The mass accuracy of unit resolution mass spectrometers (MS) is generally near 0.2 Da (~400 ppm at 500 Da) and reported to
the nearest integer mass. This is because the software reduces the profile mode spectrum to a centroid spectrum with great
loss of information. In this article we will introduce a novel way to calibrate this class of MS instruments that allows routinely
achieving 5–10 ppm mass accuracy as well as dramatically improving ion signal extraction and better ratios of signal to noise
Centroid Mode Calibration
Three parameters express the mass (σppm) for any mass spectrometer (MS) instrument. It is inversely related to the resolving power (R) of the instrument, the square
root of the signal strength (S), and a constant (C) , as shown in this equation,1,2
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.
To understand what impacts the constant, it is first necessary to understand the current approach typically used in acquiring
MS data. In most cases, data presented to the user at the end of the acquisition is centroid data, the well-known "stick"
graph, in which an ion's m/z (molecular mass/charge) positions are represented by single lines on the graph (Figure 1). This form of graph represents
the MS data after heavy processing by the instrument firmware or post-analysis software.
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.