manual vs autosampler injection

Hi all, 

Quick question here regarding manual vs using an ALS for injection. 

I have been looking at a method to injection MDEA, methyl diethanolamine in a water matrix (unfortunately its an enviromental sample so cant avoid water). 

2 scenarios below that I have seen:

1. Manual injection 1.5 uL gives an area of of ~ 150 for the high standard
2. ALS injection of 0.8 uL gives an area of ~ 12 for the high standard

Both injections are using the same pressure pulsed mode (28 psi until 0.75 min then to 10 psi). Based on calculations 1.5 uL would cause severe backflash due to liner vol being approx 870 uL. 

Can anyone help me understand this scenario ? Do i need to further optimize my ALS injection to increase sensitivity ? Or injecting 1.5 uL is the way to go despite backflash ?

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