div class="line"><a name="l00049"></a><span class="lineno">   49</span>&#160;                                              <span class="stringliteral">&#39;ARS&#39;</span>,  <span class="comment"># arsenic</span></div><div class="line"><a name="l00050"></a><span class="lineno">   50</span>&#160;                                              <span class="stringliteral">&#39;NH4&#39;</span>,  <span class="comment"># ammonium ion</span></div><div class="line"><a name="l00051"></a><span class="lineno">   51</spa