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Chem 315 > Quiz 3 Chapter 2 Material > Flashcards

Quiz 3 Chapter 2 Material Flashcards

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1
Q

What is a qTOF system?

A

Quadruple time of flight system, is a type of mass spec that can perform multiple stages of mass spec on a single sample

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2
Q

What did Francis W aston do?

A

In 1919, used a mass spectrograph to seperate ions differing in mass by 1%
found that Ne
consists of 2
isotopes (20Ne and
22Ne) &
went on to
discover 212 of
the 281
naturally
occuring
isotopes

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3
Q

What is the biggest characteristic of any mlcl?

A

Is it’s molar mass

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4
Q

What’s an isotope?

A

When neutrons are different in same atom, have diff mass to charge ratios

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5
Q

What is molecular mass?

A

Is the sum of atomic masses ex: 14.99 for phosporus

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6
Q

What is nominal mass?

A

integer mass of most abundant
isotope of each of the
constituent atoms added up for ex: have o2 be 32.

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7
Q

What is mass spectroscopy?

A

a technique for studying masses of atoms/
molecules/fragments of molecules

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8
Q

How do you obtain a mass spec?

A

ransform analyte molecules into gaseous species
* ionize them
* accelerate ions by an (electric field)
* separate ions according
to mass/charge (m/z) ratio
* detection

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9
Q

Say all species have a charge of +1, what is the mass to charge ratio?

A

m/z is equal to mass

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10
Q

Say all species have a charge of +2, what is the mass to charge ratio?

A

m/z is equal to half of mass

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11
Q

How does a magnetic sector mass spectrometer work?

A

Gas mlcls covert to positive ions at the electron beam, these are then accelerate dby ion plates and expelled into the analyzer tube. In the analyzer tube they encounter a magnetic field perpendicular to their direction of travel. The tube is maintained under a high vaccum so ions are not deflected by collison w background gas mlcls. The spectrum of masses is obtained by varying the magnetic field and ions are sperated accoridng ot mass. each ion that arrives at the electron multiplier detector starts a cascade of electrons which then hits dynodes multiplying electrons by 10^6 to 10^8, then they reach the anode where the current is measured. The MS then shows a detector response as a function of m/ selected by a magnetic field.

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12
Q

What is electron ionization?

A

molecules converted to ions by electron ionization (EI)
here e- emitted from a hot filament (like the one in a light
bulb) accelerated though 70 eV electrons before interacting with your incoming (neutral) molecules.

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13
Q

How much eV is needed to ionize molecules?

A

if 0.01% of molecules absorb 12-15 eV, that’s enough for ionization.

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14
Q

Why do we have M+?

A

since almost all stable
molecules have even
number of e-, when
one e- is kicked out we get a pos charge ion.

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15
Q

What happens if too much energy in eV hits the neutral molecules?

A

They mlcl may fragment and be small or absent in the mass spec

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16
Q

How does M+ form?

A

since almost all stable molecules have an even number of electrons when one electron is kicked out the resulting cation is designated M+.

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17
Q

What electron comes off the fastest and how many electron volts (eV) does it take?

A

The electron that comes off the fastest comes from a non bonding lone pair orbital and takes 11.0 eV to remove.

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18
Q

How much eV does it take to remove a pi bonding electron?

A

takes 14.1 eV, this is a high energy electron and can fragment M+ so much that the peaks result in being scattered or not present in mass spectrum.

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19
Q

Where do peaks occur in a mass spectrum?

A

Peaks occur at various mass to charge ratios, due to fragmentations they can occur at a bunch of diff ones but this gives clues to the molecule structure of the sample.

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20
Q

If we lower energy of the ionization source say to 20 eV what does this produce?

A

This gives a lower yield of ions and less fragmentation which leads to a higher chance to observe M+, but it might form negative ions

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21
Q

What electron volt energy is normally use din ionization and why?

A

70 eV because it gives reproducible fragmentation that can be compared with spectra in a library.

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22
Q

What is the most intense peak in a mass spectra clalled?

A

the base peak

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23
Q

How are all other peaks expressed in relation to the base peak in the mass spec?

A

the intensities of all other peaks are expressed as a % of base peak intensity

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24
Q

What is chemical ionization?

A

A different form of ionization used in mass spec

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25
Q

What produces more fragmentation, chemical ionization or electron ionization?

A

chemical ionization

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26
Q

What is the ionization source filled with in chemical ionization and at what pressure?

A

The ionization source is filled with reagent gas such as CH4, isobutane, and NH3 at 1 Torr

27
Q

What does CH4 convert into through energetic e (100-200 eV)?

A

it converts into a variety of reactive products

28
Q

why is isobutane and Nh3 used instead of ch4 sometimes in chemical ionization?

A

This is to reduce the fragmentation of Mh+ as CH4 bind more strongly to H+ and gives less energy to MH+ when the proton is transferred to M.

29
Q

How is CH5+ used in chemical ionization?

A

Is a potent proton donor that reacts with the analyte to give the protonated molecule MH+ which has biggest peak and less fragmentation.

30
Q

What does MALDI stand for?

A

Matrix assisted laser desorption ionization mass spectrometry

31
Q

What is MALDI?

A

Is a method for introducing proteins and other macromolecules into the Mass spectrometer

32
Q

How does MALDI work?

A

A laser pule is directed onto a sample in the matrix to introduce ions into the gas phase, the matrix then vaporizes and expands into the gas phase, carrying analyte with it and transfers charge to analyte (analyte may have already has some charge just by sitting in acidic matrix). Then the vapor expands into a supersonic plume. then ions detected by ms. Analyte in this case is usually protein.

33
Q

What is resolving power?

A

Is the ability of the mass spectrometer to seperate two peaks of similar mass,

34
Q

What limitations does each mass spectrometer have?

