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Flame Ionization Detector

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Mar 1, 2020

Cards (45)

Section 1

(45 cards)

Flame Ionization Detector

Front

omost common type of GC detector o"universal" detector capable of measuring the presence of almost any organic oPrinciple of operation: The production of ions when a solute is burned in a flame. Ions are collected at an electrode to create a current Measure the current produced Rugged Easy to use Sensitive (~10-13 g/s) Wide dynamic range (~107) Low noise Signal depends on # C atoms in organic analyte - mass sensitive not concentration sensitive Weakly sensitive to carbonyl, amine, alcohol, amine groups Not sensitive to non-combustibles - H2O, CO2, SO2, Nox carrier gases not detected Destructive

Back

Electrospray ionizations (ESI)

Front

-Explosion of charged droplets containing analytes solution analyte pumped through charged (1-5 kV) capillary, small droplets become charged, solvent evaporates, drop shrinks, surface charge density increases, charge density reduced by expulsion of charged analyte molecules ("Coulomb explosion") •Very important technique for large (>105 Da) thermally fragile molecules peptides proteins polymers Oligonucleotides •Analytes may accumulate multiple charges in ESI •M+, M2+, M3+... Advantages: Soft ionization - little fragmentation Form multiply charged ions detectable by a quadrupole instrument with 1500 or less. Easily adapted to CE and HPLC Disadvantages: Structural elucidation difficult Usually use tandem MS: ions from the original ionization process are separated and the ion of interest is subjected to a fragmentation step before being mass analyzed.

Back

Matrix-Assisted Laser Desorption Ionization (MALDI)

Front

Soft ionization • MWs of polar biopolymers ranging from a few thousand to several hundred thousand Da • Solution of analyte is mixed with a large amount of radiation absorbing matrix material • The solid material was placed on a metallic probe and inserted into a Time-of-Flight MS (TOF-MS) • Mixtures exposed to a pulsed laser beam • Complete MS measured between each pulse • Analyte:matrix=1:103 to 1:105. • Sample preparation: dried-droplet technique.

Back

TCD: Thermal Conductivity Detector

Front

oHot-wire detector oFirst universal detector developed for GC oThermal conductivity changes with presence of other components in the mobile phase oCarrier gas He has the 2nd highest thermal conductivity (after H2) oOrganics cause T rise in filament oMobile phase must have very different thermal conductivity then solutes being separated oSimplicity oWide dynamic range (105) oUniversal: response to both organic and inorganic species oNondestructive oInsensitive (10-8 g/s)

Back

Eluent

Front

the mobile phase in chromatography is referred to as the eluent.

Back

the general elution problem

Front

The general elution problem is the separation case where solutes of very different distribution constants are to be separated. When conditions are optimized for the separation of late eluting peaks, bad resolution is obtained for fast eluted peaks. On the other hand, when conditions are optimized for the separation of early eluting peaks, very long retention of late eluting peaks is realized, including severe band broadening. The general elution problem can be overcome using gradient elution in liquid chromatography or temperature programming in gas chromatography.

Back

Hard ionization sources

Front

impart enough energy to analyte molecules, rupture of bonds, produce fragment ions Functional group and structural information Close passage of highly energetic electrons, referred to as a hard ionization source, causes large fluctuations in the electric field around the neutral molecules and induces ionization and fragmentation.

Back

• In a Fourier Transform NMR experiment,

Front

the analyte is subjected to periodic pulses of radio-frequency radiation. After each pulse, the decay of the emitted signal is monitored as a function of time. This free induction decay signal is then converted to a frequency domain signal by a Fourier Transformation

Back

Describe the differences between off-resonance and broad-band proton decoupling in C-13 nmr.

Front

Broadband decoupling avoids the spin-spin splitting of 13C lines by 1H nuclei by irradiating sample with broadband radio frequency signal that encompasses entire proton spectral region. Off-resonance decoupling is set at 1000-2000Hz above the proton spectral region leading to a partially decoupled spectrum in which all but the largest spin-spin shifts are absent

Back

gaseious ionization sources

Front

-the sample is first volatilized (by heating if necessary) and then transmitted to the ionization area for ionization. -The advantage of gaseous ionization sources are their simplicity and speed (no need to use probe and wait for probed area to be pumped out).

