The value you are speaking of refers to a deficit in the amount of carbon 13
relative to a standard.  The unit is in per mil or parts per thousand.  Your
example of -23.5 would mean that the sample is 2.35 percent depleted in C 13
relative to the standard.  I am not sure what standard they are using.
Probably an NBS standard.  Older literature may cite the Pee Dee Belemnite
standard.  The test should be able to detect alteration due to the addition
of Cane sugar which is less depleted in C13 than most nectar sources.
 
Jay Jones
[log in to unmask]