Most current non-contact-atomic force microscopy (nc-AFM) techniques rely on vibrating the measuring lever at resonance using amplitudes that are large compared to typical interaction length scales. Here we present results of simulations that show that off-resonance, small amplitude AFM provides an alternative non-contact technique in which force gradients can be measured directly without the need of mathematical de-convolution. We show that under a wide range of reasonable conditions the measurements are linear and quantitative. (C) 2002 Elsevier Science B.V. All rights reserved.