The analytical chemist

During this phase, little thought was directed to the wider problems which confront a laboratory which changes from a regime based on manual analysis to one dependent on automation. However, many laboratories now have several years of experience with automated systems, and it is possible to address the broader issues. Brief consideration is given here to the problems which the introduction of automated analysis pose for the analytical chemist and for the laboratory management. 

In summary, the introduction of automation into an analytical laboratory leads the analytical chemist, unwillingly in some instances, away from experimental individualism 

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Hyphenated analytical methods usually give rise to iacreased confidence ia results, eaable the handling of more complex samples, improve detectioa limits, and minimi2e method development time. This approach normally results ia iacreased iastmmeatal complexity and cost, iacreased user sophisticatioa, and the need to handle enormous amounts of data. The analytical chemist must, however, remain cogni2ant of the need to use proper analytical procedures ia sample preparatioas to aid ia improved seasitivity and not rely solely on additional iastmmentation to iacrease detection levels. 

It is becoming more and more desirable for the analytical chemist to move away from the laboratory and iato the field via ia-field instmments and remote, poiat of use, measurements. As a result, process analytical chemistry has undergone an offensive thmst ia regard to problem solviag capabihty (77—79). In situ analysis enables the study of key process parameters for the purpose of definition and subsequent optimization. On-line analysis capabihty has already been extended to gc, Ic, ms, and ftir techniques as well as to icp-emission spectroscopy, flow iajection analysis, and near iafrared spectrophotometry (80). 

When John Phillips, in 1991, presented the practical possibility of acquiring a real comprehensive two-dimensional gas chromatographic separation (33), the analytical chemists in the oil industry were quick to pounce upon this technique. Venkatramani and Phillips (34) subsequently indicated that GC X GC is a very powerful technique, which offers a very high peak capacity, and is therefore eminently suitable for analysing complex oil samples. These authors were able to count over 10 000 peaks in a GC X GC chromatogram of a kerosine. Blomberg, Beens and co-workers... 

Sulfur Compounds. All crude oils contain sulfur in one of several forms including elemental sulfur, hydrogen sulfide, carbonyl sulfide (COS), and in aliphatic and aromatic compounds. The amount of sulfur-containing compounds increases progressively with an increase in the boiling point of the fraction. A majority of these compounds have one sulfur atom per molecule, but certain aromatic and polynuclear aromatic molecules found in low concentrations in crude oil contain two and even three sulfur atoms. Identification of the individual sulfur compounds in the heavy fractions poses a considerable challenge to the analytical chemist. 

In a modern industrialised society the analytical chemist has a very important role to play. Thus most manufacturing industries rely upon both qualitative and quantitative chemical analysis to ensure that the raw materials used meet certain specifications, and also to check the quality of the final product. The examination of raw materials is carried out to ensure that there are no unusual substances present which might be deleterious to the manufacturing process or appear as a harmful impurity in the final product. Further, since the value of the raw material may be governed by the amount of the required ingredient which it contains, a quantitative analysis is performed to establish the proportion of the essential component this procedure is often referred to as assaying. The final manufactured product is subject to quality control to ensure that its essential components are present within a pre-determined range of composition, whilst impurities do not exceed certain specified limits. The semiconductor industry is an example of an industry whose very existence is dependent upon very accurate determination of substances present in extremely minute quantities. 

An indication has been given in the preceding sections of a number of techniques available to the analytical chemist. The techniques have differing degrees of sophistication, of sensitivity, of selectivity, of cost and also of time requirements, and an important task for the analyst is the selection of the best procedure for... 

Element-selective detectors. Many samples, e.g. those originating from environmental studies, contain so many constituent compounds that the gas chromatogram obtained is a complex array of peaks. For the analytical chemist, who may be interested in only a few of the compounds present, the replacement of the essentially non-selective type of detector (i.e. thermal conductivity, flame ionisation, etc.) by a system which responds selectively to some property of certain of the eluted species may overcome this problem. 

In this introductory chapter, we hope to describe the properties of x-rays most useful to the analytical chemist, to trace the discovery of these properties, and to foreshadow their importance in analytical chemistry. 

To the analytical chemist, scattering is important mainly because it increases total absorption, and because it often leads to an increase in the background observed when x-ray intensities are measured. The extent to which a sample scatters x-rays (or y-rays) can sometimes be used as a means of analysis. 

Moseley found that each K spectrum of Barkla contains two lines, Ka and K(3, and that the L spectra are more complex. Later important work, especially by Siegbahn,38 has shown that M, N, and O spectra exist and are more complex in their turn. Relatively numerous low-intensity lines are now known to exist in all series. Fortunately, the analytical chemist can afford to ignore most of these low-intensity lines in his practical applications of x-ray methods at present. It generally suffices for him to know that x-ray spectra at their most complex are enormously simpler than emission spectra involving valence electrons, and that most x-ratr lines are satisfactorily accounted for on the basis of the simple selection rules that govern electron transitions between energy states. 

There is unusually strong justification for the historical approach in presenting the important facts about x-rays to analytical chemists the information these chemists need in their work is largely that discovered in the early researches. This is particularly clear in connection with absorption and emission spectra, in which more refined investigations with more powerful equipment later revealed important complexities that the analytical chemist may ignore. Several of these complexities will be recorded below. 

Up to this point, our position has been that the elementary processes by which x-rays are absorbed and emitted are free of chemical influences because these processes involve energ levels nearer the nucleus than the levels in which valence electrons are to be found. This simplified position suffices for most x-ray applications in analytical chemistry. Nevertheless, chemical influences on both types of elementary processes have been demonstrated, but only at very high resolution—at much higher resolution than the analytical chemist usually requires. 

