Environmental Sample Stability for Pharmaceutical Compound Analysis: Handling and Preservation Recommendations

Efficient and resilient techniques for handling samples are essential for detecting pharmaceutical compounds in the environment. This study explores a method for preserving water samples during transport before quantitative analysis. The study investigates the stability of 17α-ethynylestradiol (EE2), acetaminophen (ACM), oxytetracycline (OTC), sulfamethoxazole (SMX), and trimethoprim (TMP) after preconcentration within solid-phase extraction (SPE) cartridges. Through various experiments involving different holding times and storage temperatures, it was determined that four pharmaceutical compounds remained stable when stored for a month at 4°C and for six months when stored at −18°C in darkness. Storing these compounds in SPE cartridges at −18°C seemed effective in preserving them for extended periods. In addition, ACM, TMP, OTC, EE2, and SMX remained stable for three days at room temperature. These findings establish guidelines for appropriate storage and handling practices of pharmaceutical compounds preconcentrated from aqueous environmental samples using SPE.


Introduction
Te emergence of pharmaceuticals within the environment has become a topic of heightened public concern due to their potential ecological ramifcations.Tese pharmaceutical compounds fnd their way into the environment through diverse avenues such as wastewater treatment plant discharge, improper disposal of expired medications [1][2][3][4], pharmaceutical industry and hospital efuents [5], and animal waste discharge into the environment.Te ubiquity of pharmaceuticals, including acetaminophen (ACM), oxytetracycline (OTC), sulfamethoxazole (SMX), trimethoprim (TMP), and 17α-ethynylestradiol (EE2), has raised concerns regarding their environmental impact [6][7][8].In the context of this study, these selected pharmaceuticals hold particular signifcance as they are currently priority targets in Indonesia's eforts to detect their presence in aquatic environments and assess associated environmental risks [9][10][11][12].ACM, or paracetamol, is a globally widespread antipyretic and analgesic, often used as a primary treatment for headaches, including migraines, a signifcant contributor to disability, particularly among young women [13][14][15][16].ACM's availability without prescription and its afordability in the Indonesian context has led to its wide utilization [17,18].Te detection of ACM in various aquatic environments, coupled with its potential threat to aquatic organisms, warrants attention [19][20][21][22].
OTC, SMX, and TMP are widely used antibiotics for therapeutic purposes in humans, as well as in farmed aquatic animals (including fsh and shrimp) to treat bacterial diseases [23,24].Tose compounds are three of the top 10 list of generic antibiotic drugs [19].Te presence of OTC in environmental compartments has been detected worldwide [25], as well as TMP [26] and SMX [27].Previous study conducted by [28,29] demostrated OTC bioaccumulation in various organisms, including invertebrates and fsh.OTC, TMP, and SMX are often detected in seafood products, especially shrimp [30,31].Tose antibiotics have been reported to potentially cause negative impacts on the aquatic organisms [32,33].
EE2 is a constituent of frequently used contraceptive pills and an endocrine disruptive substance [34].A high risk of their occurrence in aquatic environments becomes a rise concern.Fertilization was signifcantly reduced in sea urchins when exposed to this substance.Morphological abnormalities were also observed in mussel and sea urchin embryos [35].Te synthetic estrogen EE2 also poses an adverse efect on the reproduction of aquatic organisms [36,37].Te continuous use and the evident occurrence of those selected compounds in aquatic environments as well as the high potential ecological risks on aquatic organisms attracted our attention to investigate their stability.
Efective environmental studies necessitate wellstructured sampling plans encompassing location, timing, and methodology for collection and analysis.Sample handling is paramount within the broader sampling strategy to ensure analysis accuracy [38,39].Changes in the analyte concentration during storage have been recognized as a potential source of bias that could occur prior to sample analysis [40].Storage conditions are critical, as samples can be degraded through bacterial activity, exposure to light, and/or chemical interactions [41,42].Furthermore, it is important to obtain the maximum delay between the sample collection and analysis times to provide accurate and reliable data.Tis also includes transport during and or after feld sampling and storage before analysis [43,44].For this reason, in this study, we investigated a set of conditions necessary to prevent or minimize errors introduced through sample handling and storage conditions before environmental sample analysis.In the current study, we estimated the stability of targeted pharmaceutical compounds (ACM, antibiotics, and steroid hormones) under various sample storage conditions and maximum holding times after their preconcentration involving solid phase extraction (SPE) cartridges.Given the focus on marine life, saltwater serves as the focal point.Tis choice is rooted in environmental, regulatory, and ecological considerations, acknowledging the signifcant infuence of medicinal compounds on coastal ecosystems and public health.
Te outcomes of this study hold profound relevance, particularly in regions with limited access to necessary laboratories and analytical instruments such as Indonesia, where the time span between sampling and analysis is extended.By addressing these challenges, the study aims to contribute invaluable insights to uphold data validity and reliability in environmental analysis.

