Page 135 - Illustrated Pocket Dictionary of Chromatography

134 NITROGEN-PHOSPHORUS DETECTOR (NPD)

nitrogen-phosphorus detector (NPD) Utilizes the ioniza-
tion caused by heating a cesium- or rubidium-salted bead such that it
preferentially reacts with nitrogen and phosphorus atoms. Detec-
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4
tion limits are about 10 -12 g and a linear range of 10 or 10 can be
obtained.
noise Any signal output from the detector that is not a direct
response to the analyte. There are many different sources of noise,
some that are controllable or minimizable and some that are not. The
most important aspect to noise is that it is the ultimate limiting factor
in establishing the precision of a measurement. From a more practi-
cal viewpoint noise is the determining factor in establishing system
detection limits. Noise can be experimentally determined in a number
of ways.

noise, long-term Noise is noise that cycles over long times, for
example, the duration of an analysis. This is not to be mistaken for
drift because long-term noise is not necessarily confined to moving in
one direction as drift typically is.

noise, short-term Noise that cycles or pulses over short time
intervals. Short-term noise is due to electrical detector noise (e.g.,
Johnson flicker and shot noise) and is the ultimate limit for estab-
lishing detection limits because it overlies all other signals.

nonaqueous reversed-phase chromatography (NARP)
Reversed-phase separations are typically conducted with a water/
organic mobile phase. However, for analytes that are extremely hydrop-
hobic or incompatible with water, a mobile phase that contains no
water is used and the separation is termed “nonaqueous reversed
phase.”

nonporous supports Contain no porous character at all but may
be chemically modified like other bonded phase supports. These
supports have extremely low surface areas as compared with analo-
gous porous material. Typical nonporous supports are silica- or resin
based.

normal distribution A perfectly symmetric distribution of data
that is mathematically described by the Gauss function:

s
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f x,s = [1 s÷2 p] e -( xxi)22 2