The factors and protocols that influence, accuracy, precision and uncertainty of accurate mass measurements by FT-ICR ms to validate the assignment of elemental composition
The need for very accurate mass measurements of compounds is becoming more
demanding with the expansion of the number of compounds in need of correct
identification and with the limits of elemental analysis; a fast, robust analytical solution
is sought.
Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is a very
powerful tool for undertaking accurate mass measurements because of its high resolving
power and mass accuracy. The use of FT-ICR MS in a high-throughput environment is
limited due to the need of operator intervention required to obtain optimum accurate
mass measurements. The lowest mass measurement errors (MME) are usually obtained
using internal calibration, as the sample and calibrant ions inside the ICR cell
experience the same conditions. However, internal calibration requires calibrant ions to
be selected according to the mass of the sample ion, which requires operator
intervention. External calibration is the preferred choice when performing accurate mass
measurements in a high-throughput environment, as a calibration can be acquired
independently of the sample. This study aims to demonstrate the use of ion population
balancing using a dual sprayer approach to reduce MME. Population balancing between
sample ions and calibrant ions can reduce the MME 40-fold. Ion populations across a
chromatographic run can also vary greatly. The dual sprayer can also be employed to
easily control the ion populations of both sample and calibrant entering the cell and can
help to reduce ion suppression.
The aim of the latter part of the work was to address the uncertainty of an accurate mass
measurement performed using FT-ICR MS. In order to confidently select an elemental
formula following an accurate mass measurement, a ‘cut-off’ limit or tolerance has to be
selected. This tolerance can be set by calculating the uncertainty of the measurement;
without an uncertainty component the measurement is meaningless. The work herein
demonstrates how to calculate uncertainty of an accurate mass measurement with the
aim that journals will adopt a procedure of only accepting an accurate mass
measurement which quotes an uncertainty