Spectroscopy

Question 1

Define mass spectrometry:

Answer 1

A technique used for measuring the molecular weight and determining the molecular formula of an organic molecule.

Question 2

What happens in the most common type of mass spectrometry?

Answer 2

A molecule is vaporized and ionized, usually by bombardment with a beam of high-energy electrons.

The energy of these electrons is typically about 6400 kJ, or 70 electron volts (eV). This electron beam ionizes a molecule by causing it to eject an electron.

Question 3

In spectrometry, where do the positively charged ions accelerated toward?

Answer 3

A negatively charged plate.

Question 4

Where do positively charged ions in spectrometry go after passing the negatively charged plate?

Answer 4

They pass through a curved analyzer tube in a magnetic field, where they are deflected by different amounts depending on their ratio of mass to charge (m/z).

Question 5

Why doesn’t the term spectroscopy apply to the technique specific to this chapter?

What is the more acceptable term?

Answer 5

Because the energy source termed with spectroscopy is usually only applicable for techniques that use electromagnetic radiation as an energy source.

Mass spectrometry. This is because the energy source in MS is a beam of electrons.

Question 6

What is the full name for the shorthand M+*

Answer 6

Radical cation.

Also called molecular ion or parent ion

Question 7

Why is M+* a radical?

Answer 7

This is due to it having an unpaired electron, it is also a cation because it has one fewer electron than it started with.

Question 8

Is M+* stable or unstable?

Answer 8

Inherently unstable thus leading to decomposition.

Question 9

T/F radicals and cations have a lower molecular weight than the molecular ion.

Why?

Answer 9

True. This is because single bonds break to form fragments.

Question 10

A mass spectrometer analyzes the masses of _____ only

Answer 10

cations

Question 11

What does a mass spectrum plot?

Answer 11

The amount of each cation (its relative abundance) versus its mass

Question 12

What does a mass spectrometer analyze?

Answer 12

The mass of individual molecules, not the weighted average mass of a group of molecules.

Question 13

What is the tallest peak in a mass spectrum called?

Answer 13

The base peak

also synonymous with the M peak

Question 14

The decomposition of a molecular ion into lower molecular weight fragments is termed _________.

Answer 14

fragmentation

Question 15

T/F the mass of the molecular ion equals the molecular weight of a compound.

Answer 15

True

Question 16

Fun Fact: :)

Answer 16

Hydrocarbons like methane (CH4) and hexane (C6H14) as well as compounds that only contain CHO always have a molecular ion with an even mass.

An odd molecular ion generally indicated that a compound contain nitrogen.

Question 17

What is the nitrogen rule?

Answer 17

A compound that contains an odd number of N atoms gives an ODD molecular ion.
A compound that contains an even number of N atoms ***Including zero gives an even molecular ion.

Question 18

What is an example of the nitrogen rule?

Answer 18

3-methylfentanyl (C23H30N2O)
(2 N atoms)
molecular weight = 350

MPPP (Desmethylprodine)
(C15H21NO2)
(1 N atom)
molecular weight: 247

Question 19

How can using the street drug MPPP lead to Parkinson’s disease?

Answer 19

A common impurity in the synthesis of the drug is called MPTP. This is a neurotoxin that specifically targets dopamine producing neurons. Thus leading to reduction of dopamine leading to Parkinson’s disease.

This impurity occurs due to dehydration in acidic conditions when the temperature rises above 30 degrees C.

This is why it is so important to monitor EVERYTHING during your experiment.

The smallest of changes can yield an impure product in the lab.