Significant Figures and Errors in Measurements

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Questions and Answers

Why is analytical science important?

To develop new theories about matter

To explore the origins of matter

To develop and use instruments and methods to identify matter (correct)

To develop and use instruments and methods to create matter

What makes analytical chemistry a popular field of work?

Its emphasis on kinetic theory

Its focus on theoretical plate models

Its application in gas chromatography

Its instruments and methods for identifying matter (correct)

How do significant figures and decimal places help in measurements?

They introduce errors in measurements

They provide an exact measurement

They indicate measurement certainty (correct)

They change the nature of the measurement

What is the importance of providing units when measuring anything?

It provides context and meaning to the measurement Signup and view all the answers

What is the significance of a unit when reporting a measurement?

It conveys information about the measurement scale Signup and view all the answers

Why should we pay attention to errors in measurements?

To improve measurement precision Signup and view all the answers

What is the purpose of stating errors in measurements?

To provide an idea of how significant the result is Signup and view all the answers

How is precision related to random errors in measurements?

Precision is limited by random errors Signup and view all the answers

What is the role of accuracy in measurements?

To provide an idea of how close to the true value a measurement is Signup and view all the answers

How does chromatography differ from solvent extraction?

In chromatography, one phase is stationary while the other moves past it Signup and view all the answers

What does a partition coefficient signify in chromatography?

The ratio of concentrations of a solute in two phases at equilibrium Signup and view all the answers

How does precision relate to standard deviation in measurements?

Precision is often described using standard deviation Signup and view all the answers

What is the classification of chromatography based on the physical properties of the analyte?

Molecular characteristic Signup and view all the answers

In which type of chromatography does the competition occur between a liquid stationary phase and the mobile phase?

Partition Chromatography Signup and view all the answers

What type of chromatography involves a competition between an ionic exchange resin stationary phase and liquid mobile phase?

Ion Exchange Chromatography Signup and view all the answers

Which type of chromatography is associated with the acronym LC/HPLC?

Liquid Column Ion-Exchange Chromatography Signup and view all the answers

In which type of chromatography does the mobile phase consist of gas?

Gas-Solid Chromatography Signup and view all the answers

What is the principle of competition in Permeation Chromatography?

Between a polymer matrix stationary phase and liquid mobile phase Signup and view all the answers

What is the principle of chromatography?

Separation of components by distribution between mobile and stationary phases over time Signup and view all the answers

In chromatography, what is the mobile phase?

The solvent moving through the plate or column Signup and view all the answers

Which type of chromatography involves large molecules being excluded?

Size-exclusion Chromatography Signup and view all the answers

What is the purpose of preparative chromatography?

Purify and collect one or more components of a sample for further use Signup and view all the answers

Which category of chromatography involves molecules interacting differently with the mobile and stationary phase?

Partition Chromatography Signup and view all the answers

What is the term for a substance that is going to be separated in chromatography?

Analyte Signup and view all the answers

In which type of chromatography is one kind of molecule in a mixture attached to a molecule covalently bound to the stationary phase?

Affinity Chromatography Signup and view all the answers

What does ion-exchange chromatography primarily attract?

Anions Signup and view all the answers

What does size-exclusion chromatography exclude?

Small molecules Signup and view all the answers

What does an eluate refer to?

The solvent flowing out after interacting with the stationary phase Signup and view all the answers

According to the Theoretical Plate Model of Chromatography, what are the separate layers within the chromatographic column called?

Theoretical plates Signup and view all the answers

Who proposed the Rate Theory of Chromatography in 1956?

Van Deemter Signup and view all the answers

What is the primary focus of the Kinetic Theory in chromatography?

Separation dynamics Signup and view all the answers

In Gas Chromatography, what is the role of the mobile phase?

Gas carrier Signup and view all the answers

What distinguishes Normal Phase Chromatography from Reverse Phase Chromatography?

Stationary phase polarity Signup and view all the answers

Which theory of chromatography focuses on the concept that the chromatographic column contains multiple separate layers known as theoretical plates?

Theoretical Plate Model Signup and view all the answers

How does an increase in the velocity of the mobile phase impact the band broadening in chromatography?

Increases the band broadening Signup and view all the answers

According to the Van Deemter equation, what does the term 'u' represent?

Average velocity of the mobile phase Signup and view all the answers

What is one of the main causes of band spreading in chromatography?