A

Each mass spectral peak has width that limits how closley two peaks could be spaced and still be resolved.

35
Q

What is the eqn for resolving power?

A

Resolving power = m/delta m or m/m0.5, m=smaller value of mass spec

36
Q

What are the two ways resolving power can be calculated?

A

can be taken as the separation of two peaks or as the width of a peak at max height.

37
Q

What does M+ tell you in a MS?

A

Tells us mlclr mass of unknown

38
Q

When using electron ionization does M+ always show up?

A

No because M+ breaks apart so effectively (however it gives clues to the structure of the mlcl via fragments)

39
Q

In chemical ionization do we se a strong or weak M+ peak?

A

Strong, that’s why we use it to find the molecular mass

40
Q

In which compounds does EI give a strong M+?

A

for aromatic compounds line benzene, biphenyl

41
Q

How do you calculate the theoretical intensity of M+1 peak for a compound like CnHm?

A

Use the isotope ratios (normal element/isotop per cent) and multiply it by the amount of atims to see the theoretical peak.

42
Q

What are the distinct peaks of Cl and Br? Describe them?

A

Br usually has two near equal peaks, 2 m/z away from each other, Cl has one peak at x, then one peak at x+2, then one peak at x+4, then another at x+6

43
Q

How do you deal with the fact that some ions have very similar compositions and what device do you use to distinguish between small differences in mass?

A

for ex an ion at m/z 84 could have a
variety of compositions:
C5H8O+ = 84.05696 or
C6H12+ = 84.09335
So to resolve differences we use a double focusing Mass spectrometer

44
Q

What can a double focusing MS, TOF MS, orbitrap MS, and Fourier Transform Ion Cyclotron MS do?

A

It can resolve differences of 0.001 or less at m/z 100

45
Q

How does a magnetic sector MS operate?

A

here ions with different masses but constant kinetic energy are separated by their trajectories in a magnetic field. The ion source in this system has acceleration plates, voltage from them imparts kinetic energy to ions.

46
Q

What is a limitation of the Magnetic sector MS?

A

Variation in kinetic energy of ions limits resolution to 0.1 at m/z 100.

47
Q

How does a double focussing mass spectrometer work?

A

Ions ejected from the ion source pass through an electric sector before the magnetic sector.
The electric sector only permits a narrow range of ion kinetic energies to pass to the magnetic sector.

48
Q

Why does a double focussing mass spectrometer have such a high resolution of 0.001 at 100 m/z?

A

Because the ions enter in series as they enter the electric first which increases resolving power, the electric sector also acts as a filter as it selects ions with a particular kinetic energy to pass through giving a more focused beam of ions which improves accuracy of magnetic sector.

49
Q

What is the resolving power of a double focusing mass spectrometer?

A

10^5

50
Q

How does a quadrupole mass spectrometer function?

A

analyte enters at GC column outlet under a pressure of 10^-6 Torr, ions are then accelerated and go through an electric field in the quadruple mass separator which deflects some so only certain m/z reach the detector

51
Q

What is the recording stats for a Quadrupole mass spec?

A

2-8 spectra / second
Up to 4000 m/z

52
Q

Why do we use a quadrupole filter in a quadrupole mass spec?

A

because it has a low cost

53
Q

What does the quadrupole filter consist of?

A

It consists of 4 parallel metal rods to which are applied a constant and radio frequency oscillating voltage

54
Q

How does a TOF Mass spectrometer work?

A

ions are produced at a source region by electron ionization or irradiation, they then hit the backplate which has 5000 V applied to it to accelerate ions and send it into the drift region, ions then passy by grid at the same time regardless of kinetic energy if same mass, and are then turned around by the reflectron, where they are reflected back to the grid in front of the detector to improve resolution

55
Q

What are the benefits of the TOF Mass spectrometer?

A

resolving power can be 10000 to 25000 and the m/z accuracy is 0.001
Has a high aquistion rate (is fast): 10^2 to 10^4 spectra/s
Can measure very high masses (m/z 10^6)

56
Q

How does a orbitrap mass spectrometer work?

A

Has no magnetic or radiofrequency field, has ions injected in an electromagnetic field into the orbitrap in one small package, once in they circle back and forth until they hit a mlcl, the orbitrap is held at -5 kv and 10^-10 torr.

57
Q

What are the benefits of using an orbitrap mass spectrometer?

A

Has high resolving power: 15000 at m/z 100 and 6000 at it’s upper m/z limit, however its VERY expensive

58
Q

What is the problem with trying to combine Mass spectrometry and liquid chromatography?

A

Liquid chromatography creates a hughe volume of gas (as the solvent evaporizes) but MS can only work in a vaccum

59
Q

How do u solve the problem in combining Mass spec and liquid chromatography?

A

You use an interface

60
Q

What is ESI?

A

electrospray ionization interface

61
Q

How does ESI work?

A

liquid from the chromatography column enters the steel nebulizer capillary along with n2 gas, this combined w the strong electric field at the nebulizer outlet creates a fine aerosol of charged particles , the positive ions are attracted to the glass capillary and flows to the MS where pressure is reduced by pumps.

62
Q

What are the benefits of ESI?

A

there is little fragmentation

63
Q

Why is it common to find charged ions using ESI?

A

because these ions were already in solution in the chromatography column

64
Q

What is atmospheric pressure chemical ionization? (APCI)

A

Is an interface, here heat and flow of N2 gas convert eluent coming off GC column into a fine mist and then into ions. This happens by high voltage being applied to a metal needle in the path of the aerosol, an electric corona (plasma containing charged particles) fans around the needle injecting electrons into the aerosol and creating ions.

Gives little fragmentation

Chem 315 (7 decks)

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