Evaluation of Improvements in Mass Accuracy of Q-Tof Ultima API for the Rapid and Automated Screening of Compound Libraries

The overall exact mass measurement accuracy and precision were investigated for three Waters Micromass Q-Tof Ultima API instruments on three separate sites, each equipped with the LockSpray interface, modified electronics, and operated in W-Optics mode.

Benefits

The overall aim of reliably producing exact mass confirmation for, on average, greater than 90% of the 'found' target components to better than 2 ppm was achieved

The overall exact mass measurement accuracy and precision were investigated for three Waters Micromass Q-Tof Ultima API instruments on three separate sites, each equipped with the LockSpray interface, modified electronics and operated in W-Optics mode. 

Data were acquired in real-time exact-mass centroid mode and processed automatically using the OpenLynx Application Manager post-acquisition processing software. 

The sample set consisted of a GSK test plate containing 96 samples chosen to be representative of a larger pharmaceutical compound library. The test plate was analyzed a total of eleven times using the same generic high throughput LC-MS method. 

The results obtained show that overall, in excess of 90% of detected target masses were exact mass measured to within 2 ppm of the theoretical value and 100% within 5 ppm. 

The overall RMS mass measurement error was found to be better than 1 ppm for the entire sample set.

Data Processing

Each 96-well plate analysis was processed using OpenLynx set to automatically select and average target ion spectra with the optimum signal intensity from the real-time mass measured spectra from the total ion chromatograms.

The results were compiled in the form of electronic browser reports, as in the example below, to generate an integrated chromatogram, exact mass measured spectrum with the mass measurement error annotated and the associated elemental composition search report for each well in the plate.

The browser report for each plate was exported to produce a listing of individual mass measurement errors for each target component.

The table below is a summary of the average mass measurement accuracy and reproducibility for the overall sample set.

W-Optics mode operating at an elevated resolution (>17,000 FWHM) in combination with electronic modifications to stabilize the Tof pusher characteristics confer significant improvements in exact mass measurement performance. These modifications are now a standard feature. 

The repeat analysis of the 96-well plate over 48 hours on each of three different instruments produced very consistent results across the study set. The overall aim of reliably producing exact mass confirmation for, on average, greater than 90% of the 'found' target components to better than 2 ppm was achieved. 

The overall RMS error of less than 1 ppm and a standard deviation for any individual plate not greater than 1.5 ppm represents a considerable improvement over the currently stated instrument specification of less than 5 ppm RMS for Q-Tofs operated in V mode. 

None of the 'found' target masses were measured with an error greater than 5 ppm compared to the expected mass. This allows a greater constraint to be put on elemental composition searches therefore producing considerably reduced elemental composition listings and greater confidence in the validity of the screening of compound libraries.