This question is about the reference format making letters inside maths $ $ into lower case. As they are in standard format I want them to be uppercase.
MWE:
\RequirePackage{fix-cm}
\documentclass[natbib,fleqn,smallextended]{svjour3}
\begin{filecontents}{sw_ref.bib}
@article{daon,
title={ $\delta^{18}O$ and $\delta^{13}C$: },
author={D},
journal={Ers},
volume={54},
number={3},
pages={545},
year={2005},
publisher={El}
}
@article{cow,
title={$\delta^{13}C$ measurements},
author={Coichael},
journal={A},
volume={48},
number={7},
pages={5781},
year={2256},
publisher={ACtions}
}
\end{filecontents}
%
%
\smartqed % flush right qed marks, e.g. at end of proof
%
\usepackage[version=3]{mhchem} % for upright CO2
\usepackage{graphicx}
\usepackage{floatrow}
%
%setting spacing
\usepackage{setspace}
\doublespacing
%\singlespacing
%\onehalfspacing
%For other sizes use the \setstretch command like this:
%\setstretch{1.8}
\raggedbottom
%line Numbers
\usepackage{lineno}
\linenumbers*[1]
%to set numbering format
\usepackage{enumerate}
%Maths
\usepackage{amsmath}
%for deg
\usepackage{gensymb}
%for permil
\usepackage{wasysym}
% for rotating floats
\usepackage{lscape}
%for subfigures and tables
\usepackage[font=small,labelfont=bf]{caption}
\usepackage[font=footnotesize]{subfig}
%made commands
\newcommand{\mytilde}{\raise.17ex\hbox{$\scriptstyle\mathtt{\sim}$}} %around tilde
%% Insert the name of "your journal" with
%
\begin{document}
\title{Det}
%\subtitle{}
\titlerunning{Sou} % if too long for running head
\author{Tor}
\authorrunning{et al} % if too long for running head
\institute{Tror \at
d \\
}
% \date{Received: date / Accepted: date}
% The correct dates will be entered by the editor
\maketitle
\begin{abstract}
\keywords{}
\end{abstract}
$\delta^{18}O$ and $\delta^{13}C$
\citep{cow}
\citep{daon}
\bibliographystyle{spbasic}
\bibliography{sw_ref}
\end{document}
The output (the stuff at the two ends of the red arrows should look the same):

{$\delta^{18}O$} and {$\delta^{13}C$}. BTW: the IUPAC recommends upright letters for atom symbols:{$\delta^{18}$O} and {$\delta^{13}$C}– cgnieder Sep 27 '13 at 14:02filecontentsenvironment unless you add\usepackage{filecontents}before the environment. – cgnieder Sep 27 '13 at 14:33