Gas Chromatography Instrument System - NPD Detector Operating Notes:
The Nitrogen Phosphorus Detector (NPD) is based on the Flame Ionization Detector (FID), but it includes a thermionic source made of strontium salt beads. This source uses a micro-hydrogen flame to significantly enhance the ionization efficiency of nitrogen- and phosphorus-containing compounds, allowing for selective detection of these substances. Since hydrogen is used, the safety level of the NPD is comparable to that of an FID.
The temperature of the thermionic source has a major impact on detector sensitivity. Higher temperatures increase sensitivity, but also shorten the lifespan of the salt beads. Increasing the voltage or hydrogen flow rate can boost sensitivity, but care must be taken: too low an air flow can extend the detector’s balance time, while too high a hydrogen flow may create a flame similar to FID, reducing the life of the beryllium salt beads and affecting the detector's response to nitrogen and phosphorus. Typical gas flow rates are around 3–4 mL/min for hydrogen and 100–120 mL/min for air. For packed columns or large-diameter capillary columns, the carrier gas flow is about 20 mL/min without tail gas. When using conventional capillary columns, the tail gas flow should be set to approximately 30 mL/min.
When adjusting the voltage of the thermionic source, always turn off the detector power first to prevent accidental damage to the salt beads. The active component (onium salt) in the thermionic source is prone to contamination, which reduces its lifespan. To prolong its life:
First, avoid silicon dioxide from entering the detector. Ensure proper column aging, especially with silicone-based stationary phases that have thin films. Also, prevent silicon residues from samples after derivatization from entering the column.
Second, before shutting off the carrier gas (e.g., when changing the cylinder or column), reduce the thermionic source voltage to zero. Otherwise, without carrier gas, the salt beads may burn out within minutes.
Third, use the lowest possible voltage that still meets sensitivity requirements.
Fourth, store the instrument in a dry place. When not in use, keep the detector temperature above 100°C and turn off the thermionic source voltage.
Fifth, if the system will not be used for a long period (such as overnight), lower the thermionic source voltage instead of turning it off. The salt beads remain hot, allowing for quick stabilization when the next sample is injected. If the voltage is fully turned off, the detector may require several hours to re-equilibrate.
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