using CentroidsList = System.Collection.Generic.List<...>; //put the type of the element of newSpectrum.PeakCentroids

//...

CentroidsList list = new CentroidsList();
foreach (var element in newSpectrum.PeakCentroids)
   if (SomeCondition(element))
       list.Add(element);
foreach (var element in list)
   newSpectrum.PeakCentroids.Remove(element);