RF mass spectrometer


RF mass spectrometer is also called Quadrupole Mass Spectrometer. The core component of the radio frequency mass spectrometer is a quadrupole rod. In the quadruple rod, the four electrode rods are divided into two groups, and a radio frequency (RF) inverse alternating voltage is applied to them respectively. The ions located in this potential field can reach the detector after the selected part is stable, or enter the space afterwards for subsequent analysis. The structure and circuit of the radio frequency mass spectrometer are simpler than other mass spectrometers. The cost is also relatively low. Radio frequency mass spectrometers are widely used in Chromatography-MassSepctrometry (Chromatography-MassSepctrometry).

TendemMassSpectrometry (TendemMS) can be achieved through the use of multiple quadrupoles in series to obtain the structure information of the analyte.

The radio frequency mass spectrometer only allows ions with a single charge-to-mass ratio to pass through each time. When scanning a large mass interval, the time required by the radio frequency mass spectrometer is much greater than that of the time of flight mass spectrometry (TimeofFlightMassSpectrometry, ToF-MS). ), Orbitrap MS (OrbitrapMS), Linear Ion Trap (LinearIonTrap) and other mass spectrometers that use pulsed sampling.

Radio frequency

Radio frequency (RF) is the abbreviation of RadioFrequency, which means the electromagnetic frequency that can be radiated into space, and the frequency range is from 300KHz to 300GHz. Radio frequency is abbreviated as RF. Radio frequency is radio frequency current, which is an abbreviation of high-frequency alternating current changing electromagnetic wave. The alternating current that changes less than 1,000 times per second is called low-frequency current, and the one that changes more than 10,000 times is called high-frequency current. Radio frequency is such a high-frequency current. High frequency (greater than 10K); radio frequency (300K-300G) is the higher frequency band of high frequency; microwave frequency band (300M-300G) is the higher frequency band of radio frequency.

In the theory of electronics, when current flows through a conductor, a magnetic field will be formed around the conductor; when an alternating current passes through the conductor, an alternating electromagnetic field will be formed around the conductor, which is called electromagnetic wave. When the electromagnetic wave frequency is lower than 100khz, the electromagnetic wave will be absorbed by the surface and cannot form effective transmission, but when the electromagnetic wave frequency is higher than 100kHz, the electromagnetic wave can propagate in the air and be reflected by the ionosphere at the outer edge of the atmosphere to form long-distance transmission capability , We call high-frequency electromagnetic waves with long-distance transmission capabilities as radio frequency; radio frequency technology is widely used in the field of wireless communication, and the cable television system uses radio frequency transmission.


The earliest literature on radio frequency mass spectrometers dates back to mid-1950. The inventor, Professor Wolfgang Paul, won the Nobel Prize in Physics in 1989.

Quality selector and its principle

Although most of the quadrupole quality selectors used in reality are cylindrical, the ideal quality selector is a hyperbolic shape. The size of the mass selector is usually between a few centimeters to tens of centimeters.

The four poles of the quadrupole quality selector are divided into two corresponding groups, and the inverse radio frequency high voltage is applied respectively. The expressions of the two sets of voltages are:

The two sets of voltages have only opposite signs. Where U is the direct current (DC) component, and V is the amplitude of the radio frequency (alternating current that reaches the transmit frequency, RF) component (V_rms is used here instead of Vp-p).

Under normal circumstances, the value of U is 500-2000V, and the value of V is 0-3000V.

In such an electric field environment, ions will oscillate according to the electric field. However, only ions with a specific charge-to-mass ratio can pass through the electric field stably. When the voltage on the pole is specified, the ions with too small mass will be affected by a large voltage, which will cause a very violent oscillation, which will cause the contact pole to lose charge and be pumped away by the vacuum system; It cannot be pulled by enough electric field, and eventually it will touch the pole or fly out of the electric field and fail to pass the mass selector.

In the hardware of the quadrupole mass selector, the usual method is to adjust the radio frequency working frequency w to select the mass of the ion, and adjust the ratio of U to V to adjust the ion pass rate. The corresponding picture in this section shows that the triangular area is the stable area of ​​the ion of this mass. The ratio of U to V is represented here as a slope. It can be seen that the larger the U/V, the higher the accuracy of ion selection, and the stronger the resolution of the instrument, but the number of ions that can be stably passed is reduced; while the smaller the U/V ratio, the more ions pass, but the resolution decreases. . After weighing, the resolution of most RF mass spectrometers is about 1Th, which is reflected in the mass spectrum that the half-peak width is about 1Th or 1Da.

