Sodium dodecyl sulfate (SDS) is widely utilized in biomolecules separation, but high residue SDS in biomolecules samples interfere mass analysis. Ethidium bromide (EtdBr) interacts with SDS, and the formation of EtdBr-SDS complex at low SDS concentration (0-0.1 %) results a large red shift of the n→π∗ transition of EtdBr from 480 nm to 530 nm. The ion pairs become non-emissive and cause low emission intensity. While the concentration of SDS is above 0.1 %, SDS starts aggregating to form micelle. Micelle formation destabilizes the complex and the absorption maxima shift back to 513 nm while emission intensity increases. Based on the change of absorption and emission of EtdBr, a SDS concentration assay was developed. If absorption maximizes at 480 nm, the concentration of SDS of the sample is below 0.005 %. If absorption maximum is at longer wavelength than 480 nm, a second parameter, the ratio of absorbance at 513 nm and 550 nm is introduced. If the ratio is smaller than 1.5, the concentration of SDS is between 0.01-0.1 %. If the ratio is larger than 1.5, the concentration of SDS is above 0.15 %. Despite the suitable range is small, the lower limit is around the range of no mass interference. Color of EtdBr in sodium dodecyl sulfate solution is changed according to SDS concentration. In brown color region, the SDS concentration is very low and won't interfere mass measurements. In the purple region, the samples may be diluted 10 times for mass measurements. In the red color region, the samples must go through further purification prior for mass measurements.
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