Identification and Imaging of ¹⁵N Labeled Cells with ToF-SIMS

Stable isotope labeling may provide a novel method for tracking stem cells once they have been injected into a human or animal host. Here we present a simple pilot study to determine the potential for using ToF-SIMS to detect and localize ¹⁵N labeled cells in tissue biopsies for use in cell therapy studies. For this pilot study, 3T3 fibroblasts were grown in normal media and in two different media containing ¹⁵N labeled amino acids. Samples containing a mixture of ¹⁵N labeled and unlabeled cells were prepared, fixed and dried for analysis and were then imaged using a bunched Bi₃⁺ primary ion source. The cells containing ¹⁵N labeled amino acids could be readily distinguished using nitrogen containing peaks which have been previously associated with the labeled amino acids. Contrast was sufficient to allow easy identification of labeled cells in both sparsely and densely plated cultures. Multivariate analysis showed that the image contrast could be improved by including peaks originating from characteristic fragments of the labeled amino acids as well as lower mass NH₄⁺ and CH₄N⁺ peaks. Additional work is being pursued to determine and improve the longevity of the label.