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forensic chemistry > lecture 3&4 - Polarity, Extraction and Basic Chromatography > Flashcards

lecture 3&4 - Polarity, Extraction and Basic Chromatography Flashcards

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1
Q

what is a dipole moment

A

the sum of individual bond polarities and lone pair contributions within the molecule

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2
Q

explain the difference between hydrophilic and hydrophobic

A

hydrophilic = attracted to water and repels fats
hydrophobic = fat loving, repels water

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3
Q

what’s the rule about polar and non-polar extraction

A

like dissolves/extracts into like

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4
Q

list 3 properties of polar molecules

A

hydrophilic, dissolve into polar solvents and have higher boiling/melting points than non-polar molecules (due to dipole-dipole interactions)

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5
Q

the greater the strength of a dipole interaction…

A

the greater the dipole moment and therefore the higher the boiling/melting point

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6
Q

why is hydrogen bonding so strong

A

hydrogen is very small and doesn’t have electrons to protect its nucleus

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7
Q

name 3 properties of non-polar molecules

A

hydrophobic, dissolve into non-polar (organic) solvents, and have lower bps and mps than polar molecules

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8
Q

what type of forces occur between non-polar molecules

A

Van Der Waals - very weak, temporary and easy to break

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9
Q

why do we use extraction?

A

not all samples can be directly analysed by chromatography and therefore we need to extract the analyte and discard the matrix. We can also remove contaminants or concentrate a sample

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10
Q

what is the aim of extraction

A

sample sustainability

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11
Q

name 3 problems associated with extraction

A
  1. samples large number of analytes
  2. small amounts of analytes - extraction can lead to a large loss of evidence
  3. chemical sustainability
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12
Q

what is liquid - solid extraction

A

extracting a solid powder in a liquid e.g. a street drug laced with adulterants needs to be separated

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13
Q

what is liquid-liquid extraction

A

where a liquid is partitioned into another liquid

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14
Q

explain the process of liquid liquid extraction

A

using a separating funnel (O-ring and a stand) add the sample and the solvent and shake. Open the valve to release built up pressure and place into the stand to allow to settle into separate layers (an aqueous layer and a non-polar layer). Repeat a number of times and then remove the separate layers into two conical flasks, and evaporate to dryness under nitrogen.

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15
Q

what does immiscible mean

A

do not mix

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16
Q

what does a positive log P value suggest

A

the drug will be extracted into the organic layer, and therefore is non-polar

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17
Q

what does a negative log P value suggest

A

the drug will extract into the aqueous layer and therefore is polar

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18
Q

what type of extraction produces Log P values

A

liquid liquid extraction

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19
Q

give an example of an acidic drug and why its acidic

A

aspirin - it is a carboxylic acid (only has one ionisation centre)

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20
Q

what happens to an acidic drug when its placed into water?

A

it is deprotonated and forms a COO- ion
the proton then binds to water molecules forming H3O+ ions (an acidic solution)

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21
Q

for extraction of acidic drugs what pH do you use?

A

2 units below the pKa

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22
Q

name an example of a basic drug and why?

A

amphetamine - has an amine group

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23
Q

what happens when a basic drug is placed into water?

A

the NH2 group is protonated into NH3, and the water molecules will lose a hydrogen, turning them into HO- ions ( a basic solution)

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24
Q

what pH do you use to extract basic drugs in liquid liquid extraction

A

2 pH units above pKa

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25
Q

give an example of an amphoteric drug

A

morphine

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26
Q

how many ionisation centres does an amphoteric drug have?

A

2

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27
Q

what pH do you use to extract amphoteric drugs in liquid liquid extraction

A

the average pKa of both ionisation centres - the isoelectrical point

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28
Q

list 3 disadvantages of liquid liquid extraction

A
  1. time consuming
  2. large volumes of solvents used - wastage
  3. sample reconstitution - HPLC grade solvents need to disposed of specially
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29
Q

what is the rule regarding chromatography and polarity

A

Chromatography is governed by polarity

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30
Q

what is TLC used for?

A

to separate components of a mixture such as inks

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31
Q

what is the stationary phase in TLC

A

a solid microparticulate stationary phase (silica) bound to a supportive backing (aluminium)

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32
Q

how can we view non-coloured compounds on a TLC plate

A

impregnate the silica stationary phase and use a UV lamp

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33
Q

what is the range for the diameters of the particles in the stationary phase in TLC, and why?

