spectroscopy Flashcards
molecular ion peak
peak on spectrum formed when molecule looses an electron forming a positive ion and an electron
- on far right of spectrum (small M+1 peak after it)
what does molecular ion peak tell us?
the Mr of the compound
fragment ions
when molecular ion is broken down through fragmentation producing fragment ion + RADICAL
products of fragmentation
positive ion
uncharged radical - UNDETECTED
how does IR spectroscopy work?
- atoms are joined by covalent bonds which contain energy and vibrate (increase as temp increase)
- bond absorbs radiation at same frequency as bond
- increases its vibrations
2 types of movement by covalent bonds
stretch
bend
how does global warming link to IR radiation
- most of suns radiation doesn’t disturb atmospheric gases
- some radiation re-emitted from earth and is longer wavelength IR radiation
- absorbed in bonds In CO2, water vapour and methane
- causing bonds to vibrate more and re-emit this energy as radiation which increases atmospheric temp close to the earths surface = global warming
fingerprint region
below 1500cm-1
use of fingerprint region
identify specific molecules as region is unique to each compound
O-H (alcohol) wavenumber
3200-3600
C-H wavenumber
2850-3100
C=O wavenumber
1630-1820
O-H (carb. acid) wavenumber
2500-3300 broad
applications of IR spectroscopy
- pollutants can be identified using IR fingerprint region - remote sensors used to analyse IR spectra of vehicle emissions to monitor local pollution
- breathalysers - pass beam of IR radiation through breath to detect ethanol
order of use of analysis for unknown compound
1) elemental - find empirical formula
2) mass spec - find molecular mass from molecular ion peak - fragment ions identify different sections of molecule
3) IR spec. - identify bonds and functional groups
what does NMR use (2)
- magnetic frequency
- radio frequency radiation
how does NMR work
(odd number) of nuclei spin -
has 2 different spin states, in right conditions (high frequency radiation and strong magnetic field) nucleus rapidly flips between 2 states = RESONANCE
typical NMR radiowave frequencies
100, 200 or 400 MHz
chemical shift
- units
difference in radio frequencies absorbed by nuclei in molecule being analysed and same nuclei in TMS
- ppm
why deuteriated solvents?
doesn’t contain H1 - therefore won’t have signal on spectrum for NMR
typical deuterated solvent
CDCl3
what does C13 NMR tell us (2)
- from what?
- number of C environments (number of peaks)
- types of C environments (chemical shift)
what does proton NMR tell us (4)
- from what?
- number of H environments (number of peaks)
- type of environment (chemical shifts)
- relative number of H in each environment (relative area of signals)
- number of Hs on adjacent carbons (splitting patterns)
what causes splitting?
- what does it tell us?
H atoms on adjacent carbons
N+1 rule
uses of chromatography
analysis of:
- drugs
- plastics
- flavourings
- air samples
chromatography has 2 things:
1) stationary phase - doesn’t move, usually solid or liquid in solid
2) mobile phase - does move - gas or liquid
adsorption (TLC)
process by which silica hold different substances in the mixture to its surface
how does TLC work?
- TLC plate coated in solid absorbant substance (silica) = stationary phase
- different components of sample have different affinities for absorbent and bind to its surface with different strengths
TLC method
- draw line across bottom of TLC plate 1cm from bottom = base line
- use capillary tube to spot a small amount of solution of sample onto base line
- prepare solvent in beaker - 0.5cm depth
- suspend TLC plate in solvent - place bath glass over to avoid evaporation
- when solvent is 1cm form top of plate remove plate and mark solvent front
- circle any defined dots
leave to dry - calculate Rf
purpose of gas chromatography
separating and identifying volatile compounds in organic mix.
stationary phase in gas chromatography
high boiling liquid absorbed onto inert solid support
mobile phase in gas chromatography
inert carrier gas - eg. neon/helium
how does gas chromatography work?
- small amount of mix is injected into apparatus - mobile phase carriers components through capillary column which contains stationary phase
- components slow as they enter stationary phase - more soluble component is the slower it moves
- compounds reach detector at different times depending on solubility in stationary phase - lowest retention time detected first
retention time
time taken for each component to travel through column
what does gas chromatography tell us
- identify components - from retention time
- concentration of components - through peak integrations (area under peak)
how to find concentrations form gas chromatography
- prepare standard solutions of known concs of compound being investigated
- get chromatograms for each
- plot calibration curve of peak area vs. conc. = EXTERNAL CALIBRATION
- obtain gas chromatogram for component under investigation
- use calibration curve to find conc.
what does external calibration do
converts peak area into conc.
why may peaks in NMR spec. be outside of ranges of values?
factors eg. conc, solvents move them
