Question

a) describe a quadropole mass analyzer

b) explain the electron-impact source

c) describe what is meants by a “double-focusing” instruments.

Answer 1

a. The quadrapole mass analyzer consists of four metal rods which are arranged in a square manner, where the same charged rods are placed diagonally. These metal rods are parallel to the beam of ions entering into the mass analyzer. An electrostatic field accross these rods is generated by appling Radiofrequency and electric current (DC), this electrostatic filed generated results in oscilating motion of the ions travelling through the metal rods. When ions of varied mass to charge ratio (m/z) are allowed to pass through the analyzer at a given ratio of radiofrequency and voltage, only certain particular ions with m/z will reach the detector, that is the seperation of these ions depends on the ratio of radiofrequency and DC voltage applied.

If the Radiofrequency is greater than DC voltage applied, ions with large size will hit the detector first.

If the DC voltage applied is greater than Radiofrequency, ions with smaller size hit the detector first.

However, the ions with inappropritate m/z does not reach the detector.

b. Electron-impact source is the fundamental source of ions generated in mass spectroscopy. Here, the sample molecules are broken down or fragmented into ions by colliding them with electrons generated from a filament. The electrons generated from the filament are fast moving and hence knocks out the valence electron from the sample molecule generating a positive ion (M⁺). Further, the fragmented ions are extracted from ionization region by applying electric field and allowed to pass through a slit leading to mass analyzer.

c. The double-focusing instrument is an advanced version of single-focusing instrument for better resolution. The double-focusing instrument consists of a horse-shoe shaped chamber with an inlet for the entry of sample ions with varied m/z size. This chamber consists of first magnetic field with magnetic analyzer and second electrostatic field with electrostatic analyzer. The positive ions generated in ionization chamber are accelerated into magnetic analyzer and are deflected by the magnetic field and travel in a curved path, in this process the ions are seperated depending on the mass of ions in the mass mass analyzer before reaching a Slit A. The ions with larger size reach the slit first and enter into a electrostatic analyzer consisting of a negative and positive electrode. Since the ions are positive in charge, these ions are repelled and deflected by positive charge electrode and attracted by negative charged electrode to travel in a straight path with a particular velocity, where the velocity of ion depends on it m/z ratio. The molecule with same m/z value hit pass through a Slit B and hit the detector. Here ions with large size hit the detector first. Since the double-focusing instrument has two analyzers, magnetic and electrostatic they result in better resolution.

Magnet positive electrod Slit A Inlet . Negative Electod Magnetic Analyzer Electrostatic analyzer