Chapter 4. Guide to Method Sampling Flashcards
Define the problem
Require a solid understanding of
analytical techniques available
Problem-solving skills
Experience
Intuition, logic and common sense
Define the problem
Intent of the measurement
Considerations in sampling and sample preparation
Best technique/ method for doing the analysis
Evaluation / analysis of data
Reporting of results
Resources needed to accomplish the analysis
choosing you 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.
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 the elements in the sample
polyatomic atoms, functional groups, specific molecules, molecular species
study quantitative information - rough concentration
- PROPERTIES OF THE SAMPLE/ ANALYTE:
Sample
* Phase: solid,
liquid, gas,
dissolved,
suspended
* Amount
available for
analysis
*Homogeneity
Analyte
* Chemical and
physical
properties
- 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
- ANTICIPATED CONCENTRATION
OF THE ANALYTE
linear dynamic range
limit of linearity
limit of detection
upper and lower
boundary of applicability and may not be
linear over all concentrations
limit of dynamic range
calibration curve
limit of linearity
most important part in method selection
limit of detection
The lowest concentration that can be measured with
reasonable statistical certainty (AOAC)
The lowest concentration of analyte in a sample that
can be detected, but not necessarily quantified,
under the stated conditions of the test (NATA Tech,
Note #3)
The smallest concentration that
can be determined that is
statistically different from a
blank at a specified level of
confidence (typically 95%) This
corresponds to the critical
level.(Currie, 1988, Am. Chem.
Soc.)
The true net concentration or amount of the
analyte in the material to be analyzed to the
conclusion that the concentration of the analyte in the material is larger than that of the blank matrix (ISO DIS 118431)
The output signal or value above which it can be affirmed with a stated level of confidence (e.g.,
95%) that a sample is different from a blank
sample containing no analyte of interest (ISO 13530:2009)
detection limit
three types of detection limit
- instrument detection limit (IDL)
- method detection limit (MDL)
- limit of quantitation (LOQ)/ practical reporting limit (PRL)
▪ the smallest signal above the
background noise that can be
detected reliably
▪ Typically 3 X signal/noise ratio
Instrument detection limit
random variation in signal or background
noise
net response recorded by a method for a sample
signal
a value of S/N = 2 or better is considered to be the minimum ratio needed for the reliable detection of a true signal coming from a sample.
Estimate S/N:
1) Multiple determination of
blank samples.
2) Estimation of best-fit to calibration curves
▪ 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”
LOD = 3 x SD of low sample
LOD = 5 x of blank solution (not a calibration blank)
(US-EPA definition)
Method detection limit
- 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)
study graph on what these limits mean in practice
(within batch, internal)
repeatability
- DESIRED PRECISION
s2method = s2
sample + s2
technique
(between batches, variability)
reproducibility
- DESIRED PRECISION
- 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
charts
- DESIRED ACCURACY
trueness; Is the closeness of
agreement between
a test result and the
accepted reference
or true value of the
property being
measured
ACCURACY CAN BE EVALUATED AS
“BIAS” BY:
- use of CRMs as controls
- use of traceable RM or material prepared “in house”
- use of standard/reference method with little or no systematic error
- use of the method when participating in PTs
- use of spiked samples, based on blank or positive samples
- from assessment of QC charts
STANDARDS AND
CALIBRATION CURVE
Working curve
“Best-fit” relationship between analytical signal and concentration
of analyte
Background signal
Slope
Linearity
Control of calibration standards- temp, pH, complexing properties
of sample and standards
Instrumental parameters- amplitude and frequency of input signal, sensitivity of the detector, timing of measurement of sample relative to calibration, “drift” of
measured signals
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
solve practice problem on notebook
- INTERFERENCES
- Dictates:
- instrument to be selected for
measurement - Sample preparation
- Separation
- Example:
Determination of
Na ions in potato
chips using an
ISE- interference
of K
- ROBUSTNESS/ SYSTEM
SUITABILITY
The analytical method should not be sensitive to small
changes in procedures e.g., flow rates, reagents, etc
✓Identify any variables which may have an effect on
the data
✓Set up experiments using known materials to
determine effects
✓Set performance criteria for methods to achieve
✓Minimum slope
✓Lowest standard response
✓Highest sensitivity
Robust methods (using standard methods) will generally
produce similar results for the same sample when used in
independent laboratories
- EXISTING METHODS
Standard methods
Official methods
Published methods in scientific journals
Laboratory-developed methods
Customer’s methods
- SAMPLING
- Most important step- largest source of error
- Sampling plan (lake) : topography, temperature
variation, depth - “representative sample”- reflects the true value and
distribution of analyte in the original material - In large complex samples – “sub-sampling”
- Transport, labelling, storage, documented chain of
custody - Prevent changes due to:
- Volatilization
- Absorption of moisture
- Contamination
- Absorption/desorption process with sample
container
SAMPLING CONSIDERATIONS
- Specific information on how to collect the sample
- Documentation on where and when the sample was
collected - Sampling equipment used
- Proof of maintenance and calibration of sampling equipment
- Type of sample containers
- Proper storage of samples
- Criteria for accepting and rejecting samples
- Methods for excluding or separating foreign objects- soil with
plant roots and debris - Proper treatment for sampling-drying, mixing,
homogenization & handling - Procedure for sub-sampling and
compositing - Record keeping that documents all
actions performed, traces the chain of
custody and any auxillary information
important.
