ION-SOURCE EMITTANCE INFLUENCE ON THE TRANSMISSION OF A QUADRUPOLE OPERATED IN THE 2ND STABILITY REGION

The variation of transmission (T) with resolution (R) has been calcula ted for a quadrupole mass filter operated in the second stability regi on with Mathieu parameters q = 7.547 and a = 0 to 0.02995. The fringin g fields at the entrance to the quadrupole, which can be strongly defo cusing, and which can dramatically reduce the acceptance, have been in cluded in the calculation. Even in the absence of fringing fields, at a resolution of 10,000 the acceptance in the x and y directions is les s than 2 x 10(-4) pi r(0)(4) f(2) which is about 10(-3) of the accepta nce in the first stability region at low resolution. Because the sourc e emittance can strongly effect the T(R) behavior, the calculation of ion transmission has been done for two source emittances that correspo nd to different degrees of focusing of ions into the quadrupole. The T (R) characteristics for two realistic source emittances give behavior markedly different from the decrease in acceptance with increasing res olution which previously has been used as a measure of the quadrupole transmission. Comparisons of the calculated transmission losses with i ncreasing resolution to earlier experimental results obtained with an inductively coupled plasma source show good agreement provided an emit tance which corresponds to ions being tightly focused into the quadrup ole is used. The calculation demonstrates that up to a resolution at h alf height of 7000 in the experiment, the transmission losses were a r esult of the decreasing quadrupole acceptance. At higher resolution th e experimental transmission was limited by either the residence time o f the ions in the quadrupole or the rod quality. It is also shown that the strong defocusing effects of the fringing fields at the entrance of the quadrupole can be largely overcome by accelerating the ions thr ough these fields and then decelerating the ions in the quadrupole. (C ) 1998 American Society for Mass Spectrometry.