A branch of chemistry that deals with separation, identification of chemical substances, and determination of relative amounts of the substances in a sample.
Determination of the identity of analytes in a sample
Determination of the amount of analytes in a sample
2 types of analytical methods
- 1. Classical
- 2. Instrumental-Analytical
Qualitative Classical Methods
Analytes are treated with reagents that yield products that can be recognized by their: color, boiling or melting points, solubilities, odors, or optical activities.
Quantitative Classical Methods
- amounts of analyte is determined by gravimetric or by volumetric analysis.
- 1. Gravimetric: mass of the analyte or substance is chemically related to the analyte measured.
- 2. Volumetric: The volume or mass of a standard sol'n required to react completely with the analyte is measured.
Qualitative Instrumental Methods
- 1. Light emission or absorption spectra: UV-VIS, AAS
- 2. Mass-to-charge ratio: mass spectrometry
- 3. Chromatograms: chromatography
- 4. Electropherograms: capillary electrophoresis
Quantitative Instrumental Methods
- 1. Measurement of intensity of atomic or molecular emission
- 2. Measurement of molecular absorbance, electrode-potential
- 3. Measurement of conductivity or abundance of mass-to-charge chemical species.
Instruments for chemical analysis can...
...convert physical or chemical characteristics of an analyte to information that can be manipulated and interpreted by a human.
A measure of an instruments ability to discriminate between small differences in analyte concentration.
Factors limiting sensitivity
- 1. Slope of the calibration curve
- 2. Reproducibility of the measuring device
S = mc + S(bl)
- S: measured signal of sample
- m: slope of calibration curve
- c: concentration of the analyte
- S(bl): measured signal for a blank
Analytical sensitivity (gamma)
- (gamma) = m / (sigma)
- m: slope
- (sigma): standard deviation of the measurment
minimum concentration or mass of analyte that can be detected
How to determine the detection limit
- Step1: find minimum distinguishable analytical signal S(m)
- S(m) = S(bl) + k(sigma(bl))
- k: constant (usually 3)
- (sigma(bl)): standard deviation of the blank
- Step2: use equation S=mc+S(bl) and convert S(m) to C(m)
- C(m) = (S(m) - S(bl)) / m
A range that extends from the lowest concentration to the concentration where the calibration curve departs from linearity.
degree to which the method is free from interference by other species in the sample matrix
- 1. =0 : no interference
- 2. >0 : value of S will be high
- 3. <0 : value of S will be low
study of interaction between electromagnetic radiation and matter as a function of wavelength
study of interaction between electromagnetic radiation and molecules as a function of wavelength
spectroscopic technique used to assess the concentration of the amount of a given species
instrument used to measure properties of light over a specific portion of the electromagnetic spectrum
a graph showing absorbance or molar absorptivity varying with wavelength
a region in a molecule that is responsible for light absorption
provides a stable source of radiation energy, sufficiently powerful for easy detection and measurement.
Types of Lamps
hydrogen, deuterium, tungsten, tungsten-halogen, and xenon arc lamp
H2 or D2 lamps
- 1. wavelength: 160-380
- 2. spectroscopy: UV molecular absorption
- 1. wavelength: 320-2500
- 2. spectroscopy: visible to near IR region
- 1. wavelength: 240-2500
- 2. spectroscopy: UV-Vis-near IR
Xenon Arc Lamp
- 1. wavelength: 200-1000
- 2. spectroscopy: UV-Vis-near IR
emits a temperature-dependant spectrum of light
Tungsten-Halogen lamp with Iodine
Iodine sometimes prolongs the life of a tunsten-halogen lamp by combining with gaseous tungsten and causing the metal to be redeposited.
require a radiation that consists of a limited, narrow, continuous group of wavelengths, called a band.
enhances selectivity of absorption, and provides selectivity to both emission and adsorption methods
Types of Wavelength Selectors
- 1. Filters
- 2. Monochromators
2 Types of Filters
- 1. interference filters
- 2. absorption filters
rely on optical interference to provide narrow bands of radiation
restricted to the visible region of the spectrum and usually consists of colored glass
can select a wavelength or a wavelength range and are designed for either a fixed wavelength measurement or a spectral scanning
2 Types of Monochromators
- a beam of 2 wavelengths enters the monochromator via a slit, is collimated and then strikes the surface.