A

10-60um - the smaller the particle = better the separation = better results

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34
Q

what is the acceptable range for Rf values?

A

0.2-0.8mm

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35
Q

what is the Rf equation

A

distance travelled by the component (from the baseline to the centre of the dot)/
Distance travelled by the mobile phase (solvent front)

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36
Q

whats the method to setting up a TLC plate

A

Pour a small amount of the mobile phase into the solvent tank and allow it to equilibrate draw a pencil line on the TLC plate and add a number of crosses equidistant from each other for the samples. Use a capillary tube to add to samples allowing time for them to dry in-between. Place in the solvent tank and wait 30 mins. Then measure Rfs.

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37
Q

how does the mobile phase travel up a TLC plate

A

Capillary action

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38
Q

what is the mobile phase of TLC

A

a mixture of analytical grade solvents

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39
Q

what do the locating reagents do in TLC, to allow the separated components be visible

A

bind to functional groups within your components

40
Q

what does ninhydrin bind to in TLC

A

amino acids and produces purple spots - used in fingerprint detection

41
Q

what does the locating reagent Anisaldehyde/ antimony trichloride bind to in TLC?

A

steroids - produces a variety of coloured spots

42
Q

what does the locating reagent vanillin/ sulphuric acid bind to in TLC?

A

alcohol functional groups - producing pink, blue or green spots

43
Q

what do we use TLC for in forensics

A

screening for drugs and forged documents ink analysis

44
Q

what is gas chromatography used for

A

the separation of a complex mixture of volatile components (based on boiling points)

45
Q

what are the carrier gases used in GC?

A

pressurised, purified, inert gases, such as; nitrogen (capillary), helium (capillary) or hydrogen (packed) - depend on the column used

46
Q

what are the stationary phases of GC

A

high boiling point liquids, waxes, and oils - they have different polarities

47
Q

what does an autosampler do in GC?

A

draws up the required amount of sample into the syringe and then it is injected into the injector

48
Q

where does the injector release the sample in a GC?

A

the sample inlet which will then release it into the column inside the oven

49
Q

where does separation occur during GC

A

inside the column

50
Q

name the 3 injectors that can be used on capillary columns in GC

A

split
splitless
On-column

51
Q

name the two GC injectors used on packed columns

A

flash vaporisation and On-column

52
Q

what happens when a split injector is used in GC

A

only 2% of your sample reaches the column as the rest is vented to the atmosphere to prevent overloading. It also reduces sensitivity.

53
Q

what happens when you use a splitless injector in GC

A

all of your sample is condensed on the top of your column - where the temperature is just above the boiling point of the carrier gas. Your sample is trapped until the temperature is increased at the top of the column. This allows for the experiment to be much more sensitive, however it also allows for overloading of the column.

54
Q

what happens when you use an On-column injector on a capillary column in GC

A

all of the sample is condensed in a cooled zone on the top of the column. The sample is then volatilised by programmed heating, and then can be released into the carrier gas. This minimises degradation to thermally labile components

55
Q

what happens when you use an On-column injector on a packed column in GC

A

the sample is injected onto a packed bed and it minimises degradation to thermally labile components

56
Q

what happens when we use a flash vaporisation injector in GC and what can we not use it on?

A

the sample is injected into a hot zone above the column, where the temperature is around 20-50 degrees above the column temperature, causing the sample to immediately breakdown and volatize - DO NOT USE ON THERMALLY LABILE COMPONENTS

57
Q

what are the properties of a capillary column

A

long (up to 100m), narrow (extremely small diameter), coiled tube made of quartz, with a stationary phase coating the inner wall

58
Q

which column is more efficient; packed or capillary?

A

capillary

59
Q

what are the properties of a packed column in GC?

A

short tubes made of glass with a large diameter, packed with a granular solid made of silica particles (on top of the silica particles is a thin coating of a liquid stationary phase)

60
Q

what is the isothermal method regarding GC

A

the temperature remains the same throughout the analysis

61
Q

what is temperature programming in regards to GC?

