A spatial distribution study of analyte concentration in solid substrate surface-enhanced Raman spectroscopy

As surface-enhanced Raman spectroscopy becomes more widely used as a technique for routine analysis, attention must be given to factors which affect the reproducibility of the technique. One such factor which has not been considered is the effect of the spatial distribution of molecules in an analyte spot on a solid SERS substrate. In this study, we demonstrate that the spatial distribution of analyte can be quite non-uniform due to chromatographic interactions between the analyte molecules, solvent and support used. The SERS signal obtained is highly dependent on the position of the laser beam in the analyte spot. The SERS signal is also dependent on the relative sizes of the analyte spot and the laser beam at the sample. This latter dependence can not be explained with current theoretical treatments. However, we demonstrate that a simple modification of the current theory can qualitatively explain the results we observe.     

The effect of different gloss levels on in-line monitoring of the thickness of printed layers by NIR spectroscopy

Near-infrared (NIR) reflection spectroscopy was used for monitoring the thickness or rather the coating weight of thin printed layers of transparent oil-based offset printing varnishes in a range from 0.5 to 5 g?m^?2. Quantitative analysis of the spectral data was carried out with partial least squares regression. Surface properties such as the gloss were found to strongly affect the prediction of the coating weight. This influence was minimized by the development of calibration models, which contained spectra of layers with a broad range of gloss levels. The prediction error of these models was in the order of 0.12 to 0.16 g?m^?2. In-line measurements were carried out at a sheet-fed offset printing press in order to test the performance of the models under real process control conditions. Varnishes were applied to paper at printing speeds of 90 or 180 m?min^?1. A close correlation between the predictions from in-line NIR spectra and the reference data from gravimetry was observed regardless of the specific degree of gloss of the layers (errors between 0.15 and 0.17 g?m^?2). The results clearly prove the efficiency of NIR reflection spectroscopy for quantitative investigations on thin layers in fast processes such as printing and demonstrate its analytical potential for quality and process control.

Monitoring powder blending by NIR spectroscopy

A strategy is proposed for the monitoring of powder blending. Samples are taken throughout the blender vessel and scanned by diffuse reflectance spectroscopy in the NIR range. The NIR spectra are first subjected to the Standard Normal Variate transformation (SNV). The first approach consists of overlaying the transformed spectra taken from different locations at each time. To quantify the differences between the spectra, the standard deviation spectrum at each time is calculated and the mean standard deviation value is plotted as a function of time. This plot indicates the time at which the blend is most homogeneous. Each individual spectrum can be compared with the mixture spectrum, which is an approximation of the spectrum of a true homogeneous sample. For that purpose several approaches, i.e. determination of the dissimilarity, Principal Component Analysis, are proposed. As an alternative approach to monitoring the blending the use of SIMPLISMA is recommended.Dedicated to Professor Dr. h.c. mult. J.F.K. Huber on the occasion of his 70th birthday     

Improving the sensitivity of NIR spectroscopy with an enrichment technique: determining a trace analyte of ethyl carbamate.

An exploration was made to develop a determination method of a low-concentration analyte by NIR spectroscopy. An absorber, silica gel was employed to extract and enrich a low-concentration analyte of ethyl carbamate. The solid absorber with the enriched analyte was measured by NIR spectroscopy in the range of 800 - 2500 nm. Afterwards, PLS regression was performed between the NIR spectra and the concentrations of the analyte for quantitative analysis of the low-concentration analyte. The spectra of 20 solid samples of analyte-absorbed silica gel showed a good correlation with the concentrations of ethyl carbamate in the samples. A leave-one-out cross validation was applied to evaluate the prediction ability of PLS models built with the full spectra, spectra in the region of 1920 - 1970 nm and the region of 2250 - 2430 nm, respectively. The values of the root-mean-square error of the cross validation (RMSECV) were about 0.1 mg L(-1) (0.1 ppm).     

