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Characterization of biomaterial surfaces requires analytical techniques that are capable of detecting a wide concentration range of adsorbed protein. This range includes detection of low amounts of adsorbed protein (<10 ng/cm[sup 2]) that may be present on non-fouling biomaterials. X-ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) are surface sensitive techniques capable of detecting adsorbed proteins. We have investigated the lower limits of detection of both XPS and ToF-SIMS on four model substrates each presenting unique challenges for analysis by XPS and ToF-SIMS: mica, poly(tetrafluoroethylene), allyl amine plasma polymer and heptyl amine plasma polymer. The detection limit for XPS ranged from 10 ng/cm[sup 2] of fibrinogen (on mica) to 200 ng/cm[sup 2] (on allyl amine plasma polymers). The detection limit for ToF-SIMS ranged from 0.1 ng/cm[sup 2] of fibrinogen to 100 ng/cm[sup 2], depending on the substrate and data analysis. Optimal conditions provided detection limits between 0.1 ng/cm[sup 2] and 15 ng/cm[sup 2] on all of the substrates used in this study. While both techniques were shown to be effective in detecting protein, the sensitivity of both XPS and ToF-SIMS was shown to be dependent on substrate surface chemistry and the organization of the adsorbed protein film. This study specifically highlights the applicability of ToF-SIMS in the characterization of low level protein adsorption. [ABSTRACT FROM AUTHOR]