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Experiment No.6 Atomic Absorption Spectroscopy (AAS) CHEM 127.1 - MAD Duro, Marlon Espiritu, Kevin Sotelo, Tiffany

c127.1 Expt 6 & 11 Report

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Page 1: c127.1 Expt 6 & 11 Report

Experiment No.6Atomic Absorption Spectroscopy (AAS)

CHEM 127.1 - MADDuro, Marlon

Espiritu, KevinSotelo, Tiffany

Page 2: c127.1 Expt 6 & 11 Report

INTRODUCTION

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Basics

• One of the most common methods used for analyzing metals in a sample

• Uses absorption of emitted light from free atoms

• Used for qualitative and quantitative analysis– Requires standards of known concentration

for quantitative analysis

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Basics

• Involves atomization at very high temperatures

• The light emitted from sample give a line spectrum– AAS detects the emission from first half of

excitation process (during absorption and before transition to excited state)

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Notes

• Very selective – elements have different sets of energy levels

• Follows Beer-Lambert law– Limited by quality of monochromator– Bandwidth of absorbing species must be

broader than light source– Fixed by narrowing radiation sources

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Basic Parts

1. Lamp2. Atomizer3. Nebulizer4. Monochromator5. Photomultiplier tube (PMT)

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Lamp

• Hollow Cathode Lamp (HCL)– Most common lamps– Provides constant yet intense light line for

element of interest– Cathode is made of same element analyzed

and contains low pressure inert gas– Light from HCL goes through glass

transparent to UV-Vis region– Generates very narrow spectral lines

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Lamp

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Lamp

• Electrodeless Discharge Lamp (EDL)– Contains small amount of analyte in form of

salt/metal– Narrower spectral lines

• Deuterium (D2)– Used for background correction– Limited wavelength range (190-320nm)

• Continuum sources

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Atomizers

• Flame• Destroys analyte ions and complexes• Involves the following processes:

– Desolvation, vaporization, atomization, ionization

• Creates elemental form of element of interest

• Used for liquid or dissolved samples

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Flame Atomizer

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Nebulizer

• Controls flow rate of sample• Mixes fuel and oxidant – pressure

generated sucks sample through tube– Fuel is usually acetylene

• Creates an aerosol of the sample• aerosol + fuel + oxidant

– heterogeneous mixture that goes to the burner– Leftover sample goes to glass waste container

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Monochromator

• Filters specific bands for the element of interest for entry to the PMT

• The line from the light source (usually HCL) is isolated

• Allows light not absorbed by sample to pass through

Page 14: c127.1 Expt 6 & 11 Report

Photomultiplier Tube

• The light is detected by the PMT• PMT readings comes from presence of

analyte in flame• The elemental form of the analyte absorbs

light and the corresponding decrease in the PMT reading is transformed into analytical data

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Apparatus

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Procedure• Fit the specific light source lamp to the lamp housing,

and switch on the instrument.• Light the source lamp, adjust the wavelength dial of the

spectroscope to the wavelength of the analytical line specified, and set at an appropriate current value and slit-width.

• Using the supporting gas and combustible gas specified, ignite the mixture of these gases, adjust the gas flow rate and pressure, and make the zero adjustment after nebulizing the solvent into the flame.

• Nebulize the test solution or the standard solution or control solution prepared by the method prescribed elsewhere, and measure the absorbance.

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Analysis

• Calibration curve Method– Preparation of a standard curve, followed by

measurement of the adsorbance of the unknown.• Standard Addition method

– To equal volumes of more than 2 of different test solutions, add the standard solution so that the stepwise increasing amounts of the object element are contained in the solutions, and add the solvent to make a definite volume.

