Quantitative analysis of sugar composition in honey using 532‐nm excitation Raman and Raman optical activity spectra

Raman spectroscopy based on the 1064‐nm laser excitation was suggested as a handy non‐invasive technique allowing to quickly determine sugar content in honey and similar food products. In the present study, the green 532‐nm laser radiation is explored instead as it provides higher‐quality spectra in a shorter time. The sample fluorescence was quenched by purification with activated carbon. For control mixture decomposition of Raman spectra to standard subspectra led to a typical error of the sugar content of 3%. Raman optical activity (ROA) spectra that could be measured at the shorter excitation wavelength as well provided a lower accuracy (~8%) than the Raman spectra because of instrumental sensitivity and noise limitations. The results show that Raman spectroscopy provides elegant and reliable means for fast analyses of sugar‐based food products. Copyright © 2016 John Wiley & Sons, Ltd.     

Laser wavelength dependence of background fluorescence in Raman spectroscopic analysis of synovial fluid from symptomatic joints

Gout is a disease process where the nucleation and growth of crystals in the synovial fluid of joints elicit painful arthritis‐like symptoms. Raman spectroscopy is evolving as a potential diagnostic tool in identifying such crystals; however, attainment of sufficient Raman signal while overcoming the background fluorescence remains as a major challenge. The current study focused on assessing whether excitation in 532–700 nm range will provide greater signal intensity than the standard 785 nm while not being impeded by background fluorescence. We characterized the fluorescence spectra, absorption spectra and Raman spectra of synovial fluid from patients who presented ‘gout‐like symptoms’ (symptomatic) and controls (asymptomatic). A digestion and filtration method was developed to isolate crystals from synovial fluid while reducing the organic burden. Spectral profile and photobleaching dynamics during Raman spectroscopy were observed under an excitation wavelength range spanning 532 to 785 nm. Absorbance and fluorescence profiles indicated the digestion and filtration worked effectively to extract crystals from symptomatic synovial fluid without introducing additional fluorescence. Raman spectral analyses at 532 nm, 660 nm, 690 nm and 785 nm indicated that both asymptomatic and symptomatic samples had significant levels of fluorescence at excitation wavelengths below 700 nm, which either hindered the collection of Raman signal or necessitated prolonged durations of photobleaching. Raman‐based diagnostics were more feasible at the longest excitation wavelength of 785 nm without employing photobleaching. This study further demonstrated that a near‐infrared (NIR) OEM‐based lower‐cost Raman system at 785 nm excitation has sufficient sensitivity to identify crystals isolated from the synovial fluid. In conclusion, while lower excitation wavelengths provide greater signal, the fluorescence necessitates NIR wavelengths for Raman analysis of crystal species observed in synovial aspirates. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of laser excitation wavelength and optical mode on Raman spectra of human fresh colon,pancreas, and prostate tissues

Early cancer detection is the central and most important factor for allowing successful treatment and resultant positive long‐term patient outcomes. Recently, optical techniques have been applied to this purpose, although each has inherent limitations. In particular, Raman spectroscopy applied in the pathological diagnosis of cancerous tissues has received increasing attention, with the merit of being highly sensitive to the biochemical alterations in tissue compositions and applicable in vivo. Nevertheless, its application has been impeded by the high background intensity, which masks the Raman signal of biological molecules. In this work, the influence of laser excitation wavelength (785 vs. 830 nm) and optical mode (single mode vs. multimode) on the background intensity of fresh human tissues was studied. Based on the results, laser with 830 nm excitation demonstrated better background reduction than that with 785 nm excitation for the same optical mode, but the Raman signal intensity was conversely reduced, and the signal‐to‐noise ratio (SNR) not improved. In contrast, by comparing single‐mode and multimode 785 nm excitations, it was shown that the single‐mode laser with its smaller beam waist and beam propagation factor had better background reduction ability and an improvement of the SNRs. It is speculated that this decrease in background intensity comes from the effect of the optical mode on the Mie scattering from the biological tissue. High‐quality spectra based on a careful selection of both laser excitation wavelength and optical mode will benefit Raman measurements in further research focusing on spectral interpretation and histopathological correlation ultimately aimed toward intraoperative applications. Copyright © 2014 John Wiley & Sons, Ltd.     

