TELLMETHEOLDO OR THE ANALYTICAL TRINI D. Betteridge Chemistry Department University College of Swansea Swansea SA2 8PP, UK
T h e writings of 19th century analysts are full of interest. As well as being written gracefully, they discuss analytical problems and novel techniques which are still a t t h e forefront of analysis. From them, one is tempted to conclude that, even in analysis, "plus ça change, c'est la même chose", b u t in the light of the progress which has been made during the last 100 years, this view does not bear close examination. W h a t does seem to be a constant factor is the interplay between theory, technique, and problems, which we will dub "the analytical trinity" and represent in the form favoured by alchemists and modern communicators (Figure 1).
THEORY
TECHNIQUE
PROBLEM Figure 1. The analytical trinity T h e necessity of employing new techniques to solve new problems is well recognised; indeed, it is often assumed t h a t problems generate techniques. T h e examples considered here show the converse; techniques may be developed well in advance of the problem for which they are best suited. T h e successful analytical chemist, desirous of finding solutions to practical problems posed by society, his employer, and fellow scientists, must be adept at a number of techniques and at the forefront of chemical knowledge. Such a paragon (complete with theory, technique, problem, and sponsor), as shown in Figure 2, is rare, and so the development of the subject depends on effective collaborations between industrialists, academics, and instrument makers.
It is argued that analytical chemistry depends on the interplay between technique, theory, and problems— the analytical trinity. The importance of the three to the development of the subject is illustrated by consideration of two 19th century growth points—flame spectroscopy and food analysis—and the work of Sorby, Lockyer, Accum, and Hassall. The science and the problems placed before the analyst have a very contemporary flavour.
These points are illustrated by consideration of crucial stages in t h e development of a technique—flame spectroscopy—and in the solution of a problem—food adulteration. Because analytical chemistry is responsive t o the needs of society, some social and biographical background is included. Technique—Spectrum Analysis Among all the discoveries of modern science none has deservedly attracted more attention, or called forth more general admiration, than the results of the application of Spectrum Analysis to Chemistry" said Roscoe in 1868 to the Society of Apothecaries (1). This was only eight years after Kirchoff and Bunsen had demonstrated the validity and value of the technique (2, 3). There were forerunners who have made important contributions, notably (4-8): Herschel who in 1823 recognised t h a t the colours imparted to flames by different objects "afford a ready and neat way of detecting extremely minute quantities of them"; Talbot who extended the qualitative
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work in 1826-34; Foucault who in 1849 demonstrated t h a t the D lines of sodium were coincident with the Fraunhofer lines; Wheatstone (1834), Masson (1845), and Angstrom (1853) who obtained spark spectra; and Miller (1845) and Alter (1854) who obtained the spectra of several metals. Most of these early experiments were confused by the presence of sodium, and it was not until 1856 t h a t Swan, while investigating hydrocarbons, demonstrated t h a t 0.4 p p m of sodium could be detected. This crucial measurement, which depended on the recent development of the nonluminous flame by Bunsen, was followed u p by Kirchoff and Bunsen who confirmed the extreme sensitivity of spectrum analysis. As physicist and chemist working together, they solved many of t h e theoretical and experimental problems which had been hindering progress and placed spectrum analysis on a secure foundation. Their contribution was not immediately appreciated by Miller who felt slighted by their arrogance and lack of acknowledgment of earlier contributions. He claimed t h a t ". . . it is obvious t h a t all of the principal facts in relation to the occurence of bands in the spectrum; were known before Kirchoff and Bunsen directed their attention to the subject" (7, 8). This drew from Kirchoff a devastating reply, which serves as a warning to all historians seeking evidence of antecedents. He explained why he considered much of the early work incompetent or irrelevant, e.g., Talbot could not decide whether the yellow colouration of the flame was due to sulphur or water, the spectra which Miller had reproduced were totally unrecognisable, and Swan's was peripheral to the original author's work (9). Their work did not need invective to sustain it. Within a year Crookes had discovered thallium, and several
Report
tions of colouring matters, all for which he was recognised by the award of the Royal medal of the Royal Society in 1874 and the presidency of several learned societies. Later, he branched into marine biology and equipped a yacht as a floating laboratory. He remained active in the affairs of the Sheffield Literary and Philosophical Society throughout his life, and in 1882, two years after it had opened, he became president of Firth College which later became the University of Sheffield. By 1864 Sorby had made major contributions to geology and metallurgy by novel application of the microscope. In t h a t year, he heard a lecture from Stokes on spectrum analysis, whereupon he wrote:
Figure 2. Paragon of an analyst The notebook reads, Absolutely pure cocoa. No chemicals". Possibly, this is an unauthorised likeness of A. H. Hassall; Queen Victoria was similarly used in advertisements. (Source unknown)
