Current Chromatography - Volume 6, Issue 1, 2019

Background: Application of simple regularities and general principles along with direct use of reference gas chromatography retention index data for reliable structure determination of compounds can be enhanced by determination of new regularities that are specific to certain structural elements. Objective: Revelation and interpretation of an anomaly in the elution order of alkyl esters of alkoxycarbonyl derivatives of glycine and alanine on standard and semi-standard non-polar phases. Methods: Preliminary derivatization of amino acids to alkyl esters of N-alkoxycarbonyl analogs and interpretation of their gas chromatographic characteristics. Results: Alkyl esters of N-alkoxycarbonyl derivatives of alanine (Alkyl = C2H5, n- and iso-C3H7) elute prior to the same derivatives of glycine, despite the presence of an additional methyl group at C(2) in the molecule. Elution order is reversed for methyl esters of N-methoxycarbonyl derivatives. Conclusion: It is established that the peculiar behavior of alkyl esters of N-alkoxycarbonyl derivatives of glycine and alanine agrees with the concepts of gas chromatography and the known retention index regularities of organic compounds. A decrease of retention index values is a result of an introduction of an additional methyl group to a carbon atom connected to two polar fragments in a molecule like CH2XY. The dependence of the difference of retention index values for homologs of the types of CH3-CHXY and CH2XY vs. the total mass of fragments (X + Y) is similar to those for other sub-groups of analytes.

Background: Dating of ballpoint pen manuscripts in forensic documents is still a challenge for experts. Methods of ink dating consider solvent evaporation through time, especially 2- phenoxyethanol (2-PE). Objective: The objective of the researchers was to perform a continued study with pen ink strokes of ballpoint pens commercialized in Brazil, to characterize the ageing profile of 15 brands of blue and black pens. Methods: Solvent composition analysis of ballpoint pen ink lines written in white and recycled paper was performed by gas chromatography/mass spectrometry, to quantify 2-PE, in recently written lines and in lines aged 4, 18, 27 and 36 months. Results: The results showed that the pens had initial quantities of 2-PE smaller than in the Europe despite of having similar ageing behavior. Such kind of paper had more influence on aged pen strokes than in the recently applied pen strokes. Light and heat accelerated ageing. Some pens stopped ageing at 18, 27 and 36 months. Conclusion: The results obtained were promising for elucidation of the ageing behavior of pen ink lines in tropical weather, but further statistical treatments could be done to adequate interpretation in real cases.

Background: Increased consumer demand for curry leaves free from pesticides demands fast and reliable analytical methods for the analysis of pesticide residues. Objective: The optimization of a QuEChERS based sample preparation technique with improved analytical accuracy by removing interfering matrix components for LC-MS/MS analysis of pesticide residues from curry leaves. Methods: A modified QuEChERS solid phase extraction method was developed and validated for the analysis of 26 pesticides in fresh and dried curry leaves. The effects of the sample preparation steps and column retention time on the matrix suppression of analyte ions were also evaluated. Results: Validation parameters were found within an acceptable range. The matrix effect evaluation studies showed that the QuEChERS sample preparation was able to minimize the ion suppression of analytes due to co-eluting matrix of components and that a d-SPE clean up step had major role in reducing matrix effect. The gradient mobile phase with longer retention time for analytes resulted in comparatively lesser matrix effects than the isocratic mobile phase of non-polar nature. Even after the clean up, a considerable number of compounds had more than 20% reduction in their MS response in the gradient mobile phase. Conclusion: This study emphasized the need of proper sample clean up before a LC-MS/MS analysis and the usage of matrix matched standards and mobile phase that ultimately results in an appropriate analyte separation in reasonable retention times.

Background: A major class of nitro-explosives being used in military and commercial purposes belongs to organic compounds containing nitro (NO2) groups like nitrobenzene, nitrotoluenes, and nitramines. Apart from being energetic materials, these substances are inherently toxic to human beings. These substances may find their way into different environmental matrices from the site of their military or commercial applications. They are present in nature at ultra-trace levels. Methods: Chromatographic techniques have been widely used for the detection and quantification of nitro-explosives from various environmental matrices. The current study involves a comparison of the performance of liquid and gas chromatography with and without pre-concentration techniques like Solid Phase Micro-extraction (SPME) to detect ultra-trace levels nitrobenzene and nitrotoluenes in water. Results: The addition of SPME to chromatographic techniques significantly improves the quality of the analysis in terms of ease and sensitivity. Both SPME-HPLC and SPME-GC are equally competent techniques for the analysis of nitroaromatic explosives from water. Detection limits were improved by upto 5 orders of magnitude by these methods. Conclusion: The methodology for determining nitroaromatic explosives in water samples has been optimized and validated after trying four different methods and comparing their performance. GC and HPLC techniques used alone cannot be sensitive enough to detect these compounds at ultra-trace levels. Both SPME-HPLC-UV and SPME-GC-FID are equally competent techniques. SPME method has been proved to be an excellent tool with no requirement of any tedious sample preparation and chemical processing of the samples.

The retention mechanism in reversed-phase liquid chromatography was quantitatively described using log P (octanol-water partition coefficient). The hydrophobic (lipophilic) interaction liquid chromatography was then used to measure the hydrophobicity of a variety of compounds. Furthermore, the technique has been used as an analytical method to determine molecular properties during the drug discovery process. However, log P values cannot be applied to other chromatographic techniques. Therefore, the direct calculation of molecular interactions was proposed to describe the general retention mechanisms in chromatography. The retention mechanisms in reversed-phase liquid chromatography were quantitatively described in silico by using simple model compounds and phases. The competitive interactions between a bonded-phase and a solvent phase clearly demonstrated the retention mechanisms in reversed-phase liquid chromatography. Chromatographic behavior of acidic drugs on a pentyl-, an octyl-, and a hexenyl-phase was quantitatively described in the in silico analysis. Their retention was based on their hydrophobicity, and hydrogen bonding and electrostatic interaction were selectivity of the hexenyl-phase. This review focuses on the quantitative explanation of the retention mechanisms in reversed-phase liquid chromatography and the practical applications in drug discovery.