Agilent Single Quad 5975 GCMS CI theory Overview Video
Introduction
This video will provide a brief overview of the Positive and Negative CI theory as it relates to the Single Quad.
Chemical Ionization is a different technique to create positive or negative mass spectrums in M S analysis. For some molecules, EI is not ideal since it causes too much fragmentation. In CI a reagent gas is first ionized. It then reacts with the analyte molecule forming an ion. The most common reagent gas for CI is methane. However there are many other choices, depending on the application.
Chemical Ionization is an attractive option for G C M S because it adds complementary analytical capability to the core functions of E I M S. Because it is a softer ionization approach than EI, CI provides a simpler mass spectrum with more intensity concentrated in high mass ions. The CI mass spectrum provides additional structural information which complements the EI spectrum. CI can also provide more selectivity than EI depending upon the nature of the sample matrix. For example CI is very selective against a saturated hydrocarbon background.
This graphic shows a profile of the reagent ions for methane CI in the Single Quad system. The main reagent ions for methane are observed at mass-to-charge 17, 29, and 41. Notice that water in the background of the mass spectrometer always creates some water CI reagent ions at mass-to-charge 19.
Example of CI versus EI
Cocaine is a Drug of Abuse which exhibits a weak molecular ion in EI. Using CI, the spectrum can be used to confirm the molecular weight. Let's look at the EI spectrum of Cocaine.
The chemical structure of cocaine is shown above and the EI mass spectrum below. Notice the low intensity of molecular ion and the extensive fragmentation to ions at mass-to-charge 82 and 182 due to the highly energetic EI fragmentation process.
This is the methane CI mass spectrum of cocaine. The M+1 ion at mass-to-charge 304 confirms the molecular weight of 303 suggested by the low-abundance M+ ion in EI mode.
Reagent Gas Selection
In the case of cocaine, the “softer” CI reagent ions from iso butane generate a CI spectrum dominated by the M+1 ion.
When you perform chemical ionization you may select from numerous CI reagents to meet your analysis needs. Use a Hard CI reagent such as methane for a strong response and more fragmentation. To enhance M+1 ion abundance, and reduce fragmentation, use a softer CI reagent such as iso butane or ammonia.
Here are the CI reagents in order of harder to softer
The limitations of CI is that the CI spectrum has less fragmentation and the results depend on CI gas type, CI pressure and the nature of the compound.
Formation of the reagent gas ions come From E I ionization of the methane molecule into several different reaction pathways. The predominate reagent ion formed in CI for methane is 17, 29 and 41.
The relative abundances of the different reagent ions is dependant on the methane pressure. As you can see by this plot, the lower pressure favors the formation of some of the reagent ions and the higher pressure favors formation of other ions.
The various proton affinities for the different reagent gas types are shown in this table.
Positive CI - PCI
In CI, there are four fundamental ionization processes. These include the most common which is Proton Transfer where the ion formed is usually M plus 1. Other type of CI reactions are Adduct formation and hydride abstraction.
After the formation of the reagent ion, the reagent ion reacts with the sample molecule to form an positive or negative ion. There are several different ionization pathways. The most common is the addition of a hydrogen to form an M plus one ion. Other less common pathways include Adduct formation and Hydride abstraction.
In proton transfer, if the proton affinity of the analyte sample is greater than the reagent gas, then a positively charged ion of M plus one is formed.
Negative CI - NCI
IN negative CI, the reagent ion acts to form lower energy electrons. Electro negative compounds that have a high affinity for these electrons accept them to form a negatively charged ion. Examples of compounds that show good negative CI response are molecules containing halogens, nitrogen, oxygen and phosphorus.
Here is a table showing optimal condition for negative CI. These include, low reagent pressure, low levels of oxygen and water, lower temps and lower extraction voltages.
Here is a spectra of Diazepam in both EI and NCI. Note the strong formation of the negatively charged molecular ion.
In Summary for NCI, the reagent gas forms lower energy "thermalized" electrons that react with certain types of molecules. For these compounds the response is very good and matrix interferences usually do not occur.
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