Slow equilibration Signup and view all the answers

How does the Rate Theory of Chromatography differ from the Plate Model?

It considers different paths available for solute molecules Signup and view all the answers

In chromatography, which term measures the ratio at which a substance moves randomly from a region of high concentration to a region of lower concentration?

Diffusion coefficient Signup and view all the answers

Which factor contributes to band broadening in chromatography according to the Van Deemter equation?

Average velocity of the mobile phase Signup and view all the answers

Which factor dominates at low flow rates and is more significant in gas chromatography compared to liquid chromatography?

Longitudinal diffusion Signup and view all the answers

What aspect of chromatographic peak shape is affected by the rate of elution according to the Rate Theory of Chromatography?

Width of the peak Signup and view all the answers

In Van Deemter's model, what does term C represent?

Mobile phase mass transfer resistance Signup and view all the answers

In chromatography, what does the Van Deemter equation help us understand better?

Band broadening mechanisms Signup and view all the answers

What causes broadening of the solute band in chromatography according to the text?

Difference in paths due to varying lengths Signup and view all the answers

What does Cs stand for in Van Deemter's model and what does it represent?

Stationary phase mass transfer, film thickness Signup and view all the answers

What does the plate height (H) represent in the Theoretical Plate Model?

Length of the column required for each Mobile Phase/Stationary Phase equilibrium to be established Signup and view all the answers

In chromatography, what does the Distribution Constant (KC) denote?

The ratio of solute concentrations in stationary and mobile phase Signup and view all the answers

What is the aim of varying solvent strength in chromatography?

To increase the selectivity between components in a mixture Signup and view all the answers

What does the Separation Factor (α) describe in chromatography?

The ability of experimental conditions to discriminate between two components Signup and view all the answers

What is the significance of Resolution (Rs) in chromatography?

Describes the degree of separation of two adjacent bands Signup and view all the answers

How is the Number of Theoretical Plates (N) related to the plate height (H) in a chromatographic column?

N = L/H; N is inversely proportional to H Signup and view all the answers

What is a limitation of the van Deemter theory in terms of comparison between different chromatographic techniques?

It does not allow for comparison of resolution between different techniques. Signup and view all the answers

Which factor is not considered by the van Deemter theory when comparing different types of columns?

Temperature fluctuations Signup and view all the answers

How do systems operating under different pressures compare according to the limitations of the van Deemter theory?

They cannot be compared based on resolution. Signup and view all the answers

What does the Kinetic theory fail to consider regarding system changes?

Viscosity changes Signup and view all the answers

In terms of efficiency, how does the Kinetic theory predict the change with mobile phase velocity?

Efficiency decreases with increasing mobile phase velocity. Signup and view all the answers

What aspect does the Rate Theory by van Deemter focus on primarily?

Dynamics of separation Signup and view all the answers

What modification is recommended if the Rf values in silica gel TLC are too high?

Add 5-10% of hexane Signup and view all the answers

In two-dimensional TLC, after the first run, what is the next step before re-running the plate?

Drying the plate Signup and view all the answers

What is the primary role of the layer in thin-layer chromatography (TLC)?

Promoting physical sorption of solutes to the surface Signup and view all the answers

What does multiple TLC (unidimensional multiple development) aim to achieve?

Increase resolution of components with Rf values below 0.5 Signup and view all the answers

How does varying the polarity of the mobile phase affect the separation in silica chromatography?

Reduces retention time for polar solutes Signup and view all the answers

Which method is NOT a detection and visualization technique used in Thin-Layer Chromatography?

Gas chromatography Signup and view all the answers

In thin-layer chromatography, why is alumina considered more basic than silica?

Alumina has a higher partition coefficient Signup and view all the answers

In column chromatography, what pH range is typically used with silica as the stationary phase?

pH 2-8 Signup and view all the answers

What is a characteristic feature that distinguishes reverse-phase silica from normal-phase silica in TLC?

Presence of aliphatic hydrocarbons attached to silica Signup and view all the answers

What is the recommended particle diameter range when using silica in column chromatography?

40-60 µm Signup and view all the answers

Why is thin-layer chromatography (TLC) not recommended for preparative work involving large sample quantities?

Limited loading capacity (up to 10 mg) Signup and view all the answers

What impact does an increase in mobile phase polarity have on the chromatographic strength in silica chromatography?

