Samples Preparations for AAS Analysis Notes

Samples Preparations for AAS Analysis

Introduction

This document outlines the procedures for preparing samples for analysis using Atomic Absorption Spectroscopy (AAS), specifically Flame AAS (FAAS) and Graphite Furnace AAS (GFAAS).

Sample Types and Preparation

FAAS vs. GFAAS

• FAAS can only analyze solution samples.

• GFAAS can analyze both solution and solid samples.

Solid Sample Preparation for FAAS

Solid materials (e.g., soil, animal tissues, plants) must be treated to obtain a solution before FAAS analysis.

Methods for Transferring Metals into Solution

Metals are transferred from solid samples into solution using the following methods:

• Decomposition in hot mineral acids (wet ashing): Examples include $HNO<em>3$, $HClO</em>4$, and $H<em>2SO</em>4$.

• Fusion with basic reagents at high temperature: Examples include Boric oxide, $Na<em>2CO</em>3$, $Na<em>2O</em>2$, Potassium pyrosulfate. Dissolve the resulting mixture in an appropriate solvent.

• Ashing at high temperature: Followed by dissolving the ash in an appropriate solvent.

Risks and Considerations for Decomposition

Decomposition methods increase the risk of:

• Analyte loss due to volatilization.

• Analyte loss as particulates in smoke.

• Contamination if the analyte is present as an impurity in the acids used.

• Chemical and spectral interferences introduced by the decomposition acids.

• Time-consuming decomposition and solution steps.

Direct Analysis in GFAAS

• Some materials can be atomized directly in GFAAS, avoiding the solution step.

• Liquid samples can be pipetted directly into the furnace for ashing and atomization. Examples include:

  • Blood

  • Petroleum products

  • Organic solvents

Direct Solid Sample Analysis in GFAAS

• Solid samples can be weighed directly into sampling cup type atomizers or tantalum boats for introduction into tube furnaces.

• Calibration can be difficult and requires standards that approximate the sample in composition.

  • Examples: Dry plant leaves and soil.

Use of Organic Solvents

Solvents

Low-molecular-weight alcohols, esters, or ketones enhance absorbance when used as analyte solvents.

Reasons for Absorbance Enhancement

• Increased nebulizer efficiency.

• Lower surface tension of solutions, leading to smaller drop sizes and increased sample reaching the flame.

• More rapid solvent evaporation.

Fuel-Oxidant Ratios

Leaner fuel-oxidant ratios are used to offset the added organic materials. However, leaner mixtures can produce lower flame temperatures and an increased potential for chemical interferences.

Extraction Procedures

Chelating Agents

Chelating agents are added to the sample solution to form metal chelates. Examples include:

• Ammonium pyrrolidinecarbodithioate

• Diphenyl thiocarbazone (dithizone)

• 8-hydroxyquinoline

• Acetylacetone

Immiscible Organic Solvents

Immiscible organic solvents are used in flame spectroscopy to extract chelates of metallic ions. For example, Methyl isobutyl ketone (MIBK).

Process

The resulting extract is nebulized into the flame. The sensitivity of the analyte in the extract is increased because a small volume of the organic liquid is required to remove metal ions quantitatively from large volumes of aqueous solution. Extraction leaves matrix components in the aqueous solvent, thus reducing interferences.

Learning Objectives

• Discuss the limitations of analyzing solid samples by flame AAS.

• Explain why organic compounds are used to extract metals in samples and list their merits and demerits.

Reference

Skoog, D.A., Holler, F.J. and Crouch, S.R. 2016. Principles of Instrumental Analysis. 7th Edition. Thomson Brooks/Cole.