Elodie

CI provide more abundant molecular ions. This way you can increase the sensitivity and selectivity in SIM mode. In terms of SCAN the identyfication is more problematic comparing to EI.

Regards

G

Dear Elodie,

Main difference between EI & CI can explain following way:

EI : Electron bombardment (with energy of 70 eV) takes place directly to the molecule, here no reagent gases are useed. It's also called hard ionization technique. In this case high energy electron completely broke down the parent molecule to their corresponding daughter ions. Therefore probability of getting molecular ion peak is almost negligible. This technique more significant during unknown molecule analysis.

CI : two types-a) Positive Chemical Ionisation (PCI), Negative Chemical Ionisation (NCI).

PCI : Electron bombardment takes place through reagent gas (like methane, ammonia, iso-butane etc), due to presence of highly dense (than analyte molecule) reagent gas, electron bombardment preferentially occur with reagent gas and they form reactive ions, then these ions are react with analyte molecule (known as ion molecular reaction), and provides quasi-molecular ion or adduct ion ([M+H]+ [M+C2H5]+ etc).

NCI : Ionization technique similar to PCI, here instead of positive ion negative ion form. Ion source temperature kep lower than PCI to form thermal electron (or slow moving elactron).

CI techniques are significant when molecular weight of an unknown molecule require.

Dear Elodie,

I understand that you are very familiar with EI mode of ionization. EI is a very hard ionization technique which gives more structural infiormation from the fragmentation pattern. With EI mode, you can do the library search also. EI mode of ionization is always with 70 eV energy electrons. In most of the cases, you will not be able to see the molecular ion (except in few cases) and hence the information on molecular weight will not come through. Chemical Ionization (CI) is a soft ionization technique which mainly provide molecular weight information. Here you have to use a reagent gas like Methane, Iso-butane, Ammonia etc. CH4 is the most widely used reagent gas. In most of the cases you will be able to see an M+1 peak, though it can give other ions like M-1, Adduct ions with reagent gas etc. EI and CI can be used as complimentary to each other. While EI provides structural information, CI provides molecular weight information and hence you can characterize the molecule with full confidence. Negative Chemical Ionization (NCI) is only used for trace analysis of electrophilic compounds. Please keep in mind that you can not do library search with CI analysis.

Dear Elodie,

regarding the last part of your questions: CI ion sources operate at higher pressuse (compared with EI) as they work with surplus of ionization agent (methane, butane, isobutane, ammonia...) .

For this reason, CI sources utilize close ion volumes with a small hole, while EI have open ones.

Some producers (e.g.Thermo) of MS instruments offer a very quick exchange of ion source/ion volume without neccesity of shutting down the instrument, so choose your instrument wisely...

Speaking about soft ionization in GC/MS, it is necessary to mention new "variable energy electron ionization", which was introduced by ALMSCO, UK (Markes group), and is said to offer some advantages over "classical" CI.

hope it helps,

Tomas

Allow me to join in the discussion. And what about the calibrant used during MS tuning? I'm just switching from EI to CI. Is it necessary to change the calibrant from PFTBA to PFDTD?

Thank you in advance for your suggestion :)

Monika

Hi, Monika, As long as you are in the mass range upto 600, PFTBA can be used for tuning.

Mr. Girijan, where do you find the information show that we can use PFTBA replacing PFDTD in CI method when mass range up to 600? Would you mind to introduce that book for me? I extremely want to know about it.

Thanks and Best,

Dear Elodie,

I would like to differentiate between two ionization.

CI(chemical Ionization) uses a common source for ionization as EI(Electron Impact Ionization), with some modifications. CI is soft where EI is hard ionization. An electron impact (EI) or a chemical ionization (CI) ion source can be used to operate the mass spectrometer.

Electron impact ionization (EII) is of fundamental importance in understanding the physics of the collision process associated with many electron problems involving atoms, neutral, and ions. An ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions. Electron impact ionization delivers the highest energy to the analytes leading to ‘hard’ ionization and causes a large amount of molecular fragmentation.

Chemical ionization requires a lower amount of energy compared to electron ionization (EI), but this depends on the reactant material used. This low-energy ionization mechanism yields less or sometimes no fragmentation, and usually a simpler spectrum, as a result causes less molecular fragmentation.

Compared to EI, this CI technique provides less energy resulting in “soft” ionization. Mass spectra acquired from a CI source generally contain the molecular ion. This boosts confidence in identification and assignment of the molecular ion, which is crucial in the analysis of unknowns. Individual components in complex matrices can also be identified using chemical ionization. However, in complex samples, all the individual components observed can be difficult to resolve chromatographically. In such situations, co‐eluting peaks can be deconvoluted using CI.

The absence of molecular ion (ion representing the charged analyte) in the EI spectra is quite common, and therefore it would be difficult to identify an analyte for which a database spectrum is unavailable. CI facilitates the ability to detect the molecular ion peak, due to less extensive fragmentation.[3] Chemical ionization can also be used to identify and quantify an analyte present in a sample, by coupling chromatographic separation techniques to CI.

To facilitate the reactions between the ions-gases, the chamber is kept gastight with a pressure around 1 torr. Electrons are produced through a metal filament, which is made of tungsten, rhenium, or iridium and travel to a longer distance in the ionization chamber, due to high energy it owns. In contrast to EI, the magnet and the electron trap is not needed for CI, since the electron beam do not travel to the end of the chamber. The pressure inside the chamber is kept below 10−4 torr.

Hope it work out for you.

Ashish