19 - more chemical analysis Flashcards
what is the traditional method of analysis?
determine empirical and molecular formula from combustion analysis, elemental percentage composition and characteristic reactions of functional groups
what is a limitation of traditional methods of analysis?
difficult to be sure especially when a molecule contains many atoms
what is HRMS and why is it useful?
high resolution mass spectrometry - provides a value for the relative molecular mass to four or more decimal places
what is HPLC and how is it different from column chromatography?
high performance liquid chromatography
- solvent forced through a metal tube under high pressure rather than gravity
- particle size of the stationery phase is much smaller (better separation of the components)
- sample injected into the column
- components detected after passing through the column usually by absorption of uv radiation
- whole process is automated and results are available on a computer
what is the retention time (Rf)?
the time taken from injection to detection - important in identifying the compenents
how is gas chromatography different from column chromatography?
- metal tube is several metres long and coiled to save space
- stationary phase is a solid or a liquid coated on the inside of the tube
- sample is injected into the column, as in HPLC
- components passing through the column are detected
- whole process is automated and the results are quickly available on a computer display
after sample is injected the components vaporise and move through the coiled tube with the carrier gas. they move at different speeds depending on their attraction the stationary phase. weaker attractions move more quickly and have shorter retention times
what are the limitations of HPLC and GC?
very useful for separating small quantities of components in a mixture but not good at positively identifying them (difficult to control all variables - solvent, pressure, temp) and also because some substances have the same retention factor
some components have a retention factor for which there is no reference and therefore these techniques are useless (in that case)
when do chromatography methods have to be exact?
- providing forensic evidence
- detecting banned drugs in sportsmen and women and racehorses
also used in analysis of pollutants, detecting explosives in airport baggage, in space probes on other planets
what are HPLC and GC combined with in order to remain useful?
mass spectrometry - abbreviated to HPLC-MS and GC-MS
what are anabolic steroids used for?
in medicine to treat anaemia, osteoporosis and some forms of cancer by increasing muscle growth, red blood cell production and bone density
explain NMR spectroscopy…
- nucleons have spin
- nuclei either have an even or odd number of nucleons
- residual spin causes a tiny magnetic field
- nuclei are affected by an external magnetic field
- difference in energy between these two different states of a nucleus
- the nuclei of two adjacent atoms in a molecule influence each other and electrons also have an effect
how is an NMR spectrum labelled?
y - absorption
x - chemical shift (ppm - parts per million) (values increase to the left)
which atoms are detected by NMR?
must have an odd number of nucleons - carbon-12 is even and therefore not detected however carbon-13 is odd and can be detected. hydrogen can also be detected however are referred to as protons
what solvent is used and why is it challenging to find a solvent?
most solvents contain c or h and would therefore also produce a signal
CDCl3 is used - D is deuterium (a hydrogen isotope)
it only produces one signal from the carbon however this can easily be cancelled out
what is TMS and why is it used?
tetramethylsilane - SiH4
carbon and hydrogen atoms in different chemical environments will produce different signals. TMS produces a single strong signal that is easy to identify - it is also unreactive
