Jesús Vela

SIMULTANEOUS DETERMINATION OF ARSENIC(III) AND ANTIMONY(III) BY OZONE-INDUCED GAS-PHASE CHEMILUMINESCENCE

A combined hydride-generation/gas-phase chemiluminescence (HG-GPCL) method for the determination of arsenic(III) and antimony(III) by using a conventional luminometer is proposed. The method was optimized in terms of the type of reaction chamber, the instrumental settings, and the hydride generation and reaction conditions used. It provides a linear response to As(III) and Sb(III) above a concentration of 0.05 and 0.50 mg L−1, respectively, with a relative standard deviation of ∼3% for both species. Arsenic and antimony can thus be determined simultaneously by (1) their differential effect on the signal obtained with an optical filter placed between the reaction chamber and detector, or (2) the difference between the peak height/peak area ratio of the transient signal for their hydrides. Both procedures were applied to the determination of arsenic and antimony in various synthetic aqueous samples with good accuracy and precision. The effect of potential interferences including anionic and hydride-forming inorganic species was investigated.

Hydrocarbon group-type analysis by thin layer chromatography and scanning densitometry

Hydrocarbon group-type analysis (HGTA) is a common technique for characterization of complex mixtures derived from raw materials such as coal, petroleum, or biomass. In these and other, related, samples, trying to achieve extensive separation of all the components would be very difficult at least, and most of the relevant properties of the samples can be related to the amounts of the different types of hydrocarbon. Groups of interest depend mainly on the nature of the sample, and some kind of liquid chromatography is usually involved in the most common HGTA methods. Thin-layer chromatography (TLC) can nowadays usually be used instead of HPLC, resulting in several advantages in terms of speed, cost, and general convenience. Detection and quantification of the different peaks might involve the use of special equipment, e.g. in TLC—flame ionization detection (FID) methods, although it can also be accomplished by means of UV and fluorescence scanning densitometry. In this paper, we describe a series of TLC-based HGTA methods developed for coal-, biomass-, and petroleum-derived products that give a reasonably general overview of the possibilities of TLC applied to HGTA.

Revisiting molecular weight distribution of polystyrenes using adsorption high-performance thin-layer chromatography

A repeatable separation of polystyrenes according to MW, from 1920 to 520000 u.m.a, has been obtained under adsorption conditions using a method that comprises the use of Lichrospher HPTLC plates, and a controlled, isocratic elution with a 78:22 (v/v) mixture of cyclohexane (Cy)-tetrahydrofuran (THF). Likewise, UV-densitometric quantification of polystyrenes in mixtures can be achieved, by an intra-plate or an inter-plate procedure, using the corresponding polystyrene calibration curve. In the case or overlapped, unresolved peaks, an average curve of the corresponding polystyrenes can be used. Migration of polystyrenes strongly varies with slight variations in the relative proportion of Cy and THF in the mobile phase. This allows different ranges of MW to be separated as a function of mobile phase composition. Other factors influencing repeatability have been identified. Some reasons have been advanced to explain the current lack of activity in the research on polymer characterization by HPTLC. Old literature results concerning polystyrene separation have also been discussed in the light of modern HPTLC instrumentation. Developed method provides similar information on Molecular Weight Distribution (MWD) to that obtained using Gel Permeation Chromatography (GPC). Advantages and limitations of HPTLC for obtaining polymer MWD have also been discussed.