Analysis of Free Inositol Stereoisomers in Dietary Supplements by HILIC-CAD: Detection Optimization and Analytical Performance Evaluation

Significance of the topic

Inositol stereoisomers are biologically active cyclitols involved in metabolic regulation and hormonal signaling and are commonly used as dietary supplements (e.g., for polycystic ovary syndrome and gestational diabetes). Because inositols lack chromophores, they are challenging to detect by conventional UV/fluorescence detectors, creating a need for robust, cost‑effective detectors compatible with routine quality control. Charged aerosol detection (CAD) offers near‑universal, mass‑sensitive detection suitable for UV‑transparent, non‑volatile analytes and gradient separations, making it an attractive alternative to mass spectrometry for inositol stereoisomer quantitation.

Objectives and study overview

This application study evaluated Waters Charged Aerosol Detector (CAD) coupled to HILIC separation for simultaneous quantitation of multiple free inositol stereoisomers in dietary supplements. Major aims were to: optimize CAD operating parameters for maximum signal‑to‑noise (S/N); select a robust calibration model for the inherently nonlinear CAD response; define method sensitivity, linearity, precision, and accuracy; and demonstrate applicability to commercial supplement products.

Methodology

The chromatographic separation used an ACQUITY UPLC BEH Amide column (1.7 µm, 2.1 × 150 mm) in HILIC mode with a gradient between high organic (ACN:water 90:10 v/v) and more aqueous conditions (ACN:water 50:50 v/v), both containing 0.01% ammonium hydroxide. Standards for seven inositol stereoisomers (allo-, epi-, D‑chiro‑, myo‑, scyllo‑, muco‑, neo‑) were prepared in water; working ranges were 15–600 mg/L with a mixed‑standard stock at 1500 mg/L. Dietary supplement powders were extracted in water (heating up to 50 °C if necessary), filtered and diluted into the calibration range. Spike‑recovery experiments at two concentration levels assessed accuracy. LOQs were estimated by interpolation to the concentration giving S/N = 10.

Used Instrumentation

System and detectors used in the study included:

• Arc Premier System with Quaternary Solvent Manager (QSM‑R), Sample Manager (FTN‑R), and Column Manager‑Active (CM‑A)
• Waters Charged Aerosol Detector (CAD)
• Empower Chromatography Data System (CDS) for control and data acquisition
• ACQUITY UPLC BEH Amide column (1.7 µm, 2.1 × 150 mm)

Reagents: LC‑MS grade acetonitrile, ammonium hydroxide (0.01% in mobile phases), and ultrapure water. Ammonium bicarbonate was evaluated but excluded from final mobile phase due to detrimental effects on S/N.

Optimization of CAD parameters

Key CAD parameters were systematically optimized for S/N: nebulizer gas pressure, ion trap voltage, and evaporator temperature. Principal findings:

• Optimal settings giving best S/N were nebulizer gas pressure = 40 psi, ion trap voltage = 600 V, and evaporator temperature = 95 °C.
• Ion trap voltage had the largest impact on S/N (approximate 1.5‑fold improvement when increased from 20 V to 600 V).
• Nebulizer gas pressure and evaporator temperature also affected S/N; lower pressure (40 psi) and higher evaporator temperature (95 °C) generally improved S/N for the compounds tested.
• Ammonium hydroxide (0.01%) was retained in mobile phases to ensure retention time consistency; ammonium bicarbonate decreased S/N and was omitted.

Optimization exploited the CAD’s user‑adjustable ion trap setting (accessible in advanced settings) and the system console for nebulizer pressure adjustments.

Calibration strategy and sensitivity

Because CAD response is inherently nonlinear, the study compared a power‑law fit to a second order polynomial fit. The second order polynomial provided superior regression (higher R2 and lower residual errors) for both myo‑ and D‑chiro‑inositol across the tested concentration ranges and was therefore selected for quantitation.

