Dry goods | How much do you know about VOCs testing?

What are VOCs?
VOCs are the abbreviation of volatile organic compounds.
In China, VOCs refer to organic compounds with saturated vapor pressure greater than 70.91 Pa, standard atmospheric pressure of 101.3 kPa, boiling point below 50~260 ℃ and initial boiling point equal to 250 ℃, or any volatile organic solid or liquid under normal temperature and pressure. The Standard for Uncontrolled Emission Control of Volatile Organic Compounds in China states as follows: "Organic compounds participating in atmospheric photochemical reaction, or organic compounds determined according to relevant regulations."

Main detection methods of VOCs

Main detection technologies of VOCs
01. PID photoionization detection

Basic principle: The ultraviolet ray generated by the vacuum discharge of inert gas is used to ionize the gas molecules to be measured. The concentration of the gas to be measured is obtained by measuring the current intensity generated by the ionized gas.
Technical features: the sensor is small in size, strong in anti-interference (low responsiveness to alkanes), fast in response, and a non-destructive detector. It can be applied to the detection of total VOCs.

02. FID hydrogen flame ionization detection

Basic principle: use hydrogen flame as ionization source to ionize organic molecules into positive and negative ions. In the electric field formed by polarization voltage, positive and negative ions move to opposite electrodes. The formed ion current is collected and output by the collector. After impedance conversion, the amplifier will obtain measurable electrical signals.
Technical features: It responds to almost all organic matters, especially hydrocarbons. Linear range, strong stability, can be used to monitor the total amount of VOCs, vulnerable to the interference of O2 and H2O, unable to distinguish VOC types; It is commonly used for alarm in dangerous areas and indoor ambient air detection.

03. Gas chromatography (GC)

Basic principle: firstly, the chromatographic column is used to separate the components to be tested, and then the selected detector (FID/MSD/ECD) is used to determine the quality by the retention time of chromatographic peaks and to quantify the peak height (or peak area).
Technical features: It has the advantages of high selectivity, high sensitivity and wide application range. There are many detectors to choose from, such as MS, FID, PID, etc. The disadvantage is that the analysis cycle is relatively long, which is generally used for online monitoring of fixed pollution sources.

04. Mass Spectrometry (MS)

Basic principle: high-energy electron flow bombards the sample molecule, causing the molecule to lose electrons and become positively charged molecular ions and fragment ions. These different ions have different masses. The ions with different masses arrive at the detector at different times under the action of magnetic field. The result is a mass spectrum.
Technical features: high detection accuracy, good repeatability, excellent selectivity, and elimination of cross interference between different components

05. Spectroscopy (FTIR)

Basic principle: When the infrared light with continuously changing wavelength irradiates the target compound molecule, the infrared light of a specific wavelength with the same natural vibration frequency as the molecule is absorbed. The infrared light that irradiates the molecule is dispersed with a monochromator, arranged in sequence according to the beam, and the absorbed intensity of different beams is measured to obtain the infrared absorption spectrum. Qualitative analysis is based on the fitting degree between the infrared absorption spectrum of the sample and the reference material, and semi quantitative analysis is based on the intensity of characteristic absorption.
Technical features: It has good selectivity and sensitivity, and is dynamic, non-destructive, fast and efficient. It is very suitable for on-site rapid detection and real-time online analysis.

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