质谱分析法导论

A mass spectrometer consists of an ionization source, a mass analyzer, and a detector. The ionization source converts sample compounds into gas phase ions, usually with a single positive charge. The mass analyzer separates ions based on their mass-to-charge ratio, and the detector measures the relative abundance of each ion type to generate the mass spectrum.

Hard ionization techniques cause extensive fragmentation, producing many fragments of lower mass that aid in determining molecular structure. Soft ionization techniques result in little or almost no fragmentation, preserving high molecular mass ions. The choice depends on the analyte and desired fragmentation level, as excessive fragmentation can lose structural information while insufficient ionization may fail to detect small molecules.

A magnetic sector uses a curved magnet to produce a homogeneous magnetic field that separates ions based on their mass-to-charge ratio. Ions travel in a circular path determined by accelerating voltage, magnetic field strength, and m/z ratio. By scanning the magnetic field, specific ions reach the detector at different times, enabling precise identification of each species.

A quadrupole consists of two pairs of parallel metal rods with alternating direct current voltages that keep the rod pairs out of phase. Ions travel through the center in a corkscrew-like path due to constant attraction and repulsion from the rods. Depending on mass-to-charge ratio, ions either traverse the full path to reach the detector or crash into the rods and are lost.

Inductively coupled plasma (ICP) is a hard ionization method using argon plasma at approximately 10,000°C to ionize sample molecules. The plasma dries aerosol droplets, dissociates molecules, and removes electrons from components for detection. ICP-MS is commonly used to detect and quantify metal elements in samples, such as iron in nanoparticles, with calibration curves principles and applications guiding quantitative analysis.

Sample preparation removes contaminating trace elements and ensures proper sample digestion. Samples are treated with hydrochloric acid to remove iron contamination, then digested in concentrated nitric acid overnight. The digested sample is diluted to achieve ideal parts-per-billion concentration range and centrifuged to remove residues, ensuring accurate detection and quantification of target elements.

Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) is a soft ionization technique used to analyze high molecular weight proteins by stabilizing molecules with a matrix. Electron ionization is a hard ionization technique for analyzing volatile and oxidation-sensitive compounds. Synchrotron radiation coupled with molecular beam mass spectrometry selectively ionizes gas phase molecules to explore their electronic structure.