Spectrophotometry Part 2

Question 1

Comparsion of atomic analysis methods (1)

Answer 1

See picture (1)

Question 2

Comparsion of atomic analysis methods (2)

Answer 2

See picture (2)

Question 3

Uses of Atomic Spectroscopy

Answer 3

-Flame AAS
-GFAAS
-ICP-OES
-ICP-MS
-MES

Question 4

Flame AAS

Answer 4

One of the most common methods for metal analysis, especially in
environmental testing, food, and beverage industry

Question 5

GFAAS

Answer 5

Commonly used in trace metal analysis, particularly in regulatory testing of water and biological samples

Question 6

ICP-OES

Answer 6

one of the most widely used techniques for multi-element analysis, especially in environmental and industrial applications

Question 7

ICP-MS

Answer 7

Commonly used in environmental monitoring, geological analysis, food
testing, and research due to its high sensitivity and ability to measure ultra-trace levels

Question 8

MES

Answer 8

new, not established, not routinely used

Question 9

Guide to Atomic Spectrometry Technique Selection

Answer 9

See pic

Question 10

3 different types of atomic spectroscopy

Answer 10

1. Atomic emission spectroscopy
2. Atomic absorption spectroscopy
3. Atomic fluorescence spectroscopy

Question 11

Atomic Absorption Spectrophotometry ”AAS”

Answer 11

See pic

Question 12

Flame AAS

Answer 12

See picture

Question 13

Graphite Furnace Atomic Absorption Spectroscopy “GFAAS”. Overview.

Answer 13

See picture

Question 14

Graphite Furnace Atomic Absorption Spectroscopy “GFAAS”

Answer 14

Electrically heated
Graphite conducts electricity
E = IR
Current through resistor dissipates heat

Consider incandescent light bulb

• Sample may be injected manually using micropipette
• Many instruments use autosampler
• User loads samples into vials and
  programs the autosampler
• Autosampler rotates the tray, uses
  syringe to withdraw sample from vial and inject specified volume into
  graphite furnace

Question 15

GFAAS Temperature Program

Answer 15

See picture

Question 16

Inductively Coupled Plasma – Atomic Emission Spectroscopy “ICP-OES” or “ICP-AES”

Answer 16

See picture

Question 17

ICP-OES

Answer 17

See picture

Question 18

How to measure results from ICP-OES?

Answer 18

See picture

Question 19

Hollow-Cathode Lamp

Answer 19

• The hollow-cathode lamp is filled with low-pressure Ne or Ar (1–5 Torr)
• The cathode is made of the element to be
  analyzed.
• A voltage of ~500 V applied between the anode and cathode ionizes the Ne.
• Positive Ne+ ions are accelerated toward the cathode.
• Accelerated Ne+ ions “sputter” some of the cathode atoms into the gas phase.

Question 20

Narrow Linewidths of Hollow-Cathode Emission

Answer 20

Beer’s law requirement: Hollow-cathode lamp lines should be
substantially narrower than the line width of analyte atoms in the
flame or furnace.

Atoms in the lamp are cooler than atoms in a flame and pressure
in the lamp is low, so lamp emission is sufficiently narrower than
the width of the absorption line.

The purpose of the monochromator is to select one line from the
hollow-cathode lamp and to reject as much emission from the
flame or furnace as possible.

Question 21

How do monochromators work?

Answer 21

• A grating causes angular dispersion via constructive and destructive interference.
• A planar surface with closely spaced lines
• Each line acts as a point source of light
• Light exhibits wavelike behavior

Question 22

Calculations for monochromators

Answer 22

Calculations for grating equation (angle), dispersion, and bandwidth or separation at exit slit of monochromator.

Question 23

Prisms

Answer 23

Prism disperses wavelengths based on change of refractive index with wavelength. Lever arm. Slit width. Resulting bandwidth (continuous light source), or selection of line (line source).

Question 24

Grafting monochromator

Answer 24

• Angle of Incidence, Angle of Diffraction
• Angular Dispersion
• Linear displacement

Question 25

What are some qualitative uses of spectroscopy?

Answer 25

-Screening for drug analysis or drug use -Presence or absence of a drug

Question 26

Why is Fluorescence more sensitive than Absorbance?

Answer 26

* Consider a low concentration of analyte * Absorbance: A = 𝜀bc, small c means small A. %T = 100x10-A * %T ~ 100% which means P ~ P0 * Subtraction of two very large numbers in search of a very low number * Detecting a flashlight on the western horizon right as sun is setting * Fluorescence: * Ifluorescence = Φf ∙ 𝜀bc (Iexcitation) * where Φf is the fluorescence quantum yield * Absolute signal, not a difference signal * Detecting a flashlight on a dark, moonless night