Near‐room‐temperature photon‐noise‐limited quantum well infrared photodetector

With the modern development of infrared laser sources such as broadly tunable quantum cascade lasers and frequency combs, applications of infrared laser spectroscopy are expected to become widespread. Consequently, convenient infrared detectors are needed, having properties such as fast response, high efficiency, and room‐temperature operation. This work investigated conditions to achieve near‐room‐temperature photon‐noise‐limited performance of quantum well infrared photodetectors (QWIPs), in particular the laser power requirement. Both model simulation and experimental verification were carried out. At 300 K, it is shown that the ideal performance can be reached for typical QWIP designs up to a detection wavelength of 10 µm. At 250 K, which is easily reachable with a thermoelectric Peltier cooler, the ideal performance can be reached up to 12 µm. QWIPs are therefore suitable for detection and sensing applications with devices operating up to or near room temperature.     

Comparison of near infrared laser excitation wavelengths and its influence on the interrogation of seized drugs‐of‐abuse by Raman spectroscopy

The laser excitation wavelength is an important parameter in obtaining Raman spectra from drugs‐of‐abuse. This article compares the effect of near infrared wavelengths, 785 nm, using both benchtop and portable instrumentation and benchtop 1064 nm on the Raman spectra of seized drugs‐of‐abuse, including cocaine hydrochloride, cocaine freebase (crack), methylenedioxymethamphetamine (‘ecstasy’), amphetamine, diamorphine (heroin) and cannabis. The significant benefit of using 1064 nm for the interrogation of this type of sample is highlighted. Copyright © 2009 John Wiley & Sons, Ltd.     

A portable shifted excitation Raman difference spectroscopy system: device and field demonstration

In this paper, we present a portable shifted excitation Raman difference spectroscopy (SERDS) system applied in outdoor experiments. A dual‐wavelength diode laser emitting at 785 nm is used as excitation light source. The diode laser provides two individually controllable excitation lines at 785 nm with a spectral distance of about 10 cm⁻¹ for SERDS. This monolithic light source is implemented into a compact handheld Raman probe. Both components were developed and fabricated in‐house. SERDS measurements are performed in an apple orchard, and apples and green apple leafs are used as test samples. For each excitation wavelength, a single Raman spectrum is measured with 50 mW at the sample. Strong background interference from ambient daylight and laser‐induced fluorescence obscure the Raman signals. SERDS efficiently separates the wanted Raman signals from the disturbing background signals. For the Raman spectroscopic investigations of green leafs, one accumulation with an exposure time of 0.2 s was used for each excitation wavelength to avoid detector saturation. An 11‐fold improvement of the signal‐to‐background noise is achieved using SERDS. The results demonstrate the suitability of the portable SERDS system for rapid outdoor Raman investigations. Copyright © 2016 John Wiley & Sons, Ltd.     

Forensic and security applications of a long‐wavelength dispersive Raman system

A novel dispersive system operating at 1064‐nm excitation and coupled with transfer electron InGaAs photocathode and electron bombardment CCD technology has been evaluated for the analysis of drugs of abuse and explosives. By employing near‐IR excitation at 1064‐nm excitation wavelength has resulted in a significant damping of the fluorescence emission compared to 785‐nm wavelength excitation. Spectra of street samples of drugs of abuse and plastic explosives, which usually fluoresce with 785‐nm excitation, are readily obtained in situ within seconds through plastic packaging and glass containers using highly innovative detector architecture based upon a transfer electron (TE) photocathode and electron bombarded gain (EB) technology that allowed the detection of NIR radiation at 1064 nm without fluorescence interference. This dispersive near‐IR Raman system has the potential to be an integral part in the armoury of the forensic analyst as a non‐destructive tool for the in‐situ analysis of drugs of abuse and explosives. Copyright © 2015 John Wiley & Sons, Ltd.     

