In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Automatic Low-Cost Data Acquisition from Old Polarimetric Instruments Giuseppe Alibrandi* and Santi D’Aliberti Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Università di Messina, Salita Sperone 31, Villaggio S. Agata, 98166 Messina, Italy; *
[email protected] Salvatore Coppolino and Antonino Villari Dipartimento Farmaco-Chimico, Facoltà di Farmacia, Università di Messina, Villaggio SS. Annunziata, 98168 Messina, Italy Norberto Micali Istituto per i Processi Chimico-Fisici, Sez. Messina, CNR, Via La Farina 237, 98100 Messina, Italy
A completely automatic analytical method has some significant advantages over its nonautomatic counterpart. The automatic method can be used to make many similar analyses or to continuously control an analyte with a minimal intervention from the operator and, often, with more precise and accurate results. Moreover, by interfacing the apparatus with a PC, it is possible to collect and save data and then to process, visualize, and print it. In this article we show how to convert a common physical chemistry laboratory instrument into an automatic one at low cost, making the technique economically available to undergraduate students. Many inorganic and organic compounds (1–3), some of which having pharmacological activity, such as adrenaline (4) and vitamin C, are optically active. As a consequence, polarimetry is an analytical method that is widespread in some fields, such as in the food and pharmaceutical sectors. A system that is typically employed in routine measurements is the Lippich polarimeter. It is characterized by a field split into two halves. Initially the device is set so that a perfect balance of the two lit halves is obtained. When the substance to be examined is placed between the polarizer and the analyzer, the two fields will not be balanced any more: one of them will appear darker than the other. To re-establish the balance of the two lights it is necessary to rotate the analyzer to an angle (α) that will be equivalent to the rotation angle of the polarization plane for the examined substance (5, 6). The determination of the angle that makes the lighting of the two halves identical is fraught with the possibility of errors, depending on the observer’s experience and on the capacity of the observer’s eye to distinguish different color hues. The aim of this article is to show how it is possible to convert a traditional, manual optical polarimeter into an automatic polarimeter, capable of better precision and higher accuracy in analysis and at minimal cost. This setup uses a video camera for surveillance (it is also possible to use a Web camera) and a low-cost image acquisition device (Video for Windows) connected to a PC. Through the calibration of the relative difference of brightness between the two fields according to the rotation power of the sample, it is possible to calculate the rotation power of the sample directly from the difference of relative442
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brightness intensity without the necessity of rotating the analyzer mechanically. Moreover, with this apparatus it is possible to follow a reaction continuously where there are optically active substances. For the calibration of relative difference of brightness between the two fields and the optical power, a set of fresh D-(+)-glucose and D-(−)-fructose solutions were prepared. As theoretically proved, a linear relation between optical power and relative difference of brightness were measured under one degree of rotation; this range is the most interesting from an analytical point of view. However, a nonlinear relation (cubic relation) can be used for rotatory powers up to 10⬚. This method is sensitive to a millesimal degree and precise within four millesimal degrees. Measurements of the hundredth of a degree are easily obtainable and reproducible. The modified instrument was tested by measuring the rotatory power of different solutions of (−)-adrenaline and following the hydrolysis of sucrose at acidic pH. Experimental
Materials (R)-(−)-epinephrine (adrenaline), D-(+)-glucose, D-(−)fructose and sucrose were purchased from Sigma-Aldrich Chemie (Germany). No further purifications were needed. The HCl was purchased from Carlo Erba (Milano, Italy), water from Angelini (Roma, Italy). Apparatus The experiments were performed using an ATAGO Polar X polarimeter (λ = 589 nm) equipped with a homemade 1-dm polarimetric tube. A PerkinElmer 341 Polarimeter was also used to obtain more accurate comparative data. The temperature control was achieved using a thermostated bath Haake C25 (7). For the optical couple of a CCD sensor (video signal PAL) the ocular of polarimeter was connected with a projection optic to the CCD detector. This device gives an image covering one-half of the CCD revealer (1兾3-inch color). For interfacing the CCD camera with a PC, an acquisition board, Video for Windows compatible, incorporated to a VGA video board was used. Using a Windows program,
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which uses the callings to Video for Windows functions, for example Microsoft VidCap (Microsoft Corp.), it was possible to capture and save images from the CCD as a function of time. Thus, it was possible to calculate the relative difference of brightness between the two fields as the difference divided by the sum of medium value of intensity of brightness, measured in the two fields, that is, the contrast between the fields. The projection optic consists of a 1-inch diameter lens with 10-mm focal lens that, coupled with the ocular of ATAGO Polar X polarimeter, makes a real image on the CCD detector (the projection optic mimics the crystalline lens and CCD mimics the retina). The CCD detector is a commercial CCD camera without optics and working in manual intensity-amplify mode (several cameras, also surveillance cameras, have this option). The possible CCD output signals are analog (PAL, NTC, etc.) or digital (e.g., USB) depending on the hardware and software system employed for the acquisition and processing of the image. The best hardware choice (for cost and quality) is an analog surveillance camera and a frame grabber (PCI card or USB device). For the acquisition and processing it is possible to choose a customized software program (via a Matlab environment or a C++ program) or use a shared program such as VidCad and commercial image processing program (for example, Image Pro Plus). Projection optic, CCD camera, acquisition board, software, and PC cost about $1500.
rate) PerkinElmer polarimeter, are shown in Table 1. It can be seen that the automatic measurements correspond well with those obtained manually and have a better precision. Figure 2 shows the kinetic behavior of rotatory power obtained in the same way by the modified polarimeter during the reac-
Table I. Optical Rotatory Power of Adrenaline Solutions As Obtained by ATAGO Polar X (αA), Automated ATAGO Polar X (αAA), and PerkinElmer 341 (αPE) Polarimeters
Conc/(g L᎑1)
αA/⬚
αAA/⬚
αPE/⬚
04
᎑0.20
᎑0.198
᎑0.201
08
᎑0.40
᎑0.401
᎑0.402
12
᎑0.60
᎑0.615
᎑0.613
16
᎑0.75
᎑0.811
᎑0.814
20
᎑0.95
᎑1.011
᎑1.012
NOTE: Experimental conditions were pH = 0 and T = 298.2 K.
Calibration To calibrate the apparatus standard solutions of D-(+)glucose and D-(−)-fructose were employed. Kinetic Runs For the kinetic runs 2.17 g of sucrose were dissolved in 50 mL of distilled water. During the reaction, the sucrose solution was circulated, using a closed circuit, from the reaction vessel to the optical cell with a peristaltic pump. The optical cell was homemade, and it consists of a 1-dm length polarimetric flow-through tube. The reaction was started by adding 3 mL of 12 M HCl to the circuit.
Figure 1. Calibration of contrast (circles) for standard solutions of D-(+)-glucose and D-(–)-fructose. The line refers to the best fit to a third-order polynomial function.
Hazards HCl severely irritates the eyes and respiratory system; it also irritates the skin and may cause severe burns to both the eyes and skin. (R )-(−)-epinephrine is toxic. May be fatal if inhaled, swallowed, or absorbed through the skin. Results and Discussion The measured value of contrast versus the optical rotatory power of standard solutions of D-(+)-glucose and D-(−)fructose is shown in Figure 1. It shows that deviation from the linear trend is observed for optical rotatory power greater than one degree. The best fit of analytical data by a thirdorder polynomial function allows the rotation angle in the whole calibration range of the measure of contrast to be obtained. The optical rotatory power of solutions of (−)-adrenaline, measured at 298.2 K using (i) a commercial polarimeter ATAGO, (ii) the automated instrument (obtained through the calibration), and (iii) a (more precise and accuwww.JCE.DivCHED.org
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Figure 2. Change in optical rotatory power during the acid-catalyzed hydrolysis of sucrose at 298.2 K as monitored by an ATAGO Polar X manually (circles) and automatically (line).
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tion of hydrolysis of sucrose at a temperature of 298.2 K. The noise of the signal, in this case, is enhanced by the particular experimental apparatus used (flow-through tube). Nevertheless, the kinetic profile is identical to that obtained by manual measurements but is continuous (acquisition rate of 1 s᎑1) and produced in a completely automatic way. Conclusion Using this method it is possible to easily measure the optical rotatory power and follow its variation with time (better than one measurement per second) in a completely automatic and operator-independent way. The apparatus is inexpensive compared to the commercial instruments, which add high cost to higher precision and rapidity of measurement. It is also shown that the use of the relatively modern ATAGO Polar X polarimeter is not necessary because it is possible to use a manual polarimeter.
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Literature Cited 1. von Zelewsky, A. Stereochemistry of Coordination Compounds; John Wiley & Sons: Chichester, United Kingdom, 1996. 2. Eliel, L. E.; Wilen, S. H. Stereochemistry of Organic Compounds; John Wiley & Sons: New York, 1994. 3. Atkinson, J. F. Stereoselective Synthesis; John Wiley & Sons: New York, 1996. 4. Alibrandi, G.; Coppolino, S.; D’Aliberti, S.; Ficarra, P.; Micali, N.; Villari, A. J. Pharm. Biom. Anal. 2002, 29, 1025. 5. Heller, W.; Curmè, H. G. In Physical Methods of Chemistry; Weissberger, A., Kossiter, B. W., Eds.; Wiley-Interscience: New York, 1972; Part III C, Chapter II, p 70. 6. Amandola, G.; Terreni, W. In Analisi Chimica Strumentale e Tecnica; Masson: Milano, 1995; pp 251–267. 7. An economic solution for thermostatation problem has been recently reported; Kundell, F. A.; Adkins, W. A. J. Chem. Educ. 2001, 11, 1516.
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