Which manufacturer's VOCs online monitoring equipment is good quality?

The quality of the product is a very important issue, we all want to buy cheap things, but the quality of VOCs online monitoring equipment to be considered comprehensively, such as the size of the manufacturer, brand, after-sales service and so on, Thermo Fisher, Shenzhen Huaxingcheng, Shanghai Bo Measurement of the environment, Shandong Xinze and so on.