A shifted‐excitation Raman difference spectroscopy (SERDS) evaluation strategy for the efficient isolation of Raman spectra from extreme fluorescence interference

A biochemical characterization of pathologies in biological tissue can be provided by Raman spectroscopy. Often, the raw spectrum is severely affected by fluorescence interference. We report and compare various spectra‐processing approaches required for the purification of Raman spectra from heavily fluorescence‐interfered raw spectra according to the shifted‐excitation Raman difference spectroscopy method. These approaches cover the entire spectra‐processing chain from the raw spectra to the purified Raman spectra. In detail, we compared (1) area normalization versus z‐score normalization, (2) direct reconstruction of the difference spectra versus reconstruction of zero‐centered difference spectra and (3) collective baseline correction of the reconstructed spectra versus piecewise baseline correction of the reconstructed spectra and, finally, (4) analyzed the influence of the shift of the excitation wavelength on the quality of the reconstructed spectra. Statistical analysis of the spectra showed that – in our experiments – the best results were obtained for the z‐score normalization before subtraction of the normalized spectra, followed by zero‐centering of the difference spectra before reconstruction and a piecewise baseline correction of the pure Raman spectra. With our equipment, a wavelength shift from 784 to 785 nm provided reconstructed spectra of best quality. The analyzed specimens were different tissue types of pigs, tissue from the oral cavity of humans and a model solution of dye dissolved in ethanol. © 2015 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons Ltd.     

传统陶瓷中结构羟基的光谱学研究

烧结粘土产品可以吸收水分子发生再羟基化,生成结构羟基的量与产品保存时间存在一定关系,基于该理论可以利用热重分析方法对陶器制品进行测年研究.红外与拉曼光谱技术也可以用来分析结构羟基信息,因此人们希望探索利用光谱分析方法代替热重法进行传统陶瓷无损测年分析.为了验证可行性,收集了多种典型矿物原料和可溯源的传统陶瓷样品,利用红...     

多波长消荧光拉曼光谱仪的研制及应用研究

拉曼光谱技术作为探究分子、晶体及其结构特征的有力手段,具有快速、无损、样品用量小、无需前处理且适应性强等优点,已被广泛应用于食品安全、石油化工等领域。但在拉曼光谱应用中,常常受到荧光背景干扰,导致拉曼信号降低,严重的情况下拉曼信号甚至会淹没在荧光背景中。为解决拉曼技术在实际应用中荧光背景干扰的问题,从仪器角度出发,采用二色镜对多波长拉曼光谱进行光路耦合设计,研制了近红外拉曼光谱与移频差分拉曼复合一体的多波长消荧光拉曼光谱检测系统,其中近红外拉曼光谱采用1 064 nm激光光源设计,移频差分拉曼光谱选取784.5和785.5 nm两组激光光源进行时分复用,在移频差分拉曼光谱检测的同时,亦可获得两组单波长拉曼光谱数据。通过对比同步测试和分时逐次测试的强度及峰位稳定性,验证了多波长消荧光拉曼光谱仪的同步测试性能;选取了多种荧光背景强弱不同的样品,进行了单波长拉曼、近红外拉曼及移频差分拉曼光谱的对比分析。针对丙酮、乙腈等荧光背景较弱的样品,可采用单波长拉曼光谱对样品进行定量及定性分析;针对食用油、红色塑胶微粒等荧光背景与拉曼信号强度相当的样品,可采用近红外拉曼光谱对样品进行定量及定性分析;针对红酒、棕色塑胶微粒等荧光背景较强的样品,需结合近红外拉曼光谱和差分拉曼光谱对样品进行定性分析。研究表明：通过多波长消荧光拉曼光谱检测系统的研制,在常规单波长拉曼光谱技术的基础上,将两种抑制荧光干扰技术有机结合,有效扩充了应用领域及样品检测范围。     

