BS 6200-6 1-1990 Sampling and analysis of iron steel and other ferrous metals - Guidelines on atomic absorption spectrometric techniques - Recommendations for the drafting of stand.pdf

1、BRITISH STANDARD BS 6200-6.1: 1990 Sampling and analysis of iron, steel and other ferrous metals Part 6: Guidelines on atomic absorption spectrometric techniques Section 6.1 Recommendations for the drafting of standard methods for the chemical analysis of iron and steel by flame atomic absorption sp

2、ectrometryBS 6200-6.1:1990 This British Standard, having been prepared under the directionof the Iron and Steel Standards Policy Committee, waspublished under the authorityof the Board of BSI andcomes into effect on 28 September 1990 BSI 09-1999 The following BSI references relate to the work on thi

3、s standard: Committee reference ISM/18 Draft announced in BSI News April 1989 ISBN 0 580 18179 0 Foreword This Section of BS 6200 has been prepared under the direction of the Iron and Steel Standards Policy Committee. The text is identical with Information Circular No.8 “Chemical analysis of ferrous

4、 materials: Recommendations for the drafting of standard methods of analysis employing flame atomic absorption spectrometry for the chemical analysis of iron and steel”, published by the Commission of the European Communities. In the Synopsis on page 1, reference is made to a complementary Informati

5、on Circular, No.9, the text of which forms the basis for BS6200-6.2. Both Information Circular No.8 and Information Circular No.9 were prepared by the European Committee ECISS/TC20 “Methods of chemical analysis” with the active participation and approval of the UK. The recommendations given in these

6、 documents have also been adopted by ISO/TC17/SC1, “Steel Methods of determination of chemical composition”. Most of the European and ISO standards involving flame atomic absorption spectrometry produced by these two committees have been prepared in accordance with these recommendations. Many of the

7、se methods have been implemented as Subsections of BS6200-3. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from

8、 legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1 to 20, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendmen

9、t table on the inside front cover. Amendments issued since publication Amd. No. Date of issue CommentsBS 6200-6.1:1990 BSI 09-1999 i Contents Page Foreword Inside front cover 1 Introduction 1 2 Instrument criteria 2 3 Adjustment of atomic absorption spectrometer 5 4 Preparation of solutions 5 5 Cali

10、bration procedure 6 6 Interferences 7 7 Background correction 7 8 Expression of results 7 9 Safety 8 10 Miscellaneous 8 11 Summary of recommendations 8 12 Literature 9 Appendix 1 The determination of calcium in steel, based on EURONORM 177-85 10 Appendix 2 The determination of cobalt in iron and ste

11、el 14 Publications referred to Inside back coverii blankBS 6200-6.1:1990 BSI 09-1999 1 Synopsis Standard methods of chemical analysis have traditionally been based on the techniques of classical chemistry. Generally, by specification of quantities and purity of reagents, such methods may be describe

12、d completely and thereafter followed exactly in any laboratory. Today, however, the bulk of analysis is done by instrumental methods for which different laboratories may use instruments from various manufacturers with different configurations and operational settings. It becomes of pressing importan

13、ce to consider how such methods may be specified in Standard documents with regard to both instrument quality and procedure. The present document considers these matters for methods employing flame atomic absorption spectrometry and makes recommendations. These prescribe instrument quality in terms

14、of limit of detection, curve linearity and precision. Recommendations concerning the preparation of solutions, the calibration procedure and the expression of results are also included. Methods for the determination of calcium in steel and for cobalt in iron and steel are included as illustrative ex

15、amples of the drafting recommendations. NOTEAttention is also drawn to Information Circular No9: Chemical analysis of ferrous materials; operational guidelines for the application of flame atomic absorption spectrometry in standard methods for the chemical analysis of iron and steel. 1 Introduction

16、An acceptable analytical procedure has to be written in such a form and in such detail that after faithful execution by a well trained analyst an unquestionable result will be obtained. Neither the individual analyst nor the make of apparatus used should have any influence on the final result. These

17、 requirements are particularly stringent in the formulation of standard methods used for reference purposes and for arbitration in cases of dispute. Traditionally, Standard methods of analysis have been based on the techniques of classical chemistry. While technically sound, they do not reflect mode

18、rn developments such as optical emission spectroscopy, X-Ray fluorescence and atomic absorption spectrometry, now in widespread use. New and revised standard procedures now in the course of preparation will increasingly embody these techniques and it becomes pressingly important to consider how such

19、 methods may be specified. With traditional chemical methods there was little problem. Generally, by specification of quantities and purities of reagents, the methods could be described completely and thereafter followed exactly in any laboratory. With instrumental methods, however, the situation is

20、 different. Different makes of instruments may be employed in the various laboratories, with different configurations or operational settings, and it becomes more difficult to specify the procedure precisely. Clearly, the standard should not refer to a particular make of instrument, to the implied e

21、xclusion of other satisfactory equipment, nor should it include operational settings or measurement times or solution concentrations which may be appropriate to a given instrument only. On the other hand, if matters are left in the hands of the individual analyst just to “follow the manufacturers in

22、structions” the document can no longer be regarded as a Standard, nor is there any guarantee that the instrument employed will have a quality of performance suitable for the purpose. The way forward is for the Standard document to specify instrument suitability in terms of fundamental performance cr

23、iteria such as precision, limit of detection and linearity, according to agreed definitions. Individual manufacturers would then be free to meet these requirements in any way they choose. Also, since even a good instrument may deteriorate, the Standard should contain reference to practical procedure

24、s for the determination of these criteria, to ensure that the instrument meets the specification and continues to do so during its functional years. Other essential aspects of the Standard include instrument optimization, preparation of the test sample, calibration procedures, calculation of results

