24
SECTION 3, continued
3.2.2 Sample Preparation
For colorimetric tests, sample preparation is next. The zero
solution (or blank) and sample are usually prepared at this time.
Generally, sample preparation consists of adding the contents of a
premeasured reagent pillow to a small volume of sample. Follow
the instructions in the procedure specific to your analysis.
It is important to observe the waiting period specified in the test
procedure to ensure the color from the reaction of the reagent(s)
with the target analyte develops fully. Many procedures also give
a maximum time limit after which the color may begin to fade.
The instrument has method-specific timers for color development
times pre-programmed into the software. The user is notified
when the time has elapsed by a series of short beeps.
3.2.3 Zeroing the Colorimeter
The instrument must be zeroed for each test or series of tests to
establish a zero reference for the measurement. This is done by
placing a solution recognized as the blank solution in the cell
holder, covering the sample with the instrument cap, and pressing
the
ZERO key. The next prompt will display zeros and illuminate
the
READ action icon. The instrument is now ready to take the
first sample reading.
Note: Once the zero reference point has been established, several
samples can be measured consecutively by placing each of them
into the cell holder and pressing the
READ key. The instrument can
be re-zeroed at any time by placing the zero solution (blank) into the
instrument and pressing the
ZERO key.
3.2.4 Measuring the Prepared Sample
When ready to take the reading, place the prepared sample in the
sample compartment. For best results orient the sample cell
consistently for each measurement; see Figure 5 on page 19.
Place the instrument cap (light shield) over the sample cell and
press the
READ key. After a brief pause, the results will be
displayed.
Toggle between absorbance or percent transmittance values by
successive presses of the
ABS %T key. Press the CONC key to
restore the concentration display. Successive presses of the CONC
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