3. ANALYTICAL TECHNIQUES Flashcards
Most often used to determine concentration of analytes
in Clinical Chemistry laboratory
Spectophotometry and Electrochemistry
o Is described as photons of energy traveling in waves
o Can take several forms, the most recognizable being
light and radiant energy
Electromagnetic radiation
o Is the linear distance between any two equivalent
points on a successive wave
o Unit used in the visible spectrum is nm
Wavelength
The relationship between wavelength (λ) and energy (E) is
described by
Planck’s formula (E = hv)
Planck’s constant
6.62 X 10 -27 erg sec
o The number of oscillations of the waveform in a
second
Frequency
Changes that may occur over period of time such as movement
Movement of waveform in a second
Oscillation
The relationship of energy and wavelength is that the frequency is _________________ to wavelength
inversely proportional
Visible region wavelength
400 - 700 nm
Ultraviolet region wavelength
< 400 nm
Infrared region wavelength
: > 700 nm
states that the concentration of a substance is directly proportional to the amount of light absorbed or inversely proportional to the logarithm of the transmitted light.
Beer’s law
Is used to measure the light transmitted by a solution to determine the concentration of the light-absorbing substance in the solution.
Spectrophotometer
Components of a spectrophotometer
Light Source
Monochromator
Sample Cell or Cuvet
Photodetector
Meter or read-out device
Provides polychromatic light
Light Source
light source provides light
at several wavelength
Polychromatic light
light source that provides visible and near-infrared regions continuum type
Incandescent tungsten or tungsten-iodide lamp –
light source that provides UV
region continuum type
Deuterium lamp and mercury arc lamp
2 types of Light Source
Continuum
Line
o Wide applications in the laboratory
o Emits limited number of discrete lines or
bands of radiation
o Examples: Tungsten (visible region) ,
deuterium (UV region), xenon (visible and
UV regions)
Continuum
o Emits a few discrete lines or bands of
radiation
o Examples:
Mercury and sodium vapor lamps – UV
and visible regions
Hollow cathode lamp - atomic absorption
spectroscopy / spectrophotometry
Line
Light source that provides visible and
UV regions continuum type
xenon
Light source that provides UV and visible regions line type
Mercury and sodium vapor lamps
Line type light source for atomic absorption spectroscopy / spectrophotometry
Hollow cathode lamp
Isolates individual wavelengths of light
Monochromator
Characteristics of monochromators
Nominal wavelength
Spectral bandwidth (or FWHM)
Bandpass
Represents nanometers in peak
transmittance
Nominal wavelength
Range of wavelengths about ½ peak
transmittance
Spectral bandwidth (or FWHM)
Total range of wavelengths, as seen in the chart
Bandpass
Types of monochromators
Filters
Prism
Diffraction gratings
Simple, inexpensive, and useful monochromator that requires you to determine the analyte of interest and set the wavelength at a specific point
Filters
example of filters
Interference and absorption filters
Monochromator that can be rotated, allowing only the desired
wavelength to pass through an exit slit
Prism
Most commonly used monochromator ; contain parallel grooves
Diffraction gratings
may be round or square and must be made of material that is transparent to radiation
used to hold samples; path length is 1 cm (general)
Sample Cell or Cuvet
Types of cuvet
Plastic cuvet
Fused silica or quarts
Alumina-silicate glass
cuvet for UV region
Fused silica or quarts
cuvet for 350-2000 nm
Alumina-silicate glass
cuvet for visible region
Plastic cuvet
Converts the transmitted radiant energy into an equivalent amount of electrical energy
Photodetector
Types of photodetector
Barrier-layer cell or photocell
Phototube
Photomultiplier tube (PMT)
Photodiode
Photodetector
o Least expensive; temperature sensitive
o Composed of selenium on a plate of iron
o Used mainly in filter photometers
Barrier-layer cell or photocell
Photodetector
o Contains cathode and anode enclosed in a glass tube
o Has photosensitive material that gives off electrons when light energy strikes it
Phototube
Photodetector
o Most common type
o 200 times more sensitive than the phototube
o Highly sensitive to UV and visible radiation
Photomultiplier tube (PMT)
Photodetector
o Not as sensitive as PM tube but with
excellent linearity and speed
Excellent linearity: beam of light that
strikes the photodetector reflects the
amount of analyte or concentration of
analyte present in the sample
Speed: concentration is immediately
read out on the read out device or the
meter because it can immediately
transmit the radiant energy into electrical energy
Photodiode
Displays output of the detection system
Meter or read-out device
Examples of Meter or read-out device
Digital meters, d’Arsonval meters, recorders, light-emitting diodes (LEDs), cathode-ray tubes (CRTs), and liquid crystal displays (LCDs).
uses 2 photodetectors 2 sample cuvets and 2 photodetectors
Double beam in space
– uses 1 photodetector; chopper is used to pass the monochromatic
radiation through the sample cuvet and then to the reference cuvet
Double-beam in time
a device that rotates or breaks up
radiation beams
Chopper
Wavelength or photometric accuracy
Implies that a photometer is measuring at the wavelength that it is set to
Can be checked by Special glass-type optical filters like
– didymium glass (600 nm); holmium oxide (360 nm)
Closeness of a measured value to its true or target value
Accuracy
Using glass filters or solutions that have known absorbance values for a specific wavelength
Absorbance check