Comparison of near-infrared and mid-infrared spectroscopy for the determination of distillation property of kerosene

Near-infrared (NIR) and mid-infrared (MIR) spectroscopy have been compared and evaluated for the determination of the distillation property of kerosene with the use of partial least squares (PLS) regression. Since kerosene is a complex mixture of similar hydrocarbons, both spectroscopic methods will be best evaluated with this complex sample matrix. PLS calibration models for each percent recovery temperature have been developed by using both NIR and MIR spectra without spectral pretreatment. Both methods have shown good correlation with the corresponding reference method, however NIR provided better calibration performance over MIR. To rationalize the improved calibration performance of NIR, spectra of the same kerosene sample were continuously collected and the corresponding spectral reproducibility was evaluated. The greater spectral reproducibility including signal-to-noise ratio of NIR led to the improved calibration performance, even though MIR spectroscopy provided more qualitative spectral information. The reproducibility of measurement, signal-to-noise ratio, and richness of qualitative information should be simultaneously considered for proper selection of a spectroscopic method for quantitative analysis.     

Laser ablation molecular isotopic spectrometry of water for 1D2/1H1 ratio analysis

Laser Ablation Molecular Isotopic Spectrometry (LAMIS) has been investigated for optical isotopic analysis of the deuterium to protium ratio in enriched water samples in ambient air at atmospheric pressure. Multivariate PLSR (Partial Least Squares Regression) based calibrations were carried out and validated using multiple statistical parameters. Comparisons of results are reported using two spectrometers having two orders of magnitude difference in spectral resolution. The accuracy and precision of isotopic analysis depends on the spectral resolution and the inherent isotope shift of the elements. The requirements for spectral resolution of the measurement system can be significantly relaxed when the isotopic abundance ratio is determined using chemometric processing of the spectra. Large isotopic shifts in the individual rotational branches of OH/OD molecular emission spectra were measured. Optimized temporal conditions for LAMIS measurements were established. Several sub-regions of spectra were used for PLSR calibration and the results demonstrate that both the emission intensity and degree of spectral differentiation affect the quality of the PLSR calibration. LAMIS results also were compared with traditional LIBS results obtained using PLSR and a spectral deconvolution method, demonstrating the advantages of LAMIS over LIBS with respect to isotopic composition determination.     

Experimental and Chemometric Optimization to Enhance the Performance of Near-infrared Diffuse Reflectance Spectroscopy

Approaches for improving the performance of near-infrared diffuse reflectance spectroscopy (NIRDRS) have been extensively studied. The silver mirror was found to be a substrate to enhance the detection ability of this technique. For further optimization of the method, experimental and chemometric efforts were made in this study. Lysozyme samples of different concentrations were spotted on the silver mirror for measuring the spectra. Low-resolution spectra with repeated measurements were used for increasing the signal-to-noise ratio, and the chemometric methods of variable selection and nonlinear modeling were used to make the quantitative model more effective and accurate. The results show that the quality of the spectra was improved and a satisfactory quantitative model was obtained after optimization. The maximum deviation for the prediction samples is only 2.98?µg, which is much lower than the reported values in previous papers. Significant advances were obtained for NIRDRS for trace analysis.     

Simultaneous spectrophotometric determination of paracetamol, ibuprofen and caffeine in pharmaceuticals by chemometric methods

In this study, the simultaneous determination of paracetamol, ibuprofen and caffeine in pharmaceuticals by chemometric approaches using UV spectrophotometry has been reported as a simple alternative to using separate models for each component. Spectra of paracetamol, ibuprofen and caffeine were recorded at several concentrations within their linear ranges and were used to compute the calibration mixture between wavelengths 200 and 400 nm at an interval of 1 nm in methanol:0.1 HCl (3:1). Partial least squares regression (PLS), genetic algorithm coupled with PLS (GA-PLS), and principal component-artificial neural network (PC-ANN) were used for chemometric analysis of data and the parameters of the chemometric procedures were optimized. The analytical performances of these chemometric methods were characterized by relative prediction errors and recoveries (%) and were compared with each other. The GA-PLS shows superiority over other applied multivariate methods due to the wavelength selection in PLS calibration using a genetic algorithm without loss of prediction capacity. Although the components show an important degree of spectral overlap, they have been determined simultaneously and rapidly requiring no separation step. These three methods were successfully applied to pharmaceutical formulation, capsule, with no interference from excipients as indicated by the recovery study results. The proposed methods are simple and rapid and can be easily used in the quality control of drugs as alternative analysis tools.     

