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The following message was posted to: PharmPK
Dear all,
Could anybody explain to me what is the general meaning of an external
standard?I have found in literature two quite opposite explanations:
1.An external standard is a compound different from an analysed
substance
(but with similar characteristics) and is added into a sample after an
extraction (before a measurement) - it is measured together with an
analysed
substance (similar to an IS, the only difference is that an IS is
added
into a sample before an extraction).
2. ES is an analyzed substance in known concentrations, which is
measured
separately from a sample under the same conditions, and is used for
calibration.
Which one is true?
Thanks a lot.
Anita
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The following message was posted to: PharmPK
Anita
The true is N. 2 ES is indeed an analyzed substance in known
concentrations, which is
measured separately from a sample under the same conditions, and is
used for
calibration.
Regards
Stefano Porzio
Pharmacokinetic & Tox. Dept.
Inpharzam Ricerche SA
Zambon Group
Taverne Switzerland
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Definition 2 is correct!
The "ES" from definition 1 is in fact an Internal Standard, but in that
case it cannot correct the recovery of analyte after extraction; it
correct only concentration/dilution of sample after preparation prior
to injection or variation of volume injected.
Vlase Laurian Univ. Med. & Pharmacy "I. Hatieganu" Dept. of
Pharmaceutical Technology and Biopharmaceutics Cluj-Napoca Romania
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Anita,
An external standard is one that is external to the sample, as you
state in part 2.
An internal standard is one that is internal to the samplebeing
analyzed, which includes your case 1. Internal standards can be added
before or after extraction.
The choice of how to use a standard depends upon what part of the
method it is intended to control. Usually, when an extraction is
involved, that extraction process is considered difficult to control,
and so the usual choice is to add a standardto the sample before
extraction.
Frank
Frank Bales, Ph.D.
Senior Regulatory Consultant
Worldwide Regulatory Affairs
PAREXEL Intl.
2520 Meridian Pkwy, Suite 200
Durham, NC 27713
(508) 650-8051 Phone
Email:frankbales.at.msn.com
frank.bales.-at-.parexel.com
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The following message was posted to: PharmPK
In bioanalysis of drugs in plasma, 2 is the correct definition.
Definition 1 refers to environmental analysis. If you measuring the
concentration of dioxin in estuary sediment, you cannot find any blank
matrix, and so you cannot measure extraction recovery. To correct for
this,
one reference compound is added to the matrix and a second ("external")
is
added after extraction, so that you can calculate recovery. A standard
curve is prepared by adding "additional" concentration of the analytes.
I hope this makes sense.
Ted
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