of flux densities in the three abutting [[*Spitzer*]{}]{} passbands at 16, 22 and 24[$\mu$m]{} allow redshifts of objects to be constrained as these dust features move through these filters. PAH are only 0.5-1$\mu$m wide, while the 9.7$\mu$m silicate feature is 3$-$4$\mu$m wide, depending on the continuum level adopted. As a result, adjacent passbands can show widely different flux density ratios as a function of redshift (Figure 3).

At $0.2<z<0.6$, the 9.7[$\mu$m]{} silicate feature enters the 16$\mu$m bandpass while the 22 and 24[$\mu$m]{} bandpasses are tracing the warm dust and 11 and 12[$\mu$m]{} PAH features. At these redshifts, sources show low 16[$\mu$m]{}/22[$\mu$m]{} flux density ratios. At $0.6<z<1.2$, the 9.7$\mu$m features moves out of the 16[$\mu$m]{} bandpass and into the 22[$\mu$m]{} bandpass, while the 6.2[$\mu$m]{} and 7.7[$\mu$m]{} PAH features fall in the 16[$\mu$m]{} window, boosting the 16[$\mu$m]{}/22[$\mu$m]{} ratio. At $1.2<z<1.7$, the 9.7[$\mu$m]{} feature moves into the 24[$\mu$m]{} band. Since the 22[$\mu$m]{} bandpass is $\sim$2[$\mu$m]{} wider than the MIPS 24[$\mu$m]{} band, the 7.7[$\mu$m]{} feature enters the 22[$\mu$m]{} bandpass. The net effect is to boost the 22[$\mu$m]{}/24[$\mu$m]{} flux ratio while lowering the 16[$\mu$m]{}/22[$\mu$m]{} ratio. At $z>1.9$, the PAH emission shifts out of the 16[$\mu$m]{} band, decreasing the 16[$\mu$m]{}/22[$\mu$m]{} ratio while the 22[$\mu$m]{}/24[$\mu$m]{} flux ratio 