QA Final flashcards

Nominal Mass

Integer mass of the species with the most abundant isotope of each of the constituent atoms.

Electron Ionization

Method used to create ions of gaseous molecules in the inlet of a mass spectrometer by bombardment of the gas with high-energy electrons.

Where do electrons travel in EI?

From filament to positively charged metallic conductor

How is a lower energy state achieved EI

Electrons interact with gaseous molecules to create cations via raising energy. Done to the point where the molecule will expel an electron to achieve a lower energy state.

molecular ion

cation with the same elemental composition as the original molecule

Base peak

most intense peak in a mass spectrum, intensity of 100.

Chemical Ionization

a gentle method of producing ions for a mass spectrometer without extensive fragmentation of the analyte molecule, M

Types of molecules in CI and EI

Volatile organics, gases, nonpolar compounds

What type of Ionization if EI?

Hard ionization

What type of Ionization is every other ionization technique in this course?

Soft ionization

Molecules in ESI

Polar compounds from small molecules to large biopolymers

Characteristics of ESI

high ionization efficiency; large ion charge; compatible with liquid chromatography

Characteristics of EI

structural information; possibly weak molecular ion; compatible with gas chromatography

Characteristics of CI

strong molecular ion; compatible with gas chromatography

Molecules in APCI and APPI

Nonpolar and less polar small molecules

Characteristics of APCI and APPI

strong molecular ion peak; compatible with chromatography

Molecules of MALDI

Polar compounds from small molecules to large biopolymers

Characteristics of MALDI

easy to perform; low ion charge; suitable for high throughput; high ionization efficiencies

Electrospray Ionization

an ionization method for mass spectrometry in which cations or anions in solution are transferred to the gas phase by formation and desolvation of an aerosol spray created by an electric potential

Mobile phase in chromatography

Phase where solvent flows, typically liquid or gas

Stationary Phase

Viscous stuff, stays in place

Elution

Liquid/Gas passing through chromotography column

Eluent

Fluid entering column

Eluate

Fluid emerging from column

Packed Column

Filled w/ stationary phase particles

Open tubular column

Narrow, hollow capillary w/ stationary phase coated on inside walls

Adsorption Chromatography

Stationary Phase: Solid
Mobile Phase: Liquid/Gas
Solute is adsorbed onto surface of solid particles, stronger solute=slower travel time

Partition Chromatography

Stationary Phase: Liquid bonded to solid support
Mobile: Liquid/Gas
Solute partitions between stationary liquid and mobile phase

Ion-Exchange Chromatography

Stationary Phase: Anions and Cations (SO3-, N(CH3)+
Mobile Phase: Liquid w/ Eluent Ions
Solute ions of opposite charge are attached to stationary phase.

Size Exclusion Chromatography

Stationary: Porous Gel
Mobile: Liquid/Gas
Separates particles based on size, larger solutes pass quickly. Pore only small enough to exclude large solute, not small ones. Smaller molecules take longer as they enter gel and are shielded from mobile phase.

Affinity Chromatography

Stationary Phase: Covalently bound molecule that binds w/ high specificity
Mobile Phase: Liquid
Analyte is eluted via changing pH or Ionic Strength

Baseline Resolution

Occurs in Chromatography when two adjacent peaks are sufficiently resolved that signal between peaks returns to baseline.

What is the ideal baseline resolution?

1.5

Gas chromatography

Form of chromatography in which the mobile phase is a gas, with the mobile phase being a carrier gas.

Open-tubular columns

A hollow capillary column whose inside walls are coated with stationary phase. Coiled to fit within a compact temperature-controlled oven.

Wall-coated open tubular (WCOT) column

Hollow chromatographic column in which the stationary phase is coated on the inside surface of the wall. Most common type of gas chromatography column.

Porous-layer open tubular (PLOT) column

Column containing an adsorptive solid phase coated on the inside of its wall.

Most common carrier gases in GC

Helium, H2 (both give better resolution), Nitrogen

Guard Column

A 3- to 10-m length of empty capillary. Silanized so that solutes are not retained by the bare silica wall. Placed in front of the capillary chromatography column to trap nonvolatile contaminants. Accumulates nonvolatile substances that would otherwise contaminate the chromatography column and degrade performance.

Common Gas Chromatography Detectors

•Thermal conductivity

•Flame ionization

•Electron capture

•Mass spectrometer

Universal Detector

Responds to all analytes, but not to carrier gas

What are two universal detectors?

•Flame ionization and thermal conductivity

Selective Detector and Example

Responds to limited class of analytes. Example: Electron capture

Thermal conductivity

Measures the ability of a substance to transport heat from a hot region to a cold region.

