Based on the signal to noise ratio, the detection limit for methylene chloride was determined as 7.5 pmol or 700 pg total injected substance (Carlo Erba GC with ECD 400/Ni-63 source) Is this normal or are lower detection limits possible?

My limit with Hewlett-Packard Ni-63 ECD was about 50 pg at the detector (specific response of methylene chloride relative to carbon tetrachloride was 0.006). Your message says "total injected" so I am not sure if that is comparable. Strictly speaking to compare detectors, sensitivity should be in pg/second. On my system that was about 10 pg/second.