Reagent-less and sub-minute quantification of sulfite in food samples using substrate-integrated hollow waveguide gas sensors coupled to deep-UV LED
Graphical abstract
Introduction
Free sulfites usually refer to a series of sulfur-containing chemical agents (e.g., sodium sulfite, sodium bisulfite, sodium metabisulfite, potassium sulfite, etc.) widely used as preservatives by the food industry [1]. Due to their inherent oxidation properties – they are easily oxidized to sulfates – sulfites prevent the proliferation of fungi and bacteria in various types of foods, ranging from alcoholic beverages and soft drinks to fruits, seafood, jams and processed meats [2,3].
On the other hand, an immoderate consumption of sulfite may cause innumerous health issues, especially to sulfite-sensitive people, such as dermatitis, urticaria, flushing, hypotension, abdominal pain, diarrhea, anaphylactic reactions, and inflammation of the airways (as it is an activator of neutrophils) [1].For these reasons, the majority of countries follow the recommendations by the Joint FAO/WHO Expert Committee on Food Additives (JECFA), which establishes an acceptable daily intake (ADI) of sulfite (expressed as SO2) of 0.7 mg/kg per body weight/day [1,4,5]. The control and regulation of the maximum permitted levels of sulfite in foods are established by international agencies. For instance, the European Community through the Directive 95/2/EC establishes the maximum permitted levels of sulfite for a large variety of foodstuffs, including fruit juices for bulk dispensing systems (50 ppm), beer (100 ppm), mustard (250 ppm), dried fruits (2000 ppm) and marmalades (50 ppm) [6].
Hence, analytical methods that provide accurate sulfite determination have proven to be essential. The direct determination of sulfite in food matrices is not feasible due to the sample complexity. Therefore, during sample preparation it is fundamental to separate the analyte from the matrix in order to enhance the selectivity [1]. Additionally, it may also lead to an enhancement of sensitivity caused by the increase in the final concentration of the analyte. Extraction by volatilization (i.e. distillation) is frequently employed when the analyte is likely to be volatilized (e.g. formaldehyde [7], sulfite [8], sulfide [9] and nitrite). Free sulfites are extracted from the sample by conversion to gaseous SO2 after acidification and then collected using an appropriate reagent solution – usually aqueous-based – prior to the analysis. A variety of instrumental analytical techniques based on this procedure have been used, for example, spectrophotometry [10], flow-injection analysis (FIA) [8] electrochemistry [11], and chromatography-DAD [12]. Moreover, the officially acknowledge Monier-Williams method [13] uses titration with standard NaOH solution after the conversion of SO2 to SO42− by the addition of H2O2.
However, it is a challenge to collect gaseous SO2 with high efficiency, as it needs to solubilize into the trapping solution, thus requiring that the mass transference at the gas/liquid interface be elevated. Usually, the SO2 is directed using a controlled airflow and bubbled into the trapping solution. Although this procedure decreases the analysis time, the efficiency of the sample solubilization is affected by high airflows, which in turn also affects the sensitivity. Gas-diffusion microextraction (GDME) allows the gas passage from a donor solution (sample) to an acceptor (trapping) solution through a semi-permeable membrane (e.g., PTFE) [14]. This interesting approach does not require pumps, as the gaseous SO2 is moved by diffusion. However, the time to complete the solubilization process in the trapping solution is significantly extended. Therefore, it can be concluded that the volatilization/collection approach decreases the analytical frequency, affects the accuracy and precision of the results, uses specific reagents to enhance sample collection and requires an additional step to collect the actual measurement. Additionally, most analytical methods are bulky and laborious, which limits their applicability for in situ analysis. In particular, the Monier-Williams method suffers from low repeatability and sensitivity, laborious procedures, bulkiness of instrumentation, high analysis time, and interference from other volatile compounds.
An interesting approach to eliminate the sample collection step and maintain treatment benefits from volatilization is to directly detect the gaseous SO2. A few studies following this strategy have been reported using different approaches. In one study, a conventional UV–visible molecular absorption spectrometer was employed to detect SO2 in the gas-phase using the molecular absorption band at 198 nm [15]. In another approach, the molecular fluorescence of gaseous SO2 was determined using a hollow cathode lamp (213.8 nm) and an atomic fluorescence spectrometer [16]. Oliveira et al. developed an analytical method to detect SO2 generated from total and free sulfite present in coconut water using high-resolution absorption spectroscopy [17]. To this end, a xenon arc lamp was used as a continuous source and the absorbance was measured at 215 nm. Although the obtained linear ranges using these methods were suitable for the determination of sulfite in food samples (10–1000 ppm), the complexity (automatic injectors, specific quartz tubes and extended gas cells), high cost and bulkiness of instrumentation make these methods unfeasible for routine analysis. Additionally, it has been reported that the absorption cross-sections of SO2 below 260 nm undergo significant differences due to variations in temperature [18].
