[av0930]

Hello, I have a question concerning the use of the single point internal standard method for confirming drugs of abuse in urine and was hoping someone in this forum can help.  First of all, isn't the whole premise of using an internal standard that it has a predictable retention time and area?? Secondly, shouldn't the internal standard be of the same concentration and volume in all samples tested in the batch (quality control standards included), being that the concentration of the internal standard in the calibrator is taken into account when calculating the response factor used to determine the unknown concentration of samples???  Finally, if the internal standard response varies significantly (by significantly I mean as much as double) from sample to sample, is this a problem?? What is this an indication of???  Thank you much.

[chiralic]

Hi av0930:

Before to answer yours questions, I would like ask you: What is the meaning of "internal standard method for confirming drugs of abuse in urine"? In Forensic Chemistry, we CONFIRM drugs of abuse
by using Gas-Chromatography-Mass Spectrometry. We don't CONFIRM drugs of abuse by Standard Methods.

We use the Standard Methods to determinate if you are or not into the CUT-OFF Values. In others words from this point of view: Chromatography will tell us CONCENTRATION and Mass Spec will tell us
IDENTIFICACION=> CONFIRMATION. If you don't have GC-MS you need to do before a PRESUMPTIVE TEST and then you need another method TO CONFIRM.

Well if you need more information about this topic please send a private email to chiralic@gmail.com

Regards,

Chiralic

[av0930]

Thank you.  Will do.

[mdlhvn]

   You have little confuses. GC-MS is an instrumental analysis method to determine qualitatively and quantitatively an unknown sample while "internal standard"-IS is one way of quantification, different from external standard-ES method.
   
   In ES, the factor indicating the concentration is direct height or area of peaks of samples. However, in IS, you inject a certain amount of IS substance to all sample and the factor indicating the concentration now is the ratio of height or area of peaks of samples and that of IS substance.

    But in which cases they use IS method? the characteristics of IS substance? The advantage of IS compare with ES? Try to explain, you will comprehend IS method.

[chiralic]

Hi mdlhvn:

I'm not confuse. You are right when you say that GC-MS is an Instrumental Analysis Method to do qualitative and quantitative analysis, that is totaly clear for me (GC=HOW MUCH, MS=WHO IS). Also, I Know that we can do QUALI and QUANTI only by GC or MS.

I try to explain a simple way to av0930 that with the use of a Standard Method (IS or ES) we cann't CONFIRM drugs of abuse in Urine.

My best regards,

Chiralic

[mdlhvn]

Ok!
But GC#HOW MUCH, GC=SEPRATING, MS=WHO IS AND HOW MUCH?

right.

Sincerely yours!

[Ψ×Ψ]

First of all, isn't the whole premise of using an internal standard that it has a predictable retention time and area?? Secondly, shouldn't the internal standard be of the same concentration and volume in all samples tested in the batch (quality control standards included), being that the concentration of the internal standard in the calibrator is taken into account when calculating the response factor used to determine the unknown concentration of samples???  Finally, if the internal standard response varies significantly (by significantly I mean as much as double) from sample to sample, is this a problem?? What is this an indication of???  Thank you much.

I'm assuming this is done by GC/MS?  In practice, the retention time for internal standards as well as for known analytes does not vary much at all, unless you seriously abuse your column.  A known concentration of internal standard is used, but the peak area may vary for the same standard between runs.  This is (mostly?) because the injection isn't reproducible, despite the wonderful world of autosamplers.  <a href="http://www.dreamingspirals.com/item.php/2006/10/17/internal-standards-in-gc-ms">This post</a> at Dreaming Spirals explains things better than I can.