A novel fourth-order calibration method based on alternating quinquelinear decomposition algorithm for processing high performance liquid chromatography–diode array detection– kinetic-pH data of naptalam hydrolysis

Five-way high performance liquid chromatography–diode array detection (HPLC–DAD)–kinetic-pH data were obtained by recording the kinetic evolution of HPLC–DAD signals of samples at different pH values and a new fourth-order calibration method, alternating quinquelinear decomposition (AQQLD) based on pseudo-fully stretched matrix forms of the quinquelinear model, was developed. Simulated data were analyzed to investigate the performance of AQQLD in comparison with five-way parallel factor analysis (PARAFAC). The tested results demonstrated that AQQLD has the advantage of faster convergence rate and being insensitive to the excess component number adopted in the model. Then, they have been successfully applied to investigate quantitatively the kinetics of naptalam (NAP) hydrolysis in two practical systems. Additionally, the serious chromatographic peak shifts were accurately corrected by means of chromatographic peak alignment method based on abstract subspace difference. The good recoveries of NAP were obtained in these samples by selecting the time region of chromatogram. The elution time, spectral, kinetic time and pH profiles resolved by the chemometric techniques were in good agreement with experimental observations. It demonstrates the potential for the utilization of fourth-order data for some complex systems, opening up a new approach to fourth-order data generation and subsequent fourth-order calibration.     

Fractional-continuous wavelet transforms and ultra-performance liquid chromatography for the multicomponent analysis of a ternary mixture containing thiamine, pyridoxine, and lidocaine in ampules

New chemometric approaches based on the application of partial least squares (PLS) and principal component regression (PCR) algorithms with fractional wavelet transform (FWT) and continuous wavelet transform (CWT) are proposed for the spectrophotometric multicomponent determination of thiamine hydrochloride (B1), pyridoxine hydrochloride (B6), and lidocaine hydrochloride (LID) in ampules without any separation step. In this study PLS and PCR techniques were applied to the raw spectral data, FWT-coefficients, and FWT-CWT-coefficients. These calibration models were labeled as Raw-PLS and Raw-PCR, FWT-PLS and FWT-PCR, and FWT-CWT-PLS and FWT-CWT-PCR, respectively. A new ultra-performance liquid chromatographic (UPLC) method was developed for the comparison of the results obtained by applying the chemometric calibration methods. Chromatographic separation and determination of B1, B6, and LID in ampules were performed on an Acquity UPLC BEH C18 column (50x2.1 mm id, 1.7 pm particle size) using gradient elution with a mobile phase consisting of methanol and 0.01 M HCI at a constant flow rate of 0.6 mL/min. These combined chemometric calibrations and UPLC were validated by analyzing various ternary mixtures, B1, B6, and LID. The proposed chemometric approaches (signal processing-multivariate calibrations) and UPLC method were applied to the quantitative multicomponent analysis of marketed ampules containing the vitamins B1 and B6 with LID.     

Quantitative determination of acetylsalicylic acid and acetaminophen in tablets by FT-Raman spectroscopy

