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CH304 > CH 22 Topics > Flashcards

CH 22 Topics Flashcards

1
Q

What is the most intense peak in a mass spectrum?

A

the base peak

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2
Q

What is the nominal mass?

A

It is the integer mass of the species with the most abundant isotope of each of the constituent atoms

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3
Q

Find the nominal mass of C2H5Br

A

(212)+(51)+(1*79) = 108

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4
Q

What is the monoisotopic mass?

A

the “real mass” or the exact mass of the species with the most abundant isotope of each of the constituent atoms

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5
Q

Find the monoisotopic mass of C2H5Br

A

(212 Da)+(51.007825 Da)+(1*78.91834 Da)= 107.95746 Da

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6
Q

What is the Nitrogen Rule?

A

If a compound has an odd number of nitrogen atoms (in addn to any number of C, H, halogens, O, S, Si, and P) then M+ has an odd nominal mass
For a compound with an even number of nitrogen atoms, M+ has an even nominal mass.

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7
Q

Why might the molecular ion not always be the absolutely highest m/z seen in a spectrum

A

because of adduct formation or due to isotope pattern

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8
Q

How is the elemental composition of an ion established?

A

either by accurate mass measurement or from the relative intensities of isotope peaks

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9
Q

What are the 3 types of elements in common organic compounds?

A

X, X+, X+2

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10
Q

What are X elements?

A

(F, P, I, and Na) have only a single natural nuclide

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11
Q

What are X+1 elements?

A

elements (C and N) have only one natural isotope that is 1 Da greater than the most abundant

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12
Q

What are X+2 elements?

A

have an isotope 2 Da greater than the most abundant

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13
Q

How do you know that the peak at m/z 100 represents the molecular ion?

A
  • M+ will be at the highest m/z value of any of the significant peaks in the spectrum that cannot be attributed to isotopes or background.
  • Intensities of isotopic peaks at M+1, M+2 and so on must be consistent with the proposed formula.
  • The peak for the highest m/z ion should not correspond to an improbable mass loss from M+.
  • If a fragment is known to contain a specific element or number of atoms of an element, then that element must be in the molecular ion.
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14
Q

What is the ring + double bond equation and what are its parts?

A

R+DB = c - h/2 + n/2 +1

c: number of Group 14 atoms (C, Si, and so on)
h: number of (H+ halogen) atoms
n: number of Group 15 atoms (N, P, As, etc)
group 16 atoms (O,S, etc) don’t enter the formula

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15
Q

What are the types of bond breaking?

A

Homolytic cleavage: one electron remains with each fragment

Heterolytic cleavage: both electrons stay with one fragment

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16
Q

What can fragmentation patterns do?

A

can unravel the structures of small molecules and even large biological molecules such as proteins and carbohydrates

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17
Q

What are high m/z associated with?

A

high resolving power because peaks are very narrow with high resolving power, so there is little uncertainty in locating the apex of the peak

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18
Q

What are the chromatography-mass spectrometry techniques?

A

Extracted Ion Chromatogram
Selected Ion Monitoring
Tandem Mass Spectrometry
Selected Reaction Monitoring

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19
Q

What is a total ion chromatogram?

A

a plot of combined signal from all detected ions versus time

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20
Q

What is an extracted ion chromatogram?

A

a chromatogram made by collecting consecutive full-range mass spectra, but selecting just one value of m/z for display

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21
Q

What is selected ion monitoring?

A

the mass spectrometer monitors just the a values in any acquisition style
it lowers the limit of detection for individual analytes and decreases the background signal when using a transmission quadrupole mass spectrometer

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22
Q

Why is the signal-to-background ratio in selected ion monitoring greater than that in Total ion chromatograms or extracted ion chromatogram?

A

because most of the spectral acquisition time is spent collecting data in a small range and little but the intended analyte gives a signal at selected m/z

23
Q

What is Tandem Mass Spectrometry?

A

a technique in which two m/z separators are connected in series, separated by a collision cell
uses the sequence: (m/z separation to select precursor ion) -> (collision cell to create fragments from one selected ion) -> (m/z separation of collision cell product ions)

24
Q

What is selected reaction monitoring?

