Centroid Mode Calibration
The constant is determined by factors such as signal unit conversion to real ion counts, peak sampling interval, peak analysis, and mass determination algorithms. The mass accuracy of unit resolution instruments should approach that of high-resolution instruments, but it does not. Instruments based on quadrupole or ion trap technology typically have R values of ~1,000 whereas a high-resolution instrument such as a hybrid quadrupole Time of Flight (qTOFMS) unit has an R value of ~5,000. In theory, the high ion transmitting efficiency (S) of the lower resolution unit should narrow the gap in resolution. In practice, however, the low unit resolution instruments demonstrate a mass accuracy level between 1 and 2 orders of magnitude worse, pointing to an unusually high C value on low-resolution systems.
Figure 1 is a flow diagram of a typical instrument data acquisition process. The detector signal is sampled across time (or pixel) to obtain the raw continuum data as output from the detector. A previously established linear or nonlinear calibration equation within the instrument firmware transforms the X-axis to m/z (mass only calibration). This results in what is commonly referred to as a profile mode spectrum.
While most instruments can collect data in the profile mode, it is more common when using unit resolution instruments to let the instrument firmware attempt to locate the peaks from the profile data, and then produce the centroid or stick spectrum before presenting the data to the analyst. This makes for easy data transmission, storage, and management due to the high data rate in a MS system. It made sense in the days of older computer technologies.