A mass spectrometer is disclosed wherein ions from a pulsed ion source are dispersed in a drift region so that the ions become separated according to their mass to charge ratios. The method includes the steps of: scanning a predetermined pattern; storing the predetermined pattern; scanning the documents; encryptingdecrypting the documents in accordance with the predetermined pattern. The first variable is substituted with the sum of a greater one of the first variable and the likelihood and a correction value of the difference between them. This roller assembly includes a roller for transporting a web, the roller having an axis; a web wrapped partially around the roller and applying a load on the roller transverse to the axis; and a trolley roller contacting the roller, the trolley roller configured and arranged to impart a sufficient force on the roller to keep the axis of the roller linear under said load. 70 or more and an average particle diameter of 1 to 10 \u03bcm is added as at least the alumina source material in an amount of 10 mass % or more with respect to the total mass of the cordierite forming material to obtain the cordierite forming material.