A sheet material conveying apparatus is provided with retard levers for supporting a retard roller shaft, which are rotative centering on the central axis of rotation so as to press the retard roller to a conveying roller. In an embodiment, a metric of a terminal portion of an arterial PPG waveform is determined, and a metric of an initial portion of the arterial PPG waveform is determined, and a surrogate of mean arterial pressure is determined based on the metric of the terminal portion and the metric of the initial portion. Also devices are provided for periodically discarding visual images not used. In some embodiments, the instrument may also include an x-ray film holder and an x-ray film holder support, allowing the film to be conveniently positioned and held in proper alignment during an x-ray procedure. Additionally, the method includes stopping a current guardian from being a guardian if the current guardian is less suitable than a threshold number of current guardians. These compounds are useful in a variety of applications including the PEGylation of biologically active molecules.