Analytica Chimica Acta 486 (2003) 143–148
Implementation of an automatic standard addition method in a flow–batch system: application to copper determination in an alcoholic beverage by atomic absorption spectrometry Luciano Farias Almeida a , Valdomiro Lacerda Martins a , Edvan Cirino Silva b , Pablo Nogueira Teles Moreira b , Mario Cesar Ugulino Araujo b,∗ b
a Departamento de Qu´ımica Fundamental, Universidade Federal de Pernambuco, Recife PE, Brazil Departamento de Qu´ımica, Universidade Federal da Para´ıba, CCEN, Caixa Postal 5093, 58051-970-João Pessoa-PB, Brazil
Received 27 September 2002; received in revised form 18 February 2003; accepted 22 April 2003
Abstract A novel strategy for implementing the automatic standard addition method (SAM) is described. By using a flow–batch system that presents the intrinsic favourable characteristics of the flow and batch techniques, the proposed strategy performs fast standard additions with sufficient flexibility and versatility and employs only one standard solution per analyte. To calculate the analyte concentration, a mathematical model based on a classical SAM and flow variables of the system was developed. The proposed flow–batch SAM was applied to copper determination by flame atomic absorption spectrometry (AAS) in sugar cane-made alcoholic beverages, known as “Cachaça”, available in Brazil. A SAM has been recommended for these analyses because “Cachaças” presents a significantly different composition causing matrix effects and copper determination by calibration using matrix-matching standards can yield inaccurate results. The results show good agreement between the obtained values with the proposed flow–batch SAM and a manual SAM. The mean relative errors and overall standard deviations were always 5.0 mg l−1 [2]. Thus, it is very important for manu∗ Corresponding author. Tel.: +55-83-216-7438; fax: +55-83-216-7437. E-mail address:
[email protected] (M.C.U. Araujo).
factures to monitor the copper content in a quick, accurate, precise, robust and versatile manner. Atomic absorption spectrometry (AAS) can be employed for rapid copper determination in alcoholic beverages by use of calibration matrix-matching standards [3,4]. However, this approach often fails, giving inaccurate results, because of the different matrix compositions of actual samples [3] which may yield statistically different tendencies or matrix effects, and this was observed in alcoholic beverages analyses by AAS [5]. To overcome this draw-
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back, the standard addition method (SAM) has been recommended [5]. One of the ways to implement a SAM [6] is to add increasing amounts of analyte to identical aliquots of the sample; these mixtures are then diluted to the same total volume. This ensures that any sample matrix effects should be the same throughout [7], and in effect a SAM plot is constructed with exact matrix-matching between samples and standards. Besides, different from calibration matrix-matching standards [3,4], a SAM is less susceptible to errors stemming from physical differences in the matrices of the sample [7]. Notwithstanding these advantages, when a SAM is carried out by a non-automatic procedure, it is slow and laborious. These drawbacks have been circumvented by using automatic flow systems [8–20]. Among the various proposed strategies to implement an automatic SAM, the flow systems that exploit the concentration gradients of the sample and of the standard solution [18,19] have presented good analytical performance. However, if the samples have a large variation of analyte concentration and of matrix composition, these systems often are not very versatile, especially when the analyst seeks to design an automatic SAM in order to provide a graph with the best precision [5]. The flow–batch systems have been first named, developed and proposed by our research group to automate spectrophotometric titrations of wines, exploiting a one-dimensional optimisation algorithm for end-point search by means of the Fibonacci method [21]. Recently, flow–batch systems have been used for spectrophotometric determination of total nitrogen in Kjeldahl digests [22], of aluminium in plant tissues [23] and of Fe(III) in estuarine waters [24]. These systems are characterised by the use of three-way solenoids valves or a multi-port selecting valve and an open mixing chamber and to present the intrinsic favourable features of the flow and batch techniques [21–24]. In these systems, the sampling, additions of reagents and signal monitoring are done in the same way as in a flow analyser, whereas mixing and reaction is performed inside an open mixing chamber as in batch systems. In this paper, a flow–batch system is used to implement a SAM in a fast, flexible, versatile and robust manner. This novel strategy to perform an automatic
SAM was applied to copper determination by atomic absorption spectrometry in sugar cane-made alcoholic beverages known as “Cachaça”, available in Brazil.
