...Perhaps I can help. To start things off, I'm mainly writing this because initially I had a lot of trouble with it and upon looking up some stuff, so did others. In the end I suppose it wasn't hard to solve but it's not my place to decide that for others and if they need the information, I believe it should be readily available.
So, a few things to note.
1) The sequence is different for everyone, so the solution I give is for me only. This is just to show my thinking in working it out and hopefully you can follow suit.
2) Scan every mutagen (and I will be assuming you have them all - Echo 1-4 / Solis 1-2 / Helicon 1-3 / Io 1-3 / Ovid 1-3 / Exitus 1 for a grand total of 16). To do this, allocate power to the Hollow Earth Research Complex and enter the building with the security camera on the outside. You can find the sequences at the combinator past the gate however this will not tell you positive or negative atoms and they're a massive help to know. Plus you need to go there to scan Exitus as you cannot dump that into the combinator.
3) Write all the sequences onto a piece of paper. You won't regret it.
4) The aim of the game is to replicate the sequence of your Exitus 1 mutagen. It is not enough to simply walk into mordor have the correct atoms, but to get them in the right order too.
5) Compounds (the final product, essentially) can only consist of unique positive atoms.
6) Adding a positive or negative atom will nullify previous entries in sequences.
So for example, If you add mutagen A with the atoms A1, A2, A3 and then add mutagen B with B1, B2, -A3 then you'll end up with A1, A2, B1, B2.
7) You can use your mutagen reagents multiple times to add back in an atom that was previous nullified (or to nullify an unneeded atom).
8 ) When adding mutagens to a sequence, if there are negative atoms and it doesn't nullify a previous atom, you can consider it as non-existent.
Next I'm going to list all my mutagen sequences and follow that up with my process of elimination.
Exitus 1 - S1 VZ L1 PT Q2 CT GY Q5 IL VH Y8 FX
Helicon 1 - L1 PT Q2 CT GY -D4 -AN -YI
Helicon 2 - Y8 FX Q2 L1
Helicon 3 - WR Q5 D4 VH -A9 -VZ -YI
Io 1 - FX Y8 CT IL RT -Q2 -AN
Io 2 - S1 VZ D4 -Q5 -HV -Y8 -PT
Io 3 - L1 HV VH -VZ
Solis 1 - Q5 Q2 L1 AN -WR -GY -IL
Solis 2 - VZ CT D4 -A9 -HV
Echo 1 - Q2 AN S1 HV
Echo 2 - RT VZ IL Q5 -WR -FX -VH -PT
Echo 3 - Q5 Q2 WR VZ
Echo 4 - PT VH Y8 FX -A9
Ovid 1 - RT VH PT WR A9 CT Y8 -D4 -FX -S1 -GY
Ovid 2 - A9 IL L1 -QT -WR
Ovid 3 - Y8 RT VZ QT -L1 -IL
Okay. The first thing I noticed was an abundance of RT atoms but no -RT. This immediately eliminates any mutagen containing RT as my Exitus sequence does not contain RT and I will therefore have no way to remove it once added (unless I clear the sequence and remove all of my work)! This means we need to ignore: Ovid 1 / Ovid 3 / Echo 2 / Io 1. Now I can work out the sequence using this list:
Helicon 1 - L1 PT Q2 CT GY -D4 -AN -YI
Helicon 2 - Y8 FX Q2 L1
Helicon 3 - WR Q5 D4 VH -A9 -VZ -YI
Io 1 - FX Y8 CT IL RT -Q2 -AN
Io 2 - S1 VZ D4 -Q5 -HV -Y8 -PT
Io 3 - L1 HV VH -VZ
Solis 1 - Q5 Q2 L1 AN -WR -GY -IL
Solis 2 - VZ CT D4 -A9 -HV
Echo 1 - Q2 AN S1 HV
Echo 2 - RT VZ IL Q5 -WR -FX -VH -PT
Echo 3 - Q5 Q2 WR VZ
Echo 4 - PT VH Y8 FX -A9
Ovid 1 - RT VH PT WR A9 CT Y8 -D4 -FX -S1 -GY
Ovid 2 - A9 IL L1 -QT -WR
Ovid 3 - Y8 RT VZ QT -L1 -IL
Now, my Exitus sequence starts off with S1. Looking at my list, the only mutagen that has this is Io 2. Luckily, the next atom in the sequence (VZ) is also correct. So far it looks like [green for correct, red for incorrect]:
S1 VZ D4
Because Q5/HV/Y8/PT are negatives, they do not matter and hence why they weren't listed (read point 8 above). We only need to worry about D4. So we have to find a mutagen that not only has -D4, but also contains the next atom in the sequence, which is L1. Out of my list there is only one mutagen that contains a -D4 and since we've established Io 2 has to be correct as a starting point, this must be the next mutagen. This mutagen is Helicon 1. My sequence will now look like:
S1 VZ L1 PT Q2 CT GY
What luck - all 5 of those atoms were exactly what I needed and it removed that pesky D4. Next in my sequence I need Q5. Looking at the list, Echo 3, Solis 1 and Helicon 3 all contain Q5. I don't really want to add Helicon 3 because of the -VZ, so we will rule that out. With a 50/50 chance I picked Echo 3, and my sequence ended up as:
S1 VZ L1 PT Q2 CT GY Q5 WR
Now I have to try and get rid of WR and add in IL. Looking at my list, Ovid 2 is the only mutagen that I haven't blacklisted that contains IL and also handily has a -WR. The sequence will now look like:
S1 VZ L1 PT Q2 CT GY Q5 A9 IL
We got IL but it added an atom we didn't want, A9. So now we need a mutagen that removes A9 and hopefully can add the final atoms (VH, Y8, FX). Looking at the list, Helicon 3, Echo 4 and Solis 2 all contain -A9. I therefore knew immediately that the final mutagen must be Echo 4 because it has: -A9, adds in VH, Y8 and FX and the PT was a previous positive entry so can be disregarded. This gives me the final sequence of:
S1 VZ L1 PT Q2 CT GY Q5 IL VH Y8 FX
Io 2, Helicon 1, Echo 3, Ovid 2, Echo 4. AKA Exitus 1.
I won't pretend to know tonnes about this but I hope this can help point people into the right direction like it did me and I'll try to answer questions. Thanks for reading.
