate every plasticizer for quality control. For this purpose, the near-infrared spectra were measured for 41 kinds of PVC samples with different plasticizers (DINP, DOP, DOA, TOTM and Polyester) and different plasticizer contents (0-49%). A neural-network analysis was applied to the near-infrared spectra pretreated by second-derivative processing. They were discriminated from one another. The neural-network analysis also allowed us to propose a calibration model which predicts the contents of plasticizers in PVC. The correlation coefficient (R) and the root-mean-square error of prediction (RMSEP) for the DINP calibration model were found to be 0.999 and 0.41 wt%, respectively. In comparison, a partial least-squares regression analysis was carried out. The R and RMSEP of the DINP calibration model were calculated to be 0.993 and 1.27 wt%, respectively. It is found that a near-infrared spectra measurement combined with a neural-network analysis is useful for plastic recycling.     

Post-column continuous-flow analysis combined with reversed-phase liquid chromatography and computer-aided photodiode-array detection for the characterization of phloroglucinols by second-derivative difference spectroscopy

Flow-injection analysis coupled with liquid chromatography (LC) and a photodiode-array detector was applied to the investigation of phloroglucinol derivatives (kosins) from Hagenia abyssinica. Because of the structural similarities of the kosins, it was not possible to distinguish between the individual components in the crude mixture by their spectra alone. Post-column pH changes with various reagents, added using a flow-injection system, induced a bathochromic shift of the UV absorption maxima and these were found to furnish additional qualitative information. Addition of 0.3 M potassium hydroxide induced a smaller red shift in the spectrum for α-kosin compared with the shifts for kosotoxin and protokosin. This was confirmed in the normalized difference spectra, ΔA(λ), computed for each kosin by subtracting the normalized spectrum (in eluent at pH 6) from the normalized pH-shifted spectrum (at pH 12). The small pH-induced difference in ΔA(λ) was enhanced in the second-derivative transformation of the difference spectrum. This novel technique for solute recognition permits closely similar pH-sensitive spectra to be discriminated and characterized.     

Potentialities of Quantitative Analysis by High-Performance Liquid Chromatography without Reference Standards of Analytes: Use of Reference Data on UV Spectra for Calculating Calibration Coefficients

It was shown by the examples of simple organic compounds and some medicinal substances that, in the quantitative analysis by the internal standard method in high-performance liquid chromatography (HPLC) with UV detection, one can use calibration coefficients calculated from reference data on UV spectra. Determinations should be performed at several constant wavelengths (for example, 220, 254, 280, and 320 nm) using at least two internal standards (a set of eight samples was proposed) rather than one standard. As can be judged from the estimates obtained, the systematic component of the determination error under these conditions does not exceed the contributions of random errors. The developed approach opens up new opportunities for quantitative determinations by HPLC without reference samples of analytes, instead of which reference data on molar absorptivities are used.     

A flow-through fluorimetric sensing device for determination of alpha- and beta-naphthol mixtures using a partial least-squares multivariate calibration approach

A single flow-through optosensor spectrofluorimetric system is proposed for the resolution of mixtures of alpha- and beta-naphthol at mug l(-1) levels using a partial least-squares (PLS) calibration approach. The sensor was developed in conjunction with a monochannel flow-injection analysis system with fluorimetric detection using Sephadex QAE A-25 resin as an active sorbent substrate in the flow cell and the second derivative of the native synchronous fluorescence spectra of analytes as analytical signal. In the manifold, the solutions of naphthol (at pH 10.0) were injected in a carrier stream of KCl (0.15 M)/NaOH (10(-2) M). Because of the strong spectral overlap, the mixture could not be resolved by conventional spectrofluorimetry. The non-additive behaviour of the fluorescence signals revealed an interaction in the system, which was not found by working in the solution only (without the sorbent support). This interaction, probably due to the environment of the analytes on the solid phase, made impossible their simultaneous determination. So, the use of synchronous fluorescence spectroscopy or even its derivative signal could not resolve satisfactorily the mixture. The simultaneous determination of both naphthol has been carried out by recording the signal of the second-derivative synchronous fluorescence (Deltalambda=170 nm) spectra between 200 and 450 nm and a PLS multivariate calibration treatment. The optimum number of factors was selected by using the cross-validation method. After validating the proposed method, it was applied to the determination of these compounds in natural waters with different amounts of each chemical.     

