An Efficient and Rapid HPLC-QQQ-MS Method for the Quantitation of Tropane Alkaloids in Medicinal Plants

Summary

This study developed a high-performance liquid chromatography coupled with triple quadrupole mass spectrometry (HPLC-QQQ-MS) method to identify and quantify tropane alkaloids (hyoscyamine, anisodamine, scopolamine) in Anisodus tanguticus. The method proved rapid, accurate, and reliable for high-throughput analysis of medicinal plants.

Abstract

Anisodus tanguticus is an important Tibetan medicinal plant. The tropane alkaloids (TAs), such as hyoscyamine, anisodamine, and scopolamine, exhibit significant analgesic and sedative pharmacological activities, making this species a valuable natural source for extracting these bioactive compounds. However, the chemical composition of A. tanguticus is highly complex. It includes a large number of compounds without elucidated structures, rendering systematic identification and structural characterization of its active constituents particularly important. Although high-performance liquid chromatography (HPLC) has been widely used for the separation of extracts from this plant, many unknown components remain challenging to accurately identify, even with spectral database matching.

This study established an analytical method based on high-performance liquid chromatography coupled with triple quadrupole mass spectrometry (HPLC-QQQ-MS) for the determination of tropane alkaloids in A. tanguticus. The method encompasses a series of systematic procedures, including standardized sample preparation, mass spectrometric parameter configuration, LC pre-equilibration, method establishment, acquisition of MS data, multistage mass spectrometry (MSⁿ) scanning, and manual data interpretation. Using this strategy, we successfully identified three representative tropane alkaloids in A. tanguticus: hyoscyamine, anisodamine, and scopolamine. Additionally, it validates the precision of the established HPLC-QQQ-MS method, confirming that the method is rapid, sensitive, feasible, and accurate for the quantitative determination of tropane alkaloids (TAs) in A. tanguticus. The established approach demonstrates good versatility and reliability, and is suitable for the high-throughput identification and quantification of tropane alkaloids in A. tanguticus and other medicinal plants.

Introduction

The therapeutic efficacy of traditional Chinese medicine (TCM) largely depends on its secondary metabolites, such as alkaloids, flavonoids, terpenoids, and other active constituents^1,2,3. Consequently, the precise and sensitive qualitative and quantitative analysis of specific target metabolites in these complex mixtures is a key step in ensuring the quality of TCM, elucidating the material basis of its efficacy, and advancing modern research³. However, the diversity and complexity of TCM components, coupled with the extremely low concentrations of many key active ingredients, present considerable challenges for achieving efficient and accurate detection.

Anisodus tanguticus (Maxim.) Pascher represents a major botanical source of tropane alkaloids (TAs)⁴, known for its rich content of pharmacologically active tropane-type compounds⁵. Key characteristic alkaloids include anisodamine, scopolamine, and hyoscyamine, which share a common tropane core but exhibit distinct functional group substitutions (Figure 1), leading to divergent pharmacological profiles^6,7,8. While their structural similarities complicate chromatographic resolution, they also offer a valuable model for comparative phytochemical and metabolic investigations. Recent studies have investigated the chemical composition and geographical variations of A. tanguticus using metabolomics and network pharmacology approaches^9,10. Current analytical approaches for A. tanguticus metabolites predominantly rely on high-performance liquid chromatography (HPLC)¹¹. Nevertheless, tropane alkaloid levels in seedling-stage A. tanguticus can be as low as 0.01% (one part per ten thousand), and conventional HPLC methods are often constrained by limited resolution, sensitivity, and analytical throughput. These limitations impede the complete separation of structurally analogous alkaloid homologs and hinder the detection of trace constituents, thereby compromising analytical accuracy and limiting support for more advanced research applications. In recent years, advancements in HPLC coupled with tandem mass spectrometry have provided powerful tools to address these analytical bottlenecks. High-performance liquid chromatography-triple quadrupole mass spectrometry (HPLC-QQQ-MS), in particular, offers markedly enhanced chromatographic resolution, separation efficiency, and exceptional sensitivity^12,13,14. This technology has emerged as a state-of-the-art platform for the investigation of secondary metabolites in TCM, enabling efficient and accurate qualification and quantification of trace target analytes within intricate biological matrices^11,15,16,17. As such, it provides a robust technical foundation for comprehensive chemical profiling and quality assessment of medicinal plants like A. tanguticus. In this study, we address the persistent challenges in detecting tropane alkaloids in A. tanguticus, with emphasis on the accurate identification and quantification of trace-level constituents. By systematically optimizing chromatographic and mass spectrometric conditions, we developed and validated a novel HPLC-QQQ-MS method for the simultaneous determination of multiple major tropane alkaloids. The optimized approach significantly enhances sensitivity, selectivity, and methodological robustness, thereby overcoming common limitations associated with conventional mass spectrometry-based assays-such as inadequate sensitivity, matrix effects, and co-elution interference-often encountered during the analysis of trace alkaloid compounds.

