Analytica Chimica Acta 406 (2000) 309–315 Automatic determination of fat in milk by use of a flow injection system with a piezoelectric detector Lisbeth Manganiello a,1 , Angel R´ ıos a , Miguel Valcárcel a,∗ , Angel Ligero b , Teresa Tena c a Department of Analytical Chemistry, Faculty of Sciences, University of Córdoba, E-14004 Cordoba, Spain b Departament of Physics, Faculty of Sciences, University of Córdoba, E-14004 Cordoba, Spain c Departament of Chemistry, University of La Rioja, E-26004 Logroño, Spain Received 2 June 1999; received in revised form 1 October 1999; accepted 10 October 1999 Abstract A new method for the direct determination of total fat in milk by use of a quartz crystal microbalance was developed. The method is based on a (micro) extraction procedure that is carried out in a flow injection system. Samples are diluted with an ethanol–water mixture and injected into the flow system, where they are mixed with n-hexane in an extraction coil. The analyte is determined in the organic phase. The calibration thus achieved is linear in the range 0.20–0.45% w/v, and the relative standard deviation is ±3.2% (n = 11; P = 0.05). The throughput is 12 samples per hour. The proposed method was used to determine fat matter in milk samples; the results were found to be competitive with those of official methods for the same purpose. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Flow system; Piezoelectric detection; Fat; Milk 1. Introduction The use of the piezoelectric quartz crystal as microbalance detector was originally conceived by Bruckenstein and Shay [1], who developed a circuit that allowed the simultaneous measurement of the in situ frequency change accompanying electrolysis at the working electrode as a function of either elec- trode potential or current. Also, they proposed using an oscillating quartz crystal as an electrochemical quartz microbalance. The in situ mass sensitivity of ∗ Corresponding author. Fax: +34-957-218606. E-mail address: qalmeobj@uco.es (M. Valc´ arcel). 1 Permanent address: Chemical Research Centre, Faculty of Engineering, University of Carabobo, B´ arbula, Carabobo, Venezuela. the crystal as determined by electrodeposition of sil- ver, is quite consistent with the value predicted from the Sauerbrey [2] equation. This author also inves- tigated the effects of the solution temperature and viscosity, and the thickness of the liquid film on top of the crystal, and correlated his results with theory. Subsequently Alder and McCallum [3] reviewed the theory and applications of piezoelectric crystals for mass measuring. They concluded that many of the shortcomings and advantages of piezoelectric crystal detectors had been identified and that equipment and data processing had developed to an extent allowing one to identify appropriate situations for application of the piezoelectric crystal detector under real-life conditions. Thus, applications where only a modest level of selectivity is required (e.g. to monitor fam- 0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII:S0003-2670(99)00775-8