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chem2006 - principles in analytical chemistry > Theory > Flashcards

Theory Flashcards

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1
Q

What is qualitative analysis?

A

Revealing the identity of elements and compounds in a sample.

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2
Q

What is quantitative analysis?

A

Indicates the amount of each substance in a sample.

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3
Q

What is the equation for calculating weight percent?

A

(Mass of solute / Mass of solution) * 100.

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4
Q

What is the equation for calculating volume percent?

A

(Volume of solute / Volume of solution) * 100.

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5
Q

What is the equation for calculating weight/volume percent?

A

(Mass of solute / Volume of solution) * 100.

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6
Q

How is ppm calculated?

A

(Mass of solute / Mass of solution) * 106.

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7
Q

How is ppb calculated?

A

(Mass of solute / Mass of solution) * 109.

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8
Q

What is error?

A

The difference between the measured and real values.

It is also the estimated uncertainty of a measurement or experiment.

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9
Q

What is a benefit of using the median?

A

The median is less affected by outliers.

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10
Q

What is precision?

A

Describes the reproducibility of measurements.

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11
Q

What is accuracy?

A

Indicates the closeness of the measurements to the true or accepted value.

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12
Q

What are the three types of error?

A

Random (indeterminate) error.

Systematic (determinate) error.

Gross error.

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13
Q

What is a random error?

A

Data is scattered approximately symmetrical around a mean value.

This affects precision and is dealt with statistically.

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14
Q

What is a systematic error?

A

An error that causes the mean to be displaced from the known value. All the values are either too high or too low.

This affects accuracy.

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15
Q

What is a gross error?

A

Gross errors are often large and very obvious and cause outliers.

These can be avoided by carrying out replicate measurements.

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16
Q

What are the three sources of systematic error?

A

Instrumental errors - The measuring device hasn’t been calibrated correctly so all values are too high/too low.

Method errors - Caused by inaccuracies in physical or chemical behaviour of reagents/reactions used in determination (for example, slow or incomplete reaction).

Personal errors - Where a personal judgement is used such as where the meniscus is on a titration.

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17
Q

How are the three sources of systematic error reduced?

A

Instrumental error - careful calibration and maintenance/servicing of the instruments and equipment.

Method error - very difficult to reduce as the ‘true’ value of the determination is often not known accurately.

Personal error - good technique, discipline and care over carrying out and recording measurements.

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18
Q

What are three approaches to reducing systematic error?

A

Test calibrations through analysis of certified standards.

Using two or more independent analysis techniques to cross check.

Check absolute response and baseline through the analysis of blanks.

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19
Q

Why are separations important?

A

Accurate chemical analyses can only be carried out on samples that are pure or free from other components that could interfere.

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20
Q

What are the two general categories of separation?

A

Separations that occur between two bulk phases.

Separations that occur between a bulk phase and a thin layer.

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21
Q

What are some common bulk phase separations?

A

Gas-Gas: Gaseous Diffusion
Liquid-Liquid: Distillation
Liquid-Solid: Crystallisation
Solid-Gas: Sublimation
Liquid-Gas: Liquid-gas extraction
Liquid-Liquid: Liquid-Liquid extraction

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22
Q

What is crystallisation?

A

The preferential formation of a desired solid precipitate of the analyte from a mixture that also contains interfering solutes. The interferent remains in solution and can then be discarded.

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23
Q

How is the feasibility of crystallisation determined?

A

The difference in the solubility of the interferent and the desired analyte.

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24
Q

What is inclusion?

A

Impurities occupy a lattice site within the crystal framework.

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25
Q

What is occlusion?

A

Impurities become trapped within the crystal framework.

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26
Q

What is distillation?

A

Separates molecules based on their volatility.

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27
Q

What is a liquid-liquid extraction?

A

A separation technique based on a compound’s solubility in different, immiscible solvents (usually organic and water).

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28
Q

What considerations must be taken when choosing the solvents for a liquid-liquid extraction?

A

The solubility of the analyte in each solvent.

Beware of solvents that are mutually soluble with each other or are prone to form azeotropes.

The densities must be sufficiently different otherwise an emulsion could form.

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29
Q

Is it more efficient to do one separation with 30 cm3 of solvent or three separations with 10 cm3?

A

Three separations with 10 cm3.

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30
Q

What technique can be used for compounds with extremely low distribution constants?

