INSTRUMENTATION BY RALPH H. MÜLLER
Improved Precision Needed in Molecular Weight Determinations
Tlecular HE
THREE-DAY S Y M P O S I U M
Oil IÛO-
weight determination organized and conducted b y Professor Sidney Siggia and sponsored b y t h e University of Massachusetts a t Amherst was held on August 7 to 9. Several lectures and afternoon laboratory sessions afforded a well-rounded account of t h e contemporary status of t h e subject. The lecturers a n d their topics were: Wolfgang Schulz, Esso Research a n d Engineering Co., " C u r r e n t State of D e velopment of Molecular Weight D e t e r m i n a t i o n s ; " Edward W . Westhead, University of Massachusetts, "Ultracentrifugation ; " Roger S. Porter, University of Massachusetts, "Viscosity M e t h o d s ; " Ralph H . Miiller, Louisiana State University, "Vapor Pressure Osm o m e t r y ; " Myer Ezrin, DeBell a n d Richardson, Inc., "Methods Based on Colligative P r o p e r t i e s ; " Fred W . Billmeyer, Jr., Rensselaer Polytechnic I n stitute, " M e m b r a n e O s m o m e t r y ; " J . Gordon Hanna, Conn. Agricultural E x periment Station, " ( E n d Group) D e t e r m i n a t i o n ; " Jack B . Carmichael, University of Massachusetts, "Gel Permeation ( T h e o r y ) ; " Karl B o m baugh, Waters Associates, "Gel Permeation (Practical) ; " a n d J . P . Kratohvil, Clarkson College, "Light Scattering." In keeping with contemporary interests and problems, attention was focused primarily on high molecular weights as in polymers or m a n y large molecules of biological importance. As far as colligative properties a r e concerned, low molecular weights should furnish more precise results. Despite great instrumental advances, we gain the impression t h a t further precision would be very valuable. At this stage, it does not seem t h a t a molecular weight should merely check a n d establish t h e correctness of elemental analysis. If we could determine molecular weights to a t e n t h of a p e r cent or better, a load could be taken from the microanalyst's shoulders. T h e currently accepted values for C, H , N , a n d Ο are still r e garded as permissive and about the best t h a t can be done. W e do n o t know whether the proceedings of this sym
posium will be published. Several speakers emphasized t h e need for a compilation covering recent advances. Thus the excellent a n d eminently p r a c tical monograph "Number-Average Molecular Weights" b y R . V. Bonnar, M . Dimbat, and P . H . Stross, Intersci ence Publishers, Inc., N e w York, 1958, necessarily omits developments of the last decade. D r . Siggia's comments and direction of discussion were most help ful. We were delighted b y his associ ates and research students and their en thusiasm for analytical chemistry. N o trouble about t h e status of analytical chemistry in t h a t pleasant neck of t h e woods. H e remarked, with evident pleasure—"some of m y best friends here are organic chemists." Evidently they have found some one who can and is willing to solve their analytical prob lems. W e are pleased to note t h a t he is Chairman-Elect of the Analytical D i vision of the ACS (see page 61A, this issue). This post has been held b y many eminent analysts. H e brings t o it an impressive amount of industrial research experience, a love of teaching, and an infectious zest for tackling diffi cult problems.
