Flow Injection Sensing Strategy for Determining Cationic Surfactants in Commodity and Water Samples

Abstract

The formation of stable binary water-soluble sub-micellar aggregates of cetyltrimethylammonium bromide-copper-pyrocatechol violet complex (CTAB-Cu-PCV) diminishes the stability and absorbance of the Cu-PCV complex. A new flow injection spectrophotometric sensing strategy used for the determination of CTAB in commodity personal care antiseptics and water samples has been established relying on the above-mentioned concept. Based on the reduction of the absorption of the Cu-PCV solution by the injection of CTAB solution at pH 6.0 and 430 nm, a linear absorbance decrease was observed over the CTAB concentration range of 2.0 to 100.0 µg mL⁻¹ (r = 0.987). The analysis method showed limits of detection (3.3 ơ) and quantification (10 ơ) of 0.08 and 0.27 µg mL⁻¹, respectively. The precision (RSD) for five replicate determinations was 7.9 and 3.7% at 10 and 50 µg mL⁻¹, respectively. The developed method was applied successfully to the determination of CTAB in personal care products, namely skin lotion and vaginal wash, in addition to water samples. The corresponding RSD (n = 5) values were ≤8.2%.

1. Introduction

Flow injection analysis (FIA) methodology has been employed to automate a wide range of chemical and/or biochemical analyses since the 1970s [1]. It is a simple technique that is feasible to be coupled with various types of detectors for several analytical applications. An added feature that gives another simplicity of the FIA technique is some parts of the systems can be homemade or replaced as suitable, leading to lowering the cost of the analysis compared to many other commercial separation instrumentation-based analyses [2]. Owing to their eco-friendly safety and availability, cationic surfactants (CSs) or hydrophiles, are commonly used in manufactured soaps, shampoos, detergents, antiseptics, disinfectants, and germicides or sanitizer products as surface cleaning agents [3]. Moreover, they have well-recognized emulsifier and sanitizer activities, which are important for the handle, antifriction and antistatic effects on wool, cotton, human hair, and other synthetic materials [4].

For CSs importance, many publications have been described for their quantitative determinations at the macro- and micro-concentration levels. Many reported determinations of the CSs in different samples were based on their liquid–liquid extraction with different organic solvents after reaction with anionic dyes as counter ions, more commonly Orange II (tetrabromophenolphthalein ethyl ester), bromochlorophenol blue, and quinidine. Few and Ottewill developed a method for investigation of the CSs based on the interaction between these CSs and monosulfonate dye (Orange II), and then an extraction process to the formed complex was achieved by chloroform [5]. The method was insensitive for the high concentration of the CSs and/or a wide range of the used pH, and the LOQ was 10⁻⁵ M. Similarly, Zografi et al. developed a method for their determination based on the reaction between CSs and Orange II dye over a wide pH range, then measured colorimetry at λ_max 488 nm without further extraction with a molar absorptivity of 2.097 × 10⁴ L mol ⁻¹ × cm [6]. Irving and Markham developed two methods for the determination of CSs; one method was colorimetry based on the reaction with bromocresol green dye, and measured at 615 nm, and the second one was relying on the reaction of the surfactant with ammonium erdmannate and measurement at 353 nm. In both methods, their absorptiometry was determined after extraction to an organic solvent, 1,4 dichloroethane [7]. The sensitivities of the two methods respectively were 1.4 × 10⁴ and 4.0 × 10⁴ L mol⁻¹ cm⁻¹. Sakai et al. developed a flow injection analysis (FIA) of CSs in pharmaceuticals depending on ion associates (a blue ternary ion associate) between the CSs and sulphonephthalein dyes (bromochlorophenol blue) and quinidine in an extraction solvent of 1,2-dichloroethane, in a linear concentration range of 5.0 × 10⁻⁵ mol L⁻¹ [8]. Jiro Kawase developed an automated FIA based on ion-pair extraction for the determination of some CSs [9]. The method overcame the slow extractability of the CSs by complex formation and extraction with chloroform in a longer extraction coil before its passing for its spectrophotometric detection with LOQ ranged from 0.3 to 3.0 mmol L⁻¹. The CSs, at a lower concentration than their critical micelle concentration, were determined by an extraction-free FIA as their ability to quench the emission intensity of the formed chemiluminescence systems generated after the oxidation of luminol by N-chloro- and N-bromosuccinimide in an alkaline medium of NaOH, which was used as the carrier stream through the flow injection apparatus. The sensitivity of the CL methods ranged from 0.84 × 10⁻⁵ to 3.6 × 10⁻⁵ mol L⁻¹ [10]. An extraction-free FIA technique based on the enhancement of the color intensity of the formed complex between Fe³⁺ and SCN⁻, measured at λ_max 475 nm, was developed, and their molar absorptivity ranged from (2.10 to 4.30) × 10³ [11]. Next, an extraction-free FIA method based on absorptivity enhancement of the Bi³⁺-I complex in the presence of CSs was developed [12]. The apparent molar absorptivity value of the complex in the CSs was 6.00 × 10³ Lmol⁻¹ cm⁻¹ at 505 nm with a LOD of 110 µgL⁻¹ and a throughput of FIA of more than 140 samples h⁻¹. Additionally, a non-extraction FIA with the aid of a visible detection at a determination of 708 nm of CSs was developed [13]. The method was based on a reaction between the CSs with eriochrome black-T reagent to form a colored ion-association complex. The method was sensitive in the range of 2.0 × 10⁻⁶ to 2.0 × 10⁻⁴ mol L⁻¹. Furthermore, a liquid–liquid free-extraction method for the determination of CSs was developed by Minori Kamaya et al. [14]. The method was based on the association between the CSs and tetrabromophenolphthalein ethyl ester, which gave an adsorbate onto the walls of a PTFE vessel after a vigorous shaking; thus, after discarding the resulting solution, the ion associate was dissolved in methyl cellulose and measured at 605 nm, with an absorptivity of 7.88 × 10⁴ Lmol⁻¹ cm⁻¹.

