who should a personal assistant of the ceo report to is it the ceo or the head of admin, and also what does the position- a personal assistant to the ceo comprises.

It really depends on the company policy.

For instance at my company all requests for days off or missed time or lateness must be report to the head of the admins and then passed down to person you are supporting.
It can be dubious at times but it is necessary that the person in charge of the admins/personal assistants knows who is going to be out so that they can send someone to cover their area.

A PA to the CEO should report to directly to the CEO. All leave and days off should be preapproved by the CEO before going to the head of Admin. By 'comprise' I want to believe the job description?

JD includes: In charge of admin duties in the CEO's office; taking notes at meetings, representing CEO at meetings; attending to CEO's visitors and callers; attending to mails, cordinating admin staff in CEO's office. etc.

Should you need more info, let me know.