ANACHEM II UNIT I

• Classical/Wet-Chemical Methods

  • separating the components of interest (the

  analytes) in a sample by precipitation,

  extraction, or distillation
• Instrumental Methods

  • make use of instruments for separating and

  determining chemical species
• Emission of Radiation Emission Spectroscopy (X-ray, UV, Visible, Electron);

  Fluorescence, Phosphorescence, Luminescence (X-ray,

  UV, Vis, IR)
• Absorption of Radiation Spectrophotometry & Photometry (X-ray, UV, Vis, IR);

  Photoacoustic Spectroscopy; Nuclear Magnetic

  Resonance (NMR) & Electron Spin Resonance

  Spectroscopy
• Electrical Potential Potentiometry;

  Chronopotentiometry
• Electrical Charge Coulometry
• Electrical Current Amperometry; Polarography
• Electrical Resistance Conductometry
• Precision

  • The degree of mutual agreement among

  data that have been obtained in the same

  way

  • Provides a measure of the random or

  indeterminate error of an analysis

  • Absolute standard deviation, relative

  standard deviation, coefficient of variation,

  variance, standard deviation of the mean
• Bias (Accuracy)

  • Provides a measure of the systematic or

  determinate error of an analytical method

  • Involves analyzing one or more standard

  reference materials whose analyte

  concentration is known

  Bias = μ – xt
• Sensitivity

  • A measure of the instrument or method’s

  ability to discriminate between small

  differences in analyte concentration

  • Two factors that limit sensitivity:

  1. slope of the calibration curve

  2. Reproducibility or precision of the

  measuring device
• Calibration Sensitivity

  • The slope of the calibration curve at the

  concentration of interest
• Detection Limit

  • The minimum concentration of analyte that can be

  detected at a known confidence interval

  • Depends upon the ratio of the magnitude of the

  analytical signal to the size of the statistical

  fluctuations (standard deviation) of the blank

  • Unless the analytical signal is larger than the blank

  by some multiple of k of the variation in the blank

  owing to random errors, it is impossible to detect

  the analytical signal with certainty
• Limit of Detection (LOD)

  • Typically 3 times the signal-to-noise

  (based on standard deviation of the noise)
• Dynamic Range

  • Extends from the lowest concentration at

  which quantitative measurements can be

  made (limit of quantitation, LOQ) to the

  concentration at which the calibration

  curve departs from linearity (limit of

  linearity, LOL)
• identify

  A) 1

  B) 2

  C) 3

  D) 4

  
• Selectivity

  • Refers to the degree to which the method

  is free from interference by other species

  contained in the sample matrix
• Robustness free from chemical interferences

  • can be applied to analytes in a wide variety of matrices
• Rugged method

  • insensitive to changes in experimental conditions
• All instrumental methods require

  calibration curve to relate measured signal

  to concentration (amount) of analyte.
• Calibration – the process of ensuring that the signal

  measured by a piece of equipment or an

  instrument is correct
• Standardization – the process of establishing the

  relationship between the amount of analyte and a

  method’s signal
• Calibration Curve – the result of standardization

  showing graphically how a method’s signal changes

  with respect to the amount of the analyte
• Primary reagent

  • Reagent of known purity that can be used

  to make a solution of known concentration
• Secondary reagent

  • A reagent whose purity must be relative to

  a primary reagent
• Reagent grade

  • reagents conforming to standards set by

  the American Chemical Society
• Single-point standardization

  • A single standard containing a known

  concentration of analyte, Cs , is prepared

  and its signal, Sstand , is measured

  • The value of k is calculated as

  k = Sstand / Cs
• Multiple-point standardization

  • Most commonly employed standardization

  method

  • Uses one or more external standards

  containing known concentration of analyte
• External standards - prepared and

  analyzed separately from the samples
• Three Types of Calibration Curves

  1. External Standard

  2. Standard Addition

  3. Internal Standard
• External Standard

  • Constructed by measuring signals

  over several known concentrations of

  analyte and then plotting signal vs

  concentration

  • Known analyte solutions must have

  similar matrix to that of sample
• External Standard

  Signal magnitude should not be a function

  of small sample volumes that may be

  difficult to reproduce

  • Works well for simple matrices

  • Calibration curve prepared from known

  concentration

  • Unknown solution measured
• Standard Addition Known amounts of analyte are added

  directly to sample to try to account for

  matrix effects

  • Two main types:

  – Constant Total Volume

  – Changing Total Volume'
• Standard Addition: Constant Total Volume

  Analysis conditions:

  – Constant volume of unknown analyte

  – Varying volume of added analyte of known

  concentration to each sample

  – Linear response of instrument to

  concentration
• Standard Addition: Constant Total Volume

  
• Two-point Standard Addition:

  Constant Total Volume

  Can be done with only two points using

  the unknown and one addition to sample

  
• Two-point Standard Addition:

  Changing Total Volume

  Signal from known volume of unknown

  concentration is measured (S1

  • Signal after known volume of standard

  added to the unknown sample is measured

  (S2
• Two-point Standard Addition:

  Changing Total Volume

  Signal from known volume of unknown

  concentration is measured (S1)

  • Signal after known volume of standard

  added to the unknown sample is measured

  (S2

  )

  
• How to Determine if Standard Addition is Needed

  • Have to do both ways and see if external

  standard gives numbers that are always too

  high or too low.

  • External standard

  – faster

  – More efficient if many samples and fast

  analysis time for each sample is needed

  • Standard Addition

  • Used only if external standards do not give

  correct answers
• Voltammetric Analyses almost always

  use standard addition because:

  each measurement that requires

  emptying and refilling cell takes

  several minutes

  addition of known to solution that

  has already been deoxygenated

  allows for quick measurement

  • electrode surface is very sensitive

  to changes in solutions