Spectroscopy: Chapter 20

Question 1

Define a molecular ion

Answer 1

a radical ion that has the same molecular mass as the molecule

Notes Definition: unfragmented charged ion (whole molecule)

Question 2

The largest peak in a mass spectrum is referred to as the _______ ______.

Answer 2

Base peak

Question 3

What are the two basic methods used for ion formation in Molecular Mass Spectrometry?

Answer 3

1) Gas phase sources
2) Desorption sources

Question 4

What happens to a sample when using the Gas Phase source method?

Answer 4

The sample is first vaporized then ionized

Question 5

What happens to a sample when using the Desorption source method?

Answer 5

sample (solid or liquid) is converted **directly **into gaseous ions.

Question 6

What advantage does a desorption source have over a gas phase source?

Answer 6

they can be used for nonvolatile and thermally unstable samples.

since desorption sources don’t require volatilization of the analyte molecule, it can handle analytes with larger molecular masses (as large as 10⁵ Da)

Question 7

Name the different gas phase sources

Answer 7

• Electron Impact Ionization (EI)
• Chemical Ionization (CI)
• Field Ionization (FI)

Question 8

Name the different desportion sources (that is discussed in notes/class)

Answer 8

• Field Desorption (FD)
• Electrospray Ionization (ESI)
• Matrix-Assisted Laser Desorption-Ionization (MALDI)

Question 9

Describe the process of ions produced by* **electron impact. ***

Answer 9

sample is heated and the vapor molecules are ionized by bombarding them with a beam of energetic electrons.

Question 10

Describe the process of ions produced by **Field Desorption. **

Answer 10

Electrode is physically mounted on a probe, the probe is coated with sample and placed into the instrument, then a high voltage is applied to the electrode in order to ionize the sample.

Question 11

Describe the process of ions produced by MALDI

Answer 11

solid sample is dispersed with a matrix, then deposited on a metal plate and placed into a vacuum chamber. A laser is then shot onto the sample causing it to desorb and ionize. Creating an Ion Plume

Question 12

Describe the process of ions produced by **electrospray ionization. **

Answer 12

A solution of sample is pumped through a stainless steel needle and is electrified, which results in the sample exiting the needle as fine mist droplets. Desolvation then occurs which shrinks the droplets and increases th surface charge density until the Rayleigh limit is reached.

Question 13

Name at least three types of mass analyzers

Answer 13

• Time of Flight (TOF)
• Quadrupole
• Fourier Transform-Ion Cyclotron Resonance (FT-ICR)
• Sector Instruments (magnetic/electrostatic)

Question 14

Explain the Nitrogen Rule.

Answer 14

When analyzing MS information, an even number of nitrogen’s indicate that the monoisotopic molecular ion will be even.

Vise Versa with odd number nitrogens

Question 15

Explain how exact mass is calculated

Answer 15

exact mass is calculated by the ratio of the isotopes occurring naturally.

Question 16

What is the purpose of a Quadrupole?

Answer 16

A quadrupole is a type of mass analyzer therefore its purpose is to separate ions based on m/z (mass to charge ratio)

Question 17

Define Rayleigh limit.

Answer 17

the point where electrostatic repulsion exceeds surface tension of the droplet during desolvation in electrospray ionization. (EI)

Question 18

What happens when the Rayleigh limit is reached? (two things can happen)

Answer 18

1) Coulomb explosion, which generates smaller droplets from the original droplet.
2) Smaller droplets ejected from the original droplet, but there is no explosion

Question 19

What are the advantages of using a quadrupole as the mass analyzer?

Answer 19

• high ion transmission
• light weight, compact, cheap
• high scan speeds

Question 20

How does a quadrupole work?

Answer 20

STEPS:

1. RF voltage is applied to a pair of rods respective of the other
2. DC voltage is super-imposed on the rods
3. combination of voltages causes a selective filter of masses
4. rods are connected
5. only ions with a stable directory make it through