Sample Preparation

1
Sample Preparation

• Analytical II

2
Sampling

• Process of collecting a representative sample for
  analysis.
• Real samples require sample preparation to remove
  substances that interfere with reading of the
  analyte or convert the analyte into a suitable
  form for analysis.

3
Statistics of Sampling

• For Random Error
• Overall variance, So2Sa2 Ss2
• Standard deviation in sampling operation
• ?n vnpq
• np expected number of particles

4
Choosing a Sample Size

• The bigger the sample the more sufficient your
  sample will be and a decrease the in standard
  deviation
• Sampling Constant mR2Ks
• M is the mass of sample drawn
• R is the relative standard deviation (expressed
  in percentage)
• Ks is a sampling constant 36 g

5
Sampling a Random Bulk Material

• How many 0.7 g samples must be analyzed to give
  95 confidence the the mean is known to within 4?

6
Dissolving Sample for Analysis

• Once the bulk sample is selected, a laboratory
  sample must be prepared for analysis.
• Solids are typically dried at 110oC, to remove
  absorbed water prior to analysis
• Dissolve sample for analysis
• If sample does not dissolve, acid digestion or
  fusion may be used.
• Organic material may be destroyed by combustion

7
Grinding

• Solids can be ground in a mortar and pestle
• The mortar is the base and the pestle is the
  grinding tool
• Ores and minerals can be crushed by striking the
  pestle lightly
• Boron Carbide- is the best mortar

8
Dissolving Inorganic Materials with Acids

• Insoluble inorganic materials can be dissolved in
  strong acids and heating
• Glass vessels are often useful, but Teflon,
  platinum, or silver are required for HF, which
  dissolves silicates.
• Non oxidized acid is insufficient, aqua regia or
  other oxidized acids will work
• Teflon- lined bomb heated in a microwave oven

9
Dissolving Inorganic materials by Fusion

• If Acid digestion fails, fusion in a molten salt
  will work
• Large quantity of flux adds trace impurities
• This is a last resort procedure

10
Decomposition of Organic Substances

• Organic material are decomposed by wet ashing
  with hot concentrated acids or dry ashing with
  heat
• Wet ashing- when water is involved
• Dry ashing- procedure does not involve water

11
Sampling Preparation Techniques

• Analytes can be seperated from complex matrices
  samples by
• Liquid extraction
• Analyte is dissolved in a solvent that does not
  necessarily dissolve the entire sample and does
  not decompose the analyte.
• The liquid is then analyzed by chromatography
• Supercritical fluid extraction
• Superficial fluid as the extraction solvent
  19CO2
• Solid- phase extraction
• Concentrates nonionic analytes

12
Preconcentration

• Metal ions can be preconcentrated with cation-
  exchange resin
• Trace analysis often requires preconcentration of
  analyte to bring it to a higher concentration
  prior to analysis

13
Derivatization

• Procedure in which analyte is chemically modified
  to make it easier to detect or separate.
• Ex. Aldehydes or ketones can be trapped and
  derivatized by passing air through a tiny
  cartridge containing 0.35g silica coated with 0.3
  wt 2,4- dinitrophenylhydrazine.