Question: How do I...

That's interesting. I wonder why there is such a difference in the gradient for the nitrogen water. Also interesting that the contaminated milk spectra is much fainter. Reminder me what the black red blue lines are again?

@kgradow1 awards a barnstar to akhila for their awesome contribution!

Hi, @akhila - this is a really cool experiment you're doing! OK, i think i may have some suggestions that could help;

1. i think you may have a doubled-up spectrum as we shouldn't be seeing blue mixed between green and red lines; it should be very strictly blue, green, red, from left to right. So maybe there is either a light leak, or something is reflecting inside, or something? Can you share a pic of the inside of your spectrometer to help debug?
2. it looks like you're using a CFL fluorescent light in your comparisions. This is OK and can be helpful if you are generating fluorescence from your sample, because the fluorescence will appear in areas of your spectrum that are black (zero) in your control (i.e. only water). But often people use a full-spectrum light source for this, OR they use just an ultraviolet light if they expect fluorescence, because then there's only a single UV spike of input light, and all other light would be your sample fluorescing
3. as to the big difference you're seeing, the spectrum doubling/out-of-orderness makes this hard to read, but one thing to think about is that if you see a really big difference between two spectra, you should be able to see it with the naked eye, without a spectrometer. that is, the sample should visibly and noticeably glow a different color if the difference is really that big. Does it, in your case?

I'd say maybe the ideal next steps would be to get your setup to the point where you are seeing a really consistent spectrum over 5 or more scans of plain water, and to then make a set of scans of your sample, once you figure out the doubling issue. It's always better to average a few scans to ensure you're getting repeated, consistent results, and not just some random reflection or fluke. Then we can really start to measure the difference.

Great work so far! Keep it up and tell us how it goes, we can help troubleshoot more as well!

Ok guys,I'm used to commercial instruments. So these scans always throw me. First, if you are trying to compare concentration in the commercial world, this is almost always done in the absorbance mode, not the linear mode (these scans show linear scales). This is because of Beers Lambert s law (sounds better than it is) that says concentration is usually proportional to absorbance. And almost always background scans are run to take out the effects of routine environmental differences. Would this take too much extra software? Calibration used to be done with holmium oxide glass. That is probably hard to get a hold of in the correct thickness. At least for a decent price.

Sorry for the dumb questions.

Yes you're quite correct in an ideal world we would be using absorbance (Log of transmission) and subtracting a blank. There is also a problem with focusing in this experimental setup the sharpness of the spectrum will depend on the distance from the light source (is it optimised for infinity - I don't know). And, as I have mentioned previously, it's no good trying to quantify stuff that is colourless. Nitrogen fertiliser is either ammonium nitrate or urea both of which are colourless.

The usual way to solve the ammonium nitrate problem is to form a complex (that does absorb in the visible) and analyze the complex. Off the top of my head, this seems like a reasonable approach. But then again, I've done no investigation.

We did some work with developing a quick replacement for a lab analyzer. Please do not misunderstand. I did not program. But worked with the programmers. the program was dropped. Oh well.

I m always learn ur post