A

Liquid-liquid continuous extraction.

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31
Q

What is the process of a liquid-liquid continuous extraction?

A

Solvent is continually distilled and condenses into a long tube immersed in the water containing the solute to be extracted.

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32
Q

What is a process to extract a lipid from a solid and how does it work?

A

Solid-liquid Soxhlet Extraction.

The sample is placed into a thimble and the solvent is continually delivered to it via redistillation. The solvent gradually extracts the analyte from the solid matrix and when the thimble is full of solvent it is siphoned back to the solvent reservoir to maintain maximum efficiency.

The analyte must be separated from the solvent before analysis is done.

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33
Q

When is a solid-liquid Soxhlet extraction done?

A

When the compound has a very limited solubility in the solvent.

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34
Q

What are the two basic types of chromatography?

A

Column chromatography where the stationary phase is in a tube.

Planar chromatography where the stationary phase is on a plate.

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35
Q

What is chromatography?

A

The separation of two or more substances by virtue of their different affinities toward a stationary phase and a mobile phase that moves in relation to the stationary phase.

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36
Q

What is partition chromatography?

A

The mobile phase is a liquid that is coated on a solid support.

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37
Q

What is adsorption chromatography?

A

There is a solid stationary phase and the compounds adsorb onto the stationary phase.

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38
Q

What is ion-exchange chromatography?

A

The solid stationary phase is charged and so attracts ions.

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39
Q

What is molecular exclusion chromatography?

A

Solid stationary phase consists of pores. Smaller molecules are retained as they fit into the pores whereas larger molecules do not.

40
Q

What does the area under a chromatogram peak represent?

A

The quantity of the molecule.

41
Q

Why does increasing the column length not always result in a better separation?

A

Although the distance between the peaks would be greater, the bands would be wider.

42
Q

What is the main cause of band broadening?

A

The molecules do not take a straight path down the column so some molecules may take a longer path - this gives a normal distribution as it is random.

43
Q

What is fronting?

A

Where the front of the peak is wider than it should be.

44
Q

What is tailing?

A

Where the end of the peak is wider than it should be.

45
Q

What can cause fronting?

A

Overloading the column.

The partition coefficient isn’t linear.

46
Q

What can cause tailing?

A

The partition coefficient isn’t linear.

47
Q

What are two ways that the resolution of a chromatogram may be increased?

A

The separation could be increased.

The bandwidth could be decreased.

48
Q

What are resolution values for a poor and a reasonable separation?

A

Poor resolution: 0.75.

Reasonable separation: 1.5.

49
Q

What is the plate height?

A

The length of the column that contains the fraction of the analyte between L (length of column packing) and (L-1σ).

This is equal to 34% of the analyte.

50
Q

What are the features of gas chromatography?

A

It involves the introduction of a volatilised sample onto the head of a chromatographic column.

The mobile phase is a flow of inert gas.

51
Q

What is the stationary phase in gas-liquid chromatography?

A

A liquid that is immobilised on the surface of an inert solid.

52
Q

What is the flowchart for a gas chromatography machine?

A

Carrier gas tank → Flow regulators → Sample injection chamber → Column → Detector → Data System and Flow Meter.

Over the sample injection chamber, column and detector there may be a thermostat.

53
Q

What is a lab-on-a-chip?

A

A device that integrates one or several laboratory functions on a single chip of a only millimetres to a few square centimetres in size.

54
Q

What are the features of the gas used in gas chromatography?

A

The gas must be inert (H2, He, N2).

The choice of gas depends on:
* The type of detector used
* The nature of the stationary phase
* The operation environment
* Cost of gas

55
Q

What are the features of a carrier gas tank?

A

The gas used is held in a high pressure cylinder that is fitted with regulators and valves that allow the pressure and flow rate of the gas to be controlled. This is usually between 10 and 50 psi and 25-150 mL min-1.

The gas supplies are also fitted with scrubbers that remove traces of O2 and other impurities.

56
Q

What are the features of the sample injection system in GC?

A

For maximum efficiency, the sample must be introduced rapidly as a ‘plug’ of vapour.

The sample is introduced as a solution (around 1 mg of sample in 1 mL of solvent) using a micro syringe.

The typical injection volume is between 0.1 and 1 μL.

57
Q

What are the features of the injector in GC?