other workers were engaged in research. By 1869 Roscoe was able to append an extensive bibliography (over 140 papers on terrestrial applications) to his lectures {10). Some of the credit for the rapid diffusion is due to Roscoe, a former s t u d e n t of Bunsen, who was indefatigable in correspondence, translation, and communication, and also to Crookes whose Chemical News well merited its name. B u t another powerful factor was a well-developed urge to leap onto bandwagons and to rush into print. A full account of 19th century spectroscopy has been given by McGuicken (5). Szabadvâry (11) and Grove (4) have described the developments of analytical consequence. In this article
we shall discuss some of the a t t e m p t s to apply absorption and emission spectroscopy to analysis, touched lightly, if at all, by the above authors and, in particular, the work of Sorby and Lockyer. Sorby and the S p e c t r u m M i c r o scope. H. C. Sorby (1826-1908) was the son of a wealthy Sheffield cutler. He did not wish to enter the family business, and after private tuition he became a very successful gentleman scientist (12). (At the time, private tuition was the best way to obtain a scientific training. Science was hardly taught in the universities.) He pioneered petrography, the microscopic examination of metals, and the spect r u m microscope and made investiga-
It immediately occurred to me that a spectroscope might be combined with a microscope and employed to distinguish coloured minerals in thin sections of rocks and meteorites. I was soon led to examine many other coloured substances, and found that the instrument is more useful in connection with qualitative analysis when only very small quantities of material can be obtained. He soon designed the spectrum microscope (Figure 3) (13) which was improved by Browning, the well-known London optical instrument maker. (Browning's contribution to the subject is revealed by R. Routledge in "Discoveries and Inventions of the Nineteenth Century", 8th éd., p p 330-59, Routledge, London, 1891.) T h e sample is placed on a microscope stage, light is shone through it, and the spectrum is observed with a spectroscope. With this microscope he embarked on a 10-year programme of analytical research, which amounted to an investigation of the utility of visual absorptiometry for the qualitative
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CitRMKur, NKWS, » Jtpril 21, 1805. /
Spectrum Analysis in Microscopical
Investigations.
The general construction will be more readily under· stood by reference to the accompanying Figure :—• SrECTRVM MICROSCOI'E.
Figure 3. Sorby's spectrum microscope "The only addition to the microscope itself is the prism (e) and its mounting shown by the dark lines. This fits into the bottom of the moveable tube carrying the achromatic condenser (f). Detached from the microscope is a long narrow slit, shown in section at (b c); and the light passing through this at (a) is separated by the prism (e) and passes onto (g), where the image of the slit is seen as a coloured spectrum, on looking through the microscope in the usual manner. By this arrangement we can determine the character of the light transmitted by an object placed on the stage at (g), or by one held in front of the slit at (a); and by a little adjustment, we can compare the two spectra side by side." ( 13)
analysis of small samples. Typical of the many investigations are the identification of bloodstains (14,15) and the determination of the age of wines (16). He found t h a t the absorption spect r u m of blood was diagnostic of carbon monoxide or cyanide poisoning. He managed to detect 1/1000 part of a grain of blood in a stain, to differentiate blood stains from other stains, and to deduce the age of a blood stain, but he was unable to distinguish animal from h u m a n blood. T h e forensic significance of this work was recognised and put to the test in the "Elt h a m Murder Case" of 1871 (17). T h e validity of the scientific evidence was disputed in the pages of The Lancet (against) and the British Medical Journal (for). T h e view was expressed in the The Lancet t h a t in contrast to the bright lines observed in emission spectra, the absorption spectrum amounted to "a little dimness here and there". In the event, the accused admitted the stains to be blood, produced a reasonable account for their presence, and was acquitted, but the results of the spectrum analysis had been admitted as scientific evidence. jW. B. Herapath states t h a t with the spectrum microscope he detected blood on a murder weapon which had lain in a wood for several weeks. He implies t h a t this evidence was admitted at the Swansea Spring Assizes of
1866 in the case Reg. vs. Robert Coe [Chem. News, 17,124 (1868)]; therefore, this may be an earlier forensic application.) Lockyer, AAS, and Emission Spectroscopy. T h e main developments in spectrum analysis during the
1860's and 1870's were in the realm of what we now recognise as atomic spectroscopy. Many contributed to the growth, but Lockyer's book, published in 1878, which combines accounts of analytical investigations, theoretical interpretations, and solar research, provides a convenient point of focus (18). Sir Joseph Norman Lockyer (1836-1920) was another privately educated man. He was a clerk in the War Office at the age of 21, and while assisting in the reorganisation of t h a t Office, performed some notable research in astronomy. He was encouraged by T. H. Huxley to become the founder editor of Nature. While he was making t h a t journal a notable success, he continued with his work in spectroscopy, discovering helium in the sun, advancing a dissociation (i.e., ionization) hypothesis to account for the complexity of spectra, and performing the first truly quantitative analyses by emission spectroscopy (19-21). He described the apparatus required at some length (22). T h a t for absorption measurements (Figure 4) is clearly an advance on t h a t shown by Roscoe and Schorlemmer (Figure 5), which must surely be the earliest flameless-cell AAS apparatus. With Lockyer's apparatus the interference of oxygen is greatly reduced. T h e cell, an iron tube with detachable glass
—Arrangement for observing the absorption spectra of metallic vapour*.
