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24 pages, 911 KiB

Open AccessArticle

A Critical Evaluation of Vibrational Stark Effect (VSE) Probes with the Local Vibrational Mode Theory

by Niraj Verma, Yunwen Tao, Wenli Zou, Xia Chen, Xin Chen, Marek Freindorf and Elfi Kraka

Sensors 2020, 20(8), 2358; https://doi.org/10.3390/s20082358 - 21 Apr 2020

Cited by 28 | Viewed by 4233

Over the past two decades, the vibrational Stark effect has become an important tool to measure and analyze the in situ electric field strength in various chemical environments with infrared spectroscopy. The underlying assumption of this effect is that the normal stretching mode [...] Read more.

Over the past two decades, the vibrational Stark effect has become an important tool to measure and analyze the in situ electric field strength in various chemical environments with infrared spectroscopy. The underlying assumption of this effect is that the normal stretching mode of a target bond such as CO or CN of a reporter molecule (termed vibrational Stark effect probe) is localized and free from mass-coupling from other internal coordinates, so that its frequency shift directly reflects the influence of the vicinal electric field. However, the validity of this essential assumption has never been assessed. Given the fact that normal modes are generally delocalized because of mass-coupling, this analysis was overdue. Therefore, we carried out a comprehensive evaluation of 68 vibrational Stark effect probes and candidates to quantify the degree to which their target normal vibration of probe bond stretching is decoupled from local vibrations driven by other internal coordinates. The unique tool we used is the local mode analysis originally introduced by Konkoli and Cremer, in particular the decomposition of normal modes into local mode contributions. Based on our results, we recommend 31 polyatomic molecules with localized target bonds as ideal vibrational Stark effect probe candidates. Full article

(This article belongs to the Special Issue Infrared Spectroscopy and Sensors)

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19 pages, 6646 KiB

Open AccessArticle

Development of Low-Cost Portable Spectrometers for Detection of Wood Defects

by Jakub Sandak, Anna Sandak, Andreas Zitek, Barbara Hintestoisser and Gianni Picchi

Sensors 2020, 20(2), 545; https://doi.org/10.3390/s20020545 - 19 Jan 2020

Cited by 30 | Viewed by 6805

Abstract

Portable spectroscopic instruments are an interesting alternative for in-field and on-line measurements. However, the practical implementation of visible-near infrared (VIS-NIR) portable sensors in the forest sector is challenging due to operation in harsh environmental conditions and natural variability of wood itself. The objective [...] Read more.

Portable spectroscopic instruments are an interesting alternative for in-field and on-line measurements. However, the practical implementation of visible-near infrared (VIS-NIR) portable sensors in the forest sector is challenging due to operation in harsh environmental conditions and natural variability of wood itself. The objective of this work was to use spectroscopic methods as an alternative to visual grading of wood quality. Three portable spectrometers covering visible and near infrared range were used for the detection of selected naturally occurring wood defects, such as knots, decay, resin pockets and reaction wood. Measurements were performed on wooden discs collected during the harvesting process, without any conditioning or sample preparation. Two prototype instruments were developed by integrating commercially available micro-electro-mechanical systems with for-purpose selected lenses and light source. The prototype modules of spectrometers were driven by an Arduino controller. Data were transferred to the PC by USB serial port. Performance of all tested instruments was confronted by two discriminant methods. The best performing was the microNIR instrument, even though the performance of custom prototypes was also satisfactory. This work was an essential part of practical implementation of VIS-NIR spectroscopy for automatic grading of logs directly in the forest. Prototype low-cost spectrometers described here formed the basis for development of a prototype hyperspectral imaging solution tested during harvesting of trees within the frame of a practical demonstration in mountain forests. Full article

(This article belongs to the Special Issue Infrared Spectroscopy and Sensors)

