Lect12 Instrumental Technique analytical.pptx

analytical chemistry

1. CHE 112: Inorganic & Analytical Chemistry Fall 2020/2021 Lecture 1 2 : Instrumental Technique (Principles of AAS-GC) By Dr. Samah Hawash Asst. Prof. in Chemical Engineering E-mail: [email protected]

2. Atomic Absorption Spectroscopy Atomic absorption spectroscopy is a very common. Technique for detecting metals and metalloids in samples. It is very reliable and simple to use. It can analyze over 62 elements. It also measures the concentration of metals in the sample.

3. Elements detected by atomic absorption are highlighted in pink in this periodic table

4. PRINCIPLE The technique uses basically the principle that free atoms (gas) generated in an atomizer can absorb radiation at specific frequency. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption.

5. Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration. Atomic absorption is a very common technique for detecting metals and metalloids in environmental samples.

6. Atomic absorption spectrophotometer consists of Light source Nebulizer Atomizer Flame atomizer: Monochromator: Detector: Calibration curve

7. LIGHT SOURCE Hollow cathode lamp are the most common radiation source in AAS. It contains a tungsten anode and a hollow cylindrical cathode made of the element to be determined. These are sealed in a glass tube filled with an inert gas (Neon or argon ). Each element has its own unique lamp which must be used for that analysis .

8. NEBULIZER Suck up liquid samples at controlled rate. Create a fine aerosol spray for introduction into flame. Mix the aerosol and fuel and oxidant thoroughly for introduction into flame.

9. Atomizer Elements to be analyzed needs to be in atomic state. Atomization is separation of particles into individual molecules and breaking molecules into atoms. This is done by exposing the analyte to high temperatures in a flame .

10. FLAME ATOMIZER To create flame, we need to mix an oxidant gas and a fuel gas. In most of the cases air-acetylene flame or nitrous oxide acetylene flame is used. Liquid or dissolved samples are typically used with flame atomizer.

11. Application Determination of even small amounts of metals (lead, Mercury, calcium, magnesium, etc ) as follows: Environmental studies: drinking water, ocean water, soil. Food industry. Pharmaceutical industry.

12. GAS CHROMATOGRAPHY A separation technique used to separate organic mixture into its chemical components. The mobile phase is a gas must be chemically inert (He, Ar , N 2 &H 2 ). The choice of carrier gas depends on the type of detector. Carrier gas must contain molecular sieve to remove water and impurities.

13. How a Gas Chromatography Machine Works First, a vaporized sample is injected onto the chromatographic column Second, the sample moves through the column through the flow of inert gas. Third, the components are recorded as a sequence of peaks as they leave the column.

14. Chromatographic Separation Deals with both the stationary phase and the mobile phase . Mobile – inert gas used as carrier. Stationary – liquid coated on a solid or a solid within a column.

15. Chromatographic Analysis The number of components in a sample is determined by the number of peaks. The amount of a given component in a sample is determined by the area under the peaks. The components identification can be determined by the given retention times.

16. GC Components Carrier gas Injector Oven Column Detector

17. CARRIER GAS Hydrogen Better thermal conductivity Disadvantage: it reacts with unsaturated compounds & inflammable Helium Excellent thermal conductivity It is expensive Nitrogen Reduced sensitivity It is inexpensive

18. Requirements of a carrier gas Inertness Suitable for the detector High purity Easily available Cheap Should not cause the risk of fire Should give best column performance

19. Factors Affecting Separation in GC Column length As length of column increases, increase resolution, and better separation Flow rate Increase flow decrease retention time Volatility Volatile compounds have less retention time