Mass Spectrometry

Sources – making ions can be hard or soft

Referenced MS timeline can be found at:

http://masspec.scripps.edu/MSHistory/timeline.php

“Hard” vs. “Soft” Ionization methods,

ICP sources Spark Sources Glow Discharge Secondary Ion/Neutral Mass Spec fast atom bombardment (FAB) sources desorption sources

field ionization (FI) field desorption, laser desorption . . . (especially MALDI)

plasma desoprtion sources electron impact (EI) chemical ionization (CI) electrospray ionization sources (ESI) “no-prep” atmospheric sources

ICP sources gas, liquid or solid sample is introduced into hot plasma an efficient sourceof positively charged analyte ions Ar plasma is generated and maintained at the end of the glass torch locatedinside the loops of a water cooled copper load coil. RF potential applied to the coil produces an electromagnetic

field in the part of the torch located within its loops. electrons are accelerated and collide with Ar atoms in the Ar

gas flowing through the torch producing Ar+  ions and free electrons - a plasma. 

ICP sources the ions have to be extracted from the high temperature (~ 6000K or more), atmospheric pressure (760 torr) environment of an often chemically corrosiveAr plasma into a mass spectrometer operating in a high vacuum (10-5 torr) at room temperature. interface region contains two successive cones (mm orifices) ions in the center of the plasma are sampled into the region

between two cones held at a pressure of about 1-3 torr At this stage, most of the Ar atoms are removed by a vacuum pump.

ion beam is further extracted through the skimmer cone orifice into the front section of the mass spectrometer (pressure of about 10-3 - 10-4 torr)

Spark Ionization Sources samples are physically incorporated into two conductive

electrodes (usually either carbon or silver) a high-voltage arc is produced, ionizing the material semiquantitative trace element technique for solids and liquids

samples: conducting, semiconducting and insulating solids, powders, crystals, liquids, organometallics, ash from organics, unknowns and many other sample forms.

detection capabilities encompass the periodic table (Li – U) has the ability to determine impurity levels from the sub-ppm

level to 0.1%. SSMS total simultaneous elemental coverage low detection limits high res. capabilities - eliminates many spectral interferences.

Glow Discharge MS analytical technique for the bulk elemental analysis of inorganic solid samples. capable of analyzing, conducting, semi-conducting and insulating samples. amenable to solids, powders, crystals, wafers, and many other sample forms. elemental coverage encompasses Li – U determine impurity levels from the sub-ppb to percent level advantages include

high precision and low detection limits, quantitative accuracy (+/- 25% on average), without the use of standards high resolution capabilities eliminate most spectral interferences.

Secondary Ion/Neutral Mass Spec a primary, high-energy

beam of ions (usually oxygen, argon, or cesium) is aimed at a small area of a sample, such as a mineral grain.

the primary ions have energies of ~ 10 keV

the primary ions sputter away the sample by causing the ejection of atoms and ions (called secondary neutrals and ions)

these secondary ions (approximately 1% of the sputtered material) are accelerated into a mass spectrometer to reveal the elemental and isotopic characteristics of the sample.

Fast Atom Bombardment (FAB) material to be analyzed is mixed with a non-volatile chemical protection environment called a matrix This is bombarded under vacuum with a high energy (4 – 10 keV) beam of atoms. atoms are typically an inert gas (Ar or Xe) common matricies include glycerol, thioglycerol, 3-nitrobenzyl alcohol (3-NBA), 18-Crown-6 ether,2-nitrophenyloctyl ether, sulfolane, diethanolamine, and triethanolamine.

Field Ionization Field ionization (FI) is the generation of M+ ions by removal

of electrons, primarily from gas sample molecules, using a high electric field.

This generally occurs at a sharp edge or tip that is biased to a high electrical potential

Field Desorption Field desorption (FD) is a method for emitting ions into the

gas phase. Sample spread on an emitter is heated while a high electric

field is applied. Ions are then emitted by the tunneling, ion-molecule reactions,

thermal fusion effects, and other phenomenon occurring on the emitter surface and around thewhisker ends.

The ionization phase depends strongly on the sample material and the spread condition.

Plasma Desorption

Plasma desorption ionization mass spectrometry (PDMS; also called fission fragment ionization) is a mass spectrometry technique in which ionization of material in a solid sample by bombarding it with ionic or neutral atoms formed as a result of the nuclear fission of a suitable nuclide, typically the Californium isotope 252Cf

Californium-252 plasma desorption mass spectroscopy

RD Macfarlane and DF Torgerson We have shown that 252Cf-PDMS is capable of producing mass spectra of

quasi-molecular ions for a wide variety of compounds, including amino acids, moderately large peptides, nucleotides, and natural products. Positive and negative ion mass spectra can be obtained, and in many cases quasi-molecular ions are observed in both. The method is nondestructive, as only a relatively few molecules are used and samples can be recovered after the measurement is made. Fragmentation patterns are obtained which can yield structure information. The present sensitivity of the method is at the nanogram level and there are possibilities for reducing this to picograms. The mass resolution is sufficient to give elemental identification up to mass 500. This may be extended to higher masses with improved time-of-flight techniques. There are indications that 252Cf-PDMS may extend the mass range of molecules that can be studied to as high as 3000 or more.

Science, Vol 191, Issue 4230, 920-925Copyright © 1976 by American Association for the Advancement of Science

laser desorption . . .

Especially MALDI Matrix-assisted laser desorption ionization (MALDI)

Ionization method using matrix-assisted laser desorption. a soft ionization technique

analysis of biomolecules proteins, Peptides sugars)

large organic molecules polymers, dendrimers other macromolecules

tend to be fragile and fragment when ionized by other methods.