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Standard Addition Method

• Beer-Lambert Law: A = kC• Unknown: A0 = kCx

• Unknown + standard:

T

ss

T

xx

VVC

VVC

kA

Page 19: c127.1 Expt 6 & 11 Report

Standard Addition Method• Dividing A by A0:

• Working Equation:

Tx

ssxx

0 VCVCVC

AA

sx

ssxx

s

T

0 CCVCVC

CV

AA

sxs

x

s

T

0V

C1

CV

CV

AAy

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DETERMINATION OF TRACE LEVELS OF COPPER AND LEAD IN VEGETABLE SAMPLES USING THE ATOMIC ABSORPTION SPECTROPHOTOMETER

Experiment:

Page 21: c127.1 Expt 6 & 11 Report

Copper (Cu)

• A micronutrient needed for the absorption and transport of iron in the blood stream.

• RDA: 900 μg / day• Main sources are shellfish, beans and

mushrooms.• Also found in vegetables in trace amounts.

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Experimental

• Preparation of stock solutions– 0.5 g of Cu(s) dissolved in 1:1 nitric acid– Diluted to 250 mL in a vol. flask– Take 5 mL aliquot and dilute to 100 mL– Stock solution: 100 ppm Cu

Page 23: c127.1 Expt 6 & 11 Report

Experimental

• Preparation of standard solutions– 5 100-mL vol. flasks (1-5)– 0.50,1.25, 2.50, 5.00 mL of the stock solution

added to flasks 1-4– Flask 5 is reagent blank.

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Experimental

• Preparation of sample– From 1 kg of vegetables, dried for two weeks– 2 g of dried leaves used.– Digested in conc. HNO3 for 20 min– 10 mL d. H2O added and filtered while

washing out the filter paper into 50-mL vol. flask

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Experimental

• Analysis of vegetable sample– Absorbance of standardd sol’n and each

sample recorded using the AAS using the setting for Cu.

– Solution was diluted 1:5 if absorbance is too high.

– Three trials performed

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Experimental

• Standard addition– 10 mL digested solution in 5 50-mL vol. flasks– 0.00, 1.00, 2.50, 5.00, 10.00 ppm of added

standards then diluted to the mark.– Absorbance of each solution measured.– Plot of A/Ao versus Vs made.

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Calibration Curve

Standardppm Cu

A A'

blank 0.0 0.0123 0.0000

1 1.0 0.1937 0.1814

2 2.5 0.3685 0.3562

3 5.0 0.6789 0.6666

4* 10.0 1.1751 1.1628

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

R² = 0.994078942520581

ppm Cu

A'

Page 28: c127.1 Expt 6 & 11 Report

Analysis of Vegetables

Sample A A'ppm Cu

sol’n sample

1 0.0498 0.0375 0.09945 2.486

2 0.0475 0.0352 0.08177 2.044

3 0.0475 0.0352 0.08177 2.044

Average 0.08767 2.192

Sample A A'ppm Cu

sol’n sample

1 0.1219 0.1096 0.6536 16.34

2 0.1278 0.1155 0.6989 17.47

3 0.1134 0.1011 0.5883 14.71

Average 0.6469 16.17

kangkong

kamote tops

0.1301130.02456 - 0.0375

g 2.00

mL 50.0ppm 0.09945

Sample computations:

A’ = 0.0375Vs = 50.0 mLwsample = 2.00 g

Solution: ppm Cu =

= 0.099449 ppm Cu

Sample:

ppm Cu =

= 2.486 ppm Cu

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Standard Addition

SolutionSample 1 Sample 2 Sample 3

Vs (mL) A y A y A y

1 0.00 0.0119 0.5000 0.0127 0.5000 0.0133 0.5000

2 0.50 0.1619 6.803 0.1632 6.425 0.1612 6.060

3 1.25 0.3350 14.08 0.3311 13.04 0.3282 12.34

4 2.50 0.6165 25.90 0.6147 24.20 0.6148 23.11

5 5.00 1.0865 45.65 1.0820 42.60 1.0820 40.68

slope 8.908 8.307 7.940

y-int 2.106 1.984 1.848

r 0.997 0.997 0.998

ppm CuSolution 0.1123 0.1204 0.1259

Sample 14.03 15.05 15.74

kangkong

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Standard Addition

SolutionSample 1 Sample 2 Sample 3

Vs (mL) A y A y A y

1 0.00 0.0272 0.5000 0.0261 0.5000 0.0276 0.50002 0.50 0.1696 3.118 0.1679 3.216 0.1733 3.1393 1.25 0.3308 6.081 0.3315 6.3506 0.3317 6.0094 2.50 0.6200 11.40 0.6191 11.86 0.6202 11.265 5.00 1.1073 20.35 1.1078 21.22 1.1088 20.09

