Register to reply

How to use Beer-Lampert law without knowing pathlength?

Share this thread:
rawr18
#1
Mar13-12, 12:08 AM
P: 3
So I did an experiment today using ATR-IR spectroscopy, thus I didn't know what the exact pathlength was (I should have just used traditional FT-IR!). Anyhow, I know that Beer's Law states that:

A(Absorbance)= a(wavelength) * b (pathlength) * c (concentration)

I know absorbance and wavelength, and am trying to find the concentration of my sample. However, I have no idea what the pathlength was since I used ATR. I looked up a formula online for penetration depth (which may be interchangeable with pathlength, not really sure), and it read:

dp= λ / 2(π)(n1) sqrt(sin^2θ - (n2/n1)^2)

I'm just confused as how to find the incident angle, ATR crystal refractive index and sample refractive index?

Any help would be much appreciated! thanks.
Phys.Org News Partner Chemistry news on Phys.org
Chemists create nanofibers using unprecedented new method
Nanostructured metal-oxide catalyst efficiently converts CO2 to methanol (w/ Video)
Researchers discover low-grade nonwoven cotton picks up 50 times own weight of oil
tex43
#2
Mar13-12, 04:55 PM
P: 14
Without the path length the Beer-Lambert law is redundant. One way round your problem would be to construct a calibration curve for your substance vs absorbance and then simply read off your concentration at the appropriate absorption
rawr18
#3
Mar13-12, 09:43 PM
P: 3
Thanks Text, is there a way to create a calibration curve AFTER the analyses have been done? I already have my data in the form of FT-IR graphs, how can I create a calibration curve?

Borek
#4
Mar14-12, 03:08 AM
Admin
Borek's Avatar
P: 23,406
How to use Beer-Lampert law without knowing pathlength?

Quote Quote by rawr18 View Post
is there a way to create a calibration curve AFTER the analyses have been done
If you have not recorded signals for different concentrations - no. If you did - it is not different from calibration curve for any other type of measurement.
tex43
#5
Mar14-12, 06:39 AM
P: 14
Thanks Text, is there a way to create a calibration curve AFTER the analyses have been done? I already have my data in the form of FT-IR graphs, how can I create a calibration curve?

Make solutions with a range of concentrations which include your estimated value and measure their absorbances. Then plot concentration vs absorbance.
DrDu
#6
Mar14-12, 08:55 AM
Sci Advisor
P: 3,571
Maybe you can use some absorption of your solvent for calibration?
DrDu
#7
Mar14-12, 08:58 AM
Sci Advisor
P: 3,571
Quote Quote by rawr18 View Post
I'm just confused as how to find the incident angle, ATR crystal refractive index and sample refractive index?

For the first two, RTFM of the instrument. For the latter one, if your substance is diluted, you can probably use the index of the pure solvent (in the IR, of course).
tex43
#8
Mar14-12, 10:30 AM
P: 14
Here's a dopy question - how do you get to display the originally posted questions when answering?
tex43
#9
Mar14-12, 10:34 AM
P: 14
Hang on! I think I may have it! You just click "quote" right?
Borek
#10
Mar14-12, 10:59 AM
Admin
Borek's Avatar
P: 23,406
Quote Quote by tex43 View Post
Hang on! I think I may have it! You just click "quote" right?
Yes.


Register to reply

Related Discussions
Tiddly Beer Beer Beer... General Discussion 48
How many craft beer fans we got on this? General Discussion 28
Beer's Law Biology, Chemistry & Other Homework 1
Pathlength Difference from two waves Introductory Physics Homework 2
Something in the Beer? General Discussion 22