Cluster Analysis for IR and NIR Spectroscopy: Current Practices to Future Perspectives

Supervised machine learning techniques have become well established in the study of spectroscopy data. However, the unsupervised learning technique of cluster analysis hasn’t reached the same level maturity in chemometric analysis. This paper surveys recent studies which apply cluster analysis to NIR and IR spectroscopy data. In addition, we summarize the current practices in cluster analysis of spectroscopy and contrast these with cluster analysis literature from the machine learning and pattern recognition domain. This includes practices in data pre-processing, feature extraction, clustering distance metrics, clustering algorithms and validation techniques. Special consideration is given to the specific characteristics of IR and NIR spectroscopy data which typically includes high dimensionality and relatively low sample size. The findings highlighted a lack of quantitative analysis and evaluation in current practices for cluster analysis of IR and NIR spectroscopy data. With this in mind, we propose an analysis model or workflow with techniques specifically suited for cluster analysis of IR and NIR spectroscopy data along with a pragmatic application strategy.     

红外光谱结合主成分分析对纸质快递文件袋的分类研究

目的 建立一种快速无损的检验纸质快递文件袋的分析方法。方法 利用傅里叶变换红外光谱对63个纸质快递文件袋样品进行检验,分析样品的红外光谱吸收峰的峰位,结合主成分分析对光谱数据进行了降维处理并分类。利用费歇尔判别对快递文件袋的分类结果进行分析和验证。同时建立多层感知器神经网络和径向基函数神经网络2种分类模型,进行分析和验证。结果 63个纸质快递文件袋样品可被分成四大类,利用费歇尔分类模型进行验证,准确率为100%;多层感知器神经网络分类模型准确率为95.23%,径向基函数神经网络分类模型准确率为92.06%。通过比较发现,费歇尔判别可以实现对纸质快递文件袋更加有效地分类。结论 该方法简单快速,样品用量少且无损样品,可为快递文件袋类的物证鉴定提供科学依据。     

An experimental investigation of Lewis acid-base interactions of liquid carbon dioxide using Fourier Transform Infrared (FT-IR) spectroscopy

Presented here is an investigation into the solvent properties of liquid carbon dioxide by means of FT-IR spectroscopy. A high-pressure, circulation-type apparatus was designed and built specifically for this study. The spectra for the combination bands for carbon dioxide show that there are interactions between methanol and carbon dioxide. However, the spectra of the fundamental O-D vibration of deuterated methanol in liquid carbon dioxide indicate that there is no hydrogen bonding. Therefore. we conclude that the interactions between carbon dioxide and methanol are Lewis acid-base interactions rather than hydrogen bonding. This conclusion is supported by experiments where acetone is introduced into the CO₂/methanol-d binary system. FT-IR measurements show that acetone hydrogen bonds with deuterated methanol.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Analysis of texture and microstructure of anatase thin films by Fourier transform infrared spectroscopy

This work provides the basis for a “fingerprint” analysis of the microstructure and the texture of anatase thin films by using Fourier transform infrared spectroscopy. For this, a formalism that reproduces the experimental spectra has been developed and its applicability is analysed for anatase thin films with different microstructure (columnar and porous, with varying degrees of compactness) and texture. The rather good agreement between calculated and experimental spectra observed in all cases demonstrated that a systematic use of this non-destructive technique can be very useful for the characterization of thin films.     

