D4294_16e1_12
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A quality control
sample is measured
prior to analyzing unknowns to
verify that the test method is
in control.
It is run identically
to any unknown sample.
If the chosen quality control
sample's repeatability
varies more than the
repeatability value
of this test method expected
for that concentration
(acceptance values
obtained from Table 6)
then the procedure is
deemed to be out of control
and the instrument should be
recalibrated before running
any further analysis.
A synthetic solid
quality control sample
may be used in lieu of liquid
samples (see section 15).
Fill the cup with the sample
to be measured to about 75%
of cup capacity.
Before filling the
cell, it may be
necessary to heat viscous
samples so that they
are easy to pour into the cell.
Ensure that no air bubbles
are present between the cup
window and the liquid sample.
Measure each sample once.
If the concentration
from the first analysis
is less than 100
milligrams per kilogram,
repeat the measurement using
a freshly prepared sample
cup and a fresh
portion of the sample,
and obtain the average
of the readings
for the sulfur content
in the unknown sample.
When analyzing more than
a single unknown sample,
measure a quality
control sample at the end
of each batch of unknown
samples, but no less
than 10 unknown
samples, to verify
that the method is in control.
In all situations where the
quality control samples vary
by more than the
repeatability expected
for that concentration
(Table 6),
the analysis must be
discontinued and corrective
action taken to find
the source of air.
Use a quality
control sample close
to the unknown samples'
sulfur concentration.
Refer to Section 15.
For samples containing
100 milligrams
per kilogram total
sulfur or less,
duplicate determinations
are required.
Each determination must be
made on a new portion of sample
material and analyzed
in accordance
with section 12.1 and 12.2.
The difference between
the duplicate analyses
should be equal to or less
than the repeatability values
indicated in Table 6.
If the difference is larger,
investigate sample preparation
to identify any possible
sources of sample contamination,
and repeat the analysis.
The reason for
duplicate measurements
is to identify
problems associated
with sample
contamination, leading
to improved results precision
at lower sulfur levels.
The concentrations of
ethanol and methanol
were calculated assuring
a theoretical mixture
of hydrocarbons
and dibutyl sulfide
to which ethanol
(or methanol) was
added until the sum of
the mass coefficients
times the mass fractions
increased by 5%.
In other words, the amount
of ethanol (or methanol)
that caused a negative 5%
error in the sulfur measurement
was calculated.
This information is
included in Table 1
to inform those who
wish to use Test Method
D4294 for determining sulfur
in gasohol (or M85 and M100)
of the nature of
the error involved.