There are research techniques where something is investigated under working conditions, and these techniques are commonly referred to as in situ.
For the sake of being specific to my field of study, investigation of electrode material in situ is when the material is studied in the battery that has been put in some static condition (constant voltage applied and the battery cell is in equilibrium).
Lately, there is a novel term, "operando," which is a more specific "in situ." It is a method for studying electrode material in a working cell while the system changes under an external influence. For example, while the battery charges under constant current.
Roughly speaking, in situ is for studying the state, operando is for studying the process.
The question is: should "operando" or "in operando" be used?
I tried following the literature to avoid being wrong, but it does not help much. For example, Wikipedia has an article on operando spectroscopy, and there is no "in" anywhere in the article. There is also no "in operando" page on Wiktionary, only "operando". However, in research papers things get messy and there is extensive use of "in operando", e.g., here, here, and here. And there is also a lot of "operando" e.g., here, here, and here. So, even within Nature journals, there is no clear agreement. Is there a correct way? Does it matter at this point, or now it is just a matter of personal preference?