My second source fails against my initial calibration ...

[EnvChemist]

Hello all !

I use a HP 1100 series HPLC to detect for PAH compounds inenvironmental samples like industrial wastes (EPA methods 610 and 8310). For a while now I have been having trouble getting my second source to pass against my initial calibration . The second source fails high (criteria is +/- 15%)

I have tried all kinds of trouble shooting apart from regular instrument maintenance; like making new stock solutions from unopened vials, but to no avail. Any ideas in terms of troubleshootin and what could be the possible cause of this problem ?

My curve is fine, it is the second source that is the issue ...

Thanks !

[ym3142]

what is a 2nd source?

[semivoadude]

The 2nd source is used to check the curve externally. Are you integrating individual peaks or ranges? If your failing high, it suggests carry-over or contamination. Try performing maintanance or inserting blanks to pinpoint any carryover. You also may want to reverse your initial source with your secondary source (if permitted). Perhaps your current secondary source would give you a good curve then you can try checking it with your current initial source. (Technically your still using an external source)

[Mark Tracy]

Does it fail for the entire list of components? That is a dilution error somewhere. Are both standard mixes prepared in the same solvent? If it is scattered among the components, look for adsorption, decomposition, volatilization, etc.

[Dan]

EnvChemist,

When you say "second source" do you just mean a second preparation of the standard solution (i.e. a check standard) or is there actual a second lot/batch of the reference standard material?

If it is a check standard, then follow Mark's suggestions. (I worked with PAHs some years back and, as I recall, stability/degradation, especially in solution, is an issue.)

If it is a second lot of reference material, then did you remember to correct for the purity? A different lot of reference material may not necessarily have the same purity as the first lot.

Regards,
Dan

[Mark Tracy]

"Second source" usually means an independent supply. Either a different vendor, or a different manufacturing line within a single vendor. You need to be careful because some vendors don't actually make all the stuff they sell, and re-sell other peoples' chemicals. For some compounds, there is only one original manufacturer that everyone buys from, or worse yet it is no longer made, and the world's supply is the last batch from only manufacturer who got out of that business years ago. Environmental chemists have been dipping into the last pot of Aroclor 1254 for the last 20 years.

[EnvChemist]

Eureka ! The problem has been solved.

Thank you all for your input, you provided some new troubleshooting ideas.

The problem was actually with the syringe - it was out of calibration ....... (Imagine that !!!!! ) .

I was using a 50uL syringe to spike 10uL of the ext chk solution into my vials. When I used a 10ul and 100ul instead the ext chk passed fine. Because I only used the 50ul syringe for the two lowest levels, out of six (10ul and 50ul) for my initial calibration, the curve was not not affected.

Again thanks for the suggestions ...