ajax onreadystatechange parameter

I want to pass a parameter to the callback function to identify one ajax call from another.  
This code is not doing the business.  Is it possible?  How is it done?

function runscript(page, data, element)
{
	//get ajax object
	HTTPrequest[req_no]=GetHTTPrequestObject();
	// set callback
	HTTPrequest[req_no].onreadystatechange=stateChanged(req_no);
	//check there is a connection to server
	if (HTTPrequest[req_no]==null)
	{
		alert ("Browser does not support HTTP Request");
		return;
	}
	
	info['ajax'] += 1;
	info['script'] = page;
	
	//create script url
	var url=page+".php";
	url=url+"?element="+element;
	for(i in data)
	{
		url=url+"&"+i+"="+data[i];
	}
	url=url+"&sid="+Math.random();
 
	//load the script
	HTTPrequest[req_no].open("GET",url,true);
	HTTPrequest[req_no].send(null);
}

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