Clause 7.2.1.3

You asked

"1. If I can not purchase the last version of the method right now, can I use the old one?

For a standard method, this will depend on what changed; as well as your laboratory’s application of the method. You have to consider any risk of staying with the old version by looking at the purpose of the test. You need to consider what you are required to test and report, meaning what decision does you client need to make, based on the result you provide? If you have to provide a statement of conformity and the test has a regulatory requirement, for example tolerances for drinking water, your client may need you to make a pass or fail statement based on the latest standard. If you can verify there was no known methodology change and the table of tolerances are published elsewhere with reference to the new standard, yes in principle you could continue using the old version, until you can purchase the new version.

You then asked

2. If not, what mean of "unless it is not appropriate or possible to do so." in this clause."

For certain methods, the latest version of a standard my include a technique that you cannot implement. In this case, you once again need to look at the significance technically, of staying with the previous version. You could choose to continue with the old version, effectively validating it as your laboratory’s latest valid modified standard method.  Another case where it may not be possible to change to the latest valid standard method, is where the test results are being used for research or academic projects, and the change will not be appropriate (will affect interpretation of project data).