The NO, NOx and Prereactor background corrections are determined
during zero calibration. The NO background is the amount of signal read
by the analyzer in the NO channel while sampling zero air. The NOx
background is the amount of signal read by the analyzer in the NOx
channel while sampling zero air. The Prereactor background is the amount
of signal read by the analyzer when the sample is routed through the
prereactor. The background signal is the combination of electrical offsets,
PMT dark currents, and trace substances undergoing chemiluminescence.
Before the analyzer sets the NO, NOx and Prereactor readings to zero, it
stores these values as the NO, NOx, and Prereactor background corrections,
respectively. The NO2 background correction is determined from the NO
and NOx background corrections and is not displayed. The background
corrections are typically below 1.5 ppb.
The NO, NOx and Prereactor Background screens are used to perform a
manual zero calibration of the instrument. Before performing a zero
calibration, allow the analyzer to sample zero air until stable readings are
obtained. The NO channel should be calibrated first. The NO, NOx, and
Prereactor Background screens operate the same way. Therefore, the
following description of the NO background applies to the NOx and
Prereactor background screens as well. The first line of the display shows
the current NO reading. The second line of the display shows the NO
background correction that is stored in memory. The NO background
correction is a value, expressed in the current gas units, that is subtracted
from the NO reading to produce the NO reading that is displayed.
In the example below, the analyzer is displayed 0.09 ppb of NO while
sampling zero air. A background correction is 0.0 ppb means that 0 ppb is
being subtracted from the NO concentration being displayed. Therefore,
the background correction must be increased to 0.09 ppb in order for theNO reading to be at 0 ppb, i.e., a NO reading of 0.09 ppb minus a NO
background reading of 0.09 ppb gives the corrected NO reading of 0 ppb.