DETERMINATION OF ION FREQUENCIES IN A QUADRUPOLE ION-TRAP BY USING A FAST DIRECT-CURRENT PULSE AS PUMP AND A LASER PROBE

A new technique has been developed which allows the direct measurement of frequencies of ions trapped in a quadrupole ion trap mass spectrom eter. This pump/probe method employs a fast direct current (DC) pulse (pump) to displace a kinetically cooled ion population from the center of the trap, and a laser (probe) which recognizes when ions reappear at the center of the trap by the formation of photodissociation fragme nts. The translationally excited ions undergo periodic motion within t he confines of the ion trap, and this periodic motion can be followed by recording the intensity of the photodissociation fragment as a func tion of the delay time between the DC pump and the laser probe. The DC pulse has a rise time of 15 ns; data are taken 1 ms after its applica tion to allow stable ion motion to be sampled. Sampling of the ion clo ud is done at 50 ns intervals, and fast Fourier transformation of the time-based data yields the ion frequencies and their relative magnitud es. Data are reported for ions derived from acetophenone (m/z 105) and 1,4-cyclohexadiene (m/z 80) under various trapping conditions corresp onding to different Mathieu q(z) values. The measured fundamental secu lar frequencies, f(z) and f(r), are found to agree well with those pre dicted. The presence of higher order multipole contributions to the tr apping field is evident from such ion frequencies as the drive frequen cy, f(RF). The ability to measure ion frequencies under operating cond itions provides a new tool for comparing simulated and experimental da ta. Simulation data from the program ITSIM, modified to account for th e effects of collisions, are shown to predict the major frequency comp onents observed in the experimental data.