Preparation of Standard Solutions.
Individual compound standard solutions (1000 mg•L −1 ) were prepared separately by dissolving an appropriate amount of each substance in 10 mL of MeOH.Te stock standard solutions were then vigorously shaken to dissolve the standard material.Individual stock solutions were fnally stored at −18 °C in the dark until use.Working standard solutions were prepared from individual stock solutions in 10 : 90% ACN/H 2 O at a concentration of 10 mg•L −1 .All the working solutions were stored at −10 °C.Tese mixtures were employed to generate calibration curves and perform recovery studies.

Solid Phase Extraction (SPE).
Considering the need for a "controlled experimental environment," all experiments were conducted using an artifcial seawater matrix.Tis decision ensured a controlled and replicable experimental context.Te selection of artifcial seawater aimed to establish a well-defned setting.During SPE extraction, 10 mL artifcial seawater samples were processed.Each seawater sample received 100 µL of a standard solution.Te pH was adjusted to 7 using HCl or NaOH.A 10 mM Na 2 EDTA solution was introduced to achieve a fnal concentration of 10 mM EDTA in the samples.Subsequently, a standard 2 Journal of Analytical Methods in Chemistry analyte solution was added to attain a fnal concentration of 10 mg•L −1 .Te spiked artifcial seawater samples (10 mL aliquots) underwent extraction within SPE cartridges, employing a modifed version of a validated analytical method [45] with some modifcation.Briefy, 10 mL of the seawater samples was preconcentrated in SPE cartridges.Te cartridges were conditioned by successively applying 5 mL of acetonitrile, 5 mL of ultrapure water, and 5 mL of Na 2 EDTA 10 mM with a pH of 7 at a fow rate of 2 mL/min [2].After sample loading at a rate of 1-2 mL•min −1 , the cartridges were pumped under vacuum and then stored at various temperatures (−18 °C, 4 °C, and room temperature) and holding times.Five milliliters of ACN/H 2 O (% v/v) at the selected percentage composition was used as the elution solvent.
Te extracted solvent was evaporated to an extract volume of approximately 50 µL under a gentle stream of nitrogen at 40 °C and then transferred into an amber chromatographic vial (2 mL).Te collection vial was then successively washed with 2 × 50 µL of ACN and adjusted to a fnal volume of 1.0 mL.All reconstituted extracts were passed through PTFE (0.2 μm) fltration membranes before analysis.

Optimization of Extraction.
To achieve optimal recovery of the target compounds, a range of ultrapure ACN/H 2 O solvent compositions (% v/v) (40/60; 60/40; 80/20; 90/10; and 100/0) were systematically evaluated to facilitate the separation of the analyzed compounds from the HLB cartridges.Te recovery assessment was conducted at a consistent concentration of 10 mg•L −1 .
Te extraction recovery was determined using the absolute recovery (AR) formula, as specifed by the following equation [46]: AR � Peak area of analyte in sample Peak area of analyte in external sandard . (1) Here, "Peak area of analyte in sample" represents the peak area of the analyte in the sample that had been spiked prior to extraction, and "Peak area of analyte in external standard" pertains to the peak area of the analyte dissolved in pure solvent.Each set of experimental conditions was replicated in triplicate during the recovery assessment.To ensure robust quality assurance, three procedural blanks consisting of 10 mL of artifcial seawater were included in the analysis.

