GCE. Chemistry. Edexcel Advanced Subsidiary GCE in Chemistry (8CH01) Edexcel Advanced GCE in Chemistry (9CH01) Analysis of Chemicals for AS

Transcription

1 GCE Chemistry Context study Edexcel Advanced Subsidiary GCE in Chemistry (8CH01) Edexcel Advanced GCE in Chemistry (9CH01) Analysis of Chemicals for AS October 2007

2 Edexcel, a Pearson company, is the UK s largest awarding body offering academic and vocational qualifications and testing to more than 25,000 schools, colleges, employers and other places of learning here and in over 100 countries worldwide. Our qualifications include GCSE, AS and A Level, GNVQ, NVQ and the BTEC suite of vocational qualifications from entry level to BTEC Higher National Diplomas and Foundation Degrees. We deliver 9.4 million exam scripts each year, with over 3.8 million marked onscreen in As part of Pearson, Edexcel has been able to invest in cutting-edge technology that has revolutionised the examinations system, this includes the ability to provide detailed performance data to teachers. References to third party material made in this specification are made in good faith. Edexcel does not endorse, approve or accept responsibility for the content of materials, which may be subject to change, or any opinions expressed therein. (Material may include textbooks, journals, magazines and other publications and websites.) Authorised by Roger Beard Prepared by Sarah Harrison All the material in this publication is copyright Edexcel Limited 2007

3 Contents Introduction 1 Analysis of chemicals 3 Unit 1 topic 1.5: Atomic structure and the periodic table 3 The age of the Earth 3 Determining the composition of the solar wind 4 The pharmaceutical industry 4 Drugs testing for athletes 5 Accurate relative atomic masses 5 Unit 2 topic 2.12: Mass spectra and IR 7 Analysis of organic molecules 7 References 9

4

5 Introduction This document is designed to help teachers to understand the contemporary context of analysis of chemicals. It should give teachers information on this context and on how to research it further if they wish. This document could also be given to students as introductory material. 1

6 2

7 Analysis of chemicals Mass spectrometry (MS) is an extremely valuable analytical tool used over a broad range of scientific disciplines. It is often used in conjunction with other techniques such as gas chromatography (GC), high performance liquid chromatography (HPLC). Unit 1 topic 1.5: Atomic structure and the periodic table The age of the Earth Geologists can date rocks or even obtain the age of the Earth. Here is an example using the rubidium/strontium dating method. Natural rubidium consists of two isotopes, 85 Rb and 87 Rb. The latter is radioactive with a halflife of nearly fifty billion years. It emits a β particle and turns into the non-radioactive isotope 87 Sr. Natural strontium also contains 87 Sr. Most minerals that contain rubidium also contain some strontium, so the age calculation must take into account the presence of the 87 Sr at the beginning of the time interval. As molten rock material cools, first one mineral and then another solidifies. Rubidium is crystallised with potassium compounds fairly late in the solidification process together with some strontium compounds. Different rocks of the same age will have different Rb:Sr ratios although the initial 87 Sr: 86 Sr ratio will be the same in all samples. If there is a non-radiogenic isotope of the daughter element present in the mineral, it can be used as a reference, and the ratios of the parent and daughter elements plotted as ratios with that reference isotope. The slope of the curve then gives the time interval. (Non-radiogenic means that it is not produced by radioactive decay.) The rubidium/strontium dating method uses the non-radioactive isotope strontium-86 as a comparison standard. The relative amounts of 87 Sr to 86 Sr and 87 Rb to 86 Sr in different rock samples believed to be the same age are determined with great precision using a highresolution mass spectrometer. A plot of these points, shown in figure 1, fits a straight line. The slope of this graph can be used to calculate the time since the solid rock formed from the molten material, ie the age of the Earth. The 4.5 billion year age for the Earth is consistent with the results of the potassium/argon and uranium/lead estimations. Similar results are also obtained from the study of spontaneous fission events from uranium-238 and plutonium

