The current results show that resolving powers in excess of 4,000 (FWHM) are readily achievable without the use of an ion mirror. This represents a 4-fold improvement over other recent attempts to use the oa geometry with a MALDI source [1,2].
A two-point external calibration of the mass scale was found to be stable over several days. The precision of the mass measurement has been estimated by a statistical error analysis involving about 40 measurements in the mass range specified above. The distribution of errors was found to be normal (linear normal probability plot) with a mass accuracy of about 0.1 to 70 ppm (average error) depending on how the calibration was performed while the precision is found to be 90 ppm (standard deviation) irrespective of calibration method. The observed errors are consistent with predictions based on the resolving power and the 2.5 ns digitiser frequency used in the mass accuracy studies.
The results are consistent with a delayed extraction (DE) advantage though the exact mechanism of resolving power advantage is still under investigation. One predicted advantage of the orthogonal acceleration MALDI geometry is that the high resolution and mass accuracy advantages will extend to much higher masses than do corresponding advantages in DE MALDI TOF. The reasons for this will be discussed.
Measurements are restricted in mass range with the current instrument because it has a low ion energy (3 kV) accelerator and detector configuration that is linked to the accelerator by a long narrow drift-tube (1.5 m x 20 mm). A new large-detector configuration with a 20 kV accelerator and a wider drift region is briefly described.