NIR spectroscopy of jarosites

Near-infrared (NIR) spectroscopy has been used to analyse a suite of synthesised jarosites of formula Mn(Fe3+)6(SO4)4(OH)12 where M is K, Na, Ag, Pb, NH4+ and H3O+. Whilst the spectra of the jarosites show a common pattern, differences in the spectra are observed which enable the minerals to be distinguished. The NIR bands in the 6300-7000 cm-1 region are attributed to the first fundamental overtone of the infrared and Raman hydroxyl stretching vibrations. The NIR spectrum of the ammonium-jarosite shows additional bands at 6460 and 6143 cm-1, attributed to the first fundamental overtones of NH stretching vibrations. A set of bands are observed in the 4700-5500 cm-1 region which are assigned to combination bands of the hydroxyl stretching and deformation vibrations. The ammonium-jarosite shows additional bands at 4730 and 4621 cm-1, attributed to the combination of NH stretching and bending vibrations. NIR spectroscopy has the ability to distinguish between the jarosite minerals even when the formula of the minerals is closely related. The NIR spectroscopic technique has great potential as a mineral exploratory tool on planets and in particular Mars.     

Control production of polyester resins by NIR spectroscopy

The control of the esterification reaction for production of polyester saturated resins is followed usually by determination of the acid value (AV) and hydroxyl value (OHV).These parameters are determined by titrimetry, but these methods are slow, intensity working and produce waste. In this paper an alternative methodology is proposed, based in the construction of multivariate models on NIR spectroscopic data and different models are constructed in order to apply to different steps of the production process. The ensuing methodology provides models of good predictive ability and constitute an advantageous alternative to existing titrimetric reference methods as regards expeditiousness and environmentally compatible. The multivariate calibration models established were also used with a different instrument; to this end, the spectra recorded with the original equipment were subjected to Piecewise Direct Standardization (PDS) in order to make them equivalent to those provided by the new equipment. Also, PLS calibration was reproduced by using the same samples, spectral treatment, wavenumber range and number of factors as in the original model, and the AV and OHV results thus obtained were similarly good.     

Control production of polyester resins by NIR spectroscopy

The control of the esterification reaction for production of polyester saturated resins is followed usually by determination of the acid value (AV) and hydroxyl value (OHV).These parameters are determined by titrimetry, but these methods are slow, intensity working and produce waste. In this paper an alternative methodology is proposed, based in the construction of multivariate models on NIR spectroscopic data and different models are constructed in order to apply to different steps of the production process. The ensuing methodology provides models of good predictive ability and constitute an advantageous alternative to existing titrimetric reference methods as regards expeditiousness and environmentally compatible. The multivariate calibration models established were also used with a different instrument; to this end, the spectra recorded with the original equipment were subjected to Piecewise Direct Standardization (PDS) in order to make them equivalent to those provided by the new equipment. Also, PLS calibration was reproduced by using the same samples, spectral treatment, wavenumber range and number of factors as in the original model, and the AV and OHV results thus obtained were similarly good.     

Quality control of cosmetic mixtures by NIR spectroscopy

Cosmetic preparations typically consist of mixtures of various compounds of natural origin or their derivatives. Their analysis is made rather difficult by their usually high complexity and is utterly impossible with a single analytical method; also, there is usually little to be gained by determining every individual component of the mixture. Rather, analyses are aimed at ensuring a proper balance between the contents of each component and thus require the use of methods capable of delivering global information. The combined use of near-infrared (NIR) spectroscopy and multivariate spectral processing chemometric techniques has enabled the development of effective methods for establishing the composition of complex samples with acceptable levels of analytical properties, such as accuracy, precision and throughput. In this work, we developed partial least squares calibration models for the determination of each component in a cosmetic mixture, and global indices (viz. the hydroxyl value), simply from the NIR spectrum of the sample. The models thus obtained are accurate enough for use in quality control analyses of cosmetic preparations and provide an effective alternative to existing conventional global methods. Experimental setup for measurement     

Intercalator-DNA interactions revealed by NIR surface-enhanced Raman spectroscopy