A procedure for quantitative determination of acetylsalicylic acid and acetaminophen in pharmaceuticals by PLS (partial least squares) and PCR (principal component regression) treatment of FT (Fourier transform)-Raman spectroscopic data is proposed. The proposed method was tested on powdered samples. Three chemometric models were built: the first, for samples consisting of an active substance diluted by lactose, starch and talc; the second, in which a simple inorganic salt was applied as an internal standard and additions were not taken into account; and the third, in which a model was constructed for a commercial pharmaceutical, where all constituents of the tablet were known. By utilising selected spectral ranges and by changing the chemometric conditions it is possible to carry out fast and precise analysis of the active component content in medicines on the basis of the simplified chemometric models. The proposed method was tested on five commercial tablets. The results were compared with data obtained by intensity ratio and pharmacopoeial methods. To appraise the quality of the models, the relative standard error of predictions (RSEPs) were calculated for calibration and prediction data sets. These were 0.7-2.0% and 0.8-2.3%, respectively, for the different PLS models. Application of these models to the Raman spectra of commercial tablets containing acetylsalicylic acid gave RSEP values of 1.3-2.0% and a mean accuracy of 1.2-1.7% with a standard deviation of 0.6-1.2%.     

Lineare Silizium-Photodioden-Matrizen mit parallelem Datenausgang als Strahlungsempfänger in der optischen Emissionsspektroskopie—II. Analytische anwendung in verbindung mit der glimmentladungslampe nach Grimm

A silicon photodiode array was placed in the focal plane of a spectrometer to detect the radiation. Three single diodes of the array, each 6.5 mm long and 19 μm wide, were connected to the three channels as described in Part I.The properties of the photodiodes were investigated with respect to their applicability in spectral analysis. Especially the spectral response, the dynamic range and the signal-to-noise ratio were of great interest. The quantum efficiency was found to be in the range from 300 to 800nm. The dynamic range, defined as the range from the noise level to the maximum permissible signal voltage extends over 5.5 decades and is a linear function of the radiative flux. The signal-to-noise ratio of systems using photodiodes depends on the wavelength of the radiation and, if high feedback resistors (≧10¹¹Ω) are used, reaches the same order of signal-to-noise ratio as that obtained by using a photomultiplier (EMI 9789 QB). To demonstrate the applicability of the photodiodes, the analysis of limestone was studied. The sample powder was mixed with copper powder, pressed into pellets, and the major constituents Al, C, Mg and Si were determined by means of a Grimm glow discharge lamp. The calibration curves obtained as well as the detection limits are reported.Furthermore, oxygen was determined in a series of synthetic samples using the line 777.2 nm.     

Improving the linearity of spectroscopic data subjected to fluctuations in external variables by the extended loading space standardization

In process analytical applications, spectral measurements can be subject to changes in process temperature, pressure, flow turbulence, and compactness as well as other external variations. Generally, the variations of external variables influence spectral data in a non-linear manner which leads to the poor predictive ability of bilinear calibration models on raw spectral data. In this contribution, the influence of external variables on spectral data is generally classified into two different modes, multiplicative influential mode and composition-related influential mode. A new chemometric method, termed Extended Loading Space Standardization (ELSS), has been developed to explicitly model these two kinds of influential modes. ELSS was applied to two sets of spectral data with fluctuations in external variables and its performance evaluated and compared with global partial least squares (PLS) models and Loading Space Standardization (LSS). Results show that ELSS can efficiently model the external non-linear effects in both data sets and greatly improve the accuracy of predictions with the mean square error of prediction for test samples being 2-3 times smaller than those of LSS and global PLS.     

Comparison of various chemometric evaluation approaches for on-line FT-NIR transmission and FT-MIR/ATR spectroscopic data of methyl methacrylate solution polymerization

For the evaluation of vibrational spectroscopic data acquired on-line to a chemical reaction a broad range of different chemometric algorithms is available. The present study reports the comparative results obtained by different chemometric techniques from the data acquired by light-fiber coupled Fourier-transform near infrared (FT-NIR) transmission spectroscopy and Fourier-transform mid-infrared (FT-MIR) spectroscopy in the attenuated total reflection (ATR) mode to monitor the solution polymerization of methyl methacrylate (MMA). We have found that the results obtained by the application of multivariate curve resolution (MCR) methods to the MIR spectral data acquired during the polymerization of MMA are quite comparable to the results derived by partial least-squares calibration. In the case of the NIR data univariate calibration yields somewhat poorer results than multivariate calibration and MCR, but still inside an acceptable range.     