Increases chromatographic strength Signup and view all the answers

What property of the eluted solute can be detected directly in HPLC using a detector with a range of 1-1000 pg?

Solute's mass Signup and view all the answers

Which chromatography technique requires the presence of a fluorophore for detection at concentrations between 1-10 pg?

Fluorescence Chromatography Signup and view all the answers

What is the primary purpose of an electrochemical detector in HPLC?

Detecting amperometric properties Signup and view all the answers

Which type of detector in HPLC is suitable for detecting solutes in the range of 100-1000 ng based on changes in refractive index?

Refractive Index Detector Signup and view all the answers

At what concentration range can a UV/VIS detector operate effectively in HPLC for direct solute detection?

0.1-1 ng Signup and view all the answers

What parameter is used to measure the adsorption energy per unit area of the solvent in chromatography?

Solvent strength factor Signup and view all the answers

In Thin-Layer Chromatography (TLC), why is it important for all solutes to be moderately soluble in the mobile phase?

To enhance resolution between solutes Signup and view all the answers

What is the key factor that determines if a solvent has the correct strength for the mobile phase in Thin-Layer Chromatography?

Solvent's ability to move zones near the edge Signup and view all the answers

What is the recommended pore size range for packing materials when the analyte has a molecular weight above 10,000 MW?

250 Å - 300 Å Signup and view all the answers

What action should be taken if single solvents do not provide adequate resolution in Thin-Layer Chromatography?

Use a mixture of solvents Signup and view all the answers

In chromatography, what is the purpose of using larger particles (15-20 μm)?

For preparative-scale separations Signup and view all the answers

What does the non-linear effect observed when mixing weak and strong solvents in chromatography highlight?

The complexity of solvent-solute interactions Signup and view all the answers

What distinguishes gradient elution from isocratic elution in chromatography?

Gradient elution allows varying hydrophobicity separations Signup and view all the answers

When should a packing material with a pore size of 100 Å or less be used?

For samples below 3,000 MW Signup and view all the answers

What is the main advantage of using a 10 μm packing material in chromatography?

Moderate column efficiencies Signup and view all the answers

What role does the molecular weight of an analyte play in choosing the appropriate pore size for packing material?

Guides selection of pore size Signup and view all the answers

What type of mobile phase is used in gas chromatography?

Gas Signup and view all the answers

In gas-liquid partition chromatography, what is the nature of the stationary phase?

Non-volatile liquid Signup and view all the answers

What does the stationary phase in gas-solid adsorption chromatography directly interact with?

Gaseous analyte Signup and view all the answers

What is the primary function of the carrier gas in gas chromatography?

To separate different components Signup and view all the answers

What differentiates gas-liquid partition chromatography from gas-solid adsorption chromatography?

Nature of stationary phase Signup and view all the answers

What happens to the analyte in gas-solid adsorption chromatography?

Adsorbed on solid particles Signup and view all the answers

In Size Exclusion Chromatography (SEC), how are molecules separated?

According to their shape Signup and view all the answers

What is another name for Size Exclusion Chromatography (SEC)?

Molecular Exclusion Chromatography Signup and view all the answers

Which type of chromatography involves small molecules entering the pores of particles?

Size Exclusion Chromatography Signup and view all the answers

What determines if a molecule will be attracted to an Anion-exchange resin in Ion-exchange Chromatography?

Charge of the molecule Signup and view all the answers

What does Size Exclusion Chromatography (SEC) primarily aim to separate?

Molecules by their size Signup and view all the answers

Which method of chromatography involves mobile ions being held near cations covalently attached to the stationary phase?

Ion Exchange Chromatography Signup and view all the answers

What is the primary function of the stationary phase in size-exclusion chromatography?

Separating molecules based on their size Signup and view all the answers

What is the impact of having a larger particle size in a size-exclusion chromatography column?

Reduces resolution and increases flow rate Signup and view all the answers

What happens when particles with different pore sizes are mixed in a size-exclusion chromatography column?

Improves both resolution and separation range Signup and view all the answers

What is the typical percentage of the total column volume occupied by the pores in size-exclusion chromatography?

40% Signup and view all the answers

How does the resolution of molecules in size-exclusion chromatography change as the particle size decreases?

Resolution increases and flow rate decreases Signup and view all the answers

Why does size-exclusion chromatography require long columns or multiple columns?

