As others already said, your chances to negotiate a salary change are slim.
The office policy is to provide everyone with the same salary, and you pining for more would probably lead to other problems down the road.
However, not everything is lost.
Instead of going for salary change, why not asking for some sort of compensation?
Every time you go on a trip for your company, you could ask for some sort of one time bonus, according to how well you did your job.
This way, you get some kind of gratification directly attached to your role that doesn't count as a salary raise, and you also justify your request by attaching it to some kind of performance evaluation, instead of just asking for more money.