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20 pages, 4422 KiB

Open AccessArticle

Methylated Poly(ethylene)imine Modified Capacitive Micromachined Ultrasonic Transducer for Measurements of CO₂ and SO₂ in Their Mixtures

by Dovydas Barauskas, Donatas Pelenis, Gailius Vanagas, Darius Viržonis and Jonas Baltrušaitis

Sensors 2019, 19(14), 3236; https://doi.org/10.3390/s19143236 - 23 Jul 2019

Cited by 18 | Viewed by 4518

Abstract

A gravimetric gas detection device based on surface functionalized Capacitive Micromachined Ultrasound Transducers (CMUTs) was designed, fabricated and tested for detection of carbon dioxide (CO₂) and sulfur dioxide (SO₂) mixtures in nitrogen. The created measurement setup of continuous data [...] Read more.

A gravimetric gas detection device based on surface functionalized Capacitive Micromachined Ultrasound Transducers (CMUTs) was designed, fabricated and tested for detection of carbon dioxide (CO₂) and sulfur dioxide (SO₂) mixtures in nitrogen. The created measurement setup of continuous data collection, integrated with an in-situ Fourier Transform Infrared (FT-IR) spectroscopy, allows for better understanding of the mechanisms and molecular interactions with the sensing layer (methylated poly(ethylene)imine) and its need of surface functionalization for multiple gas detection. During experimentation with CO₂ gases, weak molecular interactions were observed in spectroscopy data. Linear sensor response to frequency shift was observed with CO₂ concentrations ranging from 0.16 vol % to 1 vol %. Moreover, the Raman and FT-IR spectroscopy data showed much stronger SO₂ and the polymer interactions, molecules were bound by stronger forces and irreversibly changed the polymer film properties. However, the sensor change in resonance frequency in the tested region of 1 vol % to 5 vol % SO₂ showed a linear response. This effect changed not only the device resonance frequency but also affected the magnitude of electroacoustic impedance which was used for differentiating the gas mixture of CO₂, SO₂, in dry N₂. Full article

(This article belongs to the Special Issue Infrared Spectroscopy and Sensors)

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14 pages, 1372 KiB

Open AccessArticle

Combining Fourier Transform Mid-Infrared Spectroscopy with Chemometric Methods to Detect Adulterations in Milk Powder

by Lei Feng, Susu Zhu, Shuangshuang Chen, Yidan Bao and Yong He

Sensors 2019, 19(13), 2934; https://doi.org/10.3390/s19132934 - 03 Jul 2019

Cited by 19 | Viewed by 4128

Abstract

Adulteration is one of the major concerns among all the quality problems of milk powder. Soybean flour and rice flour are harmless adulterations in the milk powder. In this study, mid-infrared spectroscopy was used to detect the milk powder adulterated with rice flour [...] Read more.

Adulteration is one of the major concerns among all the quality problems of milk powder. Soybean flour and rice flour are harmless adulterations in the milk powder. In this study, mid-infrared spectroscopy was used to detect the milk powder adulterated with rice flour or soybean flour and simultaneously determine the adulterations content. Partial least squares (PLS), support vector machine (SVM) and extreme learning machine (ELM) were used to establish classification and regression models using full spectra and optimal wavenumbers. ELM models using the optimal wavenumbers selected by principal component analysis (PCA) loadings obtained good results with all the sensitivity and specificity over 90%. Regression models using the full spectra and the optimal wavenumbers selected by successive projections algorithm (SPA) obtained good results, with coefficient of determination (R²) of calibration and prediction all over 0.9 and the predictive residual deviation (RPD) over 3. The classification results of ELM models and the determination results of adulterations content indicated that the mid-infrared spectroscopy was an effective technique to detect the rice flour and soybean flour adulteration in the milk powder. This study would help to apply mid-infrared spectroscopy to the detection of adulterations such as rice flour and soybean flour in real-world conditions. Full article

(This article belongs to the Special Issue Infrared Spectroscopy and Sensors)

