is smaller injection equivalent to more dilute solution inje

[zimanli]

I had peak fronting. I was injecting 1uL in splitless injection mode. Then I decreased the injection to be 0.4uL and still in splitless injection mode. I had much less fronting. My question is: can I dilute my solute to get rid of fronting?

I think 1uL should be normal injection size. Why do I have fronting? Thanks a lot.

[chromatographer1]

Not knowing the concentration of your present sample, I will agree that your suggestion to reduce the concentration will help the fronting overload condition you are experiencing.

Why is 1µL necessary in your opinion? Why not 0.5 or even 0.25µL?

Is your autosampler limited to 1µL injection size?

Reduce your concentration by a factor of 10 or perhaps 5 and see if that meets your requirements.

Good luck,

Rod

[zimanli]

Thanks Rod.
1uL is special because most of the literatures injected that amount. I thought it is normal. I am doing manual injection.

[chromatographer1]

I would inject 0.2µL or any amount you can do reproducibly manually.

Dilute if you feel it necessary. A Chaney adapter will be of great value to you. Consider purchasing this inexpensive tool.

best wishes,

Rod

[KC]

To tell you the truth I would stay with 1uL injection and fix your peak. If you have the sensitivity diluting your solution may help a little but I think you will still have characteristically the same peak (unless you are shooting real high concentration). Like chromatographer said, what’s the concentration? If I was sure I had the right column and knew it was in good shape I would still try another or at least clip ends and reinstall as well as change liner and septum. Using single gooseneck? Try double. Have wool? Try without. I’d also pay real close attention to exactly what the distance was from ferrules to column ends when I took it out. Did you put it in and sure its right? Even if I was not going to change final solvents (because of method), I would still stick my analyte in another solvent and try it for clues. What is the solvent? Whats it look like if you up inj temp 20C…. or lower it? See a trend? Flows and head pressure right? God the list is endless but something else is wrong, you should be able to shoot 1 uL.

[ntruong]

To solve your problem, I would suggest you use the split mode. The reason the split mode was designed is to prevent overloading the column, which normally resulted in front peak. Keep your 1uL injection volume, but play with the split 100:1, 50:1, 20:1,etc. until you get a symmetrical peak (1 is ideal). By the way, why use manual injection? If you want to inject 0.2uL, check to see if you have nanoliter adapter installed, then configure for 10uL syringe. For 0.5uL injection volume, configure it with 5uL syringe.
Good luck,

[Peter Apps]

Fronting peaks are ones that rise slowly to the maximum and then fall very quickly. They are caused by overloading the column, in other words introducing too much of the analyte to the column.

This is one of the easiest of all problems to fix - the only thing that you have to do is reduce the quantity of analyte that is introduced to the column.

Overloading is worse when there is a "mismatch" between the polarity of the column and the polarity of the analyte. Small quantities of polar compounds such as alcohols and free acids give fronted peaks on non-polar columns when the same quantity of e.g. hydrocarbons or esters give symmetrical peaks.

In this case, unless there is a good reason to use splitless injection, use the inlet splitter to reduce the quantity of analyte that is introduced to the column. The repeatability of injection volume gets rapidly worse at volumes below 1 microlitre.

If your analytes are alchohols or acids it would be a good idea to use a polar column, one of the wax phases perhaps.

Peter

[KC]

I believe this may be why chromatographer and myself asked about the concentration. If your fronting peak is due to overloading the column then indeed it is one of the easiest problems to fix. However, this is not the only reason you may have a fronting peak. Using a deteriorating or the wrong type of column for the analyte (or one installed improperly) may also have this effect well below a concentration that is overloading the column.
It sounded to me as though Zimanli is following a method where someone else successfully chromatographed this analyte, at this or similar concentration, with this type of column, with a 1uL splitless inj, and without a fronting peak. If this is the case and he is looking at previous chromatography that appears fine and he cannot understand why his is fronting then what is the problem? Something else is wrong. Reducing the mass on column or running split is not going to fix it. I typically run just about everything splitless with a 1 or 2 uL inj and if I go to even a 10:1 split my analyte is gone. What if he cannot give up the sensitivity?
If we assume he is following conditions that should work because they have in the past, then something else is wrong?
This is why I mentioned a number of other factors that may be involved. It sounds to me as though it is the condition of the column or injector condition and/or parameters.
Think maybe if the last guy using the instrument shot 500 dirty soil extracts and never changed the liner it could be the problem?

[Peter Apps]

I think that we have a confusion of terminology here.

A fronting (front tailing) peak has a slow rise to its maximum, and a rapid decline. The "tail" is in front of (earlier than) the peak maximum. The front edge of the peak is very often almost a straight line and so the peak as a whole looks like a neat triangle. As far as I know the only way to generate such a peak shape is by concentration overloading the column.

Column deterioration, dead volumes, turbulent gas flows, dirt in the inlet, poor connections and installation etc etc etc all give peaks where the tail is after the peak maximum. The leading edge of the peak goes up more quickly than the trailing edge comes down. (Nearly ?) always the tailed back edge is curved (concave outwards) becuse the peak drops more and more slowly as it gets closer to the baseline. This is called peak tailing.

Tailing gets worse for smaller peaks. Fronting gets better when you decrease the column loading (smaller peaks), as Zimanli did when he (?)decreased injection volume and saw an improvement in peak shape.

Peter

[Rafael Chust]

I fully agree with Peter. Although we can discuss several "logical" reasons for peak fronting, 99.995% will be due to column overload...