8 Figure 1 Plot of 87 Sr v 87 Rb standardised against 86 Sr (Source: Kane 1987) For figure 1, the half-life, t 1/2, for 87 Rb = 4.88 x years, so the age of the rock = 4.53 x 10 9 years. Determining the composition of the solar wind An exciting example of the use of mass spectrometers in astronomy is to determine the composition of the solar wind. The Solar and Heliospheric Observatory (SOHO) satellite has a group of instruments aboard known collectively as the Charge, Element and Isotope Analysis System (CELIAS). Up-to-date results indicate that hydrogen and helium make up 99.9 per cent of the wind. The sensitive mass spectrometer has made measurements of the trace constituents, which include isotopes of silicon, sulphur, calcium, chromium, iron and nickel. Isotopes of neon and argon have been discovered in the wind, which were undetected in earlier satellite missions. The isotopes provide clues about where the solar wind originates in the dynamic structure of the sun. The pharmaceutical industry Mass spectrometry provides four essential requirements for the pharmaceutical industry, namely: sensitivity the accurate measurement of small samples selectivity can differentiate similar chemical environments even in stereoisomers speed fast turnaround times for analysis high throughput dealing with large volumes of samples. It is a valuable tool in each stage of drug development and has become the preferred analytical method for trace-mixture analysis. It is used in conjunction with sample preparation, and chromatographic separation using GC or HPLC. 4

9 Modern methods of surface chemical characterization play an important role in the study and development of pharmaceutical products. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is one of the most important surface analysis techniques. (Source: CSMA laboratories) The energy of an ion produced by electron bombardment is proportional to the square of its velocity, so the heavier the molecule the more slowly it passes through the spectrometer. In ToF-SIMS, short electrical pulses are applied to the material being studied. The time taken for the ions released to travel to the detector is measured. This time depends on the m/z value of the molecular ion. No magnetic deflection or focusing is required with this method. The two interesting case studies below concern analysis of contamination of pharmaceutical products. These typically arise post production and show the versatility of mass spectrometry. 1 Tablets from a commercial manufacturer were turning yellow (from white) on exposure to light. Mass spectrometry revealed that light was converting a CH 2 COOH group to an aldehyde group leading to a new structure with a molecule of molar mass 30 less than the original. 2 A certain liquid drug showed signs of micro-crystallization, in the bulk. Careful extraction of a crystalline region onto filter paper followed by ToF-SIMS revealed that a polymer additive in the bottle stopper had leached out and reacted with the drug molecule. Controlled release drug-delivery systems for heart or diabetic patients allow improved quality of life with fewer side effects and increased life expectancy. One approach is to encapsulate a drug bead within a multi-layer polymer coating where the properties of the latter control the rate of drug release. Since the production of this type of multi-component system is difficult, analysis of the final product is vital. Mass spectrometry (ToF-SIMS) has proved ideally suited for this task and can provide an assessment of the thickness and uniformity of the coating layers as well as the distribution of the drug and other excipients (released substances) within the bead. Drugs testing for athletes Unfortunately a small minority of athletes who want to enhance their performance use drugs such as anabolic steroids sometimes during competitions. The same techniques that chemists use to analyse new products can of course be applied to drug testing. Accurate relative atomic masses A traditional example of the uses of mass spectrometry is where chemists and physicists need to know accurate relative atomic masses (RAM). The relative abundances of the isotopes of an element may be obtained with a mass spectrometer. For example, the relative abundances of krypton isotopes is shown in figure 2. 5

10 Figure 2 Experimental mass spectrum of krypton (Source: Krane 1987) Krypton isotope mass relative abundance 78 Kr Kr Kr Kr Kr Kr Table 1 Calculation to obtain the relative atomic mass (RAM) (Source: A weighted average of the isotopes in table 1 above gives a value of 83.90, the accepted atomic mass of krypton which appears in the periodic table. Other isotopes of krypton are known, but they do not appear in natural samples because they are unstable (radioactive). 6

