tailing issue for one peak type

It is an inertness problem and maybe something else. Be sure your injector is at 300 °C (low vapour pressure of your analytes) and what about your starting temperature? Is it maybe too low, so that you get condensation at the beginning of the guard column? (BTW real guard column or retention gap?)

Anyway, everything UltraInert is good for this.

Regards,

Norbert

thank you, see below the last 3 runs of the LIS solution. the black one is the most recent and is developing a lump after the main peak. the guard column I am using is 10M ultimate plus deactivated fused silica tubing (part CP805310) connected to 30M DB-5 530mm ID main column. is this guard column the correct one to be using? is there other guard columns that provide better inertness? the starting temp of the oven is 35C. 

thank you, see below the last 3 runs of the LIS solution. the black one is the most recent and is developing a lump after the main peak. the guard column I am using is 10M ultimate plus deactivated fused silica tubing (part CP805310) connected to 30M DB-5 530mm ID main column. is this guard column the correct one to be using? is there other guard columns that provide better inertness? the starting temp of the oven is 35C. 

This is very long for a guard. What is your injection technique and volume injected (and what solvent)?

There is also the chance that your lump is something late eluting from an earlier run.

injection volume is 1ul and it is splitless. flow is around 5.5ml/min. unfortunately there is not much in terms of the method i can change as it is validated with set parameters. so you think the guard is too long and would benefit from a shorter length possibly? 

the solvent is acetonitrile

With ACN, even in the worst case, 2-3 m retention gap would be sufficient. That is quite some flow, so I assume you use a 0.53 mm ID column. Still I think it's more something coming off from a previous run. Just to check you might want to increase your final temp step by 20 minutes, just to check. Do 3 runs and see if the lump still elutes.