A

The injector is a hot block that volatilises the sample and introduces the appropriate quantity of the sample vapour onto the analytical column.

The sample is injected into the system with a micro syringe which contains a polymer based, chemically inert septum.

The temperature of the injector block is held 50 °C higher than the lowest boiling point.

The column also requires a sample splitter (1:100 to 1:500) to reduce the volume of vapour introduced into the column.

58
Q

What are the benefits of using an automatic injection system?

A

The volume and flow rate for each sample is reproducible as it is nearly identical each time.

59
Q

What are the two types of columns in gas chromatography?

A

Packed.

Capillary (or open tubular).

60
Q

What is the length of a column in GC?

A

2 - 50 metres.

61
Q

What are the types of column used in GC?

A

Fused silica open tubular column.

Wall-coated open tubular column.

Support coated open tubular column.

Packed column.

These have been listed from best to worst.

62
Q

What is used as the stationary phase in GC?

A

Siloxanes.

63
Q

What are the features of the column oven in GC?

A

Roughly a column temperature slightly above the boiling point of the sample will result in reasonable elution times (2 - 30 minutes).

Temperature programming can be used to increase the resolution of the chromatogram.

64
Q

What temperatures, in principle, result in the best resolution?

A

Lower temperatures, however, this will increase elution time and therefore increase zone broadening.

65
Q

What are the ideal characteristics of a detector in GC?

A

Highly sensitive.
Stable and reproducible.
Linear response to analytes over a large range of concentrations.
Operating range from room temperature to 400 °C.
A rapid response to analytes that is independent of flow rate.
Reliable and easy to use.
Similar in response to all samples OR highly predictable and selective towards one or more types of sample.
Non-destructive to the sample.

However, no detector exhibits all of these characteristics.

66
Q

What are example detectors for GC?

A

Flame ionisation
Thermal conductivity
Electron capture
Mass spectrometer
FTIR

67
Q

What are the features of a flame ionisation detector (FID)

A

A flame burns the analytes as it elutes and the ions generated by the flame are detected by monitoring the current between the grounded jet and a charged collector plate.

The number of ions produced is proportional to the number of reduced carbon atoms in the flame. Therefore, the response is to the number of C atoms entering the detector per unit of time.

The response is independent to flow rate. However, there is no response to water, carbon dioxide, sulphur dioxide and nitrogen dioxide.

It is sensitive over a large linear range.

Mass selective rather than concentration selective.

68
Q

What are the features of a thermal conductivity detector (TCD)?

A

The detector measures changes in the thermal conductivity of the eluent mixture as it flows through the cell.

Low concentrations of analytes can be detected.

Twin detectors are normally used. One acts as a reference for the carrier in the absence of analyte.

It is concentration selective.

69
Q

What is the response factor (Rf) of a detector?

A

As not all materials raise the same response in many detectors, calibration towards the material needs to occur.

This is done by preparing at least three known concentrations of the analyte and plotting a concentration curve. The gradient is equal to the response factor.

70
Q

Do LLC and LSC use partition or adsorption chromatography?

A

LLC - Partition

LSC - Adsorption

71
Q

How is plate height reduced in LC?

A

Reducing the size of the particles in the stationary phase.

This increases the backpressures which means that the mobile phase needs to be pumped through the column at a higher pressure.

72
Q

What is the instrumentation flowchart for LC?

A

Pump → Pulse damper → Drain valve → Filter → Back-pressure regulator → Pressure transducer→ Injector valve → Column → Detector.

73
Q

What are the features of pumps in LC?

A

Generate pressures up to 6000 psi.
Pulse free flow of solvent.
Large range of flow rates (0.1 - 10 mL min-1)
Reproducible flow (within 0.5%).
High resistance to corrosion.

A smooth flow without pulsing is obtained by used two reciprocating pumps in series.

74
Q

What are the features of an injector in LC?

A

Introduces sample (dissolved in solvent) into mobile phase without introducing bubbles of gas.

Produces a reproducible and rapid injection.

75
Q

What are the features of an LC column?

A

The column is constructed from stainless steel to withstand the high pressures.
It is packed with a stationary phase (usually silica particles).
Internal volume (0.5 mL - 50 mL).

76
Q

What are some types of LC detectors?

A

UV
Absorbance
Fluorescence
Electrochemical
Mass spectrometry
FTIR
Conductivity.

