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Topic: Internal Standards in GC-MS  (Read 4241 times)

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Offline Wuffe_devil

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Internal Standards in GC-MS
« on: March 21, 2013, 11:36:50 PM »
Hi!

We did a GC-MS lab last week and im not quite sure about some things.

We ran a gc-ms run for 4 standards
Each standard had 100 ng/ml of n-decane and
1st standard was 150 ng/ml of undecane and 150 ng/ml of dodecane(and 100 ng/ml of n-decane)
2nd standard was 100 ng/ml of undecane and 100 ng/ml of dodecane(and 100 ng/ml of n-decane)
3rd had 50 ng/ml of un and dodecane(and 100 ng/ml of n-decane)
4th had 10 ng/ml of un and dodecane(and 100 ng/ml of n-dcane)

then we ran 2 unknowns each having unknown conc of un and dodecane and 100 ng/ml of decane.

So it's asking us to make equation of the linear regression line for undecane and dodecane.

im not sure how exactly do we plot a line here, what is supposed to be in equation and whats supposed to be y and x.

I mean, i calculated response factor for undecane and dodecane for each concentration (150,100,50,10) and they were pretty similar, is our line supposed to be reponse factor vs concentration of analyte? im getting really confused as i think i could calculate concentration of undecane and dodecane in the unknowns, but im not sure which response factor to take in the calculation, or is the response factor always the same regardless of the concentration of the analyte?

Also in each standard , we got the data for ret. time and area. I understand why we need to use the area, but not sure whats ret time for, is it to calculate response factor?

I calculated response factors in each standard for undecane and it does seem like a linear relationship(standard 1 is like 0.96, standard 2 is 1.03, standard 3 is 1.13 and standard 4 is 1.33), so is that it?do we plot analyte concentration vs response factor?
then i still dont know which response factor to plug in the formula to find out concentration of analyte in the unknowns.
« Last Edit: March 22, 2013, 12:07:28 AM by Wuffe_devil »

Offline Borek

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Re: Internal Standards in GC-MS
« Reply #1 on: March 22, 2013, 04:46:50 AM »
not sure whats ret time for

GC101: retention time is specific for a given substance and the column, which makes it suitable for the substance identification.
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Offline Arkcon

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Re: Internal Standards in GC-MS
« Reply #2 on: March 22, 2013, 08:23:46 AM »
That's quite a block of text to read through.  Let me try to work with one question, for a start:

So it's asking us to make equation of the linear regression line for undecane and dodecane.

im not sure how exactly do we plot a line here, what is supposed to be in equation and whats supposed to be y and x.


Here's a starting point:  build a table.  The names x and y are crutches for grade school kids.  (Don't feel bad, I call them that too.)  The abscissa (the one going across the page, or the x) is the independent variable -- its what you know, or think you know.  The one you measured with a calibrated device -- a balance, or a pipette, or other volume measure (or maybe the instructor did that, and handed it to you.)  The ordinate (the one going up the page, or the y) is the dependent variable.  This is what the instrument gives you for each reading, and will change from run to run, instrument to instrument.  So build a table for each (without typing out the name of each each time!  and skip the units if they're all the same each time!) and see how you can plot it, perform regression, and draw a line.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

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