Analytica Chimica Acta 706 (2011) 328–337 Contents lists available at SciVerse ScienceDirect Analytica Chimica Acta j our na l ho me p age: www.elsevier.com/locate/aca Environmental monitoring using a conventional photographic digital camera for multianalyte disposable optical sensors A. Lapresta-Fernández, L.F. Capitán-Vallvey ∗ ECsens, Department of Analytical Chemistry, Campus Fuentenueva, University of Granada, 18071 Granada, Spain a r t i c l e i n f o Article history: Received 20 May 2011 Received in revised form 11 August 2011 Accepted 25 August 2011 Available online 1 September 2011 Keywords: Photographic digital camera Image colour analysis Potassium, magnesium and hardness determination Disposable multianalyte sensor Environmental monitoring a b s t r a c t The primary interest of this study concerns the use of an inexpensive photographic digital camera as the detection system, using its own flash as the source of light to present a new analytical procedure to measure disposable multianalyte optical sensors for potassium, magnesium, hardness and conventional pH test strips. The camera arrangement was designed in a fixed position over an optical board with controllable ambient conditions. After acquiring the digital image, the analytical information contained in each test zone is analyzed using theRGB colour space. Reflectance measurements were developed to study the colourimetric and spectral characteristics of the test zones. We obtained the following application ranges and precision in terms of relative standard deviation (RSD %): for potassium from 3.2 × 10 -7 to 0.1 M with a precision between 3.3 and 4.0%, for magnesium from 2.7 × 10 -6 to 1.5 M showing a precision between 4.7 and 7.8% and finally for hardness from 4.3 × 10 -2 to 200,000 mg L -1 CaCO 3 and between 5.1 and 7.0%. Moreover, the analytical characteristics of several optical procedures were compared with the results presented here. The proposed method was statistically validated against a reference procedure using samples of water from different sources and beverages, indicating that there are no significant statistical differences at a 95% confidence level. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The current trend for rapid and low-cost methods to assess ana- lytical information – composition, quality, discrimination – has led to the development of devices which provide relevant information that is easy to read. One suggestion to obtain methods with the lowest possible cost is the use of disposable sensor devices, mainly single analyte but also multianalyte. Visual multianalyte sensors can produce qualitative information where the change or disappearance of colour provides the yes/no answer [1] or a semiquantitative estimation of concentration is achieved by comparing the colour intensity with a colour chart [2]. Quantitative multianalyte disposable sensors can be based on the spatial resolution of different analytes on the same substrate with or without separation by lateral fluid flow. One example with- out separation is the multi-spot test zone strip developed for the simultaneous determination of several pesticides in water samples by chemiluminescence [3]. The same concept was developed by Lemke et al. [4] as a potentiometric array for pH, K(I), Na(I) and Ca(II) based on coated-film electrodes. In other cases, there are no separa- tions, but the sample is delivered using a microfluidic device such as ∗ Corresponding author. Tel.: +34 958 248436; fax: +34 958 243328. E-mail address: lcapitan@ugr.es (L.F. Capitán-Vallvey). the paper-made multianalite optical device containing hydrophilic microfluidic patterns and sensing areas [5–7]. When separation is required in disposable devices, it is usually performed by lateral fluid flow. One example of a mul- tianalyte disposable optical device is the four-band capillary fluoroimmunosensor presented by Mastichiadis et al. [8] for the simultaneous determination of pesticides. In the case of optical sensing, the usual way to acquire the analytical information from disposable multianalyte sensors is by imaging techniques. The most common are CCD cameras for the measurement of fluorescence [9], electrochemiluminescence [10] or effective absorbance [11]; although other imaging devices have been used such as scanner, including hand-held [12–15] and desktop scanners [16,17], camera phone [15], video cameras [18], webcam [19], digital photographic cameras [20,21] and digital colour analyzers [22–24]. Most of these systems make it possi- ble to acquire images of an analytical element containing potential analytical information, but the ease of use is quite different, jeop- ardising portability in many cases. In this respect, the use of a conventional digital photographic camera as an imaging device and its built-in flash as the light source is an interesting and practical alternative for the easy readout of bulk optode films where the colour indicates the uptake of the analyte. The approach presented in this paper relies on the idea of using an inexpensive photographic digital camera, placed in a fixed 0003-2670/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2011.08.042