Analysis of trifluralin and other dinitroaniline herbicide residues by zero-order and derivative ultraviolet spectrophotometry

The utility of zero-order and first- and second-derivative ultraviolet (UV) spectrophotometry for the identification of benfluralin, trifluralin, isopropalin and oryzalin is discussed. These four herbicides were determined by zero-order and first-derivative UV spectrophotometry, with linear calibration graphs established between 50 and 100 concentration units and limits of detection ranging from 1 to 7 micrograms ml-1. The application of these techniques to the residue analysis of fortified soils and niebe and peanut leaves is described. Trifluralin residues were found to be 6.7, 8 and 1.7 micrograms ml-1 in samples of fortified soils, niebe leaves and peanut leaves, respectively. Isopropalin residues were found to range from 62 to 154 micrograms ml-1 in samples of fortified niebe leaves.     

Determination of dissociation constants of sparingly soluble phenothiazine derivatives by second-derivative spectrophotometry

The dissociation constants (pK_a) for sparingly soluble phenothiazines (promazine, chlorpromazine, trifluoropromazine) in water were measured by second-derivative spectrophotometry. The intense background signals in the absorption spectra due to the turbidity caused by the precipitation of insoluble free base of the phenothiazine derivatives were eliminated in the second-derivative spectra, and the solubilities of the phenothiazine derivatives could be easily determined from the peak-to-trough lengths (D values) of the second-derivative signals. The pK_a values were calculated from the pH dependence of the D values. The pK_a values obtained agreed well with reported values and the standard deviations for 6–10 determinations were ? 0.02. The solubilities of the free bases of the phenothiazines were also determined.     

Multicomponent analyses of chiral samples by use of regression analysis of UV–visible spectra of cyclodextrin guest–host complexes

We report the first combined use of analytical spectroscopy, guest–host chemistry, and multivariate regression analysis for determination of enantiometric composition of multicomponent samples of chiral analytes. Sample solutions containing multicomponent analytes of ephedrine, tryptophan, propranolol, and proline of varying enantiomeric composition with beta-cyclodextrin (BCD) or methyl-beta-cyclodextrin (Me-BCD) as chiral host molecules were investigated using ultraviolet (UV)–visible spectroscopy. The interactions of enantiomers of chiral analytes with chiral hosts resulted in the formation of transient diastereomeric inclusion complexes with varying spectral properties. Multivariate analysis using partial-least-square (PLS) regression was used to correlate subtle changes in the UV–visible spectra of the guest–host complexes with the enantiomeric composition of the calibration samples. These PLS regressions were carefully optimized and then used to predict the enantiomeric composition of multicomponent chiral analytes of validation samples. The results of these validation studies demonstrate the predictive ability of the regression models for determination of future enantiomeric composition of samples. The accuracy of the models to correctly predict the enantiomeric composition of samples, evaluated by use of the root mean square percent relative error (RMS%RE) was analyte and chiral host dependent. In general, better prediction of enantiomeric composition of samples and low RMS%RE values were obtained when Me-BCD was used as the chiral host. The analyses procedure reported here is simple, rapid, and inexpensive. In addition, this approach does not require prior separation of chiral analytes, thus reducing analysis time and eliminating the need for expensive chiral columns.     

Simultaneous determination of Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+ by using second-derivative spectrophotometry method

A new method of simultaneous determination of Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+ is proposed here by using the second-derivative spectrophotometry method. In pH=10.35 Borax-NaOH buffer, using meso-tetra (3-methoxyl-4-hydroxylphenyl) porphyrin ([T-(3-MO-4-HP)P]) as chromomeric reagent, micelle solution was formed after Tween-80 surfactant was added into the solution containing Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+ ions. The original absorption spectrum of the above complexes was obtained after heating in the boiling water for 25 min. The second-derivative absorption peaks of five metal-porphyrin complexes can be separated from the original absorption spectrum by using chemometric tool. In this way, Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+ ions can be determined simultaneously. Under the optimal conditions, the linear ranges of the calibration curve were 0-0.60, 0-0.60, 0-0.40, 0-0.80 and 0-0.48 μg mL(-1) for Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+, respectively. The molar absorptivity of these color systems were 1.38×10(5), 1.01×10(5), 3.24×10(5), 1.07×10(5) and 1.29×10(5)Lmol(-1)cm(-1). The method developed in this paper has advantages in selectivity, sensitivity, operation and can effectively resolve spectra overlapping problem. This method has been applied to determine the real samples with satisfactory results.     

