Hello,I am currently working on a new UPLC/MS method with 4 analytes. I am having carry-over problems with one of the compounds (the other three have no or neglible carry-over).When a series of blanks was injected after a high calibration standard, carry-over peak with areas approximately 0.3-0.5%, 0.08%, 0.04%, 0.03%, 0.02% of the standard are observed. The carry-over peak area in the initial blank injection is higher than the lowest standard.Initially I was using injection mode PLNO, 25µL inj vol with 50 µL loop. I increased the needle wash volumes to 1000/1500 µL. I tried changing from PEEK needle to SS needle with no significant difference.I have tried various strong needle washes - higher organic, lower organic, higher pH, acidic pH - no significant improvement.I did see slight improvement after changing injection mode to PL(PA) then to Full Loop (20µL loop) (decrease in 1st blank from ~0.5% to ~0.3%).Method conditions are: MP A= 0.1% Formic Acid, MP B = 1:1 ACN:MeOH, WNW = 90:10 Water:ACN, SNW = 2:2:1 ACN:MeOH:Water with 5% formic acidGradient 10-95%B in 1.5 min, hold 95%B until 2 min, then re-equil to 4 min. Flow 0.4 mL/min. There was no significant difference btwn using load ahead (offline 3.0 min) vs no load ahead. Samples are dissolved in 20:80 ACN: aq Formic Acid (no difference in carry-over btwn 0.1, 0.5 or 1%).Peak with carry-over elutes at ~1.3 min.Any suggestions to solve this problem would be greatly appreciated.Thank you.