Table 2: Measurements, limit of detection (LOD) on pure standards, data obtained by LC/MS system no. 1.

Buffer/pH modifier (obtained pH) ionization methodTFA (~2) ESIWithout buffer/pH-modifier ESINH4OAc (~7) ESINH3 (~9) ESITFA (~2) APCI
PositivePositiveNegativePositiveNegativePositiveNegativePositive

FTH-Val (LOD, nM)155,200750472161108.4
FTH-MeVal (LOD, nM)515,900660210180142.74.6
DABTH-MeVal (LOD, nM)3505,0005,10079059013073055
DNTH-MeVal (LOD, nM)1,20012,00013,00012,0002800,0006,100210,000110
PTH-MeVal (LOD, nM)100,00048,00012,000130,00087,0007,80012,00010,000