Chromatography Forum

Manual says detection limit of Leco pegasus III is in the ppb range but all I could get is ppm range. Instrument pass optimization with no problem. Is that possible an instrument to pass optimization successfully but not to show the detection limit in the ppb range? If yes what could cause this?

Walsh

Any Idea about this?

Detection limits in "ppb" or "ppm" have so many undefined variables (sample volume for example, and what the Si units are) that they are meaningless.

Peter

Is specification for a specific compound under specific conditions? If so, does it meet that?
If it is just a general claim then as Peter says, the spec is meaningless.

The spec on the new Agilent 7000C we just had installed is to have a calculated detection limit of 4 femtograms on column determined from injecting 1ul of a 10 femtogram/ul standard of Octafluoronaphthalene in MRM mode. It met that spec easily, but when trying to calibrate it for volatiles, I have some problems seeing a 0.5ppb standard when purging 5ml. That is 0.5ng/ml or 2.5ng desorbed then sent through a 50:1 split which is .05ng on column.

So is the spec for the Leco for looking at ppb levels when extracted from 1 L of water into 1ml of solvent then injected or is it for 1ppb in the solvent solution injected? There is a difference of 1000x between those two examples.

Not forgetting of course that whether you are talking in volumes, masses or Moles a can add another couple of orders of magnitude to the fudge factor.

Peter

The specification for the Pegasus is S/N of 10:1 on one of the ions (286 - I think) for 2 pg HCB on column a a peak of a specified width - probably 1 second wide or less. The instrument can fail to meet specification for a number of reasons, starting at the sample vial.

If that sample stands in the GC vial over night, garbage is extracted from the septum and it may coelute with the HCB peak. A bad syringe, cored septum, or loose ferrule on the inlet may allow a significant portion of the sample to escape the system - and you have less that 2 pg on column. And, a fouled inlet liner can result in loss of HCB signal (yes, I've seen it). A degraded column can broaden the HCB peak and contribute silanes - adding noise to the chromatogram. An incorrectly placed column (the end should be 5 - 7 mm beyond the end of the transfer line, if I recall correctly, and cut squarely) will result I a loss of sensitivity. And the detector must be in good condition and the detector voltage needs to be adjusted to optimize the sensitivity. On a new instrument, specification can be met with the detector in the 1400 to 1600 v range. As the instrument ages, the detector voltage must be increased. At about 1800 v, you are not likely to get additional benefit and the next step will be to replace the detector when you can no longer make specification at this voltage.