I'm a new LC-MS user and my guide told me that I should not infuse a solution at a concentration above 0.01 µg/µl for tuning the MS Detector. He insisted, saying that in 5 years experience he always uses 0.01 µg/µl solution and this is the generally found optimum. I just read a paper where a 0.1 µg/µl solution was used. I don't know if that concentration was found as an optimum for that particular work and I would like to better understand the problem I will get if I infuse more concentrated than needed. And also, I want to know what is the criterion to decide for the right (optimum) concentration.

That depends on what you mean by "tuning", as I've heard different uses of this term. For calibration of your mass analyzer (mass and resolution), you should use the tuning solution recommended by your instrument manufacturer.

If by "tuning" you mean optimization of your instrument parameters for a given analyte, it depends on how much sensitivity you get for that analyte. I find that 10 µg/mL = 0.01 µg/µL is a good place to start. If the solution is too concentrated, you'll saturate your detector and get an ugly full scan spectrum. Also, when doing SIM or MRM on the analyte and ramping your lens voltages, you'll get a very broad optimum again due to detector saturation. If this happens, just dilute the solution and try again. If 10µg/mL is too dilute, you won't get enough signal to optimize on, but this may also mean that your analyte is a poor candidate for your chosen ionization method.

Thank u MG for ur comment. Yes, I really meant optimization. Now another concern is to decide that the detecor is saturated and that my spectrum is ugly; because, I'm not used to this technique

If you're acquiring profile data, one thing you might see if your detector is saturated, is that your mass peak will be either flat or jagged / misshapen at the top, rather than appearing gaussian as it should under normal operation.