CTAB (cetyltrimethylammonium bromide, cetrimonium bromide, or cetrimide) is an example of CSs representing a form of a quaternary ammonium product (Figure 1); it is a positive charge candidate and could be adsorbed strongly to the countered negatively charged surfaces of soil, sludge, and sediments as a mechanism of their penetration and cleaning [12]. Therefore, they are widespread with their sorption behavior and are expected to be present in many environmental compartments.

Despite being considered safe for use in rinse-off cosmetic commodities, the CTAB compromises about 5% in washes. On the other hand, it is safe only up to 0.25% in leave-on products such as hair conditioners, tonics, dressings, grooming aids, and face and neck preparations [15]. The hazard of CTAB is because it biodegrades very slowly in the environment and causes chronic health problems due to toxicity in aquatic organisms [16]. Thus, it is important to precisely determine its concentration in the water bodies for monitoring environmental pollution and for quality control of water [17]. Moreover, due to the low biodegradability and the ecotoxicity of surfactants, it is important to monitor their concentration in water bodies.

Our objective of the present study is to develop a new sensing analytical strategy that could be added to the quality monitoring profile of CSs assessment using CTAB as a model. The proposed reaction depends on two steps: initially, there is a reaction of the pyrocatechol violet (PCV, an organic sulfone phthalein dye) with copper ions (Cu²⁺) to create a stable colored complex at slightly alkaline pH [18]. In the second step, adding quantitative amounts of CTAB to the developed colored complex induces a drop-off in the developed color intensity, measured as the difference in absorbance of the original-colored complex and after addition of CTAB, as depicted in Figure 2.

2. Materials and Methods

2.1. Instrumentation

A homemade peristaltic pump furnished with two injection ports, running at a constant flow rate, was employed to propel the carrier solution of the Cu-PCV complex at a fixed flow rate of 3.5 mL min⁻¹ via Tygon tubes (0.5 mm ID) and connections were made of Teflon. The CTAB was implanted into the carrier stream using a sample injection loop of 200 µL. A mixing coil of 10 × 15 mm dimensions was adopted to mix the reactants before entering the detector’s flow cell (Figure 3). A UV-Vis spectrophotometer model UV 1650 PC (Shimadzu, Japan) was used, equipped with Helma flow cell model 174.010 of 1.5 mL bed volume and 10 mm path length. The UV2.10 probe software for kinetic data acquisition was employed to record the absorbance. The pH measurements were operated on Metrohm 780 pH meter (Herisau, Switzerland). Doubly distilled water (DDW) was obtained from two consecutive distillations using a Hamilton laboratory glass apparatus (Europe House, Sandwich Industrial Estate, Kent, UK). Agilent 1200 HPLC system with diode array detector was used as a reference technique to determine CTAB. Analyses were carried out at the National Research Center (Dokki, Giza, Egypt).