A

where the temperature of the fan oven is set to change during analysis

62
Q

what do GC detectors produce

A

a chromatogram - a plot of retention time against relative abundance

63
Q

what is retention time

A

the time taken for your separated component to elute from the column

64
Q

name 2 universal GC detectors

A

Thermal conductivity & Flame ionisation

65
Q

how do thermal conductivity detectors work?

A

they detect changes (decreases) in the thermal conductivity of the carrier gas, which can then be compared to a reference flow of carrier gas

66
Q

how does a flame ionisation detector work?

A

the carrier gas elutes from the column and is mixed with hydrogen and air and then is burnt. This results in the production of ions which are collected at a negative electrode, which produces a current which is directly proportional to the concentration of the sample

67
Q

name 3 specific GC detectors

A
  1. Nitrogen-Phosphorus
  2. Electron Capture
  3. Mass spectrometer
68
Q

Explain what a nitrogen-phosphorus detector detects and how it does this?

A

Detects nitrogen and phosphorus containing groups. It is a modified flame ionisation detector that has a ceramic bead that is heated to 800 degrees. The modification allows for Nitrogen detection to be increased by a factor of 50 and Phosphorus by a factor of 500.

69
Q

what does an electron capture detector detect

A

specific to electronegative elements - group 7 elements e.g. chlorine, iodine etc.

70
Q

how does a mass spectrometer detector work?

A

samples are broken down into ions and fragments of a specific mass to charge ratio that can be identified

71
Q

what does HPLC do

A

separate a complex mixture into its component parts at room temperature

72
Q

what column is used in HPLC

A

a packed column

73
Q

how is the sample transported to the column in HPLC

A

by a pressurised flow of liquid mobile phase

74
Q

name the two types of separation used in HPLC and which is more commonly used

A

normal phase (not commonly used) and reverse phase (95% of the time)

75
Q

what mobile and stationary phase is used in normal phase separation in HPLC

A

mobile = a non-polar liquid - a mixture of hydrocarbons with an alcohol or chlorinated solvent
stationary = a polar substance - an unmodified silica

76
Q

what mobile and stationary phase is used in reverse phase separation in HPLC

A

mobile = polar solution e.g. methanol and water (must contain water)
stationary = non-polar - a modified silica (C18 or C8)

77
Q

what type of bottle is the sample placed into for HPLC

A

Schott

78
Q

what does a degasser do in HPLC

A

pumps helium into the mobile phase

79
Q

what do the pumps do in HPLC

A

2 reciprocal pumps ensure the mobile phase is pumped around the HPLC instrument at a constant and reproducible flow rate, with a high pressure

80
Q

what does the column compartment do in a HPLC machine

A

allows for temperature control

81
Q

what detector is used with a HPLC machine

A

a UV visible detector set to 254nm

82
Q

if the analyte is polar what should the mobile and stationary phase be?

A

mobile = non-polar stationary phase = polar

83
Q

what is solvent programming in HPLC

A

changing the composition of the mobile phase during the analysis

84
Q

name the two types of HPLC columns

A

conventional & microbore

85
Q

name the 4 features both HPLC columns share

A
  1. stainless steel
  2. variety of lengths
  3. contain different stationary and mobile phases
  4. mean particle diameters 3.5 and 10um
86
Q

what is the operating pressure and flow rate of conventional column in HPLC

A

OP = 500-3000 psi
FR = 1-3 ml/min

87
Q

what is the operating pressure and flow rate of a microbore column in HPLC

A

OP = 1000-5000 - increases sensitivity
FR = 10-100µl/min - costs less

88
Q

name the 4 HPLC detectors

A
  1. mass spectrometer
  2. UV visible absorbance
  3. Fluorescence
  4. Refractive index
89
Q

what can a mass spectrometer detector do in HPLC that no other detector can

A

provide positive identification

90
Q

what detector would you use to detect steroids in HPLC

A

Fluorescence

91
Q

which is the only HPLC detector that is universal

A

Refractive index detector as it looks at the RI changes in your mobile phase as your sample elutes from the column

92
Q

what are the forensic uses of GC and GCMS

A

Drug, accelerant, paint, plastic, polymer analysis

93
Q

what are the forensic uses of HPLC and LC-MS

A

Drug and Ink analysis - without the need for chemical intervention

94
Q

give 4 examples of polar solvents used in HPLC

A

water, acetic acid, methanol, ethanol

95
Q
A

forensic chemistry (7 decks)

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