My guess Alberto doubts there are other chlorides in the ketchup.
And I'm inclined to agree...I'm no ketchup buff, and I wouldn't count on NaCl being the only
source of chlorides...but I would count on it being the PRIMARY source of chlorides. Depending on accurate you want this analysis to be...that may be good enough (of course, check to make sure there aren't other hidden sources...my word isn't THAT trustworthy lol)
If you find another method or other information, keep us posted!