Unit 3.2 - AAS Instrumentation Part 1 PDF

ATOMIC ABSORPTION SPECTROMETER INSTRUMENTATION- PART 1 By Nancy Tyrer and Ela Kogut Edits by Katie Rankin Readings: Fundamentals of Analytical Chemistry, Chapter 28 Chemical Analysis, Chapter 13 Lab Manual, Experiments 4 and 5  Atomic Absorption Spectrometer...

ATOMIC ABSORPTION SPECTROMETER INSTRUMENTATION- PART 1 By Nancy Tyrer and Ela Kogut Edits by Katie Rankin Readings: Fundamentals of Analytical Chemistry, Chapter 28 Chemical Analysis, Chapter 13 Lab Manual, Experiments 4 and 5  Atomic Absorption Spectrometer Components 2 1. Radiation (Line) Source: Hollow Cathode Lamp (HCL)  Emits resonance lines (λ’s that exactly match the λabs by the analyte of interest) 2. Atom Cell (Atomizer): a) Nebulizer: Reduces analyte solution droplet size producing an aerosol mist b) Mixing chamber: Turbulent mixing of flame gases and analyte aerosol mist c) Burner/flame: Atomizes and reduces analyte so that it possesses the full complement of valence e-s (high temperature required) 3. Detection System: a) Monochromator: Selects resonance λ for the analysis, filtering out all other λ’s emitted by HCL b) Photomultiplier Tube (PMT): Converts light signal to electrical signal c) Signal processor: Converts electrical signal to readable format  Double Beam AAS Design 3 Po P  AAS Components: Line Source 4  The most useful radiation source for atomic absorption spectroscopy is the hollow cathode lamp (HCL).  Function: Emits narrow emission lines (resonance lines) of analyte element  Construction of the HCL:  Sealed glass cylinder with quartz window  Cup shaped cathode made of element to be analyzed  Anode wire  Neon or argon (1-5 torr)  Single element and multi-element lamps exist  Single element lamps are more sensitive and have greater accuracy  Hollow Cathode Lamp (HCL) 5 Quartz window Pyrex body Anode Cathode  Hollow Cathode Lamp (HCL) 6 Ne or Ar at 1-5 torr (made of element of interest)  HCL Emission-> Analyte Absorption Process 7 Mo(gas) → M*(gas) λ3 λ1 λ2 Po P M* → Mo + hc/λ HCL Emission/Absorption Process bandwidth HCL produces line spectrum with resonance λ’s → Po Analyte in the flame absorbs portion of the radiation → P Intensity of radiation is reduced after it passes through the sample → T = P/Po 8  Emission Spectra for HCL 9 Single Element Lamp (Pd) Multi-Element Lamp (Cd, Pb, Cu)  Hollow Cathode Lamp Operation 10 1. Ionization of inert gas (Ar or Ne)  High voltage (300 V) is applied across electrodes  Inert gas (Ar) is ionized by the voltage Ar o(g) -----> Ar +(g) + e-s  Electrical energy causes e- s produced from ionization to accelerate to the anode, while Ar+ ions produced from ionization accelerate to the cathode  Collision process is continuous  Hollow Cathode Lamp Operation 11 2. Sputtering  A) Ar + ions strike the cathode Ca Cathode causing the metal atoms to be ejected from the cathode (-), Cao(g) producing an atomic vapour of sputtered metal atoms Ca(g)O  B) The metal atom vapour is Ca*(g) excited due to the energy transfer from the collision between the Ar+ ions with the solid metal cathode Ca(g)O Ar +(g) + Cao(s) + e-s ----> Ar O(g) + Ca*(g) cathode  Hollow Cathode Lamp Operation 12 3. Emission of Resonance Lines  Excited gaseous metal emits photons of discrete λ’s due to excited electrons falling back down to ground valence energy level: Ca*(g) -----> Cao(g) + hc/λ (λ = 422.7 nm for Ca)  Hollow Cathode Lamp Operation 13 4. Redeposition of Cathode Metal  Gaseous metal redeposits itself onto cup-shaped cathode Cao(g) -----> Cao(s)  HCL: Cup Shaped Cathode 14 Purpose: 1. To ensure redeposition is efficient → metal plates out onto the cathode, instead of cool glass walls. 2. To collimate the radiation in a concentrated cone.  Next Up: In Depth Look at the Atomizer 15 Po P  References 16  Tyrer, N.; Kogut, E. CHEM25415 Lecture on Atomic Absorption Instrumentation. Presented at Sheridan College, Brampton, ON, Fall 2012.  Tyrer, N. CHEM25415 Instrumental Analysis 1 Laboratory Manual; Sheridan College: Brampton, ON; Experiments 4 and 5.  Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry, 9th ed.; Brooks/Cole: California, 2014; Chapter 28.  Rouessac, F.; Rouessac, A. Chemical Analysis, 2nd ed.; John Wiley & Sons: New Jersey, 2007; Chapter 13.  Beaty, R. D.; Kerber, J. D. Concepts, Instrumentation and Techniques in Atomic Absorption Spectrophotometry. Perkin-Elmer: Connecticut, 1993.