Heat and mass transfer by using DPM model and UDF in fluent
 

I've got a problem about using the macro of TP_COMPONENT_INDEX_I(p,is)when i want to identify the vaporating component in multicomponent DPM particles. In my* case,particle materal is*the solution of CO3HNa and bulk gas consists of air and water vapor(So,only water get involved in vaporation).My code to judge vaporation or not listed below.
for (ns = 0; ns < nc; ns++)
{
int gas_index = TP_COMPONENT_INDEX_I(p, ns); /* gas species index of vaporization */
if (gas_index >= 0)
It*worked smoothly in fluent,while,no vaporation*happened.However,i found the returned value of TP_COMPONENT_INDEX_I(p, ns) is -1.I don't know why the returned value is negative.There*are no details about the using of TP_COMPONENT_INDEX_I(p, ns) in fluent udf manual.Can*anybody give me some advices？