Application of spectra cross-correlation for Type II outliers screening during multivariate near-infrared spectroscopic analysis of whole blood

In this study, a simple screening algorithm was developed to prevent the occurrence of Type II errors or samples with high prediction error that are not detected as outliers. The method is used to determine “good” and “bad” spectra and to prevent a false negative condition where poorly predicted samples appear to be within the calibration space, yet have inordinately large residual or prediction errors. The detection and elimination of this type of sample, which is a true outlier but not easily detected, is extremely important in medical decisions, since such erroneous data can lead to considerable mistakes in clinical analysis and medical diagnosis. The algorithm is based on a cross-correlation comparison between samples spectra measured over the region of 4160-4880 cm^− 1. The correlation values are converted using the Fisher's z-transform, while a z-test of the transformed values is performed to screen out the outlier spectra. This approach allows the use of a tuning parameter used to decrease the percentage of samples with high analytical (residual) errors. The algorithm was tested using a dataset with known reference values to determine the number of false negative and false positive samples. The cross-correlation algorithm performance was tested on several hundred blood samples prepared at different hematocrit (24 to 48%) and glucose (30 to 500 mg/dL) levels using blood component materials from thirteen healthy human volunteers. Experimental results illustrate the effectiveness of the proposed algorithm in finding and screening out Type II outliers in terms of sensitivity and specificity, and the ability to predict or estimate future or validation datasets ensuring lower error of prediction. To our knowledge this is the first paper to introduce a statistically useful screening method based on spectra cross-correlation to detect the occurrence of Type II outliers (false negative samples) for routine analysis in a clinically relevant application for medical diagnosis.     

Acrylamide assisted polymeric citrate route for the synthesis of nanocrystalline ZrO2 powder

Nanocrystalline ZrO₂ powders have been prepared using acrylamide assisted polymeric citrate combustion (autoignition) route. Process parameters (i.e., total metal ions to citric acid ratio) were varied for the formation of polymeric resin in order to obtain the final product with desired properties (organic free, smaller crystallite size, etc.). The effect of three different citric acid amounts in the formation of nanocrystalline ZrO₂ powder was investigated using FTIR, XRD, TG/DTA and SEM-EDS, respectively, to identify the structural coordination, phase, thermal behavior and microstructure of the polymeric intermediates as well as the final ZrO₂ powders. Organic free ZrO₂ powder with two different phases of metastable t-ZrO₂ and m-ZrO₂ were prepared by calcining the polymeric intermediates at 600 °C. The lowest crystallite was found to be 18 and 16 nm, respectively, for the t-ZrO₂ and m-ZrO₂ phases prepared with total metal ions to citric acid ratio of 1:3.     

Band gap enhancement on metal chelation: Growth and characterization of cobalt chelated glycine single crystals for optoelectronic applications

Single crystals of Diaqua Bisglycine Cobaltous Chloride (DBGCoCl) have been synthesized and grown by slow evaporation method. Grown crystals were subjected to FTIR and XRD studies for structural confirmation. The DBGCoCl crystallizes in the monoclinic system with the space group P2₁/c. The UV–vis spectrum for the pure glycine as well the cobalt chelated glycine have been recorded. The lower cutoff wavelength observed from the UV–vis spectrum is found to decrease by the chelation of cobalt with glycine and this leads to an increase in the band gap of the compound from 3.13 eV to 4.91 eV. The optical constants were calculated and illustrated graphically. Mechanical hardness of the grown crystal DBGCoCl was studied and Vickers hardness number was calculated. The work hardening coefficient (n) was determined to be 1.365 and the stiffness constants for different loads were calculated and reported. The thermal stability of the grown crystal has been studied using TGA and DTA. The crystal is thermally stable up to 116.86 °C.     

One-step approach to prepare magnetic iron oxide/reduced graphene oxide nanohybrid for efficient organic and inorganic pollutants removal

An environmentally friendly effective technique was demonstrated to prepare iron oxide/reduced graphene oxide nanohybrid (IO/RGO) at room temperature by using banana peel ash aqueous extract as the base source and Colocasia esculenta leaves aqueous extract as the reducing agent. The nanohybrid was characterized by Fourier transform infrared spectroscopy, X-ray diffractometry, transmission electron microscopy, vibrating sample magnetometry, Raman spectroscopy and thermal studies. The results indicated the decoration of superparamagnetic IO nanoparticles on the surface of the RGO. Both organic and inorganic pollutants were effectively removed from the contaminated water (for Pb²⁺ and Cd²⁺ within 10 min, whereas for tetrabromobisphenol A within 30 min) by IO/RGO. The study revealed that adsorption followed pseudo-second order kinetics and isotherms were well described by the Langmuir model in all the cases. The thermodynamics parameters (ΔG°, ΔS° and ΔH°) were calculated from the temperature dependent isotherms and indicated that the adsorptions were endothermic and spontaneous.     

