Reverse phase dispersive liquid–liquid microextraction coupled to slotted quartz tube flame atomic absorption spectrometry as a new analytical strategy for trace determination of cadmium in fish and olive oil samples

Highlights

• A sensitive and accurate analytical method was developed for cadmium.

• The optimized RP-DLLME method was used to preconcentrate the analyte from oil.

• SQT with three exit holes was coupled to FAAS system to increase the signal/noise.

• Percent recovery results were calculated between 95.6 and 105.3 %.

Abstract

In the present study, trace level of cadmium was determined by a new, sensitive and simple analytical method called as reverse phase dispersive liquid–liquid microextraction (RP-DLLME) combined to slotted quartz tube flame atomic absorption spectrometry (SQT-FAAS) system. Microextraction parameters such as type and volume of disperser solvent, volume of extraction solvent, heating effect, initial sample volume, final volume, centrifugation period and type/period of mixing were optimized. FAAS and SQT-FAAS systems were also examined to get high signal to noise ratio. After optimizing all significant parameters, the limit of detection (LOD) and limit of quantification (LOQ) of the developed analytical strategy were calculated as 0.74 and 2.48 μg/kg, respectively. In order to determine the applicability and accuracy of the RP-DLLME-SQT-FAAS method, recovery studies were performed in fish oil syrup. Matrix matching calibration strategy was used to eliminate heavy matrix effects on analytical signals, and percent recovery results were found between 95.6 and 105.7 %.

Introduction

In recent years, consumption of fish oil supplements has become widespread and popular among people of all ages and even pregnant women since they are one major source of omega-3 and omega-6 fatty acids (Hansen et al., 2017; Swanson et al., 2012). Omega-3 fatty acids contain some essential compounds such as α-linolenic acids (Swanson et al., 2012). According to British Nutrition Foundation (BNF), 0.2 g of eicosapentaenoic acid (EPA) + docosahexaenoic acid (DHA) per day is a need to support human metabolism for healthy people (Foundation, 1992). Recent studies showed that taking fish oil supplements have numerous benefits such as preventing from depression (Parletta et al., 2019), stroke (Blondeau, 2016), cardiovascular diseases, and high blood pressure (Kromhout et al., 2010). In spite of its benefits, these supplements may have toxic chemicals, so fish and fish oil samples have been investigated for their safety in terms of their heavy metal concentration (Foran et al., 2003; Sun et al., 2018). Another commonly used oil all over the world especially in Mediterranean countries for centuries is olive oil (Foscolou et al., 2018). In this regard, the quality of olive oils has become an important issue for many years. To determine both the quality and the safety of olive oils, quantitative determination of heavy metals such as cadmium, mercury and lead is crucial to protect consumers from heavy metal exposure (Gouvinhas et al., 2015). Cadmium is known as a heavy metal which occurs naturally and identified in 1817 by Stromeyer. It can easily make alloys with other elements like zinc. It is extremely toxic for humans, plants and other living organisms (Yang et al., 2018). According to recent publications, cadmium exposure can cause to development of cancer cells in lungs, prostate, kidney, liver and stomach (World Health Organization, 2010). However, it has been used in pigments, alloys, electroplating other elements, rechargeable (Ni-Cd) batteries (Garche and Dyer, 2009; Nordberg et al., 2014) and its salts are employed as stabilizers in plastics (Robards and Worsfold, 1991), antihelminthics, antiseptics, acaricides and nematicides (Blyth, 1884). Main sources of cadmium exposure are working in primer metal industries, mining operations and consuming tobacco products (Nordberg, 2004). For these reasons, it is essential to develop analytical strategies for the determination of cadmium at trace levels.

Some techniques such as inductively coupled plasma-optical emission spectrometry (ICP-OES) (dos Santos et al., 2015), inductively coupled plasma-mass spectrometry (ICP-MS) (Mędyk et al., 2017), flame atomic absorption spectrometry (FAAS) (Sixto et al., 2019) have been used for the determination of trace level cadmium in literature. Moreover, there are several methods for the determination of various elements in oil samples using inductively coupled plasma-optical emission spectrometry (ICP-OES) (Kalschne et al., 2019), liquid chromatography (LC) (Miralles et al., 2018), high-resolution continuum source flame atomic absorption spectrometry (HR-CS-FAAS) (Gómez-Nieto et al., 2018), graphite furnace atomic absorption spectrometry (GFAAS) (Kalschne et al., 2020).

Flame atomic absorption spectrometry (FAAS) is one of the most common technique to conduct heavy metal detections. Among all spectroscopic techniques, it is found in many laboratories thanks to its cheapness and easy application (Şahan et al., 2010). However, its weak points are low nebulization efficiency and short residence time of the excited atoms in flame region (Matusiewicz, 1997). In order to solve the residence time problem, slotted quartz tubes (SQT) was designed as an accessory that placed into burner head. It helps to increase sensitivity of the conventional FAAS system. It was firstly designed by Watling in 1977 (Brown et al., 1985). Basically, they have one entrance slot underneath the quartz tube to allow the entrance of analyte atoms and one exit slot on the top. Recently, slotted quartz tubes with several exit designs are used in studies (Tekin et al., 2019). In this study, SQT with three exit holes was used to increase the residence time of the atoms in the inner surface of quartz tube before their exiting. As a result of that, the interaction between the analyte atoms and light beam increases which provides sharper and higher absorbance signals.

Low detection limits and low matrix effects can be achieved by microextraction methods. These methods have been preferred by scientists to separate and preconcentrate the analyte(s) of interest (Sorouraddin et al., 2017). Dispersive liquid–liquid microextraction (DLLME) has become one of the most well-known and applied method for both organic (Berijani et al., 2006) and inorganic analytes (Zeini Jahromi et al., 2007). A water immiscible solvent named as extraction solvent is a need for this microextraction strategy but another solvent having miscibility in aqueous and organic phase should be employed to disperse the extraction solvent into the aqueous solution. Therefore, more interaction between aqueous and organic phase can improve extraction yields (Rezaee et al., 2006). However, this approach is not compatible with water-insoluble samples. A new microextraction method called as reverse phase dispersive liquid–liquid microextraction (RP-DLLME) has been introduced to literature in order to meet this necessity. In order to extract the analyte(s) from water-insoluble samples, aqueous solvents like water and nitric acid are employed as the extraction solvent (Lourenço et al., 2019). There are some studies in literature regarding this method for the determination of organic (Miralles et al., 2018) and inorganic analytes (Kalschne et al., 2019). One study proposed oleuropein extraction from olive oil extracts by water and ethyl acetate mixture which also facilitates easy analyte detection by reverse phase high performance liquid chromatography (Hashemi et al., 2010). In our study, analyte in oil samples was extracted by the help of RP-DLLME method to investigate cadmium concentration at trace levels.

In this study, RP-DLLME and SQT-FAAS systems were combined to determine trace levels of cadmium in fish and olive oil samples. SQT with three exit holes was employed to prolong the dwell time of the cadmium atoms in the light path. Significant parameters in microextraction strategy and instrumental system were adjusted to their optimum values. After validating the developed method, spiking experiments were performed in order to test the applicability and accuracy of the developed method.