There is a contour level option, I　ａｄｊｕｓｔ　ｔｈｅ　contour ｖａｌｕｅ　ｏｆ　ｔｈｅ　ｎｅｇａｔｉｖｅ　ｐｅａｋ　to ｖｅｒｙ　ｌｏｗ，　ｌｉｋｅ　10^ -9, the negative peaks will not show in the spectra. hope it is what you what to know. thank you