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3.2.2 MALDI Time-of-Flight Mass Spectrometry
James J. Filliben
Alan Heckert
Mark G. Vangel Statistical Engineering Division, ITL
Charles Guttman Polymers Division, MSEL MALDI (Matrix-Assisted Laser Desorption Ionization) Time-of-Flight Mass Spectrometry is a newly-developed methodology which will serve as the basis for future NIST synthetic bio-polymeric SRMs. Such SRMs have as their output not a number, but a table of weights and integrals corresponding to the spectrometer's spectral output. This "signature" of the polymer will change depending on whether it is a homo-polymer or a co-polymer and which specific homo- or co-polymer is being analyzed. MALDI is superior to the classic (moment-based) methods of light scattering, osmometry, and ultracentrifugation (which yield slightly better than 2% characterizations)-MALDI offers a characterization based not just on moments but on the entire distribution. Such distributional detail provides the analyst with greater insight, discrimination, and accuracy which in turn yields superior SRM values. In practice, however, the MALDI time-of-flight mass spectrometer is not yet a finished "production" device-it is a state- of-the-art piece of equipment which has instrumental and procedural factors which have not yet been characterized, understood, or optimized-such must be done in order for the full force of this new technology to be brought on line to produce improved polymeric characterizations in general and higher-accuracy SRMs in particular. Some of the more important factors which affect the quality of the MALDI spectrometer output are as follows: 1. size of crystal; 2. laser energy; 3. ion optics; 4. detector voltage; 5. accelerator voltage; 6. delayed extraction time; 7. delayed extraction voltage; 8. mode (linear or reflection); 9. number of shots; 10. evaporation rate; 11. polymeric concentration; 12. baseline calculations; and 13. scanning pattern. It is expected that bringing the device on-line will take about a year. The early stages of the shake-down (testing for stability, drift, and repeatability) are currently underway. The plots on the following page reflect such early analyses- note the existence of unexpected spikes (at a frequency and harmonic) in the (statistical) spectrum.
Future collaboration will involve the construction and execution of
orthogonal designed experiments to simultaneously and efficiently
study the large number of factors so as to provide information
about dominant factors, important interactions, best settings, and anomolous
conditions.
Figure 5: MALDI Time-of-Flight time traces.
Date created: 7/20/2001 |