It is worth pointing out that when the U value is zero, that is, when only RF voltage is applied to the quadrupole, all ions can pass. The significance of this operation is that the ion beam can be more concentrated. Usually used as an ion mirror (IonLens). The most typical extension is the appearance of octopoles and hexapoles, which are actually derived from the basic working characteristics of quadrupoles.

Vacuum system

The vacuum system of RF mass spectrometer is usually divided into two stages.

The primary vacuum system provides basic vacuum support for the secondary vacuum system. The secondary vacuum system is usually directly connected to the cavity of the mass spectrometer to make the mass spectrometer reach a vacuum state. It is worth noting that the vacuum of the RF mass spectrometer is not a high vacuum (0.001Pa). The ions move in the pole, and a large amount of energy is obtained from the electric field. In order to form a stable ion cloud, a very small amount of gas must be present in RF mass spectrometry to absorb excess kinetic energy. The vacuum of a radio frequency mass spectrometer is usually one percent of the time-of-flight mass spectrometry (1e-5Pa) and one ten billionth of the orbital ion trap mass spectrometry (1e-14Pa).

Primary vacuum

The primary vacuum is usually a mechanical pump (RoughingPump) or a scroll pump (ScrollPump). The degree of vacuum is about 1mTorr (0.13Pa).

Mechanical pumps are relatively inexpensive compared to roll pumps, but they require lubricating oil to operate. When conducting gas-sensitive analysis, especially in the field of atmospheric sciences, a roll pump is usually used instead of a mechanical pump.

Secondary vacuum

The secondary vacuum usually uses a turbomolecular pump (TurbomolecularPump) or a dispersion pump (DiffusionPump).

The molecular pump is small in size, and its efficiency is higher than that of the dispersion pump. The usual molecular pumps can support an airflow velocity of 350L/min, and the more high-end molecular pumps can achieve an ultra-high vacuum of 1e-14Pa.

The dispersing pump has a huge volume, which can reach 1-2 meters. In modern instruments, it has basically been replaced by turbomolecular pumps.

For the vacuum conditions required by the quadrupole mass spectrometer, the turbomolecular pump can usually reach within 30 minutes. The dispersion pump needs 20-80 hours.

Power system

Because the quadrupole system requires high-frequency voltage, magnetic cores are usually not used in the core power supply system of RF mass spectrometry, but air-core transformers are used to ensure the circuit For high frequency radio frequency response. The early shock-generating components used a capacitor-inductor-transistor self-oscillation method (the mass spectrometer produced by THS in Georgia, USA still uses this system), with electronic technology For the development of oscillating sources, voltage-controlled oscillators (VoltagedControlledOscillator, VCO) or direct digital synthesis (DirectDigital Synthesis, DDS) are mostly used.

Extended applications

Direct measurement


RF mass spectrometer can be used as a direct measurement instrument use.

By matching different ion sources, the RF mass spectrometer is used as a general analytical chemistry tool. Especially in the long-term measurement, the amount of data generated by the RF mass spectrometer is significantly less than other parallel measurement mass spectra (time-of-flight mass spectrometry, etc.).

Multi-stage mass spectrometry


Due to the low ResolvingPower of the RF mass spectrometer , So the ability to determine unknown substances is lacking. Through multi-stage mass spectrometry, ions pass through independent chambers in the middle of the two groups of quadrupole systems for fragmentation. Thus, the fragments produced by ions of specific mass are analyzed, and the structure information of the ions can be obtained.

The fragmentation method includes the collision fragmentation method (CollisionInducedDissociation, CID) in which the injected gas collides with ions, and the method that directly emits the electron fragmentation through the electron gun (ElectronDissociation).

Multistage mass spectrometry plays a vital role in biochemistry and organic chemistry.

Chromatography-Mass Spectrometry


The most typical application of chromatography-mass spectrometry is gas chromatography Mass Spectrometry (GasChromatography-MassSpectrometry, GC-MS) and Liquid Chromatography-MassSpectrometry (LiquidChromatography-MassSpectrometry).

The advantage lies in the combination of chromatography and mass spectrometry, which solves the problem that if the masses of ions are too similar in mass spectrometry, they cannot be distinguished. In chromatography, the retention time (RetensionTime) gives the structural information of different types of substances in the mixture, and the pre-separation operation improves the credibility of the mass spectrometer. This method is similar to ion mobility spectrometry (IonMobility Spectrometry-MS, IMS-MS).

The difficulty of applying this method lies in how to couple chromatography equipment and mass spectrometry equipment. The most commonly used method is electrospray ionization (Electrospray Ionization, ESI).

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