Charge Detection Mass Spectrometry
Charge detection mass spectrometry (CDMS) is an emerging field of mass spectrometry that offers an alternative to time of flight and FTICR-MS for measuring high mass ions. CDMS directly measures the charge state of individual ions, rather than a packet of ions, as they pass near an electrode and induce an image charge on the electrode. This advantage allows the direct measurement of the charge and the velocity of the ion. The velocity is used to directly calculate the mass of the ion. For complex systems, such as large molecules and nanoparticles, the CDMS approach provides useful information without the limitations of microchannel plates and without the need for ultra high resolution. Since the mass and charge is directly known, there is no need to deconvolute a complicated mass to charge spectrum in order to determine the masses of the ions.
Ion Flight Path Through Detection Cylinder and Corresponding Signal Output
We are interested in detecting the charge of ion fragments in mass spectrometry measurements. By doing this, we can effectively generate a true mass spectrum of large complexes (such as viruses). This yields better powers of identification where the mass (rather than just the m/z) of the individual fragments are known. This project is being carried out in two different approaches.
Image Charge Detection MS
With typical time of flight mass spectrometry, microchannel plates are used to detect the ions. For microchannel plates to work, an ion is accelerated into a plate that has a series of microchannels which generates a cascade of electrons in response to the ion’s collision. This cascade of electrons is then collected and amplified to determine when the ion hit. Microchannel plates are not very efficient at detecting slow moving ions, which is often the case with very massive ions. Since the image charge detection method relies upon charge detection on an electrode instead of the typical microchannel plates, the ion’s velocity does not determine the detection efficiency. This allows the detection of molecules and particles with very high mass to change ratios without loss of detection efficiency.
Using the single detector CDMS setup we were able to get a mass spectrum on 50nm gold nanoparticles. The particles have a mass of 6 GDa. Also note that the mass spectrum covers 8 orders of magnitude.
An extension of the single detector design incorporates a linear array of 22 detectors. This offers the advantage of multiple measurements which can be averaged to lower the detection limit.
Above is a graph of an ion that has passed through all 22 detectors. On the right a Fourier transform of the graph is shown to demonstrate the improvement in signal to noise from averaging.