The HPLC operation and data acquisition were controlled by Agilent OpenLAB CDS ChemStation software [19]. The system contains a G1315B UV-Vis diode array detector, G1312B Binary pump SL, G1329AALS auto-sampler of 2 mL loop and Superpher. A RP-18 reversed-phase column (4 µm, 12.5 cm × 0.4 cm ID). The mobile phase consists of water:acetonitrile (70:30, v/v) adjusted to an acidic pH 3 with orthophosphoric acid and flows at a rate of 1.0 mL min⁻¹. The injection volume was 10 µL, and the detection wavelength and column temperature were set at 208 nm and 25 °C, respectively.

2.2. Reagents

The CTAB of 99% purity was purchased from Merck (Darmstadt, Germany). A standard CTAB solution containing 1.0 mg mL⁻¹ was freshly prepared by proper dissolution in DDW. A stock solution of Cu²⁺ (1.0 mmol L⁻¹) was prepared by dissolving an appropriate amount of CuSO₄.5H₂O (Panreac, Barcelona, Spain) in DDW, acidified by adding 1.0 mL of concentrated nitric acid and completed to one-liter mark by DDW. PCV was obtained from Sigma Aldrich Co. (St. Louis MO, USA). Stock solution (1.0 mmol L⁻¹) of PCV was made by dissolving 0.38 g of PCV and diluting it to one liter with DDW in a calibrated flask. Citric acid, phosphoric acid, sodium hydroxide, acetic acid and sodium acetate, and hydrochloric acid were obtained from Panreac (Barcelona, Spain). A Teorell and Stenhagen buffer (pH range from 2 to 11) was prepared and it was consisted of a mixture of citric acid, phosphoric acid, and sodium hydroxide all of 1 mol L⁻¹, (adjustment of pH by 0.1 mol L⁻¹ hydrochloric acid) [20]. Carrier solution was prepared by mixing 100 mL from each of Cu²⁺ and PCV solutions and diluting with the acetate buffer to a final volume of one liter to obtain an equimolar concentration of 0.1 mmol L⁻¹ each.

2.3. Flow Injection Manifold for CTAB Determination

A schematic diagram of the FIA manifold for CTAB determination is presented in Figure 1. The flow system, operating in time-based mode, was manifested using a peristaltic pump fitted with Tygon tubes of 0.5 interior diameters. For baseline adjustment, the Cu-PCV complex solution was pumped at a steady flow rate of 3.5 mL min⁻¹. Meanwhile, the DDW carrier was injected at the same flow rate. Both streams were merged in the mixing coil and then left in the detection flow cell and the absorbance signal was recorded at a fixed wavelength of 430 nm. For CTAB measurement, a definite sample volume of 200 µL of different concentrations of the analyte was injected into the carrier water stream and reacted with the Cu-PCV in the mixing coil then the drop in the absorbance signal was recorded. Inclined absorption peaks appeared, and the decrease in the absorbance value was recorded as a function of the CTAB concentration. The downturn in the peak height absorbance was used to represent the concentration of the injected amount of CTAB. The discharge from the flow cell was released into the waste route. After a complete recording of the signal, the sample loop came back to the fill mode for the next cycle.

2.4. Sample Collection and Preparation

The specific personal care commodities, namely skin lotions and vaginal washes were analyzed by the proposed technique.

Clinso^® cleanser solution is a vaginal wash that contains triclosan, cetrimide (1% w/v), menthol, chamomile extract, thymol, camphor, betaine, and propylene glycol. This product is valued as a local antiseptic against bacterial and fungal infections for the routine female cleansing. Additionally, it has lubricating properties, anti-inflammatory and analgesic effects, a potent antipruritic action, and relieves itching and irritation. A 200 mL bottle of the commercial pharmaceutical product containing the viscous liquid was received from a nearby pharmacy, produced by Hicare Novex Pharma Company (El-Nasr City, Cairo, Egypt). Without prior treatment, a 1.0 mL aliquot was diluted 100 times with DDW in a 100-mL flask and the ultimate solution was analyzed.

