Green microextraction and preconcentration methods for heavy metal determination coupled with atomic absorption spectrometry

Abstract

A novel preconcentration method based on hybrid air-assisted solvent terminated dispersive liquid-liquid microextraction (AA-ST-DLLME) using a domestic fish-tank pump as a bubble generator was developed for the determination of Pb in water and beverage samples using graphite furnace atomic absorption spectrometry (GFAAS). Termination or demulsification was a new approach for centrifuge-less separation in DLLME, based on the demulsification process for emulsion break down. In this approach, the extraction was terminated by the addition of a second portion of the surfactant that acted as a demulsifier and encouraged physical phase separation without centrifugation. The important parameters for the microextraction process (pH, concentration of complexing agent, type of extracting solvent, type of disperser, type of demulsifier, air-assisted time, and extraction time) were optimized. Under the optimized conditions, a sensitivity enrichment factor (EF) of 58.9 was achieved, with the preconcentration factor (PF) being 60.0 and the extraction efficiency of 98.2%. The limit of detection (LOD) and limit of quantitative (LOQ) were 0.025 ng/mL and 0.084 ng/mL, respectively. The relative standard deviation (RSD) was 1.02-3.19%. The proposed method was applied for the determination of trace levels of Pb in water, fruit juice, and beverage samples. The spiked recovery at three different spiked concentrations were 82.0-117.3%. Results from the analysis of standard reference materials (SLRS-6) were not significantly different at the 95% of confidence level.

In the second approach, novel method was proposed to determine the concentration of lead (Pb) in different type of water samples by preconcentration with magnetic dispersive solid phase extraction (MdSPE) using a graphene oxide-magnetite-dithizone (GO-Fe3O4-DTZ) sorbent made of synthesized graphene oxide from the used graphite tubes of electrothermal technique, separated by external magnetic field and analyzed with flame atomic absorption spectrometry (FAAS). The synthesized sorbent was evaluated for its surface property, functional group and surface morphology by Zeta potential, Fourier Transform Infrared Spectrophotometer (FTIR), and Scanning Electron Microscope (SEM), respectively. The relevant measurement parameters such as pH, extraction time, type and concentration of eluent, sample volume and reusability were optimized. Under the optimal conditions, preconcentration factor was 13.33. LOD and LOQ obtained were 0.070 and 0.23 mg/L, respectively. %RSD was 3.41% at 1.0 mg/L of Pb. For the accuracy study, percentage recovery values determined using spiked standard solutions in drinking, tap, and river water samples were 90.1-123%. In addition, the robustness of proposed method was reported in terms of the tolerance limit obtained from interference studies.

The third microextraction method involved effervescent tablet assisted-switchable solvent based liquid phase microextraction (EA-SS-LPME) for multi-element sequential determination of Pb and Cd in drinking water, canned tuna, and canned fruit samples by high resolution continuum source flame atomic absorption spectrometry (HR-CS-FAAS). NaHCO3 and KH2PO4 were used as effervescent tablets for improving of extraction efficiency. Triethylamine (TEA) as a hydrophobic solvent was synthesized by addition of CO2 for switching to protonated triethylamine carbonate (P-TEA-C) as a hydrophilic form. The synthesized switchable solvent was applied to extract Pb-dithizone and Cd-dithizone complexes in samples. Under the optimum condition, calibration linearities were obtained from 0.06 to 10.0 mg/L (Pb) and 0.02 to 1.50 mg/L (Cd). For the analytical performance, it was found that LOD was 0.0195 (Pb) and 0.0068 (Cd) mg/L, respectively. LOQ was 0.0649 mg/L (Pb) and 0.0228 mg/L (Cd), respectively. %RSDs were 1.25%-1.69% (Pb) and 1.07%-1.64% (Cd). EA-SS-LPME method was applied for the determination of Pb and Cd in water and canned food samples. The spiked recovery at different spiked concentrations were in the range 82.3-119.0% (Pb) and 81.7-120.0% (Cd). The preconcentration factor of proposed method was 3.3 with enrichment factor of 1.4 (Pb) and 2.6 (Cd) with the analysis time of 8 minutes per extraction.

The fourth approach involved a dispersive liquid-liquid microextraction based deep eutectic solvent with air-assisted technique ( AA-DES-DLLME) have been developed and applied to the extraction and preconcentration of Pb and Cd using sequential multi-element determination by HR-CS-FAAS. In this method, mixed Pb and Cd solution were used for optimization and spiked in real samples for evaluation. First, pH 13 buffer and dithizone (DTZ) were added to sample. Pb-DTZ and Cd-DTZ were formed and dispersed by the addition of THF. Then, target complexes were extracted into organic phase by the addition of DES. Extraction efficiency was achieved by small air bubbles addition. This was a simple procedure for air-assisted technique using an inexpensive home fish tank pump. Under the optimum condition, Pb-AA-DES-DLLME were linear in the range 0.017-4.50 mg/L with LOD and LOQ at 0.0050 and 0.0168 mg/L, respectively. %RSD was in the range 1.49-2.43% with the enrichment factor of 2.60. For Cd-AA-DES-DLLME, the calibration curve was in the range 0.009-2.00 mg/L with LOD and LOQ at 0.0028 and 0.0094 mg/L, respectively. %RSDs were 1.39 to 4.19% with the enrichment factor of 3.08. Preconcentration factor was 6.67. Extraction recovery from the spiked standard solution to real samples were in the range (Pb) 84.20-114.6% and (Cd) 84.70-118.0%. AA-DES-DLLME method was applied for preconcentration and extraction of trace Pb and Cd in drinking water and canned food samples with 15 minutes per extraction by HR-CS-FAAS.

In the final work, the determination of Pb in anionic form via hollow fiber liquid phase microextraction (HFLPME) using an electric field with deep eutectic solvent (DES) carrier acceptor and GFAAS detection was firstly reported. Pb-oxyanion or hydrate form of Pb was used as a chemical form in the proposed method. Chemical forms of Pb in samples were converted to anion forms using alkali condition. Electro-assisted technique was applied to facilitate the anion passed thorough the supported liquid membrane (SLM) into the phase of DES mixed with buffer solution so called EM-DES-HFLPME. Therefore, effect of extraction parameters were carefully obtained and evaluated. Under optimum condition, linear range was observed in the range 0.0036-1.00 ng/mL. Limit of detection and limit of quantitation were 0.011 and 0.036 ng/mL, respectively. Precision defined as %RSD was 1.03-3.49%. Enrichment factor and preconcentration factor were 111 and 750, respectively. EM-DES-HFLPME was applied for the determination of Pb in water samples with the acceptable recoveries from 83.9 to 110.9%. The proposed method was successfully applied to the analysis of standard reference materials (SLRS-6) at which found concentration was not significantly different at 95% of confidence level from certified value.

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