diff --git a/src/core/peptideevidence.cpp b/src/core/peptideevidence.cpp
index d0eb2f59ce4b2a60035ea5f55c456798056c8e69..ee20bb8785c3f09c7ee66dca000bd13a539f7ffa 100644
--- a/src/core/peptideevidence.cpp
+++ b/src/core/peptideevidence.cpp
@@ -126,8 +126,8 @@ pappso::mz PeptideEvidence::getDeltaMass() const {
 pappso::mz PeptideEvidence::getPpmDeltaMass() const {
     //return (_peptide_sp.get()->getMz(1) - (_exp_mass+pappso::MHPLUS));
     pappso::pappso_double diff = getDeltaMass();
-    while (diff < -0.5) {
-        diff = diff + pappso::DIFFC12C13;
+    while (diff > 0.5) {
+        diff = diff - pappso::DIFFC12C13;
     }
     diff = (diff / getPeptideXtpSp().get()->getMz(1)) * pappso::ONEMILLION;
     return diff;