Recent advances in on-line liquid chromatography - infrared spectrometry (LC-IR)

Raman spectrometer with vertical flow method for solutions containing organic solvents

The vertical flow (VF) method improves generation and collection efficiency in Raman spectroscopy. It enhances all Raman signals, including undesired signals of organic solvents having a considerably large Raman cross section. We constructed a Raman spectrometer using the VF method to overcome this drawback and introduced a spatial line rejection mask to eliminate unnecessary bands. In addition, the design of the VF unit was improved to resist organic solvents. A VF unit with a 60-µm pinhole enhanced the signal 168 times. The spatial mask effectively eliminated the large Raman bands of the solvent and enabled a longer exposure time. The increase in the dynamic range improved the signal-to-noise ratio by 10 % in methanol and acetonitrile measurements. Raman spectrometer with the VF method and spatial mask enables us to record the Raman spectrum of solute molecules without the disturbance of solvent bands.

High Performance Liquid chromatography - Fourier Transform Infrared Spectroscopy Coupling: A Comprehensive Review

This review presents a critical examination of the interface for coupling high performance liquid chromatography (HPLC) with Fourier transform infrared spectrometry (FTIR) since 2010. This coupling offers a robust analytical approach characterized by exceptional chemical specificity and the capacity to analyze complex multi-component mixtures qualitatively and quantitatively with high sensitivity, particularly in low limit of detection ranges. This coupling enables the identification of individual components of a mixture by IR after their separation by HPLC, although challenges arise from the potential distortion of infrared spectra by mobile phase components. Addressing this issue necessitates the implementation of suitable interfaces, such as flow cells or off-line indirect measurement methods like hot inert gas streams or ultrasonic nebulizers. The key parameters influencing the coupling of HPLC-FTIR include the solvent elimination methods, mode of FTIR technique, and IR background for accurate analyte identification. Moreover, the composition of the mobile phase and the utilization of buffer solutions in the HPLC mobile phase profoundly impact analyte identification by FTIR.

Determination of sugar content in honey using LC-Raman and programmable pump-Raman methods

We combined (i) liquid chromatography and Raman spectrometry (LC-Raman) and (ii) programmable pump and Raman spectrometry (PP-Raman) to separate and identify compounds in a mixture. These techniques were applied to conduct a quantitative analysis of the sugars in honey. The spectral and temporal axes of the LC-Raman data were analyzed using the MCR-ALS analysis procedure, which enabled the separation and identification of four sugars (glucose, fructose, sucrose, and trehalose). The PP-Raman method was employed to examine the sugar concentration dependence of the intensity pattern of the Raman spectrum, and the linear concentration dependence of the intensity was obtained. The sugar contents were quantitatively determined from the integrated area of the elution peaks. The result was consistent with those derived from mass spectrometry and previous studies. The origin of the errors in the derived sugar contents is discussed. Our study presents a novel quantitative LC-Raman spectrometric method that does not rely on resonance or surface enhancement effects.

Recent Progress in the Study of Taste Characteristics and the Nutrition and Health Properties of Organic Acids in Foods

Organic acids could improve the food flavor, maintain the nutritional value, and extend the shelf life of food. This review summarizes the detection methods and concentrations of organic acids in different foods, as well as their taste characteristics and nutritional properties. The composition of organic acids varies in different food. Fruits and vegetables often contain citric acid, creatine is a unique organic acid found in meat, fermented foods have a high content of acetic acid, and seasonings have a wide range of organic acids. Determination of the organic acid contents among different food matrices allows us to monitor the sensory properties, origin identification, and quality control of foods, and further provides a basis for food formulation design. The taste characteristics and the acid taste perception mechanisms of organic acids have made some progress, and binary taste interaction is the key method to decode multiple taste perception. Real food and solution models elucidated that the organic acid has an asymmetric interaction effect on the other four basic taste attributes. In addition, in terms of nutrition and health, organic acids can provide energy and metabolism regulation to protect the human immune and myocardial systems. Moreover, it also exhibited bacterial inhibition by disrupting the internal balance of bacteria and inhibiting enzyme activity. It is of great significance to clarify the synergistic dose-effect relationship between organic acids and other taste sensations and further promote the application of organic acids in food salt reduction.

