March 22, 2023, 09:19:00 AM
Forum Rules: Read This Before Posting


Topic: Standard curve  (Read 12326 times)

0 Members and 1 Guest are viewing this topic.

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Standard curve
« on: August 02, 2014, 07:46:47 PM »
I'm trying to troubleshoot here.. What are some of the reasons my standard curve could have come out bad? My r^2 was terrible at 0.3. My cuvettes may have been contaminated since they've been used before although I cleaned them. Could that be cause?

Offline Arkcon

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7367
  • Mole Snacks: +533/-147
Re: Standard curve
« Reply #1 on: August 02, 2014, 07:52:06 PM »
That's certainly a possible reason.  However, that is really a horrible r2.  I suspect many things -- could you have calculated it wrong, and of course there be another serious fault.  Can you give a a tale table of values, and possibly a chart, because really, how do you call this a line.  Maybe you need a higher order polynomial for your data, if that's permissible.
« Last Edit: August 02, 2014, 10:46:02 PM by billnotgatez »
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Corribus

  • Chemist
  • Sr. Member
  • *
  • Posts: 3402
  • Mole Snacks: +512/-23
  • Gender: Male
  • A lover of spectroscopy and chocolate.
Re: Standard curve
« Reply #2 on: August 02, 2014, 09:49:51 PM »
Telling us what technique/experiment you're doing could also be helpful.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Re: Standard curve
« Reply #3 on: August 03, 2014, 05:27:24 PM »
I redid it and got a 0.85 by changing the cuvettes I was using but that still is not good enough

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Re: Standard curve
« Reply #4 on: August 03, 2014, 05:33:55 PM »
The concentrations are: 1650, 825, 412.5, 206.25 (ug/mL)
The absorbances I got through UV-Vis were: 0.090, 0.078, 0.08, 0.073

I redid it using 20 mL standards instead of 10 mL. My r^2 ended up worse..
Absorbances: 0.194, 0.172, 0.012, 0.005

I always struggle once I put the 412.5 solution in. I am using 15 uL of each of the solutions (concentrations above) with 1 mL of a coumassie blue reagent I made up.

Offline Corribus

  • Chemist
  • Sr. Member
  • *
  • Posts: 3402
  • Mole Snacks: +512/-23
  • Gender: Male
  • A lover of spectroscopy and chocolate.
Re: Standard curve
« Reply #5 on: August 03, 2014, 09:21:06 PM »
The R2 isn't your primary problem. I'm not sure exactly what you're doing, but if you double the concentration, you should double your absorbance - as long as you're in the linear range. You're not even close. In fact, between 412.5 and 825 ug/mL, your absorbance is going down. This is basic Beer's Law.

Are you doing a single read measurement, or are you acquiring the entire spectrum? If the former, are you sure you're reading the right wavelength (i.e., where there's a peak)? I would suggest you acquire an entire spectrum to be sure.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Arkcon

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7367
  • Mole Snacks: +533/-147
Re: Standard curve
« Reply #6 on: August 03, 2014, 09:30:04 PM »
Here's a quick and dirty chart.  Sorry I didn't take the time to label axes, or the series.  You should be able to tell by the numbers, but pink is your "20 ml" sample, and blue is your "10 ml" sample.  I'm sorry you don't like the "10 ml" sample, it gives a pretty straight line -- although not a useful one, as absorbance doesn't change much as a function of concentration.

I'm wondering if you can come up with some conclusions, looking at the data thi way.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Re: Standard curve
« Reply #7 on: August 04, 2014, 10:34:06 AM »
I've done beers law before and know how to make standards well... The only thing I can think of is my pipettes weren't calibrated correctly or my coumassie blue went bad. This is so frustrating! :/

Offline Corribus

  • Chemist
  • Sr. Member
  • *
  • Posts: 3402
  • Mole Snacks: +512/-23
  • Gender: Male
  • A lover of spectroscopy and chocolate.
Re: Standard curve
« Reply #8 on: August 04, 2014, 10:53:20 AM »
The latter can be ruled out (mostly) by taking an entire spectrum of a dilute solution of your coumassie blue and comparing it to literature. Are the peaks in the right place? Are there any extra ones that shouldn't be there? Do your extinction coefficients roughly match. UV-Vis is fairly tolerant of impurities.

I think your pipettes would have to be pretty terrible in order to be off by so much.

What wavelength are you measuring at?
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Corribus

  • Chemist
  • Sr. Member
  • *
  • Posts: 3402
  • Mole Snacks: +512/-23
  • Gender: Male
  • A lover of spectroscopy and chocolate.
Re: Standard curve
« Reply #9 on: August 04, 2014, 11:10:33 AM »
It seems the extinction coefficient at 595 nm at pH 7 is around 43,000. This should help you estimate what your absorbance should be at the concentrations you are using.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Re: Standard curve
« Reply #10 on: August 04, 2014, 12:09:40 PM »
I've been working at 595

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Re: Standard curve
« Reply #11 on: August 04, 2014, 12:11:27 PM »
I'm just completely confused at what could cause this

Offline Corribus

  • Chemist
  • Sr. Member
  • *
  • Posts: 3402
  • Mole Snacks: +512/-23
  • Gender: Male
  • A lover of spectroscopy and chocolate.
Re: Standard curve
« Reply #12 on: August 04, 2014, 12:23:52 PM »
Based on an extinction coefficient of 43,000 M-1 cm-1, which seems about right, what do you expect your absorbance to be for a 206 ug/mL solution?
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Re: Standard curve
« Reply #13 on: August 04, 2014, 12:35:11 PM »
I know beers law is A = EBC, b being path length (1 cm). How would i convert the concentration to fit this formula

Offline orgo814

  • Full Member
  • ****
  • Posts: 412
  • Mole Snacks: +11/-6
Re: Standard curve
« Reply #14 on: August 04, 2014, 01:09:42 PM »
I may have solved my problem. Did research and the coumassie I had was the reddish kind with max absorbance at 470 and I was doing mine at 595.

Sponsored Links