Zincoderm^® is a skin cream containing zinc oxide, cetrimide (CTAB, 0.5% w/v), chamomile, thymol extract, vitamin E and Glycerine ingredients. It is handled as a first-aid antiseptic ointment commodity to treat sweat, diaper rashes, itching, insect bites, dry skin, and sunburn. A 120 mL cream pack was procured from a local pharmacy, produced by Pixel Pharmaceutical Company (Nasr City, Cairo, Egypt). An exact volume of sample, 1.0 mL was transferred into a 100 mL calibrated flask and 40 mL methanol was added and then heated in a water bath at 50 °C under occasional shaking until complete dissolution. After cooling, it was filtered through a membrane filter to remove any suspensions and then diluted with DDW to the mark. Then, 200 µL from the resulting solution was injected into the flow injections manifold.

Comparatively, the content of CTAB in the mentioned samples was determined by the HPLC-UV reference method [19]. The prepared samples were filtered through a 0.45 µm cellulose acetate membrane before injection into the chromatograph.

3. Results and Discussion

3.1. Spectral Characteristics

The absorption spectra of PCV, Cu-PCV, and Cu-PCV-CTAB were compared in Figure 4. The absorbance peak decreased for the Cu-PCV complex at λ_max 430 nm by the addition of CTAB without any difference in its position. Therefore, the gradual hypochromic effect of the absorption produced by increasing the CTAB concentration was considered as an analytical signal of CTAB. This decrease in absorption was found to be not endlessly. At a particular concentration limit, the absorption remained persistent. Interpretation can be due to the stoichiometric ratio of the stable modified reagent PCV [CTAB]₂ achievement. Beyond this limit, the method’s validity diminishes. Therefore, the PCV concentration was maintained as high as possible compared to the CTAB concentration to enhance the linearity range of the developed procedure.

3.2. Optimization of Chemical Variables Studies

3.2.1. Effect of the pH Value

The influence of solution pH on the PCV-Cu-CTAB ternary complex was investigated in different buffer solutions of pH range 2.0–8.0. For this purpose, a 50 mL sample consists of an equimolar concentration of 0.1 mmol L⁻¹ both of PCV and Cu²⁺. The solution was adapted to different pHs using a Teorell and Stenhagen buffer (pH range from 2 to 11) [20]. The complex solution was pumped into the flow system and 200 µL of CTAB was injected according to the prescribed procedure. The absorbance was recorded in the presence and absence of CTAB at 430 nm. The results achieved are illustrated in Figure 5.

At pH 2, the CTAB has almost no effect on the PCV-Cu complex which might be expected due to the low stability of the complex. The absorbance increased with increasing the pH from 2 to 5 then it became relatively constant between 6 and 7. Additionally, the absorbance for samples that contain CTAB was invariably lower than those without CTAB. Beyond pH 7, the absorbance decreased steadily up to pH 8. Therefore, pH 6.0 was chosen as the optimum value for the consequent measurements.

3.2.2. PCV Concentration Study

The influence of PCV concentration on the sinking of the absorbance was studied within the range from 0.01 to 0.125 mmol L⁻¹ at 30 µg mL⁻¹ CTAB, 0.1 mmol L⁻¹ Cu²⁺ and a carrier flow rate of 3.5 mL min⁻¹. The retrieved results are presented in Figure 6. A linear decrease in the absorbance was realized by extending the PCV concentration from 0.01 to 0.075 mmol L⁻¹, and formerly it became sustained up to 0.1 mmol L⁻¹. After that, the absorbance started to increase. Therefore, the concentration of PCV was selected to be 0.1 mmol L⁻¹.

3.2.3. Effect of Cu²⁺ Concentration Study

The concentration of Cu²⁺ could directly affect the stability of the Cu-PCV complex. Thus, the concentration of Cu²⁺ was investigated from 0.01 to 0.125 mmol L⁻¹. The results obtained are presented in Figure 6. The absorbance quickly increased within the concentration from 0.01 to 0.025 mmol L⁻¹. Later, it remained constant up to 0.05 mmol L⁻¹. Beyond this limit, the absorbance was decreased until it reached a minimum using 0.1 mmol L⁻¹. Then, any increase in the concentration of Cu produced no effect on the absorbance because all PCV have been exhausted either as modified reagent PCV [CTAB]₂ or Cu-PCV. Correspondingly, the Cu²⁺ concentration was settled at 0.1 mmol L⁻¹ to obtain the finest absorbance peak heights.

