Frequency of Bracketing Standards for FID-GC Method

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Hello Everyone,

I'm in the latter stages of method development of a GC-FID method using splitless injection (trace-level work) and a 30-minute temperature program. It's been awhile since I've done GC work, so please, can anyone guide me in terms of how to decide the scheduling of bracketing standard injections around sample injections in a sequence?

For example, in one job I held, we were required to queue bracketing standard injections every 6 - 8 sample injection OR every two hours of running time, whichever time period was shorter...in HPLC work.

My thanks to you all, in advance.

(Wow...I meant to type GC-FID...it's been a long day. Pretty much, a long week.)
MattM
Depends on how tolerant you are of failed calibration checks. EPA methods usually require passing every 12 hours. Some QC protocols require it by batch, and I often see recomendations of every 10 samples to avoid large numbers of re-injections. Sounds like your in your example they wanted you to keep a close eye on things.
Worst case; every sample with a blank in between.
Hi Steve,

My thanks for your comments. I'm pretty sure we're not too tolerant of failed QC checks, I tried out a frequency of brackets every four samples, %RSD overall was less than 3. I'll run, say, four or five more sets of samples (this last one was 52 samples, so 14 bracketing QC injections) so that I feel this is an okay option going forward.

Best Wishes to You!
MattM
Surely it would depend on how stable the method is compared to your QC limits ? An FID's response is very stable, columns deteriorate but if yours is dying fast enough to affect method stability you probably need to do some trouble-shooting, so that basically leaves the inlet and how clean the samples are. If you can inject a standard, 100 samples and then another standard and get the same peak areas for the two standards, then why slow things down by injecting lots of standards along the way ?

Peter
Peter Apps
Hi Peter,

My thanks-agreed assuredly on the stability of the FID. To respond without giving away too much, I'm in a situation where I know what the standard I'm using is (I'm using an internal standard, actually BHT), but at this point I've no clue as to what the analytes I'm interested are, other than they are substances that have been extracted from a solid with a solvent. To this point, we've not yet done any other sample preparation other than adding internal standard to these sample extracts.

We're pretty early along as things are going in this process.

I suppose one may call what we're doing at this point "semi-quantitative" at best. We are working toward identifying and eventually having quantities of our analytes prepared to shore up the method...I hope!

The main source of variability is not the QC brackets, but the samples with added IS. Can't recall the variability of the IS in the samples, but I'll post this info later today. And agreed, no reason to slow things down too much with tons of QC injections--perhaps, if I can show that the IS in the QC and the samples are having similar repeatabilities over multiple analyses, then I will feel better about lessening the QC injection frequency in time.

Regards, Sir!
MattM
As long as the variability of the internal standard is from the instrument and not spiking errors then your results will be normalised. Our QC allows variability up to +/- 50% of the calibration responses.
----suffers separation anxiety----
mattmullaney wrote:
Hi Peter,

My thanks-agreed assuredly on the stability of the FID. To respond without giving away too much, I'm in a situation where I know what the standard I'm using is (I'm using an internal standard, actually BHT), but at this point I've no clue as to what the analytes I'm interested are, other than they are substances that have been extracted from a solid with a solvent. To this point, we've not yet done any other sample preparation other than adding internal standard to these sample extracts.

We're pretty early along as things are going in this process.

I suppose one may call what we're doing at this point "semi-quantitative" at best. We are working toward identifying and eventually having quantities of our analytes prepared to shore up the method...I hope!

The main source of variability is not the QC brackets, but the samples with added IS. Can't recall the variability of the IS in the samples, but I'll post this info later today. And agreed, no reason to slow things down too much with tons of QC injections--perhaps, if I can show that the IS in the QC and the samples are having similar repeatabilities over multiple analyses, then I will feel better about lessening the QC injection frequency in time.

Regards, Sir!


Bracketing standards should be used to detect drift - something that changes over multiple analyses in a consistent direction. Similar to control charts. Random run-to-run variation can put a bracketing standard out of spec when the samples both before and after it have run perfectly. With an IS, drift can only be due to causes which differ for the IS and the analytes - so for example if you use an alkane as an IS for alcohols the alcohol peaks will drift down in relation to the alkane as the inlet cruds up and the column deteriorates. At some stage the calibration based on sharp alcohol peaks from a clean inlet and new column will no longer be valid - a bracketing standard will show that has happened.

So repeatability of the IS has nothing to do with frequency of bracketing standards, because repeatability is (or should be) a measure of random variation, not drift.

A lot of variation in IS peak area is not necessarily a problem as long as you get a correlated variation in analyte peak area - the IS is then doing its job of correcting for whatever is making the peak areas vary.

Peter
Peter Apps
@ Peter and tangaloomaflyer,

My thanks for your comments--I Do Tend to Get Hung Up on Repeatability...probably because I get tired of jotting down small "x's" on SPC paper!

Your point (one Big One Taken Two Times) is well-taken! I stand corrected.

As of yet, I'm not seeing any obvious drift or bias...but I'll let everyone know. I'm only 18 QC Standard Points and 48 Spiked Samples in, not counting 4 Blanks.

Regards to you both!
MattM
If you have no drift during a batch then the only bracketing standards that you need are one at the beginning and one at the end to show that you have not drift.

Peter
Peter Apps
@ Peter, agreed wholeheartedly sir. Very Well Said. Probably I'd not be permitted to "get away" with doing that in practice, but the principle you state is sound. It'll be interesting to see how the thing plays out in the practice of my position!

And my thanks, once again, for an appropriate kick to my pants!

Regards!
MattM
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