Moment combined partial least squares (MC-PLS) as an improved quantitative calibration method: application to the analyses of petroleum and petrochemical products

A new quantitative calibration algorithm, called "Moment Combined Partial Least Squares (MC-PLS)", which combines the moment of spectrum and conventional PLS was proposed. Its calibration performance was evaluated for the analyses of three import petroleum and petrochemical products: gasoline, naphtha and polyol samples. The selected properties for these products included the research octane number (RON) and Reid vapor pressure (RVP) for gasoline, the distillation temperature at 10% (D 10%) for naphtha and the hydroxyl (OH) number for polyol. The major concept presented here used the moment to find the closest spectrum of a sample in a given dataset, and generate the difference spectrum and the corresponding difference in the property. These difference spectra and property differences were then used for PLS calibration. The moment has been employed in spectroscopic fields as a simple and effective "spectral feature characteristic" using just a few scalar values (moments). MC-PLS showed improved prediction performance over PLS for each case. In MC-PLS, the difference spectra generated using the moments were used as explained; therefore, additional detail in spectral variations can be utilized for calibrations. Additionally, the difference in the property was employed as reference data, so that its variation range was smaller when compared with that of the original property. Consequently, the MC-PLS performance could be better since the feature-enhanced spectra were used to model a narrower range of property variations. In the case of the D 10% prediction for naphtha, a non-linear prediction pattern that occurred in conventional PLS was effectively corrected using the MC-PLS method.     

CMOS arrays as chemiluminescence detectors on microfluidic devices

A simple, low-cost process to integrate complementary metal oxide semiconductor array detectors (CMOSAD) for chemiluminescence is presented, evaluated, and applied to the determination of nitrite in ground water samples. CMOS arrays of different brands (obtained from commercial image sensors) were adapted as chemiluminescence detectors on microfluidic devices. The performance of the CMOSADs was evaluated in the visible zone of the spectrum using a tungsten halogen lamp as light source. Intrinsic parameters assessed included signal stability, spectral response, dark current, and signal-to-noise ratio. Thereafter, the CMOSADs were integrated on microfluidic devices and their performances in quantitative analysis were assessed with the chemiluminometric reaction of hydrogen peroxide with luminol, catalyzed with hexacyanoferrate (III). The parameters assessed were sensitivity, linear range, detection limit, reproducibility, correlation coefficient of the calibration curves, and baseline drift during measurements. The CMOSAD with the best performance was selected to assess the applicability of the developed microfluidic devices with the integrated detector. The microfluidic system permitted the determination of nitrite with both good precision and good recovery values in the analysis of ground water samples. Integration was easily achieved and enabled the development of a simple, low-cost, and feasible alternative to conventional detectors.     

Chemometrics: the issues of measurement and modelling

In this paper, two spectral data sets have been used to illustrate the importance of maintaining chemical information whilst generating predictive multivariate calibration models. The first data set is based on 26 duplicate UV/VIS spectra for four meal ions (Fe, Ni, Co, Cu) present at varying concentrations in aqueous solution. Spectra were collected across the range 180–800 nm at a resolution of 3.5 nm generating 211 data points for each sample. Calibration was carried out using multiple linear regression (MLR) and a K-matrix approach to demonstrate the advantages the latter method has in describing real spectral features. In addition, the limitation of MLR in accommodating noise and spectral overlap in the data is also illustrated. The second data set based on NIR spectroscopy, was generated using a four-level 2 factor Factorial design strategy and consisted of two additives present at a range of concentrations in an aqueous caustic system, with the spectra being collected over the range 10,000–3000 cm⁻¹. Whilst a conventional partial least squares (PLS) model was applied to the data, it was through the use of variable selection (VS) prior to PLS and the application of weighted ridge regression (WRR) techniques that the need to develop chemometric methodology which intuitively reflected chemical information has been demonstrated. The results will also illustrate how a poorly designed experimental design protocol and missing data can limit the performance of the calibration models generated. The aims of this paper are not to prescribe ideal calibration methodology but rather to demonstrate the relevance of selecting multivariate calibration methodology that relates more to the chem rather than just the metrics in chemometrics.     

