Varietal discrimination of Australian wines by means of mid-infrared spectroscopy and multivariate analysis

This study outlines the use of mid-infrared (MIR) spectroscopy combined with principal component analysis (PCA) and linear discriminant analysis (LDA) for the varietal classification of commercial red and white table wines. Three red varieties (Cabernet Sauvignon, Shiraz and Merlot) and four white varieties (Chardonnay, Riesling, Sauvignon Blanc and Viognier) were sourced from different wine regions in Australia. Wine samples were scanned in transmission on a FOSS WineScan FT 120 from wave numbers 926 to 5012 cm⁻¹. All samples were sourced from the 2006 vintage and had not been blended with any other variety or wine from other regions. Spectral data were reduced to a small number of principal components (PCs) and LDA was then performed to successfully separate the wines into the different varieties. To test the robustness of the LDA models developed for the red wines, a set of red wines scanned in 2005 were used. Correct classification of over 95% was achieved for the validation set.     

Imino tautomers of gas-phase guanine from mid-infrared laser spectroscopy

We reinvestigated the assignment of the three major guanine conformers detected via resonance enhanced two-photon ionization (R2PI) in supersonic expansions and present IR/UV double resonance spectra in the spectral region between 1500 and 1800 cm(-1). Comparison with B3LYP/TZVPP and RI-MP2/cc-pVQZ calculations shows that both conformers B and C are 7H-keto tautomers with an imine group in the 2-position. They differ only in the local conformation of the imine group but are otherwise identical. Conformer A is an amino-enol form with the OH group in the trans position.     

Reflection and transmission mid-infrared spectroscopy for rapid determination of coal properties by multivariate analysis

In the present paper, the influence of different acquisition techniques (transmission, diffuse reflectance infrared Fourier transform and attenuated total reflectance) in the determination of nine coal properties related to combustion power plants has been studied. Raw coal samples of different origins were pooled for developing a correlation between the resultant spectra and the corresponding coal properties by multivariate analysis techniques. Thus, the existent collinearity in mid-infrared coal spectra led to the application of partial least squares regression (PLS), studying simultaneously the influence of different spectroscopic units as well as several spectral data mathematical pre-treatments. On the other hand, a principal component analysis (PCA) revealed a relationship between principal components and coal composition in both transmission and reflection techniques. Although the best accuracy and precision results were obtained for coal properties related to organic matter, the system was also able to differentiate coal samples attending to the presence of a specific mineral matter, kaolinite.     

ATR and transmission analysis of pigments by means of far infrared spectroscopy

In the field of FTIR spectroscopy, the far infrared (FIR) spectral region has been so far less investigated than the mid-infrared (MIR), even though it presents great advantages in the characterization of those inorganic compounds, which are inactive in the MIR, such as some art pigments, corrosion products, etc. Furthermore, FIR spectroscopy is complementary to Raman spectroscopy if the fluorescence effects caused by the latter analytical technique are considered. In this paper, ATR in the FIR region is proposed as an alternative method to transmission for the analyses of pigments. This methodology was selected in order to reduce the sample amount needed for analysis, which is a must when examining cultural heritage materials. A selection of pigments have been analyzed in both ATR and transmission mode, and the resulting spectra were compared with each other. To better perform this comparison, an evaluation of the possible effect induced by the thermal treatment needed for the preparation of the polyethylene pellets on the transmission spectra of the samples has been carried out. Therefore, pigments have been analyzed in ATR mode before and after heating them at the same temperature employed for the polyethylene pellet preparation. The results showed that while the heating treatment causes only small changes in the intensity of some bands, the ATR spectra were characterized by differences in both intensity and band shifts towards lower frequencies if compared with those recorded in transmission mode. All pigments' transmission and ATR spectra are presented and discussed, and the ATR method was validated on a real case study. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Analytical investigation of salivary calculi, by mid-infrared spectroscopy