Figure 4. "Arrangement for observing the absorption spectra of metallic vapours" (23) Path length, 4 ft. Furnace fuel, coke or charcoal. Temperature gauged by intensity of sodium Dline and extent of spectrum. Source, electric arc lamp
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plates at each end, is filled with hydrogen and heated by the furnace to the desired temperature. T h e n the metal sample is introduced by a side arm. At the conclusion of each experiment, one of the glass plates was removed and the "presence of excess'hydrogen in the tube, conclusively proved by igniting it at the open e n d " (25). There were variations on the apparatus shown in Figure 4. If higher temperatures were required, the cell was replaced by one of chalk, which was then heated with an oxyhydrogen flame (26). If heating could be dispensed with, a multiple p a t h reflectance cell 40 ft long could give an effective p a t h length of 120 ft. This last was used to measure the spectrum of ozone in oxygen (27). Absorption spectra were also obtained by aspirating a solution, from a perfume spray, into the flame. A few years ago we made a s t u d e n t experiment out of the construction of a primitive atomic absorption apparatus, similar to t h a t shown in Figure 5. Even with a tungsten lamp as a source, the absorption was difficult to see ("a little dimness here and t h e r e " vide supra). We greatly admired the experimental skill of those early workers whose results have stood the test of time. Lockyer's a p p a r a t u s would have given good results. He obtained emission spectra of good quality by a rearrangement of the apparatus shown in Figure 6. T h e arc lamp was used as a source, the sample being placed on one of the electrodes. He carefully investigated the changes in spectra which are dependent with amount of sample via the line length, the point a t which lines from different elements in alloys show equal intensity and changes in relative lengths of line pairs. These in-
vestigations, the first part of which was published in 1873, led in 1874 to a collaboration with W. C. Roberts, the Chemist of the Mint, and the unambiguous quantitative analysis of various Zn/Cd and Au/Cu alloys (28-33). T h e readings were taken from the micrometer settings of the spectrometer wires, and an accuracy of 0.01% was claimed. T h e composition of the alloys was determined by parting analysis, and in the one instance where a point fell off the calibration curve, the error was in the parting analysis. However, the curves shown only apply to a narrow range of concentration 50-54% Cd in Zn/Cd and 90-92.25% Au in Au/Cu alloys. Hartley confirmed the method and Lockyer's claim for originality (34, 35). By 1878 spectrum analysis had been widely published and developed to the point where atomic absorption spectroscopy (with flame and flameless cells), multiple-path length absorbance cells, micro-sample cells, and emission spectroscopy had all been placed on a sound experimental footing. Why, then, were they not used analytically for almost 100 years? In the context of the analytical trinity, we may suppose t h a t there were faults in technique and/or theory, or there were no suitable problems. T h e evidence above suggests t h a t the technique had developed at a very satisfactory rate, and further developments could have been made had there been a demand. T h e lack of a suitable theory undoubtedly caused difficulties because of the appearance of inexplicable lines. Lockyer did have an account of absorption—"particles absorb light of the same wavelength and of greater amplitude passing through t h e m " (36)—but he got hopelessly bogged down in his interpretation of emission spectra. However, one
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Figure 5. Earliest flameless-cell AAS? (24)
Figure 6. "Arrangement of electric lamp for demonstrating the existence of long and short lines." (28). Emission spectrum shown on a screen
feels an analyst with a need to use the methods would not have been held back for very long. One concludes t h a t the analytical problems for which they now seem best suited were not at the time recognised as "problems". T h e average analyst could perform the analyses requested of him with accuracy and within an acceptable time by gravimetry or titrimetry and thus had no need for expensive, fiddly apparatus. In the 19th century the technique was applied with great success to the analysis of celestial bodies, and it proves an example of an analytical technique making a major contribution to the development of science.
Problem—Adulteration of Food Inadequate sanitation and heavily polluted water and air were inevitable consequences of the increase in urban population and the rapid development of new industries, which took place during the Industrial Revolution (37-39). This gave rise to a major debate on public health, the intensity of which may be gauged from the cartoons shown in Figure 7. One part of the discussion concerned the adulteration of food which had become easier and profitable as production and marketing methods changed to meet the new conditions of society. Bread, beer, coffee, tea, wine, spirits, and other basics were adulterated on a large scale. T h e situation is revealed by Punch (Figure 8) and by Normandy's quotation of Ure (40): Some beers possess a remarkable narcotic power, by which they cause drowsiness and stuper without corresponding previous exhilaration. Such beverages may justly
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976 · 1037 A
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