CHM 302

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CHM 302

• CHAPTER 4
• Sample Treatment, Interferences, and Standards

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• Sample preparation, sample treatment or
• sample pretreatment is carried out based on
• the following principles
• No loss of analyte recovery
• Making the analyte into the most suitable
• chemical form
• (iii) Should include removing interferents from
• the matrix
• (iv) Should not add any new interferents

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• (v) Using appropriate concentrations
• Recovery
• Always want to make sure all the analyte is
• extracted from the sample
• To maximize recovery, consider
• the chemistry of the analyte
• the chemistry of the matrix
• the nature of the interactions between both

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• Analyte loss
• Common ways include
• (a) - adsorption
• - absorption
• - reactions
• (b) Chemical reactions
• - decompositions catalyzed by light
• - CO2, O2 reactions
• (c) Evaporation of volatile samples

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• Quantitative transfer is supposed to transfer
• or move all the analyte

• Optimizing the chemical form
• Some instrumental methods require the use
• of solids
• - homogenizing may be sufficient

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B. Change of state by heating or cooling -
sometimes the solid is dissolved in
an appropriate solvent C. Digestion - a common
method used to release analytes from very tough
matrix - usually involves boiling/heating a
solid with an acid, base, enzyme or redox agent
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• Flux digestions are also practiced
• - a flux is a solid at room temperature
• - usually a redox agent, H or OH- that
• forms a melt at high temperature
• Problems with digestions
• - change in chemical composition of the
• analyte
• - safety

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• D. Minimizing interferences
• This usually involves removal of the matrix
• - or exchanging the matrix for one that
• will not interfere
• Extractions
• Transferring analytes from one matrix to
• another
• - usually from solids or liquids to another
• liquid

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• The most common is solid-liquid extraction
• The liquid must be able to solubilize the
• analyte
• - or better it should be selective for the
• analyte
• This selectivity can be expressed as a
• distribution coefficient, KD

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• Multiple sequential extractions are better
• than single extractions
• - the soxhlet extraction is an example
• of a continuous multiple extraction
• method
• Liquid-liquid extractions require that the two
• liquids be immiscible

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• Solid-phase extraction is now widely practiced
• in an effort to reduce the use of VOC
• In SPE, a solid phase specific for the analyte
• is used to absorb the analyte from a solution
• passing over it
• - concentration of the analyte occurs
• simultaneously

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• Other ways to minimize interferences
• dialysis
• precipitation and floatation
• purge and trap
• Optimizing concentration
• Every assay has a range of values in which
• precision and accuracy are maximized
• - called the working concentration range

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Calibration and Standards

• With two exceptions all types of analytical
• methods require calibration
• - a method that relates the measured
• analytical signal to the concentration of
• the analyte
• The three most common calibration methods
• Preparation and use of a calibration curve

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(ii) standard addition and internal
standard methods External standards - prepared
and analyzed separately from the unknowns -
used when components of the matrix do
not interfere
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• Calibration curve
• - make standards of exactly known
• concentrations ? measure instrument response
• ? plot instrument response (corrected) vs
• analyte
• The graph is called a calibration (or analytical
• or working) curve
• - most curves are linear

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• Instrument response constant x concentration
• Note that nonlinear curves are not uncommon
• - requires more points to accurately
• establish the relationship between
• instrument response and concentration

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Standard Additions Method

• Useful for analyzing complex samples
• - the likelihood of matrix effects is
• substantial
• Can take several forms. One of the most
• common is spiking
• - adding one or more increments of a
• standard solution to sample aliquots of the
• same size

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- each sample is then diluted to a fixed volume
before measurement - measurements are made on
the original sample and on the sample plus the
standard after each addition KEY for the
standard addition method to be applicable, the
instrument response must be proportional to
concentration
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• Amount of original analyte is calculated from
• the data points by extrapolation of the graph
• Or from the slope and intercept

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• Assume several identical aliquots Vx of the
• unknown solution with concentration cx are
• transferred to volumetric flasks of volume Vt
• b) To each flask is added Vs of a standard of
• concentration cs
• c) Each solution is diluted to volume

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d) Instrumental measurements are then made
to yield an instrument response S e) If S ? ,
then f) Plot S vs. Vs ? straight line S mVs
b
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g) cx can be found from the ratio of b/m or
From extrapolation
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• One disadvantage of the standard addition
• method is the number of separate analyses
• required
• - method cannot accommodate a nonzero
• blank
• - narrow concentration range for linear
• responses
• Major advantage allows for severe matrix
• effects

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The Internal Standard Method

• Internal standard a substance that is added
• in a constant amount to all samples, blanks and
• calibration standards in an analysis
• Calibration involves plotting the ratio of
• Asignal/ISsignal as a function of analyte of
  the
• standards

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• Internal standards compensates for several
• types of random errors
• - instrument response
• - matrix effects
• Major difficulties
• - finding an appropriate substance to
• serve as an internal standard
• - introducing the substance in a
• reproducible way

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• The internal standard should
• - provide a signal that is similar to the
• analyte signal in most ways but sufficiently
• different to be distinguishable by the
• instrument
• - must be absent from the sample matrix