ICUMSA News n°15 – 1992

Message from the President

Session Recommendations

We have now had nearly two years since our Colorado Springs Session and in that time the Proceedings have been produced and new Referees and their Associates appointed. The work of the 21st Session is still slow to get underway. I believe that Referees should establish priorities for undertaking the work involved in tackling the 20th Session Recommendations. History shows that there is more work to be done than resources to do it. I believe that the most important goal of this Session is the production of our new Methods Book. Recommendations which call for the collaborative testing of methods intended to be published in the Methods Book should be accorded the highest priority. This is all the more important when it is appreciated that all international bodies like our Commission are expected to offer properly documented and collaboratively tested methods. Since it will not be possible to have all methods collaboratively tested by 1994, we should aim to have a substantial number achieve this standard. More importantly, we should have a well-devised plan to complete the task for the remaining methods.

Methods Book

Nearly all Referees have submitted lists of methods proposed for the new Methods Book. A Table of Contents is being worked up by John Dutton and we have volunteers to draft approximately half these methods. Methods will be written to conform ISO 78/2- l982(E) Layouts for Standards-Part 2: Standard for chemical analysis. The Polarisation of Raw Sugars’ method is available as an example of the required format.

Since the work of editing methods for publication is substantial, we need a steady stream of draft methods being submitted to John Dutton if he is to complete the task by 1994. Referees and Chairmen of National Committees are urged to assist in co-ordinating this work.

Some remarks about NIR-Polarimetry

by A. Emmerich, Referee for Subject 4 (Polarimetry and Quartz Plates)

In the December 1991 issue of ZUCKERINDUSTRIE, p. 1041, W. Altenburg and C. C. Chou published a paper about polarimetric determinations of raw sugar using the wavelength of 882.6 nm in the near infrared region of the spectrum, which they had presented at the 50th SIT Conference, New York, May 12-15, 1991. The results of this investigation are of great interest and I want to take this opportunity to express my views on this matter.

In Colorado Springs it was stated that all measures which lead to a reduction or elimination of lead clarification should be given high priority in the work of ICUMSA. NIR polarimetry is one possible approach to the solution of the problem.

For some years now the range of wavelengths used in polarimetry has been extended from the range of 540-633 nm presently approved by ICUMSA to 850 nm or higher in factory control applications. Perceiving the need for quartz plate values at these new wavelengths, K. Zander presented preliminary data on measurements by the PTB in Braunschweig of the rotatory dispersion of quartz in his Report on Subject 5, Quartz Plates (Proc. 20th Session ICUMSA, 1990, p. 202). The new figures for the specific rotation of quartz, [a) in °/mm, are presented below:

Specific rotation of quartz

λvac (nm) [α]m (°/mm)[α]c (°/mm)Δ (%)
1.084.75 60.63260.586-0.076
1,141.23 54.50054.459-0.075
1,152.60 53.37953.335-0.082
1,177.01 51.08151.032-0.096

[α]m is the measured specific rotation and [α]c the specific rotation calculated according to Bünnagel’s equation (Proc. 14th Session ICUMSA, I 966, p. 32). The last column shows the difference between the calculated and the measured values in %. These figures reflect at the same time the influence of the rotation of quartz on the sugar value of a quartz control plate when changing from the calculated to the measured value. The figures show that at 730 nm the rotation of quartz almost exactly follows the Bünnagel equation while at higher wavelengths the calculated values are lower than the measured ones by up to 0.1 % approximately. These results are within the limits of technical measurements.

At the 20th Session, ICUMSA recommended provisionally that conversion of rotation values of quartz to wavelengths in the range 650 to 1000 nm should be made using Bünnagel’s equation. Furthermore, ICUMSA recommended, also provisionally, to use the equation of Bünnagel for the rotatory dispersion of sucrose (Proc. 14th Session ICUMSA, 1966, p. 17) for calculating the 100 °Z point in the same range of wavelengths. This latter Recommendation implies that it was hoped that the-rotatory dispersion of sucrose follows Bünnagel’s equation with similar low deviations as have been found with quartz. It was not based on any measurements.

These Recommendations clearly were only intended to serve as a guide for the application of NIR polarimetry in practice, e.g. in factory control. In the discussion, it was stressed that Bünnagel’s formulae should not be used where measurements of higher accuracy are required. Such situations might be those involving commercial payments.

For an application of NIR polarimetry for analyses in trade and legislation applications, four conditions should be fulfilled:

1. Exact measurements of the rotatory dispersion of quartz in the neighbourhood of the wavelength used in the instruments for the analysis

2. Corresponding measurements of the rotatory dispersion of sucrose

3. Investigation of the rotatory dispersion of technical products of different origin, and

4. Investigation of the influence of different levels of non-sugars in clarified and unclarified (only filtered) solutions.

Point 1. The rotatory dispersion of quartz was the object of a preliminary study of PTB, as described above. Unfortunately, all measurements available till now cover a wavelength range far away from those used in practice. Measurements between 850 and 900 nm should be undertaken. To find suitable light sources in this range with exactly known wavelengths still presents some problems.

Point 2. Optical rotation measurements of sucrose solutions above 644 nm are not available for the purpose of sugar analysis.

Point 3. Measurements of the rotatory dispersion of impure sugar products have only been carried out with solutions clarified with basic lead acetate. Systematic differences compared with pure sucrose solutions led to a restriction of the wavelength range for sugar polarimetry from 540 to 633 nm. Unclarified solutions have not been measured till now.

