DET REPORT
No.63 JUNE
2011
background signal. However, that procedure keeps
heating the ionsource hotterand hotter,andoperating
life is greatly shortened. Longest ion source operating
life is achieved by leaving the heating current close to
that magnitude justneeded to ignite theboundary layer
chemistry. Chemistry ignition inanNPD is indicated by
a sudden increase in detector background signal as
the ion source heat is slowly increased. This was
illustrated in the data displayed in Figure 2.
It is not unusual for the absolute signal of an NPD to
decay slowly with operating time, but the noise also
decays, so signal-to-noise remains more constant.
This characteristic is illustrated in Figure 6.
To account for any NPD response decay, use of an
internal standard reference compound is usually
recommended for the NPD. As an NPD ion source is
used for long times, increases in Hydrogen flow and
ion source heat can also be used periodically to get
larger signals for extended time. DET'S TID-2 and
TID-4 NPD ionsourcescan providegoodNP response
for operating times in excess of 1500 hours, and they
have unlimited shelf lives when not in use.
NPD ion sources are consumable components, and
DET provides a recycling service as an environment
friendly way of disposing of depleted ion sources.This
recycling experiencehas revealed that NPDend users
havewidely ranging criteria when determining that it is
time to replace the ion source.
NPD (TID-4) Signal and Noise vs. Time
Signals at 64 pA full scale
N-O.65 ng Azobenzene, P
a1.0
ng Malathion
P
N
40 hours
140 hours
P
N
Noise at 2 pA full scale
~
..
140hours
Figure 6. Absolute magnitudes of bothsignalsandnoise
decreasewithNPDoperating time, butsignal-to-noise
(l.e.,detectivity) remainsmoreconstant.
DETector Engineering
&
Technology, inc.
486NorthWiget Lane
Walnut Creek,CA94598·2408USA
Phone: 925-937-4203; FAX:925·937-7581
e-mail:
Many ion sources returned to DET still have hundreds
of hours of useful life remaining when judged with
regard to signal-to-noise and selectivity. Although the
absolute response of a used NPD source may be 10
times lower than when it was new, the noise is also
lower,so there remains gooddetectivityandselectivity.
Figures 7 - 11 provide some examples from ion
sources that have had extensive prior use. For some
of theseexamples, judicious adjustments ofHydrogen
flow and ion source heat were used to gain further
enhancement of NPD responses in the later stages of
ion source operating life.
• METHAMPHETAMINE
NPD 32 pAlmV
TIO-4 Ion aource :
(750 houra prior u••)
heat-3.10 V.
pol.r1~tton.
detector
g ••••:
hydrogen-4. air-55mUmln
3,4-MDMA
COCAINE
-
4 V
HEROIN
Figure 7.
OET T10-4
WhiteCeramic ionsourceused750
hourspriorto thesedata. Signals 4.4timeslessthanwhen
new,noise5.5 times less, signal-to-noise 20%betterthan
new.
TID-2 Ion Source
(BlackCeramic)
"1600 hours
P
prloruse
32
In a890
NPD
pg
NP
"16
2 pA
r.B.
pg
Figure 8.
OET T10-2
BlackCeramicionsourceused1500
hours prior to these data. Still sharp P peaks, good
selectivity ofNand Pvs.C, and lowpicogram sensitivity.
N=AZobenzene, NP=Methyl Parathion, P=Malathion
C=32,OOO pgn-Heptadecane.
5
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