Hybrid mesoporous materials for on-line preconcentration of Cr(VI) followed by one-step scheme for elution and colorimetric determination at ultratrace levels.

An hybrid mesoporous material synthesised in our laboratories for solid phase extraction (SPE) in flow through systems has been used for analytical purposes. The solid was obtained from mesoporous silica MCM-41 functionalized with 3-aminopropyltriethoxy silane by Sol-Gel methodology. In order to exploit the large sorption capacity of the material together with the possibility of modeling it for anions retention, a microcolumn (MC) filled with the solid was inserted in a flow system for preconcentration of Cr(VI) and its determination at ultratrace levels in natural waters. The analytical methodology involved a reverse flow injection system (rFI) holding a MC filled with the solid for the analyte extraction. Elution and colorimetric detection were carried out with 1-5 diphenylcarbazide (DPC) in sulfuric acid. DPC produced the reduction of Cr(VI) to Cr(III) together with the generation of a cationic red complex between Cr(III) and 1-5 diphenylcarbazone which was easily eluted and detected with a visible spectrophotometer. Moreover, the filling material got ready for the next sample loading remaining unspoiled for more than 300 cycles. The effect of several variables on the analytical signal as well as the influence of cationic and anionic interferences were discussed. Particular attention was given to sulfuric acid interference since it is the required media for the complex generation. Under optimal conditions, 99.8% of Cr(VI) recovery was obtained for a preconcentration time of 120s (sample and DPC flow rates=1 mL min(-1)) and an elution volume of 250 micro L. The limit of detection (3s) was found to be 0.09 microg L(-1) Cr(VI) with a relative standard deviation (n=10, 3 microg L(-1)) of 1.8. Since no Cr(III) was retained by the solid material and Cr(VI) was completely adsorbed, electrothermal atomic absorption spectrometry (ET AAS) determinations of Cr(III) were also performed by simply measuring its concentration at the end of the microcolumn after Cr(VI) retention by the mesoporous solid. Applications to the determination of Cr(VI) and Cr(III) in natural waters and the validation of the methodology were also studied.

Characterization of "Yaa Chud" Medicine on the Thailand-Myanmar border: selecting for drug-resistant malaria and threatening public health.

Multidrug-resistant Plasmodium falciparum malaria is a severe public health problem on the Thailand-Myanmar border. Many villagers buy packets of 4-5 mixed medicines ("yaa chud") from shops without medical assessment as their first-line malaria treatment. In 2000-2001 a local researcher purchased 50 yaa chud from 44 shops around Mae Sot, Thailand and Myawaddy, Myanmar (Burma), for his wife who was said to be pregnant with fever and drowsiness. The tablets/capsules were provisionally identified by appearance and active ingredients determined in a subset by using mass and atomic spectrometry. The most frequently detected active ingredients were acetaminophen (22%), chlorpheniramine (13.4%), chloroquine (12.6%), tetracycline/doxycycline (11.4%), and quinine (5.1%). Only seven bags contained potentially curative medicine for malaria. A total of 82% of the bags contained medicines contraindicated in pregnancy. Inappropriate, ineffective antimalarial drugs on the Thailand-Myanmar border are likely to increase malaria morbidity, mortality and health costs and engender the emergence and spread of antimalarial drug resistance.

Kinetic control of reagent dissolution for the flow injection determination of iron at trace levels.

A novel methodology for the determination of iron at the ppb level by spectrophotometric flow injection analysis is described. The method is based on the control of the flow dissolution of the colorimetric reagent 1,10-phenanthroline. This is achieved by means of the minimization of the area of contact between the carrier and the solid reagent, thus allowing the use of the fairly soluble organic compound without affecting the reactor lifetime. The reagent is melted inside an acrylic column (3.0 x 0.5 cm id) in such a way that a hollow space is left in the center after cooling. This new design improves some aspects of the performance of the classical solid-phase reactors as no problems related to the increase in the backpressure of the system are evidenced. Furthermore, the total reagent loading of the column is increased as no inert support is needed. A comparison between the performance of this novel methodology and that of the conventional packed reactor was performed and several advantages were observed: the use of higher flow rates, an increase in the reactor lifetime and a decrease in reagent consumption. A mathematical model to fit the concentration profiles of the dissolved reagent as a function of the residence time of the sample within the column is presented. The application of this strategy to the determination of Fe(II) improves the figures of merit in comparison to those obtained with a single-line homogeneous system: the limit of detection is 2 microg Fe L(-1) (3s) and the sensitivity is similar to that of the batch procedure. Results obtained for the determination of iron in natural waters are also presented.