There are two methods for collecting emissions from the exhaust stream: raw and dilute. In raw sampling a pump draws a small stream of exhaust directly from the tailpipe (or equivalent) into the analysis instruments. Dilute sampling takes all the exhaust and dilutes it with a known quantity of ambient air, and then draws a sample. There are engineering, space and practical trade-offs between these methods but both are permitted under most regulations. Sometimes different equipment may be needed for raw vs. dilute, but both gaseous and particulate emissions can be measured by either method.
Gaseous emissions are quantified with instruments designed specifically for the gas. For instance carbon monoxide and carbon dioxide are both quantified by measuring the amount of infrared light they absorb (non-dispersive infrared), but oxides of nitrogen and hydrocarbons are each best measured by other methods. The instruments must be calibrated against known concentrations as well as several other types of adjustments and checks that are unique to each measurement principle. Newer technologies have enabled instruments that can quantify many gasses simultaneously, such is the advantage of Fourier transform infrared analyzers. However, they are not permitted for certification tests.
Particulate emissions are quantified by several methods but the most straightforward is to collect the particles on a filter and weigh them. While this sounds simple, the temperature and flow must be carefully controlled and weighing the filter requires an environment with a regulated temperature, humidity and few stray particles, as well as a high-precision electronic balance. As a result PM filter masses are not available until the filters are transported to the clean room, allowed to equilibrate with the environment therein and weighed by a technician.
Collecting PM on a filter, while precise, has two serious disadvantages. First it cannot determine at what point during that test the majority or minority of PM was emitted. Secondly, it doesn't characterize the particles themselves, especially how large or small they are. However two technologies can: particle counters and particle sizers. Both technologies may also require a PM filter to verify the total mass and possibly correct their readings after the fact.