On the noise limit of stress and temperature measurements with micro‐Raman spectroscopy

We report for the first time on the thorough experimental and theoretical assessment of the noise limit of mechanical stress and temperature measurements with micro‐Raman spectroscopy. A comprehensive study has been performed in which, for different incident laser light intensities and acquisition times, 1000 Raman spectra of mono‐crystalline silicon were acquired per setting. Curve fitting was employed to obtain the peak positions of all the spectra, from which the standard deviations of the measured peak positions were obtained versus the total accumulated amount of laser light incident on the sample during one measurement. It has been found that the noise in the obtained peak position decreases as 1/sqrt(n) over more than three decades of the incident amount of laser light. At very low light conditions, the noise decreases as 1/n. By comparing the experimental results obtained to recent theoretical work, we show that the acquisition is limited by photon shot noise over most of the range and is limited by electronic detector noise at very low light conditions only. Pixelation errors do not play a role. It is concluded that the low electronic noise of typical Raman spectroscope detectors is overkill for the investigation of mechanical stress and temperature in silicon and other materials with comparable peaks, as it has absolutely no influence on the noise level of such an experiment. Maximum Raman signal intensity on the detector and high quantum efficiency detection are more important. Copyright © 2013 John Wiley & Sons, Ltd.     

Rapid shifted excitation Raman difference spectroscopy with a distributed feedback diode laser emitting at 785 nm

A distributed feedback (DFB) laser diode emitting at 785 nm was tested and applied as a light source for shifted excitation Raman difference spectroscopy (SERDS). Due to the physical properties of the laser diode, it was possible to shift the emission wavelength by 8 cm^-1 (0.5 nm) required for our SERDS measurements by simply changing the injection current. The internal grating ensured single mode operation at both wavelength with the frequency stability of ±0.06 cm^-1 (0.004 nm) required for high resolution Raman spectroscopic applications. The shifted spectra were used for calculating enhanced Raman spectra being obscured by a strong scattering background. A 16 dB (≈38 fold) improvement of the signal-to-background noise S̄/σ_B was demonstrated using blackboard chalk as a sample. The tunable DFB laser is a versatile excitation source for SERDS, which could be used in any dispersive Raman system to subtract fluorescence contributions and scattering background. PACS 82.80.Gk; 42.55.-f; 42.64.Fi     

Instantaneous shifted‐excitation Raman difference spectroscopy (iSERDS)

Shifted‐excitation Raman difference spectroscopy (SERDS) is an experimental method to recover spontaneous Raman spectra despite the presence of strong fluorescence interference. The common scheme requires a tunable laser source and recording two spectra after each other. In this paper, an approach for instantaneous SERDS (iSERDS) is presented utilizing a broadband light source. The broadband radiation is spatially dispersed in the focal plane inside the object of investigation. The generated scattering signal is imaged onto the slit of an imaging spectrograph. The individual pixel lines on the detector represent Raman spectra with slightly shifted excitation wavelength and hence allow SERDS spectra to be derived. The proposed iSERDS technique is a suitable approach for obtaining Raman spectra from fluorescing samples provided they are homogenous on the length scale of the measurement volume. Copyright © 2014 John Wiley & Sons, Ltd.     

Analysis of alizarin by surface‐enhanced and FT‐Raman spectroscopy

This paper presents the application of Raman spectroscopy (RS) for the structural study of alizarin adsorbed on a metallic surface. As a biologically active molecule, alizarin has remarkable antigenotoxic activity like other anthraquinone dyes. Alizarin is highly fluorescent and that limits the application of RS as an investigation method; however, the Fourier transform‐RS (FTRS) can be applied since the near‐infrared excitation line lies far away from the absorption region of alizarin. The surface enhanced‐RS (SERS) technique also makes the fluorescence quenching possible. In this work, monolayers of alizarin were deposited on the surface of an electrode by the immersion of silver substrates in methanolic solution of the analyte. From such prepared samples, by using the excitation of 488, 514.5 and 647.1 nm the Raman spectra were registered. Depending on the excitation line, SERS or surface‐enhanced resonance Raman scattering (SERRS) spectra of alizarin were observed. The interpretation of experimental data was supported by theoretical calculations. Copyright © 2008 John Wiley & Sons, Ltd.     

