Posted: July 25th, 2015

Chemical Analysis

Chemical Analysis

INSTRUCTIONS TO CANDIDATES
1.    There are NINE questions on this paper worth a total of 180 marks.

2.    You should complete ALL questions.

Question 1: X-Rays (5 + 5 + 5 + 5 + 3 + 2 = 25 marks)

a)    Explain the chemical principles behind the medical imaging method known as a CT scan. Your answer should include a description of the radiation source and how that radiation interacts in order to produce a medically useful diagnostic image. What applications can this be used for, and what are the advantages and limitations?

b)    Similar spectroscopic methods utilising X-rays include XRF and XRD techniques. What do XRF and XRD stand for? Explain the difference between these two methods.

c)    Suppose you have three different formulations of paracetamol tablets, where each formulation utilises a different polymorph of paracetamol in the solid state (a polymorph is a different crystal structure of the same material). You are given the task of analysing the different formulations to work out which polymorphic state of paracetamol is used in each of the formulations. You have also been asked to analyse for metal content in these tablets. Suppose also that the only instruments you currently have available to you are XRD and XRF. Explain which technique would you use to analyse for:
i) the polymorphic state, and

ii) the metal content.

d)    How would you prepare the tablets for the analyses above?

e)    Superman is rumoured to have X-Ray vision. Apparently he can see through brick walls using this amazing superpower. But is this realistic? If I set up an X-Ray source and detector, will I really be able to see through a brick wall? Briefly explain your answer.

f)    Is Ironman better than Superman?

Question 2: Drug tests (10 + 5 + 5 = 20 marks)
a)    There are a wide range of drug-test strips available for many different applications. Such tests have been recently introduced by Victoria Police to test to see if drivers have been taking drugs and driving. Equally, there are a lot of simple assays using strips to test for compounds like glucose and cholesterol. Choose a test for a particular drug or biomarker (there are test strips for amphetamine, marijuana, cocaine, cholesterol etc), and explain how the strip works. Your answer should include:
i)       Are other ancillary devices required to analyse the strip? Explain.
ii)      Are these methods of analysis quantitative? Explain your answer.
iii)     Describe one major limitation of the method?
b)    Because blood glucose monitoring is extremely important for diabetes patients, and the instances of diabetes is increasing dramatically, a lot of research has been undertaken to develop new sensing technology that is non-invasive and does not require a blood sample form the patient. One such technology is based on the electromagnetic emissions in the near-infrared part of the spectrum. All objects emit IR radiation, and both the intensity of the radiation and spectral characteristics of the object are determined by its absolute temperature as well as by the properties and states of the object. Researchers believe that, by placing a sensor close to the skin, these IR emissions can be used to estimate the amount of glucose in blood. Recent research has shown that glucose sensors based on this technology are at least as effective as the current electrochemical sensors requiring patients to place a blood sample on a test-strip.
i) How is it possible that you could use the total IR emission spectrum from the human body to specifically measure glucose concentrations?

ii) Would such an IR-glucose device require calibration? How might this be done?

c)    The use of cannabis for medicinal purpose has become a hot topic in recent times. Both leaves and seed contain a range of compounds that can potentially be very useful from a therapeutic perspective. However, many of these compounds are also problematic and potentially dangerous to long term health.
A recent article published in the journal of Nutrition stated the following:

‘The distribution of heavy metals content in cannabis leaf and seed were investigated. The heavy metals determined in samples were arsenic, cadmium, chromium, iron, nickel, lead mercury and manganese. The results obtained revealed that metal levels in cannabis leaf exceeded those of cannabis seed except in manganese, which had the highest metal content among all the metals determined. The results further showed that the As, Cd, Cr, Fe, Ni, and Hg levels in cannabis leaf exceeded those of the cannabis seed.’

i) How might you prepare the cannabis leaves for analysis? What instrument would you use?

ii) Would you change anything in the sample preparation for the analysis of the seeds compared to the leaves?

