Chromatography Forum

Hi ,

When i go from 0.0005 ppm ( 5 ppb ) to 0.0001 ( 1 ppb ) i start to see a negative peak for my analyte !

My question :

1- Is it acceptable to carry on the determination of my analyte at this concentration ( i.e 0.0005 ppm ) .The precision of 10 injection is 25 % ?

2- what is acceptable R.S.D% FOR the lowest CONC. at ( LOQ ) BASED ON 10 injection ,please ?

Any help will much appreciated

Thanks

The presence of a negative peak at the retention time of the analyte suggests that your mobile phase has been contaminated with the analyte.

And you have the question about standard deviation and LOQ backward. The correct order is:
1. How precisely do you need to quantitate?
2. The analyte level at which your CV hits that precision is your LOQ.

Tom ,

i prepared my new fresh mobile phase and i still see the negative peak at that concentration ( 0.0005 down to 0.0001 ppm ) but if i go up than 0.0005 ( say 0.001 i don't see negative peak at this concentration ) !

Regarding the LOQ i heared that from ICH THEY SAY at the LOQ the precision must not exceeds 1% or 10 % not sure

i prepared my new fresh mobile phase and i still see the negative peak at that concentration ( 0.0005 down to 0.0001 ppm ) but if i go up than 0.0005 ( say 0.001 i don't see negative peak at this concentration ) !

Then your new batch of mobile phase is also contaminated.

What do you see if you inject a deionized water blank (I'm assuming reversed-phase chromatography here)?

My reason for suspecting contamination is that the absence of something chromatographs in the same manner as the presence of that same compound (the phenomenon has been exploited under the name "vacancy chromatography").

Many years ago when I was working with single-column ion chromatography, we used Santa Clara, California drinking water as a quick-and-dirty check sample (it contained ppm-level amounts of chloride, nitrate, and sulfate). Just for fun one day we prepared our mobile phase buffer with tap water and injected DI water. We obtained a chromatogram with *negative* peaks at the correct retention times for those ions.

The negative peak could also be a system peak which just happens to be where the analyte is. If this is the case you could try to shift the rt via mobile phase composition change.