Review of MS and Coupling Techniques

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Review of MS and Coupling Techniques

Course notes can be found at:

h<p://www.analy@k.ethz.ch/vorlesungen/ModernMS.htm

Rob Nieckarz

Oﬃce: HCI D325

nieckarz@org.chem.ethz.ch

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Lorentz Force equa@on

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Resolu@on: Smallest mass diﬀerence

∆m between two equal magnitude peaks so that the valley between them is a speciﬁed frac@on of the peak height.

Resolving power: the observed mass divided by the diﬀerence between two masses that can be separated: m/∆m.

Resolu@on vs. Resolving power

Valley

FWHM

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We actually measure the mass-‐to-‐

charge ra-o, m/z, of the ions in our mass spectrometer

Q: Do we actually measure mass?

A: Yes, but indirectly!

Lets look at the isotope pa<erns for a hypothe@cal ion of mass A and charge Z

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Calculating the mass of a multiply charged protein:

M = mass of the unknown protein

n = number of charges (corresponding to addition of n protons)

m1 and m2 two adjacent multiply charged ions:

m₁ = (M + n)/n (1) m₂ = (M + n + 1)/(n + 1) (2)

Neurotensin

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Why do we need a vacuum?

• Keep surfaces free of contaminants.

• Process ﬁlms with low density of impuri@es.

• Maintain plasma discharge for spu<ering sources.

• Large mean free path for electrons and molecules

• (λ = 1 m @ 7 x 10

^-‐5

mbar).

λ

Mean free path for air at 20 ºC:

λ = 7 x 10^-‐3 cm / P(mbar)

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Choosing an Ioniza@on method:

Points to Consider

•Vola@lity of analyte

•Polarizability

•Is your analyte pure, or part of a liquid-‐, solid-‐ or gas-‐phase mixture

•Desired degree of fragmenta@on

•Con@nuous or pulsed ioniza@on

•Sensi@vity

Main Types of Ion Sources

•Field Ioniza@on (FI) / Desorp@on (FD)

•Fast Atom Bombardment (FAB)

•Secondary Ion MS (SIMS)

•Electron / Chemical Ioniza@on (EI /CI)

•Matrix-‐Assisted Laser Desorp@on/Ioniza@on (MALDI)

•Atmospheric Pressure Ioniza@on Methods

o Atmospheric Pressure Chemical Ioniza@on (APCI) o Atmospheric Pressure Photo-‐Ioniza@on (APPI) o Electrospray Ioniza@on (ESI)

Many other modiﬁed or combined methods

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•

Can form mul@ply charged species for large analytes

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Ion Focusing:

Quadrupoles, Hexapoles, Octopoles, etc.

Einsel Lenses Ion Funnels

To MS

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Time of Flight (TOF)

Principal of Delayed Ion Extrac@on

m z ⁼

2 eVt

²

L

²

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Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

(FTICR)

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Actual mo@on:

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Q’s Sector TOF Orbitrap FTICR

Cost lowest medium medium med-‐high highest

Resolu@on lowest med., can

be high* med high highest

Mass Accuracy low med., can

be high* high high highest

Sensi@vity high med* high med-‐high med-‐high

Scan Speed fast slow fastest fast slow

Pulsed or Con@nuous

Ion Source cont. cont. pulsed pulsed pulsed

* Inverse rela@onship between resolu@on (and mass accuracy) and sensi@vity

Comparison of Mass Analysers

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Detectors

Ion multiplier (with dynodes) Faraday Cup

Channeltron multiplier

+_e_-‐

primary ion

e^-‐

e^-‐e^-‐

L

D

-‐ 1000V

-‐ 100V

L >> D

Multichannel Plate

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MS is an informa@on-‐rich method of analysis

-‐ molecular weight, chemical formula, structural informa@on Can ionize and mass-‐analyze a wide variety of analytes using our arsenal of MS tools

BUT… ion suppression in complex mixtures can pose a problem, so it would be nice to break up our sample into simpler segments

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Triple quadrupole tandem mass spectrometer (TSQ)

By courtesy of Spektrum Akademischer Verlag

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Courtesy of h<p://www.colorado.edu/chemistry/chem5181/ ⁴⁶

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Courtesy of h<p://www.colorado.edu/chemistry/chem5181/ ⁴⁷

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Courtesy of h<p://www.colorado.edu/chemistry/chem5181/

R _s ₌ ² ^!" t W _a ₊ W _b

∆t

W_a W_b

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15 – 20 scans for a single peak is a good “rule of thumb”

How many scans does one need to accurately recreate the peak shape?

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Gas Chromatography

Courtesy of h<p://academics.eckerd.edu/instructor/grove/organic2/

GC Interface MS

1 - 60 mL/min

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GC -‐ MS

GC Interface MS

Interfaces for capillary columns

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Schema@c of a direct coupling of a GC capillary column to a MS

GC -‐ MS

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MS GC

L

d₁ d₂

Schema@c of molecular jet separator

Interface for packed columns

GC -‐ MS

Molecular Jet Separator:

•takes advantage of the fact that the carrier gas is usually much lighter than the analyte gas

•when passing through a small aperture the carrier gas will disperse over a larger solid angle than the analyte

•using a second skimmer cone, the rela@ve analyte concentra@on will be enhanced since the heavier analyte molecules are more likely to travel in the straight line path towards the MS

L = 0.1 mm d₁,d₂ = 0.1 -‐ 0.5 mm

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GC -‐ MS

GC Interface MS

MS considera@ons:

Mass Analyzers

-‐ EI

-‐ CI -‐ Quadrupole

-‐ Ion Trap -‐ QQQ

-‐ Time of Flight -‐ Sector

Ion sources

-‐ Fast scan @me -‐ Sensi@ve

-‐ Con@nuous ion source

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To MS Op@onal ﬂow splixng or post-‐

column addi@on

Mobile phase solvents Solvent mixer /

degasser Pump

Column oven LC Column Injector

Schema@c of a liquid chromatography setup

Important parts:

•Solvent mixer

•Pump

•Injector

•Column

•Column oven

•Flow Splixng

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Review of MS and Coupling Techniques