Sialolithiasis is common in salivary glands, especially in the submandibular and parotid ducts. X-Ray diffractometry was the principal technique used for their analysis, sometimes associated with scanning electron microscopy. Hydroxyapatite was the most frequently described constituent, in association with whitlockite and other calcium phosphates as brushite or octocalcium phosphate. Proteic matter was detected, as mucoproteins, albumin, nucleoproteins or as degenerative bacterial matter. This study presents the identification of constituents by mid-infrared spectrometry of 74 sialoliths. Their successive layers are analyzed from their crust to the nucleus, using absorbance measurements. Spectra are compared with reference mixtures of two or more constituents. Approximately 99% of sialoliths are constituted of calcium phosphates, under carbonated forms. More than three-quarters contain proteins, in which mucins represent the majority and albumin is found in 10% of all the specimens. Only 7% calculi are an association of two constituents, 66% are made of three and 27% have four or more components. For the 74 studied sialoliths, no specimen contains hydroxyapatite; but they are composed of carbonate apatites with irregular microcrystallized forms, even if proteins are present. Some of them have a pure protein nucleus, surrounded by carbonate apatite layers; the other stones are made of internal layers of apatites and covered with a dense and varnished crust of proteins.     

Measurement of sugar content by multidimensional analysis and mid-infrared spectroscopy

The advent of more and more powerful micro-computers has allowed the introduction of multidimensional analysis in research laboratories. Complex mathematical treatments are now possible within a few seconds. Prediction equations that linked sucrose, fructose, glucose, total sugars and reducing sugars concentrations to the spectral data, were established by regression on the principal components. Very high correlation coefficient values between the first ten axes and the chemical values were obtained. The bias and standard deviation (S.D.) values obtained between reference and predicted values were good. From such aqueous biological samples containing a ternary mixture of sucrose, fructose and glucose it was possible to (i) identify the characteristic IR bands of these different sugars (and their combination: reducing sugars, total sugars)-using spectral pattern; and (ii) to specifically measure their concentrations with good accuracy.     

Metasurface-enhanced mid-infrared spectroscopy in the liquid phase

Vibrational spectroscopy is an important tool in chemical and biological analysis. A key issue when applying vibrational spectroscopy to dilute liquid samples is the inherently low sensitivity caused by short interaction lengths and small extinction coefficients, combined with low target molecule concentrations. Here, we introduce a novel type of surface-enhanced infrared absorption spectroscopy based on the resonance of a dielectric metasurface. We demonstrate that the method is suitable for probing vibrational bands of dilute analytes with a range of spectral linewidths. We observe that the absorption signal is enhanced by 1–2 orders of magnitude and show that this enhancement leads to a lower limit of detection compared to attenuated total reflection (ATR). Overall, the technique provides an important addition to the spectroscopist''s toolkit especially for probing dilute samples.

A novel surface-enhanced infrared absorption (SEIRA) spectroscopy method exploits the resonance of a dielectric metasurface to provide strong field enhancement at a microscale depth, making it suitable for analysis of dilute liquid samples.     

Continuous glucose monitoring in interstitial fluid using glucose oxidase-based sensor compared to established blood glucose measurement in rats

Glucose monitoring is of importance for success of complex therapeutic interventions in diabetic patients. Its impact on treatment and glycemic control is demonstrated in large clinical trials. Up to eight blood glucose measurements per day are recommended. Notwithstanding, a substantial number of diabetic patients cannot or will not monitor their blood glucose appropriately. Considerable progress in control of disturbed metabolism in diabetic patients can be expected by continuous glucose monitoring. The aim of the study was to evaluate the performance of a new amperometric glucose oxidase-based glucose sensor in vitro and in vivo after subcutaneous implantation into rats.For in vitro testing current output of sensors was measured by exposure to increasing and decreasing glucose concentrations up to 472 mg dL⁻¹ over a time period of 7 days. After subcutaneous implantation of sensors into interscapular region of male rats glucose in interstitial fluid was evaluated and compared to glucose in arterial blood up to 7 days. Hyper- and hypoglycaemia were induced by intravenous application of glucose and insulin, respectively. Current of each implanted sensor was converted into glucose concentration using the first blood glucose measurement only.A change of current with glucose of 0.35 nA mg⁻¹ dL⁻¹ indicates high sensitivity of the sensor in vitro. The response time (90% of steady state) was calculated by approximately 60 s. Test strips for blood glucose measurement as reference for sensor readings was found as an appropriate and rapidly available method in rats by comparison with established hexokinase method in an automated lab analyzer with limits of agreement of +32.8 and −25.7 mg dL⁻¹ in Bland-Altman analysis. In normo- and hypoglycaemic range sensor readings in interstitial fluid correlated well with blood glucose measurements whereas hyperglycaemia was not reflected by the sensor completely when blood glucose was changing rapidly.The data given characterize a sensor with high sensitivity, long term stability and short response time. A single calibration of the sensor is required only in measurement periods up to 7 days. The findings demonstrate that the sensor is a highly promising candidate for assessment in humans.     