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11 pages, 1738 KiB

Open AccessArticle

Grading and Sorting of Grape Berries Using Visible-Near Infrared Spectroscopy on the Basis of Multiple Inner Quality Parameters

by Hui Xiao, Li Feng, Dajie Song, Kang Tu, Jing Peng and Leiqing Pan

Sensors 2019, 19(11), 2600; https://doi.org/10.3390/s19112600 - 07 Jun 2019

Cited by 21 | Viewed by 3545

Abstract

The potential of visible-near infrared (vis/NIR) spectroscopy (400 nm to 1100 nm) for classification of grape berries on the basis of multi inner quality parameters was investigated. Stored Vitis vinifera L. cv. Manicure Finger and Vitis vinifera L. cv. Ugni Blanc grape [...] Read more.

The potential of visible-near infrared (vis/NIR) spectroscopy (400 nm to 1100 nm) for classification of grape berries on the basis of multi inner quality parameters was investigated. Stored Vitis vinifera L. cv. Manicure Finger and Vitis vinifera L. cv. Ugni Blanc grape berries were separated into three classes based on the distribution of total soluble solid content (SSC) and total phenolic compounds (TP). Partial least squares regression (PLS) was applied to predict the quality parameters, including color space CIELAB, SSC, and TP. The prediction results showed that the vis/NIR spectrum correlated with the SSC and TP present in the intact grape berries with determination coefficient of prediction (R_P²) in the range of 0.735 to 0.823. Next, the vis/NIR spectrum was used to distinguish between berries with different SSC and TP concentrations using partial least squares discrimination analysis (PLS-DA) with >77% accuracy. This study provides a method to identify stored grape quality classes based on the spectroscopy and distributions of multiple inner quality parameters. Full article

(This article belongs to the Special Issue Infrared Spectroscopy and Sensors)

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9 pages, 1063 KiB

Open AccessArticle

Rapid Determination of Chlorogenic Acid, Luteoloside and 3,5-O-dicaffeoylquinic Acid in Chrysanthemum Using Near-Infrared Spectroscopy

by Zhengyan Xia, Yiming Sun, Chengyong Cai, Yong He and Pengcheng Nie

Sensors 2019, 19(9), 1981; https://doi.org/10.3390/s19091981 - 28 Apr 2019

Cited by 12 | Viewed by 3389

Abstract

The feasibility of near-infrared spectroscopy (NIR) to detect chlorogenic acid, luteoloside and 3,5-O-dicaffeoylquinic acid in Chrysanthemum was investigated. An NIR spectroradiometer was applied for data acquisition. The reference values of chlorogenic acid, luteoloside, and 3,5-O-dicaffeoylquinic acid of the samples were determined by high-performance [...] Read more.

The feasibility of near-infrared spectroscopy (NIR) to detect chlorogenic acid, luteoloside and 3,5-O-dicaffeoylquinic acid in Chrysanthemum was investigated. An NIR spectroradiometer was applied for data acquisition. The reference values of chlorogenic acid, luteoloside, and 3,5-O-dicaffeoylquinic acid of the samples were determined by high-performance liquid chromatography (HPLC) and were used for model calibration. The results of six preprocessing methods were compared. To reduce input variables and collinearity problems, three methods for variable selection were compared, including successive projections algorithm (SPA), genetic algorithm-partial least squares regression (GA-PLS), and competitive adaptive reweighted sampling (CARS). The selected variables were employed as the inputs of partial least square (PLS), back propagation-artificial neural networks (BP-ANN), and extreme learning machine (ELM) models. The best performance was achieved by BP-ANN models based on variables selected by CARS for all three chemical constituents. The values of r_p² (correlation coefficient of prediction) were 0.924, 0.927, 0.933, the values of RMSEP were 0.033, 0.018, 0.064 and the values of RPD were 3.667, 3.667, 2.891 for chlorogenic acid, luteoloside, and 3,5-O-dicaffeoylquinic acid, respectively. The results indicated that NIR spectroscopy combined with variables selection and multivariate calibration methods could be considered as a useful tool for rapid determination of chlorogenic acid, luteoloside, and 3,5-O-dicaffeoylquinic acid in Chrysanthemum. Full article

