Enhancement of Detection Sensitivity

Luminescence detection can be improved in MLC because many solutes show enhanced fluorescence and in some cases room temperature liquid phosphorescence when associated with micelles. The fluorescence intensity of certain compounds in micellar media can be drastically increased as a result of solubilization in the micelle. The location of a solute in the anisotropic medium of micelles, which have a large microenvironment viscosity and different polarity from the aqueous bulk solvent, would result in a decrease in the freedom of movement, shielding of the compounds from nonradiation deactivation, and/or an increase in quantum efficiency. This leads to intensified fluorescence signals and thus to better sensitivity and lower detection limits. Room temperature phosphorescence in solution is possible in the presence of ionic micelles and heavy atom counter-ions, which increase the population of the triplet excited state molecules and protect them from radi-ationless deactivation.

Furthermore, many metal-dye complexes show increased absorbance in the presence of micelles. This is

Figure 6 Separation of a seven-component test mixture. Mobile phase: 0.30 mol L"1 SDS, 0.02 mol L"1 phosphate buffer, pH 2.5 with propanol added. (A) Isocratic separation with 3% 2-propanol, (B) gradient separation with 3 to 15% 2-propanol, and (C) isocratic separation with 15% 2-propanol. Key: 1, aspartic acid-phenylalanine; 2, phenylalanine; 3, lysine-phenylalanine; 4, phenylalanine-phenylalanine; 5, triphenylalanine; 6, tetra-phenylalanine; and 7, pentaphenylalanine. Column: Nucleosil C18 (15 cm x 4.6 mm i.d.). (Reproduced with permission from Madamba-Tan LS, Strasters JK and Khaledi MG (1994) Gradient elution in micellar liquid chromatography II. Organic modifier gradients. JournalofChromatographyA 683: 335-345, copyright Elsevier Science Publishers B.V.)

Time (min)

Figure 6 Separation of a seven-component test mixture. Mobile phase: 0.30 mol L"1 SDS, 0.02 mol L"1 phosphate buffer, pH 2.5 with propanol added. (A) Isocratic separation with 3% 2-propanol, (B) gradient separation with 3 to 15% 2-propanol, and (C) isocratic separation with 15% 2-propanol. Key: 1, aspartic acid-phenylalanine; 2, phenylalanine; 3, lysine-phenylalanine; 4, phenylalanine-phenylalanine; 5, triphenylalanine; 6, tetra-phenylalanine; and 7, pentaphenylalanine. Column: Nucleosil C18 (15 cm x 4.6 mm i.d.). (Reproduced with permission from Madamba-Tan LS, Strasters JK and Khaledi MG (1994) Gradient elution in micellar liquid chromatography II. Organic modifier gradients. JournalofChromatographyA 683: 335-345, copyright Elsevier Science Publishers B.V.)

due to the capacity of the micelles to produce hyper-chromic and bathochromic displacements. Generally, these displacements result in greater sensitivity. In UV/Vis spectrophotometry, the upward displacement of the Amax of the complex, together with the effect that micellar solutions also have on the Amax of the ligand, normally enable a more sensitive metal ion determination than that possible in nonmicellar media.

Figure 7 shows a comparison of the detected fluorescent peaks of identical concentrations of three aromatic solutes separated by HPLC. The enhanced fluorescence obtained with an SDS mobile phase with respect to that obtained with a methanol/water mobile phase is observed.

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Solar Panel Basics

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