Chromatographic Analysis.
All analyses were conducted using an ultraperformance liquid chromatography (UPLC) instrument (PerkinElmer Altus 30) paired with an ultraviolet (UV) detector.Chromatographic separation was performed using a Zorbax Eclipse XDB C 18 column (2.1 mm × 150 mm, 3.5 µm) at 45 °C with HPLC-grade acetonitrile enriched with 0.1% formic acid for mobile phase A and ultrapure H 2 O with 0.1% formic acid for mobile phase B. A 5 μL sample of the analytes was injected at a 0.4 mL•min −1 fow rate.Te sample reservoir was set up at a temperature of 4 °C.Te elution gradient was as follows: initial time-0% A; 2 min-5% A; 13.5 min-0% and maintenance until minute 16; and 17.0 min-5% A and maintenance up to 18 min.All the analytes were measured using UV detection at wavelengths of 280 nm for ACM, 218 nm for TMP, 355 nm for OTC, 248 nm for SMX, and 200 nm for EE2.
2.6.Method Validation.For validation, the ICH guidelines of Q2 [40] served as the established protocol, encompassing assessments of linearity, accuracy, precision, and extraction recovery.Linearity test solutions were prepared spanning concentrations from 2 to 500 mg•L −1 , with each predefned concentration being independently prepared in triplicate.
Linearity assessment was executed through regression analysis, yielding intercept and correlation coefcient values.Accuracy and precision were determined based on recovery percentage and relative standard deviation (RSD%), respectively, both for intraday and interday assessments.Intraday precision was gauged by analyzing six replicate samples within the same day, while interday precision was evaluated over three days with samples spiked to identical concentrations, with results expressed as RSD%.Accuracy was quantifed through the relative error (RE), representing the percent deviation from the nominal concentration.According to [47], intra-and interday precision criteria can be accepted with values ≤15%.Te limit of detection (LOD) and limit of quantifcation (LOQ) were calculated by multiplying the average concentration of ten blank replicates by 3 and 10, respectively [48].LOD values were determined as 0.19 for ACM, 0.14 for TMP, 0.12 for OTC, 0.01 for SMX, and 0.04 for EE2.Corresponding LOQ values were established at 0.62, 0.46, 0.4, 0.04, and 0.14 for ACM, TMP, OTC, SMX, and EE2, respectively.To ensure that no pollutants were present and that no signifcant drift in the instrument sensitivity occurred, a blank sample and a standard at a concentration of 10 µg•L −1 were employed.Moreover, in this study, the instrument sensitivity drift was assessed and validated considering a threshold with a 10% maximum drift [49].

Study of the Holding Time and Storage Temperature.
After the preconcentration step, a stability study was conducted by exposing the samples, which remained in their packs (SPE cartridges), to likely exposure conditions.Te SPE cartridges were then stored at diferent commonly adopted temperatures, i.e., frozen (−18 °C), cold (4 °C), and room temperature (25 °C), in the dark and for various lengths of time (from day 0 up to 6 months after preconcentration).
Initial sample preparation occurred on day 0 for each study, with analysis commencing within three hours of preparation.Subsequent samples were immediately stored under designated conditions until analysis.Storage at 4 °C involved refrigeration until the morning of analysis, while −20 °C-stored samples were thawed overnight in the refrigerator after being removed from the freezer the previous evening.To evaluate potential losses, the target compounds Journal of Analytical Methods in Chemistry extracted from the SPE cartridges were analyzed and compared with freshly prepared samples.

Optimal Wavelength and Retention Time.
Te UPLC-UV reversed phase technique was harnessed for the quantifcation of ACM, TMP, OTC, SMX, and EE2.To ascertain the most suitable conditions for the concurrent determination of these compounds, preliminary experiments were conducted.Diferent wavelengths were evaluated, leading to the identifcation of the most efective wavelengths for each analyte: 280 nm for ACM, 218 nm for TMP, 355 nm for OTC, 248 nm for SMX, and 200 nm for EE2.Te retention times (in minutes) observed under our experimental settings were 2.49 for ACM, 5.22 for TMP, 5.48 for OTC, 6.70 for SMX, and 9.6 for EE2.A representative UPLC-UV chromatogram showcasing the selected pharmaceuticals and hormone in a mixed standard solution is depicted in Figure 1.

