1.0 Scope
This method note describes the inductively coupled plasma mass spectrometry (ICP-MS) procedure applied to heavy metal screening of synthetic research peptides manufactured by Nexphoria. The method targets the four elemental impurities designated as Class 1 (oral or parenteral route) under USP <232> (Elemental Impurities — Limits) and USP <233> (Elemental Impurities — Procedures): arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg) [1, 2].
This method note applies to all lyophilized peptide lots produced at Nexphoria and is performed on a per-lot basis as part of the standard lot release testing package. Results are reported on the Certificate of Analysis as µg per gram of lyophilized material (µg/g), equivalent to ppm on a mass basis.
2.0 Instrumentation
| Instrument | Agilent 7900 ICP-MS (or equivalent quadrupole ICP-MS with collision/reaction cell technology) |
|---|---|
| Collision/reaction cell | Octopole Reaction System (ORS3); operated in He mode for polyatomic interference removal (ArCl on As-75, ArO on Fe-56 matrix), and H₂ mode for Hg-202 |
| RF power | 1550 W |
| Plasma gas flow | 15 L/min Ar (plasma); 0.8 L/min Ar (auxiliary); 1.05 L/min Ar (nebulizer) |
| Nebulizer | MicroMist glass concentric nebulizer; Scott-type spray chamber at 2°C |
| Sample introduction | Peristaltic pump at 0.3 rps; PFA sample tubing; rinse between samples: 2% HNO₃, 60 s |
| Cones | Nickel sample cone and skimmer cone; replaced per manufacturer PM schedule |
| Monitored isotopes | As-75, Cd-111, Pb-208, Hg-202; internal standards Sc-45, Y-89, In-115, Bi-209 |
3.0 Sample Preparation
Lyophilized peptide samples are digested by microwave-assisted acid digestion prior to ICP-MS analysis. This approach achieves complete matrix destruction and converts all elemental species to soluble ionic forms [3].
| Sample mass | 100 ± 1 mg of lyophilized material, weighed on calibrated analytical balance (readability: 0.01 mg) |
|---|---|
| Digestion vessel | PTFE-lined microwave digestion vessel (e.g., Anton Paar Multiwave PRO or equivalent); pre-cleaned with 10% HNO₃ and rinsed 3× with ultrapure water |
| Digestion acid | 5 mL concentrated HNO₃ (Optima-grade, ≤0.01 ppb metals) + 1 mL H₂O₂ (30% v/v, trace-metal grade) |
| Digestion program | Ramp to 200°C over 15 min; hold at 200°C for 20 min; cool to ≤50°C before venting |
| Post-digestion | Transfer quantitatively to 50 mL volumetric flask; dilute to volume with ultrapure water (18.2 MΩ·cm). Final acid concentration approximately 2% HNO₃. |
| Blank | Reagent blank prepared in parallel (all reagents, no sample). Procedural blank accepted if all analytes are below LOD. |
| Matrix spike | One matrix spike (spiked sample) per batch at 2× LOQ level; recovery 80–120% required for batch acceptance. |
4.0 Calibration
Calibration is performed with a multi-element external calibration standard prepared from NIST-traceable single-element stock solutions or certified multi-element standard. Internal standards are added to all solutions (calibrators, blanks, samples) at a fixed concentration of 10 ppb each to correct for instrumental drift and matrix-induced signal suppression or enhancement [4].
| Calibration range | 0 (blank), 0.5, 2.0, 5.0, 20.0, 50.0 ng/g (ppb) for all target analytes |
|---|---|
| Linearity requirement | R² ≥ 0.9995 across the calibration range for each analyte |
| Internal standards | Sc-45 (for As), Y-89 (for Cd, Pb), In-115 (general), Bi-209 (for Hg); 10 ppb in 2% HNO₃ |
| CCV (continuing calibration verification) | At mid-range concentration; acceptance: 90–110% of expected value; run every 10 samples |
| Standard source | NIST SRM 3100 series (single-element) or equivalent certified reference material; expiry verified before use |
5.0 Detection Limits
Limits of detection (LOD) and quantitation (LOQ) were determined by the signal-to-noise method (LOD = 3σ of blank signal; LOQ = 10σ) using ten replicate reagent blank measurements. Values below represent instrument LOD in the final diluted digest solution, converted to equivalent µg/g in the lyophilized material at the standard dilution factor of 500× (100 mg / 50 mL).
