I have worked with an elzone and have found that you have to make sure it is very clean to get good results.
You should clean it every day before you start and analyzing your samples.
Also keep the tubes in a solution of clean deionized water.
I learned this when i visted the rudolph sring company in Switzerland.
Let me know how you make out.

We used this anaylizer to grade Industrial Diamond Powders.
Also allways check your standards before start.