Gas materime GCMS double ion source EI and CI differential performance

For the same mass spectrometer, several different ion sources such as electron bombardment ionization sources (EI) and chemical ionization sources (CI)
can be used.

EI is currently commonly used GCMS ionization method, because its rich mass spectrometry library is widely used in qualitative analysis and quantitative analysis, but this hard ionization method usually does not get the molecular ion information of the compound, especially for the target compound that is not in the mass spectrometry library, it is more difficult to use EI to characterize it more accurately, at this time it is necessary to use the CI source This soft ionization method, by obtaining molecular ion information and then inferring the structure of the compound, It is very useful for characterization, and some compounds can be analyzed with CI to obtain higher sensitivity
.

Principle of the EI source: Its main working principle is that the filament emits electrons, which are focused and pass through the ionization chamber under the action of a magnetic field to reach the collection electrode
.

At this time, the sample molecules entering the ionization chamber are ionized under the action of certain energy electrons, and the ions are focused and accelerated into an ion beam into the mass spectrometry analyzer
.

Principle of CI source: consists of an ionization chamber (ion box), a filament, an ion-focused lens and a pair of magnetic poles
.

The difference between the application of the EI source and the CI source is as follows: the main difference is that the air tightness of the CI source ionization chamber is better than that of the EI source to ensure that the reaction time into the ion source has sufficient pressure
.

One is an electronic source, one is a chemical source, for different molecular weights, CI analysis of molecular weight will be larger, of course, the principle of the two source analysis is also different! The two methods of ionization are different
.

The EI source is that the electron bombardment makes the sample charged, and the structure will be more broken
.

CI is ionized by reacting with the sample by reacting with the reaction gas, and the molecular ions are generally well
retained.

The EI source is mainly composed of an ionization chamber (ion box), a filament, an ion-focused lens and a pair of magnetic poles
.

The filament emits electrons, which are focused and pass through the ion cosine theorem under the action of a magnetic field to reach the collection electrode
.

At this time, the sample molecules entering the ionization chamber are ionized under the action of certain energy electrons, and the ions are focused and accelerated into ion beams into the mass analyzer
.

Advantages of EI: non-selective ionization, as long as the sample can be gasified can be ionized; High ionization efficiency and high sensitivity; EI spectra provide rich structural information and are the "fingerprint" of compounds; There is a huge standard spectral library for retrieval, the spectral map is obtained under 70 eV conditions, the spectral repeatability is good, called the classical EI spectrum (refers to the proportion of isotopic peaks in the spectra can reflect the natural isotopic abundance distribution of
the ion.

Disadvantages of EI: the sample must be vaporizable, not suitable for volatile, thermally unstable samples; Some compounds are unstable and fragile in the EI mode, and molecular weight information cannot be obtained, and it is difficult to interpret the complex spectra.
The EI method can only detect positive ions, not negative ions
.

The structure of the CI ion source is similar to that of the EI source, which is also composed of an ionization chamber (ion cartridge), a filament, an ion focusing lens and a pair of magnetic poles
.

The main difference is that the airtightness of the ionization chamber is better than that of the EI source to ensure that the reaction time into the ion source is under sufficient pressure
.

CI and EI, the filament emitted electrons to ionize neutral molecules, the difference is that the sample and the reaction reagent together into the ionization chamber, the reaction concentration is higher than the sample concentration, the first ionization is the reaction reagent neutral molecules, due to the higher pressure, the ion-molecule reaction occurs, producing a variety of active reactive ions, these ions and sample molecules and then ion-molecule reaction, to achieve sample molecule ionization
.

Commonly used reaction gas reagents are methane, isobutane, ammonia and the like
.

Advantages of CI: CI is not only an important means of obtaining molecular weight information, but also can be used to control the reaction, according to the ion affinity and electronegativity to select different reaction reagents for the selective detection
of different compounds.

Disadvantages of CI: Like EI, the sample must be vaporizable, not suitable for difficult volatile, thermally unstable samples; Moreover, ci spectra are not as reproducible as EI, and there is no standard spectral library
.

In addition, the reaction reagent is easy to form a higher cost and a low cost, which affects the detection limit
.

The pressure of the reaction reagent needs to be explored
.

Relative to the EI source, ci source requires additional methane, isobutane and other reactant gases ionized in the ion source, and the target compound molecular-ion reaction, so the traditional ion source is an independent EI source and CI source, when the need to replace the ionization mode needs to remove the vacuum for hardware replacement, as you know the replacement process and the stability of the vacuum system takes a long time, which brings great inconvenience to the experimental work, dual source EI / CI composite ion source, Under the premise of ensuring that the sensitivity of the EI source is not affected, it is easy to seamlessly switch between the two ionization modes of an ion source EI and CI, which meets the needs of customers in different fields of qualitative and quantitative analysis
.

1, save a lot of time, improve experimental efficiency: no need to remove vacuum when switching ionization mode, no need to change the ion source, 2, based on the similarity of molecular weight retrieval, strengthen the accuracy of identification results: obtain molecular ion peaks through CI mode to determine the molecular weight of the target compound, and then obtain qualitative results of similarity retrieval by comparing EI mode to strengthen the accuracy
of identification.

3, to ensure high sensitivity, with independent ion sources have the same qualitative quantitative effect