Report for Analytical Chemists
plenum U PUBLISHING
CORPORATION
The second volume in a new series of monographs and texts
PHYSICAL METHODS IN ORGANIC CHEMISTRY*
NMR SPECTROSCOPY IN ORGANIC CHEMISTRY SECOND EDITION By B. I. lonin and B. A. Ershov
Translated from Russian by C. Nigel Turton and Tatiana I. Turton Beginning with a comprehensive re-
view of the fundamentals of NMR spectroscopy, the book continues with a consideration of the broadest possible application of the technique to structural studies, to reaction mechanisms, reactivity, and kinetics in general organic chemistry. CONTENTS: Preface · The Fundamentals of NMR Spectroscopy · Chemical Shift Spin-Spin Coupling · Analysis of Complex Nuclear Magnetic Resonance Spectra · NMR Spectra and the Structure of Organic Molecules · Application of NMR Spectroscopy in Various Fields of Organic Chemistry · Literature cited · Appendix ·
Index.
April
Approx.
376 pages SBN 306-30424-4
1970
$25.00
GUIDE TO FLUORESCENCE LITERATURE
VOLUME 2 Director, By Richard A. Passwater, American Applications Research Laboratory, Instrument Company, Silver Spring, Maryland
When Volume 1 of the Guide appeared, the Journal of the Optical Society of America praised it as “a carefully prepared guide to a very voluminous literature.” Applied Optics lauded the volume and stated that its arrangement ‘‘facilitates the survey of the publications of any year or of any author." As a guide to the literature, this series saves valuable time for the scientist. In addition, it shows trends in research, suggests applications, opens channels of communication, and provides for an interchange of ideas between scientists. March 1970 374 pages $22.50 SBN 306-68262-1 * place order today for your continuation books in this series. It will ensure the delivery of new volumes immediately upon publication; you will be billed later. This arrangement is soley for your convenience and may be cancelled by you at any time.
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At very low concentrations it is uncertain whether an observed value for x is due to the presence of the substance to be determined or to uncontrolled chance perturbations. Such may have many different causes. Examples are impurities in reagents, losses by sputtering or absorption, errors of weighing or titration, temperature fluctuations in light sources, secondary reactions, thermal electronic noise in amplifiers, reading blunders, and dirt of all kinds. All such uncontrolled fluctuations are subsumed under the concept of “analytical noise.” The size of such fluctuations and their effect on the measure, x, can be grasped only in statistical terms, their distribution must be known— at least by its first and second moments, average and standard deviation. In general, these two moments cannot be predicted theoretically. The decisive step is therefore an experimental approach. In practice the magnitude of the fluctuations for each analytical procedure can be found numerically by making a sufficiently large number (at least 20) of blank analyses and then treating statistically the measures of xbl found in the course them. The average xbl and the standard deviation s&¡ are calculated. Such a series of analyses on a blank sample must be planned with critical consideration so that all causes for perturbations involved in the analytical procedure can play their full part. (Beware of idealized model analyses!) A criterion must be given to decide which observed value of the x can be accepted as genuine and which must be rejected because it is suspected to be only an accidental high value of the blank measure. This means the analytical signal, x, to be accepted must amount to a stated multiple (k) of the standard deviation sbi above the average xbi of the blank measures. This leads to the equation for the measure, x, at the limit of measure
detection: ?
=
Xti
+ k-Sbi
(18)
the analytical calibration function, c = f(x), the concentration at the limit of detection follows, c = f(x). This is therefore the lowest value of concentration From
Service Card
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
which the particular analytical procedure can ever yield. The definition of the limit of detection as given by Equation 18 seems to be generally accepted recently. It looks simple and well suited to routine application. However, its interpretation and correct use in the many different cases of analytical work require insight, imagination, and critical judgment. These problems cannot be treated in this article. To come to a common uniform understanding and to get comparable numerical values for the detection limits of the many
analytical procedures,
one
conven-
tion
is needed—the appropriate choice of the factor, k, in Equation 18. Some propose k = 3, some k 2, others wish to put this to a vote. The discussion is lively but, I am afraid, not always enlightened. The trouble arises from the misuse of the concept of “statistical confidence” which we discussed previously. It is entirely wrong to say that with respect to our problem the factor fc = 3 gives 99.86% confidence (one-sided!) or fc = 2 gives 97.7% confidence. (The author regrets that he also had this wrong opinion 15 years ago.) With fc = 3, we may only expect a confidence level of say 95% in most cases. Why? First of all, we are not allowed to presuppose a strictly normal distribution; we must take into account —
broader or asymmetric distributions. Tschebyscheff’s theorem gives for fc = 3 a confidence level of at least 89%, while fc 2 would yield only 75%, which is too low. Second, the values for xu and sbi in Equation 18 are estimates only, gained from a limited series. This approximation will cause an uncertainty of a few per cent that a measure x near to x will be wrongly accepted or rejected, even in the favorable case of a normal distribution of the measures. Third, experience by a number of authors, gained with different types of analyses over a number of years, has proved that the values for the limit calculated with fc = 3 in general agree well with estimates from inspection (spectrum lines, turbidity, color, etc.). On the other hand, there is no reason to choose a higher value of —
