Chromatography Forum

Hi etaxene

You can calculate your RSD with:

RSD= 50 * N/S

Where N is the Noise of your baseline and S your signal both expressed in the same units.

You can find a paper about this subject and others about validation:

VALIDATION OF HPLC TECHNIQUES FOR PHARMACEUTICAL ANALYSIS by N. A. Epshtein. 2004

I found this in the internet and I think is one of the most comprehensive papers I have read!

Best Luck!

Fernando

Hi again

I forgot when you determine the cleanlines, your goal is a concentration less than LOQ and NO zero. You inferm below LOQ I think.

Best Luck!

Fernando

By definition the rsd at the LOQ is 10%, so you will never get a 2% repeatability at levels close to or below the LOQ.

Peter

Sorry Peter but i don't understand.
what do you mean?

If you do repeated injections of a standard with a concentration equal to the (lower) limit of quantitation, the relative standard deviation (rsd) of the peak area will be 10%. By the definition of LOQ.

2% is less than 10%.

To get a smaller rsd (e.g. 2% instead of 10%) in this particular instance you need to increase the size of the peak. That requires a higher concentration than the concentration at the LOQ that gave you a 10% rsd.

Peter

I think the confusion is happening because of the way the problem has been posed. In effect, you're doing it the wrong way round at the moment!

You would like to demonstrate that the amount present is zero, but you're worried that you need to measure it +/- a certain percentage, so you're trying to achieve this percentage. The difficulty is that because your sample contains nothing, you are trying to generate the result "0" with an RSD of 2%, which means no error whatsoever (clearly impossible, as you recognised). Therefore quite reasonably you're trying to guess a concentration a bit higher than zero, where you can achieve the RSD of 2%, so you can check that the precision of your measurements is acceptable.

The difficulty is (1) that you're obliged to guess a concentration and hope that you can achieve a target %RSD, and (2) that the target %RSD isn't helpful to the underlying problem. If 1ng of X is going to cause trouble, being knowing you can measure 10ng with 2% RSD doesn't help. What you want to know is whether you can detect 1ng.

This is why it's better to measure the Limit of detection, and simultaneously estimate how much of the problem peak you can actually tolerate in your application before it causes trouble. If the LOD is appropriately less than the estimated amount of X that causes trouble, then you can detect problematic levels of X with adequate reliability. As a 'bonus', if the LOQ is less than or equal to your measured value, then you can promise a certain level of precision to your quantitative answer about how bad the problem is.

The point about the %RSD approach is that you use it on reasonably large peaks, in order to be able to claim that you can measure the amount of a chemical that is present (rather than a chemical that probably isn't, but might be!) with the precision someone has required.

Lovely case. I unite myself to have an opinion. I start from the beginning.
I am writing an internal POS where I want to indicate various system syitability. According to you what parameters can I use for the 'RSD?

I thought: max 2% For assay. Max 15% for Impurities (HPLC and GC) and max 20% for the cleaning validation.

IS IT OKAY?

Lovely case. I unite myself to have an opinion. I start from the beginning.
I am writing an internal POS where I want to indicate various system syitability. According to you what parameters can I use for the 'RSD?

I thought: max 2% For assay. Max 15% for Impurities (HPLC and GC) and max 20% for the cleaning validation.

IS IT OKAY?

If "Supplier imposes a limit of 2%. " (your post 16 Nov) then 2% it must be, what we think and what you think makes no difference.

Peter

Lovely case. I unite myself to have an opinion. I start from the beginning.
I am writing an internal POS where I want to indicate various system syitability. According to you what parameters can I use for the 'RSD?

I thought: max 2% For assay. Max 15% for Impurities (HPLC and GC) and max 20% for the cleaning validation.

IS IT OKAY?

If "Supplier imposes a limit of 2%. " (your post 16 Nov) then 2% it must be, what we think and what you think makes no difference.

Peter

Peter I would write a general SOP. Not a specific case.
When I do not have a customer-specified limit.

Lovely case. I unite myself to have an opinion. I start from the beginning.
I am writing an internal POS where I want to indicate various system syitability. According to you what parameters can I use for the 'RSD?

I thought: max 2% For assay. Max 15% for Impurities (HPLC and GC) and max 20% for the cleaning validation.

IS IT OKAY?

If "Supplier imposes a limit of 2%. " (your post 16 Nov) then 2% it must be, what we think and what you think makes no difference.

Peter

Peter I would write a general SOP. Not a specific case.
When I do not have a customer-specified limit.

For me...
RSD
<2% for Assay
<10% for Cleaning
<15% for Impurity

If "Supplier imposes a limit of 2%. " (your post 16 Nov) then 2% it must be, what we think and what you think makes no difference.

Peter

Peter I would write a general SOP. Not a specific case.
When I do not have a customer-specified limit.

For me...
RSD
<2% for Assay
<10% for Cleaning
<15% for Impurity

The supplier specifies an rsd of 2% (according to your post of 16 Nov). What are they going to say when you take them a methods with rsd 10% ?

Peter

Hi etaxene

Why are you injecting 10 µl, your standard is very little, I'll try 100 or 200 µl.

Best regards.

Fernando

I don't know. The customer told me that I have to use this method without any changes.

Did you ask the customer if THEY are able to achieve <2% RSD?
If they manage to do so, you should not think about tinkering with acceptance criteria, concentrations or injection volumes, but find out why it's not possible in your lab. If it's written in the SOP, follow the SOP. Analytical considerations unfortunately have limited persuasive power in a regulated environment.

Ok. I will ask them.

For the future I would also write an internal SOP where I establish the limits of RSD different depending on the type of analysis (Assay, Purity, Cleaning, etc.)

For example:

<2% for Assay
<10% for Cleaning
<15% for Impurity

There are guidelines?

No guidelines except the number of reference standards used during system suitability as per USP <621>. However, as 10% RSD is the statistical LOQ I would suggest using a reference standard concentration at 25% of the top end of the linearity curve. Also be sure to use truncating and numbering (significant digits) as per USP, so 2% is 2.0% (see general chapter, page 4?).

Ok. I will ask them.

For the future I would also write an internal SOP where I establish the limits of RSD different depending on the type of analysis (Assay, Purity, Cleaning, etc.)

For example:

<2% for Assay
<10% for Cleaning
<15% for Impurity

There are guidelines?

Guideline number 1: The customer is always right.

Peter