Hi, can you give some numbers, please?

Georg

if you was selecte perfact column, I suppose due to solvent affect, may be if you reduce the solvent quantity in prepartion for the low concentration will be get about good results.

Hi Rajeev,

I have observed distortions on the peak shape in high concentrations of an analyte. Frequently, if the concentration is too high the peak is asymmetrical due peak talling. Are you sure that the peak you are focusing is really your target analyte? When you say that the RT significantly differs do you mean exactly what?

it happens because the adsorption isotherm of the analyte is not linear at high concentrations...if the isotherm is Langmuir like at high loadings its slope, that represents the distribution equilibrium constant, decreases, hence the higher the concentration of the injected analyte, the lower the retention time will be, and....the higher it asimmetry factor will be! this phenomenon shoud be reduced if you work with reasonable concentration (try to dilute the sample)

There are several potential reasons for the observed shift in chromatographic retention time. It is likely that the observed shift is due to a combination of factors.

1. If you were using a selective detector (e.g., ECD), you could only be observing a small portion of the analytes in a sample. If your chromatographic peak of interest is eluting on the tail of a larger (undetected) peak, you may oberve a retention time shift with analyte concentration. The magnitude and direction of the observed RT shift will depend on both the observed analyte and undetected analyte peak concentrations. As you are using an MS detector, this should not be an issue.

2. Vapor volume - increasing the concentration of the solutes (both analytes and matrix) in a sample can effect mass transfer of the bulk sample to the column, resulting in RT shifts (higher concentration causes shift to later RT).

3. Organochlorine pesticides commonly interact with active sites in the GC system. At low concentrations, the bulk of the molecules in a chromatographic peak will have some interaction with these sites, shifting the retention time. As the concentration increases, the bulk of the molecules in the chromatographic peak do not interact adsorptively with these sites and the retention time becomes stable. Adsorption would be characterised by later retention times increased peak tailing for lower analyte concentrations.

4. Chromatographic peaks deviate from the theoretical gaussian peak shape due to adsorption of the analyte to active surfaces in the inlet and column. and the stationary phase or column support.

As a result, almost all peaks are asymettric to some extent and the actual peak shape is best modelled using an alternative mathematical model such as the exponentially modified gaussian (EMG) peak model.

The effect of this asymmetry is a difference between the observed (modal) retention time (Tr) and the mean retention time (Tm) - the difference between these being the EMG time constant (Tau).

Since both the asymmetry factor, Tr and Tau for a peak are fixed, increasing the peak area (e.g., with analyted concentration) of an EMG peak results in a shift in the modal retention time (Tr), for a given peak. In theory, the mean retention time equates to the retention time for a gaussian peak or true retention time; though in practice, this generally

Chromatographic Integration Methods by Norman Dyson (RSC chromatography monographs series) explains the EMG peak model in detail.

To minimise problems, ensure you are using the optimal phase thickness (and phase ratio) for your application, use good quality MS grade columns (try different manufacturers columns (e.g., Restek, Agilent) and use deactivated inlet liners (SGE quartz wool deactivated FocusLiner; packed unless your applications demands an open liner; consider Siltek coating for really active analytes). Also, ensure the column is inserted to the correct depth in the inlet and cut cleanly and square.

I agree with Teresa. You are probaly injecting high concentrations and the stationary phase of the column gets saturated. Are the peaks bell shaped?

Are you sure you are not overloading the column?

Question is how much? One of the reason: Most of the chromatographic separation systems (GC, GC/MS, etc.) are using a retention time calculation from the top of the peak. You suppose that if you have two different concentrations for a compound. Top of the concentrated compounds peak will be reached later than less concentrated one. If the concentration difference is not so much (like 500-1000 ppb - almost double), it gives almost same Rt. In case of 100 ppb or lower concentration, it is almost 10 times smaller than before and it creates Rt difference. Could you check the start points of the peaks. If Rts of the peak start points are also different, then other parameters should be discussed.

I agree that column overloading is the most likely problem.

Hello everyone,

dear Rejeev Jain, tis is normal in gas chromatography (this lower conc. peaks have lowest RT than peaks from higest conc.) i observe this same in GC, HPLC even column chromatography, its normal. U have much more analytes and this analytes needs more time (migrating trough the stationary phase) or eluting from column. This is normal in chromatogrpahy. Of course its depend from which type of detector u use. It will be different with MS and FID (vacum versus high pressure) , u can try and comparise RT's from this detectors (U will see). The second u can make experiment with on colum injection or if u have Gerstel injector with solvent free mode injection (with concentration). General colegue Nathaniel Hawkins explain almost everything very clear.

Hello everyone!

Thanks for your suggestions. I am agree with Teresa Cecchi and Nathaniel Hawkins. As far as the stationary phase is concerned I am using TG-5MS column which is the best suited column for the organochlorine analytes. I am using PTV large volume injection for the trace analysis of OC pesticides.

Thank u all for your suggestions!!

Thermodynalic nonideality of the analyte might be a reason. In addition, the adsorption isotherms can be effected by sample concentrations as the solvent characteristics change with solute concentrations.

Due to column packing variation and peak eluting height

I agree with Mr Hawkins and Mariusz

Concentration for MS analysis should be around 100 ppm If you use higher concentration your EI filament may get damaged .

@Usha Prasad. Suitable concentration for GC MS analysis is 10-500ppb???

Depends on your instrument

to understand if the retention time shift depends on the isotherm non-linearity under overloaded condition or if it has a kinetic origin (active sites) you may observe the Asimmetry factor with decreasing concentration: in the former case asimmetry decreases, in the latter case it increases; moreover you may change the flow rate: kinetic tailing usually increases with increasing flow rates. I expect that your peak is asimmetric and, at variance with what stated by Nathaniel Hawkins at (#2), you should experience and increased retention time with dilution if the origin of tailing is the isotherm non-linearity, please let me know

I am really sorry but in my world retention shifts should not happen if the same solvent for injection and the same amount of solvent is used.

I experience only shift in retention time if the the composition of solvent is changed.

E:G: Toluene heavily interacts with the column phase, once toluene is injected it is always phase plus tolune you are chroamtographing on. Now if a stock solution is made in toluene and the dilutions are made in hexane, the whole thing might work fine, BUT as the fraction toluene is shifting through the calibration you will indeed experience RT shifts of up to 10-20 secs.

You are saturating your column or there is a leak in your system.

Dear Jain,

Because you are using PTV large volume injection for the trace analysis of OC pesticides. In my opinion, you should think about affects of solvent.

good luck!

I agree with Mr. Hawkins. The lower the concentration the greater the shifts in RT, asymmetry, etc.

As pointed out by other respondents, it would be useful to know the direction of Rt shifts: do they increase or, otherwise, decrease?

If Rt increases with decreasing concentration (all other variables being constant, including the kind of solvent and its amount) it is very probably an active site (adsorpive) interaction problem.

You are either about of saturate the column or you have a system leak

The retention time (Rt) of OC at lower concentration varies due to leakage of gas and your column may be oxidized then first you have to do condtioning the GC MS for at least 8 hrs and clean the glass column. if the same problem persist then your cloumn may need to be new replacement.

I have seen this before, and the reason was that I had saturated the column!! Woops

With PTV injections you have to be careful with both the solvent choice as well as the drying time. If you go too dry (oops!) you will get significant variations in both response factors as well as retention times. If you use the wrong solvent (such as toluene) the results will be all over the map. As noted by many, you could also easily be saturating the column, which I agree with.

Those difference might be due to the mass of the molecules which are carried by the inert gas in the chromatographic column.