An acetylated softwood lignin fiber having a diameter of 5 to 100 microns. The present invention is a compound represented by the formula :wherein each symbol is as defined in the specification, or a salt thereof. Advantageously, substantially all of the solvent can be evaporated as pure solvent, which enhances process efficiencies and reduces cost. Visualization of defects is possible. A rinse portion blows pure water over the portion of the substrate from which inorganic substances have been removed, and sucks its vapor through a third suction mouth.