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Analytical chemistry 2018 > Barbara KH > Flashcards

Barbara KH Flashcards

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1
Q

What are mixed mode SPE sorbents?

A

Chemically modified silica or polymer based materials that have multiple retentive sites on an individual particle
Non polar eg C18
Ion exchange: weak or strong base: primary or quarternary or carboxylic or sulphonic
That exploit different retention mechanisms
As a result it is characterised by a higher selectivity than reversed phase
Can facilitate:
Interactions with different functional groups on a single analyte
Interaction of different functional groups on multiple analytes

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2
Q

Classification of SPE sorbents according to their selectivity

A 
Normal Phase (v low selectivity)
Reverse Phase (v low selectivity)
Ion exchange (anion/cation) (low selectivity)
Mixed mode ion exchange (medium selectivity)
Polymer based materials (eg molecularly imprinted polymers that are synthesised using template molecules) (high selectivity)
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3
Q

What steps are involved in analytical toxicology?

A

Pre- analytical
Analytical
Post-analytical

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4
Q

What is involved in the pre-analytical step?

A

Obtain details of suspected poisoning episode
Obtain the patients medical and occupational history
Decide priorities for the analysis

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5
Q

What is involved in the analytical step?

A

Perform agreed analysis

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6
Q

What is involved in the post analytical step?

A

Interpreting the results

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7
Q

What 5 things should be considered in analytical toxicology?

A
  1. Which sample should be collected/ is relevant?
  2. How it should be prepared?
  3. Which analytical method should be chosen?
  4. Qualitative and quantitative analysis - screening tests or target analysis?
  5. Disposition of xenobiotic in the body - look for parent compound or metabolite?
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8
Q

What is phase 1 metabolism and the scientific name?

A

Functionalism
Chemical modification of the original xenobiotic molecule by oxidation, reduction or hydrolysis (may ‘activate’ the compound)

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9
Q

What is phase 2 metabolism and the scientific name?

A

Conjugation reactions in which a second hydrophilic molecule such as glucuronic acid (a more polar compound) is added to the molecule

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10
Q

What process occur in functionalism?

A
  1. Oxidation
  2. Reduction
  3. Hydrolysis
  4. Hydration
  5. Dehalogenation
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11
Q

What processes occur in conjugation?

A
  1. Sulphation
  2. Glucuronidation
  3. Glutathione conjugation
  4. Acetylation
  5. Amino acid conjugation
  6. Methylation
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12
Q

What is glucuronidation?

A

A larger more polar molecule (UDP-Glucuronic Acid) is added to substrates to make them more polar and water soluble making them easier to transport and more likely to be excreted.

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13
Q

What characteristics are there of lipophilic xenobiotic molecules?

A

Rapidly absorbed
Rapidly and widely distributed in the body
Extensively metabolised before its excreted

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14
Q

Give information on rapid screening tests

A

Rapid analysis
No sample preparation required
Low sensitivity and selectivity
Usually allow for detection of only drugs/ poisons in the samples

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15
Q

Give general information on advance analytical techniques (HPLC, GC, MS, NMR)

A

Takes time
Requires sample preparation
Very sensitive and selective
Allow for identification and qualification of drugs/poisons

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16
Q

What steps are involved in extraction?

A
  1. Extraction - present samples directly to the chromatographic system
  2. Concentration - to increase the concentration of the sample
  3. Clean up - remove potentially interfering matrix compounds (particularly important for GC, LC/MS)
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17
Q

What extraction processes are there for non/semi volatile compounds from liquids?

A

Liquid liquid extraction
Solid phase extraction
Solid phase microextraction

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18
Q

What extraction processes are there for non-/semi- volatile compounds from solids?

A 
Soxhlet extraction
Ultrasonic extraction
Supercritical fluid extraction
Accelerated solvent extraction
Microwave extraction
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19
Q

What extraction methods are there for volatile compounds from solid s and liquids

A

Static headspace extraction
Dynamic headspace extraction - purge and trap
Solid-phase microextraction

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20
Q

What is the mechanism for solid phase extraction?

A

Sorption of analytes from solution onto solid phase based on the affinity of the analytes to the solid phase

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21
Q

What is the application of normal phase SPE?

A

Extraction of polar analytes from non-polar samples

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22
Q

What sorbents are used in normal phase SPE?

A

Activated alumina, silica gels

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23
Q

Wht interactions are exploited in normal phase SPE?

A

Polar interactions, (hydrogen bonding, dipole-dipole)

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24
Q

What eluents are used in normal phase SPE?

A

Polar solvents

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25
Q

What are the applications of reversed phased SPE?

A

Extraction of non-polar analytes from polar samples eg water

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26
Q

What sorbents are used for RP SPE?

A

Bonded phase silica, copolymers

27
Q

What interactions are exploited in RP SPE?

A

Non polar interactions (van der waals)

28
Q

What eluents are used in RP SPE?

A 
Non polar solvent
Hexane
Ethyl acetate
Acetone
Methanol
Acetonitrile
29
Q

What is the application of ion exchange SPE?

A

Extraction of ionic analytes from polar samples

30
Q

What sorbents are used in ion exchange SPE?

A

Silica gel or polymers containing cation or anion functional groups

31
Q

What interactions are exploited in ion exchange SPE?

A

Cation and anion exchange

32
Q

What eluents are used for ion-exchange SPE?

A

High ionic strength required pH
Buffers
Salt solutions

33
Q

What is the dissociation of a weak acid with a pH > pKa by 2 units?

A

100% A-

0% HA

34
Q

What is the dissociation of a weak acid with pH

A

100% HA

0% A-

35
Q

What is the dissociation of a weak base with pH>pKa by 2 units?