Optimization of Extraction.
Te extraction of the studied compounds was carried out using solid-phase extraction (SPE), a technique based on liquid-solid sorptive extraction through a chromatography column where analytes are selectively retained in a solid adsorbent [50].Te choice of solvent plays a crucial role in the extraction recovery.In this study, we employed acetonitrile (ACN) as the extraction solvent for ACM, SMX, TMP, OTC, and EE2 due to its proven efectiveness, especially in two-phase water pharmaceutical compound extraction.In a previous work, the authors in [51] examined methanol, ACN, ethyl acetate, and dichloromethane as sulfonamide and trimethoprim extraction solvents.Teir results indicated that higher recoveries could be obtained with acetonitrile or ethyl acetate than with the other solvents.In line with these fndings, we evaluated elution solutions with diferent ACN/H 2 O compositions: 40/60, 60/40, 80/20, 90/10, and 100/0 (% v/v).Among these compositions, 90/10 (% v/v) ACN/H 2 O provided the optimal compromise, yielding high recoveries of the targeted pharmaceuticals (Figure 2), thus being chosen for subsequent water sample analysis.SPE is a commonly used method to preconcentrate and separate drug residues [52].Because of its versatility in terms of purifcation, trace preconcentration, salt elimination, derivatization, and class fractionation, SPE is suitable for sample preparation of liquid samples [53].
To retain the maximum number of target analytes and to prevent sample loss in the SPE column, the pH of the water samples must be adjusted [54].Most of our target compounds are acidic substances that most likely exist in solution in their ionized form, which might result in poor retention in the lipophilic column, especially under neutral pH conditions [55].Te authors in [54] found that acidic groups of tetracyclines and sulfonamides were best retained in SPE columns under acidic conditions.Te pKa value of these compounds might be responsible for this phenomenon.For instance, tetracycline (pKa � 3.3) and sulfamethoxazole (pKa � 5.6) have been efciently extracted due to high retention efects in the SPE column under acidic conditions.
However, acetaminophen is a basic and neutral antiinfammatory/analgesic drug.Tus, to ensure better retention of this group of compounds, we used Oasis lipophilic/hydrophilic balanced (HLB) packing, which is the most widely used chromatography column for the monitoring traces of basic pharmaceutical compounds.
Te Oasis HLB column comprises a combination of divinylbenzene (lipophilic) and N-vinylpyrrolidone (hydrophilic) [55].Due to the chemical composition of the Oasis HLB cartridge, it retains both polar and nonpolar emerging compounds [56].Furthermore, using this method, a favorable performance has been achieved in the extraction of acidic to basic as well as neutral compounds.
Furthermore, in the case of tetracyclines, silanol groups might generate interference since they tend to bind irreversibly to tetracyclines.In addition, to prevent chelation of metals, a strong chelator, such as Na 2 EDTA, must be added to the sample, which is highly soluble in water [55].Te improvement involving Na 2 EDTA addition to samples to better extract pharmaceuticals, i.e., antibiotics, was suggested by Hernández, Sancho [57].For this reason, we added a certain volume of a 100 mM Na 2 EDTA solution to the samples to ensure reproducibility and high recovery improvement for all target compounds.

Method Validation.
Method validation was conducted according to [47] regarding the recovery, linearity, precision, and accuracy of each analyte.

Linearity.
Te observation linearity of the results was evaluated through linear regression analysis of the calibration curves within the 2-500 mg•L −1 range using seven calibration points.Values of the linear regression coefcient of determination R 2 were calculated for each analyte with values ≥0.995, indicating a suitable linearity (R 2 ≥ 0.9900).