QCL-IR Spectroscopy for In-Line Monitoring of Proteins from Preparative Ion-Exchange Chromatography

In this study, an external cavity-quantum cascade laser-based mid-infrared (IR) spectrometer was applied for in-line monitoring of proteins from preparative ion-exchange chromatography. The large optical path length of 25 μm allowed for robust spectra acquisition in the broad tuning range between 1350 and 1750 cm^–1, covering the most important spectral region for protein secondary structure determination. A significant challenge was caused by the overlapping mid-IR bands of proteins and changes in the background absorption of water due to the NaCl gradient. Implementation of advanced background compensation strategies resulted in high-quality protein spectra in three different model case studies. In Case I, a reference blank run was directly subtracted from a sample run with the same NaCl gradient. Case II and III included sample runs with different gradient profiles than the one from the reference run. Here, a novel compensation approach based on a reference spectra matrix was introduced, where the signal from the conductivity detector was employed for correlating suitable reference spectra for correction of the sample run spectra. With this method, a single blank run was sufficient to correct various gradient profiles. The obtained IR spectra of hemoglobin and β-lactoglobulin were compared to off-line reference measurements, showing excellent agreement for all case studies. Moreover, the concentration values obtained from the mid-IR spectrometer agreed well with conventional UV detectors and high-performance liquid chromatography off-line measurements. LC–QCL–IR coupling thus holds high potential for replacing laborious and time-consuming off-line methods for protein monitoring in complex downstream processes.

Current status and contemporary approaches to the discovery of antitumor agents from higher plants

Higher plant constituents have afforded clinically available anticancer drugs. These include both chemically unmodified small molecules and their synthetic derivatives currently used or those in clinical trials as antineoplastic agents, and an updated summary is provided. In addition, botanical dietary supplements, exemplified by mangosteen and noni constituents, are also covered as potential cancer chemotherapeutic agents. Approaches to metabolite purification, rapid dereplication, and biological evaluation including analytical hyphenated techniques, molecular networking, and advanced cellular and animal models are discussed. Further, enhanced and targeted drug delivery systems for phytochemicals, including micelles, nanoparticles and antibody drug conjugates (ADCs) are described herein.

Atmospheric compensation in Fourier transform infrared (FT-IR) spectra of clinical samples

A new method is proposed for the elimination of the spectral contribution of two atmospheric gases (CO2 and H2O) in Fourier transform infrared (FT-IR) spectra of clinical samples. The algorithm is based on the measurement of reference spectra of H2O followed by an automatic calculation of the spectral contribution of the above-mentioned gases to the sample spectra. Then this contribution is compensated by spectral subtraction. Attenuated total reflectance FT-IR spectra of serum and urine samples in the presence of atmospheric gases were corrected and compared with spectra obtained with an N2 purge. Visual inspection of the spectra as well as calculated noise levels confirmed that the method compensated for the contribution of CO2 and H2O to serum and urine spectra to a great extent. As compared with the correction performed by a commercial software package, the proposed method clearly improved the correction of CO2 and provided similar results for H2O compensation. Correlation values between corrected spectra and those obtained under N2-purged conditions in the fingerprint region were higher than 0.995, and noise values in regions where samples did not absorb confirmed the suitability of the correction. The proposed method is fast, easy, and extensible to other gaseous interferences, devices, and acquisition conditions.

Morphological weighted penalized least squares for background correction

Backgrounds existing in the analytical signal always impair the effectiveness of signals and compromise selectivity and sensitivity of analytical methods. In order to perform further qualitative or quantitative analysis, the background should be corrected with a reasonable method. For this purpose, a new automatic method for background correction, which is based on morphological operations and weighted penalized least squares (MPLS), has been developed in this paper. It requires neither prior knowledge about the background nor an iteration procedure or manual selection of a suitable local minimum value. The method has been successfully applied to simulated datasets as well as experimental datasets from different instruments. The results show that the method is quite flexible and could handle different kinds of backgrounds. The proposed MPLS method is implemented and available as an open source package at http://code.google.com/p/mpls.

Hyphenated techniques and their applications in natural products analysis

A technique where a separation technique is coupled with an online spectroscopic detection technology is known as hyphenated technique, e.g., GC-MS, LC-PDA, LC-MS, LC-FTIR, LC-NMR, LC-NMR-MS, and CE-MS. Recent advances in hyphenated analytical techniques have remarkably widened their applications to the analysis of complex biomaterials, especially natural products. This chapter focuses on the applications of hyphenated techniques to pre-isolation and isolation of natural products, dereplication, online partial identification of compounds, chemotaxonomic studies, chemical finger-printing, quality control of herbal products, and metabolomic studies, and presents specific examples. However, a particular emphasis has been given on the hyphenated techniques that involve an LC as the separation tool.