A detection apparatus includes a sample holding section, an irradiation means, a detection means, a calculation means, and an evaluation means. For example, encryption of an analog signal converts the analog signal to an intermediate digital signal that is converted back into a scrambled analog signal. Phosphate anions are precipitated in form of tri-ammonium phosphate upon introduction of excess amounts of ammonia. The fluid delivery element is moved to scan the fluid stream over the wall to remove a volume of tissue surrounding the lumen. The band may demarcate the perimeter region from the generally planar portion of the reflective element. The laser beam is directed to interact with a gas in the cell that undergoes an optical transition within the spectral tuning range of the laser.