  In this paper, we study the blow-up analysis of an affine Toda system
corresponding to minimal surfaces into ${\mathbb S}^4$ [19]. This system is an
integrable system which is a natural generalization of sinh-Gordon equation
[18]. By exploring a refined blow-up analysis in the bubble domain, we prove
that the blow-up values are multiple of $8\pi$, which generalizes the previous
results proved in \cite{Spruck, OS, Jost-Wang-Ye-Zhou, Jevnikar-Wei-Yang} for
the sinh-Gordon equation. Let $(u_k^1,u_k^2, u^3_k)$ be a sequence of solutions
of \begin{align*} -\Delta u^1&=e^{u^1}-e^{u^3},\\ -\Delta
u^2&=e^{u^2}-e^{u^3},\\ -\Delta u^3&=-\frac{1}{2}e^{u^1}-\frac{1}{2}e^{u^2}+
e^{u^3},\\ u^1+u^2+2u^3&=0, \end{align*} in $B_1(0)$, which has a uniformly
bounded energy in $B_1(0)$, a uniformly bounded oscillation on $\partial
B_1(0)$ and blows up at an isolated blow-up point $\{0\}$, then the local
masses $(\sigma_1,\sigma_2, \sigma_3) \not = 0$ satisfy \begin{align*}
\begin{array}{rcl} \sigma_1&=&m_1(m_1+3)+m_2(m_2-1)\\ \sigma_2&=&
m_1(m_1-1)+m_2(m_2+3)\\ \sigma_3 &=& m_1(m_1-1)+m_2(m_2-1) \end{array} \,
\qquad \hbox { for some } \begin{array} {l} (m_1, m_2)\in {\mathbb Z} \hbox {
with }\\ m_1, m_2= 0 \hbox{ or } 1 \hbox{ mod } 4,\\ m_1, m_2 = 2 \hbox { or }
3\hbox { mod } 4. \end{array} \end{align*} Here the local mass is defined by $
\sigma_i:=\frac 1{2\pi}\lim_{\delta\to
0}\lim_{k\to\infty}\int_{B_\delta(0)}e^{u_k^i}dx.$