Protocol

1. Sample preparation

1. Preparation of standard solutions
   1. Accurately weigh 10.0 mg each of hyoscyamine, anisodamine hydrobromide, and scopolamine hydrobromide reference standards using a one-ten-thousandth analytical balance, and place them into separate 10 mL volumetric flasks, respectively.
   2. Add chromatographic-grade methanol to the marked line, and thoroughly dissolve the standards using ultrasonic treatment to prepare single standard stock solutions with a concentration of 1.0 mg/mL.
   3. Transfer the stock solutions to brown reagent bottles, store them at 4 °C in the dark.
      NOTE: The alkaloids used are highly toxic. Take safety precautions when handling the standard samples. All solutions were prepared freshly prior to analysis and stored at 4 °C in the dark to avoid degradation.
2. Preparation of mixed working solutions
   1. Pipette 100 µL of each aforementioned stock solution into a 10 mL volumetric flask, and dilute to the marked line with chromatographic-grade methanol to prepare a mixed standard intermediate solution of 10 µg/mL.
   2. According to the linear range requirements, further dilute the intermediate solution stepwise with methanol to obtain a series of mixed working solutions.
   3. Take 50 µL, 200 µL, and 500 µL, respectively. Obtain concentrations of 0.5 µg/mL, 2 µg/mL, 5 µg/mL, and 10 µg/mL. Prepare fresh before use.
   4. Transfer 100 µL of the 10 µg/mL intermediate solution into a 1 mL volumetric flask to prepare a 1 µg/mL dilute solution.
   5. Then, transfer 100 µL of the 1 µg/mL solution into another 1 mL volumetric flask and dilute it to the volume with methanol.
3. Sample pretreatment
   1. Carry out freeze-drying of fresh materials of Sophora flavescens and subsequently pulverize into a homogeneous powder using a high-speed grinder.
   2. Sieve the powder through a 0.25 mm standard test sieve. Accurately weigh 20 mg of this powder, transfer it into a 100 mL volumetric flask, and add chromatographic-grade methanol to nearly the marked line.
   3. Perform ultrasonic extraction at room temperature for 30 min. After extraction, take out the volumetric flask, allow it to cool to 25 °C at room temperature, replenish chromatographic-grade methanol to the 100 mL marked line, and vortex mix for 2 min.
   4. Pipette 1 mL of the above extract into a 100 mL volumetric flask, and dilute to the marked line with methanol (100-fold dilution). After shaking well, it serves as the mother solution for HPLC injection (Figure 2).
      NOTE: When operating the high-speed grinding machine, it is essential to wear protective goggles and a dust mask; control the grinding amount and duration to prevent overheating.
4. Sample filtration and storage
   1. Prepare a 1 mL disposable needle-free syringe (chromatography-specialized, non-adsorptive) and a 0.22 µm organic phase microporous membrane.
   2. Aspirate 1.2 mL of the mother solution for injection, and push slowly for filtration.
   3. Collect the subsequent filtrate into a 2 mL sample vial with an inner insert, and seal tightly with a cap.
      NOTE: The frequency of the ultrasonic cleaner is fixed at 40 kHz. Avoid loosening of the volumetric flask cap during extraction to prevent solvent volatilization. Adjust the dilution factor according to the sample peak response in the pre-experiment to ensure the target peak area falls within the linear range of the standard curve. All samples must be immediately stored in a refrigerator at 4 °C.

