Ionization detection methods are the most universal, with high sensitivity, used for determining small amounts of analyzed substances, suitable for connection with both capillary and packed columns.
These methods are based on the dependence of the electrically ionized gas medium on its composition. The signal of ionization detectors is the change in current caused by the introduction of the analyzed substance into the detector. The ion current is the electric current generated between the electrodes of the detector by all charged particles (ions or electrons) in the gas.
Ionic current is generated in the detector under the influence of some source of ionization (radioactive isotope, flame, discharge, photoionization, electron and ion emission) and an electric field (potential difference) between the detector electrodes.
At any moment in time, equilibrium is reached in the detector, characterized by the fact that the rate of formation of charged particles equals the sum of the rates of recombination and collection of charged particles at the electrodes of the detector. The collection rate determines the current of the detector. In ionization detectors, conditions are created where either the density (concentration) of charged particles or their transport rate in the electric field depends on the composition of the gas.
a weak field - incomplete collection of charged particles with a significant part of their recombination. With a constant rate of formation and recombination of charged particles in the detector and constant voltage on the electrodes, the detector current is determined by the drift velocity of the charged particles in the direction of the field. The velocity of charges in the direction of the field is characterized by mobility, which is numerically equal to the velocity acquired by a charge in a field of intensity 1 V/cm. Mobility is proportional to the magnitude of the charge and inversely proportional to the mass of the particles.
When a substance enters the detector, it causes an increase in the number of recombinations and a decrease in the mobility of charged particles. As a result, the current of the detector decreases. This reduction in current is recorded on the chromatogram as a peak of the substance. The operation of the electron capture detector (ECD) is based on this principle.
The next zone is the saturation area (Zone II), characterized by the absence of recombination and complete collection of all generated charged particles. In this case, the ion current is determined only by the rate of charge formation. The signal from detectors operating in this section of the voltage-current characteristic is an increase in current caused by a significant increase in the rate of charged particle formation due to the ionization of the analyzed components entering the detector. In this case, ionization of the carrier gas must be absent, and the background current level must be minimal. Representatives of this type of detectors include the DIP, thermionic detector (DTI), flame photometric detector (FPD), among others.
In a quadrupole mass spectrometer, mass separation is achieved in a different way. A high-frequency electric field is formed between four permanent magnets. When an ion beam enters this field, only ions with a specific mass-to-charge ratio have a stable trajectory and reach the detector (collector). Detection of beams with different mass-to-charge ratios is conducted by varying the electric field.
Comments
Post a Comment