1 ert 207 analytical chemistry ert 207 analytical chemistry alina rahayu mohamed school of...

1ERT 207 Analytical Chemistry

ERT 207 ANALYTICAL CHEMISTRY

ALINA RAHAYU MOHAMEDSchool of Bioprocess Engineering

University Malaysia Perlis 02600, Kangar

Perlisemail: [email protected]


LECTURE 213th July 2007


1.3 STEPS IN QUANTITATIVE ANALYSIS

1. Define the Problem

3.Obtain a Representative

Sample

2.Select a Method


STEPS IN QUANTITATIVE ANALYSIS (continue)

4.Prepare the sample for analysis 5. Perform the

Measurement

5.Perform Any Necessary Chemical Separations


STEPS IN QUANTITATIVE ANALYSIS (continue)

6. Calculate the Results and

Report


1. Define the Problem Before we begin defining the problem for

an analysis procedure, we must have some information;

A)Who is the client(EPA, engineers) B)The purpose of analysis C)What type of sample to be analyzed


1. Define the ProblemFactors to consider: What is the problem—what needs to be found? Qualitative and/or quantitative? What will the information be used for? Who will use it? When will it be needed?How accurate and precise does it have to be?What is the budget?


Once the problem is defined, next questions:

A) how sample is to be obtained B) how much is needed C)What separations may be required to

eliminate interferences?


How to select a Method?


2.Select a MethodFactors to be considered: Sample type Size of sample Sample preparation needed Concentration and range (sensitivity needed) Selectivity needed (interferences) Accuracy/precision needed Tools/instruments available Cost Are methods available in the chemical literature? Are standard methods available?


3.Obtain a Representative Sample

Factors• Sample type/homogeneity/size

• Sampling statistics/errors


The gross sample must be reduced in size to obtain a laboratory sample of several grams, from which a few grams to miligrams will be taken to be analyzed.


Sampling

Obtaining a representative sample is the first step of an analysis.

The gross sample is several small portions of the sample.

This is reduced to provide a laboratory sample.

An aliquot of this sample is taken for the analysis sample.

Obtaining a representative sample is the first step of an analysis.

The gross sample is several small portions of the sample.

This is reduced to provide a laboratory sample.

An aliquot of this sample is taken for the analysis sample.


Some precautions should be taken during handling and storing samples to prevent or minimize contamination, loss, decomposition or matrix change.

We must prevent contamination or alteration of the sample by (1)light (2)atmosphere (3) container.


For example, some samples have to be protected from the atmosphere or light.

It maybe an alkaline substance that will react with CO2 in the air.

Blood samples to be analyzed for CO2 content shlould be protected from the atmosphere.


Next example, corrosive sample may react with the container.

In automobile exhaust SO2 is lost by dissolving in condensed water vapour from the exhaust.


4.Prepare the Sample for Analysis

Factors• Solid, liquid, or gas?• Dissolve?• Ash or digest?• Chemical separation or masking of interferences needed?• Need to concentrate the analyte?• Need to change (derivatize) the analyte for detection?• Need to adjust solution conditions (pH, add reagents)?


4.Prepare the Sample for Analysis

Step 1: Measure the amount being analyzed

(volume or weight of the sample) Replicate samples are taken for analysis Why?


1) to obatin statistical data on the precision of the analysis

2) to provide more reliable results.


Step 2: Sample pretreatment Example: The organic materials sample are

analyzed for inorganic constituents. The organic constituents maybe destroyed

by dry ashing. How?


The organic materials is slowly combusted in a furnace at 400 oC to 700 oC.

Organic materials escape out, leaving behind an inorganic residue which is soluble in dilute acid.

Aim of step 2: to remove unwanted constituents that make up the whole sample.


Step 3: Optimizing sample condition Aim of step 3: to prepare sample for the

next stage of analysis (The separation or measurement step)

The solution condition is optimized. How?


For example, the pH may have to be adjusted or reagent is added to mask interference from other constituents.

The analyte may have to be reacted with a reagent to convert it to a form suitable for measurement or separation.

For example, a coloured product maybe formed that will be measured by spectrometry.


