Gas chromatograph NPD use matters needing attention

Flame photometric detector (FPD) is a highly selective and highly sensitive detector for phosphorus-containing and sulfur-containing compounds used in gas chromatography. When the sample is burned in the hydrogen-rich flame, the phosphorus-containing organic compound emits light with a wavelength of 526 nm mainly in the form of HPO fragments, and the sulfur-containing compound emits characteristic light with a wavelength of 394 nm in the form of S2 molecules. The photomultiplier tube converts the optical signal into an electrical signal, which is recorded by micro-electronic exile. This type of detector has a sensitivity of tens to hundreds of coulombs per gram, with a minimum detection of 10-11 grams. At the same time, the ratio of the response of organic phosphorus and organic sulfur to the response value of hydrocarbons in this detector is up to 104. Therefore, large amounts of solvent peaks and hydrocarbon interferences can be eliminated, which is very favorable for the analysis of trace amounts of phosphorus and sulfur. , is the main tool for detecting organophosphorus pesticides and sulfur-containing pollutants.

Use caution

1.NPD is developed on the basis of FID, which differs from FID by the addition of a thermionic source (consisting of yttrium salt beads) with a micro-hydrogen flame. Under the conditions of energization of the hot ion source, the ionization efficiency of the nitrogen-containing and phosphorus-containing compounds is greatly improved, so that the two types of compounds can be selectively detected. Due to the use of hydrogen, NPD has the same safety issues as FID.

2. The temperature change of the thermal ion source has a great influence on the sensitivity of the detector. With high temperatures and high sensitivity, the life of the barium salt beads will be shortened. Increasing the voltage of the hot ion source and increasing the hydrogen flow can increase the sensitivity. However, it must be noted that too low an air flow rate will cause the detector to equilibrate for too long. When the hydrogen flow rate is too high, a flame like the FID will be formed, which greatly reduces the service life of the helium salt beads, and destroys the nitrogen and phosphorus. The choice of responsiveness. The gas flow rate is generally set to be 3 to 4 ml/min for hydrogen and 100 to 120 ml/min for air. Packed columns and large-caliber columns are used. The carrier gas flow rate is about 20 ml/min. The tail gas is not used, and the conventional capillary column is used. Blowing is set to about 30 ml/min.

3. When adjusting and setting the thermal ion source voltage, remember to close the detector's power supply so that you do not accidentally burn the helium salt beads.

4. The active element of the thermal ion source (barium salt) is easily contaminated and shortens its service life. To extend its service life should pay attention; First, to avoid the SiO2 into the detector, the column should be a good aging, especially the siloxane type liquid, the liquid film should be thin. Also avoid SiO2 residue in the sample after derivatization to enter the column. Second, before the carrier gas is sealed (such as changing the cylinder or changing the column), the voltage of the hot ion source should be adjusted to 0, otherwise no carrier gas passes and the helium salt beads will be burned within a few minutes. Third, use a low thermal ion source voltage as much as possible while meeting sensitivity requirements. Fourth, the instrument must be stored to avoid wetting. When the instrument is not in use, it is best to keep the detector temperature above 100 (thermal ion source voltage must be closed). Fifth, if the sample is not analyzed (for example, overnight) for a period of time, the thermal ion source voltage should be reduced, but not closed. Since the salt beads are still hot after the voltage is reduced, the increase in voltage at the time of reinjection can be stabilized quickly. If the voltage is closed after the closure, the detector needs several hours of equilibration time.