slope 3.934 4.105 3.875

y-int 1.013 1.035 1.025

r 0.999 0.999 0.999

ppm CuSolution 0.2542 0.2436 0.2581

Sample 31.78 30.45 32.26

kamote tops

Page 31: c127.1 Expt 6 & 11 Report

Standard AdditionSample computations:Slope = 8.908 ppm-1

Solution: Cx = 1/(8.908 ppm-1) = 0.1123 ppm

Sample: ppm Cu =

2.00g50.0mL

10.0mL50.0mL0.1123ppm

=14.03 ppm Cu

Average ppm Cu in kangkong: 14.94 ppm (1.494 mg per 100 g)Average ppm Cu in kamote tops: 31.49 ppm (3.149 mg per 100 g)

Page 32: c127.1 Expt 6 & 11 Report

Experiment No.11Nuclear Magnetic Resonance (NMR)

Spectroscopy

CHEM 127.1 - MADGroups 5 and 6

Page 33: c127.1 Expt 6 & 11 Report

INSTRUMENTATION

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Wide-Line NMR

• Wide-line NMR1. Electromagnet2. Oscillator3. Modulation and lock-in detection4. Signal acquisition

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Pulse NMR

1. Programmable Pulse Generator2. Transmitter3. Probe circuit4. Reciever

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NMR Spectrometer

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NMR Spectrophotometer

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Video Clip

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Sample Quality Affects Spectra

• Low NMR sample quality increases every peak’s width, making it hard to resolve small couplings and frequency differences.

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Factors Affecting Sample Quality

• NMR spectra are strongly affected by both the sample contents and the NMR tube. These factors include:

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Boundaries Between Materials Cause Problems

• We shim to make the field more uniform across the sample, but sample factors can limit shimming’s effectiveness.

• Every material gets magnetized in a magnetic field, and the strength of its response is its magnetic susceptibility (χ). To ensure field uniformity, one must minimize such interfaces.

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Sample Height

• Sample should at least exceed the detection region.

• “Three fingers” rule

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• Solid particles must be absent in the sample.

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NMR Tube Straightness (Camber) and Concentricity

• A tube is held at the top, but the sample must be aligned precisely in the center of the probe for maximum performance.

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• The centers of the inner and outer surfaces of the tube may not coincide well.

• Poor positioning of the sample in the coil and nonuniformity of the glass wall thickness create shimming problems.

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NMR Tube Cleaning

• Rinse 1x with sample’s solvent• Rinse 5-10x with non- chlorinated solvent, e.g.

acteone &/or H2O• Rinse 1x with D2O• Store inverted on a lab wipe• Dry with stream of N2• If absolutely necessary, dry flat in oven ≤ 125

°C, ≤ 45 min• NEVER STORE IN AN OVEN!

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Processing of NMR

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RESULTS

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Compound 1

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Compound 1

Methyl 4-(2-hydroxyethyl)benzenesulfonate (1). C9H12O4S, 1H NMR (500 MHz, CHLOROFORM-d) δ ppm 2.42 (s, 3 H) 2.58 (br. s, 1 H) 3.77 (t, J=4.76 Hz, 2 H) 4.09 (t, J=4.40 Hz, 15 H) 7.33 (d, J=8.06 Hz, 15 H) 7.77 (d, J=8.43 Hz, 15 H).

S

O

O

OCH3

OH

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Compound 2

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Compound 2

Biphenyl-4,4'-diyldimethanediyl dimethanesulfonate (2). C16H18O6S2, 1H NMR (500 MHz, CHLOROFORM-d) δ ppm 2.45 (s, 6 H) 4.17 (s, 4 H) 7.33 (d, J=7.69 Hz, 4 H) 7.72 (d, J=8.06 Hz, 4 H).

O

OS

O

CH3O

S

CH3

OO