Question 3: pH, sodium chloride and cheese (5 + 10 + 5 + 5 + 5 + 5 + 5 + 5 = 45 marks)

a)    If I measure the pH of a particular phosphate buffer, the meter reads 7.2. If I add NaCl to this solution, the pH changes. Why does this happen? Is it that the NaCl is acidic or basic?

b)    Suppose the ionic strength of the pH 7.2 phosphate buffer mentioned above is 30 mM. Estimate the pH of this buffer solution if I add enough NaCl to give an ionic strength equivalent to that of blood (assume blood has an ionic strength equivalent to 0.9% w/v NaCl). Hint: OK, even though this is an open book exam, you might need to realise that this is a question about calculating and applying activity coefficients! (MW NaCl = 58.5 g mol-1)

c)    If I now try to measure the pH of blood plasma I find after a few measurements that the response of the electrode becomes very slow. Suggest a possible cause (I don’t want a list of every possible thing you may have written in your book, I want the most likely problems for this situation!).
In a determination of sodium chloride in a cheese sample, the Volhard method was used.

d)    After sample preparation, 50.00 mL of 0.1040 M AgNO3 was added. The titration of excess silver required 21.05 mL of 0.067 M KSCN with. If 10.0 g of cheese was used for the analysis, estimate the %w/w of NaCl in the cheese sample.

e)    The endpoint of the Volhard titration requires Fe3+ to be present in solution.  Suppose the solubility product for Fe(OH)3(s) is 1.0 x 10-28 (the fact that I have given you this information is a hint in this question). Is the pH of the solution to be titrated likely to be an issue in the Volhard method? Should a low or high pH be used? Explain.

f)    Refering to part e) above, red ferric thoicyanate complex becomes visible when the concentration of Fe is around 1 x 10-7 M. Estimate the maximum pH of a solution containing both Fe and SCN- so that the red FeSCN complex still forms.

g)    The general method for the Volhard method requires removal of the AgCl precipitate prior to titration with SCN-. Why?

h)    For titrations of Br and I- via the Volhard method, no filtration step is required. Why might we have to remove ACl, but not AgBr or AgI?

Question 4: A really big chromatography question (8 + 3 + 2 + 5 + 10 + 5 + 5 + 5 + 2 + 5 = 50 marks)

Barbiturates are drugs that act as central nervous system depressants, and can therefore produce a wide spectrum of effects, from mild sedation to total anaesthesia. They are also effective as anxiolytics, hypnotics, and anticonvulsants. Barbiturates also have analgesic effects; however, these effects are somewhat weak, preventing barbiturates from being used in surgery in the absence of other analgesics. They have addiction potential, both physical and psychological. Barbiturates have now largely been replaced by benzodiazepines in routine medical practice – for example, in the treatment of anxiety and insomnia – mainly because benzodiazepines are significantly less dangerous in overdose. However, barbiturates are still used in general anaesthesia, for epilepsy, and occasionally for the treatment of recurrent migraines and cluster headaches (under stringent protocols with mandatory physician monitoring for addiction and abuse). Barbiturates can also be obtained illegally, and clandestine laboratories around the world produce ‘cocktails’ of barbiturates to supply the illegal drug market.

Suppose you work in the ‘CSI’ laboratory, and you have been asked to quantify and identify a powder obtained from an illegal laboratory. It contains a mix of barbiturate compounds. In order to complete the quantification step you decide to use a HPLC method using a C8 reverse-phase column with an 60:40 acetonitrile:water mobile phase buffered at pH 3. A series of standards of five different barbiturates was run and the chromatogram below was produced. Detection by UV at 228 nm. The compounds corresponding to each of the peaks are given in Table 1.

Table 1: Peak identification of the barbiturates (with their concentrations) in standard solution. Numbers correspond to the peak in the chromatogram.

1.    Barbital  (0.4 mg mL-1)
2.    Phenobarbital  (0.1 mg mL-1)
3.    Talbutal  (0.4 mg mL-1)
4.    Amobarbital  (0.4 mg mL-1)
5.    Mephobarbital  (0.1 mg mL-1)

The structures of the barbiturate compounds used in the standard are below (in no particular order).

a)    Using the relative retention times in the chromatogram, and the information in Table 1, assign a name (barbital, phenobarbital, talbutal, amobarbital, mephobarbital) to the structures above (redraw the structures in your exam book).

b)    You will note from Table 1 that the standard solution contained phenobarbital and mephobarbital at a concentration of 0.1 mg mL-1, whilst all of the other compounds are present at the higher 0.4 mg mL-1 concentration. Despite the different concentrations used, the peak areas are approximately the same, explain why this might be the case.

c)    The pKa of all these barbiturates is about 4.5. Are they acidic or basic compounds? What is the functional group responsible for this behaviour?

d)    The pH of the mobile phase was set to 3. Why would this be the case? What would happen to the chromatography if the pH was set to 6?

