Photoionization Detector (abbreviated PID) andflame ionization detector(Abbreviations FID) is very good for low concentration gases and organic vapors. The sensitivity of the detector, optimized configuration can detect different gases and organic vapors. Both technologies can detect concentrations at the ppm level, but they use different detection methods. Eachdetection technologyHas its advantages and disadvantages, for special applications will choose the most suitable detection technology to detect. In general,PID small size, light weight, easy to use, so it has a good portable performance.

 

 

 

1. PID control principle and FID control principle are different

PID uses an ultraviolet lamp to ionize the sample gas to detect its concentration. When the sample molecules are absorbed intoAt high UV energy, the molecules are ionized into positive and negative charge ions, which are sensed by the charge sensor, forming a current signal.UV ionizes only a small part of VOC molecules, so they can be combined into complete molecules after ionization for further analysis of the sample.

FID uses a hydrogen flame to ionize the sample gas. These ionized ions can be easily detected by electrodes, and these sample gases are completely burned out. Therefore, the detection of FID is destructive to the sample, and the sample discharged after the detection can not be used for further analysis.

 

 

 

II. PID has different sensitivity arrangements for FID to different gases

PID: aromatics and iodides> paraffins, ketones, ethers, amines, sulfides> esters, aldehydes, alcohols, fats> halogenated fats, ethane> methane (no response).

FID: Aromatic compounds and long-chain compounds> short-chain compounds (methane, etc.)> chlorine, bromine and iodine and their compounds.

Therefore, under the same airflow conditions, we use PID and FID to detect will get different data. In general, PID is the response to the functional group and FID is the response to the carbon chain. Only molecules like propane, isobutylene, acetone, PID and FID have very similar response sensitivity to them. In addition, different PID lamps will have different sensitivity. For example, the sensitivity of butanol under lamps of 9.8, 10.6 and 11.6eV is 1, 15 and 50 respectively. In addition, most portable FIDs used in the field have a flame isolation device to control the flame and make the sensor explosion-proof. When there are large molecules slowly diffuse to the FID sensor, the lack of response is often compensated. PID can avoid the interference of some compounds by selecting lamps with different energy, or select the lamp with the highest energy to detect the broadest spectrum of compounds. Therefore, FID can be said to be a broader spectrum detector compared with PID, which has no selectivity.

 

 

 

III. Different calibration gases and measurement limits

FID is often calibrated by methane, but PID has no response to methane and needs a 12.6eVUltraviolet light sources can ionize methane, currently. PID can't do that. Therefore, FID is a powerful weapon for detecting natural gas (mainly composed of methane). On the other hand, PID can detect toxic VOCs in landfills well. If FID is used to detect VOCs in landfills, methane gas on site will greatly interfere with FID.

FID can detect 1-50000ppm;PID can detect VOC of 1ppb-4000ppm or 0.1 ppm-10000ppm. PID can detect VOC of lower concentration. FID has better linearity at high concentration (>1000ppm).

In general, the effect of humidity on The FID has no effect because the flame will remove the humidity unless there is water directly into the sensor. PID will reduce the response under high humidity conditions, and the hysteretic response due to humidity can be avoided by cleaning and maintaining the sensor. PID can detect directly in inert gas environment like nitrogen or argon VOC, the response will not have any effect with the change of inert gas concentration. The working principle of FID requires the presence of a fixed concentration of oxygen, and the oxygen source of portable FID is usually from the sample gas. Therefore, if you want to measure a stable gas in a pipe or container, the FID must use the surrounding oxygen to dilute the sample before it can be successfully detected. PIDs tend to be smaller in size, lighter in weight, and simpler in structure than FID. FID also requires hydrogen cylinders, which brings certain safety hazards during transportation and use. And The use of PID in heavily contaminated areas requires cleaning of the lights and sensors.

 

 

 

 

IV. PID and FID Performance Comparison Table