77
Q

What are the features of a UV detector?

A

The analyte must contain a chromophore.
Dual wavelength (Hg discharge at 254/280 nm).
Multi-wavelength detectors.
Diode array detectors.

78
Q

What is the difference between a liquid-liquid partition and a liquid-bonded partition?

A

In liquid-liquid partition, the stationary phase is held in place by physical adsorption on to the solid support.

In liquid-bonded partition, the stationary phase is covalently bonded to the solid support.

Bonded phase is now the most common.

79
Q

How are bonded-phase packing prepared?

A

Through the reaction of organochlorosilanes with hydroxyl functionalities on the surface of silica particles in hot dilute HCl.

80
Q

What are the advantages and disadvantages of bonded-phase packing?

A

They are very stable and there is no requirement to re-coat the solid support periodically.

However, they have a limited sample capacity.

81
Q

What is normal phase chromatography?

A

There is a polar stationary phase and a non-polar mobile phase.

For example, triethylene glycol or water (stationary phase) and hexane (mobile phase).

The least polar component is eluted first and increasing the polarity of the mobile phase decreases elution time.

82
Q

What is reverse phase chromatography?

A

There is a non-polar stationary phase and a polar mobile phase.

For example, a hydrocarbon stationary phase and water/methanol/THF as the mobile phase.

The most polar component is eluted first and increasing the polarity of the mobile phase increases the elution time.

83
Q

What is temperature?

A

A measure of kinetic energy of molecular motion.

84
Q

What are the basic principles of thermal analysis?

A

Analysis of a change in a property of a sample that is related to a change in the temperature.

The sample is heated at a constant heating rate (or controlled temperature programme).

Sample is usually in the solid state and changes such as melting occur.

85
Q

What are some thermal analysis techniques?

A

Thermogravimetric analysis (TGA).
Differential scanning calorimetry (DSC).
Differential thermal analysis (DTA).

86
Q

What is thermogravimetric analysis?

A

Analysis of the change in the mass of sample on heating it.

The measurements are carried out by a thermobalance.

The temperature is usually increased linearly but can follow more complex heating schemes.

87
Q

What can be responsible for changes in mass using TGA?

A

Mass losses are due to decomposition, sublimation and reactivity/desorption of products.

Mass gains are due to oxidation.

88
Q

How is a thermogram presented?

A

The y-axis contains the mass or percentage mass. The x-axis is the temperature.

89
Q

What are the principle uses of TGA?

A

Measurement of a material’s thermal stability and its composition.

More specific examples include:
* Filler content of polymer resins.
* Residual solvent content.
* Decomposition temperature.
* Moisture content of organic and inorganic materials.
* Oxidative stability.

90
Q

What can TGA be paired with?

A

The vapours released from TGA can be sent into an IR or MS or GC for identification.

91
Q

What is differential thermal analysis (DTA)?

A

DTA is the most widely used thermal analysis method.

The temperature of a sample is monitored with respect to a reference material while they are both being subjected to the same heating procedure.

The sample and the reference are placed in low thermal conductivity sample holders held in a block in the furnace.

The temperature of the furnace is increased linearly and the difference in temperature between the sample and reference is plotted against the furnace temperature.

92
Q

What happens when an exo or endothermic process occurs in DTA?

A

If an endothermic process takes place within the sample, the temperature will lag behind the reference and a minimum will occur on the DTA curve. The opposite is true for an exothermic process.

The area under the exo/endotherm is related to the enthalpy of the thermal event.

93
Q

What is differential scanning calorimetry?

A

A sample and reference are maintained at the same temperature throughout the procedure by using separate power supplies.

Any difference between the power supplies is recorded against the furnace temperature.

Generally, the temperature program for a DSC analysis is designed so that the sample holder temperature increases linearly as a function of time.

94
Q

What are the differences between DSC and DTA?

A

DSC is more reliable for calculating enthalpy changes whereas DTA can be used up to a higher temperature.

95
Q

What transitions can DSC be used to detect?

A

Glass transitions.
Phase changes.
Crystallisation.

96
Q

What does the standard deviation show?

A

The probable error in a single measurement.

97
Q

What does the standard error of the mean show and how is it calculated?

A

The probable error in a series of measurements.

Sm = s / √N

chem2006 - principles in analytical chemistry (2 decks)

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