GCMS Ionization Modesfor the QP2010 Plus:EI, CI and NCI

Bill Dougherty – September 30, 2008

© 2007 SHIMADZU2

GCMS Ionization Modes

EI: Electron IonizationCI (or PCI): Positive ion Chemical IonizationNCI: Negative ion Chemical Ionization

All 3 ionization mode use the same filaments.No additional (secondary) rotary pump is required to perform CI or NCI.

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QP2010 Plus Models

EI: + Ion Detection Just uses filament for ionization Positive ion detection

EI and PCI: + Ion Detection Needs extra reagent gas plumbing, electronics, ion box and reagent gas

EI, CI and NCI: + and – Ion Detection Needs third ion box and extra power supplies and boards to change polarity of lens, quadrupole and conversion dynode voltages.

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Modifications to FilamentModifications to FilamentThe filament wire is heavier gauge. Possible warping of filaments is reduced.Aperture is enlarged to allow transmission of more electrons.Improved filaments now being used on GCMS-QP2010 and GCMS-QP2010s

Result: Filaments can be operated at higher emission current (150µA) to produce more ions.

More ions = increased sensitivity225-10340-91

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New Filament vs. Old Filament

New Filament: 225-10340-91Changed coil windings, support distance and size of shield window to make less susceptible to bending.Average lifetime of 1000 hours at 150 microamps.

Old Filament: 225-10197-91Average lifetime of 1500 hours at 60 microamps.

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EI – Electron Ionization

The electron source filament has high current that puts electrons on its surface.

Electrons on the filament surface are accelerated into the ion chamber because it is 70 Volts more positive than the filament.

The “Ionization Voltage” is 70 Volts.The average “Electron Energy” is 70eV (Electron Volts).Determines how much the molecule is fragmented, the appearance of the mass spectrum.70eV is the standard set for all GCMS.All GCMS library spectra are acquired at this energy.

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EI – Creation of + Ions

High energy filament electron (70eV) passes by and transfers energy to Benzophenone.

Benzophenone internalizes this energy then dissociates electrons within bonds of its atoms.

Bonds come apartFragments are createdFragments are missing electrons therefore they are + chargeThe more energy that is transferred from the filament electron to the molecule; the more fragmentation of molecule.

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EI – Ion Box open to vacuum

For the molecules to dissociate electrons and form ions they must stay isolated from other species or they will give away energy to other species and not form ions.

The space in the ion chamber must be at low pressure to keep the molecules isolated.The EI on box and repeller plate have large openings so that the ion chamber is at low pressure

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EI Mass Spectrum of Benzophenone

The EI mass spectrum for Benzophenone below shows the distribution of ions created by dissociation of electrons.

Benzophenone molecules that absorb more energy from the filament electrons fragment more, like m/z 51 and 77.

Benzophenone molecules that fragment less produce the base peak ion at m/z 105

The benzophenone molecular ion is m/z 182, it dissociated only one electron and did not fragment.

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Positive ions are attracted to -70 Volt Filament without shielding and increased distance. Positive ion transmission to the lenses, quadrupoles and detector is increased.

GCMS-QP2010 GCMS-QP2010 Plus

:Filament : Electrical field : Flow of ions

: Ion source (set temperature) : Ion source (Above set temperature)

: Heat rays

　 shield plate

Ion Source

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QP2010 PlusQP2010

The ions are extracted by the electric potential formed by the second lens (Lens 2).The filament electric potential (-70V,10V)does not penetrate the ion box.

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Huge Improvement in ion extraction efficiency

QP2010 PlusQP2010

The ions are extracted only in the direction of the mass analyzer. Ion transmission is greatly improved.

Mass analyzer

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Chemical Ionization – CI & NCI

Ion Box must be pressurized with reagent gas with high vacuum on its outside. CI (PCI) box needs higher pressure; holes in it are smaller than NCI box.

Filament holes are very small PCI has 2 holes; NCI has 4 holes

Ion exit hole is small Repeller electrode “spacer” seals ion box. Spacer has no hole for DI probe if not used.

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Ion Box and Spacer – EI, CI & NCI

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Chemical Ionization Reagent Gases

3 possible types of reagent gas Methane (CH4)

cheapest, most common, cleanest

Isobutane (C4H10) softer ionization for more fragile molecules expensive, “hard to come by,” purity not so good

Ammonia (NH3) negative ion for alcohols, destructive to rotary pump oil and rotary pump

2 Reagent Gas Ports on back of MS Can select from 2 types of reagent gas when perform autotune.

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Chemical Ionization – CI & NCI

Definition of Chemical Ionization: Some part of the reagent gas molecule must end up as part of the ion that is detected.

Positive Ion Chemical Ionization Called CI (or PCI) Addition of proton or + ion from reagent gas

Negative Chemical Ionization Called NCI Addition of electron from filament Jokingly referred to as “Not Chemical Ionization.”

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CI – Positive Ion Chemical Ionization

CI adds positively charged ions of the reagent gas to the sample molecules with much less or no fragmentation, compared to EI.

The ion chamber must be pressurized with the reagent gas. (To reduce fragmentation and create abundant reagent gas ions.) Filament electrons ionize the reagent gas producing + charged reagent gas ions. Sample molecules to be ionized must be able to capture a proton or + charged reagent gas ion. A “molecular adduct ion” is created when the + ion is created with no fragmentation. This identifies the molecular ion; the main reason for chemical ionization.

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CI with Methane Reagent Gas

Methane, CH4, produces these ions:

CH5+ (17 amu) Adds a proton (+1 amu) to the sample molecule

C2H5+ (29 amu) Combines with sample molecule

C3H5+ (41 amu) Combines with sample molecule

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Benzophenone – EI vs. CI Spectra

EI

M = 182

CI

M+1=183

M+29=211

M+41+223

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NCI – Negative Chemical Ionization

NCI adds negatively charged electrons from the filament to the sample molecules with less fragmentation than EI.

The ion chamber must be pressurized with the reagent gas to reduce fragmentation.

Reagent gas pressure does not need to be as high as for CI. Filament electrons lose energy to the buffer gas. Sample molecules form negative ions by capturing low energy filament electrons. Sample molecules to be ionized must be able to capture an electron. The same types of molecules that are detected by an ECD are also detected by NCI. NCI is “ECD with Quadrupoles.”

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Hexachlorobenzene – EI vs. NCI

The mass of an electron is negligible compared to 1 amu.

The ion is the same mass whether it loses (EI) or captures (NCI) an electron.

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NCI Ion Box “Sub” modes

You need to break vacuum to change between the EI, CI or NCI ion box. Without changing the ion box you can use the NCI ion box to run:

SEI, SCI and NCI (all 3 ionization modes) SCI is CI using the NCI ion box SEI is EI using the NCI ion box

The software will run autotune for each or all of the 3 modes NCI, SCI and SEI. The ionization mode is changed in the method file. Reagent gas in the tune file.

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Sensitivities of Ionization Modes

Most sensitive to least sensitive:

NCI (- ion, capture electron) Extremely sensitive; like ECD

EI (+ ion, lose electron)

SEI (+ ion, lose electron)

CI (+ ion, add reagent gas ion)

SCI (+ ion, add reagent gas ion)

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A Final Question

What is another name for a positively charged ion?

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Thank You for Coming

A “Cat” ion (Lion King+1)