- angular dispersion can result from diffraction
- a beam of 2 wavelengths enters via a slit, is collimated and then strikes the prism
- refraction results in angular dispersion
Rules for choice of slit width
- 1. minimal slit width is desirable when narrow absorption or emission bands must be resolved
- 2. radiant power decreases when slits are narrow
- 3. wider slit widths may be used for quantitative analysis rather than for qualitative
Cells or Cuvettes
- 1. quartz: expensive, used in the 180-3000nm region
- 2. glass: low cost, used in the 350-2000nm region
- high sensitivity
- high signal-to-noise and a constant response over a considerable range of wavelengths
- fast response time
- zero output signal in the absence of illumination
- electrical signal produced is proportional to radiant power
- phototubes and photomultiplier tubes: work based on the photoelectric effect
- silicone photodiode
a device which converts energy other than heat energy into heat energy
All Photon Transducers
- based on the interaction of radiation with a reactive surface to produce electrons or to promote electrons to energy states where they can conduct electricity
- only occurs in the UV-Vis region
consist of a semicylinder cathode and a wire anode sealed inside an evacuated transparent glass or quartz envelope
How a Phototube Works
when a voltage is applied across the electrodes, the emitted photoelectrons are attracted to the positively charged wire anode. A photocurrent then results that is readily amplified and measured.
like a phototube except in place of a single wire anode, the PMT has a series of electrodes called dynodes
How a Photomultiplier Tube Works
the electrons emitted from the cathode are accelerated towards the first dynode that is maintained 90-100V positive with respect to the cathode. Each photoelectron that strikes the dynode surface produces several electrons, called secondary electrons, that are accelerated to dynode 2. By the time this process has been repeated at each dynode, 10^5 to 10^7 electrons have been produced. All electrons are finally connected at the anode to proved an average current.
Silicon Photodiode Transducer
- photons striking the depletion layer of the device create electrons and holes that can be attracted across the junction giving rise to a current proportional to the flux of photons
- is a pn junction device
- operates under reverse biased conditions
material that has an electrical conductivity between that of a conductor and an insulator
Producing N-type and P-type Semiconductors
the addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties
- pure semiconducting materials which are doped with atoms capable of providing extra conduction electrons to the host material
- creates an excess of negative electron charge carriers
pure semiconducting materials which are doped with atoms capable of providing extra positive charges to the host material
- a junction formed by joining p-type and n-type semiconductors together in a very close contact.
- when formed, some of the free electrons in the n-region diffuse across the junction and combine with holes of the p-region forming a depletion region at the junction
Forward Based P-N Junction
- occurs when the p-type semiconductor material is connected to the positive terminal of a battery and the n-type semiconductor material is connected to the negative terminal.
- 1. drives holes to the junction from the p-type material and electrons to the junction from the n-type material.
- 2. at the junction, electrons and holes combine so a continuous current is maintained
Reverse Biased P-N Junction
- occurs when the p-type semiconductor material is connected to the negative terminal of a battery and the n-type semiconductor material is connected to the positive terminal.
- 1. will cause a transient current to flow as both electrons and holes are pulled away from the junction.
- 2. when the potential formed by the depletion layer equals the applied voltage, current will cease.
Photodiode Array (PDA)
- a linear array of discrete photodiodes on an integrated circuit chip (IC)
- in spectroscopy, a PDA is placed in the image plane of a spectrometer to allow a range of wavelengths to be detected simultaneously
an electronic device that amplifies the electrical signal from the transducer
Signal Processors can be used to:
- 1. remove unwanted signals.
- 2. convert from DC to AC and vice versa.
- 3. perform mathematical operation on the signal as differtiation, integration, or conversion to a logarithm.
- the output from a photomultiplier tube consists of a pulse of electrons for each photon that reaches the detector surface.