2. HPLC prerun, method establishment, and MS acquisition

1. Preparation of mobile phases
   1. Prepare mobile phase A: Measure 1000 mL of ultrapure water, add 1.0 mL of chromatographic-grade formic acid (volume fraction: 0.1% v/v), and mix uniformly using a magnetic stirrer for 10 min.
   2. Prepare mobile phase B: Use chromatographic-grade methanol directly without additional treatment.
   3. Pour mobile phases A and B into ultrasonic degassing devices, respectively, and perform ultrasonic degassing at 40 kHz for 15 min.
   4. Immediately transfer the degassed mobile phases to solvent bottles after degassing, seal them, and connect them to the corresponding A and B solvent lines of the HPLC system.
      NOTE: All solvents are of chromatographic grade and must be filtered through a 0.45 µm organic phase microporous membrane before use. Monitor the system pressure regularly. If the pressure fluctuation exceeds ±±5%, check for potential column clogging or air bubbles in the pipeline.
2. Setting of HPLC conditions
   1. Use a C₁₈ column (2.1 mm × 100 mm, 2.7 µm) for chromatographic separation. Equilibrate the column with methanol:water = 80:20 (v/v) for 30 min before use, with a flow rate of 0.3 mL/min during equilibration. Set the column oven temperature to 30 °C, and start preheating 10 min in advance.
   2. Optimize a gradient elution program as follows: 0–10.0 min, 5% to 10% B; 10.0–15.0 min, 10% to 35% B; 15.0–20.0 min, 35% to 60% B; 20.0–25.0 min, 60% to 95% B. Set the flow rate to 0.60 mL/min, the injection volume to 2 µL, and detection wavelength to 210 nm.
3. Mass spectrometric conditions
   1. Perform mass spectrometric (MS) detection using a triple quadrupole (QQQ) mass spectrometer equipped with an electrospray ionization (ESI) source operating in positive ionization mode (ESI+).
   2. Set the operation parameters as follows: nebulizer pressure: 15 psi; capillary voltage: 4000 V; gas temperature: 300 °C; and gas flow: 11 L/min.
   3. Systematically optimize the MS/MS parameters for each analyte.
      1. First, identify precursor ions by directly infusing individual standard solutions in MS2 SCAN mode.
      2. Optimize the Fragmentor voltage for each precursor ion to maximum signal intensity using MS2 SIM mode.
      3. Subsequently, employ the Product Ion Scan mode to determine the characteristic product ions. Systematically optimize the Collision Energy (CE) for the precursor ion to maximize the product-ion signal intensity.
      4. Select the most intense product ion for quantification, and use the second-most-intense ion for qualitative confirmation.
         NOTE: Identify precursor ions via direct infusion of individual standard solutions in MS2 SCAN mode. Ensure standard solution concentration is appropriate to avoid signal saturation. Calibrate the instrument regularly to avoid parameter drift.
4. Method validation
   1. Specificity
      1. Inject blank solvent (methanol), mixed standard working solution, and sample extract separately, and compare the chromatograms.
      2. Ensure the following requirements are met: No interfering peaks in the blank solvent; the retention time of target components in the sample is consistent with that of the standard (deviation ≤ ±±0.1 min); the peak area ratio of quantifier ion to qualifier ion conforms to that of the standard (deviation ≤ ±±10%).
   2. Linearity
      1. Inject the series of mixed working solutions prepared in step 1.1 (0.1, 0.5, 2, 5, 10 µg/mL) sequentially under the aforementioned HPLC-MS conditions, with 3 injections per concentration.
      2. Plot the standard curve with the concentration (x, µg/mL) of each component as the abscissa and the average peak area (y) of the quantifier ion as the ordinate. Perform linear regression analysis, requiring a correlation coefficient (R²) ≥ 0.995.
   3. Precision and accuracy
      1. Intraday precision (repeatability): Analyze 6 replicates of quality control (QC) samples at three concentration levels (2 µg/mL, 5 µg/mL, and 10 µg/mL) within one day, and calculate the relative standard deviation (RSD%).
      2. Interday precision (intermediate precision): Independently prepare QC samples from the same batch of dried plant raw material and analyze them by different operators on three consecutive days. and calculate RSD%. Ensure RSD% ≤ 10% for both.
      3. Evaluate accuracy using a spike-recovery test, and express as a recovery rate (%). Ensure the average recovery rate of low, medium, and high concentrations ranges from 85% to 115%, with RSD% ≤ 5%.
         NOTE: The linear range must cover the expected concentration range of target components in actual samples (including low, medium, and high levels) to reflect practical applicability. All validation experiments were performed in triplicate, and the results were expressed as the average value.

