Any help is greatly appreciated

5mL of a solution A (unknown concentration) was transferred into sic 25mL volumetric flask. The following volumes of a standard solution of A with with a concentration 75ppm were added to the flask: 0mL, 0.5mL, 1mL, 1.5mL, 2mL, and 2.5L. The excitation spectrum and and emission spectra is provided. Determine the unknown concentration of A in that solution?

So far, I have come up with this solution
- I first calculated the concentration of standard solution. Please help me to check if my calculation is right

Stard 1: 0
Stard 2: 0.5*75ppm/25=1.5
Stard 3: 1*75/25 = 3
and so on

- Then I use Excel spreadsheet to make a best fit line between concentration and fluorescence intensity. The intensity would be a number which has a highest emission value
My equation is y=7.617x + 4.7449 and R^2=0.9797

-From the excitation spectrum, the wavelength at which maximum intensity is obtained is 375nm
-From the emission spectrum, the wavelength at which there is maximum fluorescence is 472nm

Now I am stuck. I do not know how to find the fluorescence intensity of the unknown solution to plug it in the equation.
One more question, my R^2 value is not really close to 1, should I omit some value to make a perfect line?
Thank you in advance for any help

6 answers

There is a b) question that I am stuck too. If a synchronous experiment is to be performed what would be the offset that you would use
What you've done so far appears to be ok. You must be plotting with data not posted.
As for R, that looks pretty close to 1 to me.
Thank you DrBob222. Can you explain further what you mean by saying "Plotting with data not posted"? I think when I can find the intensity of the unknown, I can plug it in the equation. But my problem is that I don't know how to do?
From what you've described you must have plotted a "standard addition" plot. Your straight line equation does not include the b of y = mx + b. You will tell be it's zero but it isn't. Those concentrations you plotted are not 1.5 ppm, 3 ppm, etc but unk+1.5 ppm; unk + 3 ppm, etc. What I'm saying is that the straight line you drew does not (at least should not) go through zero. My post about not including other data is that I didn't see any absorbance readings for those concentrations. What was the A reading for the zero (the sample alone). When the straight line is extrapolated back to the x axis that will be the concn of the unknown (and that will be b in the y = mx + b. We can't draw diagrams on this forum but here is a site that does a much better job of explaining all of this than I can do here.
http://en.wikipedia.org/wiki/Standard_addition#/media/File:Standard_addition.gif
Hi DrBob222. Thank you for your answer
However, I feel like this method requires absorbance to plot a graph. Unfortunately, My question does not provide any absorbance data.
You're right. It requires something and absorbance is ok to use. You may not have absorbance but you plotted something to get the equation you posted. What did you use; ? vs concentration. What's the ?
If you want to pursue this further I recommend you start a new thread and repost the question at the top of the page but provide answers to the above.