- 1. sometimes converts to digital pulses that may be counted
- 2. is a measurement of pulse number per unit time.
- fine strands of glass or plastics that transmit radiation for distance of several hundred feet or more.
- used to transmit radiation and images from 1 component of an instrument to another
radiation passes through either the reference or the sample cells
a device that allows light to pass for a determined period of time
radiation from the filter or monochromator is split into 2 beams that simultaneously pass through the reference and sample cells before striking two matched photo detectors.
the beam is alternately sent through the reference and sample cells before striking a single photodetector
Single vs Double Beam Instruments
- 1. Single: single beam irradiation, fluctuation in energy, simple, inexpensive ($2,000-$8,000)
- 2. Double: simultaneous two beam irradiation, compensate the source intensity fluctuation in energy source intensity both of the input/ouput, sophisticated, expensive ($10,000-$15,000)
Filters vs Monochromators
- 1. Filters: provide low resolution wavelength selection that is often suitable for quantitative work but not for qualitative work or structural study.
- 2. Monochromator: produces high resolution (narrow bandwidths) for both qualitative and quantitative work.
Photodiodes vs Photomultiplier Tubes
- 1. Photodiodes: better suited for small, portable instruments because of their size and ruggedness.
- 2. Photomultiplier Tubes: more sensitive but require a higher voltage supply compared to photodiodes. And are larger than photodiodes.
Spectrometers vs Photometers
- 1. Spectrometers: usually have monochromators for wavelength selection, and can be used for wavelength scanning or for multiple wavelength selection.
- 2. Photometers: usually have filters for wavelength selection, and are restricted to one or a few wavelengths.
Double Beam: In-Space vs In-Time
- **both split the beam into 2 portions to pass through the reference cell and the sample cell.
- 1. Space: both beams travel at the same time through the two cells, and then strike 2 separate photodetectors. Simple but requires two detectors.
- 2. Time: beams travel at different times through the cells, but are later recombined to strike 1 photodetector, at different times. Complicated but uses only one detector.
Rules for Choosing a Solvent
- 1. a solvent does not have absorption bands in a wavelength range where an analyte absorbs
- 2. no interaction between solvent molecules and analytical molecules
- 3. same solvent used in blank and analyte solution
Characteristics of Spectrophotometric Methods
- 1. applies to both organic and inorganic compounds
- 2. working range of 10-4 to 10-5 M
- 3. 1-3% accurate
- 4. easy and convenient
- 5. simple operation
Variables that Influence Absorbance
pH, ionic strength, temperature, and interference species.
- detects entire spectral range essentially simultaneously and can produce a spectrum in 1 second or less
- adv: speed and reliability
- uses mechanical methods to scan the spectrum, and requires several minutes to do so.
- adv: higher resolution, and lower stray light characteristics
science that investigates the interaction of EM radiation with atoms, usually by means of absorption or emission
devices used to convert analytes into gaseous atoms and ions
process that converts an analyte into gaseous atoms or ions
Atomic Emission of Radiation
EM radiation produced when excited atoms relax to lower energy levels by giving up their energy as photons
Atomic Absorption of Radiation
paritcular wavelength of EM radiation absorbed by gaseous atoms of a particular element
Atomic Emission Theory
based on the characteristic radiation energy emitted by excited atoms when they return to ground state or a low energy level
- Analyte ---atomization---> A*
- A*---relaxation---> A + hv
Atomic Absorption Theory
based on the absorption of a particular wavelength of EM radiation by atoms when the atoms jump from an orbital with a lower energy level to that of a higher energy level
Atomic Absorption Theory is governed by Beer's Law
- A = kbNo = K'C
- k/K' : constants
- b : thickness of gaseous atoms
- No : number of gaseous atoms in ground state
- c : concentration of analyte (proportional to No)
Energy Level Diagram
- convenient method for describing the processes that atomic absorption/emission is based on
- Show the different transition of elements between atomic orbitals
Difference in the Energy of 3P Orbitals
- rationalized by assuming that an electron spins about its own axis and the direction may be either the same or opposed to its orbital motions
- 1. energy is high if the direction of self-spin is the same as the direction of orbital motion.