Using 1064 nm excited surface-enhanced Raman spectroscopy (SERS) a well known intercalator, ethidium bromide (EB), and a structurally related compound, 4-methyl-2,7-diamino-5,10-diphenyl-4,9-diazapyrenium hydrogensulfate (ADAP), have been studied. Concentration dependent SERS spectra of both aromatic species (1 × 10⁻⁷-5 × 10⁻⁵ M) indicated existence of dimeric associates at high concentration and an equilibrium shift towards monomers with a concentration decrease. Interactions of the intercalating molecules with DNA have been studied for various intercalator/DNA (base pair) molar ratios ranging from 10/1 to 1/10. In colloidal samples containing an intercalator in excess relative to DNA binding sites (from 10/1 to 2/1) enhancement of the Raman scattering gradually weakened, indicating a decrease in a number of free molecules adsorbed on the metal surface due to binding with DNA. At the drug/DNA ratios of 1/2 and 1/5 weaker but observable SERS bands indicated insertion of the drug molecules between the base pairs (intercalation strongly diminished interaction of the drug molecules with metal surface) as well as non-intercalative binding of the drug molecules able to stay in closer contact with a metal surface. A total intercalation of EB and ADAP molecules (intercalator/DNA of 1/7 and 1/10) resulted in almost complete loss of the SERS signal. Intensity of the SERS spectra of the intercalator/DNA complexes relative to the SERS intensity of the free intercalating molecules diminished to a lesser degree for ADAP/DNA than for EB/DNA. The obtained difference was attributed to a larger aromatic surface of the ADAP molecules which, although intercalated, could be positioned near the enhancing nanoparticles, unlike the smaller EB molecules which were deeply inserted within the DNA helix.     

The effect of column characteristics on the minimum analyte concentration and the minimum detectable amount in capillary gas chromatography

The need for faster and more efficient separations of complex mixtures of organic compounds by gas chromatography has led to the development of small inner diameter open tubular columns. Owing to their decreased plate height, extremely narrow peaks are obtained. When differently sized columns with equal plate numbers are compared, injection of a fixed amount of a solute will give the highest detector signals for the smallest bore columns. When P is defined as the ratio of the column inlet and outlet pressures, it can be seen from theory that under normalized chromatographic conditions the minimum detectable amount (Q_º) for a mass flow sensitive detector increases proportionally to the square of the column diameter for P = 1. In the situation of greater interest in the practice of open tubular gas chromatography where P is large, a linear relationship is derived between Q_º and the column diameter. It is a widespread misunderstanding, however, that narrow bore capillary columns should be used for this reason in trace analysis. If a fixed relative contribution of the injection band width to the overall peak variance is allowed, a decreased plate height drastically restricts the maximum sample volume to be injected. It is shown that the minimum analyte concentration in the injected sample (C_º) is inversely proportional to the column inner diameter when a mass flow sensitive detector is used. For actual concentrations less than C_º, sample preconcentration is required. The effect of peak resolution and selectivity of the stationary phase in relation to C_º and Q_º will be discussed as well. The validity of the given theory is experimentally investigated. Minimum analyte concentrations and minimum detectable amounts are compared using columns with different inner diameter.     

Polymorphic analysis of a pharmaceutical preparation by NIR spectroscopy

A method for the characterization and determination of the crystalline from present in the amorphous miokamycin of a pharmaceutical preparation using near-infrared spectroscopy (NIRS) is proposed. The study leading to the development of the proposed method involved both qualitative (classification by residual variance analysis) and quantitative aspects (the determination of both total miokamycin and its crystalline form using multivariate calibration). Samples containing less than 5% of the total amount of miokamycin in its crystalline form can be accurately classified. Partial least-squares regression (PLSR) allows low contents of the crystalline form to be determined with absolute errors less than 1.5%, which is comparable to the limit of detection of the X-ray diffraction technique used as reference. Also, the total miokamycin content is determined by NIRS with errors less than 1%. The simplicity, expeditiousness and robustness of the proposed method make it an effective tool for determining both total and crystalline miokamycin in solid dosage forms.     

