GIF interval time array not taken -

how pass array of intervals when creating gif frames

nsdictionary *prep = [nsdictionary dictionarywithobject:[nsdictionary dictionarywithobject:[nsnumber numberwithfloat:intervals] forkey:(nsstring *) kcgimagepropertygifdelaytime] forkey:(nsstring *) kcgimagepropertygifdictionary];