High Dynamic Range Absorbance and Simultaneous Deep UV Fluorescence Measurement in a Liquid Core Waveguide Flow Cell Coupled to Liquid Chromatography

In liquid core waveguide (LCW) flow cells, light introduced into the lumen undergoes total internal reflection due to the lower refractive index at the tube wall leading to high light transmission. This is conducive to absorbance detection as it reduces relative transmitted light noise and increases sensitivity. Thin-walled fused silica (i.d. 0.53) has <3% per cm even at 200 nm with cells 6 cm in length commercially available for liquid chromatography. However, the dynamic range is unchanged and making both trace and major constituent determination in a single sample difficult. As the LCW consists of a UV-transparent capillary, simultaneous radial absorbance increases the dynamic range by >2 orders of magnitude using the shorter pathlength. This was demonstrated using fiber-optic coupled 280 nm LEDs and paired photodiode detectors placed 1 cm apart along the length of the capillary for a series of aliphatic ketones. We also discovered that broadband fluorescence was also simultaneously measured at the photodiodes; the low loss of the LCW acts as an excitation filter minimizing stray light reaching the detectors. The D2 lamp is brightest at ~200 nm and was efficient for deep UV excitation; conventional Xenon lamps used for fluorescence notably have poor output in this range. We discovered that many unconventional fluorophores with no appreciable absorbance >225 nm excite in this range and emit at ~300 nm corresponding to a Stokes shift of ~18,000 cm-1 which is one of the largest if not the largest solution phase shift measured to date. By modulating the radial LEDs, we obtain both the transmittance and background/fluorescence signals which are then deconvolved. Absorbance noise was as little as 10 µAU. We discovered that dispersion measured at the axial cell underestimate the true peak shape leaving the cell measured at the radial detectors and that the flow profile through the cell is asymmetric due to minor differences in geometry at the inlet/outlet.