Nanoparticle agglutination: acceleration of aggregation rates and broadening of the analyte concentration range using mixtures of various-sized nanoparticles

SiO(2) particles of various sizes were prepared and surface modified with biotin-chain-end-functionalized poly(ethylene glycol). Dispersions of these particles were prepared, and their aggregation was induced upon the addition of avidin. The aggregate size and growth rate were monitored by DLS analysis, and SEM and TEM images of freeze-dried samples of the aggregate solutions were used to confirm the DLS data and to image the aggregate size and dimension. A linear correspondence between apparent diameter and time was observed, and both the 20 and 300 nm particles aggregated at slower rates than for the 40 nm particles. These observations were attributed to differences in the average functionality of the systems and the different initial concentrations of particles and avidin. The observed aggregation rates of binary combinations of the three particle sizes (i.e., 20 + 40 nm or 40 + 300 nm) were faster than those of the single-sized mixtures. These results were attributed to the faster flux of smaller particles toward larger particles in the mixture solutions as well as to the potentially larger number of productive collisions possible between mixtures of small particles and large particles versus only similarly sized particles. Combinations of the three sizes of particles were studied to find an empirical optimum formulation for generating large aggregates on a short time scale and over a wide range of analyte concentrations.     

Durum wheat adulteration detection by NIR spectroscopy multivariate calibration

In the present work, we explored the possibility of using near-infrared spectroscopy in order to quantify the degree of adulteration of durum wheat flour with common bread wheat flour. The multivariate calibration techniques adopted to this aim were PLS and a wavelet-based calibration algorithm, recently developed by some of us, called WILMA. Both techniques provided satisfactory results, the percentage of adulterant present in the samples being quantified with an uncertainty lower than that associated to the Italian official method. In particular the WILMA algorithm, by performing feature selection, allowed the signal pretreatment to be avoided and obtaining more parsimonious models.     

Fuel concentration measurement of premixed mixture using spark-induced breakdown spectroscopy

This study determined the local equivalence ratio of a CH₄/air mixture in a laminar premixed flame using spark-induced breakdown spectroscopy (SIBS) with a fiber-coupled intensified charge coupled device (ICCD) spectrometer. Spectrally resolved emission spectra of plasma generated by a spark plug were investigated for their potential to measure local fuel concentrations in a premixed mixture. The influence of key parameters, such as the camera gate timing and spark energy, on the intensity of radical emission was illustrated. The intensity ratio of CN/NH had a greater sensitivity to the equivalence ratio than did that of CN/OH, and the local equivalence ratio could be obtained with high resolution by measuring the local intensity ratios of CN/NH. Moreover, a spark-plug sensor with an optical fiber was developed for application in spark-ignition engines. The atomic emission intensity during the breakdown and arc phases of spark discharge could be obtained using the fiber-optic spark-plug sensor. The H_α/O intensity showed better linearity than the CN/NH intensity ratio in lean mixtures. The results presented here confirm the use of SIBS as a diagnostic tool for spark-ignition engines.     

Inductively coupled plasma-optical emission spectroscopy : Excitation temperatures experienced by analyte species

Spatially resolved, radical excitation temperature distributions experienced by analyte species injected into the axial channel of a toroidally shaped, inductively coupled argon plasma are presented. Typical axial temperatures experienced by the thermometric species (FeI) ranged from 6500 K to 5500 K in the analytical zone of 15–25 mm above the load coil. A comparison of temperatures calculated with different sets of transition probabilities is also given.     

Determination of low analyte concentrations by near-infrared spectroscopy: effect of spectral pretreatments and estimation of multivariate detection limits

Near infrared spectroscopy (NIRS) was used in combination with partial least squares (PLS) calibration to determine low concentrated analytes. The effect of the orthogonal signal correction (OSC) and net analyte signal (NAS) pretreatments on the models obtained at concentrations of analyte near its detection limit was studied. Both pretreatments were found to accurately resolve the analyte signal and allow the construction of PLS models from a reduced number of factors; however, they provided no substantial advantage in terms of %RSE for the prediction samples. Multiple methodologies for the estimation of detection limits could be found in the bibliography. Nevertheless, detection limits were determined by a multivariate method based on the sample-specific standard error for PLS regression, and compared with the univariate method endorsed by ISO 11483. The two methods gave similar results, both being effective for the intended purpose of estimating detection limits for PLS models. Although OSC and NAS allow isolating the analyte signal from the matrix signal, they provide no substantial improvement in terms of detection limits. The proposed method was used to the determine 2-ethylhexanol at concentrations from 20 to 1600 ppm in an industrial ester. The detection limit obtained, round 100 ppm, testifies to the ability of NIR spectroscopy to detect low concentrated analytes.     

