I am making a dynamic query however i cannot see how to add range values dynamically, with the fluent api you do

            var criteria = _searcher.CreateSearchCriteria(IndexTypes.Content);
            criteria.Range(..)

However I am doing 

var criteria = _searcher.CreateSearchCriteria(IndexTypes.Content);
IBooleanOperation query = criteria.NodeTypeAlias("Product");

foreach (string key in Request.QueryString.Keys)
                {
                    if (Request.QueryString[key]!=string.Empty){
                        //field is in lucene as eg __theme 
                        if (RangeFields.Contains(key) && !skipFlag)
                        {
                //stuck here do i add range
                        }
                        else if(skipFlag)
                        {
                            skipFlag = false;
                        }
                        else{
                            query = query.And().Field("__" + key, Request.QueryString[key]);
                        }
                    }
                }

Regards

Ismail