A

100% B

0% BH+

36
Q

What is the dissociation of a weak base with pH

A

100% BH+

0% B

37
Q

What are the steps in HPLC?

A 
Mobile phase
Pump
Sample injector
Column
Detector
Data system
38
Q

What does a high pH do to the silica bonded phase?

A

Dissolution of silica backbone

39
Q

What does a low pH do to the silica bonded phase?

A

Hydrolysis of the siloxane bond

40
Q

What happens to the column lifetime of silica bonded phase at a high temp and buffer concentration

A

Decreased

41
Q

What two terms are used to describe the different mobile phases you can have in the mobile phase of HPLC?

A

Isocratic - the mobile phase remains the same over the course of the separation
Gradient - alteration of the composition of the mobile phase during the course of the chromatographic run

42
Q

What are volatile buffers needed for?

A

Light scattering detection
Coupling with mass spec
Preparative separations

43
Q

Why is the pH of the mobile phase so important for ion exchange?

A

Ion exchange is controlled by pH

44
Q

Why is the pH of the mobile phase so important for reversed phase?

A

Influences retention time and peak shape
PH affects the degree of dissociation of acidic/basic compounds and free residual silanol groups on the surface of the stationary phase

45
Q

What are the advantages of increasing the temperature in HPLC?

A

The performance of the column often increases because of decrease in mobile phase viscosity which improves mass transfer
Analysis time decreases due to the possibility of using higher flow r ages of the mobile phase due to the increase in diffusion coefficients
It is necessary to work at higher temperatures if the mobile phase is viscous - less pressure is needed to pump the mobile phase

46
Q

What are the disadvantages of increasing temperature in HPLC

A

The solvent or sample are more likely to decompose
The vapour pressure of the solvent rises this increasing the risk of bubbles in the detector - uneven baseline, ghost peaks

47
Q

What is the maximum temperature of silica

A

120

48
Q

What is the maximum temperature of chemically bonded stationary phases

A

80

49
Q

What different HPLC detectors are there?

A 
Ultraviolet detector
Fluorescence detector
Refractive index detector
Electrochemical (amphoteric) detector
Conductivity detector
Light scattering detector
Mass spectrometry detector
50
Q

Principles of uv detector

A

Allows for the choice of one wavelength in order to accommodate the absorption characteristics of a particular solute or a group of solutes
Samples will need to have chromophores

51
Q

Principles of a photodiode array detector

A

Allows access to all wavelengths simultaneously
An entire spectrum of light is passed through the detector cell. The light is then bounced off a grating, and detected using a multiple array of photodiodes

52
Q

Principles of a fluorescence detector

A

Measures radiation from a molecule which has attained an excited electronic state after the absorption of radiation
Excitation source: mercury vapour, Xenon, deuterium, lasers

53
Q

applications of fluorescence / uv detector

A

Compounds that fluorescence or of which fluorescing derivatives can be obtained
Due to small number of molecules that can fluorescence thi type of detection can be used in the detection of trace quantities of analytes in complex matrices

54
Q

What ionisation techniques are available

A

Electro spray ionisation
Atmospheric pressure chemical ionisation
Matrix assisted laser desorption ionisation

55
Q

What mass analysers are available

A

Quadrupole
Ion trap
Time of flight

56
Q

What tandem mass analysers are available

A

Triple quadrupole

Quadrupole time of flight

57
Q

What are the five main characteristics of an analyser in mass spectrometry?

A
  1. Mass limit - determines the highest value of m/z that can be determined
  2. Transmission - the ratio between no of ions reaching detector and number ions produced at source
  3. Resolving power - ability to distinguish between two ions with small mass difference
  4. Scan speed - rate at which analyser detects ions
  5. Mass accuracy - difference between theoretical and observed mass
    4.
58
Q

What are the requirements for a GC-MS Interface

A

Band broadening is minimised by a min dead volume
The carrier gas pressure has to be reduced at column exit
There are no discrimination effects against thermally labile compounds owing to active sites or heating the interface
There is efficient transfer of the entire sample to the ionisation chamber

59
Q

Why is the knowledge of drug metabolism important in the diagnostics of its abuse?

A

No parent compound could be detected in the biological matrix if the drug is readily metabolised leading to a false negative measurement
Therefore in order to confirm abuse analytical protocols should include characteristic metabolites and parent compounds where possible
Decide which metabolites are characteristic

60
Q

Explain what is meant by the term end capping. Outline the importance in reverse phase chromatography

A

Reduces the number of silanol groups that remain unreacted for steric reasons
Treatment with trialkylchlorosilane
Remove polar OH group that will increase polarity of stationary phase
Minimise secondary silanol reaction

61
Q

Propose types of reactions that could be used to make chemically modified silica C18 stationary phase

A

Reaction with octadecyl ODS

Reaction with mono or dichlorosilane

62
Q

Liquid chromatography coupled with mass spectrometry. Discuss key steps required to convert a liquid chromatography sample to a mass spectrometry sample for efficient interfacing

A

Change state of matter from a liquid to a gas
Change of pressure reduce using a high vacuum
Charge state ionisation needs ionic species
Electron spray ionisation transfer ions in solution to gas and charge molecules of interest

63
Q

Name three types of detector

A

UV/vis target analyte has a chromosphere
Mass spectrometry quadrupole, time of flight, or tandem MS (with ESI interface) for high sensitivity and selectivity
All target analytes will be ionised with ESI and separated according to m/z
Fluorescence may have to derivatize by adding a fluorescent agent

Analytical chemistry 2018 (3 decks)

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