液-液-液微萃取-高效液相色谱法测定人尿液中的美沙酮

建立了液-液-液微萃取与高效液相色谱联用技术快速分析尿样中美沙酮的方法.对有机溶剂种类、体积、样品溶液的pH值、萃取时间、搅拌速度进行了优化.方法的线性范围为0.05～10 mg/L,检出限为0.025 mg/L,相对标准偏差小于5%.     

Acetone extraction and HPLC determination of acrylamide in potato chips

A new sensitive method using high performance liquid chromatography (HPLC) and liquid extraction for the analysis of acrylamide (AA) in potato chips is reported. The method comprises extraction with acetone using ultrasonic bath and reversed phase C18-AQ (2 × 250 mm) column with water as eluent. Flow rate was 0.15 ml min^-1 and the column temperature was kept constant at 40 °C. The analysis was performed using a 20 μl injection loop and a UV detector adjusted at 202 nm. In this condition, the retention time for AA was 8 min. A linear calibration curve (regression coefficient = 0.999) in the range of 20–400 ng g^-1 was used for quantitative purposes. Limit of detection (LOD) (signal-to-noise ratio of 3:1) and limit of quantification (LOQ) (signal-to-noise ratio of 10:1) for the method was 2.46 and 3.14 ng g^-1, respectively. Extracted samples and standard solutions with different concentrations of AA were analyzed repeatedly in one day and different days to estimate the repeatability and reproducibility of the method. Analysis of variance on the obtained data showed no significant difference between variances in different days. Using the proposed method, different potato chips samples were analyzed in different days in another laboratory. Paired t-test showed no significant difference between the obtained results from the two laboratories.     

Quantitative Determination of Cr in Ink by Laser-induced Breakdown Spectroscopy(LIBS)Using ZnO as Adsorbent

Laser-induced breakdown spectroscopy(LIBS) technique was applied to detecting chromium in ink with ZnO as adsorbent, and the LIBS spectra were preprocessed by wavelet denoising. The laser energy and delay time were optimized depending on the signal-to-noise ratio(SNR) and intensity of three analysis atomic lines(Cr 425.43 nm, Cr 427.48 nm and Cr 428.97 nm). Compared with other analysis lines, atomic line of Cr 427.48 nm was selected as the analysis line for the quantitative analysis of Cr in ink as the calibration curve of it showed a better linear relationship (correlation coefficient R²=0.9778), and the relative error of Cr in the measured ink was 52.96%. Since the single spectral line used for calibration curve method is often influenced by matrix effect and other factors, partial least squares regression(PLS) as multivariate calibration method has been applied to predicting the concentration of Cr in ink, and the relative error of Cr in the measured ink was 10.48%. The result obtained from the PLS method was better than that from the calibration curve when comparing the relative error, demonstrating that, based on adsorbent, LIBS combined with PLS provides an effective, practical and convenient technique for the determination of trace element in aqueous solution.     

Strategies for solving matrix effects in the analysis of sulfathiazole in honey samples using three-way photochemically induced fluorescence data

A widely employed compound for honey treatment, sulfathiazole (ST), was determined in commercial honey samples, employing a combination of photochemically induced fluorescence excitation-emission matrices (EEMs) and chemometric processing of the recorded second-order data. Parallel Factor Analysis (PARAFAC) and Self-Weighted Alternating Trilinear Decomposition (SWATLD) methods were used for calibration. An appropriately designed calibration with a set of standards composed of 18 samples, coupled to the use of the second-order advantage offered by the applied chemometric techniques, allowed quantitation of sulfathiazole in spiked commercial honey samples. No previous separation or sample pretreatment steps were required. The results were compared with other calibration methods such as N-PLS and PLS-1 that produced good results on synthetic samples but not on the investigated commercial honey samples.     