MALDI cont’d identity of suitable matrix compounds is determined using

specific molecular design considerations fairly low molecular weight (to allow facile vaporization) large enough (with a high enough vapor pressure) not to evaporate

during sample preparation or while standing in the spectrometer are acidic / act as a proton source to encourage ionization of the analyte have strong absorption in the UV so they rapidly and efficiently absorb

the laser irradiation functionalized with polar groups - allowing use in aqueous solutions

matrix solution is mixed with the analyte (e.g. protein-sample) organic solvent allows hydrophobic molecules to dissolve water allows for hydrophilic molecules to do the same

solution is spotted onto a MALDI plate solvents vaporize, leaving only the recrystallized matrix analyte molecules spread throughout the crystals in co-crystallized

MALDI spot

laser desorption . . .

UV MALDI Matrix List

Compound Solvent (nm) Applications

2,5-dihydroxy benzoic acid

acetonitrile, water, methanol, acetone,

chloroform

337, 355, 266

peptides, nucleotides,

oligonucleotides, oligosaccharides

3,5-dimethoxy-4-hydroxycinnamic acid

acetonitrile, water, acetone, chloroform

337, 355, 266

peptides, proteins, lipids

4-hydroxy-3-methoxycinnamic

acid

acetonitrile, water, propanol

337, 355, 266

proteins

α-cyano-4-hydroxycinnamic acid

acetonitrile, water, ethanol, acetone

337, 355peptides, lipids,

nucleotides

Picolinic acid Ethanol 266 oligonucleotides

3-hydroxy picolinic acid

Ethanol 337, 355 oligonucleotides

Electron Ionization (EI)

EI (formerly known as electron impact) is an ionization technique widely used in mass spectrometry, particularly for organic molecules.

The gas phase reaction producing electron ionization is: M + e- M+ + 2e- low energies (around 20 eV), the interactions between the

electrons and the analyte molecules do not transfer enough energy to cause ionization

at around 70 eV, the de Broglie wavelength of the electrons matches the length of typical bonds in organic molecules (about 0.14 nm), and energy transfer to organic analyte molecules is maximized, leading to the strongest possible ionization and fragmentation

Electron Ionization (EI)

Chemical Ionization (CI)

Chemical ionization (CI) is an ionization technique used in mass spectrometry

ionization is achieved by interaction of its molecules with reagent ions

the analyte is ionized by chemical ion-molecule reactions during collisions in the source

the process may involve transfer of an electron, a proton or other charged species between the reactants.

a less energetic procedure than electron ionization and the ions produced are, for example, protonated molecules: [M + H]+.

These ions are often relatively stable, tending not to fragment as readily as ions produced by electron ionization.

Chemical Ionization (CI) typical reagent gases (ex. CH4, isobutane, or NH3) are present

in a millionfold excess with respect to the analyte. analyte is ionized by ion-molecule chemical reactions:

Primary Ion Formation: CH4 + e- CH4

+ + 2e-

Secondary Reagent Ions: CH4 + CH4

+ CH5+ + CH3

CH4 + CH3+ C2H5

+ + H2

Product Ion Formation: M + CH5

+ CH4 + [M + H] + (protonation) AH + CH3

+ CH4 + A+ (H− abstraction) M + CH5

+ [M+ CH5] + (adduct formation) A + CH4

+ CH4 + A+ (charge exchange)

electrospray ionization sources (ESI)

A soft ionization technique

Solvated drop of analyte is ionized

Solvent is removed in vacuuo

Charged analyte left for MS analysis

High m/z analytes easily

examined

Electrospray results

Figure 2. Total ion chromatogram (TIC) and the full scan mass spectra of GTI-2040 and major metabolites (M1-M5) (A) shows the TIC of GTI-2040 and

M1 to M5 metabolites; (B) shows the mass spectrum of M1,

the putative 3′N-1 metabolite with retention time (RT) of 14.8, which contains an ion envelope including [M-6H],6- [M-5H],5- and [M-4H]4- ions;

(C) shows the mass spectrum of M2, the putative 3′N-2 metabolite at RT 14.2 minutes, which contains the most abundant ion of [M-3H]3-; and

(D) shows the mass spectrum of M3, the putative 3′N-3 metabolite at RT 12.9 minutes containing the most abundant ion of [M-3H].3-

No-prep MS (DART) DART is a mass spectrometry system that:

can analyze samples in the gas, liquid, or solid phase operates at 0 potential operates in the open air (atmospheric pressure) does not require solvents obtains mass spectra from sampleon the surface of anything imaginable; i.e. commodities, ball caps, glass rods,plastics, adhesive tape, etc.

It is a form of chemical ionization that takes place at atmospheric pressure.

No-prep MS (DART) an electrical potential is applied to a gas (ex. N2 or He),

generates a plasma, & interacts with sample and atm. different ionization mechanisms can be favored by

changing operating conditions H+ transfer is dominant mode of positive ionization.

metastable He atoms react with H2O to produce ionized water clusters that can protonate the sample molecule, forming MH+

electrons can be formed if the carrier gas can form metastable species with high enough internal energy. He reacts with atmospheric H2O forming negative-ion clusters

that react with analytes to form negatively charged ions. Forensic note:

in negative-ionization mode, nitrate and nitrite ions are not produced because plasma formation by the carrier gas is isolated from air. Those ions interfere with the detection of nitrogen-based explosives and reduce the sensitivity of anion detection.