Table 1.

Comparison of sample preparation protocols.

Protocol Nr.	1	2	3	4	5	6
Step 1	Dilute 100 times with MilliQ					Dilute 50 times with MilliQ
Step 2	Centrifuge at 14000 rpm for 10 min
Step 3	Filter through 0.2 µm filter	Dilute 2 times with MeCN: MilliQ mixture containing 13C ISTD		Pass through 1 kDa MWCO filter	Pass through 3 kDa MWCO filter	Dilute 4 times with MeCN: MilliQ mixture containing 13C ISTD
Step 4	Dilute 2 times with MeCN: MilliQ mixture containing 13C ISTD	Pass through Isolute PLD+ cartridge	Pass through Isolute NH2 cartridge	Dilute 2 times with MeCN: MilliQ mixture containing C13 ISTD		Pass through Isolute NH2 cartridge