Vocab 6 Flashcards
Units used in MS to express atomic or molecular masses; defined relative to the mass of the carbon isotope 12/6 , so 1/12 the mass of one neutral 12/6 C atom
Atomic Mass Unit (amu) / Dalton (Da).
Refers to the rounded, whole-number precision of an amu measurement
nominal mass
Naturally occurring mass of an element in nature
Chemical Atomic Mass / Average Atomic Weight (A).
Common abscissa units used for plotting mass spectra; obtained by dividing the atomic or molecular mass of an ion (m) by the number of charges the ion carries
mass-to-charge ratio
Method of introducing gaseous or liquid samples directly into MS ionization chamber without the need for preliminary separation stages; usually accomplished by direct injection of small gaseous or liquid volumes into ionization chamber with continuous vacuum; heated inlets are sometimes used to volatilize the sample.
Batch Inlet Sample Introduction
Method of introducing solid samples into the MS ionization chamber; direct insertion probe is used where the sample is held onto the end of the probe; probe and sample are introduced into the MS and vacuum then introduced; inlet can be heated to help volatilize the sample.
Direct Probe INlet Sample Introduction
Method of introducing gaseous samples into the MS ionization chamber by interfacing the output of a gas chromatograph to the MS ionization chamber
gas chromatographic inlet sample introduction
Method of introducing liquid samples into the MS ionization chamber by interfacing the output of a liquid chromatograph to the MS ionization chamber
liquid chromatographic inlet sample introduction
Relatively simple MS detector; uses an aligned or tilted collector electrode that is connected to ground through a resistor; voltage drop is amplified using high impedance amplifier
Faraday Cup
Most common MS detector; analogous to a photomultiplier tube; ions strike cathode, emitting multiple electrons; each secondary electron strikes a series of intermediate dynodes held at successively higher voltages
electron multiplier
MS detector that uses microchannel plate and image intensifier; individual elements in microchannel plat act as electron multipliers; electrons emitted from microchannel plate can be captured and directed to two-dimensional optical array detector
Electro-optical Ion Detector
MS source in which the sample is first volatilized into the gas phase, then ionized.
Gas-Phase Ionization Source
MS source in which the sample is not first volatilized; rather, the sample probe is used to ionize the sample directly from a liquid or solid state into the gaseous ionic state.
Desorption Ionization Source
Highly energetic MS source that imparts large energies to the analyte molecule, resulting in bond cleavage and extensive fragmentation; molecular ion peak may be reduced or absent.
Hard Ionization Source
Less energetic MS source that produces simpler spectra with relatively little fragmentation; molecular ion peak predominates.
Soft Ionization Source
Most common MS ionization method; hard, gas phase source that ionizes molecules due to electrostatic repulsion; primary products are singly charged positive ions; not an efficient ionization process
Electron Ionization Source
radical ion that corresponds to the same molecular weight as the parent molecule
Molecular Ion
The largest abundance peak, or the one with the highest response, in the mass spectrum.
base peak
Large numbers of positive ion peaks that have m/z values less than that of the molecular ion
daughter ions
Peaks in the mass spectrum that occur at m/z values greater than that of the molecular ion; these peaks are attributable to ions having the same chemical formula as the molecular ion, but with different isotopic compositions.
Isotope Peaks
Peaks in the mass spectrum due to ion-molecule collisions; most common is the peak that gives the protonated molecular ion due to hydrogen ion exchange.
Collision Product Peaks
Second most common MS ionization method; gas phase and soft source; ionization process based on gas phase ion-molecule reactions; most commonly used reagent gases are methane, isobutane, and ammonia; main ionization reactions occur in CI through proton transfer (most common), adduct formation, or charge transfer; CI is much gentler ionization source than EI; less fragmentation seen, simpler spectra, much stronger molecular ion peak.
Chemical Ionization Source
type of gas phase solvation mechanism used by polar molecules in chemical ionization, in which ions can result from association of analyte molecule M with reagent gas RH+, resulting in (R + M)+, with protonated molecular ion MH+, resulting in (2M + H)+, or with a fragment ion F+, resulting in (F + M)+.
