Compensated Cell Design
The capability for accurate mass measurements is an important attribute of Fourier transform mass spectrometry (FTMS). Unlike other instrumental methods in mass spectrometry, FTMS still offers significant opportunities to improve mass measurement accuracy (MMA), making it an active area of research. Although FTMS can measure frequencies to nine significant figures, the best MMA are on the order of a part per million. This discrepancy arises in the conversion of frequency to m/z. The difficulty in conversion is due to the fact that an ion’s frequency depends not only on the magnetic field strength, but also on the electric field and space charge effects within the ion cyclotron resonance (ICR) cell. We are working on methods to shim the electric field (electrical compensation) and to correct for the effects of space charge (calibration).
Theoretical calculations have guided the development of a compensated cylindrical ICR cell, which removes the 4th, 6th, and 8th order spherical harmonic terms in the electric field. This compensated cell design was fabricated by IonSpec Corporation (Lake Forest, CA) and installed as a replacement for the original cylindrical cell in the 7-Tesla ProMALDI FTMS instrument located in our lab. Experiments show that the new trap corrects the electric fields sufficiently to remove peak shape features that were present with the use of the original cylindrical trap and which were due to the anharmonic nature of the original trapping electric field. Small changes in peak position as a function of cyclotron orbit size, or z-mode amplitude, indicate the presence of small residual inhomogeneities in the electric field. Once the trapping electric field is fine tuned to remove the residual inhomogeneities, the cell will be coupled with a high-pressure event, known as the rf-only mode event, for use in collisional focusing of the ions, MS/MS, and to test the effectiveness of the high-pressure event and electrical compensation in detection of the high m/z ions generated by MALDI.
The relationship between ion abundance and observed ion frequency is generally believed to be linear and current calibration models are based on this belief. In our laboratory we have observed a complex non-linear relationship between ion abundance and observed ion frequency with the standard cylindrical cell. This discrepancy between the actual behavior of the ions and current models is one source of error in mass measurement. To date, the compensated cell corrects this problem. A linear relationship between ion abundance and observed ion frequency, seen when using the compensated cell, facilitates frequency correction for ion abundance.
In addition to the above improvements, the compensated cell should provide increased resolving power and an increase in the upper mass limit of detection, improvements which would normally be attained by going to higher field magnets. More recently, experimental observations indicate that the cell is more sensitive when run in the compensated mode, as compared to the uncompensated mode.
Adam M. Brustkern, Don L. Rempel, and Michael L. Gross, An Electrically Compensated Trap Designed to Eight Order for FT-ICR Mass Spectrometry, J. Am. Soc. Mass Spectrom., 19, 1281-1285 (2008).