3.2.4. Carrier Flow-Rate Study

In flow injection protocols, the flow rate of the carrier solution can greatly affect the sensitivity and precision. Therefore, to obtain as high analytical throughput as possible, the carrier flow rate should be optimized as a compromise between analytical sensitivity and time expenditure. The Cu-PCV solution was recommended as a carrier since it would not disturb the background stability. The flow rate was regulated by the rolling pump speed. For this purpose, a fixed aliquot of 200 µL from 30 µg mL⁻¹ CTAB solution was injected into a stream of Cu-PCV complex using the sample loop. The stream was examined at different flow rates varying from 0.75 to 4.5 mL min⁻¹. The absorbance drop decreased linearly by increasing the flow rate within the range of 0.75 to 3.0 mL min⁻¹. At higher flow rates than 3.0 mL min⁻¹, the absorbance was relatively constant. Thus, a flow rate of 3.5 mL min⁻¹ was selected as an optimum sample flow rate for the subsequent experiments to obtain the maximum lowering in the inclined peaks.

3.2.5. Injected Sample Volume Study

The volume of the injected CTAB sample was examined by changing the size of the injection loop in the manifold. This criterion is important since it governs the mass transfer of the analyte through the mixing coil and the recorded analytical signal. Adjustment of this parameter is essential to permit maximum mass transfer without suffering the analytical throughput. Various sample volumes within the range of 100–500 µL were injected into the Cu-PCV stream (Figure 7 and

Table 1. Variation of absorbance of Cu-PCV with the volume of 30 µg mL⁻¹ CTAB solution.

Sample Volume, µL	Absorbance
100	−0.024
150	−0.037
200	−0.0555
250	−0.0751
300	−0.0994
350	−0.1
400	−0.1458
500	−0.1558

). A gradual decrease in absorbance was obtained throughout the whole studied range. The possible dilution of the carrier stream with the analyte solution can be a predominant factor. Therefore, the dilution factor for mixing should be investigated. A significant physical phenomenon in the flow procedures is the diffusion of the analyte zone when injecting a large volume of the sample solution into the reagent stream which can affect the reproducibility of the proposed non-segmented flow stream during transport of the zone to the detector. The dispersion can be deserved to the dilution of analyte concentration by molecular dispersion and convection. However, in most cases, molecular diffusion may be ignored. On the other hand, convection is most effective as a result of differences in the linear flow rate of compounds located at different points along the radial axis of the tube. The centrifugal forces generate secondary flows perpendicular to the flow path in case of non-straight connections. The great difference in polarity and molecular size of the reacting Cu²⁺, PCV, and CTAB species possess a strong effect on the zone dispersion. Finally, the 200 µL sample loop was selected for subsequent experiments to achieve a relevant analytical signal with less zone dispersion.

3.2.6. Tube Diameter Optimization

Since the diffusion of the analyte zone is enhanced by increasing the inner diameter (ID) of conducting tubes. Therefore, the inner diameter of the connection tubes was examined in the range of 0.51–3.18 mm. The results obtained are displayed in Figure 8 and

Table 2. Variation of absorbance of Cu-PCV with the inner diameter of the conducting Tygon tubes.

Tube ID, mm	Absorbance
0.51	−0.0358
0.85	−0.02809
1.14	−0.0247
1.52	−0.02023
3.18	−0.01182

. The lowering in absorbance peak decreases with increasing the tube diameter. So, the 0.51 mm tube inner diameter was chosen for the next investigations.

3.2.7. Effect of CTAB Concentration on Cu-PCV Complex

Under a dynamic process, the absorbance of the Cu-PCV complex was investigated in the presence of different concentrations of CTAB in the range of 0–200 µg mL⁻¹. For this purpose, identical concentrations of Cu²⁺ and PCV of 0.10 mmol L⁻¹ were mixed and pumped into the online system. A definite volume (200 µL) from the CTAB solution at different concentrations was individually injected into the stream and the absorbance drop was recorded. The Cu-PCV served as the carrier solution into which the CTAB was inserted at a flow rate of 3.5 mL min⁻¹, and the absorbance was measured at 430 nm. Figure 9 shows the influence of CTAB concentration on the absorbance of the Cu-PCV complex.

