THE OPERATION OF A QUADRUPOLE MASS FILTER WITH THE DC VOLTAGE REMOVED WHEN SAMPLING IONS FROM A SOURCE AT HIGH-PRESSURE AND TEMPERATURE

Flames contain a variety of positive and negative ions, as well as free electrons. Here flames at atmospheric pressure and temperatures up to about 2700 K have been sampled directly and continuously into a quadrupole mass filter, operated without any d.c. supply. The advantage of using this r.f.-only mode is that for all ions (of one sign) above a particular threshold mass there is 100% transmission, but otherwise lighter ions are not observed. The position and ''sharpness'' of this cut-off have been investigated by gradually increasing the amplitude of the r.f. voltage when sampling a flame of H-2 + O-2 + Ar, seeded with trace amounts of the alkali metals Na, K, Rb and Cs. The resulting plot of detected current for the ions of these alkali metals shows a series of steps, corresponding to another ion being no longer detected. These steps in the observed total ion current do not lie exactly at the expected mass; nor are they as narrow as predicted. The cause seems to be the considerable spread of ionic velocities when sampling ions from a very hot, high pressure source like a flame.