Important Aspects concerning the Quantification of Biomolecules by Time-of-Flight Secondary-Ion Mass Spectrometry

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Abstract
Fundamental aspects regarding the use of time-of-flight secondary-ion mass spectrometry (TOF-SIMS) as a quantitative tool for the analysis of organic compounds are reported. The following factors are discussed: (1) the use of Poisson's law to correct for dead-time in single-ion data collection; (2) practical considerations concerning the analysis of “real world” samples; and (3) the effect of the etching process on the reproducibility of the intensity ratio (analyte/internal standard) of Ag-cationized species. To evaluate the importance of these factors, we used cocaine and cyclosporin A (CsA) as analytes because they show protonated and Ag-cationized species, respectively, in their SIMS spectra. Correction for detector dead-time using Poisson's law of single-ion counting expanded the dynamic range for cocaine by ∼2 orders of magnitude. For analyses requiring only a small dynamic range (i.e., CsA), the correction improved the % RSD of the slope from 2.43 to 0.87%. The maximum secondary-ion (SI) yield of CsA (Ag-cationized species) occurs at a CsA concentration ∼3 orders of magnitude higher than the therapeutic levels in blood (25–2000 ng/mL). It is discussed how this problem should be addressed. Analysis of variance (ANOVA) indicates that Ag substrates must be etched under identical conditions to obtain quantitative results when species requiring cationization are being analyzed.