Adduct Formation
Chemical ionization mechanism that uses gases with high ionization potential, e.g. rare gases (Xe), N2 or CO as the reagent gas; reaction occurs by charge transfer; less commonly used ionization method.
Charge Transfer
Most common mechanism in chemical ionization; gas phase acid-base reaction with reagent ion RH+ (acid) and analyte molecule MH (base) resulting in R and MH2+; results in peak (M + 1)+.
Proton Transfer
Gas phase acid-base reaction mechanism seen in chemical ionization with reagent ion RH+ (base) and analyte molecule MH (acid) resulting in M+ and RH2; results in peak (M – 1)+.
Hydride Transfer
Gas phase, soft ionization method in which the ions are formed under the influence of a large electric field; special fine tungsten wires with carbon dendrites are used as electron emitters; results in very little fragmentation, mostly forms molecular ions
Field Ionization Source
Ionization method in which sample solution is pumped at atmospheric pressure through stainless steel capillary needle at a rate of 1-10 μL min-1; needle is maintained at 3-6 kV with respect to surrounding electrode (electric fields of ~10e6 V m-1); high electric field results in charge accumulation in droplet spray of molecules; as solution droplets become smaller as a consequence of solvent evaporation, charge density becomes greater and desorption of ions into ambient gas phase occurs; ions formed are multiply charged so that their m/z values are small enough to detect with analyzers such as quadrupoles; little fragmentation occurs, application to large biomolecules.
Electrospray Ionization (ESI)
Desorption method very similar to field ionization, except that it is applicable to more than just gas phase samples; electrode is mounted on a probe that is removed from sample compartment and coated with the sample, which can be either liquid or solid; electrode is then replaced in MS; application of high electrical potential leads to sample desorption; good technique for high MW non- polar species.
Field Desorption Source
Desorption technique in which condensed-phase sample is bombarded with energetic (keV) Ar0 atoms; sample is typically in form of glycerol mull; used for high molecular weight (10 kDa) polar samples, especially good for biomolecules and polymers.
Fast Atom Bombardment (FAB)
Desorption technique in which sample is mixed into a UV- absorbing organic matrix and applied to the surface of a metallic probe; probe surface is exposed to UV (250 – 350 nm) excimer laser pulses; matrix absorbs laser pulses causing rapid heating and sublimation of matrix along with ionization of analyte; ion is directed into TOF MS for analysis; spectrum is recorded between laser pulses; little fragmentation occurs, molecular ion peak predominates; very useful for large (>100 kDa) polymers and biomolecules.
Matrix Assisted Laser Desorption (MALDI)
High vacuum surface analysis technique in which surface is bombarded with high energy (5 – 20 keV) ions (e.g. Ar+); impact of primary Ar+ ions hitting surface causes surface layer of atoms to be stripped or sputtered off; these secondary ions are then directed into a mass spectrometer; very surface sensitive – samples only the first few atomic or molecular layers.
Secondary Ion Mass Spectrometry (SIMS)
The ratio of the velocities of two molecules in the gas phase is inversely proportional to the square root of the ratio of their masses
Graham’s Law
The ability of a mass analyzer to yield distinct signals for two ions with a small m/z difference.
Mass Resolution, R
Single-focusing mass analyzer that uses a permanent magnet or electromagnet to steer the beam of ionized molecules from the MS source; magnet causes ion beam to travel in a circular path of 60° / 90° /180°; ions of different mass are scanned by varying the field strength of the magnet; ions are separated spatially, i.e. sorted in space.
Magnetic Sector Mass Analyzer
The product of the mass and velocity of a molecule
Momentum
The circular path that ions travel in the magnetic field of a magnetic sector MS instrument; most common values are 60°, 90°, or 180°
degree of deflection
Single-focusing mass analyzer that uses a pulsed ionization source (e.g. a laser) to generate ions and accelerates them in an electrostatic field to constant kinetic energy; after leaving source, ions enter a field-free drift tube; since all ions entering the drift tube have the same kinetic energy, their velocities vary inversely with their masses; lighter masses reach detector first, heavier masses later; ions are sorted temporally, i.e. sorted in time.
