A Complementary Study of Mechanisms and Behaviors in Chromatography via Modeling.

Evolution in preparation of chromatographic columns has created the need for studying and evaluating them with the use of smart software. This research is an attempt to compare the retention mechanism between two stationary phases (butyl and phenyl) with the use of multivariate analysis for a large number of probes. Partial least squares has the ability to spot either major or minor differences in the chromatographic behavior of probes, with regard to changes in the stationary or mobile phases. The models developed refer to a total of 108 miscellaneous chemical compounds, described by 63 X variables (physicochemical properties and structural features) and one Y variable (retention time). The results showed that in both columns and mobile phases (40% methanol or 40% acetonitrile) the retention of an analyte is mainly affected by its lipophilicity, molar volume, and refractivity, which tend to cause delayed elution. On the contrary, solubility in water, polar surface area, and hydrogen bond donor or acceptor properties promote faster elution. The most important difference proved to be the effect of the presence of the carboxylic group and the solubility that affected the retention in a similar way in both columns but not with both mobile phases.

An automatic countercurrent liquid-liquid micro-extraction system coupled with atomic absorption spectrometry for metal determination.

A novel and versatile automatic sequential injection countercurrent liquid-liquid microextraction (SI-CC-LLME) system coupled with atomic absorption spectrometry (FAAS) is presented for metal determination. The extraction procedure was based on the countercurrent flow of aqueous and organic phases which takes place into a newly designed lab made microextraction chamber. A noteworthy feature of the extraction chamber is that it can be utilized for organic solvents heavier or lighter than water. The proposed method was successfully demonstrated for on-line lead determination and applied in environmental water samples using an amount of 120 µL of chloroform as extractant and ammonium diethyldithiophosphate as chelating reagent. The effect of the major experimental parameters including the volume of extractant, as well as the flow rate of aqueous and organic phases were studied and optimized. Under the optimum conditions for 6 mL sample consumption an enhancement factor of 130 was obtained. The detection limit was 1.5 µg L(-1) and the precision of the method, expressed as relative standard deviation (RSD) was 2.7% at 40.0 µg L(-1) Pb(II) concentration level. The proposed method was evaluated by analyzing certified reference materials and spiked environmental water samples.

Properties of ultrasound extracted bicomponent lignocellulose thin films.

Agricultural and forest residues obtained after harvesting are promising renewable sources, suitable as a source of pulp for cellulose nanocrystal manufacturing. Cavitation-assisted softening of the lignin-carbohydrate matrix offers sample opportunity for cellulose fibril liberation and degradation of amorphous cellulose. The present work addresses cavitation assisted cellulose fibril and crystal liberation and film forming properties of the supernatant phase of treated agricultural and forest residues. The effectiveness of this method has been evaluated according to crystallinity indices and hydrogen bond energies, as measured by FT-IR analysis. It has been observed that the use of cavitation increased the crystallinity and caused partial removal and degradation of the lignin matrix. Overall, it appears that considerable improvement of crystallinity can be obtained from agricultural and forest residues through the use of cavitation.

On-line liquid phase micro-extraction based on drop-in-plug sequential injection lab-at-valve platform for metal determination.

A novel automatic on-line liquid phase micro-extraction method based on drop-in-plug sequential injection lab-at-valve (LAV) platform was proposed for metal preconcentration and determination. A flow-through micro-extraction chamber mounted at the selection valve was adopted without the need of sophisticated lab-on-valve components. Coupled to flame atomic absorption spectrometry (FAAS), the potential of this lab-at-valve scheme is demonstrated for trace lead determination in environmental and biological water samples. A hydrophobic complex of lead with ammonium pyrrolidine dithiocarbamate (APDC) was formed on-line and subsequently extracted into an 80 µL plug of chloroform. The extraction procedure was performed by forming micro-droplets of aqueous phase into the plug of the extractant. All critical parameters that affect the efficiency of the system were studied and optimized. The proposed method offered good performance characteristics and high preconcentration ratios. For 10 mL sample consumption an enhancement factor of 125 was obtained. The detection limit was 1.8 µg L(-1) and the precision expressed as relative standard deviation (RSD) at 50.0 µg L(-1) of lead was 2.9%. The proposed method was evaluated by analyzing certified reference materials and applied for lead determination in natural waters and urine samples.

On-line micro-volume introduction system developed for lower density than water extraction solvent and dispersive liquid-liquid microextraction coupled with flame atomic absorption spectrometry.

A simple and fast preconcentration/separation dispersive liquid-liquid micro extraction (DLLME) method for metal determination based on the use of extraction solvent with lower density than water has been developed. For this purpose a novel micro-volume introduction system was developed enabling the on-line injection of the organic solvent into flame atomic absorption spectrometry (FAAS). The effectiveness and efficiency of the proposed system were demonstrated for lead and copper preconcentration in environmental water samples using di-isobutyl ketone (DBIK) as extraction solvent. Under the optimum conditions the enhancement factor for lead and copper was 187 and 310 respectively. For a sample volume of 10 mL, the detection limit (3 s) and the relative standard deviation were 1.2 µg L(-1) and 3.3% for lead and 0.12 µg L(-1) and 2.9% for copper respectively. The developed method was evaluated by analyzing certified reference material and it was applied successfully to the analysis of environmental water samples.

On-line cleavage of disulfide bonds by soluble and immobilized tris-(2-carboxyethyl)phosphine using sequential injection analysis.

Reduction of disulfide bonds is - in many cases - a critical pretreatment step for the determination of thiols in real samples. This study reports the first systematic investigation of the potentials of the on-line reduction of disulfide bonds under flow conditions in a sequential injection setup. One of the most promising reducing agents, tris-(2-carboxyethyl)phosphine (TCEP) was selected for this purpose while the Ellman's disulfide (DTNB) was used as model compound. The study involved the investigation of several parameters that affected the kinetics and efficiency of the reaction, including stopped-flow experiments. Both soluble and immobilized TCEP on agarose beads were examined. The results confirmed that both forms of TCEP can be used as an advantageous on-line reducing reagent for disulfide bonds under flow conditions.