Principles of Trace Analysis

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Principles of Trace Analysis

• The practical effectiveness of any geochemical
  method of exploration depends in large part on
  the availability of an analytical procedure that
  is properly suited to the problem at hand
• Rose, Hawkes and Webb (1979), p.42

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Characteristics of a Good Procedure

• Necessary
• Sensitive - detect low concentrations
• reliable - precise and accurate
• economical - large samples _at_ low
• Desirable
• Simplicity - use by untrained operators
• portable - for on site use

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Modes of Occurrence I

• Trace Mineral
• major element of mineral
• e.g. anglesite, malachite, casserite
• mobilized by
• solubility
• physical transport

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Modes of Occurrence II

• Trace element in well crystallized mineral
• e.g. Zn in magnetite, Cu in biotite, Pb in
  feldspar
• mobility controlled by host

5
Modes of Occurrence III

• In poorly crystallized material
• Fe-Mn oxides, clays, organics
• occluded trace minerals
• strongly adsorbed

6
Modes of Occurrence IV

• Weakly adsorbed
• surfaces of colloids, organics and clays
• ion exchange control
• mobilized by changing solution

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Types of Decompositions

• Total Methods
• Total - solid analysis or HF digestion
• Strong - fusion or hot concentrated acids 80 -
  100 released
• Partial Methods
• hot extractable - hot acids of gt 0.1 M
• cold extractable - weak acids ( pH 4-5) or
  complexing agents

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Analysis

• Resistate mineral - total or strong analysis of
  coarser size fraction
• Secondary mineral - strong or partial
• In host lattice - total or strong
• Adsobed ion - partial anaylsis of finer size
  fraction

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Steps of Procedure

• Sample preparation
• Decomposition
• Separation from interferences
• Analysis

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Types of Preparation

• Drying
• Pulverizing
• Sieving
• Cutting, grinding, ashing

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Separation

• Purpose
• Remove interferences
• Increase concentration
• Methods
• Solvent Extraction
• Ion Exchange
• Precipitation

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Strong Decomposition

• Acid Attack
• hot concentrated acid mixtures
• HF digestions
• Fusion
• acid - potassium bisulfate
• alkaline - sodium carbonate, lithium tetraborate
• reductive - fire assay

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Partial Decomposition

• Nonselective
• cold dilute acids
• buffers, citrate, acetate, EDTA
• Selective
• organic - hydrogen peroxide, Na-hypochlorite
• Fe-Mn oxides - hydroxylamine hydrochloride,
  ammonium oxalate, Na-dithionate
• sulfides - potassium chlorate-HCl, hydrogen
  peroxide, bromine

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Rock Samples
Split Trim 3cm size
Crush 0.5cm
Grind 60-200 mesh
Field Sample
Partial Extraction
Strong Acid Attack
Fusion
XRF
AAS
Plasma Emission
INNA
Color. Spec.
Separation
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Soils and Sediments
Dry
Sieve
Field Sample
Partial Extraction
Strong Acid Attack
Fusion
XRF
AAS
Plasma Emission
Separation
Colori- metric
INNA
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Vegetation
Dry _at_ 90-110C
Field Sample
Macerate
Ignite _at_ 450C
Wet Ashing
Strong Acid Attack
Dry Ashing
XRF
AAS
Plasma Emission
INNA
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Water Samples
ISE
Buffers
Filter
Acidify Preserve
Field Sample
Ion Exchange
Solvent Extraction
Evaporation
AAS
Plasma Emission
Color. Spec.
Ion Chrom.
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Comparison of Methods
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Comparison of Methods