Back

What is temperature programming and how does it work?

Front

•As column temperature raised, vapor pressure analyte increases, eluted faster •Raise column temperature during separation - temperature programming - separates species with wide range of polarities or vapor pressures

Back

Cross linking

Front

involves treating the stationary phase while it is in the column with a chemical reagent that creates cross links between the molecules making up the stationary phase

Back

a. Time of Flight (TOF)

Front

Generate pulse of ions (by laser, electrons) with same kinetic energy •Ions travel down field-free tube separate according to mass -light ions arrive first, heavy ions arrive later •Unlimited mass range m/zmax > 100 kDa •Poor resolution Rmax < 1000 •Poor sensitivity •Flight time: 1~50 μs

Back

Selectivity factor

Front

the ratio of the retention factors of two species is called their selectivity factor where the value of the selectivity factor is equal or greater than unity.

Back

Stationary phase

Front

a solid or liquid phase through which a mobile phase passes where solutes in the mobile phase interact or partition differently with the stationary phase.

Back

Eddy diffusion

Front

band broadening process results from different path lengths passed by solutes.

Back

In a continuous wave NMR experiment

Front

the intensity of the absorption signal is monitored as the frequency of the source or the field strength of the magnet is scanned.

Back

ECD: Electron Capture Detector

Front

oElectrons from β-source ionize carrier gas oOrganic molecules capture electrons and decrease current oSimple and reliable oSelective in its response oSensitive (10-15 g/s) to electronegative groups (halogens, peroxides, quinones, nitro. sulfur groups) oInsensitive to amines, alcohols and hydrocarbons oLargely non-destructive oUseful for environmental testing oChlorinated pesticides oPolynuclear aromatic carcinogens oLimited dynamic range (102

Back

Soft ionization sources

Front

little fragmentation Accurate information about molecular mass soft ionization' methods such as matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) tend to produce mass spectra with little or no fragment-ion content.

Back

Fast Atom Bombardment (FAB)

Front

•Hard ionization - fragmentation •Sample in glycerol solution •Bombarded by high energy Ar or Xe atoms (several keV) •Atoms and ions sputtered from surface •Both M+ and M- produced •Applicable to small (3000 Da) or large (>104 Da) unstable molecules •Disadvantages: Limited molecular mass range The need for larger sample quantities The necessity for finding an appropriate matrix Baseline noise resulting from formation of ion clusters from the matrix

Back

Longitudinal diffusion

Front

-diffusion of concentrated solute zones into more diluted zones -is a source of band broadening, especially in gas chromatography.

Back

How to determine retention indexes.

Front

To obtain the retention index of an analyte, the log of the adjusted retention times for the two alkanes and the analyte are determined. The retention index for the analyte is then derived by interpolation between the two logarithmic retention indexes of the alkane.

Back

In a desorption source

Front

-a probe is used and ionization takes place directly from the condensed phase -The advantage of desorption ionization is that it can be applied to high molecular weight and thermally unstable samples.

Back

method for determining the number of plates in a column.

Front

The number of plates in a chromatographic column can be calculated by finding the retention time of analyte as well as the width of the peak at half height then applying the equation: N = 5.54 {tR/W1/2}2

Back

Retention factor

Front

the ratio of the number of moles of solute in stationary and mobile phases and is also defined as the distribution constant multiplied by the volume ratio of stationary and mobile phases.

Back

How can spin-spin splitting be differentiated from chemical shift lines

Front

By varying the magnetic field strength. Spin-spin splitting is independent of the magnetic field strength, whereas chemical shifts increase with increases in field strength.

Back

Plate height

Front

the ratio of the column length to the number of theoretical plates and is defined as the length of column which contains 34% of the solute as solute leaves from the column.

Back

advantages of using a magnet with as great a field strength as possible

Front

-First, if the line width is constant, resolution improves with field strength. -Second, sensitivity improves with field strength according to Equation 19-8. -Third, as the Δν/J ratio increases, spectral interpretation becomes easier.

