Chapter 5 Quality Assurance and
Calibration Methods
5. Quality Assurance and Calibration
Quality assurance is
• We do to get the right answer for our purpose.
• Have Sufficient accuracy and precision to support
decisions
Standard addition and internal standards.
5-1 Basics of Quality Assurance
Use Objectives
ex) Therapeutic dose is just a little less than the lethal
dose, you should be more careful than if you make
spaghetti sauce.
• Quality assurance is making sure that results meet the
customer’s needs.
• Writing clear, concise use objectives for data and
results is a critical step in quality assurance and helps
prevent misuse of data and results.
Specifications:
• What level of accuracy and precision will satisfy the use
objectives?
• Quality assurance begins with sampling.
• A false positive
• A false negative
Selectivity and sensitivity
• Selectivity(also called specificity) means being able to
distinguish analyte from other species in the
sample(avoiding interference).
• Sensitivity is the capability of responding reliably and
measurably
• A detection limit(discussed in Section 5-2)
• Method blank is sample containing all components
except analyte through all steps.
• Reagent blank has not been subjected to all sample
preparation procedures.
• Field blank exposed to the site of sampling.
• Matrix; everything else except analyte in an sample.
• Spike also called a fortification, is a known quantity of
analyte added to a sample to test whether the response
to a sample is the same as that expected from a
calibration curve.
• Performance test samples (quality control samples or
blind samples) : to help eliminate bias. Unknown sample
in known composition.
• Standard operation procedures what steps will be
taken and how they will be carried out for quality
assurance.
- Meaningful analysis requires a meaningful sample that
represents what is to be analyzed.
Assessment
• Collecting data to show that analytical procedures are
operating within specified limits
• Verifying that final results meet use objectives.
• Control carts can be used to monitor performance on
blanks, calibration checks, and spiked samples
5-2 Method Validation
Method Validation
• Process of proving that an analytical method is
acceptable for its intended purpose.
• Method specificity, linearity, accuracy, precision, range,
limit of detection, limit of quantitation, and robustness.
Specificity
• The ability of an analytical method to distinguish the
analyte.
Linearity
• Linearity measures how well a calibration curve follows
a straight line.
• LINEST function in Excel
• A value of R2 above 0.995 or, perhaps, 0.999
Accuracy
Nearness to the truth.
1. Standard Reference Material
2. Different analytical methods.
3. Analyze a blank sample spiked with a known addition of
analyte.
4. Make standard additions(Section 5-3) of analyte to the
unknown
Precision
• Instrument precision
• Intra-assay precision is evaluated by analyzing aliquots
of a homogeneous material several times by one person
on one day with the same equipment.
• Ruggedness, intermediate precision
• Interlaboratory precision
Range
• Range is the concentration interval over which linearity,
accuracy, and precision are all acceptable.
[Box 5-2] The Horwitz Trumpet: Variation in Interlaboratory
Precision
The coefficient of variation (CV) is the standard deviation divided by
the mean : CV = s/ x Usually the coefficient of variation is
expressed as a percentage of the mean : CV(%) = 100 ⅹ s/ x
Limits of Detection and Quantitation
• Detection limit, the smallest quantity of analyte that is
“significantly different” from the blank.
• The signal from samples near the detection limit is
similar to the standard deviation from blanks.
1. sample whose concentration is ~1 to 5 times the detection limit.
2. signal from n replicate samples(n ≥ 7)
3. deviation (s) of the n measurements.
4. measure the signal from n blanks (containing no analyte) and find
the mean value, yblank.
5. Signal detection limit :
ydl = yblank + 3s
6. Calibration line :
ysample – yblank = m ⅹ sample concentration
Detecton limit
• Minimum detectable concentration = 3s/ m
• Lower limit of quantitation ≡ 10s/m
• Robustness is the ability of an analytical method to be
unaffected by small , deliberate changes in operating
parameters.
5-3 Standard Addition
Standard addition
• A matrix effect is a change in the analytical signal caused by anything.
5-4 Internal Standards
• Internal standard is a known amount of a compound , different from
analyte, that is added to the unknown.
• Response factor, F ( eq. 5-11)