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Acquisitions of Molecular-weight Information Using Soft-ionization Techniques (PI and CI)

Product: JMS-Q1500GC GC/MS System

Introduction

Electron ionization (EI), a hard-ionization technique that generates many fragment ions, is the most widely used ionization technique in gas chromatography–mass spectrometry (GC-MS). Since EI mass spectra have good reproducibility, qualitative analysis is possible by comparing an EI mass spectrum of a sample with that of the known compound recorded in the database. However, sometimes EI mass spectra lack molecular ions, which are of key importance to molecular-weight (MW) determination and correct compound assignment. Soft ionization is a useful way to determine MW. The JEOL JMS-Q1500GC offers two soft-ionization techniques: chemical ionization (CI) and photoionization (PI).

In this application note, the MW information of diethyl phthalate and n-tetradecane were estimated from measurement results using both of these soft-ionization techniques.

What is Chemical Ionization?

The CI ion source consists of a chamber with higher hermeticity than the EI ion source, as well as a filament and a repeller. The reagent gas introduced into the CI chamber is ionized; ionization of the sample occurs by electrons, protons, and other charged species transferring from the ionized reagent gas to the sample molecule (M). Generally, methane, iso-butane, or ammonia is used as the reagent gas. In the case of a proton transfer reaction, we can predict protonated [M+H]+ ion generation based on the proton affinity (PA) of both the reagent gas and M. When M has higher PA than the reagent gas, a proton transfer reaction occurs. However, CI can be unpredictable, in that generated ion species differ depending on the reagent gas. In the case of methane or iso-butane as the reagent gas, [M+H]+ ions are generally produced by proton transfer. Other ions such as [M-H]+, M+, or [M + reagent ion]+ may also be detected.

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