Study of Raman spectrum of uranium using a surface‐enhanced Raman scattering technique

Raman spectroscopic investigation on weak scatterers such as metals is a challenging scientific problem. Technologically important actinide metals such as uranium and plutonium have not been investigated using Raman spectroscopy possibly due to poor signal intensities. We report the first Raman spectrum of uranium metal using a surface‐enhanced Raman scattering‐like geometry where a thin gold overlayer is deposited on uranium. Raman spectra are detected from the pits and scratches on the sample and not from the smooth polished surface. The 514.5‐ and 785‐nm laser excitations resulted in the Raman spectra of uranium metal whereas 325‐nm excitation did not give rise to such spectra. Temperature dependence of the B_3g mode at 126 cm⁻¹ is also investigated. Copyright © 2011 John Wiley & Sons, Ltd.     

UV‐resonance Raman micro‐spectroscopy to assess residual chromophores in cellulosic pulps

Ultraviolet‐resonance Raman (UV‐RR) micro‐spectroscopy is an appropriate and sensitive tool to assess the chromophore structures in bleached cellulosic pulps used for papermaking. The particular selectivity in detection and identification of chromophores in pulps is achieved by acquiring the UV‐RR spectra in the solid state with laser excitation at 325 nm. This wavelength corresponds to absorption of poly‐unsaturated chromophore structures in partially bleached/fully bleached pulps, and linearly correlated with the signal at ca 1600 cm⁻¹ in the UV‐RR spectra. The characteristic vibrations from particular pulp chromophore structures have been assigned from experiments with model compounds, thus allowing the establishment of a UV‐RR database. Among the components of bleached pulp, the xylan–lignin complex was suggested to be an important source of chromophores. The monitoring of pulp bleaching by UV‐RR allowed us to suggest that it is the formation of new polysaccharide‐derived chromophores upon bleaching that hinders development of further brightness and is co‐responsible for the brightness reversion of fully bleached pulps. Copyright © 2010 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman micro‐spectroscopy of DNA/RNA bases adsorbed on pyroxene rocks as a test of in situ search for life traces on Mars

SERS spectroscopy with 785‐nm laser excitation minimizing fluorescence emission is exploited for remote analysis of life traces in an extraterrestrial environment. Adenine and guanine, nucleobases present in both DNA and RNA strands, and microRNA containing adenine and guanine have been used as testing ligands for identifying traces of nucleic acids in Martian rocks and sediments. SERS spectra of these nucleobase samples adsorbed on pyroxene substrates have been investigated with micro‐Raman apparatus in the absence of sample manipulation and inducing the signal enhancement by deposition of silver colloidal nanoparticles over the pyroxene/nucleobase substrates. An order‐of‐magnitude estimate of the sample amount responsible for the SERS spectra is given. Copyright © 2009 John Wiley & Sons, Ltd.     