Investigation of kinetics and morphology development for polyurethane-urea extended by DMTDA

The relationship between the reactions kinetics and morphology development during the polyurethaneurea (PUU) curing process has been investigated simultaneously by in situ Fourier transform infrared spectroscopy (FTIR). The data of the FTIR spectra showed that with the increase of conversion, the absorbance of NH bands increases and its band sites shifts to lower wavenumbers; the absorbance of free urethane carbonyl kept nearly constant at low conversion, and then decreased much because of the interaction of the formed urea links, and then changed little at high conversion owing to the diffuse control. The band sites of hydrogen bonded urea carbonyl similarly shifted to lower wavenumbers and the absorbance of the hydrogen bonded urea carbonyl, associated with the phase separation of hard segments, became stronger with buildup of hydrogen bond between urea links. The carbonyl bands available during curing process were further assigned. Both interactions, such as hydrogenised effect and phase separation, played a major role in the matrix formation of the PUU polymer.     

Thin plasma-polymerized layers of hexamethyldisiloxane for humidity sensor development

The response of resistive-type sensors based on thin hexamethyldisiloxane layers to relative humidity (RH) was evaluated. Humidity sensitive layers were plasma polymerized at low frequency glow discharge using a capacitively coupled parallel plate reactor. The sensor design comprises the absorbing layer deposited on clean glass substrate with comb-shape aluminum electrodes (interdigitated structure). The change in electrical impedance of the sensing film was monitored as the device was exposed to humidity. The variation of the plasma-polymerization parameters resulted in different humidity sensing properties which could be correlated to the results of Fourier transform infrared spectroscopy (FTIR). The deposited films exhibited a detectable response to RH ranging from 30 to 95% with low hysteresis, good reproducibility and stability in long-term use. Films with a greater thickness showed a significant decrease in the humidity sensing capability. FTIR analysis revealed the presence of SiH bonding groups, which are frequently linked to the film density. The increase in the plasma discharge power induced also a significant decrease in the diffusion process of water vapor inside the sensitive layer bulk.     

Synthesis of carbon nitride powder by selective etching of TiC0.3N0.7 in chlorine-containing atmosphere at moderate temperature

We reported the synthesis of carbon nitride powder by extracting titanium from single inorganic precursor TiC_0.3N_0.7 in chlorine-containing atmosphere at ambient pressure and temperature not exceeding 500 °C. The TiC_0.3N_0.7 crystalline structure acted as a template, supplying active carbon and nitrogen atoms for carbon nitride when it was destroyed in chlorination. X-ray diffraction data showed that the obtained carbon nitride powders were amorphous, which was in good agreement with transmission electron microscope analysis. The composition and structure of carbon nitride powders were analyzed by employing Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Results indicated that disorder structure was most likely for the carbon nitride powders and the N content depended greatly on the chlorination temperature. Thermal analysis in flowing N₂ indicated that the mass loss started from 300 °C and the complete decomposition occurred at around 650 °C, confirming the low thermal stability of the carbon nitride material.     

Discrete Fourier Transform for Short Samples in the Presence of Noise

We consider the dependences of the error of a discrete Fourier transform on the length of the realization of the considered signal. The introduction of a procedure of making a preliminary determination of the period makes it possible to consider signals in short samples and this significantly reduces the computing and apparatus costs.     