A

a technique that enhances specificity for a particular analyte and increases the signal-to-background ratio to provide decreased limits of detection

25
Q

Electrospray of Proteins

A

well suited for the study of charged macromolecules such as proteins
electrospray ejects protein ions from solution into the gas phase by the charge Residue Model
generally gives proteins that are multiplicharged

26
Q

Electron Transfer Dissociation for Protein Sequencing

A

a selective way to cleave polypeptides into fragments in a mass spectrometer. Involves the exothermic transfer of an electron from the gas phase anion to a gas phase polypeptide with concomitant bond breaking

27
Q

Tandem Mass Tags

A

molecules designed to be covalently attached to peptides from multiple sources in a mixture
allow us to determine what fraction of a particular peptide came from each source

28
Q

Direct Analysis in Real Time (Dart)

A

A direct analysis in real time (DART) source produces electronically excited He or vibrationally excited N2 which are directed at the surface of an object to be sampled in open air.
Heated gas flows through a needle held at +1 to +5 kV with respect to a grounded perforated disk upstream in the source.
The glow discharge plasma contains electrons, ions, and excited neutral species.
Electrodes 1 and 2 are held at positive potentials for positive ion mass spectrometry and at negative potentials for negative ion mass spectrometry.
At positive potential, electrodes 1 and 2 prevent cations from exiting the DART source. At negative potential, anions and electrons are retained

29
Q

Explain the Chemistry of Dart

A

The DART gun is directed at an object to be sampled.
With a helium source, excited He atoms react with atmospheric H2O to produce molecular ions, H2O+.
These ions react with H2O to give H3O+ and hydroxyl radicals (OH•).
The H3O+ reacts with more H2O to produce protonated water clusters.
These clusters can react with analyte M on the surface of an object to create MH+.
Other chemistry can produce (M-H)-, M- or adducts such as (M+NH4)+ or (M+Cl)-.

30
Q

Low-Temperature Plasma

A

In a technique related to DART, a low-temperature plasma is created by passing or ambient air through a glass tube with a grounded wire at the center.
The tube is wrapped on the outside with a copper sheath to which is applied a 3-kV alternating current.
Excited-state species in the plasma ionize and dislodge molecules from a surface such as human skin into the source of a mass spectrometer.
There is no electrical shock to the surface under study.
A battery-operated, hand-held plasma has been developed

31
Q

Desorption Electrospray Ionization (DESI)

A

In desorption electrospray ionization (DESI), micron-sized, charged droplets created by electrospray of analyte-free solvent are directed onto the surface of an object under study.
Analyte on the surface dissolves in the droplets.
Further bombardment knocks droplets into the air toward a mass spectrometer inlet.
As in conventional electrospray, it is common to observe multiply-charged ions and alkali metal adducts in the mass spectrum

32
Q

Matrix Assisted Ionization

A

Matrix-assisted ionization is a surprising method to create ions for mass spectrometry without applying an electric field or a laser.
Similar to MALDI, a matrix is used to assist in the ionization.
But in this case, instead of firing a laser to desorb the matrix + analytes, the matrix (+ analytes) is simply allowed to sublime into the mass spec
Simply exposing the matrix-analyte deposit to the subatmospheric pressure of the mass spectrometer produces ions because the matrix emitted light when its crystals were cracked. The light emission is accompanied by movement of charge across the faces of the crystal. That charge is eventually passed to the analytes

33
Q

Ion Mobility spectrometry

A

Although functionally similar to mass spectrometers, ion mobility spectrometers are operated in air at ambient pressure, and ion mobility spectrometry is not a form of mass spectrometry.
Ion mobility spectrometry does not measure m/z and provides no structural information.

34
Q

What is Mass Spectrometry?

A

Mass spectrometry measures the mass-to charge ratio, m/z, of atomic or molecular ions.

A mass spectrum displays the number of ions detected at each value of m/z.

The m/z value is treated as dimensionless, with m expressed in unified atomic mass units, u (also called daltons, Da), and z as multiples of the elementary charge, e.

35
Q

What are the 3 components of a mass spectrometer?

A

Three essential components of any mass spectrometer are:
a source of ions
a separator
a detector

36
Q

What are the Mass Spectrometry Tecchniques?