2. Experimental 2.1. Reagents, samples and solutions The 1000 mg l−1 Cu stock solution was prepared from a Titrisol (Merck, Darmstadt, Germany) ampoule in 0.12 mol l−1 HCl. A 30.0 mg l−1 Cu working standard solution in 0.12 mol l−1 HCl and the 2.0, 3.0 and 4.0 mg l−1 Cu synthetic samples in 40% (v/v) ethanol (Merck) were prepared by appropriate dilution of the stock solution. All solutions were prepared with analytical grade chemicals and freshly distilled-deionised water. “Cachaça” samples were purchased in local supermarkets and analysed without any previous treatment. 2.2. Flow–batch SAM system The proposed flow–batch SAM system is shown in Fig. 1. A model 503 Perkin-Elmer atomic absorption spectrometer, operated according to the recommendations of the manufacturer for maximum sensitivity with an air–acetylene flame and furnished with a copper hollow cathode lamp (324.7 nm), was employed. A model MCP Ismatec peristaltic pump equipped with four pumping channels and employing a 1.85 mm i.d. Tygon® pumping tube was used. The 2.0 ml laboratory-made mixing chamber (MC) was constructed in PTFE. The transmission lines linking the valves VD , VSS , VW and VS to MC and VD to the detector were as short as possible using 0.8 mm i.d. PTFE tubing. Four Cole Parmer three-way solenoid valves were used: three (VW , VS and VSS ) to direct the water, sample and standard solution into the MC, respectively, and the fourth (VD ) to select the stream flowing (water or MC mixture) through the spectrometer. A Pentium 166 MHz microcomputer furnished with a laboratory-made parallel interface card was used to control the proposed system and to perform data acquisition and treatment. The software was developed in Labview® 5.1 graphic language [25]. An electronic actuator (EA) increased the power of the signal sent
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Fig. 1. (a) Diagram of the flow–batch system at initial configuration: electronic actuator (EA); mixture chamber (MC); magnetic stirrer (MS); atomic absorption spectrometer (AAS); microcomputer (PC); solenoid valves (VW , VS , VSS and VD ); water (W); sample (S); standard solution (SS). The arrows indicate the direction of the fluids and the aspiration by the AAS. (b) Dimensions of the MC (values in cm). Stirring bar (SB).
by the microcomputer in order to control the magnetic stirrer (MS) and valves. 2.3. Procedure Two steps are inherent to flow–batch SAM implementation: obtaining the SW , SSS and SS signals that are used to correct the responses for volume changes or the flow-rates of the channels (see Section 2.4) and then performing the standard additions. Before starting the procedure, the solution in each channel is pumped and recycled towards its flasks (Fig. 1). Each valve is switched ON for a time interval of 2 s and the solutions are pumped towards MC to fill the channels between VW , VS and VSS and MC. Next, VD is immediately switched ON and the excess of the solutions in MC are aspirated to waste over 3 s. This process, here denominated as “fill channels”, takes a total time interval of 5 s and should be always accomplished when the solution in each channel is changed. 2.3.1. Measurement of SW , SSS and SS signals In sequence, the standard solution is placed in the water channel and water in the remaining channels. Afterwards, VW , VS and VSS are simultaneously switched ON during a pre-selected time interval (4 s) and the fluids are pumped towards MC. A VD is then switched ON and the resultant solution is aspirated by the spectrophotometer, obtaining signal SW , SS
and SSS measurements are performed using the same procedure, but the standard solution is pumped in the sample channel and after in the standard channel, while water is pumped in the remaining channels. SW , SSS and SS measurements should be carried out only sporadically or when some flow parameter is changed. 2.3.2. Performing the standard additions To accomplish the standard additions all the values are initially switched OFF (Fig. 1) so that the standard solution, sample and water are continuously pumped into their channels, returning to their recipient vessel, while water is aspirated by the spectrophotometer for baseline acquisition (Fig. 2). The VW , VS and VSS values are then simultaneously switched ON during previously defined time intervals for each values (tW , tS and tSS ) and aliquots of each fluid are pumped towards the MC. After MS is switched ON for homogenisation, the mixture is aspirated towards the spectrometer, by switching ON VD , and the standard addition signal is recorded (Fig. 2). This last step is repeated for each standard addition level. For successive analysis of other samples, only the standard addition step is needed. In the experimental design for standard additions, the total volume that is added to MC should be the same at all standard addition levels in order to maintain a constant matrix composition or matrix effect, thus avoiding inaccurate
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Fig. 2. Peak tracings related to the proposed flow–batch-SAM. For details and symbols descriptions, see text.
