Biomedical Engineering Reference
In-Depth Information
yielding 10
M H
2
S with each step. These stepwise injections produced a stepwise
increase in current that differed by less than
µ
2% for each injection. Response accu-
racy was determined by periodically sampling the chamber solution and determining
the H
2
S concentration with 2-PDS [43]. Calibration of the PHSS was within
1% of
the 2-PDS assay.
8.5.6 Linearity and dynamic response range
From the lower detection limit near 10 nM to approximately 200
M H
2
S, the PHSS
signal is best fi t by a linear regression (see Fig. 8.7), with coeffi cient of regression
typically
µ
0.98. However, the amperometric signal loses linearity at higher H
2
S con-
centrations. This behavior has also been observed in other micro polarographic sulfi de
sensors [42]. It is likely that the small electrolyte volume at the sensor tip becomes
saturated at higher H
2
S concentrations as the rate of H
2
S diffusion becomes more rapid
than the rate at which ferrocyanide is reoxidized at the platinum anode.
8.5.7 Response time
In response to step H
2
S changes by addition or removal, the PHSS typically takes less
than 10 s to reach 90% of the new signal (see Fig. 8.5), in agreement with previous
experiments performed with the macro PHSS at 20ºC [36]. With a freshly prepared
PHSS, we often observe that the fi rst H
2
S injection to the chamber results in a some-
what slower response time, approximately 30 s, to reach 90% of the new signal. This
slower response is not observed with subsequent injections, suggesting that for a newly
prepared PHSS, initial electrolyte conditioning occurs upon fi rst exposure to H
2
S.
8.5.8 Reliability (maintenance-free working time)
The time frame of PHSS use prior to requiring new electrolyte and membrane replace-
ment depends on solution type and H
2
S concentrations to which the PHSS has been
exposed. Generally, using the PHSS on a daily basis with physiological buffer solu-
tions and H
2
S concentrations less than about 50
M provides about 2 to 3 weeks of
service, after which the membrane and electrolyte require replacement to refurbish the
PHSS. A loss in H
2
S sensitivity and an elevated background current compared to ear-
lier calibrations are symptoms of needed refurbishment. Performance decline can be
hastened if higher H
2
S concentrations are routinely used since elemental sulfur will
more rapidly accumulate in the electrolyte. Likewise if experimental solutions promote
rapid bacterial growth coating chamber surfaces as well as the PHSS membrane with a
biofi lm, the refurbishment time frame will be shortened.
µ
8.5.9 Biocompatibility
Because the PHSS electrochemical components are separated from sample solution
by the H
2
S-permeable polymer membrane which is essentially inert with respect to
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