Solvent-dependent photoacidity state of pyranine monitored by transient mid-infrared spectroscopy.

We investigate with femtosecond mid-infrared spectroscopy the vibrational-mode characteristics of the electronic states involved in the excited-state dynamics of pyranine (HPTS) that ultimately lead to efficient proton (deuteron) transfer in H2O (D2O). We also study the methoxy derivative of pyranine (MPTS), which is similar in electronic structure but does not have the photoacidity property. We compare the observed vibrational band patterns of MPTS and HPTS after electronic excitation in the solvents: deuterated dimethylsulfoxide, deuterated methanol and H2O/D2O, from which we conclude that for MPTS and HPTS photoacids the first excited singlet state appears to have charge-transfer (CT) properties in water within our time resolution (150 fs), whereas in aprotic dimethylsulfoxide the photoacid appears to be in a non-polar electronic excited state, and in methanol (less polar and less acidic than water) the behaviour is intermediate between these two extremes. For the fingerprint vibrations we do not observe dynamics on a time scale of a few picoseconds, and with our results obtained on the O-H stretching vibration we argue that the dynamic behaviour observed in previous UV/Vis pump-probe studies is likely to be related to solvation dynamics.     

Simplified Fourier-transform mid-infrared spectroscopy calibration based on a spectra library for the on-line monitoring of bioprocesses

In order to significantly reduce the time involved in mid-infrared spectroscopy calibrations, a novel approach based on a library of pure component spectra was developed and tested with an aerobic Saccharomyces cerevisiae fermentation. Instead of the 30-50 standards that would have been required to build a chemometric model, only five solutions were used to assemble the library, namely one for each compound (glucose, ethanol, glycerol, ammonium and acetate). Concentration profiles of glucose, ethanol and ammonium were monitored with a fair accuracy, leading to standard error of prediction (SEP) values of 0.86, 0.98 and 0.15 g L⁻¹. Prediction of the two minor metabolites, acetate and glycerol, was less accurate and presented a detection limit of around 0.5 g L⁻¹. The overall performance of the library-based method proved to be very similar to a 49-standard chemometrics model. The model was shown to be very robust and uncorrelated, since it was able to predict accurately the concentration changes during a spiking experiment. Even though simple, this method allows more advanced calculations, such as determination of the explained variance and detection of unexpected compounds using residuals analysis.     

Infrared matrix-assisted laser desorption and ionization by using a tunable mid-infrared free-electron laser

Initial results of infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry of proteins by using the Vanderbilt free-electron laser as the source of selective vibrational excitation are reported. The ability of this laser to initiate desorption and ionization by excitation of specific vibrational modes is demonstrated. For the first time it is shown that IR-MALDI mass spectrometry at wavelengths other than those available from conventional fixed-frequency IR lasers, that is, 2.79 (Er:YSGG), 2.94 (Er:YAG), and 9.3-10.6 μm (CO₂), is feasible and exhibits similar performance. IR-MALDI mass spectra were taken in the wavelength ranges 2.8-4 and 5.5-6.5 μm, covering the absorption bands of the O-H and C=O stretch vibrations typical of many organic compounds such as succinic acid, fumaric acid, or nicotinic acid, which were used as matrices in these studies. A comparison between these results and Er:YAG/YSGG MALDI data are given. The potential of IR-MALDI at wavelengths near the C=O stretch vibration and the possibilities for studies of the IR-MALDI mechanisms by using this kind of tunable source are discussed.     