Molecular spectroscopy – including vibrational and electronic spectroscopy – has been used extensively used for sensing because it provides valuable information about structure and concentration of molecules [19,20]. In 2013, Mizaikoff and collaborators designed a new generation of miniaturized and non-flexible hollow waveguides – the so-called substrate-integrated hollow waveguides (iHWG) [21] as gas cells. The interaction between the molecules and radiation in the mid-infrared and ultraviolet range provided by the iHWG enabled the development of several arrangements for the determination of numerous gases, such as O3, NOx, CH4, CO2 and H2S among others [[22], [23], [24], [25], [26]]. Its highly versatile (i.e. adaptable) geometry favors the fabrication of a variety of sensing systems via the combination of iHWG with a wide range of light sources (e.g. conventional broadband IR sources, tunable lasers and miniaturized lamps) and detectors. The designable optical path length and the low volume of sample required facilitates close to real-time measurements and confers portability to the device, rendering it a promising for in situ applications.
Recently, deep-UV light emitting diodes (deep-UV LEDs) have been applied in analytical platforms due to their high output stability, robustness, low power consumption and low-heat production [27]. Their relatively narrow emission bandwidth (∼30 nm) allows their direct use as a light source for spectrometric devices without the need for optical filters or monochromators, resulting in a significant decrease in complexity and cost [28]. Several recent studies have demonstrated the development of analytical platforms based on deep-UV LEDs emitting at 235, 250 and 280 nm for absorbance measurements, including HPLC [29] and CE [30] detectors as well as gas sensors [31].
Therefore, in this study we report the combination of a deep-UV LED (@280 nm) and an aluminum-coated iHWG gas cell to produce a compact, portable and low-cost analytical platform. This device was employed for monitoring gaseous SO2 generated from acidified food samples containing free sulfites via absorbance measurements. The obtained analytical signal was associated with the concentration of sulfite in food samples, providing a rapid, simple and accurate analytical method.
Section snippets
Solutions and analysis protocol
A series of Na2SO3 standard solutions (Synth, Brazil) for calibration were daily prepared by serial dilutions of a stock solution at a concentration of 5 g L−1. The sulfite determination was performed as follows: 5 mL of the standard solution or the beverage sample was initially transferred to a glass flask. Then, 2 mL of a 2 M H3PO4 (Synth, Brazil) was added, and the flask was immediately sealed. Subsequently, a purified airflow of 200 mL min−1 of air provided by a 5V mini-pump (RS-385) was
Principle of SO2 detection
Sulfur dioxide has a strong signature in the ultraviolet range, with maximum absorption wavelengths at 190 and 280 nm. Its absorption cross-sections are 3 × 10−17 and 1 × 10−18 cm2 molecule−1 for each WL, respectively [32]. For comparison purposes, ozone – a well-known strong UV absorber – has an absorption cross-section of 1.12 × 10−17 at 254 nm23. Therefore, the development of optical methods for the monitoring of the absorbance of gaseous SO2 at 190 nm has been preferable due to the inherent
Conclusion
The employment of an extraction step based on volatilization is a powerful tool used as a sample preparation technique that minimizes interference and sample matrix effects. This fact is particularly important for food analysis, where the complexity of the sample is usually high, thereby limiting the application of direct analysis protocols. However, the volatilization method is usually followed by gas trapping and analysis using a suitable analytical technique, which implies the use of
CRediT authorship contribution statement
Diandra Nunes Barreto: Methodology, Validation, Investigation, Writing – original draft. Gabriel Martins: Methodology, Validation, Investigation. Peter C. Hauser: Conceptualization, Writing – review & editing. Boris Mizaikoff: Conceptualization, Writing – review & editing. João Flávio da Silveira Petruci: Conceptualization, Supervision, Writing – original draft, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
D.N.B., G.M.F. and J.F.S.P. wish to thank the Brazilian agencies CAPES (Coordination for the Improvement of Higher Education Personnel – Finance Code 001), FAPEMIG (APQ-00196-22), and CNPq (National Council for Scientific and Technological Development – Proc. 403929/2021–0) for the scholarships and their financial support.
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2023, Food ChemistryCitation Excerpt :This method presents some disadvantages, such as the need of bulky instruments for analyte volatilization (i.e., distillation), reduced analytical frequency, low reproducibility, and accuracy. Additionally, other reagents have been employed to collect the extracted SO2 and enabling the measurement using different analytical techniques, such as spectrophotometry (Tavares Araújo, Lira De Carvalho, Ribeiro Mota, De Araújo, & Coelho, 2005), flow-injection analysis (Melo, Zagatto, Mattos, & Maniasso, 2003), gaseous UV-spectroscopy (Barreto, Fernandes, Hauser, Mizaikoff, & da Silveira, 2022) and voltammetry (Araújo, Oliveira, Pradela-Filho, Takeuchi, & Santos, 2021). Usually, the generated gas is directly bubbled to the trapping solution (i.e., acceptor) by using a constant airflow of carrier gas.