25、 and, where appropriate, how these should be related to the actual instrument used.BS 6200-6.1:1990 2 BSI 09-1999 The present document considers these matters in relation to Standard methods of analysis employing flame atomic absorption spectrometry and makes recommendations for the drafting of such

26、 methods. Non-flame atomic absorption spectrometric methods involving instrumental parameters whose definitions and descriptions require further study are not included in the present document. The determination of calcium in steel for which performance data are available is included as an illustrati

27、ve example of the drafting recommendations. A method for the determination of cobalt in steel covering a wide concentration range has also been included in order to illustrate dilution procedures, optimization for minimum matrix interference and correction procedures where cobalt may be present in t

28、he iron used for the calibration solutions. 2 Instrumental criteria In the past, attempts have been made to specify instrument performance in a variety of different ways under such headings as minimum sensitivity, curve linearity and minimum stability. It is important that terms used in Standard doc

29、uments should be in accordance with agreed definitions. The term “minimum sensitivity”, for example, has been used to imply a stipulation that the absorbance of the most concentrated calibration solution measured in a flame path length of10cm must be at least0.3. Useful as this requirement may be as

30、 a practical guide, it is not the accepted definition of sensitivity in instrumental analysis which is, more properly, the change in instrument reading for unit change in concentration (dx/dc), or the slope of the analytical curve at a given concentration. In atomic absorption spectroscopy, the term

31、 “sensitivity” has also been used very widely in manufacturers literature and elsewhere to imply the concentration of the analyte which will produce a change in the absorbance reading, compared with that of pure solvent, of0.0044, i.e.1% net absorption. The IUPAC Commission has drawn attention to th

32、is misuse of the term sensitivity and while recognizing the usefulness of the value it represents, has suggested replacement of the name by the term characteristic concentration. The present document recommends that instrument suitability as specified in a given Standard document should be based on

33、three performance criteria, according to agreed definitions limit of detection, acceptable curvature of the calibration graph and precision. 2.1 Limit of detection The limit of detection is the smallest concentration in solution of the element of interest which may be detected with confidence. It is

34、 obtained from the calibration graph and is generally taken in instrumental analysis as that value on the concentration axis which corresponds to an instrument reading of two or three times the standard deviation above the mean reading of the blank (analytical background). The IUPAC proposals recomm

35、end that this background measurement should be derived from a sufficient number of replicate determinations on the blank solution. Purely to facilitate the practical measurements, the present report suggests a minor modification, i.e. that the measurement should be based on a concentration value sel

36、ected to give an absorbance of just above the zero and that the standard deviation should be calculated on the basis of10 replicate determinations. In taking these measurements, as with all other measurements, it is very important that the instrument should be fully optimized. It is not sufficient t

37、hat it should be adjusted merely to meet the detection limit quoted in the standard. Instrument optimization is considered in some detail in Information Circular No9. It is not unusual for the experimentally derived limit of detection to be a few times higher than that quoted by the manufacturer bec

38、ause the latter will have been derived, in all probability, from a new instrument under particularly carefully controlled conditions using pure solutions. If the discrepancy is large, however, the instrument should be overhauled.BS 6200-6.1:1990 BSI 09-1999 3 The value placed on this detection limit

39、 in the standard should be specified with reference to the lowest concentration of the element of interest likely to be encountered in the application envisaged. In the ideal case, the specified limit of detection should be less than one-tenth of the lowest concentration level to be determined and s

40、hould be measured in the same matrix. The analysis of drinking water which is subject to legal requirements provides a convenient example of this doctrine. If, for example, the maximum legally permitted concentration of lead in potable water were, say,0.05ppm Pb (mg/l) and a result very close to thi

41、s were obtained, it would be necessary to know what confidence could be placed on the result. It would be little comfort to the analyst to feel that his instrument was working at the very limit of its capabilities. Since the limit of detection of many flame atomic absorption spectrometers is only of

42、 the order of0.01 to0.02mg/l Pb, special techniques would be required to give the necessary confidence when operating near the statutory limit. In the case of manganese in potable water, on the other hand, for which the maximum permitted concentration is of the same order, the flame atomic absorptio

43、n spectrometric technique is very sensitive (limit of detection typically0.005mg/l or better), thus meeting the criterion mentioned above and being eminently suitable for direct application. It is of course recognized that there may be situations where flame atomic absorption spectrometry might stil

44、l be the method of choice even though the above requirements could not be met. Such situations might exist where the technology does not require residual element determination with a high degree of precision, and a much higher limit of detection relative to the concentration being determined would b

45、e acceptable. In this sense, each analytical requirement could be considered on its own merits. Nevertheless, for a standard referee method, as distinct from a routine procedure, specification of a limit of detection of less than one-tenth of the lowest concentration to be determined is a useful gui

46、deline. The exception would be if, despite not meeting this requirement the method were still the most accurate one available. 2.2 Curvature of the calibration graph It is widely accepted in instrumental analysis that the ideal analytical curve is one which is linear throughout the concentration ran

47、ge of interest. Not only does this situation imply that the measurement system is showing its highest sensitivity but the graph is easy to locate accurately either by a line drawn manually through all the points or by a simple linear “least squares” mathematical treatment. Most practising analysts f

48、eel instinctively that best results are obtained under these conditions. In practice, however, non-linear calibration graphs are commonplace in atomic absorption spectrometry, even though the inherent curvature may in some cases be concealed by electronic manipulation of the signal before display, i

49、.e. with inherently slightly curved calibration graphs there is the risk that a linear graph is forced through the calibration points which clearly introduces errors. Potential risks associated with a heavily curved region of calibration are that absorbance measurements are insensitive and there may be problems in defining the shape of the curve on the basis of relatively few points in the curved region. It is therefore necessary to consider what constraints should be written into the standard to safeguard analytical accuracy under the conditions of non-linear calibration g