“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 source of positively charged analyte ions Ar plasma is generated and maintained at the end of the glass torch located inside 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 corrosive Ar 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 the whisker 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 252 Cf
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
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
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. CH 4, isobutane, or NH 3 ) are present in a millionfold excess with respect to the analyte. analyte is ionized by ion-molecule chemical reactions: Primary Ion Formation: CH 4 + e - CH 4 + + 2e - Secondary Reagent Ions: CH 4 + CH 4 + CH 5 + + CH 3 CH 4 + CH 3 + C 2 H 5 + + H 2 Product Ion Formation: M + CH 5 + CH 4 + [M + H] + (protonation) AH + CH 3 + CH 4 + A + (H − abstraction) M + CH 5 + [M+ CH 5 ] + (adduct formation) A + CH 4 + CH 4 + 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 sample on 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. N 2 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 H 2 O 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 H 2 O 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.