Chapter 4 Flashcards
(45 cards)
Require a solid understanding of
analytical techniques available
Problem-solving skills
Experience
Intuition, logic and common
sense
DEFINE THE PROBLEM
CHOOSING YOUR TEST METHOD:
THE SAMPLE AND THE ANALYTE
- What is the nature and background of
the problem? - What is known about the history of the
sample? - What analyte is important in the
sample? - What is the concentration range of the
analyte? - What degree of accuracy and
precision is demanded? - What other components are present in
the sample
CHOOSING YOUR TEST METHOD:
THE ANALYTE
Have prior, similar efforts been documented in the
literature?
* What instruments and equipment are available for
the determination?
* How much time is needed to perform the work?
- How soon does the work need to
be done? - How much money is available to
accomplish the work? - How many samples must one
measure? - Are there limitations to the amount
of sample that can be used?
What Factors to Consider?
✓ What type of information does the method provide?
✓ What are the advantages or disadvantages of the technique
versus other methods?
✓ How reproducible and accurate is the technique?
✓ How much or how little sample is required?
✓ How much or how little analyte can be detected?
✓ What types of samples can the method be used with?
✓ Will other components of the sample cause interference?
✓ Other factors: speed, convenience, cost, availability, skill
required.
Green chemistry processes are
preferred for modern analytical procedures
The goal: minimize
waste and pollution
Use of less toxic
or biodegradable solvents
Use of chemicals that can be recycled
GREEN CHEMISTRY…A CONSIDERATION
CHOOSING YOUR TEST METHOD
(FOR WATER ANALYSIS)
- Is an approved/ regulatory method based on AWWA/ APHA available for use?
- What will the results be used for? (Regulatory compliance or Process Control)
- Is the LOQ achievable?
(at least 1/10th of the regulatory value) - Use a standard method/ test kit/ or developed
“in-house” method?
QUALITATIVE INFORMATION ON THE
ANALYTE
Elemental Composition
Oxidation states
Structural
information
Isotopic
distribution of
elements in the
sample
Polyatomic ions,
Functional groups,
Specific molecules,
Molecular species
Major: >1%
Minor: 0.01 – 1%
Trace: 10-2 - 10-6 %
Ultra-trace: 10-6 – 10-9 %
QUANTITATIVE INFORMATIONROUGH CONCENTRATION
FACTORS IN SAMPLE
PREPARATION:
✓Phase of the sample- solid, liquid, gas
✓Properties of the analyte:
❑Organic or inorganic
❑Pure substance or mixture
❑Homogeneous or heterogeneous
✓Instrumentation- elemental or molecular
✓Decomposition or dissolution of a solid sample
✓Dilutions made prior to measurement
✓Approaches taken to prevent analyte losses or contamination
✓Separation of interferences from the matrix- element or
compound that respond directly to measurement; gives a false
signal; signal may be enhanced or suppressed
PROPERTIES OF THE SAMPLE/ ANALYTE:
Sample
Phase: solid,
liquid, gas,
dissolved,
suspended
* Amount
available for
analysis
*Homogeneity
Analyte
* Chemical and
physical
properties
Signal-to-noise Ratio (S/N):
random variation in signal or background
net response recorded by a method for a sample
Noise:
Signal:
▪ the smallest signal above the
background noise that can be
detected reliably
▪ Typically 3 X signal/noise ratio
❖ Instrument Detection Limit (IDL)
ANTICIPATED CONCENTRATION
OF THE ANALYTE
upper and lower
boundary of applicability and may not be
linear over all concentrations
calibration curve
most important part in
method selection
Linear dynamic range
Limit of linearity
Limit of detection
▪ minimum concentration reportable to 99%
confidence level that the analyte is > zero.
–Determined from the analysis of a low
sample concentration in a given matrix.
–MDL is the “criterion for detection”
Method Detection Limit (MDL)
It is a multiple of LOD at a concentration of the
analyte that can reasonably be determined with
an acceptable level of accuracy and precision
* Can be calculated using an appropriate
standard or sample, and may be obtained from
the lowest concentration on the calibration
curve (excluding the blank)
* LOQ is 10 SD.
LIMIT OF QUANTITATION (LOQ) /
PRACTICAL REPORTING LIMIT (PRL)
(within batch, internal)
(between batches, variability)
- Influenced by changes in analyst, instrument
conditions, reagents, etc - Can be assessed by analysis of at least 10X,
calculate SD within batch and between
batches - Long term assessment can be derived from QC
chart
- Repeatability
- Reproducibility
Is the closeness of
agreement between
a test result and the
accepted reference
or true value of the
property being
measured
Trueness
The way in which the result or signal of a method
varies with the amount of compound or property being
measured.
Response
A plot of the result or signal vs. the known amount
of a known compound or property (standard) being measured.
Calibration Curve:
Dictates:
* instrument to be selected for
measurement
* Sample preparation
* Separation
INTERFERENCES
The analytical method should not be sensitive to small
changes in procedures e.g., flow rates, reagents, etc
Robust methods (using standard methods) will generally
produce similar results for the same sample when used in
independent laboratories
ROBUSTNESS/ SYSTEM
SUITABILITY
EXISTING METHODS
SAMPLING
Type of sample preparation
depends on:
➢Nature of sample
➢Technique/ method
chosen
➢Analyte to be measured
➢The problem to be
solved
- Samples may be:
➢Dissolved in water or
other solvents
➢Pressed into pellets
➢Cast into thin filmssurface analysis
