THE TECHNICAL ADVANTAGE OF A 3-D CALIBRATION IS THAT IT REMOVES AN OBLIGATION ON THE PART OF A PROBE OPERATOR TO POSITION THE PROBE PRECISELY ON THE DIFFUSIONAL AXIS OF THE ION SOURCE, AN EXACTING AND TIME-CONSUMING TASK IN THREE DIMENSIONS. THE THEORETICAL ADVANTAGE OF THE 3-D CALIBRATION IS THAT IT SHOULD CONFORM TO THE LAW OF CONSERVATION OF CHARGE MOVING THROUGH CONCENTRIC SHELLS ABOUT THE SOURCE. THE ARTIFICIAL DIFFUSIONAL SOURCE OF IONS PROVIDES A PHYSICAL STANDARD FROM WHICH AN OBSERVED DOSE-RESPONSE CURVE AND THE EFFICIENCY OF THE PROBE CAN BE ESTIMATED. THE CONCENTRATION OF THE ION OF INTEREST CAN BE PREDICTED KNOWING THE RADIAL DISTANCE FROM THE SOURCE TO THE MEASUREMENT POINT. SUCH A SOURCE HAS BEEN USED TO CREATE A CALIBRATION IN A SINGLE DIMENSION. A CORRECT 1-D CALIBRATION IS ONLY OB-TAINED WHEN MEASUREMENTS ARE MADE ON A DIFFUSIONAL AXIS EMANATING FROM THE SOURCE. UNTIL NOW THE 3-D VERSION OF THE NVPI HAS BEEN CALIBRATED BY USING THIS ARTIFICIAL SOURCE AND APPLYING THE 1-D PREDICTIVE EQUATION. HOWEVER, A METHOD INCORPORATING ALL OF THE STRENGTHS OF THE 3-D APPROACH WOULD NOW BE BENE-FICIAL. THE DERIVED EQUATIONS CAN BE APPLIED TO BIOLOGICAL DATA COLLECTED IN A FEW PLANES TANGENTIAL TO THE SOURCE SUCH THAT A POINT AND A DISC SOURCE OF IONS CAN BE DISTINGUISHED. SUCH SOURCES ARE VERY OFTEN QUITE WEAK, SO A SIMPLE ONE-DIMENSIONAL TRANSECT AWAY FROM A SOURCE DOES NOT YIELD MUCH DATA.