Enhanced dehydrogenation of hydrazine bisborane for hydrogen storage

NiCl₂ and CoCl₂ were adopted to enhance the dehydrogenation of hydrazine bisborane (HBB), respectively, of which NiCl₂ showed better performance. By adding 2.0 mol. % NiCl₂, the dehydrogenation property of HBB was significantly improved, for example, the impurity of NH₃ during the dehydrogenation of HBB was totally suppressed with more than 13.0 wt. % of pure hydrogen evolved. By Kissinger method, the apparent activation energies of the first step for HBB and Ni-doped HBB were calculated to be 143.2 and 60.7 kJ mol⁻¹, respectively. DSC result showed that the addition of NiCl₂ did not change the enthalpy change of HBB dehydrogenation. Based on theoretical analysis and literature review, the improved dehydrogenation property of HBB was potentially ascribed to the solid state interaction of Ni²⁺ with the electronegative N in the NH₂ group of HBB.     

Polariscopic imaging and vibrational characterization of hybrid films for packaging

Polymeric films (low‐density polyethylene and oriented polypropylene) for packaging applications coated with organic–inorganic nanocomposite hybrids have been prepared by sol‐gel process. Tetraethoxysilane and two different organic phases, polyvinyl alcohol and a diblock copolymer polyethylene‐polyethylene glycol were used as precursors. Information about the hydrolysis and polycondensation degrees was obtained by investigating the vibrational features of the coatings by means of attenuated total reflectance‐Fourier transform infrared and Raman spectroscopies. Through a new polariscopic imaging technique, the bi‐dimensional maps of the optical anisotropy of the samples, both as‐prepared and under mechanical strain, were obtained. The maps showed inhomogeneous retardation and nearly flat azimuth distributions. The anisotropy properties of the samples depended mainly on the substrates and were not influenced by the coatings, while the optical transmission was slightly better for the tetraethoxysilane‐polyvinyl alcohol sample. Copyright © 2008 John Wiley & Sons, Ltd.     

Detection of an en masse and reversible B- to A-DNA conformational transition in prokaryotes in response to desiccation

The role that DNA conformation plays in the biochemistry of cells has been the subject of intensive research since DNA polymorphism was discovered. B-DNA has long been considered the native form of DNA in cells although alternative conformations of DNA are thought to occur transiently and along short tracts. Here, we report the first direct observation of a fully reversible en masse conformational transition between B- and A-DNA within live bacterial cells using Fourier transform infrared (FTIR) spectroscopy. This biospectroscopic technique allows for non-invasive and reagent-free examination of the holistic biochemistry of samples. For this reason, we have been able to observe the previously unknown conformational transition in all four species of bacteria investigated. Detection of this transition is evidence of a previously unexplored biological significance for A-DNA and highlights the need for new research into the role that A-DNA plays as a cellular defence mechanism and in stabilizing the DNA conformation. Such studies are pivotal in understanding the role of A-DNA in the evolutionary pathway of nucleic acids. Furthermore, this discovery demonstrates the exquisite capabilities of FTIR spectroscopy and opens the door for further investigations of cell biochemistry with this under-used technique.     

红外光谱结合统计学方法对纸质包装盒的检验研究

目的 建立一种高效准确的红外光谱检验纸质包装盒的研究方法。方法 利用傅里叶变换红外光谱仪测得了56个不同类型不同来源的纸质包装盒的红外光谱数据;根据纸盒主要填料的不同,将样品初步分为三大类;利用主成分分析对初步分类后的数据降维,提取出4个主成分,再利用系统聚类将样品最终分为6组,使用K-means聚类算法结合Pearson卡方检验进行验证,与最终分类的结果基本吻合。基于该分组,训练朴素贝叶斯分类、随机森林模型、XGBoost分类3种判别模型,实现对新样品组别的分类预测。结果 56个快递包装纸盒样品被分为3类,而后进一步细分为6组,3种判别预测模型均有较高的准确率,其中随机森林模型的准确率最高。结论 该方法快速方便地实现了对样品的区分,并且可以实现无损检验,为犯罪现场纸质包装盒的鉴别提供依据,从而为公安侦查工作的开展提供帮助。     

