July 16, 2019, 04:34:10 AM
Forum Rules: Read This Before Posting


Topic: Internal Standard for GC quantification  (Read 266 times)

0 Members and 1 Guest are viewing this topic.

Offline coolman50544

  • Regular Member
  • ***
  • Posts: 9
  • Mole Snacks: +0/-0
Internal Standard for GC quantification
« on: July 03, 2019, 10:53:22 AM »
I'm trying to quantify the yields of indole functionalization to ethyl indole-3-acetate via GC. The reactions are worked up via liquid-liquid extraction from water/ethanol mixture to CHCl2.

I've made a calibration curve of the product, but how can I know that my internal standard ideal?  What are some good internal standards to try?

Offline hypervalent_iodine

  • Chemist
  • Full Member
  • *
  • Posts: 184
  • Mole Snacks: +27/-1
Re: Internal Standard for GC quantification
« Reply #1 on: July 03, 2019, 08:15:20 PM »
Is there a reason why you want to use both external calibration and an internal standard? I don't believe this is usual practice, i.e. you would typically use one or the other.

Offline coolman50544

  • Regular Member
  • ***
  • Posts: 9
  • Mole Snacks: +0/-0
Re: Internal Standard for GC quantification
« Reply #2 on: July 03, 2019, 11:14:01 PM »
Is there a reason why you want to use both external calibration and an internal standard? I don't believe this is usual practice, i.e. you would typically use one or the other.

This is the common practice in my laboratory which I have adopted since I am an undergrad.

I would imagine that it is assumed that the efficiency of liquid-liquid extraction of ethyl indole-3-acetate and benzedioxole are constant or change proportionally to each other. Extrapolating yields from the ratio of area of the product to area of the internal standard would account for any loss in extraction if the prior assumption is true. Our calibration curve is also based on the ratio of product of known concentration to area of the internal standard (extracted from a water/ethanol solution to DCM). Internal standard amount for known standards and reactions to be quantified are constant, of course.

I am worried that the prior assumption isn't true, however, since 1,3-benzedioxole is structurally quite different to the indole derivative.

Offline hypervalent_iodine

  • Chemist
  • Full Member
  • *
  • Posts: 184
  • Mole Snacks: +27/-1
Re: Internal Standard for GC quantification
« Reply #3 on: July 04, 2019, 12:37:08 AM »
Is there a reason why you want to use both external calibration and an internal standard? I don't believe this is usual practice, i.e. you would typically use one or the other.

I would imagine that it is assumed that the efficiency of liquid-liquid extraction of ethyl indole-3-acetate and benzedioxole are constant or change proportionally to each other. Extrapolating yields from the ratio of area of the product to area of the internal standard would account for any loss in extraction if the prior assumption is true. Our calibration curve is also based on the ratio of product of known concentration to area of the internal standard (extracted from a water/ethanol solution to DCM). Internal standard amount for known standards and reactions to be quantified are constant, of course.

I am worried that the prior assumption isn't true, however, since 1,3-benzedioxole is structurally quite different to the indole derivative.

In cases where you might use an internal standard in conjunction with an external calibration curve, the point is generally to account for random errors and fluctuations in the detector response. It allows you to measure the response factor so you can adjust the response of your samples and get more precise readings, although I think it is easier to do as you describe and plot the ratios (works out the same). It isn't to account for loss during prior purification steps, and the method I think you are describing would not be able to do that since I assume that you add the standard during sample prep and not during your purification. As for what you use for the standard, you are looking for something that behaves similarly in GC, but produces a clear and distinct response. They don't have to be chemically similar. 
« Last Edit: July 04, 2019, 01:09:19 AM by hypervalent_iodine »

Offline coolman50544

  • Regular Member
  • ***
  • Posts: 9
  • Mole Snacks: +0/-0
Re: Internal Standard for GC quantification
« Reply #4 on: July 05, 2019, 09:40:23 AM »
Is there a reason why you want to use both external calibration and an internal standard? I don't believe this is usual practice, i.e. you would typically use one or the other.

I would imagine that it is assumed that the efficiency of liquid-liquid extraction of ethyl indole-3-acetate and benzedioxole are constant or change proportionally to each other. Extrapolating yields from the ratio of area of the product to area of the internal standard would account for any loss in extraction if the prior assumption is true. Our calibration curve is also based on the ratio of product of known concentration to area of the internal standard (extracted from a water/ethanol solution to DCM). Internal standard amount for known standards and reactions to be quantified are constant, of course.

