How to adjust the font in a \newcommand math expression to the environment

Question

I'm writing my thesis about the gas NO2. The correct way to write this is $NO_2$. Currently, I have a macro defined to make typing easier:

\newcommand{\notwo}[0]{\chem{NO_2}\xspace}

\chem is a macro defined in the Copernicus package:

\def\testbx{bx}
\DeclareRobustCommand*{\chem}[1]{\ensuremath{%
\mathcode`-="0200\mathcode`\=="003D% no space around "-" and "="
\ifx\testbx\f@series\mathbf{#1}\else\mathrm{#1}\fi}}

However, I'd like to expand this a bit more. Especially, when I'm in sans-serif, I want \notwo to be sans-serif, and when I'm in sans-serif-bold, I want \notwo to be sans-serif-bold.

I'm using xelatex, and I'm using system-wide installed Myraid Pro font (OTF) via the line

\setsansfont[Mapping=tex-text,ItalicFont={MyriadPro-It},BoldFont={MyriadPro-Bold}]{Myriad Pro}.

How can I achieve this?

Answer 1

Look at the mhchem package for a general approach to chemical formulas. This allows the simple syntax

\ce{NO2}

Answer 2

You need to expand the tests and define a new math alphabet:

\makeatletter
\def\test@bx{bx}
\edef\test@sf{\sfdefault}
\DeclareRobustCommand*{\chem}[1]{\ensuremath{%
  \mathcode`-="0200 \mathcode`\=="003D % no space around "-" and "="
  \ifx\f@family\test@sf
    \ifx\test@bx\f@series
      \let\next\mathsfbf
    \else
      \let\next\mathsf
    \fi
  \else
    \ifx\test@bx\f@series
      \let\next\mathbf
    \else
      \let\next\mathrm
    \fi
  \fi
  \next{#1}}}

\DeclareMathAlphabet{\mathsfbf}{\encodingdefault}{\sfdefault}{bx}{n}
\makeatother