Synthesis of densely grafted copolymers with tert-butyl methacrylate/2-(dimethylamino ethyl) methacrylate side chains as precursors for brush polyelectrolytes and polyampholytes

Methacrylate-based densely grafted copolymers were synthesized by atom transfer radical polymerization (ATRP) and activators generated by electron transfer for atom transfer radical polymerization (AGET ATRP) techniques. The linear poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) PBIEM prepared by ATRP served as a macroinitiator backbone. The “grafting from” strategy was used to initiate polymerization of tert-butyl methacrylate (tBuMA) from PBIEM under ATRP and/or AGET ATRP conditions yielding densely grafted copolymers PBIEM-graft-P(tBuMA). The low polydispersity indices (PDI) of the synthesized brushes evidenced by SEC analysis were consistent with a controlled/living radical polymerization (CLRP) mechanism. The chlorine-terminated PBIEM-graft-P(tBuMA)-Cl macroinitiators were subsequently employed for chain extension with 2-(dimethylamino ethyl) methacrylate (DMAEMA) yielding densely grafted copolymers with diblock copolymer side chains PBIEM-graft-P(tBuMA)-block-PDMAEMA. Further, PBIEM macroinitiator was used to initiate the copolymerization of a binary mixture of tBuMA and DMAEMA through both ATRP and AGET ATRP initiating systems, yielding densely grafted copolymers with statistical distribution of the side chains. The reactivity ratios for random graft copolymerization of tBuMA and DMAEMA from PBIEM backbone established by three different methods (Finemann-Ross, Kelen-Tüdös and Error-in-Variable) did not substantially differ from literature values for conventional free-radical copolymerization of the same monomers. Polyampholyte brushes with PMAA-stat-PDMAEMA side chains were eventually synthesized by hydrolysis of the shielding tert-butyl groups.     

Stability and degradation of plasma deposited boron nitride thin films in ambient atmosphere

Boron nitride (BN) thin films were deposited at 296 K, 398 K, 523 K and 623 K by low power radio frequency plasma enhanced chemical vapor deposition with nitrogen (N₂) and hydrogen diluted diborane (15% B₂H₆ in H₂) source gases. Fourier transform infrared and UV–visible spectroscopies were used to investigate the stability and degradation of BN films under ambient air conditions. The action of moisture on the films is reduced with increasing substrate temperature (T_s) to the detriment of the film growth rate. This has been interpreted as related to the decrease in porosity and relative volume fraction of B–O containing disordered tissue at higher T_s. The thickness of the unstable films increases logarithmically with the air exposure time. Parallel to this, although the E₀₄ gap increases logarithmically with time, the Tauc gap remains the same. The increase of subgap absorptions and the decrease of Tauc slope with time indicate reduction of structural order. Crystallites of ammonium borate hydrates, the main product of the chemical reactions, are initially formed within the bulk. At a later time, as a result of increased porosity and disorder, the film thickness decreases while the islands of micro-crystallites rapidly grow above the surface of the film. Stability dependence on other deposition parameters was also studied: it is found that the 1260/1360 cm⁻¹ (O–B–O/B–N) infrared peak area ratio plays an indicator role to reveal the stability of BN films.     

Optical Properties of Polyimide Films in the Infrared

The objective of this study is to determine the infrared optical constants of polyimide films in the spectral range between 2000 and 7000 cm^-1using a five-oscillator Lorentz model. Model parameters are presented, in addition to the derived values of the complex refractive index and dielectric constant. The parameters were obtained using electromagnetic theory for thin films to model reflectivity data from two film samples with different thicknesses (5.17 and 12.4 m) on gold substrates examined at two incident angles. Measurements were taken using a polarizable reflectometer device in a Fourier transform infrared (FTIR) spectrometer. The real part of the refractive index, n, is shown to be about 1.67, while the imaginary part, k, is less than 0.01 over the spectral range examined. Results are consistent with findings of other experimentalists, and new data presented here show that polarization effects on thin film layers are predictable from the proposed model.