Pressure drop in between injections in 1200 LC

Question

Facing the pressure drop issue in two inections out of 10 injections. the pressure dropped from 100 bar to 50 bar at 5th minute and again came to 100 bar at 6th minute. it was happened same at different time interval in other injection. total run time is 10 minutes. here customer expecting the exact root cause. 
 verified the inline filters are in good condition, there no leakages in flow path. 
 Performed the Pump Leak test, pressure test are Passed. 
 verified the mcgv function also good. 
 
 here needed probable root causes for resolving the issue.

fgunawan · Answer

Hello pathagun , 
 
 First thing you can double check is if any bubbles are present in the solvent lines between the solvent bottles and the degasser or the pump. You can also check if the solvent inlet filters in the bottles are blocked as these can generate bubbles from the filters themselves. You can try purging the pump at a higher flowrate of 3 mL/min 100% of 1 channel at a time to see if any bubbles come out of the filters. 
 Next thing is the pump back pressure ripple % visible within the pump section of acquisition SW. Ripple should be below 2%. 
 Just in case there is an intermittent leak from the pump inlet and/or outlet valves, or the pump piston seals, you can repeat the pump leak test a few times to make sure they consistently pass. If there is consistently no leak in the MCGV, we can try a few things with the pressure test. With the pressure test that passed, where was the blank nut placed? If you haven't tried the pressure test with the blank nut in the column compartment already, please try this. 
 - Please confirm that the autosampler 6 port valve is in the mainpass flow path, then place the blank nut at the outlet of the column compartment's heat exchanger and run the pressure test. - If this passes, it means your system is leak tight up to the column compartment heat exchanger. We can then focus on diagnosing other areas (check MCGV again, try another column and etc.) 
 - If failed, there may be a leak around the blank nut, sampler needle and needle seat, sampler rotor seal, column compartment heat exchanger, or any of the capillary connections between pump purge valve and column compartment heat exchanger. Another potential reason for failed pressure test is air trapped in the flow path. To troubleshoot, you can take off the blank nut, connect capillary from column compartment heat exchanger to waste beaker, flush the system with 100% fresh LC grade water for 10 mins, then flush with 100% LC grade IPA 10 mins. You may then return the blank nut to the outlet of heat exchanger and repeat the pressure test selecting the channel containing 100% IPA. 
 - If the test still fails, repeat pressure test with 100% IPA and blank nut at outlet of autosampler (Port number 6 of the six-port injection valve). If this fails as well, the leak is most likely within the sampler (needle, needle seat, sample loop, and/or rotor seal. If this time it passes, leak may be around the column compartment (column compartment valve if installed, heat exchanger or capillaries between sampler and column compartment heat exchanger).