The drop height of the inclined absorbance peaks correlates linearly with the CTAB concentration. The corresponding calibration curve is depicted in Figure 10. A linear response was obtained for CTAB concentration in the range of 2.0–100 µg mL⁻¹. A typical calibration graph corresponding to the regression equation A = −0.025–0.0002 [CTAB µg mL⁻¹] with a correlation coefficient of R = −0.993 was obtained where A is the drop in the absorbance peak height.

3.3. Interference Effect Study

The influences of inorganic and organic species which may interfere with the determination of CTAB were examined. For this purpose, the effect of cations such as sodium, calcium, magnesium, Fe²⁺, cobalt, and zinc ions and anions such as chloride and sulfate ions and organic substances such as acetate, oxalate, citrate, and humic acid were examined (

Table 3. Interfering effect of organic and inorganic compounds with the CTAB determination by the proposed flow injection procedure. Cu-PCV (0.1:0.1 mmol L⁻¹), CTAB (30 µg mL⁻¹), sample loop 200 µL, carrier flow rate 3.5 mL min⁻¹, and measurement at 430 nm.

Foreign Substance	Absorbance
Added concentration, mol L−1	0.0	0.01	0.025	0.05	0.075
Na	−0.0425	−0.0443	−0.0448	−0.0476	−0.0491
Ca	−0.0425	−0.046	−0.0543	−0.0503	−0.047
Mg	−0.0420	−0.0454	0.044	−0.0442	0.045
Fe (II)	−0.0422	−0.0793	−0.087	−0.089	-
Co (II)	−0.0428	−0.0571	−0.0583	−0.0588	−0.0590
Zn (II)	−0.0426	−0.0573	−0.059	−0.0586	−0.0584
Added concentration, mg L−1	0.0	30	60	90	-
Oxalate	−0.0375	−0.0291	−0.0259	−0.0231
Citrate	−0.040	−0.0297	−0.0288	−0.027
Acetate	−0.043	−0.0456	−0.06283	−0.0631
Humic acids	−0.0385	−0.0339	−0.0321	−0.0304

; n = 3). Solutions containing 30 µg mL⁻¹ CTAB and the interfering ion were prepared and injected into the carrier stream to determine the selectivity of the developed procedure. The reported tolerance limit is defined as the ion concentration producing a relative error ≤5%. Alkali- and alkaline-earth elements showed low interference as well as chloride and sulfate ions. The maximum tolerable concentration is ≥1000 mg L⁻¹. The Fe²⁺ ions exhibited no adverse effect while Zn²⁺ and Co²⁺ slightly reduced the absorbance peak but rendered no strong interference with the analytical signal. Acetate and citrate ions showed high tolerance which can be due to their ability to chelate with Cu²⁺. Humic acids and oxalate ions showed a similar tolerance level but less than that of acetate and citrate species. All these organic ions contributed to a decrease in absorbance drop which might be owing to the competition in the interaction between Cu²⁺ and PCV.

Moreover, to study the selectivity of the proposed method, the effect of the cationic surfactants, namely cetylpyridinium chloride (CPC) and dodecyl trimethylammonium bromide (DTAB), and the anionic surfactants, sodium dodecyl sulfate (SDS) and sodium lauryl sulfate (SLS) were individually studied at a concentration range of 0–40 mg L⁻¹ on the absorption profile of Cu-PCV (n = 3). As depicted in

Table 4. Effect of different surfactants on the absorbance change of PCV-Cu chelate. Cu-PCV (1.0:1.0 mmol L⁻¹), surfactant concentration (0–40 µg mL⁻¹), sample loop 200 µL, carrier flow rate 3.5 mL min⁻¹, and measurement at 430 nm.

Surfactant Added, µg mL−1	Absorbance Change (%)
	CTAB	CPC	DTAB	SDS	SLS
0	0	0	0	0	0
5	−5.65401	0	0	0	0
10	−7.173	0	0	0	0.84388
20	−24.05063	−4.21941	−1.26582	+0.34	0
30	−33.75527	−8.43882	−1.56118	−0.95	−0.84388
40	−44.72574	−13.37553	−5.2616	+2.3	0

, all cationic surfactants showed a decrease in the absorbance signal but less than that of CTAB. The maximum drop reached 44, 13, and 5% for CTAB, CPC, and DTAB, respectively, at a concentration of 40 mg L⁻¹. This may be due to the chelate formed in presence of CTAB being able to improve the sensitivity and stability of the analytical procedure.