A

Electron Impact
Chemical Ionization
Electrospray
Atmospheric Pressure Chemical Ionization (APCI)
Matrix Assisted Laser Desorption/Ionization (MALDI)

37
Q

Electron Impact Ionization

A

The electron ionization source is heated to 300°C to prevent condensation of analyte and solvent.
Electrons (black dots) emitted from a hot (~2000 °C) rhenium or tungsten filament are accelerated through the 70 V potential difference between the filament and the ionization chamber wall. We say that the electron energy is 70 electron volts.
Electrons travel from the filament to the positively charged metallic collector.
Permanent magnets in line with the filament and collector on either side of the source cause electrons to travel in narrow, helical paths ~1 mm in diameter.
Electrons interact with gaseous molecules (open circles) to create some cations (colored circles) by raising the energy of a molecule to the point that it will expel an electron to achieve a lower energy state.
A cation with the same elemental composition as the original molecule is called a molecular ion. Some cations have enough vibrational energy to break into fragments with smaller values of m/z.
The slightly positive repeller plate and negative extraction plates accelerate cations toward the m/z separator.

38
Q

Why is EI a hard ionization technique?

A

After ionization, some M+• loses its energy by collisions with other molecules and remains intact.
Some M+• retains enough internal vibrational energy (~1 eV) for a long enough time to break into fragments including ions of lesser m/z and neutral molecules or radicals with an unpaired electron. For simplicity, we will not write dots on fragments with unpaired electrons.
The EI technique is very reproducible, and the chemical reactions are generally well behaved

39
Q

Why is it useful that the EI technique is very reproducible, and the chemical reactions are generally well behaved?

A

Because EI spectra are very similar instrument to instrument, databases of fragmentation can be compiled and used to identify unknown compounds

40
Q

Chemical Ionization

A
  • For chemical ionization, the electron ionization chamber contains a reagent gas such as methane, isobutane, or ammonia at a pressure of ~1 mbar.
  • Energetic electrons (100-200 eV) convert CH4 into a variety of reactive products
  • CH5+ is a potent proton donor that reacts with analyte to give the protonated molecule, MH+, which is usually the most abundant ion in the chemical ionization mass spectrum if proton affinity of analyte is greater than that of the reagent gas.
  • The molecular ion, M+•, can be formed by reactions
  • MH+ is the protonated molecule, not the molecular ion!
  • Chemical ionization gives less fragmentation than electron ionization and is considered a soft ionization technique
41
Q

Electrospray Ionization

A

Electrospray ionization interface can be used to connect the outlet of a liquid chromatograph to the inlet of a mass spectrometer.
Electrospray is also a stand-alone ionization source for mass spectrometry without chromatography. In that case, analyte solution is pumped directly into the steel nebulizer capillary at the upper left in the figure, along with a coaxial flow of N2 gas.
For positive ion mass spectrometry, the nebulizer is held at 0 V and the spray chamber is held at -3500 V.
For negative ion mass spectrometry, voltages are reversed. The strong electric field at the nebulizer outlet, combined with the coaxial flow of N2 gas, creates a fine aerosol of charged particles.
In general, electrospray ionization is only applicable to polar molecules.
There is little fragmentation in electrospray. It is considered a soft ionization technique.

42
Q

Collision Induced Dissociation

A

Fragmentation can be intentionally increased by in-source collision-induced dissociation (also called collisionally activated dissociation) with 3 mbar of background N2 gas in the region between the glass capillary and the skimmer cone.

43
Q

Atmospheric Pressure Chemical Ionization

A

In atmospheric pressure chemical ionization, heat and a coaxial flow of convert eluate into a fine mist from which solvent and analyte evaporate.
Like chemical ionization in the ion source of a mass spectrometer, atmospheric pressure chemical ionization creates new ions from gas-phase reactions between ions and molecules.
The distinguishing feature of this technique is that a high voltage is applied to a metal needle in the path of the aerosol.
An electric corona (a plasma containing charged particles) forms around the needle, injecting electrons into the aerosol and creating reagent ions from atmospheric O2, N2, and H2O and from mobile phase components in liquid chromatography.
Reagent ions then undergo gas-phase ion/molecule reactions with gas-phase analytes.
The analyte must not be thermally labile and must have some volatility when heated.

