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Analytics · Method validation

Why one laboratory's 98.4% and another's 96.1% may not contradict

Analytics › Method validationstability-indicating method LOD & LOQ linearity
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Solved by f.chowdhury in post #5
The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number. I have kept the units in throughout, for the obvious reason.

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LC
l.cabreraTL28 Mar 2025#1
Community wiki post. Any member at trust level 3 or above can edit this post; every edit is recorded. Last edited by revision_history on 5 Apr 2025.
  • 22 May 2025 — outline_first: Replaced an unsourced figure with the published one and cited it.
  • 26 Jun 2025 — ms_holloway: Removed a claim that the cited source did not support.
  • 28 May 2025 — v.szabo: Clarified the distinction that was causing repeat questions below.
  • 5 Apr 2025 — revision_history: Added the worked example and a unit label to the table header.
Editors: outline_first, ms_holloway, v.szabo, revision_history

Why one laboratory's 98.4% and another's 96.1% may not contradict I have a specific reason for asking rather than idle curiosity, and the context is below.

Posting the method first, because I know what the first three replies will otherwise be.

  • Column: C18, 4.6 x 250 mm, 5 um
  • Mobile phase: 0.1% TFA in water / 0.1% TFA in acetonitrile
  • Gradient: 12% to 57% organic over 22 minutes
  • Detection: 214 nm
  • Injection: 8 uL
  • Sample: semaglutide, reconstituted to 1.0 mg/mL, injected within an hour

The main peak integrates at 96.7% of total area. There is a small feature on the trailing edge that I cannot decide is a shoulder or a baseline artefact, and that is what I am actually asking about.

6 likes 17mo
ME
m.eriksenTL227 Mar 2025#2

Following this. I have the same question and no better information than the first post.

0 likes 16mo
HM
h.mensahTL29 Apr 2025 · edited#3

Coming back to the opening post, because the follow-up matters more than the original answer.

The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number.

Adding a source would improve this post and I do not have one to hand.

23 likes 16mo
LP
l.piresTL220 Apr 2025#4

The opening post is right about the mechanism and I think understates the practical bit.

The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.

10 likes 15mo
FC
f.chowdhuryTL2 Solution1 May 2025#5

The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number.

I have kept the units in throughout, for the obvious reason.

6 likes 15mo
CI
c.inglethorpeTL3Regular11 May 2025#6
h.mensah, post #3: Coming back to the opening post, because the follow-up matters more than the original answer. The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number. Adding a source would improve this post and I do not have one to hand. Go to post

The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.

Not the answer, but possibly the question that gets there.

1 like in reply to #3 15mo
MR
m.restrepoTL221 May 2025#7

I read post #5 twice before replying, because I had assumed the opposite.

Stability-indicating method: one that can separate a compound from its degradation products. Critical for assay methods that claim to measure actual degradation (as opposed to purity, which is orthogonal).

31 likes 14mo
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ThibodeauTL3Regular30 May 2025#8

Transfer between laboratories: a method can be transferred from one lab to another, but the receiving lab needs to demonstrate that they can achieve the same performance. This requires comparative testing and sometimes small method refinements.

15 likes 14mo
JC
j.cabreraTL28 Jun 2025#9
l.cabrera, post #1: Why one laboratory's 98.4% and another's 96.1% may not contradict I have a specific reason for asking rather than idle curiosity, and the context is below. Posting the method first, because I know what the first three replies will otherwise be. Column: C18, 4.6 x 250 mm, 5 um Mobile phase: 0.1% TFA in water / 0.1% TFA in acetonitrile… Go to post

No notes. Posting so the count is not one.

10 likes in reply to #1 14mo
F
FFaulknerTL3Regular17 Jun 2025#10

Building on post #8 rather than restating it.

System suitability is the ongoing evidence that a validated method is still performing. Validation is done once; suitability is done every run, and it is the one that appears on a certificate.

3 likes 13mo
MA
mi.amankwahTL226 Jun 2025#11

The arithmetic in post #10 is right; the assumption feeding it is the part to check.

Why two laboratories may disagree: after validating the same method, they may still report different purity on the same sample due to integration differences, column age differences, subtle differences in mobile phase pH or temperature. This is normal and not a sign that one is wrong.

8 likes 13mo
L
LJankowiakTL3Regular4 Jul 2025#12

Answering the question post #8 raises rather than the one it answers.

An independent laboratory's method being different from the supplier's is not a discrepancy. It becomes one only when the results differ by more than both methods' demonstrated precision.

A modest claim, modestly supported.

19 likes 13mo
AC
a.cardosoTL212 Jul 2025#13

The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.

Worth one more sentence than it usually gets.