(This article belongs to the Special Issue Infrared Spectroscopy and Sensors)

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12 pages, 5484 KiB

Open AccessArticle

Characterization of Gas Absorption Modules Based on Flexible Mid-Infrared Hollow Waveguides

by Kewang Chen, Zeqiao Zhao, Xuewen Zhang, Xian Zhang, Xiaosong Zhu and Yiwei Shi

Sensors 2019, 19(7), 1698; https://doi.org/10.3390/s19071698 - 10 Apr 2019

Cited by 12 | Viewed by 2670

Abstract

A new gas absorption module, the substrate-embedded hollow waveguide (eHWG) model, is proposed. It consists of a substrate with a curved channel and a hollow waveguide. The hollow waveguide is curved into the channel and works as a gas absorption cell as well [...] Read more.

A new gas absorption module, the substrate-embedded hollow waveguide (eHWG) model, is proposed. It consists of a substrate with a curved channel and a hollow waveguide. The hollow waveguide is curved into the channel and works as a gas absorption cell as well as a transmission medium for mid-infrared light. Owing to the low loss property of the hollow waveguide, the signal-to-noise ratio (SNR) was improved for the sensing system. A polycarbonate (PC) base tube was used to obtain flexibility in the fabrication of the hollow waveguide. A silver (Ag) layer and a silver iodide (AgI) layer were inner-coated to ensure a low loss property at the fingerprint wavelength of methane gas. A sensing system was established using a Fourier transform infrared spectrometer (FTIR), an external detector, and an eHWG. Experimental investigations were carried on the sensing performance of eHWGs with various channel shapes. Comparison studies were made on eHWGs embedded with Ag-coated or Ag- and AgI-coated hollow waveguides. The Ag- and AgI-coated hollow waveguides with inner diameters of 0.7, 1.4, and 2.0 mm were used in the eHWGs. The large bore waveguide had low loss but high bending additional loss. The large bore waveguide had a low detection limit due to high coupling efficiency with the light source. A limit of detection (LOD) as low as 2.7 ppm was attained for the system using the eHWG with the long and large bore waveguide. Full article

(This article belongs to the Special Issue Infrared Spectroscopy and Sensors)

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12 pages, 1900 KiB

Open AccessLetter

An Automatic Baseline Correction Method Based on the Penalized Least Squares Method

by Feng Zhang, Xiaojun Tang, Angxin Tong, Bin Wang and Jingwei Wang

Sensors 2020, 20(7), 2015; https://doi.org/10.3390/s20072015 - 03 Apr 2020

Cited by 22 | Viewed by 4702

Abstract

Baseline drift spectra are used for quantitative and qualitative analysis, which can easily lead to inaccurate or even wrong results. Although there are several baseline correction methods based on penalized least squares, they all have one or more parameters that must be optimized [...] Read more.

Baseline drift spectra are used for quantitative and qualitative analysis, which can easily lead to inaccurate or even wrong results. Although there are several baseline correction methods based on penalized least squares, they all have one or more parameters that must be optimized by users. For this purpose, an automatic baseline correction method based on penalized least squares is proposed in this paper. The algorithm first linearly expands the ends of the spectrum signal, and a Gaussian peak is added to the expanded range. Then, the whole spectrum is corrected by the adaptive smoothness parameter penalized least squares (asPLS) method, that is, by turning the smoothing parameter λ of asPLS to obtain a different root-mean-square error (RMSE) in the extended range, the optimal λ is selected with minimal RMSE. Finally, the baseline of the original signal is well estimated by asPLS with the optimal λ. The paper concludes with the experimental results on the simulated spectra and measured infrared spectra, demonstrating that the proposed method can automatically deal with different types of baseline drift. Full article

(This article belongs to the Special Issue Infrared Spectroscopy and Sensors)

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