qPCR standard curve slope (m) - (May/31/2012 )

I am using quantitive Real Time PCR for detection of etiologic agents from patients samples. Our current target is 33 pathogens. One company has provided us plasmids to be used as positive control. For each target we need to run standard curve per quater month and use the slope (m) and Y-intercept (c) for quantification of our samples.

For each target we are getting different slope (m). My question is "what is the ideal range for slope of qPCR standard curve?"

Surely it is 1.

I just find it.. The ideal slope should be -3.3. But the range is -3.1 to -3.6. But my problem is as I am dealing with 33 targets for some the slope is above -3.6. As the slope indicate PCR efficiency, I am worried about efficiency of my qPCR.

Could it be due to any problem with positive plasmids?