Question about UV response

[HPLCaddict]

[quote="M. FarooqI am afraid yes . System peaks arise due to the presence of analyte in the sample, competing with the species present in the eluent. The insidious system peaks can only be properly assigned when we are either using several detection modes or more easily, couple of wavelengths. But this is more of an academic curiousity. In principle, there should be no system peak, when the composition of the blank is identical as the eluent.
[/quote]

I think we're using terms in different manners. For me, "system peaks" (some call them "ghost peaks")refer to anything that's visible during a blank run (blank meaning no injection - some are referring to solvent injections as blanks). In isocratic elutions, you usually don't have any system peak. With gradient elution, they're quite common due to traces of eluent impurities that accumulated on the column at the beginning and elute when the eluent gets strong enough.
Any peak visible with a solvent injection that's not a system peak as described above I'd call a solvent peak. This is mainly at t0 ("Solvent front", "t0 noise"), but may be also visible later in the chromatogramm.
The thing you described ("System peaks arise due to the presence of analyte in the sample, competing with the species present in the eluent") sounds what I would call "vacancy peaks". They might be positive or negative depending on the absorption characteristics of the analyte and the eluent and the detection wavelength. I think this is what you were referring to?

[M. Farooq]

[quote="M. FarooqI am afraid yes . System peaks arise due to the presence of analyte in the sample, competing with the species present in the eluent. The insidious system peaks can only be properly assigned when we are either using several detection modes or more easily, couple of wavelengths. But this is more of an academic curiousity. In principle, there should be no system peak, when the composition of the blank is identical as the eluent.

I think we're using terms in different manners. For me, "system peaks" (some call them "ghost peaks")refer to anything that's visible during a blank run (blank meaning no injection - some are referring to solvent injections as blanks). In isocratic elutions, you usually don't have any system peak. With gradient elution, they're quite common due to traces of eluent impurities that accumulated on the column at the beginning and elute when the eluent gets strong enough.
Any peak visible with a solvent injection that's not a system peak as described above I'd call a solvent peak. This is mainly at t0 ("Solvent front", "t0 noise"), but may be also visible later in the chromatogramm.
The thing you described ("System peaks arise due to the presence of analyte in the sample, competing with the species present in the eluent") sounds what I would call "vacancy peaks". They might be positive or negative depending on the absorption characteristics of the analyte and the eluent and the detection wavelength. I think this is what you were referring to?[/quote]


You are right. There is apparently no established definition of system peaks (or vacancy peaks as you call them), but in the theoretical literature they are related to equilibrium disturbances with reproducible shapes and positions. Ghost peaks would refer to something unexpected or irreproducible. I was told, in esoteric terminology, to refer to injection/ solvent peaks as eigenpeaks. I think the concept matters rather than semantics.

[lmh]

Off-topic: No offence to anyone here, but I personally think semantics matter a great deal.

(1) If everyone calls things by different names, no one can understand anyone else.
(2) Excessively complex terminology clouds thought.

[M. Farooq]

Off-topic: No offence to anyone here, but I personally think semantics matter a great deal.

(1) If everyone calls things by different names, no one can understand anyone else.
(2) Excessively complex terminology clouds thought.

True, there are 10 more names for system peaks in the literature. Hope someone realizes the need of systematizing the chromatographic nomenclature like IUPAC. Probably the reason is that chromatography is still largely empirical despite its age.

[Klaus I.]

I see two issues with peak area comparison of a sample with UV under different conditions and using different mobile phases.

No different mobile phases were used, when I made this observations. We are talking about the influence of the sample solvent.

[zheyin]

A 0.2 mg/mL solution of an API in MeOH gives me 100000 area response, in buffer pH 5.7 gives me 80000 and the API is soluble in both at concentrations far greater than what is being prepared, what is this? I know there is a technical name for this. On the other hand, a dilution of the API in MeOH into buffer pH 5.7 (giving 1% MeOh content) gives me 100000 area response, same as in methanol, what is going on? It can not be a solubility problem as the compound is at least 10 times more soluble in buffer pH 5.7 than the concentration that is being prepared, please any help would be really appreciated?

What devices do u use for transferring liquid?
Pipettors? The tips are not good at holding methanol due to its low surface tension causing the volume inaccurate each time.
Try volumetric flask to compare response btw meoh and buffer.