Investigation of chemometric calibration performance based on different chemical matrix and signal-to-noise ratio.

The chemometric calibration performance was systematically investigated by two parameters (changing the chemical matrix as well as the signal-to-noise ratio) of the NIR (near-infrared) spectrum. Three different analytes (hexane, cyclohexane, toluene) were selected and heptane was used as a solvent. The degree of spectral difference significantly affected the calibration performance. The largest structural difference between the analyte and the solvent provided the best calibration result for a given signal-to-noise ratio. Additionally, the signal-to-noise ratio of the spectra also directly influenced the calibration performance. Overall, the spectral difference and signal-to-noise ratio were the major factors for governing the chemometric calibration performance, especially in the low-concentration range.     

Rapid and selective determination of UV filters in seawater by liquid chromatography-tandem mass spectrometry combined with stir bar sorptive extraction

Fast liquid chromatography coupled to triple-quadrupole tandem mass spectrometry was employed for the determination of six UV filters in seawater. The separation of the analytes was achieved in less than 5 min; polarity switching was used as four of the analytes were ionized in positive mode and the remaining two in negative mode. Two ionization sources were employed and compared: atmospheric pressure chemical ionization (APCI) gave better results than electrospray ionization (ESI) for all analytes, with higher reproducibility and lower detection limits. Therefore APCI was chosen for the determination of the analytes in seawater samples using stir bar sorptive extraction-liquid desorption (SBSE-LD).Quantitative analysis was performed in multiple reaction monitoring (MRM) mode; fragmentation pathways of the analytes with regard to the formation of the MRM ions were also proposed.For the analysis of seawater samples, calibration curves were drawn using SBSE in spiked seawater. All figures of merit of the method were satisfactory; limits of detection were particularly low for the four analytes ionized in positive mode, being in the range 8-31 ng/L. The method was applied to the determination of the six UV filters in seawater samples from Liguria, Italy. Only benzophenone-3 (BP-3) and ethylhexyl methoxycinnamate (EHMC) were measured in the analyzed samples; some of the remaining analytes were also detected but always below the limit of quantitation.     

Determination of β-sitosterol and cholesterol in oils after reverse micelles with Triton X-100 coupled with ultrasound-assisted back-extraction by a water/chloroform binary system prior to gas chromatography with flame ionization detection

Ultrasonic back-extraction of Triton X-100 reverse micelles by a water/chloroform binary system and gas chromatography with flame ionization detection (GC-FID) was developed for extraction and determination of β-sitosterol and cholesterol in soybean and sunflower oil samples. After the homogenization of the oil samples with Triton X-100, an aliquot of 200 μL of methanol was added to the samples to form two phases. The clear Triton X-100 extract obtained by centrifugation was treated with a mixture of water (1000 μL) and chloroform (300 μL) for back-extraction of the analytes into the chloroform phase by ultrasonication. After centrifugation, the sedimented chloroform layer was withdrawn easily by a microsyringe and directly injected into the GC-FID system. The influence of several important parameters on the extraction efficiencies of the analytes was evaluated. Under optimized experimental conditions, the calibration graphs were linear in the range of 1.0–30.0 mg L⁻¹ with coefficient of determination more than 0.994 for both analytes. The method detection limit values were in the range of 0.2–0.7 mg L⁻¹. The lower limit of quantification values were in the range of 0.7–2.4 mg L⁻¹. Intra-day relative standard deviations were in the range of 1.0–2.7%. This procedure was successfully applied with satisfactory results to the determination of β-sitosterol and cholesterol in spiked oil samples. The relative mean recoveries of oil samples ranged from 93.6% to 105.0%.     