11 Unit 2 topic 2.12: Mass spectra and IR Analysis of organic molecules In the analysis of organic molecules the technique of mass spectrometry, infrared spectroscopy and nmr spectroscopy are all used in the detective work of elucidating a structure. As a common example, a simple molecule ethyl ethanoate will be analysed here. Bear in mind that a microanalysis giving the empirical formula will invariably be available to the chemist. In this example the empirical formula is C 2 H 4 O. Figure 3 The mass spectrum of ethylethanoate The mass spectrum is given in figure 3. The final peak at 88 is the molecular ion M + and is extremely useful information. This confirms C 4 H 8 O 2 as the molecular formula, but there are four structural isomers within this formula, ethylethanoate, CH 3 COOC 2 H 5, methylpropanoate, CH 3 CH 2 COOCH 3, 1-propylmethanoate, HCOOCH 2 CH 2 CH 3 and 2-propylmethanoate, HCOOCH(CH 3 ) 2. (A very small peak at M+1 may be seen in some spectra. This is due to the presence of one atom of the C 13 isotope in a molecule, giving a mass of one more than the molecular ion.) Under the electron bombardment in a mass spectrometer, esters break at the C-O bond producing a stable fragment of formula RCO +. This is usually the most intense (longest) line in the spectrum. Ethylethanoate will therefore have a major line at m/z = 43 due to the CH 3 CO + ion, methylpropanaoate will have a major line at m/z = 57 due to the CH 3 CH 2 CO + ion and the two methanoate esters will have a major line at m/z = 29 caused by the HCO + ion, as will the others, but this line is then caused by the CH 3 CH + 2 ion. 7

12 Further fragmentation of the main fragments is likely and a common peak at 15 is due to the methyl ion, CH + 3, which again is almost invariably seen. The mass spectrum does not give the order of the groups but does tell which of the main groups are present. A trial guess at the spectrum say ethyl ethanoate can explain the fragmentation pattern, whereas another guess say ethanal could be eliminated. The infrared spectrum is presented in figure 4. Figure 4 The IR spectrum of ethylethanoate (ethyl acetate) For convenience the wave numbers of the absorptions are shown. The key peak to focus on occurs at 1743 cm -1. This is a very large absorption and is characteristic not only of the carbonyl group but more specifically the carbonyl component of the ester group range ( cm -1 ). C-H stretching at around cm -1 will be almost invariably be present but in this case is indicative of the CH bonds in alkanes. Sometimes this absorption is seen as a shoulder on an O-H peak. Often it is useful to note missing peaks such as a broad peak around , which eliminates O-H or COOH. Whilst C-O stretching ( cm -1 ) might be a useful peak to look for. The remaining peaks in the fingerprint region (ie less than 1400 cm -1 ) may be used to confirm the exact compound being analysed by comparing the spectrum with a database such as SDBS. 8

13 References Lee M S Spectroscopic methods of analysis Mass spectrometry Vol 1 issue 1, pp (available online ) Morrison R T and Boyd R N Organic Chemistry, 5th edition (Allyn and Bacon, Inc, 1987) Structure spectroscopy examples for GCE Chapman B Transition Metal, Quantitative Kinetics and Applied Organic Chemistry (Nelson, 2001) Houghton J Global warming: The complete briefing, 3rd edition (Cambridge University Press, 2004) Extra reading A detailed and in-depth overview of current MS technologies and applications can be obtained from the recent proceedings of the American Society for Mass Spectrometry Conference on Mass Spectrometry and Allied Topics ( and the Association of Biomolecular Resource Facilities ( Websites Dr Rod Beavon s Chemistry pages Extended periodic table rl181007S:\LT\PD\SUPPORT\GCE in Chemistry Analysis of Chemicals AS CS.doc.DOC.1-14/0 9

14 October 2007 For more information on Edexcel and BTEC qualifications please contact Customer Services on or or visit our website: Edexcel Limited. Registered in England and Wales No Registered Office: One90 High Holborn, London WC1V 7BH. VAT Reg No