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Optimization of Instrument Conditions for the Analysis of Hg, As, Sb and…
Optimization of Instrument Conditions for the Analysis of Hg, As, Sb and Se by AAS
What is this?
To adjust some instrument parameters:
• Reagent, sample and argon flow rate
• Acetylene/ air ratio
• Concentration of reagent
• Read delay time
Why do so?
To improve the sensitivity and recovery of the elements.
What is this technique?
To separate the analyte from matrix by conversion to volatile hydride or vapor.
How they develop the technique?
By coupling VGA (Vapor Generation Assembly) accessory to AAS and optimizing the instrument parameters.
A range of value is selected for each parameter.
Determination of the concentration for reducing agent and acid.
• 30 ppb standard for each metal wad used to analyze.
• Run the deionized water through the entire system to prevent cross contamination.
Determination of the sample, acid and reducing agent flow rate.
• Volume of reagent used in 60s determine the flow rate whereby the respective solution is put in 25 mL measuring cylinder.
• The adjustment knob was tightened and released to modify the flow rate.
• Pump tubing internal diameter is also studied to control the flow rate.
Acetylene flow rate is adjusted by spectra AA software
Argon flow rate is adjusted from the flow gauge on cylinder.
And finally they get the optimum parameters for each element being analyzed.
found that reductant concentration has effect on signal intensity.
• As & Sb: 0.6 %(w/v) NaBH4 in 0.5 %(w/v) NaOH
• Se: 0.1 %(w/v) NaBH4 in 0.5 %(w/v) NaOH
• Hg: 25 %(w/v) SnCl2 in 20 %(v/v) HCl
Higher concentration of acid, higher hydride generation process.
• But large amount of H2 will be produced if concentration of acid is too high.
• It will dilute the hydride vapour and reduce the intensity.
• Thus, optimal acid for As is 5 M while 10 M for Sb and Se.
Sensitivity is affected by flow rate.
• Although high flow rate will increase signal intensity, the concentrated yet unstable vapor will be produced.
• The flow rate for reducing agent and acid is the same for all while the sample flow rate for:
As, Sb & Se: 2.0 – 2.5 mL/min
Hg: 6.5 – 6.7 mL/min
Argon flow rate should be high enough to strip the volatile hydrides from liquid mixture and carry them to sample cell.
• Not too high to dilute hydride formed.
• Ar & Sb: 0.2 min/mL
• Hg & Se: 2.0 min/mL
Read delay time.
• Sometimes the metal ions in hydride may cause memory effect.
• Therefore, flushing time is needed between sample to remove these ions to avoid getting too high signal which is a false positive.
• As, Sb & Se: 20 s
• Hg: 70 s as it is prone to memory effect.
What type of AAS is suitable for each element being analyzed?
• Hydride generation AAS: As, Sb, Se
• Cold vapor AAS: Hg
2 additional parameters, acetylene flow rate and acid concentration are optimized in HGAAS