44
Q

MALDI Process

A

Typically, 1 μL of a 10 μM solution of analyte (perhaps protonated by 0.1% trifluoroacetic acid) is mixed with 1 μL of a 1 to 100 mM solution of an ultraviolet-absorbing compound such as dihydroxybenzoic acid (the matrix) directly on a probe that fits into the source of the mass spectrometer.
Evaporation of the liquid leaves an intimate mixture of fine matrix crystals plus analyte.
When the laser pulse is absorbed, matrix vaporizes and carries analyte along with it at a velocity of ~400 m/s.
High matrix-to-sample ratio inhibits association between analyte molecules and provides protonated ionic species that create singly charged analyte ions by a variety of mechanisms

45
Q

what are the Mass Separation Technologies?

A 
Magnetic Sector
Transmission Quadrupole
Quadrupole Ion Trap
Orbitrap
Linear TOF
Reflectron TOF
46
Q

Magnetic Sector Mass Spectrometer

A

Magnetic sector, a continuous MS, has been used historically the longest.
As the name implies, the mass analyzer uses magnetic field to separate ions of different m/z values.
High voltage is first applied to the ions to accelerate them into the magnetic sector.
Once the ions enter, are exposed to the magnetic field.
As a result, ions are deflected according to Fleming’s left-hand rule.
The deflections differ based on their m/z where lighter ions (of the same charge) will experience more deflection.

47
Q

Transmission Quadrupole Mass Spectrometer

A

A transmission quadrupole mass spectrometer, also called a quadrupole mass filter, is a relatively simple, relatively inexpensive mass spectral detector for gas and liquid chromatography.
The quadrupole consists of four parallel, hyperbolically or cylindrically shaped metal rods to which are applied both a constant voltage and a radio-frequency oscillating voltage.

48
Q

Orbitrap Mass Spectrometer

A

The Orbitrap mass spectrometer is a high-resolution mass analyzer that does not require a magnetic field or a radio-frequency field.
The Orbitrap provides a resolving power of ~140,000 to 480,000 at m/z 100, accuracy of 0.5-3 ppm with external calibration, an upper m/z limit of ~6000 and a dynamic range of several thousand. With internal calibration standards, sub-part-per-million m/z accuracy is attainable.
The cutaway view of an Orbitrap shows precisely machined central and outer electrodes.
The central electrode is held at -5 kV while the two outer electrodes are close to ground potential and electrically isolated from each other.

49
Q

Linear Time of Flight Mass Spectrometer

A

When an ion of mass m and charge +ze (where e is the elementary charge) is accelerated through a potential difference V = 20 kV its electric potential energy, zeV is converted into kinetic energy
differing m/z will have different velocities and therefore travel down the drift tube and reach the detector at different times.
Ions created by laser desorption/ionization have a distribution of velocities and locations in the ion source before the voltage pulse is applied to grid 1 to extract ions.
The spread of velocities and locations gives a distribution of drift velocities, causing different ions with equal m/z to reach the detector at slightly different times, thereby decreasing resolution of isotopic variants of ions with closely spaced values of m/z.

50
Q

Reflectron Time of Flight Mass Spectrometer

A

A reflectron is also sometimes called an ion mirror because it reverses the trajectory of an ion.
The purpose of the reflectron is to minimize the spread of kinetic energy of ions with the same mass to charge ratio.

51
Q

Types of mass spectrometer detectors

A

Electron Multiplier

Microchannel Plate Detector

52
Q

Electron Multiplier

A

The discrete dynode electron multiplier is analogous to the photomultiplier tube.
Each cation striking the cathode unleashes a cascade of electrons, just as a photon starts a cascade of electrons in a photomultiplier tube
The mass spectrum shows detector response as a function of m/z selected by the mass separator.
Mass spectrometers can measure negative ions by reversing voltages where the ions are formed and detected

53
Q

Microchannel Plate Detector

A

The faces of the plate have a conductive coating to allow a potential difference of ~800 V to be applied between the faces.
Each electron striking a channel wall unleashes multiple electrons that are accelerated toward the right.
Each of the two plates provides a gain of ~103 to 104 giving ~106 to 107 outgoing electrons at the right side of the second plate for every electron entering the first plate.
Pores in the two plates are offset in angle so that neutrals or photons cannot go straight through to the detector anode

CH304 (9 decks)

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