Quantifying pesticides - very basic questions

[lusi]

Can someone please tell me which is better to use when quantifying pesticides - internal or external? Also I don't have access to that many standards so I was wondering is it possible to save a calibration curve based on different concentrations of your pesticide, to be used to quantify other samples ran at a later date? Thanks so much.

[skunked_once]

Internal standards are considered more accurate than external standards but external standards are often used and are perfectly acceptable.

Using calibrations from a previous time is not acceptable. Standard curve calibrations must be run with each sample set at the time of analysis.

[lusi]

Thanks for you reply. I thought as much.

[lusi]

Hey Sunked_once. Thanks once again for your reply. I would appreciate if you can give your opinion to this reply on the same topic. Thanks:
you don't have to develop curves for each sample if u have an idea what is in the sample then you can develop one cure using external standards and store it on the system.anytime you runing any sample which might have that component you can simply use that cure providing the method of analysis is the same.
you can do both form of standards and store them on the machine the external standards give you the control of seting up whatever the limits of quantification you want that is both the maximum detection and minimum detection limits
so the best thing to do is develop the calibration curve for a method looking at particular pesticides.when making up standards you can have more than one standard in a sample providing they are detected by the same method of analyses.

[skunked_once]

Lusi,

I suggest that you search out some of the posts on method validation. Every method must be validated for each pesticide and sample matrix combination in order for the method to be valid.

anytime you runing any sample which might have that component you can simply use that cure providing the method of analysis is the same.

Even with a validated method, you must run a standard curve at the same time as the sample analysis because there may be day-to-day variations in instrument response.

the best thing to do is develop the calibration curve for a method looking at particular pesticides.when making up standards you can have more than one standard in a sample providing they are detected by the same method of analyses.

Yes you can have more than one pesticide analyzed in a method as long as you use the appropriate standards. This is part of the method validation process.

[lusi]

Thanks once again for your reply. As you realise I don't know much about these things. From reading I gathered pretty much the same things you indicated. To be honest the response was from someone who is supposed to be versed in this area. As you see some of it was not inline with other sources, so I just wanted your opinion. Thank you so much. Enjoy the rest of your day!!!