Fragmentation processes of molecular-related ions M+· and [M-H]- of phenolic endocrine disruptors, 4-n-octylphenol, 4-(1,1,3,3-tetramethyl-butyl)phenol, and 4-(1-ethyl-1-methyl-butyl)phenol, which are octylphenol isomers, have been studied using high- and low-energy collision-induced dissociation (CID) mass spectrometry. The product ion spectra revealed the isomeric structures of octyl group C8H17 found in CID studies of both precursor ions, M+· and [M-H]-. The high-energy CID spectra of the deprotonated molecules [M-H]- were compared with the corresponding low-energy CID spectra. Although the fragmentation pattern of low-energy CID differed markedly from that of high-energy CID, both product ion spectra were useful in identifying the isomeric structures of the octyl group. The negative-ion products from the precursor [M-H]- could be explained by two mechanisms with homolytic cleavage, called charge-remote fragmentation (CRF), while the positive-ion products from M+· were understood as homolytic radical-initiated fragmentation (RIF) and heterolytic charge-initiated fragmentation (CIF).
Novel aspects of laser desorption/ionization mass spectrometry were demonstrated using a recently developed 8-ch capillary-based dispensing workstation with a variable capillary pitch mechanism. Its application to improve mass accuracy in external mass calibration for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was investigated using angiotensin-I, as both of reference and sample compounds positioned close to each other on a target plate. The workstation's high-density dispensing was found to provide an 8×8 spot array, with a spot interval of 1 mm, and a standard deviation of mass errors of 0.02 (16 ppm) for 64 spots in the array. Thus, the mass accuracy was comparable to that normally observed in internal mass calibration. The workstation was also applied to dispensing small volumes of sample solutions of octylphenol polyethoxylate and α-cyclodextrin with high-density to substrates with self-assembled germanium nanodots (GeNDs) for surface-assisted laser desorption/ionization (SALDI). An 8×8 spot array, with a spot interval of 1 mm, was successfully prepared on the GeND substrates. Previously, this process required difficult preparation of multiple spots, either by dispensing them manually, or using conventional dispensers based on pipette tips.
A new technique for minor isotope analysis that uses a rotating electric field and an imaging detector is described. The rotating electric field is generated by six cylindrically arranged plane electrodes with multi-phase sinusoidal wave voltage. When ion packets that are discriminated by time-of-flight enter the rotating electric field, they are circularly deflected, rendering a spiral image on the fluorescent screen of the detector. This spiral image represents m/z values of ions as the position and abundance of ions as brightness. For minor isotopes analyses, the micro channel plate detector under gate control operation is used to eliminate the influence of high intensity of major isotopes.
Infrared matrix-assisted laser desorption/ionization (IR-MALDI) using an infrared laser is promising, because most biomolecules have a specific absorption in the infrared range. IR-MALDI is expected to provide the following advantages: (1) use of various matrices; (2) use of biomolecules such as water and lipids as the matrix; and (3) super-soft ionization, i.e., high-molecular-weight protein analysis without fragmentation. However, IR-MALDI is not widely used because of its low sensitivity, complexity, high cost, and low-compatibility with commercial MALDI time-of-flight mass spectrometers. We evaluated the wavelength dependence of ionization in IR-MALDI to clarify the ionization mechanism. The molecular mass of angiotensin II was obtained at a wavelength between 5.8 and 6.2 μm, which corresponds to the >C=O stretching vibration mode. In IR-MALDI, we believe that the strong molecular bond attracts an electron from a neighboring hydrogen atom, possibly protonating it.
The Japanese word “masu” originates from the English word “mass.” It is occasionally used as an alias for units of mass and m/z. However, mass of molecules and ions should be expressed with the unified atomic mass unit (unit symbol: u) or dalton (unit symbol: Da). Furthermore, the m/z value indicated in the abscissa of mass spectra is dimensionless; therefore, it should not have any units.