Back

Magnetic sector (single focusing and magnetic-electrostatic (double focusing)) mass analyzers

Front

•Ions exiting the source with same m/z but with small diverging directional distribution will be acted on by the magnetic field and a converging directional distributions is produced. •Magnetic field bring ions with different directional orientations to focus •Limits the resolution, Rmax < 2000 •(a) energy aberrations •(b) directional aberrations

Back

Explain why it is necessary to spin liquid samples in an nmr spectrometer.

Front

samples are spun to avoid homogeneities of line broadening in the source field.

Back

What are the advantages of fused-silica capillary columns compared with glass or metal columns?

Front

Fused silica columns have greater physical strength and flexibility than glass open tubular columns and are less reactive toward analytes than either glass or metal columns.

Back

Quadrupole

Front

•ions travel parallel to four rods which are attached to dc voltages. •opposite pairs of rods have rapidly alternating potentials (ac) •ions try to follow alternating field in helical trajectories •stable path only for one m/z value for each field frequency •Harder to push heavy molecule - m/zmax< 2000 •Rmax ~ 500 •The resolution of a quadrupole is determined by •Vrfac/Vdc ratio •the length of the rods. •Rmax when Vrfac/Vdc slightly <6 •Most common mass analyzers in use today •Low scan time (<100ms)

Back

Bonding

Front

involves attaching a monomolecular layer of the stationary phase to the packing surface by means of chemical bonds.

Back

What are retention indexes?

Front

The retention index for an analyte is a measure of the rate at which it is carried through a column compared with the rate of movement of two normal alkanes, one that moves faster than the analyte and the other thatmoves more slowly.

Back

What is a rotating frame of reference?

Front

A rotating frame of reference consists of a set of mutually perpendicular coordinates (usually labeled x′, y′ and z′) in which the x′ and y′ coordinates rotate at a constant rate around the z′ coordinate

Back

Mobile phase

Front

a solvent of specific composition that moves through a column containing a stationary phase.

Back

Why are gas chromatographic stationary phases often bonded and cross-linked?

Front

stationary phases are generally bonded and/or cross-linked in order to provide thermal stability and a more permanent stationary phase that will not leach off the column.

Back

What variables are likely to affect the selectivity factor ἀ for a pair of analytes?

Front

The selectivity factor for a pair of analytes is affectedby: a. Mobile phase composition. b. Nature of stationary phase. c. Column temperature. d. Use of special chemical effects like complexing agents, surfactants, ion pairing reagents, etc.

Back

what is the packing material used in most packed GC columns?

Front

The packing material most often used is diatomaceous earth particles having diameters ranging from 250 to 170 um or 170 to 149 um.

Back

Understand factors affecting H: the van Deemter equation H = A + B/u + Cu. Know and understand terms, and how each varies with flow rate (u).

Front

•H = A + B/ u + Cu = A + B/u + (CS + CM)u •H - plate height (cm) •u - linear velocity of mobile phase (cm/s) •A - multipath coefficient •B - longitudinal diffusion coefficient •C - mass transfer coefficient for mobile and stationary phase

Back

Elution

Front

washing a species through a column by continuous addition of fresh mobile phase

Back

Retention time

Front

the time interval between analyte injection at one end of the column and the appearance of ite maximum peak at the other end.

Back

how the retention factor for a solute can be manipulated

Front

• The retention factor can be manipulated by the following: • In LC: a. Mobile phase composition. b. Nature of stationary phase. c. Use of special chemical effects like complexing agents, surfactants, ion pairing reagents, etc. d. Change column length. • II. In GC: a. Change column length. b. Nature of stationary phase. c. Column temperature

Back

Column resolution

Front

how well two peaks are resolved from each other is called resolution. This can be defined as the ratio of the separation distance (in time units) between two peaks to have their total peak widths at the baseline

Back

Distribution constant

Front

the ratio of the analyte concentrations in both stationary and mobile phases at equilibrium.

Back