Anyone have experience with GC7890 dual channel, FID for TPH using LVI?
Have you looked at US EPA 3511? https://www.epa.gov/sites/production/files/2015-12/documents/3511.pdf
It uses 40mL EPA sample vials with a single in-vial extraction of 35mL of water with 2mL of DCM. Other suitable solvents like hexane or pentane can be used. I did a fair bit of work in the 1980s using similar methods and similar volumes with similar single extractions. Solvents that worked well were Freon 113 (Pre Montreal protocol!), DCM, pentane, hexane, CS2, and CCl4. I used cool Grob on-column injections of up to 10µL on GC-FID and GC-SectorMS instruments, and later a PTV. Some of the work was on GROs with the volatile solvents, but mostly diesel/lubricating oil with Freon and DCM. We also went up to C60+ with CS2 with GC-FID.
US EPA 3570 is a generally similar micro-scale method for soils etc: https://www.epa.gov/sites/production/files/2015-07/documents/epa-3570.pdf As you are are looking at C10-C40 the potential loss of volatiles is less important and you should not need the KD concentration in 3570.