Please enable JavaScript.
Coggle requires JavaScript to display documents.
Journal article method optimization CVAAS and HG AAS, ., . - Coggle Diagram
Journal article method optimization CVAAS and HG AAS
use
Chemical vapor generation atomic absorption spectrometry
technique trace analysis specifically hydride generation continuous flow systems for
arsenic, antimony, selenium
Cold vapor atomic absoprtion spectroscopy
technique trace analysis specifically hydride generation continuous flow systems for
mercury
Sensitivity and reliability of the results obtained can be improved by:
Optimizing the instrument parameters as well as the hydride generating reactions
Optimizing these conditions also can:
Increases the production of hydrides or vapor species
formed thereby improving recoveries.
Helps to reduce chemical interferences from other species
that may compete with the analyte of interest for hydride
formation
Method details
Chemical vapor generation in conjunction with atomic absorption spectroscopy
Purpose
: determination of trace elements in a variety of matrices
Hydride generation atomic absorption spectroscopy (HGAAS)
3 distinct process:
Hydride generation and release of the element of interest from the sample solution (sample
converted to hydride transferred to gaseous phase) with the aid of a reducing agent.
Hydride collection (optional)
Atomization- transport of the released hydride to the atomizer by the flow of an inert purge gas.
Factors that will influence the yield of the hydrides and the sensitivity of the technique include:
The concentration of the acid
The concentration and type of the reducing agent (the facile production of hydrides by reaction with reducing agent)
The flow rate of the carrier gas
Flow rate of the reducing agent
Flow rate of the acid
The read delay time of the sample
use of the separation of the analyte from the matrix by
conversion to the volatile hydrides and offers a pathway to trace analysis of elements such as As, Sb
and Se that cannot be determined by conventional methods.
Cold vapor atomic absorption spectroscopy
(CV-AAS)
Sensitivity and precision can be maximized by:
5 Read delay time
4 Flow rate of the purge gas
3 Flow rate of the sample
2 Flow rate of reducing agent (SnCl2)
1 Concentration of the reducing agent
These include:
Optimizing the parameters essential for increasing the supply rate of gaseous mercury and consequently the analytical signal depends on the transport efficiency of the gaseous chemical species (elemental Mercury)
The principle of this
technique is as follows;
3 The Mercury vapor is transferred into the optical path of the atomic absorption spectrophotometer where Mercury absorbs light of a specific wavelength at room temperature.
2 Mercury having a high vapor pressure is liberated and entrained in an inert gas of Argon after passing the liquid gas separator
1 A reducing agent typically stannous chloride (SnCl2) is used to convert ionic metal ions of Mercury2+ (aq) in solution to Hg0 (g) which produces a higher analytical absorbance signal
Cold vapor atomic absorption spectroscopy (CVAAS) is one of the most sensitive method for mercury determination than flame atomic absorption spectroscopy (FAAS) gives poor sensitivity of mercury determination
Discussion
Reductant concentration
influenced signal intensity because it
affects the hydride generation process, converting the oxidation state of the element to the more stable lower oxidation state
Argon as carrier gas has flow rate which important in the hydride generation and cold vapor
Flow rate of the argon should be high enough to strip the volatile hydrides from the liquid mixtures & carry them up to the sample cell
BUT
must not be so high so as to dilute the hydride formed.
Mercury and Selenium produced higher signals with
higher flow rates of Argon
Arsenic and Antimony produced lower signals because might be dilution of Argon have occured
Optimum argon flow rates
for:
Arsenic and Antimony = 0.2 mL/min
Selenium and Mercury = 2.0 mL/min
Mercury: Hg2+ ---> Hg0 = stable Mercury vapor
Arsenic: As5+ ---> As3+
Antimony: Sb5+ ---> Sb3+
Selenium: Se6+ ---> Se4+
Reduce ions:
form the
stable hydrides
which are needed for HG-AAS
produced by the reaction with an appropriate
reducing agent
Arsenic and Antimony: 0.6% (w/v) NaBH4 in 0.5% (w/v)
NaOH
Selenium: 0.1% (w/v) NaBH4 in 0.5% (w/v) NaOH
Mercury; 25% (w/v) SnCl2 in 20% (v/v) HCl
Antimony & Mercury: using
high concentrations of the reductant
achieved better signal intensity*
Arsenic and Selenium: greater signal noise was produced with
increasing concentrations of sodium borohydride
Higher
concentrations of acid reagent, hydride generation processes should
occur more rapidly*
BUT
High up acid concentrations ---> produces larger amounts of hydrogen gas which can
dilute the hydride vapor
thus
reducing the concentration of the hydride
!! CRUCIAL TO ESTABLISH AN OPTIMAL CONDITION !!
5 M HCl for Arsenic & 10 M HCl for Antimony and Selenium
Flow rates of the reducing agent, acid and sample had an effect on sensitivity:
Higher flow rates, higher signal intensity
BUT
Too high a flow rate can result in a concentrated yet unstable vapor
Optimum flow rate for reductant and acid for all the metals As,Sb, Se and Hg was 0.96 mL/min
Memory effects
The VGA accessory uses chemical reactions to produce the elemental hydrides. The metal ions
present in these hydrides can adhere to the surface of the various tubes that make up the VGA77
To remove adhered ions effectively:
An appropriate
flushing time
is needed between samples
Flushing time
can be controlled by the instrument delay time
The longer the
delay time
, the more flushing can occur.
Delay time
was optimized to ensure that possibility of memory effects was eliminated
Arsenic, Antimony and Selenium
produced maximum signals at lower delay times of
20 s
Mercury
required longer delay times of up to
70 s
to eliminate the memory effect.
.
.