By taking into account the correlation between the positions and velocities of desorbed ions in the source region of a time-of-flight mass spectrometer and by including this relationship in the equations used to determine spacefocusing conditions, it is predicted that matrix-assisted laser desorptionlionization time-of-flight (MALDI-TOF) mass spectral resolution can be improved significantly. Experiments involving MALDI of two proteins, insulin and ubiquitin, demonstrate the validity of this concept. Adduct formation does not limit mass spectral resolution in these two cases.Since the 1987 introduction by Karas and Hillenkamp of matrix-assisted laser desorption/ionization (MALDI) using organic matrices, this method has proven to be a remarkably generic way of producing gas-phase ions of large biomolecules and polymers.'-'3 Time-of-flight mass spectrometers (TOF-MS) are ideally suited for use with the MALDI technique because of their theoretically unlimited mass range, high ion transmission, and the pulsed nature of this ionization method. Unfortunately, the resolution in TOF-MALDI spectra is often poor and the severity of this problem appears to increase with ion m a~s . ' -~* '~ In previous studies using small volatile molecules, we investigated how temporal, spatial, and energy distributions combine to limit the mass resolution of TOF-MS .I4-l7 The relative contributions of these three factors vary, depending on the ionization method and the type of molecule being studied (e.g. volatile gasphase species or desorbed crystalline samples). The two most common approaches to reducing the effects of these distributions on the mass resolution are spacefocusing, introduced by Wiley and McLaren," and the use of reflectron mass analyzers, invented by Mamyrin and co-w~rkers.'~ Other techniques that have been applied include time-lag focusing to reduce the peak broadening caused by the velocity distribution,'8 and the use of molecular-beam sources located at a distance from the ionization region to reduce the component of velocity that molecules have along the direction of ion flight.'4. 2" In laser desorption experiments and particularly in the case of matrix-assisted laser desorptionl ionization, the most common method of reducing the effect of the broad energy distribution has been to increase the drift velocity so that the contribution of the initial kinetic energy is small compared to that due to acceleration. A particularly effective method that has recently been introduced involves cooling ions that have been desorbed from a matrix by storing them for a period of time in an ion trap and then extracting them into a reflectron mass a n a l y~e r .~ The matrix itself, which is essential in the MALDI Author for correspondence.technique, may be partly responsible for the low mass resolution that is commonly achieved. Since MALDI's inception, a wide range of matrices that absorb between 193 nm and 10.6 pm has been successfully employed.', In some experiments, analytelmatrix adducts may not be resolved, contributing to the...
There is compelling evidence to believe that some gold ornaments in the South Mexican International Style found outside the Basin of Mexico, or without provenience, were made by goldsmiths in the urban centers of the Aztec empire, rather than by the Mixtec artisans to whom they are commonly attributed. The weight of gold dust and bullion recorded as tribute to the Aztec rulers in the Codex Mendoza and other sources was sufficient to produce 22,000–27,000 small and medium-sized ornaments annually, implying a large number of goldworkers in these urban centers. Ethnohistorical documents indicate that many gold ornaments were given to provincial leaders by Aztec rulers who used gifts of luxury items as an important part of their diplomatic strategies, and political patronage system; many more were exported by means of long-distance trade.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.