The selectivity of PCV-Cu towards CTAB over other cationic surfactants might be due to the ability of the latter to form aggregates at a concentration far below its critical micelle concentration. By contrast, anionic surfactants showed no significant effect on the signal drop. They showed very little increase or decrease in the absorbance value. The SDS and SLS showed a change in absorbance of −0.9 and +2.3%, respectively. This can be due to the anionic dye PCV interacting strongly with the surfactant with an oppositely charged surfactant. Therefore, the proposed method is considered sufficient selective for CTAB analysis. It is worth mentioning that real samples such as river water generally contain more anionic surfactants, e.g., SDS, than cationic surfactants; the cationic surfactants are present as ion associated with the anionic surfactants. Therefore, the interference of SDS on the determination of CTAB is likely to be significant, but this depends on the degree of interaction of surfactants associated with the Cu-PCV ensemble.

3.4. Analytical Performance of the Proposed Technique

The optimized chemical and flow variables of the proposed manifold are presented in Figure 3. The analytical characteristics are given in

Table 5. Analytical performance data of the proposed Cu-PCV for determination of CTAB.

Parameter	Value
Sample volume, µL	200
Sample flow rate, mL min−1	3.5
Linear range, µg mL−1	2.0–100.0
Detection limit, µg mL−1	0.08
Precision, %, CTAB (10 µg mL−1)	3.7
Regression equation	A = 0.03492 − 1.2 × 10−4 [CTAB µg mL−1]
Sampling frequency (h−1)	30
Recovery (%)	87.0–104.0
RSD (n = 5)	3.3–8.4

. The detection limit of CTAB, corresponding to three times the standard deviations of the blank absorbance (3.3 σ) for five replicate measurements was 0.08 µg mL⁻¹ (2.0 × 10⁻⁴ mmol L⁻¹). The limit of quantification (10 σ) was 0.27 µg mL⁻¹ (7.4 × 10⁻⁴ mmol L⁻¹). The precision of the proposed method, corresponding to the relative standard deviation (RSD) of five measurements of 10.0 µg mL⁻¹ CTAB was 3.7%. Each analysis consumed 2.0 min for sample injection and for the signal to record. Accordingly, the sample throughput of the proposed method is 30 h⁻¹.

The recovery of the proposed Cu-PCV method was estimated by the determination of CTAB in the spiked distilled water. Good agreement of the results obtained by the proposed method with the spiked value was achieved, as is shown in

Table 6. Accuracy measurement by the add-found test for CTAB determination in spiked distilled water using the developed Cu-PCV procedure.

Amount Added, (µg mL−1)	Found (Mean ± SD, n = 5), µg mL−1	Recovery (%)	RSD (%)
10	8.9 ± 0.7	89.0	7.9
30	27.5 ± 2.3	91.7	8.4
50	48.3 ± 1.8	96.6	3.7

. Recovery ranged from 89.0 to 96.2% with corresponding RSD values from 3.7 to 8.4%. This confirmed the reliability of the developed analytical method for the determination of CTAB in natural water samples.

3.5. Analysis of Antiseptic Products and Water Samples

The analytical application of the proposed method to real samples was evaluated by applying it to the determination of CTAB contents in personal care products. The developed method was applied for the determination of CTAB in skin lotion and vaginal wash. The concentration obtained from CTAB is given in

Table 7. Analysis of CTAB in personal care products using the developed Cu-PCV flow injection procedure.

Analysed Sample	Declared Conc., (mg mL−1)	Proposed Method				HPLC Method Found a, (mg mL−1)
		Found a, (mg mL−1)	Recovery (%)	Error (%)	RSD (%)
Zincoderm® Skin lotion	5.0	4.6 ± 0.30 t = 2.11 F = 1.13	92	−8.0	6.5	4.9 ± 0.10
Clinso® Vaginal wash	10.0	9.8 ± 0.26 t = 0.38 F = 1.02	98	−2.0	2.7	9.9 ± 0.15

^a Mean value ± SD (n = 5). The t and f values refer to a comparison of the proposed method with the HPLC method [19]. The critical (one-tailed) t and F values were 2.31 and 6.39, respectively, at a confidence interval of 95% and four degrees of freedom.

.

The proposed method was accurate, with high recovery values of (92 ± 6.5)% and (98 ± 2.7)% for the skin lotion and vaginal wash, respectively. These values are considered satisfactory compared with the requirement of ± 10%. Using the HPLC reference method {19}, the retention time for CTAB was 2.27 and 2.32 min, respectively. The calculated concentration of CTAB was 4.9 and 9.9 mg mL⁻¹, respectively. Evidentially, the peak area for CTAB in Clinso^® is approximately double that in Zincoderm^®. Moreover, to check the method accuracy, the t-student and F-value methods were investigated using the results obtained by the proposed method compared to those for the reference HPLC determination [19]. As can be seen in Table 3, the results indicated the calculated t and F values are lower than the corresponding critical values. Thus, the means for the development method are not significantly different from that for the HPLC method.

The recovery and RSD values obtained for water samples are shown in

Table 8. Analysis of CTAB in spiked tap, river, and domestic wastewater samples using the developed Cu-PCV flow injection procedure.

Analysed Sample	Amount Added, (µg mL−1)	Found a, µg mL−1	Recovery (%)	RSD (%)
Tap water	0.0	ND b	-	-
	20.0	17.4 ± 1.3	87.0	7.5
	40.0	36.8 ± 3.0	92.0	8.2
River water	0.0	ND	-	-
	20.0	18.2 ± 0.6	91.0	3.3
	40.0	37.5 ± 2.7	93.8	7.2
Domestic wastewater	0.0	5.5 ± 0.2	-	3.6
	20.0	26.3 ± 1.4	104.0	5.3
	40.0	45.0 ± 1.9	98.8	4.2

^a Mean value ± SD (n = 5); ^b ND: not detected.

. In all samples, good recoveries of CTAB could be achieved, which varied from 87.0 to 92.0%, 91.0 to 93.8% and 98.8 to 104.0% for tap, river, and wastewater water, respectively. The RSD was in the range of 3.3–9.5%. The obtained analytical results confirmed the proposed Cu-PCV method satisfactory for real analytical applications.

3.6. Comparison of the Obtained Data to Other Methods

The performance of the developed flow injection Cu-PCV procedure for CTAB determination as compared to the previously reported methods. The data are compiled in

Table 9. Comparison of the developed flow injection Cu-PCV procedure to other reported methods for the determination of CTAB.

Technique	Sample Analyzed	LOD, µg mL−1	Analytical Range, µg mL−1	Ref.
Flow injection	Personal care products, water sample	0.08	2.0–100	This work
Flow injection	Wastewater	0.2	0.7–72.8	[13]
Flow injection	Water, sediment, and soil	0.11	–	[12]
Flow injection	Water, detergent, soap, and shampoo	0.25	0.5–30	[11]
Flow injection	Natural water	0.035	0.34–10.2	[23]
Spectrophotometry	Conditioner shampoo and water	9.1 a	29.1–1820 a	[21]
Spectrophotometry	Gold nanoparticles	2.7	7.2–36.4	[24]
Potentiometry-ISE	Sea water	0.2	–	[25]
Ion chromatography	Raw water, domestic wastewater, and cooling water	2.0	–	[22]
HPAEC-PAD b	Polysaccharides	0.04	0.1–5.0	[26]
HPLC-UV	Pharmaceuticals	4.0	20–200	[19]

^a µg L⁻¹; ^b HPAEC-PAD: high-performance anion-exchange chromatography-pulsed amperometry.

. The present procedure showed a lower detection limit than the reported spectrophotometric [21], ion-chromatographic [22], and HPLC [19] methods. Advantageously, the analytical range of the present approach is wider than the flow injection [11,13,23], the spectrophotometric [21,24], and the chromatographic [19] methods. This confirmed that the present method is alternative and convenient for the analytical determination of CTAB. Further, it can be versatile due to its low cost and speed and can facilitate overcoming the problems due to the micelle formation which mars most chromatographic methods.

4. Conclusions

The flow injection of different CTAB concentrations into a carrier stream composed of Cu-PCV has led to a linear decrease in the absorbance over a dynamic analytical range of 2–100 µg mL⁻¹. Analytical figures of merits revealed adequate sensitivity and allowed the accurate determination of CTAB in personal care products and different water samples. The possible coexisting substances with CTAB or other surfactants could be tolerated at high concentrations. Comparison to other reported methods showed the developed method is robust against interfering species and has good applicability. The results show the suitability of the method for the analysis of environmental water samples and personal care products.