Procedure - Refractometer

The temperature of the juice is important in determining the accuracy of the reading. If the juice is heated, the particles in the solution expand, resulting in a lower reading, or if the solution is cooled, the solution becomes denser and the reading is higher. Some refractometers have a temperature compensation capability or a thermometer attachment so they can be adjusted for temperature variations. 

Sucrose is the major sugar in apricots, followed by glucose then fructose, while other sugars such as maltose are present only in minute amounts (Wills et al. 1983). Therefore methods for determination of glucose, fructose and sucrose only are given. 

Typical sugar values (per 100 gram edible portion) found in apricots are  

A kit to carry out the sucrose / glucose / fructose test based on the principal outlined above, is available from Boehringer Mannheim. Cat. No. 716-260. 

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Procedure Prepare precisely 2.5, 5.0, 7.5, 10.5, 15.0 and 20.0% solutions of butyric acid in water by measuring suitable volumes (from a stock solution of 25% w/v) with the help of a burette into six 50 ml volumetric flasks, and finally making up the volume with DW. Using Abbe refractometer measure the refractive indices of all the above six solutions besides the stock solution (25%) at 25°C. Measure also the refractive index of DW. 

Describe the optical system of Abbe s Refractometer, its optical path for upper prism and its operational procedure. 

The basic method is to use a manual Abbe refractometer to determine refractive index. Various automated or electronic instruments exist which automatically perform some of the steps of the manual procedure. The first requirement is that the sample be a solution. In some instruments, the solution is placed between two prisms, and the image of the critical ray boundary is adjusted to meet a reference mark for this adjustment, the refractive index and equivalent °Brix can be read from a scale. The sample temperature must be known, or the instrument must have temperature compensation. Some automatic digital refractometers use the same methodology of sample presentation, but automate the matching of the critical boundary to the reference marker. 

The separation and purification of ethyl esters of EPA, DHA, and the heretofore-minor unreported polyunsaturate octadecatetraenoic acid (C18 4 3, OTA) on a preparative scale by modification of an analytical RP-HPLC procedure has been described by Beebe et al. (48). They used a liquid chromatograph equipped with a differential refractometer as detector operated at room temperature and an ST Macrobore column (350 X 4.6-mm ID) of C18 reverse-phase material, 25-yum particle size. 

An RP-HPLC procedure for the analysis of w-3 fatty acid esters that could eventually be adapted to the preparative isolation of quantities of EPA and DHA was also developed by Beebe et al. (48). The separations were performed at room temperature on an ODS-3 RAC II column (100 X 4.6-mm ID, 5-/zm particle size) with a guard column (30 X 4.6-mm ID dry-packed with SUPELCOSIL LC-18 40-yttm material). The esters were eluted with a mobile phase of acetonitrile/tetrahydrofuran/water (9 5 11) at a flow rate of 2.0 ml/min (detector differential refractometer). 

Bellingham and Stanley provide detailed instructions on the operation of the Abbe refractometer the above account is a general outline of the procedure. 

Procedure (See Chromatography, Appendix BA.) Use a suitable high-performance liquid chromatograph equipped with differential refractometer, autosampler injection unit, mobile-phase degasser, column heating block or oven, and a computing integrator. The column is Lichrosorb RP-18 250-mm x 4.5 mm (id) (GL Science, Inc., or equivalent) and YMC-Pack ODA-A A-303 250-mm x 4.5 mm (id) (YMC Company, Ltd., or equivalent) connected in a series, or equivalent, maintained at 50°. Use 80 20 acetone acetonitrile as the eluent, at a flow rate of 2 mL/min. 

Procedure Transfer 2.00 mL of each sample to separate test tubes, and equilibrate in the 37° water bath for at least 10 min. At the same time, equilibrate the Substrate Solution in the same water bath. At zero time, transfer 2.0 mL of the equilibrated Substrate Solution to the first sample tube, mix thoroughly, and return the tube to the 37° bath. Repeat the process for each sample. After exactly 30.0 min, transfer the test tube to a boiling water bath for 15 min, then remove and cool to room temperature. Add approximately 100 mg of Amberlite MB-1 Ion Exchange Resin to each tube, place the tubes on the shaker, and mix for at least 15 min. Filter the treated solution through a 0.45-p.m filter. Use a separate filter for each sample. Inject a 5-p.L portion of each filtered sample into a previously equilibrated high-performance liquid chromatograph equipped with an HPX 87C column (Biorad, or equivalent) and a differential refractometer. Filtered, degassed water is the mobile phase. Record the elution curve. 

The indexes of refraction should be measured and recorded as soon as possible (the samples decompose on standing). The refractometer and the procedure for its use are... 

The USDA published what may have been the first practical procedure for quickly determining if a truckload of seed is a linoleic or oleic variety (138). It involves squeezing a few seeds in a small hand-powered press to obtain a few drops of oil. A drop of oil is placed on the glass prism cell of a hand-held refractometer. The refractive index has a straight-line relationship with the iodine value or fatty acid distribution of the oil, hence it is easy to determine if the seed in question meets an oleic standard or not, so long as a temperature correction is applied. Recently, it has become simpler to compare the unknown sample to a known oil standard, eliminating the need to apply a temperature correction. Temperature corrections are difficult to measure accurately in the field under the time pressure of harvest. 

Salinity was obtained using the Harvey method (9) and was checked with a hand-held refractometer (9). Sihcate and phosphate analysis were obtained by a Technicon AutoAnalyzer II using standard procedures described in the EPA Methods Manual (10). 

ISO R489 suggests two methods based on Abbe refractometer and the Becke line methods. It does not recommend any specimen conditioning procedure prior to the test. DIN 53491 provides practical details relevant to refractometer measurements. It recommends test temperature... 

ASTM D542 [8] includes only the refractometer method, and this is similar to the ISO procedure. The specified conditions are 23 2°C and 50 5% r.h. In cases where the material has a high thermal coefficient of refractive index, the temperature has to be accurately controlled at 23 0.2°C. 

Occasionally, the refractometer will be so far out of adjustment that it may be difficult to measure the refractive index of an unknown. When this happens, it is wise to place a pure sample of known refractive index in the instrument, set the scale to the correct value of refractive index, and adjust the controls for the sharpest line possible. Once this is done, it is easier to measure an unknown sample. It is especially helpful to perform this procedure prior to measuring the refractive index of a highly volatile sample. 

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