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

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.

To find the unknown concentration of solution A, you can follow these steps:

1. Calculate the concentration of the standard solution correctly. Based on your calculations, it seems to be correct:

Standard 1: 0 ppm
Standard 2: (0.5 mL * 75 ppm) / 25 mL = 1.5 ppm
Standard 3: (1 mL * 75 ppm) / 25 mL = 3 ppm

2. You have already obtained the equation of the best-fit line between concentration and fluorescence intensity. The equation you provided is y = 7.617x + 4.7449, where y is the fluorescence intensity and x is the concentration. The R^2 value of 0.9797 indicates a good fit, although it is not exactly equal to 1.

3. To determine the fluorescence intensity of the unknown solution A, you need to measure the fluorescence intensity at the wavelength of maximum fluorescence (472 nm). Use a spectrophotometer or fluorometer to measure the fluorescence intensity of your unknown solution at this specific wavelength. The value obtained will be your y in the equation.

4. Substitute the fluorescence intensity (y) of the unknown solution into the equation y = 7.617x + 4.7449.

Let's assume the fluorescence intensity of the unknown solution is y_unk.

y_unk = 7.617x_unk + 4.7449

Solve this equation for x_unk (the unknown concentration).

x_unk = (y_unk - 4.7449) / 7.617

The resulting x_unk will be the unknown concentration of solution A.

Regarding the R^2 value, usually, the closer it is to 1, the better the fit of the line. However, an R^2 value of 0.9797 is still considered to be a good fit. You don't necessarily need to omit any values unless you have a strong reason to believe that some of the data points are outliers or incorrect.