Iron oxide nanoparticles modified with oleate have been employed for the extraction of peptides and proteins from aqueous solution before matrix-assisted laser desorption/ionization (MALDI) mass spectrometric (MS) analysis. Adsorption of peptides and proteins onto the nanoparticles were mainly through electrostatic attraction and hydrophobic interaction. The analyte-adsorbed iron oxide nanoparticles could be efficiently collected from solution using a magnet. No elution step was needed. With this preconcentration strategy, the lowest detectable concentration of angiotensin I, insulin, and myoglobin in 500 L of aqueous solution were 0.1 nM, 0.1 nM, and 10.0 nM, respectively. In addition, the nanoparticles could extract the analytes from solution with a high content of salt and surfactant, thus eliminating suppression effect during MALDI MS analysis. This method was successfully applied to concentrate the tryptic digest products of cytochrome c. In addition, the tryptic digestion of cytochrome c can be directly conducted on the iron oxide nanoparticles. atrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) has become a routine analytical tool to determine the molecular mass of biomolecules [1][2][3]. However, samples containing excessive amounts of salts, surfactants, or other contaminations suffer from ionization suppression and adduct formation [4,5]. This limits the application of MALDI technique. Therefore, a simple and selective procedure for extraction and concentration of analyte from complex samples before MALDI MS is required.Various methods have been developed to isolate the analyte from complex sample matrix. In surface-enhanced laser desorption/ionization (SELDI), the sample target played an active role in the extraction, purification, or concentration of the analyte of interest [6 -9]. The target surface was derivatized for the selective retention of analyte while removing interferences through on-target washing. Several surface derivatizations have been designed to extract and concentrate the analyte through hydrophobic interaction [10,11], ionic interaction [12,13], or immunoaffinity [14,15]. However, the sensitivity improvement was limited by the number of binding sites on the target. In another approach, the so-called surface-enhanced affinity capture (SEAC), micrometer-sized beads made for chromatography column were used to capture peptides and proteins from sample solution [16]. Various types of beads have been used, which include reverse-phase chromatographic beads [17,18] and immobilized metal ion beads [19 -21]. To speed up the collection of analyteadsorbed beads from sample solution, magnetic particles, which can be simply collected using a magnet, were developed [22][23][24]. After collection, the microbeads were washed and placed on sample target, followed by analyzing with MALDI MS. Unfortunately, the presence of those particles on the sample target was reported to cause decrease in mass accuracy and resolution [25,26].Recently, nanoparticles have become interesting p...