Substitution of phosphorus in the framework should show a similar change but, because of a shorter P-O distance the shift in the main stretch band near 1000 cm-1 should move toward a higher frequency. As shown in Fig.4.18 and 4.19 , for example, the infrared spectra for zeolites P-A and P-L are compered with the phosphorus – free zeolite. In creased absorption in the higher frequency portion of the asymmetric stretch band is observed due to the presence of PO4 groups in the framework.