Two-photon absorption properties of chromophores in micelles: electrostatic interactions.

Two-photon absorption (2PA) cross sections of neutral Coumarin 485 (C485) and anionic Coumarin 519 (C519(-)) solubilized in Triton X-100 (Tx-100, neutral), sodium dodecyl sulfate (SDS, anionic), and cetyltrimethylammonium bromide (CTAB, cationic) are reported. The objective of the study is to probe the influence of local electrostatic fields in micelles on the 2PA properties of chromophores. The 2PA measurements have shown that the cross sections of neutral C485 are unchanged in different micellar environments, although the local micropolarities of chromophores are different. On the other hand, the 2PA cross sections of C519(-) are unchanged or slightly decreased in Tx-100 and SDS micelles when compared to water while 100% increase in 2PA cross sections was observed for C519(-) in CTAB micelles. The enhancement in 2PA cross section is attributed to the electrostatic fields arising in the Stern layer of CTAB, where C519(-) is solubilized. The titration measurements have shown that the 2PA enhancement is due to the organized medium only and not because of the simple association of C519(-) and the quaternary ammonium group of CTAB. From the analysis, local electric field of 0.7 ± 0.3 MV/cm is estimated for the Stern layer of CTAB.

Surface sampling by spray-desorption followed by collection for chemical analysis.

Desorption electrospray ionization (DESI) directly analyzes soluble chemical components present on surfaces when a pneumatically assisted electrospray is directed at the sample. Here we demonstrate that the same spray desorption mechanism that operates in DESI can be used as a general technique to collect soluble materials present on surfaces. After desorption analytes are collected on a suitable collection surface, large areas can be scanned and collected onto a small collected area, which allows for preconcentration of low abundance material before analysis. This collection surface can then subsequently be analyzed by DESI but also by many other techniques such as gas chromatography-mass spectrometry or UV-vis spectroscopy. In addition this technique can be used to study desorption mechanisms in DESI independently from ionization mechanisms. Preliminary results indicate that the optimized conditions in DESI are a compromise between those conditions that are optimum for desorption and conditions that lead to efficient ionization.

Diamond paste-based electrodes for determination of Cr(III) in pharmaceutical compounds.

A new class of monocrystalline diamond paste-based electrodes is proposed for the determination of chromium(III) at trace levels in vitamins. Three types of monocrystalline diamond-natural diamond 1mu (natural diamond), synthetic diamond 50mu (synthetic-1), and synthetic diamond 1mu (synthetic-2)-were used for electrode construction. The linear concentration ranges are between 10(-10) and 10(-8); 10(-9) and 10(-7), and 10(-10) to 10(-8) mol L(-1), with limits of detection of 10(-12), 10(-12), and 10(-11) mol L(-1), when natural diamond, synthetic-1, and synthetic-2, respectively, are used as electrode materials. For electrodes based on natural diamond and synthetic-1 it was found that Cr(III) yields a peak at about +0.275+/-0.015 V (vs. Ag/AgCl) within a predetermined positive potential range situated between +0.4 and +0.2 V, while for the electrode based on synthetic-2 the peaks are found at +0.300+/-0.015 V (vs. Ag/AgCl). The proposed method is reliable for the determination of chromium(III) at trace levels in two vitamin tablets (RSD<0.2%).

Monocrystalline diamond paste-based electrodes and their applications for the determination of Fe(II) in vitamins.

A new class of electrochemical sensors, namely, electrodes based on diamond paste, was designed using monocrystalline diamond (natural diamond 1 microm and synthetic diamond, 50 microm (synthetic-1) and 1 microm (synthetic-2)) powder and paraffin oil. The characterization of the electrodes was performed using cyclic voltammetry and differential pulse voltammetry. Fe(II) was determined by differential pulse voltammetry (DPV) at 75 mV (vs Ag/AgCl) using all diamond paste-based electrodes. The linear concentration range was between 10(-8) and 10(-4) mol/L for both the natural diamond and synthetic-2 with detection limits of 10(-10) and 10(-9) mol/L, respectively, whereas the linear concentration range for synthetic-1 was between 10(-7) and 10(-3) mol/L with a detection limit of 10(-8) mol/L Fe(II) was determined successfully from four types of pharmaceutical products. The recovery values of Fe(II) in the pharmaceutical products were higher than 98.00% with relative standard deviation values < 5%.