ToF-SIMS quantification of albumin adsorbed on plasma-deposited fluoropolymers by partial least-squares regression

Abstract

Albumin adsorbed on a radiofrequency glow discharge (RFGD) thin film from hexafluoropropylene was characterized by x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The amount of protein adsorbed as a function of the protein concentration was estimated with XPS by following the atomic percentage nitrogen compared to fluorine, carbon and oxygen. The amount of albumin increases steeply between 10−4 and 10−3 mg ml−1 and very slowly thereafter. Intensities of protein-related peaks in the ToF-SIMS spectra are influenced by a matrix effect induced by the fluorinated surfaces. This effect is most pronounced at the lowest protein concentrations. The logarithms of the intensities of the fluorine-containing peaks decrease linearly with increasing protein concentration. Multivariate analysis was used to correlate the data obtained from ToF-SIMS and XPS. Partial least-squares regression was applied using the nitrogen content from XPS as the dependent variable and ToF-SIMS data as the independent variable. A good linear relation is observed using the logarithm of the ToF-SIMS intensities. The ToF-SIMS limitations for quantitative analysis due to the matrix effect, instrumental drift and surface sensitivity are discussed. A comparison of XPS data at different take-off angles with ToF-SIMS data confirms the surface sensitivity of the latter, but also demonstrates that some fluorinated fragments are sampled at depths deeper than might be expected from ToF-SIMS. Copyright © 2000 John Wiley & Sons, Ltd.