mst deck Flashcards
what’s the difference between quantitative and qualitative analysis?
qualitative = what is in a sample quantitative = how much is in a sample
6 stages of analytical process
- definition
- sampling
- selecting method of analysis
- sample preparation
- quantitation
- presentation and evaluation
define sampling
obtaining a small fraction of material from a large bulk such that the fraction is representative of the bulk in every way
3 considerations when deciding method of sampling
- size of gross sample
a. heterogeneity
b. accuracy/precision desired
c. cost of sampling - physical state of fraction to be analysed: solid, liquid, gas
- chemical composition of material to be analysed
4 conditions for storing samples
- storage container is clean and airtight
- avoid adsorption effect on walls of containers esp for analysis of trace metals
- store in rooms not too hot and humid
- avoid prolonged periods of storage time to prevent decomposition/evaporation
define wavelength
distance of one complete cycle
wave number formula and unit
1 / λ
unit: cm^-1
define frequency
number of cycles passing a fixed point per unit time
velocity formula
what is the speed of light
v = f x λ c = 3.0 x 10^8 m s^-1
energy formula
E = hf or hv/λ
define absorption
process where atoms or molecules acquire certain discrete quantise amounts of energy from incoming radiation and become excited
what is a photon/quanta
minute, discrete amount of energy of electromagnetic radiation
difference between atomic and molecular spectrum
when atoms absorb radiation, only electrons get excited = only electronic transition
molecules absorb whole lines of radiation, and have 3 processes of absorbing energy: rotational, vibrational, electronic transition
transmittance and absorbance formulae
T = I / I0 A = log10 (1 / T)
Beer-Lambert law
A = abc or εbc
2 assumptions of Beer-Lambert law
- monochromatic light is used: only a single wavelength is employed in absorbance measurement
- there are no concentration-dependent interactions present: each absorbing particle absorbs independently of each other, true in very dilute solution i.e. < 10-3 M
2 deviations of Beer-Lambert law
true deviation: when concentration is so high that refractive index changes from that of blank, affecting values of a / ε
apparent deviation: when nature or concentration of absorbing species changes, causing non-symmetrical chemical equilibria and hence nonlinearity.
4 major components of a uv-vis spectrometer
- radiation source (deuterium lamp for ultraviolet, tungsten lamp for visible)
- monochromator (prism or diffraction grating)
- sample cell (quartz for both, glass for visible)
- detector (photocell or photomultiplier)
compare pros and cons of single and double-beam uv-vis spectrometer (3)
- single-beam: risk of analytical error from line voltage changes resulting in fluctuation in intensity of radiation reaching detector; double-beam: beam ratio is obtained and used constantly, minimising analytical error
- single-beam: risk of analytical error from solvent absorbing some radiation; double-beam: analytical error is corrected
- both single and double-beam: risk of analytical error from loss of radiation at reflecting surfaces
describe procedure for selection of wavelength and obtaining calibration graph in uv-vis spectrometry (2)
- spectrum scan:
conduct spectrum scan at from 800 to 200 nm
take wavelength at highest absorbance = highest sensitivity = ideal choice - photometric scan:
conduct photometric analysis at λmax to obtain calibration graph
what are the quantitative applications of uv-vis spectrometry (2)
- spectrum scan: to determine wavelength that can be absorbed appreciably by sample
- photometric scan: determine unknown concentration of sample
what are the qualitative applications of uv-vis spectrometry (3)
- detect presence or absence of certain functional groups
- detect impurities
- identification by matching spectra of unknown and standard
what are the problems with qualitative analysis using uv-vis spectrometer? (2)
- broad spectrum gives little details
2. spectra of different compounds may overlap to give complex spectrum
what are chromophores?
functional groups that can absorb radiation in uv-vis region i.e. π electrons (multiple bonds) or n electrons (N, O, S, halogens)
4 components of infrared spectrometer
- radiation source (Nernst Glower or Globar)
- monochromator (except FT-IR which uses interferometer)
- sample cell (metal halide)
- detector (bolometer or thermocouple)
3 conditions for radiation source of infrared spectrometer
- must be able to emit radiation over infrared region i.e. 400-4000 cm^-1
- must have sufficient intensity such that transmitted energy can be detected at the end of the optical path
- must be stable (I0 = 100%)
3 conditions for radiation source of uv-vis spectrometer
- must be able to emit radiation over uv-vis region i.e. 800 to 200 nm
- must have sufficient intensity such that transmitted energy can be detected at the end of the optical path
- must be stable (I0 = 100%)
3 problems with using metal halides as sample cell/prism in infrared spectrometer
- mechanical instability
- thermal instability
- water solubility
how to prepare samples for infrared spectrometry for solid (3), liquid (2) and gas (1)
solid sample preparation
- KBr pellet technique
- mull technique
- cast film technique
liquid sample preparation
- KBr disc technique
- liquid cell technique (CaF2 is used if water is present in sample)
gas sample preparation
KBr or NaCl sample cell technique
*difficult to analyse as concentration is low. we can use a cell with longer path length, which will increase absorbance.
theory of infrared absorption (2)
each molecule has a complete absorption spectrum unique to its functional groups.
in molecules, atoms are held by chemical bonds and are in continuous synchronised movement.
3 conditions for molecule to absorb infrared radiation
- change in bond length or angle due to vibrational or rotational motion causing net change in dipole moment
- molecule must be vibrating at a natural frequency equal to frequency of incoming radiation
- radiation energy must correspond to energy difference between ground state and excited state of molecule
4 precautions when doing sample treatment/sample preparation
- should be done without losing any of the analytes
- should be done without adding new interferences
- should bring analyte into appropriate chemical form for method to be used
- should dilute/concentrate analyte for detectable results by the instrument
