ERRORS IN ANALYSIS Flashcards
Analytical results are often used in the diagnosis of
disease, in the assessment of hazardous wastes and
pollution, in the solving of major crimes, and in the
quality control of industrial products. Errors in these
results can have serious personal and societal
effects.
ERRORS IN THE LABORATORY
TYPES OF ERRORS
DETERMINATE OR SYSTEMATIC ERROR
errors that can be determined or eliminated.
It affects the accuracy of results.
- DETERMINATE OR SYSTEMATIC ERROR
- INDETERMINATE OR RANDOM ERROR
have a definite value, an
assignable cause, and are of the same
the magnitude for replicate measurements made in
the same way. They lead to bias in measurement
results.
Systematic errors
have a definite value, an
assignable cause, and are of the same
the magnitude for replicate measurements made in
the same way. They lead to bias in measurement
results.
Systematic errors
the deviation from the target value.
- measures the systematic error
associated with an analysis. It has a
negative sign if it causes the results to be
low and a positive sign
otherwise
Bias
TYPES OF SYSTEMATIC ERRORS:
INSTRUMENTAL ERRORS, METHOD ERRORS, METHOD ERRORS
errors that cannot be determined or
controlled. It affects precision.
INDETERMINATE OR RANDOM ERROR
Random errors are the cumulative effect of
many small, uncontrollable variables and
personal judgments that lead to uncertainty in
a measured value.
INDETERMINATE OR RANDOM ERROR
Presence of bubbles in one of the
trials, presence of bubbles in the reading in the
instruments, microclots in a plasma due to
particulate matter
INDETERMINATE OR RANDOM ERROR
A volumetric flask or pipette was graduated
at 20oC and used at 25oC.
INSTRUMENTAL ERRORS
In gravimetric method, if precipitate is not
sufficiently insoluble, a weight is less than the
correct one.
METHOD ERRORS
Color blindness is a good example of a
limitation that could cause a personal error in a
volumetric analysis.
PERSONAL ERRORS
EEFECTS OF SYSTEMATIC ERRORS:
CONSTANT ERRORS, PROPORTIONAL ERRORS,
errors that are independent of the size of the
sample being analyzed.
CONSTANT ERRORS
Here the value of absolute error is constant with
sample size, but the relative error varies when the
sample size is changed
CONSTANT ERRORS
loss on the amount of precipitate due to large
amount of liquid.
CONSTANT ERRORS
Presence of contaminants does not depend
on the size of the sample
PROPORTIONAL ERRORS
- errors that decrease or increase in proportion to the
size of the sample.
PROPORTIONAL ERRORS
Here the value of absolute error varies with sample
size, but the relative error stays constant when the
sample size is changed.
PROPORTIONAL ERRORS
It is impossible to have a perfect chemical analysis,
one that is free from errors and uncertainties
QUANTIFYING EXPERIMENTAL METHODS
Does that mean your results are unreliable? So, what
do you do?
QUANTIFYING EXPERIMENTAL METHODS
Evaluate the error
- Where did it come from? How?
Perform same procedure from known values
Compare from standard values
Calibration of equipment
Statistical treatments
BASIC STATISTICAL TREATMENT
In order to improve the reliability and to obtain
information about the variability of results, two to five
portions (replicates) of a sample are usually carried
through an entire analytical procedure.
BASIC STATISTICAL TREATMENT
________are samples of about the same size
that are carried through an analysis in exactly the
same way.
Replicates
Individual results from a set of measurements are
seldom the same, so we usually consider the____________
estimate to be the central value for the set.
“best”
