[cavegirl]

Ok, so i'm trying to write a little about why use external vs internal for a monthly lab progress report in work-but have now realised how little i know myself.

so what are the advantages and disadvantages of each?

P.s. This is not a homework question! It's a small part of a bigger report on my GC-MS method development.

Please excuse me, i'm a synthetic chemist and not an analytical bod!

[jdurg]

External standards come from an outside company whose main goal is to produce standards for use by any person at any place in the world.  Any 'errors' they have in their standards will be seen by many people and they will make purchasers of those standards aware of any contamination.  So while external standards can save people a LOT of time and provide more accurate/confident results, they are fairly expensive.

An Internal Standard is made by the people who are using the GC/MS machine themselves.  The purity of the standard can vary on a day to day basis depending on who made the standard, what glassware they used, what bottle of chemicals they used, etc. etc.  Also, the 'error' of said standard isn't really known.  It's MUCH cheaper to use an internal standard, but the accuracy of your results can suffer.

[cavegirl]

Ah, have I got my terminology muddled then?

I'm thinking of the method which involves spiking samples from the beginning of a work up with a know amount of a chemical (usually a deuterated analogue of what you want to quantify). A ratio of the response of non deuterated to deuterated is plotted on the cal curve. It accounts for losses in the sample work up etc. I thought this was an internal standard method?

 

[jdurg]

Ahhh.  I think I may have gotten my terminology confused.    It's been a few years since I did my GC/MS work.

[asqa]

In external standard method std injected sepeartely and std areas were compared to sample area for quantitation.
In internal std(which is not a actual std but its structure is closely related to sample component structure) method, internal standared is spiked in actual std and sample, then area ratios were used for quantitation.
Pls. feel free for further query

[Dude]

Asqa's definition is my understanding of internal and external standards also.  Most Analytical chemistry or Instrumental analysis type books include a page or so on this subject.  In short, internal standards provide the best precision because the error associated with differing injection volumes is taken into account when measuring the sample area or peak height.  Additionally, GC injections are typically done on a volumetric basis.  If your unknown sample has a slightly different density, the amount of mass actually entering your GC will be different than your standard (a systematic error).  Spiking the calibration standards and unknown with a known amount (carefully added) of reference material will allow peak ratioing and can minimize those two effects.    

[asqa]

you are right Mr.Dude