I am running hplc/pda for metolachlor and notices that a few of the runs had metolachlor coeluting with other compounds that had similar absorption envelopes but at slightly different peak-maxes. I did a scan across the peak and found that Metolachlor was on the long-retention time of the peak. Have tried longer column (c18 5 micron) to no avail. Gradient is Water (40%) to Acetonitrile (100%). no luck. I have limited sample to experiment with and wondered if anyone has had similar problems?