e)    If the molar mass of barbital is 184 g mol-1, and the pKa is 4.5, calculate the percent of the compound that would be ionised in a standard solution where the concentration is 0.4 mg mL-1.

f)    Would the proportion ionised change from that you calculated above if the barbital is dissolved in the 60:40 acetonitrile:water mobile phase instead of pure water? Explain your answer.

g)    If instead of using a C8 column you switched to a cation exchange column where the mobile phase was set to pH 2, what would happen to the chromatography? You should comment on peak shape and retention time.

h)    OK, so now you switch to an anion exchange column where the pH is buffered to 8. What would you expect to happen to the chromatogram? Again comment on peak shape and retention time.

i)    Do you think you could use a GC method for this analysis? Explain your answer.

j)    If you look at the structure of these 5 molecules, would you be able to distinguish between them using i) IR spectroscopy and ii) HNMR spectroscopy. Comment on what differences you might observe in the spectra in order to differentiate the molecules.

Question 5: Multiple choice (1 mark each = 10)

Find the correct answer to each of the following.
i)     How many signals would you expect to see in the 1H NMR spectrum of the following compound?

A)  6
B)  3
C)  5
D)  4
E)  2

ii)     If a chemical shift of an NMR signal is 7.2 ppm measured on a 60 MHz NMR spectrometer, how many Hz would this signal be from the TMS signal?
A)  8.3 Hz
B)  432 Hz
C)  0.12 Hz
D)  72 Hz
E)  60 Hz

iii)     Using a 60 MHz spectrometer, the protons in dichloromethane appear at 5.30 ppm. When the same sample is placed in a 100 MHz instrument, where does the signal appear?
A)  8.33 ppm
B)  3.18 ppm
C)  5.30 ppm
D)  cannot be determined from information given

iv)    Which of the methyl groups in the compounds below will exhibit the most downfield (highest) chemical shift in 1H NMR spectroscopy?
A)  methyl amine
B)  methanol
C)  propanone
D)  propane
E)  toluene

v)    What is the ratio of the protons in the following compound?

A)  3:3:3:2
B)  6:3:2
C)  9:2
D)  3:2
E)  6:2

v)    What splitting pattern is observed in the proton NMR spectrum for the CH3 hydrogens in the compound below?

A)  singlet
B)  doublet
C)  triplet
D)  quartet
E)  septet

vi)    What splitting pattern is observed in the proton NMR spectrum for the CH2 hydrogens in the compound below?

A)  singlet
B)  doublet
C)  triplet
D)  quartet
E)  septet

vii)    How many signals will be observable in the DEPT 13C NMR spectrum of naphthalene (structure below)?

A)  0
B)  1
C)  2
D)  3
E)  10

viii)    Which of the following statements best explains the information we can gain from infrared spectroscopy?
A)  It allows us to determine the number of protons in a compound.
B)  It allows us to determine the kinds of functional groups in a compound.
C)  It allows us to determine the molecular weight and the mass of some fragments of a compound.
D) It allows us to determine the presence and nature of a carbocation in the compound.
E) It allows us to determine the presence and nature of a free radical in the compound.

ix)    Which of the compounds below would be expected to show intense IR absorption at 1715 cm-1?

A) CH3CH2CO2H
B) 1-hexene
C) 2-methylhexane
D) CH3CH2CH2NH2

x)    Which of the technique(s) listed below can readily distinguish between these two compounds?

A) 1HNMR
B)  IR
C)  13CNMR
D)  A and B
E)  A and C

Question 6: Some short answer ones (5 + 5 + 5 = 15 marks)

a)    Deduce the identity of the following compound from the spectral data given. C9H10O2: 13C NMR, d 18.06 (quartet), 45.40 (doublet), 127.32 (doublet), 127.55 (doublet), 128.61 (doublet), 139.70 (singlet), 180.98 (singlet); IR, broad 3500-2800, 1708 cm-1

b)    Deduce the identity of the following compound from the 1H NMR data given.
C8H10O: d 3.4 (3H, singlet), 4.5 (2H, singlet), 7.2 (5H, singlet)

c)    Spectra A and B below are infrared spectra of 2-methyl-1-butanol and tert-butyl methyl ether. Assign each compound its correct spectrum. Explain your reasoning.

Spectra A

Spectra B

Question 7: Putting it together    (15 marks)

Use the spectral data below to identify the major structural features evident in the following spectra, and hence identify the compound (C8H10O2). You should explain your reasoning and cross-reference between the IR, 1H, and 13CNMR, for justifying the proposed structure.