Any help is greatly appreciated

Question

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

Answer 1

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

Answer 2

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.

Answer 3

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?

Answer 4

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

Answer 5

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.

Answer 6

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.