The effects in question are often translated into electric currents, pulsed or continuous. For the convenient reading or recording of these currents, complex electronic circuitry (2.3) may be needed. Modern methods of measuring x-ray intensity are therefore primarily a concern of the experimental physicist. Nevertheless, the analytical chemist must know something about them because x-ray detectors are now among the tools of his trade. This chapter, which cannot hope to do justice to modern x-ray detection, will attempt to provide him with an acceptable minimum of knowledge. 

A primary concern of the analytical chemist is the range of elements over which a given detector is useful. Unfortunately, such a range cannot be rigidK specified not only does it depend upon the characteristics of the detector and the rest of the optical system but it is determined also by the concentration of the element in a sample, by the composition of the rest of the sample (the matrix ), and by the precision desired. Nevertheless, the usefulness of detectors is so important that an operational comparison is worth while even if it is hedged about with restrictions that limit its applicability. Such a comparison has been carried out41 on eight representative elements with four detectors. 

To the analytical chemist, a standard deviation31 is a logical figure of merit for the comparison of detectors. We shall merely introduce the important subject of counting errors (10.2) here. For present purposes, it suffices to know that these errors are predictable, and that they set a lower limit to the standard deviation in an analytical method that depends upon measuring the intensity of an x-ray beam by an integrating detector. 

A brief interpolation The analytical chemist is accustomed to seeing... 

The spectrum from a Coolidge tube often contains lines traceable to impurities, those of copper, nickel, and iron being the most common. The impurities may be present in the target of the new tube, but they are more likely to be deposited on the target during operation. It is consequently desirable that the analytical chemist maintain current acquaintance with the spectrum of his x-ray source. 

The diversity and the effectiveness of the means for coping with deviations from proportionality attest the great interest in x-ray emission spectrography and the resourcefulness of the analytical chemist. Only representative references can be cited. The methods used to ensure reliable results in the face of the three classes of deviations are described briefly below. The obvious measure of separating the element to be determined from the matrix is omitted. 

The analytical chemist normally ought not to strive for a detailed knowledge of the components in Figure 9-1, spectrograph and detector excepted. But he needs a minimum of information, which the following paragraphs are intended to give. 

Equation 10-6 is the well-known Poisson distribution,5 which is important in counting whenever the number of counts taken is low enough to make a count of zero fairly probable. The analytical chemist, except occasionally in trace determinations, wrill deal with counts so large that he need not concern himself with the Poisson distribution. 

The significance of the standard counting error to the analytical chemist is increased because it can be used as a criterion for judging the operating conditions for" x-ray emission spectrography. 

Inasmuch as sc results from fluctuations that cannot be eliminated so long as quanta are counted, this standard deviation is the irreducible minimum for x-ray emission spectrography under ideal conditions. Not only is it a minimum, but it is also a predictable minimum. When the standard deviation, s, significantly exceeds the standard counting error, sc, it is likely that errors resembling those the analytical chemist usually encounters are superimposed upon the random fluctuations associated with the emission process. 

The analytical chemist often wishes to express the counting error relative to the amount present of the element sought in a simple case we have... 

This fusion of disciplines, though desirable and inevitable, complicates the writing of books in fields where it occurs. Spectrochemical analysis by means of x-rays is definitely such a field. For whom shall a book on this subject be written Our answer is clear. This book was written for the analytical chemist who wants to use these x-ray methods and to understand them. We have striven for correctness in physics, electronics, and statistics but we have tried first of all to help the analytical chemist in his work. 

Nomenclature is bound to flourish in a field that rests on several disciplines. We have pruned the growth, judiciously we hope, after considering all the various shoots, even the more exotic. To keep the book within bounds, equipment for description and literature for reference had to be selected. The choice was made with the analytical chemist in mind. In general, references later than August 1957 could not be included. 

Again with the analytical chemist in mind, we have not treated all topics equally. The electronics expert is likely to feel we have skimped, especially in Chapter 2 Chapter 4 is oversimplified statisticians will find much missing from Chapter 10 and other important developments could have been treated in Chapters 9 and 11. 

The scope of my comments will cover not the development of analytical methods but rather the process of choosing methods which give useful answers to the questions posed by the research chemist, the process engineer or the product marketing manager. The analytical chemist is always faced with the paranoia causing problem of not being able to be confident in a purity measurement until it can be shown that impurities do not interfere. 

In an outreach to the medicinal chemists at Lilly, a one-week workshop was created and taught in the research building where the organic chemists were located. (The computational chemists were initially assigned office space with the analytical chemists and later with the biologists.) The workshop covered the basic and practical aspects of performing calculations on... 

The PSP toxins represent a real challenge to the analytical chemist interested in developing a method for their detection. There are a great variety of closely related toxin structures (Figure 1) and the need exists to determine the level of each individually. They are totally non-volatile and lack any useful UV absorption. These characteristics coupled with the very low levels found in most samples (sub-ppm) eliminates most traditional chromatographic techniques such as GC and HPLC with UVA S detection. However, by the conversion of the toxins to fluorescent derivatives (J), the problem of detection of the toxins is solved. It has been found that the fluorescent technique is highly sensitive and specific for PSP toxins and many of the current analytical methods for the toxins utilize fluorescent detection. With the toxin detection problem solved, the development of a useful HPLC method was possible and somewhat straightforward. 

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