3. Data acquisition

1. Perform data acquisition in multiple reaction monitoring (MRM) mode. Refer to Table 1 for the optimized MRM parameters-including precursor ion, product ion transitions, fragmentor voltage, and collision energy (CE)-for each analyte.
2. Calculate the dwell time to ensure an adequate number of data points across each chromatographic peak for each transition while maintaining an appropriate cycle time.

4. Data analysis

1. Launch the qualitative analysis software, click File > Import Data, and import all collected data files (.d format).
2. Extraction of extracted ion chromatograms (EICs): In the Quantitation module, select MRM Transition, input the precursor ion-product ion pairs of each component, and extract the corresponding EICs.
3. Peak validation and integration: Check the peak symmetry and retention time consistency (deviation ≤ ±±0.1 min compared with the standard) of the target peaks in each EIC.
4. The software performs automatic integration by default; if baseline drift or interfering peaks exist, manually adjust the integration parameters (integration start and end points) to ensure accurate integration.
5. Calculate the concentration of each component in unknown samples using the standard curve. Click Report > Generate Report, select Excel format to export the report. The report includes the compound name, retention time, peak area, concentration, RSD%, etc.

Results

HPLC-QQQ-MS chromatographic separation and identification of TAs
A typical MRM chromatogram obtained from the analysis of a standard mixture and an A.tanguticus root extract is presented in Figure 3 and Figure 4. Under the optimized HPLC conditions, all three target tropane alkaloids: hyoscyamine, anisodamine, and scopolamine, were successfully separated within 20 min, demonstrating the high efficie...

Discussion

With growing interest in plant secondary metabolites, analytical techniques for alkaloid detection have garnered significant attention. While a foundational chromatographic system for analyzing tropane alkaloids (TAs) has been established¹⁸, recent advances, such as the adoption of capillary electrophoresis (CE), enhanced instrumental performance in GC–MS and HPLC–MS, and the identification of novel structural compounds including convolvine and tropinone-like toxins underscore the need...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Agilent MassHunter software	Agilent Technologies
Agilent ZORBAX SB-C18 column	Agilent Technologies	880975-902	C18 reversed-phase column, 4.6 × 250 mm, 5 μm
Anisodamine hydrobromide	Chengdu glip Biotechnology Co., Ltd	101-31-5	HPLC>98%
Formic Acid	Sigma-Aldrich	5.33002
freeze dryer	EYELA, Tokyo, Japan	FDU-2110
High-speed universal grinder	Taisite Instrument Co., Ltd., Tianjin, China	6010210100	FW-100,24,000 rpm, 100 g capacity, 80 mesh
Hyoscyamine	Chengdu glip Biotechnology Co., Ltd	55449-49-5	HPLC>98%
Methanol	Thermo Fisher Scientific	022909.K2
Microporous Membrane 0.22μm	Millipore	SLGV033RB
Milli-Q Direct 8 water system	MilliporeSigma, Burlington, MA, USA	ZRQSVP0JP
Scopolamine hydrobromide	Chengdu glip Biotechnology Co., Ltd	114-49-8	HPLC>98%