Time-of-Flight Mass Analyzer
Electric- and magnetic-field free region of length L in a time-of-flight mass spectrometer; section of TOF MS in which ion separation takes place
Drift Tube
Type of TOF MS that incorporates a static or time-dependent electric field as an ion mirror to reverse the direction of travel of the ions entering the TOF drift tube; substantially diminishes the spread of flight times of all ions with the same m/z caused by the spread in kinetic energy of these ions, thereby increasing resolution.
Reflectron Time-of-Flight Mass Analyzer
Refers to the use of two types of mass analyzers in series – electrostatic analyzer followed by a magnetic analyzer – that takes into account both the directional and kinetic energy distributions of the ions to increase resolution; ion beam passes first through electrostatic analyzer, which limits the kinetic energy of ions reaching the magnetic sector analyzer to a small range; magnetic sector analyzer then provides a homogeneous B-field to directionally focus a given mass by momentum & radius of curvature onto the detector; resolution can be up to R ~ 10e5 in some instruments.
Double-Focusing Mass Analyzer
consists of 4 parallel metal rods that serve as electrodes; these 4 rods are held at a DC voltage, and modulated with AC RF frequencies; At any given set of AC/DC operating conditions, it will transmit only ions within a narrow range of m/z; all other ions are neutralized and carried away as uncharged molecules; mass is tuned by tuning the AC RF frequency; varying the electrical signals to the quadrupole makes possible the variation of the range of m/z values transmitted, therefore spectral scanning is possible.
Quadrupole Mass Analyzer / Mass Filter
usually 3 quadrupole mass analyzers placed in series; first quadrupole : output is largely molecular ions; serves to separate parent ions by mass selection; second quadrupole : introduce collision gas here (e.g. N2 or He) to fragment parent ions selected by first quadrupole; third quadrupole : allows mass selective detection of daughter ion fragments, provides a spectrum rich in structure.
Tandem Mass Spectrometer
Device in which ions can be formed and confined for extended periods by electric and/or magnetic fields; formed from central doughnut-shaped ring electrode and pair of end-cap electrodes; ions confined between electrodes of a particular shape that resembles operation of a quadrupole; analyte ions admitted to cell through grid in upper end cap; RF voltage applied to ring electrode: confines ions in trap; variable RF voltages applied to ring electrode: destabilizes lighter ions; lighter ions swept from cell, pass through lower end cap into detector; mass spectrum obtained by increasing RF amplitude, which destabilizes ions of increasing mass
Ion Trap Mass Spectrometer
Determines m/z of ions based on the cyclotron resonance frequency of ions in a fixed magnetic field; uses trapped ion analyzer cell; gaseous molecules in cell are ionized by pulsed electron beam from source filament; ions held in place by 1 - 5 V potential applied to trap plate; ions are accelerated by RF-frequency pulse applied to transmitter plate; after RF pulse, image current is detected as FID; time domain FID is then Fourier transformed into mass domain spectrum; very high resolution MS technique that can determine masses with high accuracy.
Fourier Transform Mass Spectrometer
Phenomena related to the motion of gaseous ions in a magnetic field; path of these ions becomes circular in a plane perpendicular to the B field direction
Ion Cyclotron Resonance (ICR)
Angular frequency of the motion of gaseous ions in a magnetic field perpendicular to the field direction; depends inversely on the m/z value
Cyclotron Resonance Frequency ωc
Result of the coherent motion of all resonant ions with a particular cyclotron resonant frequency ; induced by circular motion of charged ions interacting with detector plates in ICR cell; coherent character of circulating ions eventually lost due to collisions; decay provides time domain signal detected by detector plates in ICR cell.
Image Current
FT-MS ICR cell which is conceptually similar to an ion trap; ion source, analyzer and detector are all incorporated into the same cell; gaseous molecules in the cell are ionized by pulsed electron beam from source filament; ions held in place by 1 – 5 V potential applied to trap plates; ions accelerated by RF pulse applied to transmitter plate; ions circulate for extended periods; after RF pulse, image current detected by detector plates.
Trapped Ion Analyzer Cell
Time domain representation of the intensity decay of the image current as detected by the trapped ion ICR cell; Fourier transform of the FID provides the frequencies, and therefore the m/z values, of the ions that are present in the cell.
Free Induction Decay
