U4AOS1: analysis techniques I (spectroscopic)

Question 1

define m/z

Answer 1

mass to charge ratio

Question 2

what is a mass spectrometer

Answer 2

an instrument that ionises, deflects and detects an atom or molecule so that its mass to charge ratio can be charged - atom can only be detected if POSITIVE

Question 3

instrumentation of mass spec

Answer 3

filament provides bombarding of electrons which cause ionisation and formation of positive ions
heavier fragments = travel slower, smaller = quicker

Question 4

define base peak

Answer 4

the peak formed from an ion fragment with the greatest relative intensity (greatest abundance)

Question 5

define molecular ion peak

Answer 5

peak formed from the ionised un-fragmented form of a molecule. on the right most side

Question 6

why can there be multiple peaks

Answer 6

appearance of additional peaks in mass spectrum can be due to presence of isotopes that occur in relatively high abundance

Question 7

what can you get from infrared spec?

Answer 7

-info about covalent bonds
-which functional groups are present

Question 8

what does IR spec involve?

Answer 8

IR light has lower energy and a longer wavelength than visible and uv light. energy from IR light is no enough to promote electrons to very high energy levels but is enough to change the vibration of the bonds in the molecules

Question 9

what happens to covalent bonds with IR spec?

Answer 9

covalent bonds are like springs - can undergo specific amount of bending or stretching. atoms in a molecule can change position due to the bending or stretching of the bonds and molecule vibrates

Question 10

trends with IR absorption

Answer 10

-lowest molecular mass of what is attached to carbon = stronger absorption frequency.
-higher molecular mass = weaker frequency
-a bond that vibrates at a higher frequency absorbs IR radiation with a higher wavenumber and greater energy than a bond that vibrates at a lower frequency

Question 11

what peaks are present in IR spec for N-H group?

Answer 11

3350-3500 cm-1
and 1560-1650 cm-1

Question 12

what is NMR?

Answer 12

-nuclear magnetic resonance
-operates in the radio wave region of the electromagnetic spectrum
-only qualitative as structure of compound can be determined but not concentration or amount

Question 13

what is chemical shift?

Answer 13

chemical shift = the difference in energy needed to change spin state in a sample compared to the energy needed to change spin states in TMS (tetra methyl silane) and measured in parts per million (ppm)

Question 14

what can 13CNMR and 1HNMR tell us?

Answer 14

1. number of different C/H environments in a molecule - number of peaks
2. the relative number of C/H atoms in each environment (ratio of area of peak/height)
3. the type of each environment (from chemical shift)

Question 15

splitting pattern in H-NMR

Answer 15

-tells us the structure of the compound by signalling how many hydrogens are on neighbouring carbons.
btw CNMR does not include splitting

Question 16

what is the n+1 rule?

Answer 16

number of peaks of that hydrogen atom = number of h atoms attached to neighbouring carbons atoms + 1

Question 17

fragmentation equation for positive charged molecule in mass spec

Answer 17

M+ –> X+ + R.

Question 18

wavelength vs wavenumber

Answer 18

wavelength = specific length of one wave
wavenumber = number of waves in given unit of space

Question 19

why do the peaks not go up in IR?

Answer 19

they measure transmittance - so how much IR w still escaped/transmitted through the sample

Question 20

how to describe IR peaks?

Answer 20

1. wavenumber
2. depth
3. width

Question 21

difference between O-H alcohol and acid on IR spec

Answer 21

O-H alcohol has lesser width, and rounded/deeper shape
O-H acid has larger width and range, also has jagged point

Question 22

is the peak height/area proportional to number of atoms in CNMR?

Answer 22

NO. only in HNMR

Question 23

acronym for mass spec

Answer 23

Vaporisation
Ionisation
Acceleration
Deflection
Detection

Question 24

when there is a absorption on an infrared spectrum…

Answer 24

there is a change in the energy level of one of the bond types present

Question 25

HPLC chromatogram: the peak at __ minutes is beyond the scope of the vertical axis. how to solve the issue?

Answer 25

the concentration of the substance is very high. the area under the peak will probably be higher than that of standards used. the sample should be diluted

Question 26

why is high pressure required in HPLC?

Answer 26

the column is very narrow –high pressure is required to push the solvent and sample through this column. the mobile phase must be carried through the column

Question 27

what process is used in analysis by HNMR spectroscopy?

Answer 27

the use of radio waves to align hydrogen nuclei with an applied magnetic field

Question 28

how can an IR spectrum be used to determine the purity of a sample?

Answer 28

-using the fingerprint region
-can compare region of sample tot hose of pure compounds

Question 29

explain why one molecule can produce multiple peaks on a mass spectrum

Answer 29

-using bombarding of electrons, can produce positively charged fragments that are detected on the spectrometer, so produces peaks at different m/zs
-fragmentation pattern that occurs is bc initial parent ion is unstable
-molar masses of different isotopes present in parent ion = different molecule ion peaks

Question 30

what you need to be careful with when circling IR peaks…

Answer 30

-have to show full range of bond
-have to be very close to margins
-go a moderate length up the bond

Question 31

principle of IR spec

Answer 31

measures vibrations of atoms in a molecule and gives info about types of bonds present

Question 32

principle of H NMR

Answer 32

measures change in spin state of nuclei and gives info about H environments present

Question 33

explain how peaks in mass spectrum relate to molecule mass of ____

Answer 33

multiple peaks means isotopes present but both parent ions

relative molar mass is the weighted avergae of 2 parent ions based on relative abundances