Charge Detection Mass Spectrometry with Resolved Charge States

Charge detection mass spectrometry (CDMS) measurements have been performed for cytochrome c and alcohol dehydrogenase (ADH) monomer using a modified cone trap incorporating a cryogenically cooled JFET. Cooling the JFET increases its transconductance and lowers thermal noise, improving the signal to noise (S/N) ratio. Single ions with as few as 9 elementary charges (e) have been detected. According to simulations, the detection efficiency for ions with a charge of 13 e is 75 %, and for charges above 13 e the detection efficiency rapidly approaches 95 %. With the low limit of detection achieved here, adjacent charge states are easily resolved in the m/z spectrum, so the accuracy and precision of the image charge measurements can be directly evaluated by comparing the measured image charge to the charge deduced from the m/z spectrum. For ADH monomer ions with 32 to 43 charges, the root mean square deviation of the measured image charge is around 2.2 e. Ions were trapped for over 1500 cycles. The number of cycles detected appears to be limited mainly by collisions with the background gas.