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

Pereira et al.
fi g u rE 1. in f l u E n C E o f aBsorBing s o l u t i o n s f o r Cl d E t E r m i n a t i o n u s i n g h 2 o,
(nh 4 ) 2 Co 3 a n d nh 4 oh (10, 25, 50 to 100 m m o l l -1 ) w i t h rEflux stEP.
dE t E r m i n a t i o n By iC; E r ro r Bars arE thE standard dEviation, n = 3)
Recoveries better than 98% were obtained for Cl
using a reflux step after the combustion using water,
(NH 4 ) 2 CO 3 and NH 4 OH solutions for all concentrations
studied (exceptionally for 10 mmol -1 NH 4 OH, Cl recovery
was about 90%). Relative standard deviation (RSD)
values for MIC with reflux were lower than 3%, however
using water RSD values were higher. Taking into
account that diluted NH 4 OH is a suitable solution for
IC and ICP-MS, a 25 mmol l -1 NH 4 OH solution was arbitrarily
selected for further Cl determination allowing
quantitative recoveries for Cl. Blanks for Cl using 25
mmol l -1 NH 4 OH were always lower than 25 µg l -1 .
3.3. ch l o r In e determInatIon In atmospherIc r e s Id u e a n d
g a s o Il samples after mIc
After RAT and gas oil samples digestion, the resultant
solutions were analyzed by IC and also by ICP-MS and
results are shown in Table II.
taBlE ii. Ch l o r i nE C o n C E n t ra t i o n in atmosPhEriC
rEsiduE a n d g a s o i l samPlEs aftEr miC d i gE s t i o n
The concentration of chlorine in RAT samples varied
from 8 to 4050 µg g -1 . In general, Cl concentration in
gas oil samples was lower than the LODs obtained by IC
and also by ICP-MS, with the exception for gas oil samples
1 and 2. No statistical difference (t-test, confidence
level of 95%) was observed between results obtained
using IC and ICP-MS. The limit of detection (LOD) for Cl
by IC and ICP-MS was 1.2 µg g -1 and 6.6 µg g -1 , respectively.
A typical chromatogram obtained by IC for Cl
determination in RAT sample is shown in Figure 2.
fi g u r E 2. Ch r o m a t o g r a m o B t a i n E d a f t E r miC d i g E s t i o n o f rat: (1) systEm
P E a k, (2) C h l o r i n E, (3) nitratE a n d (4) s u l f a t E.
The retention time for Cl was about 5 min and other
signals, correspondent to nitrate and sulfate, were also
observed in the chromatogram. This fact could be explained
since these types of samples generally contain
high sulfur concentration, as also described in literature.
[10] In addition, the high nitrate concentration could
be explained by the addition of ammonium nitrate as
igniter and also due to the cleaning step of the vessels
that is performed using nitric acid. However, despite
the presence of these ions in digests, no interference
on Cl signals was observed.
In order to check the accuracy, MIC was applied for
CRMs of coking coal and fuel oil. After digestion, Cl
was determined by IC and ICP-MS. Results are also
shown in Table 2. No statistical difference (t-test, confidence
level of 95%) was observed for Cl concentration
obtained by IC and ICP-MS. In addition, the
agreement with the certified value was better than
97% for Cl in all the evaluated CRMs.
3.5. re s Id u a l c a r b o n c o n t e n t
The efficiency of the sample decomposition by MIC
was evaluated by the residual carbon content (RCC) determination
in digests. In this work, RCC was evaluated
in samples digested by MIC following the procedure
reported in previous work [26]. Residual carbon content
were always below 1% for atmospheric and gas
oil digests obtained by MIC due to high temperature
reached (about 1400 ºC) during the combustion. In this
condition practically all the organic compounds could
be completely decomposed.
122 Br J Anal Chem