Application of Solid-Phase Extraction Coupled with Dispersive Liquid–Liquid Microextraction for the Determination of Benzaldehyde in Injectable Formulation Solutions

Solid-phase extraction followed by dispersive liquid–liquid microextraction (SPE-DLLME) technique has been developed as a new analytical approach for extracting, cleaning up and preconcentrating benzaldehyde, a toxic oxidation product of the widely used preservative and co-solvent benzyl alcohol, in injectable formulation solutions. SPE of benzaldehyde from samples was carried out using C₁₈ sorbent. After the elution of benzaldehyde from the sorbent by using acetonitrile, DLLME technique was performed on the obtained solution. Benzaldehyde was preconcentrated by using DLLME technique. Thus, 1.5 mL acetonitrile extract (disperser solvent) and 55.0 µL 1,2-dichloroethane (extraction solvent) were added to 5 mL ultra pure water and a DLLME technique was applied. Several variables that govern the proposed technique were studied and optimized. Under optimum conditions, the method detection limit (LOD) of benzaldehyde calculated as three times the signal-to-noise ratio (S/N) was 0.08 µg L^?1. The relative standard deviation (RSD) for four replicates was 5.8 %. The calibration graph was linear within the concentration range of 0.5–500 µg L^?1 for benzaldehyde. The proposed method has been successfully applied to the analysis of the benzaldehyde in injectable formulation solutions (diclofenac, vitamin B-complex and voltaren) and the relative recoveries were between 88 and 92 % and show that matrix has a negligible effect on the performance of the proposed method.     

Chemometric determination of naproxen sodium and pseudoephedrine hydrochloride in tablets by HPLC

A new chemometric determination by high-performance liquid chromatography (HPLC) with photodiode array (PDA) detection was implemented for the simultaneous determination of naproxen sodium and pseudoephedrine hydrochloride in tablets. Three chemometric calibration techniques, classical least squares (CLS), principle component regression (PCR) and partial least squares (PLS) were applied to the peak area at multiwavelength PDA detector responses. The combinations of HPLC with chemometric calibration techniques were called HPLC-CLS, HPLC-PCR and HPLC-PLS. For comparison purposes the HPLC method called the classic HPLC method was used to confirm the results obtained from combined HPLC-chemometric calibration techniques. A good chromatographic separation between two drugs with losartan potassium as an internal standard was achieved using a Waters Symmetry C18 Column 5 microm 4.6+/-250 mm and a mobile phase containing 0.2 M acetate buffer and acetonitrile (v/v, 40:60). The multiwavelength PDA detection was measured at five different wavelengths. The chromatograms were recorded as a training set in the mobile phase. Three HPLC-chemometric calibrations and the classic-HPLC method were used to test the synthetic mixtures of naproxen sodium and pseudoephedrine hydrochloride in the presence of the internal standard. The HPLC-chemometric approaches were applied to real samples containing drugs of interest. The experimental results obtained from HPLC-chemometric calibrations were compared with those obtained by a classic HPLC method.     

Improved accuracy of on-line heavy water measurement using infrared spectroscopy by investigation of signal-to-noise ratio and temperature influences

Experimental protocols for the on-line measurement of heavy water concentration in nuclear power plants have been established, and important parameters, such as the temperature and signal-to-noise ratio, which govern the accuracy of measurement, have been studied. The temperature of a sample should be controlled tightly because the temperature variation introduces non-linear baseline variations and leads to an increase of the partial least squares calibration error. Furthermore, the variation in the signal-to-noise ratio of spectra sensitively influences the calibration. For reliable prediction, it is critical to maintain the signal-to-noise ratio at a certain level. When the sample spectra were collected at a higher temperature, it was possible to acquire spectra with an improved signal-to-noise ratio and better calibration. In addition, a single beam spectrum of water shifts to a lower frequency, and the maximum transmission intensity at around 2500 cm(-1)(the heavy water band location) increases at an elevated temperature. Overall, an on-line infrared spectroscopic scheme is presented for measuring heavy water. The scheme can be applied to an actual process without practical difficulties. If the spectra could be collected at elevated temperature over 2 min with the use of a high throughput light source, the prediction error could reach to 1.0 ppm.