My LC is 1260 Infinity II with 100uL loop and MassHunter Acquisition 1.2.
I am working on a method that uses standard additions for quantitation. The column is HILIC but diluent is 80% aqueous, so dilution in acetonitrile is required along with the standard additions. It would be very nice if we could automate this part of the sample prep. I have tried doing dilutions/additions using the injector program both in a second vial and directly in the needle (examples below). In the examples below, p2-a1 is acetonitrile and p2-a2 is diluent. The sample is diluted 1/5 in diluent, and then diluted 1/5 in acetonitrile (or initial mobile phase which is mostly acetonitrile for the first example). The examples do not include standard additions yet, but they would be added during the first dilution.
The problem with the first example is that precision is VERY poor (%RSD > 15% for 5 reps). I am guessing that the sample is not being mixed thoroughly by bubbling air through the sample after everything is added to the vial.
Precision is better with the second example, but the injection volume is very large resulting in poor peak shape. I can't find any way to inject only a portion of what is in the needle, and can't decrease the total volume because 1.5 uL is already very small volume.
Any suggestions on how to make this work are greatly appreciated.
Dilution/Addition in Vial:
Dilutions/Additions in Needle:
