DETERMINATION OF BETAINE IN SUGARBEET BYPRODUCTS USING MID-RANGE INFRARED SPECTROPHOTOMETRY

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DETERMINATION OF BETAINE IN SUGARBEET BYPRODUCTS USING MID-RANGE INFRARED SPECTROPHOTOMETRY INTRODUCTION D. Eugene Rearick* and Cheri McKay Amalgamated Research Inc.

1 DETERMINATION OF BETAINE IN SUGARBEET BYPRODUCTS USING MID-RANGE INFRARED SPECTROPHOTOMETRY INTRODUCTION D. Eugene Rearick* and Cheri McKay Amalgamated Research Inc. Twin Falls, Idaho Mid-range infrared (JvfIR) spectrometry, which covers the spectral range between 2.5 and 25 microns (4000 cm-! and 400 cm-!), has been used by organic chemists to obtain qualitative information on chemical structures or the identity of compounds for over 50 years but its application as a quantitative analysis method has lagged behind some more recently developed techniques. For example, near infrared (NIR) spectrometry, which covers the spectral range of 780 nm to 2,500 nm or 12,800 cm- I to 4000 cm-! (between the l\1ir and visible light region), has been extensively applied quantitatively in the are offood and agricultural product analysis since the early work of Karl Norris at the US Department of Agriculture in the 1950's.! The relative scarcity of quantitative applications ofwr spectrometry is due to a number of reasons among which are: 1. Mid-IR absorbances are very strong, necessitating dilution of samples before an IR beam can be passed through any conventional type of sample cell. 2. Common optical materials, glass and quartz, absorb strongly in the l\1ir range and cannot be used at all. Optical components are usually either based on the use of mirrors or, if beam transmission is necessary, constructed of a non-absorbing material such as potassium bromide. Sample cells or other sample containing devices were commonly constructed of potassium bromide, sodium chloride, cesium iodide, or silver chloride. 3. For the analysis of water-based materials, such as foods, the two disadvantages given above combine to produce a seemingly impossible situation because the obvious substance for sample dilution, water, absorbs very strongly in the WR region and, in addition, dissolves the common materials used for cell construction. Even if sample dilution were not necessary, the water content of most food-related samples is not compatible with cell components constructed with potassium bromide or sodium chloride. Silver chloride cell components were available but this material, although insoluble in water, has the disadvantage of darkening on exposure to visible light. In the last ten to fifteen years, several developments in the field of mid-ir spectrometry have made the analysis of water-based materials much more attractive. One of the most important has been the advent of sample cells made of zinc selenide, which is both transparent in the MIR region and water-insoluble. A wide variety ofsample cells and other sample-containing devices constructed from this material is now available. A particularly valuable application of zinc selenide has been to the construction of cells for measurements by attenuated total reflectance (ATR), a relatively old technique for recording spectra ofthin layers ofrelatively concentrated materials. A simple AI R cell 61

2 (see Figure 1) consists of a zinc selenide plate, on which the sample is placed. The infrared beam, entering from the left, is reflected from a mirror onto the bottom of the plate. The beam travels through the plate and is reflected off the upper surface, which is in contact with sample, to the second mirror and back into the optical path of the instrument. During the reflection of the beam from the upper surface ofthe zinc selenide plate it actually travels a very short distance (on the order of several microns) into the sample material on the plate and the resulting refl ected beam then contains information on the infrared absorbance of the sample. The ATR technique thus allows measurement of the lv1ir spectrum of a sample layer that is of very low (and easily reproducible) thickness. This advantage is quite valuable in the analysis of concentrated and high-absorbing materials such as aqueous solutions. ATR cells are available in a variety of configurations, from the simple horizontal, single-bounce configuration described here, to modifications allowing multiple bounces ofthe beam from sample. Finally, the general availability ofreasonably low-priced Fourier transform infrared (FTIR) instruments, with the capability of rapidly recording and averaging multiple scans of a sample, now allows the determination of much more reliable and noise-free infrared absorbance data than was available with older dispersive infrared instruments. FIGURE 1 ATR TECHNIQUE SAMPLE IRBEAM With the expanding application of the NIR technique, the question that naturally comes up is "why bother with development ofmir methods?" However, in spite of the difficulties discussed, the mid-jr range does have several definite advantages: (l) Mid-range IR spectra usually have a large number of absorbances which are frequently assignable to fundamental vibration modes of specific chemical bonds in a compound. Thus for a compound being determined quantitatively, peaks specific for that compound can usually be identified and their absorbances measured directly. NIR spectra, in contrast, consist of overtone and combination bands derived from the fundamental absorptions of the MIR range. NIR peaks are often quite broad and may not have features readily assignable to a particular compound. In practice, the use ofnir spectral data may require rather sophisticated mathematical data treatment, including the calculation ofderivative spectra and the use of principal component analysis to look for spectral data 62

3 that relates to a particular compound concentration. Since :MIR peaks can be readily identified, it might be expected that simpler data treatments could be used on peak data and that advanced data treatment techniques might give even better results. The work to be described in this paper uses a simple data treatment, linear regression, to relate component concentrations to absorbances of l\1ir peaks for specific components. (2) Mid-range FTIR spectrometers are, at present, somewhat less expensive than NIR spectrometers and, perhaps more importantly for a small laboratory on a limited budget, can also be used for other applications such as the routine identification of unknown materials 2,3 RESULTS AND DISCUSSION N1R spectrometry has been investigated for a number of sugar industry applications including, but not limited to, sugarbeet brei and factory materials, 4,6 sugarcane factory materials; refinery materials, 8 on-line sugarbeet juice analysis/ and ion exclusion separation proflles IO In contrast, NllR spectrometry has not generally been applied to quantitative analysis in the sugar industry although a very recent paper does describe the determination of sucrose in cane juice. II In the Research Laboratory of The Amalgamated Sugar Company (now Amalgamated Research Inc.), investigations of the application of:rvtir spectrometry to quantitative determinations ofinterest in the sugar industry have been carried out occasionally over the last several years. These studies are preliminary and were carried out without the use of any sophisticated software for the statistical evaluation of data. Although the determination of sucrose is ofgreat interest and is a potential useful application ofmlr spectrometry, most of the studies in our laboratory have been aimed at the determination of betaine, primarily because alternative methods of betaine determination are somewhat slow. The infrared spectrum of betaine, in aqueous solution, contains a number of peaks in the 800 to 1500 cmregion, as shown in Figure 2. The spectrum (Figure 3) of a concentrated betaine-containing I byproduct from sucrose recovery by ion exclusion chromatography shows many ofthe same peaks plus peaks centered at around 1050 cm- I which are due to su(;;rose. Linear regression correlations were carried out between absorbanees ofseveral ofthe more prominent peaks of the betaine spectrum and the betaine content of the byproduct as determined by high performance liquid chromatography (HPLC). Generally, as a baseline correction, the value of a nearby absorbance minimum was subtracted from the peak absorbance value. In the course of these calculations, it was found that the highest betaine absorbances, which at first seemed attractive to use as analytical measurements, did not necessarily give the best correlation with betaine content. For example, the intense betaine peak at 1399 cm- I seems to be subject to interferences from coincident peaks due to unknown components and gives only a poor correlation with known betaine content. Of the betaine peaks in the region shown in Figure 3, the most consistent, interference-free absorbances are the smaller peaks at 1475 cm- I and 897 cm- I. Figure 4 shows the correlation between betaine concentration, as measured by HPLC, and the absorbance at 1475 cm- 1 (corrected for baseline shifts by subtracting the absorbance at 1439 em-i). Data for the peak near 897 cm- 1 (corrected for baseline shifts by subtracting the absorbance at

4 FIGURE 2 t.5064 '" ; I O r ,----r--r < /02/ 16 14: 36 blltan2!il; 16 Icanl, ".Oc.-1 J Blltaine lilli/iii, liiater) FIGURE i I r r _ _ _ to / 12/29 15: 48 be t9521: 26!lClSn!!, -4. OCII-1. IIpod nonll Deteln. Conc, Und lluted 64

5 cm-!) is even better, with a correlation coefficient of (Figure 5). These correlations are reasonably good considering the relatively simple treatment of absorbance data and given that the correlation also reflects any uncertainties in the HPLC betaine determination. Figure 6 shows evidence that, in the absence of comparison measurement uncertainties or interferences, the relationship between betaine concentration and IR absorbance is extremely good. In this test, the IR spectra of betaine standards in the range of 5 g/100 ml to 25 g/100 rnl were recorded and known betaine concentration was plotted versus the absorbance at 1475 cm-! (corrected for the absorbance of water at 1475 cm-!). The correlation coefficient in this test was Clearly the poorer correlations in the case of betaine concentrate samples are due either to interfering infrared absorbances of other sample components or uncertainties in the comparison HPLC determination. FIGURE 4 Betaine Concentrate = E I : Ql c:.iij Q)!II a.. I 30.80, Abs 1475 em ' - Abs 1439 em ' FIGURE 5 Betaine Concentrate ,,..., 38 o 36 :9 34 iii Q)!II 032 a.. -' I L----L---L Abs 898.5erlr' - Abs 916 em ' 65

6 FIGURE 6 Betaine Standard , E 8 15 o O Abs 1475 em ' - Abs Wate' (1475) Due to possible uncertainties in HPLC betaine determination, several attempts were made to determine betaine directly by MIR spectrometry using a standard addition technique. Betaine was added, at levels varying from 2 to 10 gl100 ml final betaine concentration, and samples were diluted to a constant volume. In all cases MIR peak absorbances gave a very good linear fit with added betaine concentration but extrapolation to determine original betaine concentration did not always give a reasonable value. This is undoubtedly due to interfering materials which have a finite absorbance at the wavelength being used. One ofthe better standard addition tests, shown in Figure 7, made use of absorbance values at cm-! to give an extrapolated value of g/ I 00 rnl for a diluted sample which had been found to have a betaine content of gi 100 ml by HPLC. FIGURE 7.Betaine Standard Addition 0.40 r , ,-, AIls Olff (addad cone) r '" :Ii e c Standard Addition Value g/100 ml HPLC g/100 ml 0.00 '-----'""- ---'- ---'- --''-- -<-- --L-. --'- --.! o 4 12 Added Betaine (gil OOmQ 66

7 Preliminary attempts were made to correlate MJR absorbances with both betaine concentration and sucrose concentration in molasses. In this case, spectra of normal weight molasses solutions were recorded and peak heights were correlated with values determined by other methods. At this lower concentration, many ofthe smaller betaine peaks used in the betaine concentrate studies were too small to measure but the cm- I peak was large enough and oot subject to the interferences that had made it unusable in the case of betaine concentrates. Figure 8, shows a correlation between betaine concentration in the normal weight solution (as determined by HPLC) and absorbance at 1400 cm- I (corrected for baseline absorbance at 1250 cm- I ). The correlation coefficient, , is excellent in this case (partly due to the fact that betaine concentration varies by a factor of nearly 3 in the sample set). The major sucrose peak (1054 cm- I ), corrected for background absorbance at 1250 cm- I, correlates well (coefficient of as shown io Figure 9) with sucrose as determined by gas chromatography, especially considering that the samples have a fairly narrow range of sucrose concentrations (12.6 to 13.6 g/100 mt). W FIGURE 8 Betaine in Twin Falls Molasses (Normal Weight Solution) 4.00, :-J r IT 2.50 e. :! l t.oo '--_---L.--'-- --' --' ' Abs 1400em". Abs 1250 em" FIGURE 9 Sucrose in Twin Falls Molasses (Normal Weight Solution) W S..., u '" Cl r Abs 1054cm". Abs 1900 em" 67

8 These results indicate that absorbances of sucrose and betaine in the mid-range infrared spectrum do correlate well with quantities determined by other methods. In addition, absorbances for betaine are quite linear with added betaine concentration. Since the basic data evaluation methods used here give good correlations with concentration it seems likely that more advanced techniques, using commercial software for data evaluation, would give even better results. Such a test is the next goal of this study. EXPERIMENTAL METHODS Infrared spectra were recorded on a P erkin-elmer Model 1650 Fourier transform infrared spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector. Sixteen scans were averaged to give the final spectra used for quantitative measurements. Samples were analyzed using a Baseline (Spectratech, Inc.) horizontal attenuated total reflectance cell with a zinc selenide plate. ACKNOWLEDGMENTS The authors would like to acknowledge the technical assistance of Carol Wambolt. REFERENCES 1. Ciurczak, E. W., Today's Chemist at Work, October, 1995, p Rearick, D. E., Zuckerindustrie, 114, 405 (1989). 3. Rearick, D. E., paper presented at the Twenty-sixth General Meeting, A S.S.B. T., Vaccari, G., G. Mantovani, G. Sgualdino, and P. Goberti, paper presented at 24th General Meeting, AS.S.B.T., Edye, L. A and M. A Clarke, paper presented at 27th General Meeting, AS.S.B.T., Edye, L. A and M. A Clarke, paper presented at 28th General Meeting, AS.S.B.T., Clarke, M. A, E. R. Arias, and C. McDonald-Lewis, Sugar y Azucar, June, 1992, p Edye, L. A and M. A Clarke, S.I.T. Proceedings (Paper No. 687), Kienzle, W. B., paper presented at 26th General Meeting, AS.S.B. T., McGillivray, T. D. and J. Wallevand, Proceedings of the Workshop on Separation Processes in the Sugar Industry, M. Clarke, ed., SPRI, New Orleans, 1966, p Cadet, F. and B. Offmann, J Agric. Food Chem., 45, 166,

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