Brazilian Journal of Analytical Chemistry - BRJAC - Brazilian Journal ...

fi g u rE 2. EffECt o f rE a g E n t slug v o l u m E o n aBsorBanCE s i g n a l. ExPErimEntal
C o n d i t i o n s: 0.1 s/0.1 s (on/off), 100 Cm C o i l l E n g t h, 40 μl s a m P l E slug
v o l u m E. sa m P l E/rE a g E n t: (a) 5:1 (v/v); (B) 5:2 (v/v); (C) 5:3 (v/v); (d) 5:4
(v/v); (E) 5:5 (v/v).
3.2. eff e c t o f sample s l u g v o l u m e
The recommendations accompanying the Vacu-vials ®
kit suggest 25 mL for the sample solution in the vial
containing the reagent solution. If the volume of reagent
is 5 mL, the stoichiometric relation between the
sample and reagent solution would be 5:2. The volume
of sample solution was tested, by varying the number
of pulses of micro-pump P1 switch ON/OFF in order to
change the sample slug volume from 8.0 to 80 µL (Figure
3). Better results were obtained when the volume
of the sample solution was 40 µL (Figure 3, curve c).
These assays were carried out using standard solutions
with concentrations within the range of 0.5 to 5.0 mg
L −1 PAA, obtaining a correlation coefficient of 0.9995
(n = 6).
fi g u rE 3. EffECt C o n C E r n i n g to thE ratio BEtwEEn v o l u m E s o f s a m P l E a n d
rE a g E n t s o l u t i o n s. ExPErimEntal C o n d i t i o n s: 0.1 s/0.1 s (on/off), 100
Cm C o i l l E n g t h. sa m P l E/rE a g E n t: (a) 1:2 (v/v); (B) 1:1 (v/v); (C) 2.5:1 (v/v);
(d) 3:1 (v/v); (E) 5:1 (v/v).
3.3. eff e c t o f reactIon c o Il l e n g t h
The results discussed above were obtained us-
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ing a 100 cm reaction coil. Because the dimension of
the reaction coil may affect detectivity, precision and
sampling rate, a set of assays was performed varying
the length of the coil from 50 to 150 cm. A slightly
increased signal magnitude of 6 % was observed for
reaction coils that were longer (120 cm). A coil with a
length of 150 cm decreased the signal magnitude by
15% when compared with the 100 cm reaction coil.
It is possible that the decrease in signal was caused by
a dispersion effect, which increases with the length of
the reaction coil. Because the best results considering
detectivity, linearity and precision, were obtained with
a reaction coil with a length of 100 cm, this was the
reaction coil used.
3.4. an a l y t Ic a l f Ig u r e s o f m e r It
Using the best operational conditions determined
previously (volumes of 200 μL for sample and 80 μL
for reagent solutions) additional experiments were performed
in order to establish the linear dynamic range
and detection limit. The analytical curve equation was
Abs = (0.0725 ± 1.12 x 10 -3 ) mg L -1 PAA + (0.0017 ±
0.0018) with a correlation coefficient of 0.9995. The
limit of detection (LOD) and limit of quantification
(LOQ) were 0.10 mg L -1 and 0.33 mg L -1 , respectively.
LOD and LOQ were estimated using the equation: LOD
= 3s b /S and LOQ = 10s b /S, where S is the slope and s b
is the blank solution standard deviation (n b = 8). The
results obtained by the system were compared with
those found using the Vacu-vials ® kit and the reference
method based on permanganate-thiosulfate titration
[12], as shown in Table II.
taBlE ii. Co m P a r i s o n o f thE analytiCal fEaturEs aChiEvEd By thE
P ro P o s E d m E t h o d, vaCu-vials ® ProCEdurE a n d rEfErEnCE m E t h o d
[12] f o r PEraCEtiC aCid d E t E r m i n a t i o n in d i s i n fE C t a n t s samPlEs.
Pa ra m E t E rs
Pr o P o s E d
m E t h o d
kit va C uv
i a l s ®
rEfErEnCE
m E t h o d
Sample volume (µL) 200 25000 50000
Reagent volume (µL) 80 2000 - a
Linear range (mg L -1 ) 0.5 - 5.0 1.0 - 4.0 150 - 300
LD (mg L -1 ) 0.10 NI NI
RSD (%) 4.0 (3.0 mg L -1 ) NI NI
Analytical throughput
(h -1 )
80 10 2
Waste volume b (mL) 1.5 27 > 50
NI: not informed. a Varies with the sample concentration. b Per
determination
Silva et al.
The coefficient of variation (obtained using eight replicates
of a 3 mg L -1 peracetic acid solution) was 4.0%.
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