Measurement Methodology; Baseline Stability; Calibration - Xylem YSI 2500 User Manual

11.2

Measurement Methodology

The 2500 Analyzer employs a steady state measurement methodology. A typical enzyme sensor response is shown in
Figure 11-1.
When sample or calibration standard is dispensed into the sample module, it is diluted into approximately 600 microliters
of buffer. The enzyme sensor response increases and plateaus. After several seconds, the sample module is flushed with
buffer and the sensor response decreases.
The net response is the difference between the plateau current (i
responses for the 2500 Analyzer are between 10 and 25 nA (nanoamps) for YSI calibration solutions.
11.3

Baseline Stability

The 2500 Analyzer monitors the probe baseline activity and stability. If an unstable baseline is detected, the instrument
will continue to flush the sample module with buffer. When a stable baseline is established, an automatic calibration is
initiated.
After every calibration and sample, the final baseline value (i
flush cycle. If a significant shift is detected, the sample module continues to be flushed with buffer. As soon as the
baseline recovers, buffer flushing ceases and the instrument performs its next command. There is a limit of about 3
minutes, at which time the instrument displays a baseline error message.
11.4

Calibration

To maintain a sample ready status, the 2500 Analyzer self-calibrates. Calibrating establishes the sensors' response to a
known concentration of substrate.
The enzyme sensors calibration response must be above 5 nA. A response below this value will result in an error (low PL
current).
The 2500 Analyzer self-calibrates enzyme sensors every 5 samples or 30 minutes. However, default calibration
parameters can be altered to tighten or loosen calibration specifications. A manual calibration can be initiated from the
[Run], [Calibrate] tab.
A STABLE CALIBRATION IS IMPORTANT. The instrument re-establishes a calibration reference point after every
calibration. If a difference of more than 2% between the present and previous net calibration values occurs, the instrument
repeats calibration. The sensors' net value for a calibration (PL) is displayed and printed. An unstable calibration is
Typical Enzyme Sensor Response
Figure 11-2
) and the initial baseline current (i
plat
) is compared to the initial baseline value (i
fb
11-2
). Typical net
ib
) during the
ib