Vocab 6

Question 1

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

Answer 1

Atomic Mass Unit (amu) / Dalton (Da).

Question 2

Refers to the rounded, whole-number precision of an amu measurement

Answer 2

nominal mass

Question 3

Naturally occurring mass of an element in nature

Answer 3

Chemical Atomic Mass / Average Atomic Weight (A).

Question 4

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

Answer 4

mass-to-charge ratio

Question 5

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.

Answer 5

Batch Inlet Sample Introduction

Question 6

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.

Answer 6

Direct Probe INlet Sample Introduction

Question 7

Method of introducing gaseous samples into the MS ionization chamber by interfacing the output of a gas chromatograph to the MS ionization chamber

Answer 7

gas chromatographic inlet sample introduction

Question 8

Method of introducing liquid samples into the MS ionization chamber by interfacing the output of a liquid chromatograph to the MS ionization chamber

Answer 8

liquid chromatographic inlet sample introduction

Question 9

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

Answer 9

Faraday Cup

Question 10

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

Answer 10

electron multiplier

Question 11

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

Answer 11

Electro-optical Ion Detector

Question 12

MS source in which the sample is first volatilized into the gas phase, then ionized.

Answer 12

Gas-Phase Ionization Source

Question 13

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.

Answer 13

Desorption Ionization Source

Question 14

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.

Answer 14

Hard Ionization Source

Question 15

Less energetic MS source that produces simpler spectra with relatively little fragmentation; molecular ion peak predominates.

Answer 15

Soft Ionization Source

Question 16

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

Answer 16

Electron Ionization Source

Question 17

radical ion that corresponds to the same molecular weight as the parent molecule

Answer 17

Molecular Ion

Question 18

The largest abundance peak, or the one with the highest response, in the mass spectrum.

Answer 18

base peak

Question 19

Large numbers of positive ion peaks that have m/z values less than that of the molecular ion

Answer 19

daughter ions

Question 20

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.

Answer 20

Isotope Peaks

Question 21

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.

Answer 21

Collision Product Peaks

Question 22

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.

Answer 22

Chemical Ionization Source

Question 23

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)+.

Answer 23

Adduct Formation

Question 24

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.

Answer 24

Charge Transfer

Question 25

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)+.

Answer 25

Proton Transfer

Question 26

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)+.

Answer 26

Hydride Transfer

Question 27

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

Answer 27

Field Ionization Source

Question 28

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.

Answer 28

Electrospray Ionization (ESI)

Question 29

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.

Answer 29

Field Desorption Source

Question 30

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.

Answer 30

Fast Atom Bombardment (FAB)

Question 31

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.

Answer 31

Matrix Assisted Laser Desorption (MALDI)

Question 32

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.

Answer 32

Secondary Ion Mass Spectrometry (SIMS)

Question 33

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

Answer 33

Graham’s Law

Question 34

The ability of a mass analyzer to yield distinct signals for two ions with a small m/z difference.

Answer 34

Mass Resolution, R

Question 35

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.

Answer 35

Magnetic Sector Mass Analyzer

Question 36

The product of the mass and velocity of a molecule

Answer 36

Momentum

Question 37

The circular path that ions travel in the magnetic field of a magnetic sector MS instrument; most common values are 60°, 90°, or 180°

Answer 37

degree of deflection

Question 38

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.

Answer 38

Time-of-Flight Mass Analyzer

Question 39

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

Answer 39

Drift Tube

Question 40

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.

Answer 40

Reflectron Time-of-Flight Mass Analyzer

Question 41

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.

Answer 41

Double-Focusing Mass Analyzer

Question 42

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.

Answer 42

Quadrupole Mass Analyzer / Mass Filter

Question 43

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.

Answer 43

Tandem Mass Spectrometer

Question 44

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

Answer 44

Ion Trap Mass Spectrometer

Question 45

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.

Answer 45

Fourier Transform Mass Spectrometer

Question 46

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

Answer 46

Ion Cyclotron Resonance (ICR)

Question 47

Angular frequency of the motion of gaseous ions in a magnetic field perpendicular to the field direction; depends inversely on the m/z value

Answer 47

Cyclotron Resonance Frequency ωc

Question 48

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.

Answer 48

Image Current

Question 49

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.

Answer 49

Trapped Ion Analyzer Cell

Question 50

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.

Answer 50

Free Induction Decay