SAMPLING
Grab samples - samples taken at a
single point in time
Composite samples -samples taken over a
period of time or from
different locations
GAS SAMPLES
▪ Generally considered
homogeneous
▪ Samples are mixed before
portions are taken for analysis
▪ May be filtered if solid materials
are present
➢Scrubbing- trapping an analyte out of the gas
phase
* passing air thru activated charcoal to adsorb
organic vapors
* Bubbling gas samples thru a solution to absorb
analyte
➢Use of gas-tight syringes
LIQUID SAMPLES
May be collected as grab or composite
Adequate stirring is necessary, but is some
cases, stirring may not be desired- analysis of
oily layer in water
Undesired solid materials are removed by
filtration or centrifugation
Layer of immiscible solids may be separated
by separatory funnel
SOLID SAMPLES
Most difficult, not
as
homogeneous
as gas and liquid
Large amounts
are difficult to
mix
Must undergo
size reduction
(milling, drilling,
crushing, etc
Adsorbed water
is often removed
by oven-drying
SAMPLE PREPARATION
- 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
SAMPLE PREP METHODS:
DISSOLUTION
➢Homogeneously distributing the analyte in a solvent
* Aqueous
* Non-aqueous: for organic compounds and
polymers
* Acid-water mixture: HCl, HNO3, H2SO4 (HF, HClO4
with special care & supervision)
* Complexing agents-water : ligands aid solubility
SAMPLE PREP METHODS:
DECOMPOSITION
➢Chemically converting the sample into a form
that can be dissolved in a solvent
inorganic analyte into soluble
form with concentrated mineral acids (HCl, HNO3,
H2SO4 (HF, HClO4 with special care & supervision)
acid decomposition
fused with acidic or basic salt (K2CO3 or
K2S2O7) at high T, cooled melt is dissolved
fusion
sealed oxidation with O2, followed by
absorption of product in a solvent
combustion
organic sample in hot oxidizing reagent,
then elemental analysis
wet ashing
heating an organic sample in flame or
furnace, followed by dissolution of the ash
dry ashing
high pressure in sealed vessel
(automated)
microwave assisted
SAMPLE PREP METHODS:
FILTRATION
➢Removal of a solid substance from solution by
exclusion process
Paper and glass fiber
Membrane filters- polymeric
structure with fine pores (0.3 um)
Hollow fiber membrane- use of pressure to
force a solution through a membrane,
ultrafiltration and reverse osmosis (0.25 um)
SAMPLE PREP METHODS:
EXTRACTION
➢Selective removal of an analyte from a mixture by
partitioning between two immiscible phases
distribution into two
immiscible liquid phases
liquid-liquid extraction (LLE)
analyte retention on
solid sorbent; followed by elution
solid phase extraction (SPE)
analyte sorbed
on a thin layer of sorbent (solid, liquid) coated on the outer
surface of a fiber exposed to a liquid mixture, followed by
redissolution or volatilization of analyte
solid phase microextraction (SPME)
a supercritical
fluid is created using a gas (like CO2) above the critical
temperature; the resulting supercritical fluid is typically used to extract organic analytes from a solid sample followed by its collection by depressurization, on a sorbent or in a solvent
supercritical fluid extraction (SFE)
microwave-accelerated extraction of organic
analytes from a solid sample with a liquid
microwave assisted extraction (MAE)
a sample is placed in
contact with a membrane that allows the sample to
selectively permeate into a new gas or liquid
membrane extraction
volatile organic compounds
are allowed to diffuse from a liquid into a headspace
above the liquid; analyte-containing gas in the
headspace is sampled
headspace extraction
SAMPLE PREP METHODS:
DISTILLATION
➢Removal or enrichment of a volatile substance
based on differences in boiling point
batch
azeotropic
fractional
vacuum
determine the mass of the analyte or some compound chemically related to it
gravimetric methods
measure the volume of a solution containing
sufficient reagent to react completely with the analyte
volumetric methods
involve the measurement of such electrical properties as voltage, current, resistance, and quantity of
electrical charge
electroanalytical methods
are based on the measurement of the
interaction between electromagnetic radiation and analyte atoms or
molecules, or the production of such radiation by analytes
spectroscopic methods
include the measurement of such quantities as mass-to-charge ratio, rate of radioactive decay, heat
of reaction, rate of reaction, sample thermal conductivity, optical activity, and refractive index
miscellaneous methods
study techniques to charts
