Chromatography Forum

HI,

What is the effect of DC voltage in quadrapoles? If we make DC voltage is Zero then we will run the instrument in SIM mode.

If the DC voltage is zero means what will happens in the instruments.
Some instrument engineer replied that if we keep DC voltage Zero also the two analyzers in the triple quad will works.

Can anybody explain this why?

For quadrupole MS, the ratio of dc voltage to rf voltage is appropriately adjusted so that the mass scan line crosses the boundaries near the apex of the stability diagram to allow a small mass range to fall into the stability diagram (reach the electron multiplier). The ions of different m/z are separated by ramping dc and rf amplitudes (keeping the ratio constant).

When you run quadrupole in SIM mode, both the ratio and amplitudes of the dc voltage and rf voltage are kept constant during the time window so that only ions with same m/z value can reach the electron multiplier.(If more than one m/z are selected in SIM mode, I believe, several sets of parameters of dc and rf voltages (keeping the ration constant) are changed back and forth during the run.

If dc voltage is zero, I'm not sure how ions can be separated based on their m/z in quadrupole MS. But most quadupole ion traps are operated with dc voltage at zero, the ions are ejected from the trap in the order of their m/z by raising rf voltage.