Diblock copolymer adsorption from the aqueous micellar phase to solid surfaces: real time monitoring by ATR spectroscopy in the mid-infrared

Attenuated Total Reflectance (ATR) in the mid-infrared is employed to monitor the adsorption of poly(tert-butyl styrene - b - styrene sulfonate), PtBS-PSS, copolymers with a small hydrophobic head as well as of an analogous poly(styrene sulfonate), PSS, homopolymer on germanium and Au-plated germanium surfaces from aqueous NaCl solutions. The surface density of the adsorbed polyelectrolyte is monitored via the growth of characteristic infrared absorptions of the sulfonated ring at 1036- and 1008 cm⁻¹ in the 2nd derivative mode. These probe bands do not exhibit shifts or changes in bandshape over very prolongued adsorption experiments. Pronounced differences in the kinetics of adsorption are observed between the PSS homopolymer and the PtBS-PSS copolymers in agreement to previous investigations by phase-modulated ellipsometry on similar systems. Adsorption of the diblocks above the critical micelle concentration is found to involve a sequence of diffusion, micellar relaxation and brush-limited processes.     

Near-infrared diode laser spectroscopy in chemical process and environmental air monitoring

This review covers the rapidly expanding field of near-infrared tunable diode laser spectroscopy where the availability of new lasers has led to the development of simple and inexpensive spectroscopic systems for the detection and monitoring of gas species. The latest diode lasers and the specific techniques associated with diode laser spectroscopy are described. Specific examples covering chemical vapour deposition reaction diagnostics, remote vehicle emission sensing, balloon-borne atmospheric monitoring and combustion diagnostics then illustrate the technique advantages of rapid, highly selective, in situ monitoring.     

Continuous reagent-free bed-side monitoring of glucose in biofluids using infrared spectrometry and micro-dialysis

Quasi-continuous glucose monitoring has been realised for a bed-side device based on reagent-free transmission spectroscopy of microliter dialysate sample volumes. Aqueous glucose solutions and serum ultrafiltrates were used for in vitro testing, whereas in vivo samples were provided by a subcutaneously implanted micro-dialysis probe. Both sample types were transported to a thermostated flow-through micro-cell, housed within the sample compartment of a Fourier-transform mini-spectrometer, by using a programmable fluidic system consisting of a six-port valve with sample loop and a syringe pump. Different options were implemented for spectral evaluation based on multivariate calibration using either partial or classical least squares. The latter approach includes the additional quantification of most relevant dialysate components such as urea, lactate, bicarbonate and others. Automated operator-free in vitro operation with reliable glucose quantification over at least 72 h was tested for analytical performance characterisation. Routines for sensitive air bubble detection by infrared spectrometry including automated removal by the fluidic system have also been implemented.     

Folding structures of isolated peptides as revealed by gas-phase mid-infrared spectroscopy.

To understand the intrinsic properties of peptides, which are determined by factors such as intramolecular hydrogen bonding, van der Waals bonding and electrostatic interactions, the conformational landscape of isolated protein building blocks in the gas phase was investigated. Here, we present IR-UV double-resonance spectra of jet-cooled, uncapped peptides containing a tryptophan (Trp) UV chromophore in the 1000-2000 cm(-1) spectral range. In the series Trp, Trp-Gly and Trp-Gly-Gly (where Gly stands for glycine), the number of detected conformers was found to decrease from six (Snoek et al., PCCP, 2001, 3, 1819) to four and two, respectively, which indicates a trend to relaxation to a global minimum. Density functional theory calculations reveal that the O-H in-plane bending vibration, together with the N-H in-plane bend ing and the peptide C=O stretching vibrations, is a sensitive probe to hydrogen bonding and, thus, to the folding of the peptide backbone in these structures. This enables the identification of spectroscopic fingerprints for the various conformational structures. By comparing the experimentally observed IR spectra with the calculated spectra, a unique conformational assignment can be made in most cases. The IR-UV spectrum of a Trp-containing nonapeptide (Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu) was recorded as well and, although the IR spectrum is less well-resolved (and it probably results from different isomers), groups of amide I (peptide C=O stretching) and amide II (N-H in-plane bending) bands can still be recognised, in agreement with predictions at the AM1 level.     

