A device and method for accurately performing quantitative diffuse optical spectroscopy on a sample includes a light source and a source optical fiber that is optically coupled to the light source. The said valve controls the direction of the fluid circulation in the cylinders which effect the steering. The guide assembly is moved lateral to the flow direction of the substrate to guide the elongate member onto the substrate along a securement path, at least a portion of which is generally oblique to the flow direction of the substrate. In one embodiment, the previous QT interval is estimated based upon the measured preceding R-R interval.