GCMS Ionization Modes

for the QP2010Plus:

EI, CI and NCI

© 2007 SHIMADZU2

GCMS Ionization Modes

EI: Electron Ionization

CI (or PCI): Positive ion Chemical Ionization

NCI: Negative ion Chemical Ionization

All 3 ionization mode use the same filaments.

No additional (secondary) rotary pump is required to perform CI or NCI.

© 2007 SHIMADZU3

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.

© 2007 SHIMADZU4

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

© 2007 SHIMADZU5

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.

© 2007 SHIMADZU6

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.

© 2007 SHIMADZU7

Ion Box and Spacer – EI, CI & NCI

© 2007 SHIMADZU8

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) softer ionization than methane, better results for some compounds, 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.

© 2007 SHIMADZU9

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.”

© 2007 SHIMADZU10

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.

© 2007 SHIMADZU11

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

© 2007 SHIMADZU12

Benzophenone – EI vs. CI Spectra

EI

M = 182

CI

M+1=183

M+29=211

M+41+223

© 2007 SHIMADZU13

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.”

© 2007 SHIMADZU14

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.

© 2007 SHIMADZU15

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.

© 2007 SHIMADZU16

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)

Endosulfan Mass Spectra

50 100 150 200 250 300 350 400 450 5000.0

1.0

(x100,000)

241207170 19315975 229 26369 109 33989 162 33719961 91 216 307252126 407 429 479463

50 100 150 200 250 300 350 400 450 5000.0

2.5

(x1,000)

407277 373

405279411243 375307 343111 325207 271135 369161 197 435269229 253 401105 383 497

50 100 150 200 250 300 350 400 450 5000.0

2.5(x1,000,000)

242

234 270 336101 300 372198 406169130 431148 481

EI

CI

NCI

Chromatogram of Organochlorine Pesticide Mix, EI Mode

7.5 10.0 12.5 15.0 17.5 20.0

0.25

0.50

0.75

1.00

1.25(x10,000,000)

TIC

Chromatogram of Organochlorine Pesticide Mix, CI Mode

5.0 7.5 10.0 12.5 15.0 17.5 20.0

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

(x100,000)TIC

Chromatogram of Organochlorine Pesticide Mix, NCI Mode

5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25(x10,000,000)

TIC

Endosulfan EI and SEI Mass Spectra

50 100 150 200 250 300 350 400 450 5000.0

0.5

1.0

(x10,000)

241265204

207195 243

229 269 33919317057 78 27275 201 27923011953 305136 180 26164 337165 182 323254 36061 223 391 499472449418288

50 100 150 200 250 300 350 400 450 5000.0

0.5

1.0

1.5

(x100,000)

241195 237

243170193159 22975 263

231 26785 10969 27299 121 33916250 169 33719961 216 307181 32355 14791 252 343

126 407 429 463

EI

SEI

Endosulfan CI and SCI Mass Spectra

100 150 200 250 300 350 400 450 5000.0

1.0

2.0

3.0

(x1,000)

407409

277 373

371275

243 307 343111 325281207 309 345245135 323291107 377 413161 329197 239 435293 401313 349 383 421 497477

100 150 200 250 300 350 400 450 5000.0

2.5

5.0

7.5

(x100)

407277109

243 409275207107

373209117 281 371245 307161 327103 135 187 289115 131 309235201 265 287165 313249 413152 447353333 495431

CI

SCI

BHC isomer chromatograms, EI, CI, and NCI

5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.50.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0(x100,000)

183.00 (1.00)

5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.50.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75(x10,000)

219.00 (1.00)

5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

(x100,000)255.00 (1.00)

EI

CI

NCI

BHC isomer chromatograms, EI and SEI

5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.50.0

0.5

1.0

1.5

2.0

2.5

3.0

(x10,000)183.00 (1.00)

5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.50.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0(x100,000)

183.00 (1.00) EI

SEI