Water quality nitrate detector

I’d like to check for nitrate levels using online optic system.

Nitrates (NO3-) in liquid absorb mostly around 220nm , most water quality devices looking for nitrate check around there.

I found a ~20$ diode that detects down to 215nm.

I can compare the diode signal thru ‘bad’ water with signal thru clean water and subtract but a lot of signal is shared thru both samples (last fig shows both detector response (blue) and nitrate abs (orange) curves)

So I thought to try to limit the source freq. to eg. 240 or below.

There are some specialty LEDs that transmit at around 240 but the’yre around 300$.

So I thought maybe I can frequency-triple a 632nm, HeNe laser , but the crystals involved (BBO,LBO) seem to also be a few hundred $.

I could also try a diffraction grating to select around 220nm from a Xe flashlamp but I’m not sure which kind of grating , they do seem cheap.
Can someone recommend which grating I want (lines/mm and kind of blaze seem to affect efficiency vs wavelength)

You can buy optical bandpass filters for a lot less than a specialised lamp if you shop around. I’m not entirely sure of the sharpness of the dropoffs, but there are lots of datasheets online.

in keeping with the $300 theme, all the bandpass filters I found are also around $300 - that’s why I’m leaning towards diffraction grating . I just need to choose which , there are variations in lines/mm and blaze type. The xenon lamp is COTS for camera flashes, a few dollars for lamp and circuit

Edmund Optics inc are high quality and have a bandpass filter for 175 USD
See link: https://www.edmundoptics.com/p/220nm-cwl-10nm-fwhm-125mm-mounted-diameter/22402/
I think it is good for you bandpass frequency and could be a good choice for build dimensions.

Amitai