How appropriate/adaptable is HPLC for in situ sample analysis on Titan?
It is a very good question because this is an active area of research. Having heard talks from the actual people who are making analytical instruments for space analysis, I can mention the actual issues before coming to your actual question so that you are aware of the latest problems. As far as I know, HPLCs have not been sent, but GC systems have been successfully used.

Key problems

(i) Weight. It costs an enormous amount (in millions) of money per kg to send anything in space. Routine HPLCs are very heavy. We need two heavy duty persons to lift the stack of standard equipment.

(ii) Latest HPLCs can now be reduced to the size of a small brief case. Check capillary chromatography. Also check HPLC on a chip. It is the size of a hand palm.

(iii) Elecric power consumption: Think how much power is consumed by the motors in the pump (huge) and other analytical equipment including the mass spec. You don't have lot electrical power in space. This the biggest problem besides weight issues.

(iv) Think of excessive radiation. Can the plastics and equipment tolerate huge amount of radiation from the Sun and elsewhere.

(v) Temperatures: Think of temperature extremes with liquid as solvents.

(vi) There is no HPLC column (or GC column) in the world which can separate everything. May be you would attach at least 7-8 columns at once and see which one separates the sample (having a system with multiple columns at once is commercially available. This is done by pharma people)


  Our motivation for adopting HPLC is that we can keep all substances in
  their original phase for analysis.


Yes, HPLC is a non-destructive technique. Mass spectrometer is not. However, UV-detection is useless for unknowns because you have no clue about their absorption spectra. Even if you have an absorption spectrum, it provides very little information about the full structure. Only mass spectrometer can be used in case of complete unknowns. Keep in mind that a typical HPLC C-18 column can only separate hydrophobic compounds under ideal conditions. The sole job HPLC or GC is to simplify the mass spectrum of a sample. Imagine you inject a sample containing 50 analytes into a MS. Imagine the complicated mess. With an HPLC, only compound will reach the detector one at a time. Mass spectrum is very simple then.


  My question then is, is there a way to use HPLC to identify unknown compounds suspended in the lake without disturbing the state of the sample? Any advice would be appreciated.


Short answer no. HPLC cannot detect or separate suspended matter. If you meant substances dissolved in the lake, yes, then it is possible. Every sample has to be carefully filtered to remove any particles.

Since you have two chemists, why don't you just do a real experiment on lake water.