双波长LD模块的拉曼光谱测试系统

荧光干扰是拉曼光谱检测过程中常见的干扰因素之一,而移频激发法是一种有效的克服荧光干扰的检测手段。移频激发法利用两个波长相近的激光分别激发被测物质,并将获得的拉曼光谱进行差谱。由于两次激发的荧光背景相同,而拉曼特征峰会产生平移,因此可有效地消除荧光背景的干扰,进而利用一定的算法还原拉曼特征峰。移频激发法的关键在于两个激发光波长的稳定性,不稳定的波长差将严重影响对拉曼特征峰的还原效果。本文研制了一种拉曼光谱测试系统,该系统的双波长LD模块能够产生两个波长稳定的激发光(分别为784.7和785.8 nm),满足移频激发法的测试要求。影响激发光波长稳定性的因素主要是光功率和温度,本系统中对这两个因素均进行了实时的监控,以保证激发光波长的稳定。系统的硬件部分主要包括ARM主控板、双波长LD模块及其驱动电路、温度控制板、数字光开关、光谱检测光路和光纤探头(两个高功率的蝶形封装激光器);软件部分可自动获取被测物质的拉曼光谱图,并对其进行后续的处理。在稳定性测试实验中,对系统驱动电源电流和激光器温度的稳定性均进行了测试。测试结果显示,电流波动范围小于0.01 mA、温度变化范围小于0.004 ℃,能够有效地保证激发光波长的稳定性。最后,对某品牌花生油进行了拉曼光谱检测,并对检测结果进行了处理,获得了良好的效果。     

Near‐infrared Raman spectroscopy for gastric precancer diagnosis

Raman spectroscopy is a molecular vibrational spectroscopic technique that is capable of optically probing the biomolecular changes associated with neoplastic transformation. The purpose of this study was to apply near‐infrared (NIR) Raman spectroscopy for differentiating dysplasia from normal gastric mucosa tissue. A total of 65 gastric mucosa tissues (44 normal and 21 dysplasia) were obtained from 35 patients who underwent endoscopy investigation or gastrectomy operation for this study. A rapid NIR Raman system was utilized for tissue Raman spectroscopic measurements at 785‐nm laser excitation. High‐quality Raman spectra in the range of 800–1800 cm⁻¹ can be acquired from gastric mucosa tissue within 5 s. Raman spectra showed significant differences between normal and dysplastic tissue, particularly in the spectral ranges of 850–1150, 1200–1500 and 1600–1750 cm⁻¹, which contained signals related to proteins, nucleic acids and lipids. The diagnostic decision algorithm based on the combination of Raman peak intensity ratios of I₈₇₅/I₁₄₅₀ and I₁₂₀₈/I₁₆₅₅ and the logistic regression analysis yielded a diagnostic sensitivity of 90.5% and specificity of 90.9% for identification of gastric dysplasia tissue. This work demonstrates that NIR Raman spectroscopy in conjunction with intensity ratio algorithms has the potential for the noninvasive diagnosis and detection of precancer in the stomach at the molecular level. Copyright © 2009 John Wiley & Sons, Ltd.     

Microanalysis of clay‐based pigments in painted artworks by the means of Raman spectroscopy

FT Raman spectroscopy and micro‐Raman spectroscopy with lasers of three different wavelengths (1064 nm, 785 nm and 532 nm) were used for analysis of reference samples of natural clay pigments including white clay minerals (kaolinite, illite, montmorillonite), green earths (glauconite and celadonite) and red earths (natural mixtures of white clay minerals with hematite). In addition, eight micro‐samples obtained from historical paintings containing clay pigments in ground and colour layers have been examined. Powder X‐ray diffraction and micro‐diffraction were used as supplementary methods. It was found that laser operating at 1064 nm provided the best quality Raman spectra for distinguishing different white clay minerals, but the spectra of green and red earths were affected by strong fluorescence caused by the presence of iron. Green earth minerals could be easily distinguished by 532 or 785 nm excitation lasers, even in small concentrations in the paint layers. On the other hand, when anatase (TiO₂) or iron oxides (such as hematite) were present as admixtures (both are quite common, particularly in red earths), the collection of characteristic spectra of clay minerals which form the main component of the layer was hindered or even prevented. Another complicating factor was the fluorescence produced by organic binders when analysing the micro‐samples of artworks. In those cases, it is always necessary to use powder X‐ray micro‐diffraction to avoid misleading interpretations of the pigment's composition. Copyright © 2013 John Wiley & Sons, Ltd.