results [7]. Thus, tS should always be the same and, while tSS increases, tW decreases (and vice versa). 2.4. Theoretical The classical expression for a manually performed SAM [6,18] is: R(m) = k C(m) + kC0 (m = 0, 1, 2, 3, . . . , n)
(1)
where R(m) are the responses of the detector for the mth standard addition, C(m); C0 is the analyte concentration in the sample; k, the linear response constant and n the number of addition levels. Since the sample volume, vS , is maintained constant and the volumes of standard solution, vSS (m), and water, vW (m), change at each mth standard addition level, Eq. (1) can be re-written as: vSS (m) R (m) = k CSS vS + vSS (m) + vW (m) VS + C0 (2) vS + vSS (m) + vW (m) where CSS is the standard solution concentration. In the automatic system proposed here since v = Qt (where Q is the channel flow-rate), the valve timing courses, t, define the volumes, v, added to the MC. So, Eq. (2) can be adapted regarding time rather than volume and tS (m), tSS (m) and tW (m) can be used instead of vS (m), vSS (m) and vW (m). R (m) =
k tS QS + tSS (m)QSS + tW (m)QW × (tSS (m)QSS CSS + tS QS C0 )
(3)
To guarantee the same matrix composition at each standard addition level, the total volume should be constant [7]. As the flow-rates in each channel are not strictly the same and tSS (m) and tW (m) are different at each standard addition level, the correction volume should be implemented by multiplying the (tS QS + tSS (m)QSS + tW (m)QW ) term in Eq. (3) in order to avoid systematic errors. In addition, by dividing Eq. (3) by QSS , Eq. (4) is found: QS QW tS + tSS (m) + tW (m) R (m) QSS QSS QS = k tSS (m)CSS + tS (4) C0 QSS Providing there is a linear relationship between absorbance and concentration of the standard solution during the measurements of SW , SSS and SS (see Section 2.3), QS /QSS = SS /SSS and QW /QSS = SW /SSS may be observed. Assuming: SS SW R (m) = tS R (m) + tSS (m) + tW (m) SSS SSS α = kCSS
and
β = ktS
SS C0 SSS
the following equation is obtained: R (m) = αtSS (m) + β
(5)
Finally, the analyte concentration is calculated from: C0 =
βCSS SSS αtS SS
(6)
where α and β are the slope and intercept coefficients of Eq. (5), respectively, estimated by linear least-squares regression fitting.
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3. Results and discussion Since the total volume of the MC is 2.0 ml, a flow-rate of 3.0 ml min−1 was selected for each channel in order to achieve a low sample and standard solutions consumption, as well as a good sample throughput. Thus, the ON switching time intervals of VS , VSS , VW valves were tS = 6.0 s, tSS = 0.0–4.0 s, tW = 0.0–2.0 s; the homogenization time interval was 3 s and for the aspiration of the mixture and measurements of the analytical signal 4 s was necessary. The uncertainties resulting from the fluid delivery by the solenoid valves were studied by employing a 3.0 ml min−1 flow-rate for each channel. Water was delivered into weighing bottles at time intervals of 1.0–12 s and their mass measured by an analytical balance. The relative standard deviations for 10 replicate measurements in each time intervals were always