Point 4. Substances which are eliminated by the normal clarification procedure may influence the optical rotation when unclarified solutions are used. This introduces further problems which can only be assessed by comparative measurements involving a large number of samples.

The results of the study by Altenburg and Chou referred to at the beginning are now considered in the light of the above considerations.

Their main conclusion is that the difference between the NIR polarisation at 882.6 nm without clarification and the “conventional method” at 589.44 nm (sodium light) for 1510 raw sugar samples is on the average +0.005 with only 73 of the individual differences (5 % approximately) exceeding 0.2. This difference cannot be assigned specific units because two different sugar scales are compared. For 589.44 nm the results are given in °S after substraction of 0.1 °S according to the “New York Coffee, Sugar & Cocoa Exchange’s raw sugar Contract No. 14”.

However, when comparing unclarified with clarified solutions at 882.6 nm, the difference was 0.24 °Z for Australian raw sugars of about 97.9 °Z and 0.15 °Z for those of about 98.8 °Z, with the unclarified solution being lower. Obviously, both the amount and nature of the non-sugars eliminated by the clarification influence the result. This effect can also be deduced from the data of Altenburg and Chou. According to the Australian results dextran seems to have a considerable effect. Dextran-affected Fiji raw sugar of about 97.6 °Z showed a difference between unclarified and clarified solutions of only 0.03 °Z compared with 0.24 °Z for the equivalent Australian sugars mentioned above and which were low in dextran. Also, in this respect, the results of Altenburg and Chou show the same tendency.

Summarizing, one should say:

1. Investigations of NIR polarimetry are of major interest to ICUMSA and therefore the experiments published by Altenburg and Chou are welcome. We need experimental material of this kind urgently.

2. The evaluation of the authors must be called in question. One should not compare corrected °S with °Z. At least the results of such an evaluation are no justification for the 0.1 °S deduction from raw sugar polarisations, still used in USA in contrast to the ICUMSA Recommendations valid since 1986.

3. The sugar values of quartz plates in the NIR region, between 650 and 1000 nm, should be established on the basis of exact measurements of the rotatory dispersion of quartz and sucrose. Preliminary measurements in Australia indicate that the extrapolation of Bünnagel’s equations leads to values suitable for technical use, e.g. for factory control, but obviously not exact enough for analyses involving trade and legislative purposes.

4. The most important problem for trade purposes is the influence of non-sugars which were hitherto eliminated by the clarification process and which remain in the solutions used for polarimetry in the NIR region.

5. ICUMSA members are invited to carry out measurements in order to collect material about NIR polarimetry compared with the traditional procedure. Special attention should be paid to the influences of wavelength (same solution at different wavelengths) and impurities (solutions of different preparation techniques at the same wavelength).

For the polarisation values obtained at 882.6 nm the instrument was standardised with a quartz control plate. The sugar value of the plate was calculated for 882.6 nm according to Bünnagel’s equations for the rotatory dispersion of quartz and sucrose solutions in °Z. That means °S – 0.1 °S are compared with °Z or in other words: at 882 nm the valid ICUMSA Recommendations were followed, while at 589 nm this was not the case.

There is no difficulty in re-calculating the original (uncorrected) results at 589.44 nm to °Z. This is not a matter of measurement but only of calculation. First one has to add 0.1 °S and then multiply by 0.99971. The average polarisation of the samples (not explicitly given in the paper) is 98.4 °Z approximately. Starting from a corrected value of 98.400 °S, one obtains (98.400 + 0.100) °S x 0.99971 °Z/°S = 98.471 °Z. This is to be compared with the result at 882.6 nm giving on the average 98.405 °Z. So, the difference for the two series of measurements becomes -0.066 °Z, the 882.6 nm value being lower, instead of 0.005.

These results of measurements and calculations are based on the assumption that the Bünnagel equations can be extrapolated from the measured range of 408–644 nm to 882 nm without important error.

This question could have easily been answered by simply measuring a normal solution of sucrose at the two wavelengths (a very pure refined sugar is sufficient). Such measurements have been recently perfom1ed in the CSR’s Central Laboratory (private communication from M.R. Player) where 26 sucrose solutions were compared at the two wavelengths. The result when using Bünnagel’s equations for the standardisation of the 882.6 nm instrument was a polarisation which was too low by 0.061 %, the standard deviation of the difference being ± 0,02 %.

Correcting the 882.6 nm polarisation of Altenburg and Chou accordingly, we obtain: 98.405 °Z + 0.060 °Z = 98.465 °Z compared with the 589.44 value of 98.471 °Z.

Thus, comparing, as one should do, °Z with °Z, the experiments of Altenburg and Chou result in an average difference of -0.006 °Z.

This apparently excellent result when comparing the valid ICUMSA method (measurement between 540 and 633 nm in °Z) with measurements in the near infrared does by no means justify a replacement of the traditional method by NIR polarimetry. We must keep in mind that in the first case clarified solutions have been measured while in the NIR experiments unclarified, i.e. filtered, solutions were used.

Some experiments in the CSR’s Central Laboratory show that the matter is much more complicated. Clarified raw sugar solutions gave the same results at the two wavelengths if the 882.6 nm instrument was standardised with sucrose. That means, clarified solutions of the investigated raw sugars show the same behaviour as pure sucrose solutions. Their rotatory dispersion is identical within the limits of experimental error. So, problems discussed above under point 3 seem of no major importance even up to 880 nm.

Editor: Dr. R. Pieck. Donystraat 85, B-3300 Tienen, Belgium – Tel.: +32 16 823 096 – Telefax: +32 16 820 826 – Telex 271 05