I am worried that the prior assumption isn't true, however, since 1,3-benzedioxole is structurally quite different to the indole derivative.

In cases where you might use an internal standard in conjunction with an external calibration curve, the point is generally to account for random errors and fluctuations in the detector response. It allows you to measure the response factor so you can adjust the response of your samples and get more precise readings, although I think it is easier to do as you describe and plot the ratios (works out the same). It isn't to account for loss during prior purification steps, and the method I think you are describing would not be able to do that since I assume that you add the standard during sample prep and not during your purification. As for what you use for the standard, you are looking for something that behaves similarly in GC, but produces a clear and distinct response. They don't have to be chemically similar.

Could you provide any further readings/sources of the use of internal and external standards, especially in conjunction?

Offline Corribus

  • Chemist
  • Sr. Member
  • *
  • Posts: 2669
  • Mole Snacks: +429/-20
  • Gender: Male
  • A lover of spectroscopy and chocolate.
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 hypervalent_iodine

  • Chemist
  • Full Member
  • *
  • Posts: 184
  • Mole Snacks: +27/-1
Re: Internal Standard for GC quantification
« Reply #6 on: July 05, 2019, 11:41:44 AM »
Is there a reason why you want to use both external calibration and an internal standard? I don't believe this is usual practice, i.e. you would typically use one or the other.

I would imagine that it is assumed that the efficiency of liquid-liquid extraction of ethyl indole-3-acetate and benzedioxole are constant or change proportionally to each other. Extrapolating yields from the ratio of area of the product to area of the internal standard would account for any loss in extraction if the prior assumption is true. Our calibration curve is also based on the ratio of product of known concentration to area of the internal standard (extracted from a water/ethanol solution to DCM). Internal standard amount for known standards and reactions to be quantified are constant, of course.

I am worried that the prior assumption isn't true, however, since 1,3-benzedioxole is structurally quite different to the indole derivative.

In cases where you might use an internal standard in conjunction with an external calibration curve, the point is generally to account for random errors and fluctuations in the detector response. It allows you to measure the response factor so you can adjust the response of your samples and get more precise readings, although I think it is easier to do as you describe and plot the ratios (works out the same). It isn't to account for loss during prior purification steps, and the method I think you are describing would not be able to do that since I assume that you add the standard during sample prep and not during your purification. As for what you use for the standard, you are looking for something that behaves similarly in GC, but produces a clear and distinct response. They don't have to be chemically similar.

Could you provide any further readings/sources of the use of internal and external standards, especially in conjunction?

What sort of detector are you using?

Offline coolman50544

  • Regular Member
  • ***
  • Posts: 9
  • Mole Snacks: +0/-0
Re: Internal Standard for GC quantification
« Reply #7 on: July 05, 2019, 08:20:44 PM »
Is there a reason why you want to use both external calibration and an internal standard? I don't believe this is usual practice, i.e. you would typically use one or the other.

I would imagine that it is assumed that the efficiency of liquid-liquid extraction of ethyl indole-3-acetate and benzedioxole are constant or change proportionally to each other. Extrapolating yields from the ratio of area of the product to area of the internal standard would account for any loss in extraction if the prior assumption is true. Our calibration curve is also based on the ratio of product of known concentration to area of the internal standard (extracted from a water/ethanol solution to DCM). Internal standard amount for known standards and reactions to be quantified are constant, of course.

I am worried that the prior assumption isn't true, however, since 1,3-benzedioxole is structurally quite different to the indole derivative.

In cases where you might use an internal standard in conjunction with an external calibration curve, the point is generally to account for random errors and fluctuations in the detector response. It allows you to measure the response factor so you can adjust the response of your samples and get more precise readings, although I think it is easier to do as you describe and plot the ratios (works out the same). It isn't to account for loss during prior purification steps, and the method I think you are describing would not be able to do that since I assume that you add the standard during sample prep and not during your purification. As for what you use for the standard, you are looking for something that behaves similarly in GC, but produces a clear and distinct response. They don't have to be chemically similar.

Could you provide any further readings/sources of the use of internal and external standards, especially in conjunction?

What sort of detector are you using?

FID

Sponsored Links