Characteristics of Thermal Conductivity Measuring

•Useful for packed columns and capillary porous layer columns

•Less sensitive than other detectors for open tubular columns

•Does not alter the sample

Requirements and carrier gas used in Thermal Conductivity

•For thermal conductivity detection, carrier gas must have a very different thermal conductivity than the analytes.

•Helium is most common used.

•Any analyte mixed with He lowers the conductivity of the gas stream.

Thermal Conductivity—Principal of Operation

•Eluate from the column flows over a hot tungsten-rhenium filament.

•Pure carrier gas flowing over the filament establishes a baseline temperature of the filament.

•When analyte emerges:

•Thermal conductivity of the gas stream decreases

•Causes the filament to get hotter è an increase in electrical resistance è a change in voltage across the filament

•The detector measures the change in voltage.

Flame ioniziation

Gas chromatography detector in which solute is burned in an H₂-air flame to produce CHO⁺ ions. The current carried through the flame by these ions is proportional to the concentration of susceptible species in the eluate.

Principal of Operation for Flame Ionization

•Eluate from the column is burned in a mixture of H₂ and air.

•Analyte containing carbon atoms (except carbonyl and carboxyl carbons) produces CH radicals, which produce CHO⁺ ions and electrons in the flame.

CH + O → CHO⁺ + e⁻

•Only approximately 1 in 10⁶ C atoms produce an ion.

•Ion production is proportional to the number of susceptible carbon atoms entering the flame.

•In the absence of analyte, a current of ~10⁻¹⁴ A flows between the flame tip and the collector.

•Analytes produce a current of ~10⁻¹² A, which is converted to a voltage, amplified, filtered, and converted to a digital signal.

Electron capture detector

Gas chromatography detector that is particularly sensitive to compounds with halogen atoms, nitro groups, and other groups with high electron affinity. Most popular, useful for environmental samples.

Principals of Operation for Electron Capture Detection

•A makeup gas of very-high-purity N₂ or 5-10% CH₄ in Ar is added to He or H₂ carrier gas.

•Gas entering the detector is ionized by high-energy electrons (β⁻ particles) emitted from a foil containing radioactive ⁶³Ni.

•Electrons in the plasma thus formed are attracted to an anode, producing a small current that is maintained at a constant level by variable frequency pulses between the cathode and anode.

•When analytes with a high electron affinity enter the detector, they capture some electrons and decrease the conductivity of the plasma.

•The detector responds by varying the frequency of voltage pulses to maintain a constant current. The frequency of the pulses is the detector signal.

Advantages of Split Injection

•Best for high concentrations of analyte or gas analysis.

•Offers high resolution and can handle dirty samples if glass wool is added to the injection liner.

Advantages of Splitless Injection

•Required for very dilute solutions.

•Solvent trapping or cold trapping is required, and temperature programming is necessary.

Advantages of On-column injection

•Best for thermally sensitive compounds and for the most accurate quantitative analysis.

•Limited to clean samples containing little or no nonvolatile components.

It can handle dilute or concentrated solutions and relatively large or small volumes

Nitrogen-Phosphorus Detector

Modified flame ionization detector that is especially sensitive to compounds containing N and P.

Photoionization detector

Uses a vacuum ultraviolet source to ionize aromatic and unsaturated compounds.

•It has little response to saturated hydrocarbons or halocarbons.

•Electrons produced by the ionization are collected and measured.

Stir-bar sorptive extraction

sample preparation method similar to solid-phase microextraction, except the sorptive layer is coated on the outside of a stirring bar. Analyte is removed from the coating by thermal desorption for chromatography.

Thermal desorption

Method for releasing volatile compounds from solid samples

Principal of Operation for Thermal Desorption

•A weighed sample is placed in a steel or glass tube and held in place with glass wool. The sample is purged with carrier gas to remove O₂, which is vented into the air, not the chromatography column.

•The desorption tube is then connected to the chromatography column and heated to release volatile substances, which are collected by cold trapping at the beginning of the column. The column is heated to initiate chromatography.

Purge and trap

Method for removing volatile analytes from liquids or solids, concentrating the analytes, and introducing them into the gas chromatograph

Process of Purge and Trap

1.A carrier gas bubbled through a liquid or solid extracts volatile analytes.

2.These analytes are then trapped in a tube containing adsorbent.

3.After analyte has been collected, the adsorbent tube is heated and purged in the reverse direction to desorb the analytes, which are collected by cold trapping in a gas chromatography column.

High-performance liquid chromatography

Uses high pressure to force the solvent through closed columns containing fine particles that give high-resolution separations.

Why is GC preferred over HPLC?

•Is normally less expensive

•Yields greater separation efficiency

•Generates less waste

Most Common Mode of HPLC

Reversed-phase chromatography

Isocratic elution

Liquid chromatography using a constant solvent mixture for the mobile phase

Gradient elution

Chromatography in which the composition of the mobile phase is progressively changed to increase the eluent strength of the solvent