Yes and no, depending on how old is your system and at which pressure you are operating I believe you will still see a perturbation of the baseline due to the switch of position of the injection valve

The system is very new, the pressure is about 90 bar, and its performance its excellent....the question is theoretical and deals with the perturbation of the system upon injection

In teoria no, ma visto la differenza di pressione tra il loop di iniezione e la posizione che bypassa il loop di iniezione vedrai una piccola perturbazione. 90 bar sono pochi e l'effetto è ridotto.

Ci sono casi in cui avrai comunque picchi di iniezione, ad esempio se inietti un acido come sale e usi chromatografia a scambio ionico/fase inversa.

It is usually a good idea to use the eluent mobile phase to dissolve your analyte. As well as reducing system peak (you will probably never eliminate it completely) it can sometimes prevent missshaped peaks for your analyte especially with ion pairing.

Injecting the analyte dissolved in the mobil phase you can avoid the peaks belong to the solvent if you method is isocratic, but not if you method is gradient.

Dear Teresa Cecchi,

Regardless how many peaks we observe in chromatograms, when we compare our standard peak with sample peak we can easily distinguish between our peak of interest and other (excipients) or in your case the peak of additive that is retained by the stationary phase.

Regards

AMIR HAIDER

yes if your method is isocratic mode.Dissolving the analyte in the mobile phase will reduce system peak to an extent. However if you use gradient elution this will not work since the mobile phase composition changes with time.

It depends on the detector you have in your HPLC...

As has been said before, injecting the analyte dissolved in the same composition as the mobile phase is primarily to improve peak shape, most frequently by limiting dispersion at the head of the column. If the additive is retained by the stationary phase, and is detectable by whatever detector you are using, I would be concerned if I didn't see any sign of your additive. That would mean that the additive may still be on the column or you may have co-elution. Typically, additional peaks can be ignored if you have confidence in the ID of your analyte peak. If the additive peak is causing problems such as interferences, you can either work to improve your chromatography or adjust your detection system (change UV wavelength, ions monitored, transitions etc.) to improve selectivity in order to reduce the impact of the interference.

If you add a small amount and detect by a reaction you might be OK. But the refractive index must change if the addition is a significant amount, depending on how closely it matches the eluant in RI. This is comparable to the magnitude of a blank injection in most cases. So it depends on volume also. And the notes above about sorption/desorption are correct too

Scott Clifton got it all right, but let me complement his answer. First I assume that by system peaks you refer to those ghost peaks that appear in the chromatogram but are not due to your analyte. This is a different thing from the solvent peak. Sometimes these definitions are admittedly a bit arbitrary...

If the additive present in your mobile phase is retained, no matter whether you dilute your sample with the mobile phase: the additive pumped by the system will build up in the column until it can be eluted. This means that a slight difference in equilibration times between 2 injections may already give system peaks of different heights regardless of the injection solvent. Still, if the analyte and the additive are resolved, you are on the safe side.

Not in theory and you will have peaks with less broadering

even if the analyte is dissolved in the mobile phase comprising the additive, the activity of the additive will be slightly different from that in the mobile phase, since the molar fractions would be different, hence according to the equilibrium dispersive model of chromatography I should see the same number of "analyte system peaks" and true analyte peaks (coeluting, and visible only if the detector is selective for the analytes and the additives) and a number of "additive system peaks" that equals the number of additives (1 in my case)...it follows that dissolving the sample in the mobile phase should not prevent their formation. Am I right?

You r right @teresa . when we inject even MP AS blank the system peaks also appear at the starting 2-3 minutes they are unavoidable because they arose from the injector valve closing gap and of the opening valve of column gap ...regardless of sample preparation in MP or in other diluent , but some times when diluent having different ion pairing reagent or salt the peaks are appear , but they can be neglected after running the blank .

It could help, But I don't think you could do much about the valve switch noise, where most of the system peaks come from.

Ya nobody willbe able to clear off them but this perturbation is acceptable as we all know it also appears in blank so can be avoided .

When we inject MP only, getting system peaks in 2-3 mnts...And dissolving analyte in the same MP is right method for good results...

One major question in chromatography process is the sample application. The sample must be preferentially dissolved in mobile phase, in the same pH, same buffer ionic concentration, degas, same temperature, and so on. The disturbance must be at minimum. If you use a additive in mobile phase aiming to release the bound target protein and if this molecule is also detectable, a blank (absence of sample) can to resolve the problem since the addictive molecule no co-elutes with the sample. On the other hand your protein chromatographic profile can be determined by several different ways. Moreover if your addictive molecule can be detected by other system different of, that concerned to proteins the question can be resolved.

Use an internal loop loading valve with a volume of 0.5 microlitres and you will not see a system peak. You will also get a sharper peak.

Minimize: for sure; avoid: perhaps