[nexisrocks]

Hello,

I'm trying to do a lab, and I'm stuck on a few questions.

To change the wavelength of maximum sensitivity of a compound, would you just change the type of lamp (ie from xenon to mercury)?  I have the equation: Measured Response = (Lamp Intensity)*(Cathode Response) -- where measured response is in transmittance.  My train of thought was that changing the lamp would give different intensities of light at different wavelengths (which would then change the measured response at those wavelengths), but I don't know if I'm flawed in thinking that or not.

Assuming that there is no stray light in the monochromator, will the instrument make more accurate measurements at 500 nm or 350 nm?

Does the accuracy of the monochromator depend on it's resolving power?  If so, then it depends on the order of light of interest and the number of grating lines that are illuminated.  So how is the order of the light related to wavelength?  If it's not that, then what is it?  Something to do with the exit slit width and spectral bandwidth?

Thanks!

[Arkcon]

These are tough questions, if asked exactly as you've asked them.  Switching lamps will give you different intensities at different wavelengths, but what wavelength are you measuring at?  Do they want you to talk about other ways of getting more signal, or not?  As for it's accuracy at two wavelengths, I can't help but feel there should be more info to the question, like you ask.  Is there really no more info?

[nexisrocks]

I was measuring the transmittance of water, and found the wavelength of maximum intensity to be 632nm; the exact question here was, "If necessary, how would you change the wavelength of maximum sensitivity?"

As for the second question, the exact question was, "Assuming there is no stray light in the monochromator, will the instrument make more accurate measurements at 500 nm or 350 nm? Explain."

Unfortunately, that's really all we were given.

[Arkcon]

OK, a little better ...

I was measuring the transmittance of water, and found the wavelength of maximum intensity to be 632nm; the exact question here was, "If necessary, how would you change the wavelength of maximum sensitivity?"

Hmmm...still a tough one, written like that.  How to make water transmit better at another wavelength?  Don't know how that's going to work.  Switching light sources for better transmission at you optimal wavelength.  Maybe you can find something that makes one better than the other?

As for the second question, the exact question was, "Assuming there is no stray light in the monochromator, will the instrument make more accurate measurements at 500 nm or 350 nm? Explain."

Can you pick one with any of your experimental data, given your limited adjustments allowed?

[nexisrocks]

Can you pick one with any of your experimental data, given your limited adjustments allowed?

I'm not sure... the data I have for the two wavelengths are as follows:

Wavelength = 350
Diode Response = 66
Measured Response = 2
Lamp Intensity = 0.0303
Relative Lamp Intensity = 0.5682%

And:

Wavelength = 500
Diode Response = 89
Measured Response = 28
Lamp Intensity = 0.3146
Relative Lamp Intensity = 5.8989%


Could it just be 350 nm because the measured response is lower there, and thus more light is being absorbed? (The measured response is transmittance).  With more light, is it easier for the monochromator to get a more accurate measurement?

But then, I thought the monochromator in a spectrophotometer came before the diode (ie you select the wavelength, then amplify the use the diode to generate current and measure the intensity of the light).

[ARGOS++]

Dear Nexisrocks;

It could be that there are several things wrong!

But to determine that I need a few Definitions from you!:
   a.)  What you define with: “Diode Response”, and what are their Units?
   b.)  What are the Units of your “Lamp Intensity”?
   c.)  For “Measured Response” did you use your Formula above, and what is in this moment “Cathode Response”?
   e.)  How you define: “Relative Lamp Intensity”? (- And relative to what?)

I hope if I know all these that I will be able to answer you.
In the mean time you may also compare your measurements with the first picture on the Spectroscopy Page on:   "Corrected Beer's Law”

About the Monochromator we will talk a little bit later, after you have you informed on the page above.

Good Luck!
                    ARGOS⁺⁺