Comparison and accuracy of methods to determine the confocal volume for quantitative fluorescence correlation spectroscopy.

Single molecule detection based on fluorescent labels offers the possibility to gain not only qualitative but also quantitative insight into specific functions of complex biological systems. Fluorescence correlation spectroscopy is one of the favourite techniques to determine concentrations and diffusion constants as well as molecular brightness of molecules in the pico- to nano-molar concentration range, with broad applications in biology and chemistry. Although fluorescence correlation spectroscopy in principle has the potential to measure absolute concentrations and diffusion coefficients, the necessity to know the exact size and shape of the confocal volume very often hampers the possibility to obtain quantitative results and restricts fluorescence correlation spectroscopy to relative measurements mainly. The determination of the confocal volume in situ is difficult because it is sensitive to optical alignment and aberrations, optical saturation and variations of the index of refraction as observed in biological specimen. In the present contribution, we compare different techniques to characterize the confocal volume and to obtain the confocal parameters by fluorescence correlation spectroscopy curve fitting, a fluorescence correlation spectroscopy dilution series and confocal scanning of fluorescent beads. The results are compared in the view of quantitative fluorescence correlation spectroscopy measurement and analysis. We investigate how unavoidable artefacts caused by a non-ideal confocal volume can be experimentally determined and validated.