Is extinction coefficient wavelength-specific? - (Dec/12/2011 )

Substrate A's extinction coefficient is x at 340nm, if i measure it at 405 nm, can i use x as the extinction coefficient in my calculation (A=ecl)?
Since it is the slope of absorbance vs concentration, will different wavelengths result in different slope?

the extinction coefficient is expressed at the wavelength for which it has been determined and can only be used for that wavelength. it would have to be independently determined for other wavelengths.