Dual-pulse laser induced breakdown spectroscopy: Time-resolved transmission and spectral measurements

Spectral analysis of laser-induced plasmas for surface ablation has demonstrated the possibility of analyte signal enhancement with dual-pulse configurations as compared with traditional single-pulse LIBS. Using an orthogonal dual-pulse arrangement, measurements were performed using glass microscope slides to allow both spectral analysis as well as optical transmission measurements. Order of magnitude enhancements in Mg and Si atomic emission signal peak intensities were recorded along with similar enhancements of the continuum emission for dual-pulse LIBS as compared to single-pulse. Peak-to-base measurements showed a roughly 50% increase, while signal-to-noise ratios were enhanced by a factor of 2–3. Temporal analysis of the measured transmitted laser pulse waveforms showed no significant differences between dual-pulse and single-pulse LIBS configurations, providing additional insight into the possible laser coupling processes for the dual-pulse configuration.     

Monitoring PDT by means of superficial reflectance spectroscopy

Monitoring of relevant parameters during photodynamic therapy (PDT) and correlating these with treatment response is necessary to guarantee optimal and reproducible treatment outcome. In this paper we study the correlation between changes in the local tissue optical properties (absorption and scattering coefficients) during ALA-PDT and changes in PpIX fluorescence. The optical properties are measured extremely superficially by employing a single fiber for the delivery and collection of white light to and from the tissue. The measured reflectance spectrum is modeled in terms of four relevant parameters: blood saturation, relative blood volume fraction, scattering intensity and wavelength dependence of the scattering. All these parameters, except the relative blood volume fraction, are shown to correlate with the rate of photobleaching of PpIX, which in turn has previously been shown to correlate with the response of tissues to PDT. These results yield valuable insight in the behavior of these parameters during PDT and their suitability to predict PDT-response for other photosensitizers for which monitoring through photobleaching is not possible.     

Primary reactions of bacteriophytochrome observed with ultrafast mid-infrared spectroscopy

Phytochromes are red-light photoreceptor proteins that regulate a variety of responses and cellular processes in plants, bacteria, and fungi. The phytochrome light activation mechanism involves isomerization around the C(15)═C(16) double bond of an open-chain tetrapyrrole chromophore, resulting in a flip of its D-ring. In an important recent development, bacteriophytochrome (Bph) has been engineered for use as a fluorescent marker in mammalian tissues. Bphs covalently bind a biliverdin (BV) chromophore, naturally abundant in mammalian cells. Here, we report an ultrafast time-resolved mid-infrared spectroscopic study on the Pr state of two highly related Bphs from Rps. palustris , RpBphP2 (P2) and RpBphP3 (P3) with distinct photoconversion and fluorescence properties. We observed that the BV excited state of P2 decays in 58 ps, while the BV excited state of P3 decays in 362 ps. By combining ultrafast mid-IR spectroscopy with FTIR spectroscopy on P2 and P3 wild type and mutant proteins, we demonstrate that the hydrogen bond strength at the ring D carbonyl of the BV chromophore is significantly stronger in P3 as compared to P2. This result is consistent with the X-ray structures of Bph, which indicate one hydrogen bond from a conserved histidine to the BV ring D carbonyl for classical bacteriophytochromes such as P2, and one or two additional hydrogen bonds from a serine and a lysine side chain to the BV ring D carbonyl for P3. We conclude that the hydrogen-bond strength at BV ring D is a key determinant of excited-state lifetime and fluorescence quantum yield. Excited-state decay is followed by the formation of a primary intermediate that does not decay on the nanosecond time scale of the experiment, which shows a narrow absorption band at ～1540 cm(-1). Possible origins of this product band are discussed. This work may aid in rational structure- and mechanism-based conversion of BPh into an efficient near-IR fluorescent marker.     

Electron affinities of double bond π* orbitals determined by means of electron transmission spectroscopy

The electron transmission spectra of small molecules containing C=C, C=N, C=O, C=S and N=N double bonds are reported. The electron affinities of these functional groups, associated with electron capture into their empty π^* orbitals, are discussed in terms of heteroatom electronegativities, geometrical variations and localization properties of the π^* orbitals. The largest electron acceptor properties were observed in the thioketone derivative, which generates a stable π anion state. The ionization energy values relating to the heteroatom lone pair and the filled π orbitals are also reported.