ESCA Electron Spectroscopy for Chemical Analysis
We have just received our copy of the large monograph on this subject from Sweden; t h e sub-title: "Atomic, Molecular, a n d Solid State Structure Studied b y Means of Electron Spectros copy." Kai Siegbahn and ten associates have compiled this record of researches b e gun by Siegbahn in 1951. T h e work emanates from the Institute of Physics and t h e Institute of Chemistry of t h e University of Uppsala, the Research Division of Pharmacia A B , Uppsala and t h e D e p a r t m e n t of Physics, Chalmers University of Technology in Goteborg. I t is published by Almquist and Wiksells Boktryckeri A B , U p p sala, 1967. T h e electron energies which can be measured a t present range from 1 MeV down t o 0.01 eV—i.e., 26
octaves. T o quote from the introduc tion, "ESCA is based on a magnetic or electrical analysis a t high resolution of the electrons which are emitted from a substance on irradiation with X rays. ESCA reproduces directly the electronic level structure from the in nermost shells to t h e atomic surface. All elements from lithium to t h e heav iest ones can be studied even if the element occurs together with several other elements a n d represents only a small p a r t of the chemical compound. This spectroscopy is characterized b y sharp electron lines a n d by high sensi tivity. T h e precision has been brought to the limit set b y t h e inherent widths of the atomic levels themselves. ESCA is applicable in a variety of fields of r e search in physics and chemistry. A p plications are, for example, found in organic chemistry, since the light ele ments carbon, nitrogen, oxygen, etc. are easy to study. Shifts of inner levels due to chemical structure effects are characteristic features a n d ESCA provides information on t h e chemical bonding in molecules." I t is curious t h a t although photo electric emission as a consequence of X - r a y excitation was known and ex amined 50 years ago b y Robinson in England a n d M . de Broglie in France, the accurate delineation of their en ergies had to await the development b y Siegbahn of a high resolution elec tron spectrometer. This development in 1954 revealed the existence of very sharp lines corresponding t o t h e bind ing energy of the relevant inner shells. Also the Auger effect which was largely ignored b y theoreticians as well as ex perimentalists accounts for electrons emitted in a radiationless return from an excited state produced b y X-rays. The Auger electrons have the advantage over X - r a y fluorescence methods for the very light elements. As is well known, the fluorescence yield is very low below atomic number 12 ( M g ) . A low fluorescence corresponds t o a high yield of Auger electrons, since t h e sum must equal unity. The experimental requirements for VOL 40, NO. 12, OCTOBER 1908 ·
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INSTRUMENTATION
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ANALYTICAL CHEMISTRY
these measurements are very critical. In the double focussing, iron-free spec trometer, resolution of these fine lines requires a precision of 1 in 105 in the measurement of momentum. Elegant instrumentation involving reference to a bank of standard cells and a Tinsley potentiometer of 1 ^.V sensitivity achieves this precision. The potentiometric system is automated for pro gressive scanning. Electrical counting is supplemented by photographing the electron spectrum, with a television microscope for scanning the plates. We select a few random examples from this extensive series of researches. An outstanding example of the sensi tivity is illustrated by the electron spectrum of a large molecule like vita min B 1 2 which contains only one cobalt atom among 180 atoms of other ele ments. A 100 Â layer of B 1 2 thus contains very few cobalt atoms. Nevertheless cobalt is easily observed in the electron spectrum; furthermore, its value state can be determined. Cobalt appears as Co (3s) and Co(3p) along with lines from oxygen (O Is) nitrogen (N Is) and carbon (C Is). Insulin has a molecular weight of about 6000 and contains 51 amino acids with three disulfide bridges, two of which bind the peptide chains together. There is approximately one sulfur atom for every 140 other atoms. A good signal was recorded from sulfur, and the binding energy obtained was consistent with the value for simple disulfides. The relative intensity was in accordance with the known content of sulfur. The electron spectrum from carbon in solidified benzene using the freezing technique shows that the Is electron line has a width of 1.5 eV. Also four sharp and well defined peaks appear in the general binding energy region of 20 eV arising from the molecular orbitals. Any high school student these days can throw orbitals around with great ease. Here we have definite experimental measurements of high precision defining their nature and magnitude. This is a refreshing and most welcome contribution to this branch of metaphysics. We expect to devote a considerable part of the next six months in digesting this vast amount of information and in consulting some of the 299 references to similar work. Siegbahn is a revered name in the physics of X-rays. The earlier one, Manne, received the Nobel prize. Kai Siegbahn is well known in this country and much work on ESCA is going on here. Scientists in a dozen fields will be looking into its manifold applications for the next 20 years.
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