To compensate for limited molecular size separation Signup and view all the answers

What is the role of anion exchangers in ion-exchange resins?

Attract solute anions Signup and view all the answers

In ion-exchange selectivity, the selectivity coefficient K is calculated using the concentrations of which ions?

[Na+] and [Li+] Signup and view all the answers

What kind of ions do ion exchangers generally favor the binding of?

High charge, small hydrated radius, low polarizability Signup and view all the answers

What is the purpose of gradient elution in ion exchange processes?

To cause a gradual increase in ionic strength Signup and view all the answers

What should be done to ensure the quality of the sample in size exclusion chromatography?

Use a pre-column when in doubt Signup and view all the answers

What application is NOT typically associated with ion-exchange resins?

Separation of non-ionic species Signup and view all the answers

How does increasing the concentration of anion C- impact the elution process in an ion exchange column?

Anion B- gets displaced first Signup and view all the answers

How can a combination of packings with different separation ranges be achieved in size exclusion chromatography?

By using columns of different porosity or mixed bed columns Signup and view all the answers

What is crucial for the chemical nature of column packing in chromatography?

Coating the silica with a hydrophilic phase Signup and view all the answers

In which type of chromatography is retention based on the attraction between solute ions and charged sites bound to the stationary phase?

Ion-exchange chromatography Signup and view all the answers

What did Benoit show in 1967 regarding the calibration for polymers of different types?

A universal calibration curve is possible based on hydrodynamic volume Signup and view all the answers

What does the Universal Calibration Principle underestimate when analyzing polymers?

Branching in polymers Signup and view all the answers

Study Notes

Module Overview

Module CH3F2 consists of 24 lectures, 6 workshops, and a lab (33% of the module) and an exam (67% of the module)
Module leader: Mark Barrow

Preparing for the Exam/Revision

Lecture slides will be color-coded to indicate the importance of each topic:
- Green: Must-know topics (basic material)
- Yellow/Amber: Should-know topics (moderate material)
- Red: Useful but not necessary to know (advanced material)

Chromatography Overview

Chromatography is a family of laboratory techniques used to separate, identify, and quantify the components of a mixture
It operates on the principle of distribution of components between a mobile phase and a stationary phase over time
Types of chromatography:
- Based on stationary phase support:
  - Planar (thin layer chromatography, TLC)
  - Column
- Based on the physical properties of the analyte:
  - Volatility (gas chromatography, GC)
  - Solubility (partition chromatography)
  - Adsorptivity (adsorption chromatography)
  - Charge (ion exchange chromatography)
  - Size (size-exclusion chromatography)
- Based on the chromatographic principle:
  - Adsorption
  - Partition
  - Ion exchange
  - Permeation

Importance of Units and Significant Figures

Units are essential to provide meaning to measurement results
Significant figures and decimal places indicate the certainty of a measurement
Errors and significant figures are important to understand the reliability of a measurement result

Precision and Accuracy

Precision: how reproducible a measurement is
Accuracy: how close to the true value a measurement is
Errors can be classified as:
- Random (affecting precision)
- Systematic (affecting accuracy)

Chromatography Terminology

Analyte: substance being separated
Eluent: solvent used to move the analyte through the stationary phase
Eluate: the mixture of analyte and solvent that emerges from the column
Mobile phase: the phase that moves through the stationary phase
Stationary phase: the phase that is held in place
Chromatograph: the equipment used to perform chromatography
Chromatogram: the detector signal vs. retention time or volume

Theoretical Plate Model

Theoretical plate model: a concept used to measure column efficiency
Plate height (H or HETP): the length of column required for each mobile phase/stationary phase equilibrium to be established
Number of theoretical plates (N): the length of the column divided by the plate height
Theoretical plate model limitations:
- Assumes infinitely fast equilibration
- Does not account for the dynamics of the separation

Rate Theory of Chromatography

Proposed by van Deemter in 1956
Accounts for the dynamics of the separation
The Van Deemter equation for plate height (HETP) = A + B/u + Cu
B term: longitudinal diffusion
C term: resistance to mass transfer
HETP: the smaller the better, indicating a more efficient column

Kinetic Theory of Chromatography

Proposed by Giddings in 1965
Widely used in the last 10 years
Limitations of van Deemter's theory:
- No allowance for comparison between different chromatographic techniques
- No allowance for comparison between different types of columns
- No consideration of viscosity changes with pressure/temperature system changes

