Hi everyone.
In the context of this project (https://www.researchgate.net/project/Large-scale-variation-of-doc-cdom-from-lakes-to-oceans), we are calculating CDOM metrics on a large number of absorption spectra.
I have noticed that some spectra have very high slope ratio (SR).
For example, in these two spectra, SR are equal to 15.67 and 5.70. However, looking in my bibliography, I do not think I have seen values higher than 1.x.
Anyone has ever reported such high values?
Cheers,
Philippe
Hi Philippe,
High SR is not common as you said.
And I think the SR here is not trustworthy because the low signal-to-noise ratio in the longer wavelength region (350-400 nm).
This kind of UV-Vis spectra are common for ocean samples when using short pathlength cells. And I suggest you do some statistical analysis of the spectral slope before you calculate the ratio. I doubt that the S350-400 is not convincing here due to it's not significant different from zero (p-value of slope >0.05). I have validated this point using my oceanic samples data.
Best Regards
Penghui
Hi Penghui.
I agree with you. Maybe a simple criteria could be the p-value as you suggest. I will do some tests and probably implement this in the cdom package.
Cheers,
Philippe
Interesting, preliminary results shows that the S350-400 slope is rarely significant. However, is I use a log approach it is. This probably comes form the fact that most of the time 350-400 constitutes the tail of the spectra and the exponential approach is not suited.
More tests are required, but I think the use of linear regressions for SR might be a better option.
Maybe an idea for a little technical paper? :)
Yes, a little technical paper is possible. Not only spectral slope, but absorbance and absorbance ratio could meet this kind of problem. Absorbance and absorbance ratio, especially in long wavelength range, (A400/A600, A465/A665 etc.) should be reported carefully, considering the signal/noise ratio and the threshold value.
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