The ash content may vary from 8 to 35% depending upon the types of paper. The ash is derived mainly from the pulp from which the paper was made, and from various filler and/or coating/colouring materials used during manufacture. Depending upon the manufacture process e.g. pulp making (kraft or sulfite pulping) and processing (filler material, coating, colouring), the chemical composition may vary from paper to paper. The most common chemical estimation in paper ash is of titanium dioxide that is used as coating/colouring material. The other elements of interest may be Na, Ca etc. The following links may be seen:

http://www.tappi.org/content/sarg/t211.pdf

http://e-science.ru/sites/default/files/upload_forums_files/bj/TiO2_determination.pdf

http://www.tappi.org/content/sarg/t266.pdf

The following books may be also used for reference:

Roberts JC (1966) The chemistry of paper, The Royal Society of Chemistry, UK.

ASTM (1963) Paper and Paperboard Characteristics, Nomenclature, and Significance of Tests, ASTM Technical publication No 60-B.

Thank you for your answer but I have already found these standards during my research.

An ion-selective electrode may be used in the case of chloride content, but is less recommended for other elements. It is not suitable to determine the total N content, taking into account your sample preparation method. Consider the use of advanced atomic spectroscopic methods, like Flame or Graphite Furnace Atomic Absorption Spectroscopy or Inductively Coupled Plasma (when this is at your disposal) that are capable of measuring Ca, Mg, K and metals accurately, even in the low concentration range.

Account that volatile elements like Cd, Hg will be lost by first ashing the paper sample. Include at least one blank to correct for main elements that are omnipresent in the (ultrapure)water.

X-Ray Fluoresence may also give you an indication of the content in the solid phase of the paper, without the need for ashing or dissolution. First, define your research question more precisely to your specific needs.