Thin-Layer Chromatography (TLC)

Usually liquid/solid chromatography
Stationary phase: alumina or silica gel coated onto a flat surface
Mobile phase: solvents (usually fairly non-polar)
Microscale, rapid (10-20 min)
Not suitable for preparative work (10 mg max)
Choice of mobile phase for silica chromatography:
- Vary polarity of mobile phase to increase or decrease time polar solutes spend on stationary phase
- Chromatographic strength of solvent: how fast it moves analytes
- Solvent strength parameter: εo = measure of adsorption energy / unit area of solvent### Thin-Layer Chromatography (TLC)
Ethyl acetate is a good solvent for silica gel TLC
To reduce polarity, add 5-10% hexane
To increase polarity, add 5-10% methanol
Adding a percentage of acetic acid or ammonia (or pyridine) can help maintain acidic or basic solutions and prevent tailing of zones

Two-Dimensional TLC

Used for examining complex mixtures
Plate is run in one direction, removed, dried, and then rotated 90° and run in another solvent
The line of partially resolved components from the first run becomes the origin for the second development

Multiple TLC (Unidimensional Multiple Development)

Increases resolution of components with Rf values below 0.5
After a single run, the plate is removed, partially dried, and then placed again in the same solvent and run in the same direction
Final Rf = 1 – (1 – Rf)n

Detection and Visualization

Self-absorption in UV (254 nm)
Fluorescence (UV excitation)
Iodine vapor (1% alcoholic solution or iodine crystals in a jar)
Charring reagents
Geiger counter for radioactive materials
Rhodamine B solution (50 mg/100 ml ethanol) for organic compounds
Spraying with water for lipophilic solutes (e.g., steroids)

Column Chromatography

Usually a preparative technique rather than analytical
Silica or alumina used as stationary phase
Particle diameter: 40-60 μm for silica, and 5 μm for alumina

Stationary Phase Pore Size

Affects separation efficiency
Choose pore size based on analyte molecular weight:
- <3,000 MW: 100 Å or less
- 3,000-10,000 MW: 100 Å - 130 Å
- >10,000 MW (peptides and proteins): 300 Å

Mobile Phase Solvents

Normal phase: heptane, 1-chlorobutane, chloroform, methylene chloride, isopropyl ether, ethyl acetate
Reverse phase: methanol, acetonitrile, acetone, ethanol, 2-propanol, THF

Mobile Phase Elution

Isocratic: constant mobile phase composition
Gradient: changing mobile phase composition during analysis
Gradient elution: for mixtures with wide ranges of hydrophobicity

HPLC Detectors

UV/VIS (0.1-1 ng)
Fixed Variable (VWD)
Photodiode array (DAD)
FTIR (1mg)
Refractive Index (100-1000 ng)
Mass Spectrometry (1-1000 pg)
Conductivity (500-1000 ng)
ELSD (100-1000 pg)
Electrochemical (10-1000 pg)
Fluorescence (1-10 pg)

Gas Chromatography

Gaseous analyte transported through the column by a gaseous mobile phase (carrier gas)
Stationary phase: non-volatile liquid coated on the inside of the column or on a fine solid support
Types: gas-liquid partition chromatography and gas-solid adsorption chromatography

Types of Chromatography

Adsorption chromatography
Partition chromatography
Ion-exchange chromatography
Size-exclusion chromatography

Size Exclusion Chromatography (SEC)

Also called molecular exclusion chromatography, steric exclusion chromatography, gel filtration chromatography, or gel permeation chromatography
Separation based on molecular size
Stationary phase: cross-linked gels or porous silica
Pore size determines separation range
Columns: 7-8 mm diameter (analytical), 20-25 mm diameter (preparative)

Ion-Exchange Chromatography

Retention based on attraction between solute ions and charged sites bound to the stationary phase
Anion exchangers: positively charged groups on the SP attract solute anions
Cation exchangers: negatively charged sites attract solute cations
Ion-exchange resins: amorphous particles of organic material

Applications

Converting a salt into another
Pre-concentration of trace components of a solution
Purification of water
Separation of proteins

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Description

Learn about the importance of significant figures and errors in measurements. Understand how precision and accuracy play crucial roles in determining the reliability of measurement results. Explore how errors in measurements can affect the trustworthiness of the final outcome.