 |
 |
 |
|
|
|
 |
|
7. Matrix Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-TOF) Mass Spectrometry MALDI-TOF mass spectrometry is an emerging technique offering promise for the fast and accurate determination of a number of polymer characteristics. The MALDI technique is based upon an ultraviolet absorbing matrix pioneered by Hillenkamp and Karas(Karas & Hillencamp, 1988). The matrix and polymer are mixed at a molecular level in an appropriate solvent with a ~104 molar excess of the matrix. The solvent prevents aggregation of the polymer. The sample/matrix mixture is placed onto a sample probe tip. Under vacuum conditions the solvent is removed, leaving co-crystallized polymer molecules homogeneously dispersed within matrix molecules. When the pulsed laser beam is tuned to the appropriate frequency, the energy is transferred to the matrix which is partially vaporized, carrying intact polymer into the vapor phase and charging the polymer chains. Multiple laser shots are used to improve the signal-to-noise ratio and the peak shapes, which increases the accuracy of the molar mass determination. In the linear TOF analyzer (drift region), the distribution of molecules emanating from a sample are imparted identical translational kinetic energies after being subjected to the same electrical potential energy difference. These ions will then traverse the same distance down an evacuated field-free drift tube; the smaller ions arrive at the detector in a shorter amount of time than the more massive ions. Separated ion fractions arriving at the end of the drift tube are detected by an appropriate recorder that produces a signal upon impact of each ion group. The digitized data generated from successive laser shots are summed yielding a TOF mass spectrum. The TOF mass spectrum is a recording of the detector signal as a function of time. The time of flight for a molecule of mass m and charge z to travel this distance is proportional to (m/z)1/2. This relationship, t ~ (m/z)1/2, can be used to calculate the ions mass. Through calculation of the ions mass, conversion of the TOF mass spectrum to a conventional mass spectrum of mass-to-charge axis can be achieved.
|