| Analyte | Isotope | LOD (µg/g) | LOQ (µg/g) | USP <232> Limit (µg/g) |
|---|---|---|---|---|
| Arsenic (As) | As-75 | 0.005 | 0.015 | 1.5 |
| Cadmium (Cd) | Cd-111 | 0.001 | 0.003 | 0.5 |
| Lead (Pb) | Pb-208 | 0.002 | 0.006 | 0.5 |
| Mercury (Hg) | Hg-202 | 0.001 | 0.003 | 3.0 |
USP <232> limits cited above apply to oral daily dose ≤10 g/day as a reference. The applicable limit for each lot is determined based on the dosage route and maximum daily mass specified in the research use documentation. Nexphoria applies the oral limit as the default conservative threshold for research-grade materials; investigators using these materials in parenteral research protocols should apply the corresponding parenteral limit from USP <232> Table 1.
6.0 System Suitability
System suitability must be demonstrated at the start of each analytical sequence and after any instrument interruption. The following criteria must be met before sample analysis proceeds [2, 5].
| Parameter | Criterion |
|---|---|
| Calibration linearity (R²) | ≥ 0.9995 for all four analytes |
| Instrument sensitivity (cps/ppb for In-115) | ≥ 200,000 cps per ppb under He mode (instrument-specific threshold verified at qualification) |
| % RSD (replicate injections of CCV) | ≤ 5.0% for all analytes across 5 replicate injections |
| Matrix spike recovery | 80–120% for all analytes |
| Reagent blank | All analytes ≤ LOD in reagent blank; if above LOD, a blank correction is applied and documented |
| Internal standard recovery | 70–130% of initial signal for Sc, Y, In, Bi throughout the run; results for samples with IS recovery outside this range are flagged and re-analyzed |
7.0 Acceptance Criteria per USP
A lot is released for the elemental impurity test when all four target analytes are below the applicable USP <232> permitted daily exposure (PDE) limit converted to concentration based on the maximum daily mass. Nexphoria reports values in µg/g and evaluates compliance using the 30% threshold rule: the measured concentration must not exceed 30% of the J-value (permitted concentration = PDE / maximum daily dose in grams) to provide margin relative to the specification limit [1].
| Reporting format | Each analyte reported as µg/g (ppm, mass basis) in lyophilized material; values below LOQ reported as "<LOQ" |
|---|---|
| Pass criterion | Each measured concentration ≤ applicable J-value derived from USP <232> Table 1 (oral or parenteral, as applicable) |
| Fail criterion | Any analyte exceeding the J-value; lot is placed on hold and QA investigation initiated per deviation procedure |
| Reference method | USP <233> Procedure 1 (microwave acid digestion with ICP-MS detection) |
8.0 References
- [1] United States Pharmacopeia (2024). USP <232> Elemental Impurities — Limits. In USP–NF. United States Pharmacopeial Convention.
- [2] United States Pharmacopeia (2024). USP <233> Elemental Impurities — Procedures. In USP–NF. United States Pharmacopeial Convention.
- [3] Kingston H.M., Haswell S.J., eds. (1997). Microwave-Enhanced Chemistry: Fundamentals, Sample Preparation, and Applications. American Chemical Society, Washington DC. ISBN: 978-0-8412-3144-5.
- [4] Thomas R. (2013). Practical Guide to ICP-MS: A Tutorial for Beginners, 3rd ed. CRC Press, Boca Raton. ISBN: 978-1-4665-5415-8.
- [5] ICH Harmonised Guideline Q3D(R2): Guideline for Elemental Impurities. (2022). International Council for Harmonisation. https://www.ich.org/page/quality-guidelines