Simultaneous determination of the colorants tartrazine, ponceau 4R and sunset yellow FCF in foodstuffs by solid phase spectrophotometry using partial least squares multivariate calibration

A method is proposed for the simultaneous determination of the colorants tartrazine (TT), sunset yellow FCF (SY) and ponceau 4R (PR) in foods. The colorants were fixed in Sephadex DEAE A-25 gel at pH 2.0 and then packed in a 1-mm silica cell. The spectra of the analytes fixed in the solid support were recorded between 400 and 800 nm against a blank and the partial least squares (PLS) multivariate calibration was used to obtain the results. The linear dynamic ranges of the calibration graphs were from 50.0 to 650.0 ng ml(-1) for the three colorants and these ranges were taking in account the optimisation of the calibration matrix using the PLS-1 algorithm. The experimental results showed that the optimum number of factors for the calibration matrix was four in all instances and the residual means standard deviation was 5.5267 for SY, 6.3878 for TT and 6.9816 for PR. The square of the correlation coefficients were 0.9977, 0.9978 and 0.9954 for SY, TT and PR respectively. The method was applied to the determination of the colorants in foods and results were compared with those obtained by means of HPLC as reference method. The results obtained can be considered as acceptable in most cases (eight of nine commercial samples). The relative standard deviations ranging between 0.5 and 10.8 for the commercial samples analysed.     

Micellar electrokinetic chromatography with on-line Fourier transform infrared detection

Micellar electrokinetic chromatography (MEKC) was successfully coupled to Fourier transform infrared (FTIR) detection, using a micromachined IR-transparent flow cell with an optical path length of 15 micro m for the on-line detection of five neutral analytes. Tight connections between the flow cell and the capillaries were achieved by creating a small O-ring of UV-curing epoxy adhesive on the sharply cut capillary ends. The background electrolyte consisted of 15 mM phosphate buffer at pH 7 and 40 mM sodium dodecyl sulfate (SDS). Five analytes (paracetamol, caffeine, p-nitro benzyl alcohol, m-nitrophenol and p-nitrophenol) were successfully separated, yielding detailed IR stack plots that could be used for quantification and identification. Linear calibration graphs were obtained for each individual analyte present in mixtures at concentrations up to 10 mM. The limit of detection (3 S/N) ranged between 1.1 and 1.5 mM (1.2-1.8 ng). Analytes were identified by comparing spectra obtained during the MEKC separation with those resulting from completely filling the capillary with each individual analyte dissolved in the micelle-containing electrolyte. Information on the specific functional groups of all analytes could be elucidated from the spectra. Since FTIR is a nondestructive detection technique, a conventional on-line UV detector was introduced directly after the developed IR flow cell to test the system's performance and to demonstrate that tandem FTIR and UV detection is feasible.     

NIR analysis of pharmaceutical samples without reference data: improving the calibration

Using an appropriate set of samples to construct the calibration set is crucial with a view to ensuring accurate multivariate calibration of NIR spectroscopic data. In this work, we developed and optimized a new methodology for incorporating physical variability in pharmaceutical production based on the NIR spectrum for the process. Such a spectrum contains the spectral changes caused by each treatment applied to the component mixture during the production process. The proposed methodology involves adding a set of process spectra (viz. difference spectra between those for production tablets and a laboratory mixture of identical nominal composition) to the set of laboratory samples, which span the wanted concentration range, in order to construct a calibration set incorporating all physical changes undergone by the samples in each step of the production process. The best calibration model among those tested was selected by establishing the influence of spectral pretreatments used to obtain the process spectrum and construct the calibration models, and also by determining the multiplying factor m to be applied to the process spectra in order to ensure incorporation of all variability sources into the calibration model. The specific samples to be included in the calibration set were selected by principal component analysis (PCA). To this end, the new methodology for constructing calibration sets for determining the Active Principle Ingredients (API) and excipients was applied to Irbesartan tablets and validated by application to the API and excipients of paracetamol tablets. The proposed methodology provides simple, robust calibration models for determining the different components of a pharmaceutical formulation.     