0 likes 13mo
BR
buffer_reviewTL3Regular20 Jul 2025#14
m.restrepo, post #7: I read post #5 twice before replying, because I had assumed the opposite. Stability-indicating method: one that can separate a compound from its degradation products. Critical for assay methods that claim to measure actual degradation (as opposed to purity, which is orthogonal). Go to post

Reading back through, this was answered upthread and I missed it. My fault.

2 likes in reply to #7 12mo
AW
ai.wikstromTL228 Jul 2025#15
TS
t.steenkampTL2Member5 Aug 2025 · edited#16

Everything in post #12 holds. The case it does not cover is the one I have.

Range and working range are different things and a certificate rarely distinguishes them. The relevant one is the range over which this particular sample was measured.

26 likes 12mo
AK
ak.kravchenkoTL213 Aug 2025#17

Where a pharmacopoeial monograph exists, a method that follows it inherits a great deal of assurance. Almost nothing discussed here has one.

If that reads as pedantic, it is, and it has saved me twice.

0 likes 11mo
AD
ambient_draftTL3Regular21 Aug 2025#18
Thibodeau, post #8: Transfer between laboratories: a method can be transferred from one lab to another, but the receiving lab needs to demonstrate that they can achieve the same performance. This requires comparative testing and sometimes small method refinements. Go to post

The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number.

On reflection I would soften that slightly.

4 likes in reply to #8 11mo
FD
f.demirTL2Regular28 Aug 2025#19

The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.

The uncertainty is in the assumption, not in the calculation.

2 likes 11mo
AI
a.iyerTL25 Sep 2025#20

The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number.

I have said this before in a thread nobody could find, so it is worth repeating.

8 likes 11mo
FC
f.chowdhuryTL212 Sep 2025#21
l.pires, post #4: The opening post is right about the mechanism and I think understates the practical bit. The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it. Go to post

Validation is compound-specific and matrix-specific. A method validated for one peptide is a starting point for another and not a validated method for it.

Someone will know this better than I do and I hope they say so.

6 likes in reply to #4 10mo
O
OstrowskiTL2Member19 Sep 2025 · edited#22

Seconded. It reads as careful rather than confident, which is the right register.

1 like 10mo
MR
m.restrepoTL226 Sep 2025#23

Coming back to post #21, because the follow-up matters more than the original answer.

Stability-indicating method: one that can separate a compound from its degradation products. Critical for assay methods that claim to measure actual degradation (as opposed to purity, which is orthogonal).

I am describing what is, rather than arguing for what should be.

0 likes 10mo
T
ThibodeauTL3Regular4 Oct 2025#24

Transfer between laboratories: a method can be transferred from one lab to another, but the receiving lab needs to demonstrate that they can achieve the same performance. This requires comparative testing and sometimes small method refinements.

21 likes 10mo
JC
j.cabreraTL211 Oct 2025#25
m.restrepo, post #7: I read post #5 twice before replying, because I had assumed the opposite. Stability-indicating method: one that can separate a compound from its degradation products. Critical for assay methods that claim to measure actual degradation (as opposed to purity, which is orthogonal). Go to post

Post #21 and I disagree about the size of the effect, not about the direction.

Why two laboratories may disagree: after validating the same method, they may still report different purity on the same sample due to integration differences, column age differences, subtle differences in mobile phase pH or temperature. This is normal and not a sign that one is wrong.

Happy to expand any of that if it is the useful part.

9 likes in reply to #7 10mo
F
FFaulknerTL3Regular18 Oct 2025#26
l.cabrera, post #1: Why one laboratory's 98.4% and another's 96.1% may not contradict I have a specific reason for asking rather than idle curiosity, and the context is below. Posting the method first, because I know what the first three replies will otherwise be. Column: C18, 4.6 x 250 mm, 5 um Mobile phase: 0.1% TFA in water / 0.1% TFA in acetonitrile… Go to post

Taking post #25 at face value and following it one step further.

A stability-indicating method is one demonstrated to resolve the analyte from its degradation products, usually through forced degradation. Calling a method stability-indicating without that work is a claim rather than a property.

2 likes in reply to #1 9mo
HN
h.nwosuTL225 Oct 2025#27

The limit of quantitation determines what the impurity table can honestly contain. Peaks below it can be reported as detected and cannot be reported as a number.

0 likes 9mo
TS
taper_shiftTL3Regular31 Oct 2025#28

The most useful single question about a method: what would it fail to detect? Every method has an answer and few documents state it.

29 likes 9mo
BA
b.adeyemiTL27 Nov 2025#29

Understood. Thank you for being specific about the limits of it.

14 likes 9mo
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IMainwaringTL3Regular14 Nov 2025#30

System suitability is the ongoing evidence that a validated method is still performing. Validation is done once; suitability is done every run, and it is the one that appears on a certificate.

5 likes 8mo
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