Environmental Engineering Reference
In-Depth Information
water and nitrogen dioxide. However, some
excess oxygen passes the catalyst. The resulting
mixture now
optimize combustion control parameters.
Interfering elements (halogens) are removed.
2. CHNS Mode: The CHNS mode is designed
to handle conventional organics. This mode is
speci
ows through a silica tube packed
with copper granules. In this zone, the tempera-
ture is around 500
fl
C, where the remaining
oxygen is bound and nitric/nitrous oxides are
reduced. The resulting gas stream now includes
the analytically important species carbon diox-
ide, water and nitrogen. Eventually included
sulphur dioxide or hydrohalogenides are absor-
bed at appropriate temperature. In CHN analyser,
high-purity helium is used as carrier gas
°
cally designed to include sulphur,
which reduces the universality of the CHN
capability. This includes the capability for
limiting the range of sample types and sample
size (1
2 mg is recommended). Metal cations
are excluded. Special precautionary measures
are to be taken in calibration and blank
detection for lower levels of sulphur.
3. Oxygen Mode: The oxygen mode excludes
compounds containing phosphorous,
-
nally
and the gas mixture is brought
nite
pressure/volume state and is passed through a gas
chromatographer system. Separation of the spe-
cies is done by so-called zone chromatography.
In this technology, a staircase-type signal is
registered. Step height is proportional to the
substance amount in the mixture. In CHN ana-
lyser, calibration is done by elemental analysis of
standard substances, whose percentage of C, H
and N are known. This is supplied by the man-
ufacturer of the instrument.
While assessing the percentage of C, H and N,
the researchers must keep in mind about various
problem and interferences that may occur during
operation, e.g., the weighing of oily or
to a de
uorine,
silicon and metal cations. Samples containing
mineral matter must be demineralized prior to
analysis. The user may choose any or all
modes. The CHN analyser may be upgraded
at any time to add additional mode capability
to suit the needs of the laboratory.
fl
3.2.3 Precision
The analyser optimizes the time of analysis for
each analysis mode and minimizes maintenance
time, changeover in analysis modes and down-
time. There is an in-built microprocessor diag-
nostics, which continually monitor instrument
conditions. Analysis time in each mode of
operation has been optimized for ef
uid sub-
stances is impossible in tin capsules and, for this
purpose, aluminium pans with a lid are available.
These pans are tightly closed by cold welding to
prevent loss of sample by minimizing evapora-
tion. Phosphorous can interfere in the process of
mineralization,
fl
ciency and
precision. The analysis time is six minutes in
CHN mode, eight minutes in CHNS mode and
4 min in oxygen mode. The unique steady-state
measurement of all signals improves precision in
all modes of operation. Modern organic ele-
mental analysis consists of a series of automated
steps: combustion and reduction, homogeniza-
tion of product gases, separation and detection.
Combustion (complete) is the most critical step
to the success of the process and ultimately
affects the accuracy and precision of the
leading to the formation of
P
2
O
5
·
yC. This effect can be controlled by
the addition of vanadium pentoxide. The analysis
of numerous
xH
2
O
·
uorine containing compounds may
cause errors in the hydrogen result.
fl
3.2.2 Analysis Mode Options in CHN
Analyser
nal
result in terms of the weight percentage of the
elements or the element to be measured in the
sample. The modern analyser not only provides
for advanced combustion conditions of temper-
ature, time and available oxygen (or pyrolysis
Analysis of elements can be done through several
modes that are available in the instrument.
1. CHN Mode: The CHN mode is the most
universal of the analysis modes because of the
combination of the reagent design and the
