However, insulating samples undergo charge buildup. Sample charging diffuses the primary beam and diverts it from the analytical area, often eliminating the secondary ion signal entirely. Sample charging also changes the energy distribution of the secondary ions, which effects their transmission and detection by the mass spectrometer. When the sample is a thin dielectric on a conducting substrate, a strong electric field develops. Mobile ions such as sodium and lithium migrate in the electric field and depth profiles no longer reflect the original compositions of the layers.
Several techniques are available to manage sample charging, and they are often used in combination.
Low energy electron beams work better because higher energies produce more than one secondary electron for every incoming electron. Low energy electron beams are more easily implemented in quadrupole SIMS instruments, making quadrupoles the system of choice for insulating materials. In contrast to quadrupoles, magnetic sector instruments maintain the sample at high positive potential for positive ion spectroscopy, making it difficult to bring in a low energy electron beam. High energy electron beams, though less effective, are widely used. Adjacent Conductors Conducting grids placed over the sample reduce the effects of charging on ion optics and bring a source of electrons near to positively charged areas of the sample.