Glass or fused silica

XIV

Glass or fused silica? Comparisons of Fused Silica and Other Glass Columns in Gas Chromatography, by W. G. Jennings, Hiithig, 1981. $19.001 DM38.00 (viii + 80 pages) ISBN 3 7785 0729 x

High resolution gas chromatography has been revolutionized by the appearance of glass and fused silica capillary columns. Both offer the advantages of very high resolution, faster analysis times and greater sensitivity. The big question is which column to use. This booklet tries to answer that question. The author, a well-known authority in the field, compares glass and fused silica with respect to: chemical composition, structure, inertness, methods of pretreatment, methods of evaluating inertness and manufacturing costs. The answer offered by this book is fused silica. The fact that fused silica columns are inherently straight, flexible and extremely durable make them extremely convenient to handle and install in both GC and GC/MS systems. Fused silica is produced by the reaction of SiCl+ and water vapor in a flame and the product, Si02, contains as much as 0.1% hydroxyl groups and less than one ppm of other impurities. This very high purity leads to a more regular lattice structure, a more uniform and inert surface that means less chemical pretreatment and a final product that is more inert, more reproducible and cheaper to manufacture (fewer rejects). Regular glass, on the other hand, contains large amounts of fluxes (oxides of sodium, potassium and boron) which decrease the viscosity and so lower the melting and working temperatures. Stabilizers, such as the oxides of calcium, magnesium and alumina, are also added to increase corrosion resistance and to inhibit crystal formation. Unfortunately, these impurities produce Lewis acid sites which result in several undesirable chromatographic properties: irreversible adsorption of some compounds, non-homogenous surface activity which makes uniform film coatings difficult, and incompatibility with some polar liquid phases. Prag-

trendsin analytical chemistry, vol. I, no. 7,198.?

matic techniques of pretreatment such as acidic leaching, addition of surfactants, salt deposits and thermal decomposition of deposited films, have been developed and result in inert glass columns. The author presents a series of very good chromatograms made on fused silica columns. Unfortunately, he offers none on glass. The book is well written, easy to read and well referenced (except for the section on micropacked columns, where the pioneering work of Cramers is omitted altogether). The obvious convenience in handling fused silica is unquestionable. The arguments for a more inherently inert surface of fused silica are logically presented (but not proven) and the real question as to which

column is more inert with respect to a variety of sample probes and several levels of concentration is not addressed. There are abundant publications with very good chromatograms made on glass columns. None of these is shown or referenced. It is a disservice to interested readers that both sides of the issue are not addressed equally. It might well be that fused silica will become the column of choice, but the reviewer feels that not all the questions have been asked, or answered. HAROLD M. McNAlR

Harold M. McNair

is Professor of Analytical Chemistry at Virginia Polytechnic Institute, Blacksburg, VA, U.S.A.

Essential reading for the 15N-NMR spectroscopist

15N-NMR $ectroscopy: NMR Basic Principles and Progress Vol. 18 by G. J. Martin, Maryvonne L. Martin and J.-P. Gousenard, Springer- Verlag, 1981. DM148.OOl$70.50 (vii + 382 pages) ISBN 3 540 10459 3

In this very useful series of books edited by P. Diehl, E. Fluck and R. Kosfeld, attention has now been focussed on another nucleus whose NMR spectroscopy was, until recently, almost impossibly diflicult. This was due to a combination of low abundance, low sensitivity and adverse relaxation characteristics. To this reviewer, at least, it came as a suprise to find 15N spectral information listed for some 900 compounds; this tabulated data alone makes this book worthwhile. However, what I particularly liked, in a pleasantly unpretentious book, was the clear and succinct explanation of the difficulties of *5N spectroscopy and how these may be overcome experimentally. In some very economic writing the authors establish the importance of relaxation, discuss the contributions from the various different mechanisms and make useful contrasts with tsC phe-

nomena. They point out the crucial fact for biochemical studies that the dipolar mechanism which is relatively unimportant in small molecules becomes efficient as the molecular correlation time increases (although not beyond a certain limit) and this allows rapid signal averaging of free induction decays. The important nuclear Overhauser effect is also discussed in some detail. But it would be wrong to give the impression that this book accentuates unduly the basic physical principles involved in observing t5N-NMR spectra. In fact much more practical, but equally important, considerations receive attention. These include a brief review of experimental techniques, including conventional single pulse methods, INDOR, double resonance in the Fourier Transform mode and methods of polarization transfer. Sample preparation, the co-axial cell system used for referencing and the whole problem of references are covered. Further chapters consider medium effects on both chemical shifts and coupling constants, the chemical shifts themselves and the coupling constants themselves. The final chapter discusses the application of i5N spectroscopy to the study of