Precision and Accuracy. Te precision and accuracy
were assessed by determining repeatability and reproducibility, respectively, throughout the experiments.Tus, repeatability (intraday precision) among the artifcial seawater samples was studied at concentrations of 5, 7.5, and 10 mg•L −1 of each analyte, using three replicates for each selected concentration level.Reproducibility (interday precision) was evaluated using the highest concentration level of 10 mg•L −1 , while the samples were spiked on three distinct days.Precision and accuracy were evaluated by determining the extraction recovery on three consecutive days at the following three levels: 5, 7.5, and 10 mg•L −1 .Te extraction recovery values ranged 80.1-98.8%,and the RSD values ranged 0.45-4.01%,as indicated in Table 1.
Satisfactory extraction recoveries were obtained for all targeted compounds, as well as at all three concentrations, with the values ranging from 80% to 98.8%.Te method accuracy was evaluated by the RSD values of three replicates and was in the range of 1.08-3.66%at low concentrations, 4 Journal of Analytical Methods in Chemistry 1.04-4.56%at medium concentrations, and 0.45-4.01%at high concentrations, indicating favorable method accuracy.Te reproducibility (interday precision) of the method is summarized in Table 2.
Te methods yielded satisfactory results regarding both the intra-and interday precision levels of the analytical procedure, with RSD values lower than 15% (ICH acceptance criteria) for all the targeted compounds.Journal of Analytical Methods in Chemistry 3.6.Stability Study.Establishing a set of conditions to ensure that a given analyte remains stable during transportation and storage without statistically signifcant concentration changes is an essential step in pharmaceutical compound analysis.Terefore, the study of the stability of target compounds is important.In the case of a certain process or analytical method, a stability study can provide evidence regarding how the concentration of a given analyte in a sample or container varies over time.Tis is infuenced by a variety of environmental factors, such as temperature, storage conditions, and light exposure.Te stability of ACM, TMP, OTC, SMX, and EE2 in the SPE cartridge was evaluated after storage at −18 °C, 4 °C, and room temperature in the dark at various holding times, and the results are shown in Figure 3. Preventing light exposure is important for certain pharmaceuticals, such as OTC, since light exposure can cause faster photodecomposition than that under dark conditions [58].It was found that almost all target pharmaceuticals, when protected from light, remained stable after 6 months of storage at −18 °C, indicating no signifcant degradation.
Generally, pharmaceuticals and personal care products require lower-temperature storage conditions to maintain chemical integrity over long periods [43].Liu et al. [59] reported that OTC remained stable for 3 months when stored at −18 °C.Doi and Stoskopf [58] determined the kinetics of OTC degradation under the infuence of various environmental factors, and the results indicated that low temperatures favored high OTC stability levels, while high temperatures accelerated OTC degradation.Tus, it could be concluded that the samples must be stored at −18 °C to maintain the initial concentration.
In contrast, we found that sample freezing increased the 17-alpha-ethynilestradiol concentration over the initial concentration, i.e., from 8.3 to 9.3 mg•L −1 (12% of the initial value).Terefore, −18 °C is the best storage temperature for the samples.
Examination of the samples stored at 4 °C revealed that no major changes in the analyte concentration occurred within 1 month of storage among all of the target compounds that could explain the magnitude of loss of the analyte.Similarly, the authors in [58] found no change in the OTC concentration after 77 days at 4 °C.
Figure 3(c) shows that ACM, TMP, OTC, SMX, and EE2 could be safely stored at room temperature for 3 days since there was no degradation indication (<15% change in the initial concentration).Te authors in [58] reported that OTC remained thermolabile at 43 °C, and in contrast, OTC Journal of Analytical Methods in Chemistry exhibited a signifcantly higher stability at 4 and 25 °C.Te authors in [59] reported that half of OTC was degraded when stored at room temperature for 1 week.

Conclusion
Stability studies provide insights into the duration pharmaceutical compounds can maintain their concentration under varying environmental factors before analysis.Our study suggests that ACM, TMP, SMX, OTC, and EE2 remain stable when stored for 6 months at −18 °C, protected from light.Similarly, stability is maintained for 1 month at 4 °C.At room temperature, ACM, TMP, SMX, OTC, and EE2 can be stored for up to 3 days without signifcant degradation.However, it is important to note that freezing at −18 °C did not result in signifcant changes in most compounds, except for EE2.In summary, the recommended conditions for environmental sampling are as follows: (a) feld sampling and storage conditions: in the dark at ≤4 °C.(b) Transportation and storage conditions: in the dark, for a maximum of 2 days at room temperature and/or ≤4 °C.(c) Holding times: in the dark, for a maximum of 1 month at 4 °C or frozen at −18 °C for long-term storage.
It is worth noting that our study did not investigate the efects of freeze-thaw cycles, which could be a consideration for future research.

Figure 2 :
Figure 2: Extraction recovery for the diferent compositions of the elution solvent (ACN/H 2 O).

Table 1 :
Recovery of extraction (RE, %) of the target compounds obtained during the repeatability (intraday precision) test.Average of three replicate SPE extractions of a spiked artifcial seawater sample.

Table 2 :
Evaluation of the intraday and interday precision (reproducibility) levels of the method.