3.3. LC–MS
To avoid the desalination procedure of sample in LC–MS
analysis, we chose NH4OH instead of NaOH solution in the process
of PMP derivatization. It not only simplified the experimental
process but also reduced the loss of sample. The LC–MS total ion
chromatogram in negative mode and the characterization of the
peaks were shown in Table 3, respectively. Eight peaks were all
with 132 Da mass smaller than the peak ahead, which was the
exact molecular mass of a xylose residue. According to the molecular
mass, Peak 2 to peak 6 were confirmed to be X2–X6 tagged
with two PMP. The mono-charged molecular ion m/z 1405.7 in
the positive ESI-MS mode (Fig. 3C) and m/z 1403.5 in the negative
ESI-MS mode (Fig. 3D), indicating that the molecule mass of peak 8
was 1404 Da. Thus we confirmed that the peak 8 was X8 tagged
with two PMP. Similarly, peak 7 was confirmed to X7 tagged with
two PMP (Fig. 3A and Fig. 3B), and peak 1 was confirmed to Xyl
tagged with two PMP. Therefore, the proposed HPLC method was
demonstrated to separate Xyl and XOS with DP of 2–8 with
satisfactory resolution.