- 2. energy is low if the direction of self-spin is the opposite of the direction of orbital motion.
Narrow Lines for AAS and AES
preferred because it reduces the possibility of interference due to overlapping spectra
Factors Causing Atomic Line Broadening
- 1. uncertainty effect
- 2. doppler effect
- 3. pressure effect
natural line width is determined by the lifetime of the excited state and Heisenberg's uncertainty principle
arises because atoms moving toward or away from a photon detector give rise to absorption or emission lines at slightly different frequencies
- arises from collisions of the absorbing or emitting species with other atoms or ions in the heated species
- causes small changes in energy levels and hence a range of absorbed/emitted wavelengths.
Hollow Cathode Lamp
- a cathode constructed of the metal whose spectrum is desired, or served to support a layer of that metal. Anode is made of tungsten. Electrodes are sealed in a glass tube that is filled with Neon or Argon at a pressure of 1-5 torr.
- *has a lamp for each element, restricts multielement detection, maximizes probability of redeposition on cathode and restricts light direction
gaseous cations of an inert gas acquire enough kinetic energy to dislodge some of the metal atoms from the cathode surface and produce an atomic cloud
- a nebulizer introduces the sample in the form of a fine spray of droplets, called aerosols.
- continuous introduction of samples produces a steady-state population of atoms, molecules and ions.
- solution-sample is converted to spray
Types of Pneumatic Nebulizers
- 1. concentric tube
- 2. cross-flow
- 3. fritted disk
- 4. babington
Concentric Tube Nebulizer
liquid sample is drawn through a capillary tube by high pressure stream of gas flow around the tip of the tube
high pressure gas flow across a capillary tip at right angles
Fritted Disk Nebulizer
sample solution is pumped onto a fritted surface through which a carrier gas flows
- high pressure gas is pumped through a small oriface in a hollow sphere's surface. The extending jet of gas nebulizers the liquid sample flowing in a thin film over the sphere's surface.
- *useful for high salt content samples because it is less subject to clogging
used for introduction of samples containing As, Sb, Sn, Se, Bi, and Pb into an atomizer as a volatile hydride gas.
Volatile Hydride Gas
- generated by adding an acidified sample solution to a small volume of 1% sodium borahydride (NaBH)
- swept into an atomization chamber and heated in a tube furnace where decomposition of the hydride takes place
- increases detection limits by 10 to 100 times
flame and plasma atomizers
Processes Occurring During Atomization
- 1. sample solution ---> nebulization
- 2. analyte + gasfuel ---> dissolvation
- 3. solid/gas aerosol ---> volatilization
- 4. gaseous molecules ---> dissociation
- 5. atoms ---> ionization ---> ions
Primary Combustion Zone
initial decomposition of molecules. recognized by its blue luminescence.
- commonly used for spectroscopy, hottest, mostly atomic fragments.
- used for emission/absorption/fluorescence
Secondary Combustion Zone
cooler, conversion of atoms to stable molecules and oxides
Characteristics of Flame Atomizers
- 1. laminar flow-burners: long path length for max absorbance
- 2. provides most reproducible methods and system provides a continuous output signal
- 3. low sensitivity due to residence time of individual atoms in the optical path and low sampling efficiency.
- 4. detection limits: 0.2-500ng/mL
- 5. relative precision of measurements: 1%
- few microliters of sample is introduced into a graphite furnace
- evaporated at low temperature and ashed at high temperatures in the graphite tube
- after ashing, atomization occurs when temperature rises rapidly to 2000-3000oC
Electrothermal Atomization occurrs
- in a cylindrical graphite tube that is open at both ends and that has a central hole for sample introduction.
- the graphite tube fits into a water-cooling metal housing
Characteristics of Electrothermal Atomizers
- 1. high sensitivity and high sample efficiency
- 2. detection limits: 5-0.001ng/mL
- 3. poor reproducibility: relative precision of 5-10%
- 4. slow: few minutes per sample
- 5. method of choice: when flame or plasma atomization provides inadequate detection limits