diff --git a/psm_utils/io/xtandem.py b/psm_utils/io/xtandem.py
index 5305f6c..6b4358b 100644
--- a/psm_utils/io/xtandem.py
+++ b/psm_utils/io/xtandem.py
@@ -163,7 +163,7 @@ def _parse_entry(self, entry, run: str) -> list:
                     score = -np.log(peptide_entry[self.score_key])
                     if self.score_key == "expect"
                     else peptide_entry[self.score_key],
-                    precursor_mz = entry["mh"] - mass.nist_mass["H"][0][0],
+                    precursor_mz = entry["mh"] / entry["z"],
                     retention_time = entry["rt"],
                     run = run,
                     protein_list = [ protein_entry["note"] ],