instrumental analysis

• Difference between qualitative and quantitative analysis of a sample

  • Qualitative: determines presence of substances in a sample (what)
  • Quantitative: determines amount of substances present (how much)
• elaborate on definition in analytical processes

  • define the problem using 5W1H
• elaborate on sampling in analytical processes

  obtain representative sample (small fraction that represents the entire bulk)
• Six essential stages of an analytical process

  definition

  sampling

  method

  sample preparation

  quantitation

  presentation and evaluation
• elaborate on method in analytical processes

  selecting appropriate analytical methods

  important factors like accuracy, detection limit, selectivity of instrument/method, testing speed, availability of reagents/equipment etc.
• elaborate on sample preparation in analytical processes

  prepare sample into correct size and form

  done without losing analytes/introducing interferences

  eg. dissolving/ concentrating
• elaborate on presentation and evaluation in analytical processes

  present in appropriate units

  evaluate using statistical tools like standard deviation
• homogenous vs heterogenous samples

  Homogenous: uniform appearance and chemical composition throughout Heterogenous: consisting of different substances or phases
• 3 considerations when deciding sampling procedure

  size of gross sample

  physical state

  chemical composition
• size of gross sample

  • Depends on homogeneity/heterogeneity of bulk material,
  • Desired level of accuracy
  • Precision in analysis 
  • Cost associated with sampling
• sampling solids

  • Challenging due to inhomogeneity of composition and variation in particle size
• homogenous solid sample sampling

  • Homogenous samples like flour: grab sampling 
  • Grab sampling: randomly taken sample, assumed to be representative
• heterogenous solid sampling

  systematic/statistical sampling

  1. Divide the land into 100 equal slots/squares to establish a systematic grid pattern 
  2. Collect grab samples from every 5th slot, ensuring the samples are taken at regular intervals
  3. Combine the grab samples obtained to create a gross sample around 20 kg
  4. Ensure uniformity in particle size

  • Combined gross samples should be ground/crushed 

  1. Take a laboratory sample (around 1 kg) for testing/analysis
• sampling homogenous liquids

  grab sampling
• heterogenous liquid sampling (less than 1L)

  mix and grab sampling
• heterogenous liquid sampling (more than 1L)

  sampling thief device

  allows collection from dif. locations, depths, times within the liquid
• sampling gas

  easier due to higher homogeneity

  1. displacement of liquid -- passing gas through impingers with an absorbing solution (gas bubbles through liquid, desired components absorbed)
  2. grab sampling using an evacuated bag/syringe
• Appropriate conditions/containers for storing

  1. air tight and clean containers
  2. minimise absorption effects of container
  3. control temp and humidity
  4. minimise storage duration
• spectroscopy

  interaction between electromagnetic radiation and other matter
• advantages of spectroscopy in QA

  less time consuming

  very little sample needed

  cost effective in the long run
• wavelength(𝞴)

  usually denoted in metres

  inversely proportionate to energy (higher wavelength= lower energy)
• frequency (Hz)

  no. of cycles passing through a fixed point per unit time

  directly proportional to energy

  
  

  f=c/𝞴
• velocity of light in a vacuum


  c=𝞴f

  units= cm/s
• electromagnetic spectrum (highest to lowest energy)

  Cosmic rays

  gamma rays

  x rays

  UV

  vis

  IR

  micro waves

  radio waves
• dual properties of light

  particle like and wave like
• energy

  E=hf or hc/ 𝞴

  units= joules (J)

  E = E1- E0
• atomic spectrum

  line spectrum

  atoms only undergo electronic transition
• molecular spectrum

  band spectrum: whole regions of lines absorbed

  3 transition:

  1. rotational
  2. vibrational
  3. electronic
• lambert's law

  intensity of transmitted energy decreases as path length of the cell increase exponentially

  A=kb, k is constant, b is path length of the cell
• beer's law

  concentration of an absorbing species is proportional to its absorbance

  A=k'c, k' is a constant, c is concentration
• beer-lambert's law

  A=ɛbc’ , where ɛ = molar absorptivity (L/mol x cm/1) and c= mol/L

  A=abc, where a= absorptivity (L/g x cm/1), c = g/L

  b= path length of the cell (cm)
• assumptions of beer's law

  1. monochromatic light used (single wavelength employed)
  2. no concentration dependent interactions present (ie. each particle absorbs independently of every particle)
  3. only true for very dilute solutions (less than 10-3 M)
• deviation from beer's law

  1. true deviation (conc is too high, refractive index of solution changes from that of blank, results in molar absorptivity and absorptivity changing)

  2. apparent deviation ( chemical deviations or change in absorbing species' nature/conc eg. pH change)
• radiation source for UV-VIS

  deuterium (D2) lamp for UV (emits wavelength of 200-400 nm )

  tungsten (W) lamp for VIS (emits wavelength of 400-800 nm)

  must be

  1. able to emit radiation over wavelength of 800-200 nm
  2. have sufficient intensity for transmitted energy to be detected at the end of the optical path (detector)
  3. be stable such that Io = 100%
• monochromator in UV-VIS

  entrance slit

  • blocks out stray light such that only light from radiation source allowed in

  dispersion element

  • prism or diffraction gratings

  exit slit

  • isolates specific wavelengths of the beam
• prisms in UV-VIS

  • radiation will refract as refraction index of prism differs from air
  • splits radiation into different wavelengths

  material

  • vis= glass
  • UV= quartz or fused silica
• diffraction gratings in UV-VIS

  • highly polished surface material like alumina with large numbers of parallel straight lines (grooves)
  • no. of grooves range from 15 to 30k per inch, acting as scattering centres
  • equal dispersion of all wavelengths, known as linear dispersion

  widely used, better resolving power than prisms
• sample/ cuvette cell in UV-VIS

  holds sample

  must be transparent to UV/VIS

  materials

  • UV= quartz
  • vis= quartz or glass
  • plastic cuvettes only for samples of specific wavelengths, widely used in industry as its cheap and disposable
• detector in UV-VIS

  measures intensity of radiation by converting radiation energy to electrical energy, before amplifying the signal

  2 types:

  • photocell
  • photomultiplier (widely used)
• photomultiplier in UV-VIS

  radiation hits metal surface (PMT), emits electrons.

  dislodged electrons attracted to another surface (dynode= maintained at higher positive voltage)

  at dynode 2 (D2), electron strikes surface and causes more electrons to be emitted etc etc

  
  

  process repeated to give a measurable effect at collector