Quantification of lysine clonixinate in intravenous injections by NIR spectroscopy

Migraine is a very painful and somewhat unpredictable ache that affects millions of people worldwide and for which no definite medicine exists, yet. New drugs and/or pharmaceutical forms are being developed, for which new quantitation methods are required. Lysine clonixinate (LC) has proved very advantageous to alleviate migraine episodes although, so far, no analytical procedures have been reported to quantify it in pharmaceutical dosage forms usually employed by physicians, i.e., injectable solutions. In this paper a NIR spectral method was developed and validated against international pharmaceutical standard guidelines and a new UV-based method to quantify LC in intravenous injection solutions. Both methods are almost inexpensive, fast, simple and suitable for LC routine determination. In addition, they provide analytical protocols less time-consuming than other reported HPLC methods (developed for other matrices), proved to be specific, accurate, precise and linear within the typical working range, according to the Harmonized Tripartite Guideline of Validation of Analytical Procedures from the International Conference on Harmonization. Both methods yield equivalent results and they are useful to monitor the concentration of LC in injectable solutions in routine analysis.     

Quantification of lysine clonixinate in intravenous injections by NIR spectroscopy

Migraine is a very painful and somewhat unpredictable ache that affects millions of people worldwide and for which no definite medicine exists, yet. New drugs and/or pharmaceutical forms are being developed, for which new quantitation methods are required. Lysine clonixinate (LC) has proved very advantageous to alleviate migraine episodes although, so far, no analytical procedures have been reported to quantify it in pharmaceutical dosage forms usually employed by physicians, i.e., injectable solutions. In this paper a NIR spectral method was developed and validated against international pharmaceutical standard guidelines and a new UV-based method to quantify LC in intravenous injection solutions. Both methods are almost inexpensive, fast, simple and suitable for LC routine determination. In addition, they provide analytical protocols less time-consuming than other reported HPLC methods (developed for other matrices), proved to be specific, accurate, precise and linear within the typical working range, according to the Harmonized Tripartite Guideline of Validation of Analytical Procedures from the International Conference on Harmonization. Both methods yield equivalent results and they are useful to monitor the concentration of LC in injectable solutions in routine analysis.     

In-line determination of the thickness of UV-cured coatings on polymer films by NIR spectroscopy

We report on the development of a measuring method based on near-infrared (NIR) spectroscopy, which is able to determine the thickness of UV-cured coatings and which can be used for in-line monitoring in technical coating processes. In particular, acrylate coatings, which were applied to transparent polymer films with a typical thickness of 5–35 μm, were investigated. NIR spectra were recorded in transflection mode. Quantitative analysis of the spectral data was carried out with partial least square (PLS) regression. In-line measurements were performed on a pilot-scale roll coating machine at web speeds up to 50 m/min. It was shown that quantitative data with excellent precision (i.e. with a standard deviation lower than ±1 μm) and high time resolution (2.5 spectra/s) can be obtained.     

Detection of protein deposits using NIR spectroscopy

The aim of this study was to determine if it is possible to distinguish between the groups of spoiled and unspoiled soft contact lenses using near-infrared spectroscopy and new analytical approach – Aquaphotomics. Using the principal component analysis, it was established that the absorbance spectra of worn and new contact lenses are differed at water absorption band related to hydration of proteins. Detection of proteins thus was performed indirectly by using vibrations of water molecules. This exploratory study showed that near-infrared spectroscopy and Aquaphotomics have potential for non-invasive, chemical-free detection of protein deposits on hydrated soft contact lenses.