Sensitivity and calibration performance summary:

• Calibration linearity: coefficient of determination R² ≥ 0.995 for representative analytes (allo‑, epi‑, D‑chiro‑, myo‑, scyllo‑inositol).
• Estimated limits of quantitation (LOQs): approximately 17–21 mg/L (based on S/N = 10 interpolation).
• Calibration ranges extended up to ~600 mg/L; higher concentrations produced peak distortion from column overloading.

Analytical performance: accuracy and precision

Method performance was assessed via spike‑recovery and repeatability studies on four supplement matrices:

• Accuracy (spike recoveries): majority of recoveries (34/36) fell within the 90–110% acceptance window; overall recoveries ranged about 86–112% at the lower spike level and 96–110% at the higher level.
• Repeatability (intra‑day precision): RSD ≤ 3.6% (n = 3) across analytes and spike levels.
• Intermediate precision (inter‑day/sample RSD): generally ≤ 4.6% over two days (n = 6); one exception was D‑chiro‑inositol in sample DS D showing RSD = 7.2%, attributable to measurement near the LOQ.

Considering CAD’s gas‑nebulization mechanism (which can introduce greater variability than optical detectors), the observed precision and accuracy indicate robust analytical performance suitable for QC contexts.

Application to commercial supplements

The validated HILIC‑CAD method was applied to four commercially available dietary supplements containing declared myo‑ and D‑chiro‑inositol.

• Myo‑inositol results matched label claims closely, at 99–109% of declared amounts.
• D‑chiro‑inositol exhibited substantial variability: two products matched or slightly exceeded labels (108% and 117%), one product contained no detectable D‑chiro‑inositol, and one product measured only ~35% of the labeled amount.

These findings highlight that the method is fit for routine content verification and can reveal manufacturing or labeling inconsistencies, particularly for lower‑dose stereoisomers.

Benefits and practical applications

Key benefits and practical aspects of the HILIC‑CAD approach include:

• Cost‑effective alternative to MS for UV‑transparent analytes in routine QC laboratories.
• Compatibility with gradient HILIC separations and non‑volatile analytes.
• User‑adjustable CAD parameters (notably ion trap voltage) enable tailored sensitivity improvements for specific analytes.
• Demonstrated accuracy and precision appropriate for supplement QC and formulation verification.

Operational caveats: avoid non‑volatile mobile phase additives with CAD; be mindful of CAD’s nonlinear response and ensure appropriate calibration modeling; and recognize potential variability from gas nebulization, especially near LOQs.

Future trends and potential applications

Potential directions to extend and improve the approach include:

• Refinement of calibration algorithms and software integration (e.g., advanced polynomial or weighted fits, automated residual analysis) to further reduce quantitation error across wide dynamic ranges.
• Preconcentration or on‑line enrichment strategies to push LOQs lower for low‑dose stereoisomers.
• Broader adoption in food and pharmaceutical QC for other UV‑transparent saccharides and cyclic polyols where MS may be cost‑prohibitive.
• Evaluation of isotope‑dilution or internal standard strategies adapted to CAD to mitigate matrix effects and improve quantitative robustness.
• Hybrid workflows combining CAD screening with targeted MS confirmation for suspect samples or trace‑level investigations.

Continued instrument control improvements (e.g., expanded software access to nebulizer settings) and detector refinements will further increase CAD utility in regulated QC environments.

Conclusion

This study demonstrates that HILIC separation coupled with Waters CAD — after systematic optimization of ion trap voltage, nebulizer gas pressure, and evaporator temperature — provides a robust, sensitive, and precise method for simultaneous analysis of multiple free inositol stereoisomers in dietary supplements. A second order polynomial calibration model improved quantitation over a power‑law fit. The method achieved LOQs of ~17–21 mg/L, excellent linearity (R² ≥ 0.995), and reliable accuracy and precision for routine QC testing. Application to commercial products revealed both label conformity for myo‑inositol and notable inconsistencies for D‑chiro‑inositol, illustrating the method’s value for quality assurance.

References

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