Chromatography Forum

Hello every one
I'm working now on captopril development on LC-MS/MS API5000
I use Ascorbic acid as antioxidant in spiked plasma samples by adding 100ul of 0.1M ascorbic acid in 2.00 ml plasma
I do precipetation using ACN for my samples and enalapril as internal standard , the strange thing is like this
ppt of blank alone give good result(clean)
ppt of blank+IS give good result(clean)
ppt of blank+ascorbic acid give good result(clean)
ppt of blank+ascorbic acid+IS(enalapril) give me a huge peak at the drug transition!!!!!!
repeat the trial with all possiblities and gives me same result and so bad linearity
but the linearity of spiked plasma without ascorbic acid give me avery good result
i change IS and get same result
why this happen and
PLEASE can anyone answer ME?!!

enalapril can give very strange peak shapes due to slow changes in molecular conformation. If your other internal standard is a related compound (with a proline function), it could be the same thing. Look in this forum for further information or in my book on HPLC Column in the section on Troubleshooting.

Hello Uwe Neue
You means that imust use internal standard different in structure , which internal standared you suggest it to be used please
the problem not in the peak shape but that when i do ppt for blank contain internal standard and ascorbic acid i get peak for my analyte(captopril)

How about: you mislabeled the sample...???

As wrote Uwe the "-pril"-s are a "mixture" of cis/trans rotamers. You can observe peak broadening especially by lowering the temperature.
Raising the temperature you will get a single peak.
I don't think is a good choice to use as internal standard, unless your analyte is another 'pril'

@smhk

When your peak is too big take in consideration a workup method that includes dilution of the samples.
From what you wrote I understand that you are facing an ionisation enhancement in the presence of ascorbic acid. If you use ascorbic acid as antioxidant you should put in every sample as well as IS (excepting DBl).
Why don't you smiply lower the conc. of IS? API5000 is a sensitive instrument .....
And check what Uwe says - mislabeling. Maybe you are injecting Stock A...

Best Regards