Using Transmission “Dip” probes in gases in the deep UV – TI300 Industrial Probe
Fiber Optic transmission dip probes or transflection probes utilize one optical fiber and a lens to project light across a gap to a mirror. The reflected light is focused and collected by a second fiber. This setup is somewhat analogous to making a transmission measurement in a cuvette with some important differences.
The light from the illumination fiber partially reflects off of the surface of the lens, back into the read fiber and effectively by-passing the sample. When the probe is used in liquids the close matching of refractive index of the lens with the solvent minimizes this back reflection. If the probe is used in gases however, back reflection can be very substantial.
The TI300 transmission probe features removable tips to accommodate a variety of different optical path lengths up to 50mm. The ability to remove the tip also provides a means to eliminate errors from back reflection. To make a measurement, remove the tip and with the light source turned on, store the resulting back reflected spectra as the DARK. This spectra will be subtracted in calculations of %T or A in the software, and that back reflected light will be eliminated from the result. It is important that the back reflection spectra only includes light from the lens, not from room light, or back reflection from particles or container walls.
The TI300 also employs an AR coated lens, to reduce the back reflection. However, the AR coating consists of multi-layer dielectric materials which also can absorb light in the deep UV. Spectrecology can supply these probes with un-coated lens to provide better signals in the deep UV. The data below show the results obtained for a probe with the standard AR coated lens and with the same probe with a lens without a coating. The tip is the 50mm tip and the measurements are recorded in air. The light source is a DH2000-DUV and the spectrometer is a USB4000-UV-VIS.
The first figure shows the raw signal from the two probes. The lower trace for each probe is with the tip removed (back reflection only). The upper trace is with the tip on (back reflection + reflection from the tip mirror). In air, the back reflection is very substantial, about equal to the reflection from the mirror. If left uncorrected, this would be the equivalent of a stray light value of 50%!
The effect of the AR coating is also evident, causing an increase in signal around 240nm, and a decrease at the higher wavelengths. There is also a sharp cutoff in signal at wavelengths <230nm.
In the second graph we show the total spectra – the back reflection. The un-coated lens has higher signal overall, and much better signal below 230nm. We can see signal as low as 190nm.
This net signal is what is available for detecting absorbance by the sample.
While not as dramatic, the same phenomena occurs in liquids as well. Making dark measurements with light source on and tip removed will give better answers that a simple light off measurement.