Analysis of mixtures of purines and pyrimidines by first- and second-derivative ultraviolet spectrometry

Zero-order, first-derivative and second-derivative ultraviolet absorption spectra of a series of purines, pyrimidines and their binary mixtures in aqueous solution have been recorded at 298 K. It is shown that second-derivative spectra can be used for the identification of eight mixtures of purines and pyrimidines. Several graphical procedures are tested for evaluating derivative spectra in quantitative measurements of single compounds and mixtures. Linear log-log calibration plots are obtained with correlation coefficients generally larger than 0.99. Second-derivative spectra appear to provide a precise and simple method for determination of purines and pyrimidines, at concentrations ranging between 5 x 10(-6) and 5 x 10(-4)M.     

Wavelet analysis used for spectral background removal in the determination of glucose from near-infrared single-beam spectra

Wavelet analysis is developed as a preprocessing tool for use in removing background information from near-infrared (near-IR) single-beam spectra before the construction of multivariate calibration models. Three data sets collected with three different near-IR spectrometers are investigated that involve the determination of physiological levels of glucose (1-30 mM) in a simulated biological matrix containing alanine, ascorbate, lactate, triacetin, and urea in phosphate buffer. A factorial design is employed to optimize the specific wavelet function used and the level of decomposition applied, in addition to the spectral range and number of latent variables associated with a partial least-squares calibration model. The prediction performance of the computed models is studied with separate data acquired after the collection of the calibration spectra. This evaluation includes one data set collected over a period of more than 6 months. Preprocessing with wavelet analysis is also compared to the calculation of second-derivative spectra. Over the three data sets evaluated, wavelet analysis is observed to produce better-performing calibration models, with improvements in concentration predictions on the order of 30% being realized relative to models based on either second-derivative spectra or spectra preprocessed with simple additive and multiplicative scaling correction. This methodology allows the construction of stable calibrations directly with single-beam spectra, thereby eliminating the need for the collection of a separate background or reference spectrum.     

Simultaneous spectrophotometric determination of phenanthridine, phenanthridinone and phenanthridine N-oxide using multivariate calibration methods

The multivariate calibration methods, partial least squares (PLS) and principle component regression (PCR) have been used to determine phenanthridine, phenanthridinone and phenanthridine N-oxide in spiked human plasma samples. Resolution of binary and ternary mixtures of analytes with minimum sample pre-treatment and without analyte separation has been successfully achieved analyzing the UV spectral data. The net analyte signal (NAS) concept was also used to calculate multivariate analytical figures of merit such as limit of detection, selectivity and sensitivity. The simultaneous determination of three analytes was possible by PLS and PCR processing of sample absorbance in the 210–355 nm region. Good recoveries were obtained for both synthetic mixtures and spiked human plasma samples.     

Comparison of three spectrophotometric methods for the determination of gamma-oryzanol in rice bran oil

A comparison of the results obtained by applying three spectrophotometric methods (at fixed wavelength, second-derivative and multicomponent analysis) to the determination of gamma-oryzanol in rice bran oil is reported.At fixed wavelength the results are more accurate when using isopropyl alcohol, rather than n-heptane, to dilute the oil samples, because the absorption bands of gamma-oryzanol are red-shifted and the absorbance, measured at lambda(max)=327 nm, is less affected by the interference of the oil "matrix" (lambda(max)=314 nm in n-heptane).However, to obtain accurate results also in oils with a low content of gamma-oryzanol, it is necessary to perform the analysis using second-derivative ((2)D330.365) or multicomponent (lambda=310-360 nm) methods. The first one fully removes the interference of oil matrix whilst the second, which needs a specific computational program to process the spectrophotometric data, furnishes evidence the presence of some unexpected interference in the analysis and/or standards which are not representative of the analysed samples, from the square root of the sum of the squared differences at each point between the linear combination of the standards and the unknown spectra (RMS error).Finally, some aspects of the chemical, spectroscopic (UV, IR) and thermoanalytical (TG, DSC) behaviour of gamma-oryzanol and the values of the parameters which enable "computation" of its UV spectra are reported.