003 Mass Spectrometry

Question 1

instrument where a small sample of a
compound is introduced, vaporized and
then ionized as a result of an electron’s
being removed from each molecule…

separation is based on the mass-to-charge (m/z) ratio of the molecule

Answer 1

Mass Spectrometer

Question 2

1. ion source, liquid feed, gives elements, metals (mostly used)
2. gas, lots, used in tandem with GC
3. gas, some
4. liquid, very little, mostly used in nat prod, good for polar compounds
5. liquid, some, mostly expensive
6. solid, some, requires small sample size
7. portable, very little

Answer 2

1. Inductively Coupled Plasma (ICP)
2. electron impact (EI)
3. chemical ionization (CI)
4. electrospray (ESI)
5. Atmospheric pressure chemical ionization (APCI)
6. Matrix assisted laser desorption ionization (MALDI)
7. desorption electrospray ionization (DESI)

Question 3

Mass analyzers
1. Acceleration in magnetic field
2. Magnetic plus electric field
3. Passage through ac electric field
4. Orbit in quadrupole
5. time to travel through tube
6. various, usually involving orbits

Answer 3

1. Magnetic Sector
2. Double focusing
3. Quadrupole
4. Ion trap
5. time-of-flight
6. Newer high resolution

Question 4

a graph of the relative abundance of each fragment plotted
against its m/z value; can be shown either as bar graphs or in tabular form.

Answer 4

Mass spectrum

Question 5

tallest peak, assigned an intensity of 100%, have the greatest
relative abundance

Answer 5

Base peak

Question 6

corresponds to the unfragmented cation
radical (M+)

Answer 6

Parent peak or molecular ion peak

Question 7

peaks with smaller m/z values; represent positively charged fragments of the molecule

Answer 7

Fragment ion peaks

Question 8

the molecular mass to the nearest whole number.

Answer 8

Nominal molecular mass

Question 9

Mass is defined in three
different ways:

Answer 9

Monoisotopic,
average and nominal

Question 10

Mass accuracy can be stated in two main ways:

Answer 10

ppm and percentage

Question 11

Formula in solving ESI

Answer 11

m/z = (MW + nH+)/n

Question 12

State the nitrogen rule

Answer 12

if a compound has an odd-mass molecular ion, the compound contains an odd number of nitrogen atoms

if a compound has an even-mass molecular ion, the compound contains an even number of N atoms (including zero N since 0 is considered an even number in this context).

For ESI, an even-mass molecular ion, the compound contains an odd number of N atoms.

Question 13

responsible for the M+1 peak

Answer 13

C13

Question 14

Just a note!

The presence of a large M+2 peak is evidence of a compound containing either chlorine or bromine, because each of these elements has a high percentage of a naturally occurring isotope that is two units heavier than the most abundant isotope.

if the M+2 peak is one-third the height of the M peak, then the compound contains a chlorine
atom (the natural abundance of 37Cl is one-third that of 35Cl).
■ if the M and M+2 peaks are about the same height, then the compound contains a bromine
atom (the natural abundances of 79Br and 81Br are about the same).

Answer 14

NA