Post by: year12 on May 29, 2010, 03:30:11 pm
I had a question to ask about drawing calibration curves.
Let's say once the data is plotted and it appears to be non-linear,
- do I draw a straight line (a line of best fit) or do I draw a curve, trying to pass through all the given points?
- & when trying to draw a line of best fit, what happens if it doesn't go through the origin? Or should I try to pass it through the origin?
Because it really doesn't make sense especially if they give us an absorbace of 0 when the conentration is 0 ... and my graph does not show this.
Your advice will be helpful!
Thanks.
Post by: stonecold on May 29, 2010, 03:33:33 pm
I couldn't imagine how they would mark us on drawing a non linear calibration curve.
Tbh, I just wish they gave us a calibration equation. Easier, and wayy more accurate. :D
Post by: superflya on May 29, 2010, 03:34:10 pm
Post by: Martoman on May 30, 2010, 03:13:56 pm
mmmm i smell duuuurty VCAA examiners getting sweaty palms just discussing such a nasty question. They would have a tasty tasty galvanic skin response, im sure. :idiot2:
Post by: crayolé on May 30, 2010, 03:38:06 pm
This is a good question. If it is non-linear then it has to be DILUTED. The question would almost certainly have to ask you: "Explain why this curve is non linear" or something like that, then ask you to dilute it by some factor and THEN with these transformed data do you graph a calibration curve.Could you please explain this in more detail? ;]
mmmm i smell duuuurty VCAA examiners getting sweaty palms just discussing such a nasty question. They would have a tasty tasty galvanic skin response, im sure. :idiot2:
Post by: Martoman on May 30, 2010, 03:47:52 pm
Say you had a very concentrated sample. At low concentrations this would still appear linear on a calibration curve. But at higher concentrations... it will start to go all over the shop (ie: most likely exponential). This is why in these types of questions we are usually told that a 20ml sample was diluted to 100ml or 200 ml or whatever so that a linear plot will form.
I think there was a great example of this in the NEAP 2010 paper asking about by what factor do you dilute a sample by to get it in the appropriate region for analysis. hit it up. ;)
Post by: stonecold on May 30, 2010, 03:50:31 pm
Why does the sample need to be diluted?
SO the absorbance reading for the sample falls within the calibration curve, as it is inaccurate to extrapolate above the curve.
Post by: Martoman on May 30, 2010, 04:00:39 pm
Given the context of this thread and the fact that no one mentioned it I thought it apt to interject.
Post by: olly_s15 on May 30, 2010, 10:17:33 pm
Yeah.
Say you had a very concentrated sample. At low concentrations this would still appear linear on a calibration curve. But at higher concentrations... it will start to go all over the shop (ie: most likely exponential). This is why in these types of questions we are usually told that a 20ml sample was diluted to 100ml or 200 ml or whatever so that a linear plot will form.
I think there was a great example of this in the NEAP 2010 paper asking about by what factor do you dilute a sample by to get it in the appropriate region for analysis. hit it up. ;)
to me this seems wrong - how can you go from an exponential graph and then multiply it by a dilution factor (which is a constant) and come out with a linear graph? just doesn't make sense; please explain.
Post by: Martoman on May 30, 2010, 11:05:02 pm
Post by: olly_s15 on May 30, 2010, 11:13:49 pm
what you are saying contradicts this... you're saying that the sample of higher concentration would deviate from a linear shape which essentially means you cannot multiply by a constant
Post by: Martoman on May 30, 2010, 11:16:46 pm
Post by: stonecold on May 30, 2010, 11:18:12 pm
And also why it is called a curve :P
Post by: Martoman on May 30, 2010, 11:21:41 pm
... because we do a somewhat watered down version of chem....................
:2funny: :2funny: :2funny: :2funny: :2funny: :2funny: :2funny: :2funny: :2funny: :2funny:
Post by: olly_s15 on May 30, 2010, 11:22:05 pm
Martoman is right. It is called Beers law I think. At high concentration, the graph is actually exponential, but because we do a somewhat watered down version of chem, we only get concentrations in the lower part of the graph where it appears linear. That is why you should NEVER extrapolate above the curve.
"beer's" law, lol!
do you understand about what type of questions i'm talking about though? i'm not saying you guys are wrong, it just contradicts some of the questions i've done - although yes, this is VCE level, watered down etc.
Post by: TrueTears on May 30, 2010, 11:25:16 pm
Post by: Martoman on May 30, 2010, 11:25:58 pm
this doesn't really have anything to do with chem but i just wanna point out a line is a curve :2funny: so u are actually drawing a curve :P
BAHAHAAHHAHAHAHAHAAHHA
Oh TT. You are a legend. You know that right.
Post by: TrueTears on May 30, 2010, 11:26:49 pm
Post by: olly_s15 on May 30, 2010, 11:32:05 pm
lololololol haha i just had an urge to point that out LOL
yes because euclidean geometry was considered when they named it the calibration curve
haha
Post by: Martoman on May 30, 2010, 11:32:46 pm
Post by: TrueTears on May 30, 2010, 11:33:19 pm
lol shows why chemists will never be true mathematicianslololololol haha i just had an urge to point that out LOL
yes because euclidean geometry was considered when they named it the calibration curve
haha
Post by: olly_s15 on May 30, 2010, 11:44:45 pm
lol shows why chemists will never be true mathematicianslololololol haha i just had an urge to point that out LOL
yes because euclidean geometry was considered when they named it the calibration curve
haha
thought that was pretty obvious - they are chemists, not mathematicians lolzzzzzz
ok i'll stop now
Post by: TrueTears on May 30, 2010, 11:54:42 pm
Post by: Mao on May 31, 2010, 02:13:24 am
/me loads shotgunlol shows why chemists will never be true mathematicianslololololol haha i just had an urge to point that out LOL
yes because euclidean geometry was considered when they named it the calibration curve
haha
Nah I kid.
but yes, kudos to kids who've taken their time to research the topic. The Generalized Beer-Lambert law only holds when there's no (or very small amount) of intermolecular interactions (i.e. the compound you are studying do not interact with each other).
As concentration gets large, these interactions become significant, and can often result in polymerization, coordination and all sorts of crazy things, which changes just about every parameter in the generalized Beer-Lambert law.
The accuracy (and linearity) of a spectrometer thus is really limited by the sensitivity of the optical detector. More sensitive optical detectors can detect smaller changes in concentrations, thus allow the use of lower concentrations, and more empirically correct data.
Post by: TrueTears on May 31, 2010, 02:17:54 am