• Distillation• Precipitation• Solvent extraction• Solid phase extraction• Chromatography (may be done as part of themeasurement step)




Why conduct chemical separation? 1.to eliminate interference 2.to provide suitable selectivity in the

measurement 3. to preconcentrate the analyte for more

sensitive or accurate measurement


6. Perform the Measurement Factors • Calibration • Validation/controls/blanks • Replicates


6. Perform the Measurement Methods of carrying out the

measurements: 1.gravimetric analysis 2.volumetric analysis 3. instrumental analysis. What are these?


• Statistical analysis (reliability) • Report results with

limitations/accuracy information

7. Calculate the Results and Report


Quizzz time! Close all of your notes! 10 minutes test.


Quiz 1: 1. Give one example of the application of

analytical chemistry.(25 marks) 2.State the steps of carrying out tests using

instrumentation for quantitative analysis. (25 marks)

3.Discuss each step in detail. (25 marks) 4. State the steps in quantitative analysis.(25 marks)


Answers: 1.Any acceptable example like: carbon

monoxide determination in public air or in trapped air (in car), blood glucose level in diabetics

2. Phase 1: Fast screening phase Phase 2: Identification phase Phase 3: Possible quantification


3. Fast screening phase: Urine samples are tested rapidly for the presence of

classes of compounds different from normal samples. Techniques use: gas chromatography (GC), liquid chromatography (LC).

If samples have unknown compounds, that may or may not be prohibited, therefore further testing is needed for identification purpose.

Identification phase: Using techniques such as gas chromatography-mass

spectrometry (GC-MS). Complex mixtures are separated by GC. Then, detetcted by MS that gives molecular structural data on the compound.


Possible Quantification: Precise quantification of some compounds.

Meaning that, exact amount of compound must be determined. Therefore, we can verufy whether the substance is present is present at low level (permissible level) or high level (unpermissible level)


4. 1.Define the Problem 2.Select a Method 3.Obtain a Representative Sample 4.Prepare the sample for analysis 5.Perform Any Necessary Chemical Separations 6. Perform the Measurement 7. Calculate the Results and Report


Basic tools in Analytical Chemistry


Modern balances are electronic. They still compare one mass against another since they are calibrated with a known mass. Common balances are sensitive to 0.1 mg.

Modern balances are electronic. They still compare one mass against another since they are calibrated with a known mass. Common balances are sensitive to 0.1 mg.

Fig. 2.1. Electronic analytical balance.


Fig. 2.8. Volumetric flask.

Volumetric flasks are calibrated to contain an accurate volume. See the inside back cover of the text for tolerances of Class A volumetric glassware.

Volumetric flasks are calibrated to contain an accurate volume. See the inside back cover of the text for tolerances of Class A volumetric glassware.


Fig. 2.9. Transfer or volumetric pipets.

Volumetric pipets accurately deliver a fixed volume.

A small volume remains in the tip.

Volumetric pipets accurately deliver a fixed volume.

A small volume remains in the tip.


Fig. 2.10. Measuring pipets.

Measuring pipets are straight-bore pipets marked at different volumes.

They are less accurate than volumetric pipets.

Measuring pipets are straight-bore pipets marked at different volumes.

They are less accurate than volumetric pipets.


Fig. 2.11. Hamilton microliter syringe.

Syringe pipets precisely deliver microliter volumes.

They are commonly used to introduce samples into a gas chromatograph.

Syringe pipets precisely deliver microliter volumes.

They are commonly used to introduce samples into a gas chromatograph.


Fig. 2.12 Single-channel and multichannel digital displacement pipets and microwell plates.

These syringe pipets can reproducibly deliver a selected volume.

They come in fixed and variable volumes. The plastic tips are disposable.

These syringe pipets can reproducibly deliver a selected volume.

They come in fixed and variable volumes. The plastic tips are disposable.


Fig. 2.14. Meniscus illuminator.

Position the black field just below the meniscus.

Avoid parallax error by reading at eye level.

Position the black field just below the meniscus.

Avoid parallax error by reading at eye level.


Fig. 2.20. Wash botltles: (a) polyethylene, squeeze type; (b) glass, blow type.

Use these for quantitative transfer of precipitates and solutions,

and for washing precipitates.

Use these for quantitative transfer of precipitates and solutions,

and for washing precipitates.


Next topic: accuracy and precision


2.1.1 ACCURACY

2.1.2 PRECISION

2. HANDLING OF DATA ANALYSIS


Thank you Q& A session.

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