if you lower the concentration your signals will get lower too. I would suggest some kind of enrichment strategy depending on the compound you are looking for

Dear Tobias,

What do you mean by enrichment strategy?

Your system has an injection loop of some fixed volume installed (e.g. 50 uL). A partial loop injection is something less than 50 and in general one keeps it less that half the loop volume (e.g. 25) to get good reproducbility. Note: If you do a full loop injection most autosamplers will sample 3 to 4 times the loop volume to get good reproducibility(e.g. 150-200 uL) but your column will only see 50 uL. Now if you were injecting 10 uL of concentration X, in theory you can inject 20 uL of concentration X/2, but it depends on the chromotography. Double the volume could lead to a column overload and poor recovery of your analyte or in MS maybe signal suppression and again poor recovery. Enrichment that Tobias suggests means some way of concentrating your sample in a smaller volume (high concentration) while often removing interfering compounds.

I agree with all the above and think it is generally better to inject a higher concentration with a small loop than to use a bigger sample loop and a low concentration. The larger the loop the wider your sample application becomes and therefore you will get less resolution, conversely a smaller loop and higher concentrations will give sharper, higher and more symmetrical peaks and better sensitivity. While column or detector overload can occur if the concentration is high enough, a very large injection of a very low concentration can, in an extreme case, cause peaks that are low in intensity, broad and with flat tops due to the width of the sample application onto the column. Partial filling of the loop is less precise than using the full loop and if smaller injection volumes are desired, rather change to a smaller loop.

Enrichment for example by solid phase extraction. The phase to use depends very much on the compound you are looking for.

So: what are you looking for?

Agree to most of it is said above. Regarding enrichment: for example if you extracted 1 ml plasma and you reconstituted the dried extract in 0.1 ml of your reconstitution buffer it would give you a 10 fold enrichment. You must make sure that the enrichment step is not affecting the impurities in the region of your interest b/c this step would increase detection of desired and could also for undesired analytes in your sample.

If you inject more samples rather than your loop capacity, your system pump is automatically stop. So, never inject overload samples in sample loop. If your pump is stop, you need to re -inject only mobile phase solvents less than loop capacity and turn on pump immediately.

I agree with all of the above statements. Additionally, you should remember that the sample dilution / large volume may have an effect on the column mobile phase (I.e. pH shift, organic composition, etc.). I have found that a large o/c vol. Injection will work if you have a high column flow and use similar mobile phase to dilute the sample. I have used 100 ul injection with an approximate 1.0 ml/min.

preconcentration methods seems a good choice in such circumstances. you need to enrich your analyte as suggested above.