Determination of Sugars and Organic Acids in Fruit Juices by FT Mid-IR Investigation of Dry Extract N. DUPUY, M. MEURENS, B. SOMBRET, P. LEGRAND,* and J. P. HUVENNE LASIR (UPR A 2631 L CNRS), Universit6 des Sciences et Techniques de Lille Flandres Artois, Bdt.CS, 59655 ViUeneuve d'Ascq Cedex, France (N.D., B.S., P.L., J.P.H.); and AGRO/BNUT/NIR-Universit~ Catholique de Louvain, Place Croix du Sud, 2/8, 1348 Louvain-la-Neuve, Belgique (M.M.) In an extension of the approach adopted by Meurens et al. for dispersive NIR spectroscopy, the principle of using dry extracts has been applied to FT-IR spectroscopy, to make use of three advantages: the rapidity of Fourier transform spectroscopy, the solvent elimination, and the better peak resolution in the mid-IR region. However, sampling appears more difficult in mid-IR than in NIR spectroscopy. The feasibility of quan- titative analysis has, in a first step, been tested on synthetic samples before application to natural fruit juices. The performance of our dry extract method is reported in terms of spectroscopy as well as of mnl- ticomponent quantitative analysis of sugars and organic acids in fruit juices. Index Headings: Quantitative analysis; FT-IR; Dry extract. INRODUCTION The high intensity and the wideness of the water ab- sorption bands in the infrared spectrum are important obstacles to the accuracy and sensitivity of quantitative analysis by FT mid-IR spectroscopy. In efforts to avoid the water problem, several methods of sample prepara- tion based on solvent elimination have been proposed. Kuehl and Griffiths 1 have described a method where the solvent of HPLC effluents is eliminated by evaporation of the mobile phase after eluate deposition on the surface of a heated mid-IR transmitting powder. In 1982, Meurens 2 presented an equivalent approach for near-IR spectroscopy where fiberglass advantageously replaces the diamond powder proposed for the aqueous liquids. The fiberglass disks are NIR translucent, easy to use, and appropriate for the quantitative analysis of dry ex- tract by NIR spectroscopy. A special drying system named DESIR has been conceived for the fiberglass disks. Its characteristic is to ensure uniform drying of the liquid sample and therefore a reproducible detection of the analyte by reflectance or transmittance measurement. Other systems of dehydration are described in the lit- erature, but their use is not very evident because they are either not practical or not adapted to quantitative analysis. We have experimented with a new system of solvent elimination which allows a fast, easy, and accu- rate multicomponent analysis of aqueous solutions by FT mid-IR spectroscopy of dry extract. This dry extract system is based on the use of silicon windows covered by a thin layer of CaF2 powder. The liquid sample is spotted on the CaF2 layer and dried. The performance of this Received 23 July 1991; revision received 22 November 1991. * Author to whom correspondence should be sent. new method is reported here in terms of spectrum quality and multicomponent quantitative analysis a of principal fruit juice components. MATERIALS AND METHODS Our FT-IR measurements were performed on an IFS48 Bruker spectrometer. The instrument is allowed to purge for two minutes prior to the acquisition of spectra in order to minimize spectral contribution due to atmo- spheric carbon dioxide and water vapor. We coadded 64 scans of symmetrical interferograms at 4 cm -1 resolution for each spectrum. We computed absorbance spectra from 2000 to 700 cm -1 by the triangular apodization function of the standard Bruker software. For the synthetic mixtures, we diluted pure Aldrich products--sucrose (120 g/L), glucose (150 g/L), fructose (150 g/L), citric acid (50 g/L), and malic acid (20 g/L)-- in the range of natural fruit juice concentration given by the book of RSK values. 4 The ATR spectra are obtained with a horizontal ZnSe crystal accessory. The pure water ATR spectrum, which is subtracted from the whole fruit juice, is measured under the same conditions for pure water. The dry ex- tract spectra are obtained by transmission measurement through a Si window and CaF2 thin layer. A reference spectrum of the thin CaF2 layer on the silicon window is collected before each dry extract sample. The sample preparation consists of three operations: (1) the deposit of a CaF2 thin layer on a Si window; (2) the impregnation of the layer with the liquid sample; and (3) the drying of the layer in a microwave oven. For the reference values, the analysis of the natural fruit juices is made by HPLC. The reproducibility of the signal, at each wavelength, is defined by the relative standard deviation according to the following formulas: RSD = (a/x,,)*lO0 (1) 2 ' \1/2 = (y_, (x,- :,m)/N) (2) (7 where xi is the absorbance of the samples, xm is the av- erage absorbance of the samples, and N is the number of samples. The software used for quantitative analysis includes Multilinear Regression (MLR), 5 Principal Component Regression (PCR), and Partial Least-Squares (PLS) 6 programs. The principle of the MLR program consists 860 Volume 46, Number 5, 1992 ooo3-7o2819m6o5-o8~o$2.ooio APPLIED SPECTROSCOPY © 1992 Societyfor AppliedSpectroscopy