Method 314.0 Perchlorate

I am running method 314.0 for Perchlorate on an ICS 5000. The parameters are: ICS 5000 Eluent: KOH 45 mM Column: AS 20 Injection Volume: 500 uL Retention time: 14.5 min (16.5 min when new) We run samples from a groundwater treatment system containing Perchlorate. We run Effluent samples (treated groundwater) which are almost always non detect, but lately a peak shows up at where Perchlorate's retention time. These samples have always been run at a temperature of around 24-25 degrees C. I've been getting 5-13 ppb of Perchlorate showing up. After being chilled in the refrigerator for several hours I ran the samples again and then the same samples came out non detect. We always have a small peak showing up to the left of where Perchlorate comes out, but in this situation a peak comes out right at where Perchlorate is and the peak disappears when running the samples at a cold temperature (5 degrees C). I have never seen this before. I composite the samples (7 days' worth) and run at 24-25 degrees C and the result is non detect.

Equipment: ICS 5000