SoloVPE (slope spectroscopy)
Introduction & Summary
The SoloVPE system (C Technologies, a Repligen company) enables rapid, accurate UV-Vis concentration measurements of proteins and nucleic acids without dilution, using Variable Pathlength Technology (VPT) and Slope Spectroscopy. By acquiring absorbance at multiple pathlengths and regressing A vs. pathlength, it maintains linearity at high concentrations—ideal for at-line/in-process control (IPC) and formulation development. Typical results are returned in <30–60 s.
Key Quality Attributes Assessed
Method Evolution: Superseded, Current Standard, and Emerging
Legacy: Fixed 1-cm cuvette UV-Vis—narrow linear range, serial dilutions.
VPT (SoloVPE): Measures directly without dilution; slope-based linearity with R² ≥ 0.999 acceptance criteria; widely adopted for at-line concentration checks.
Scientific Principle
SoloVPE varies the optical pathlength 5 µm–15 mm while measuring at selected wavelengths across 190–1100 nm. A disposable Fibrette optical component moves within the sample vessel to change pathlength.
Per Beer-Lambert, the slope of Absorbance vs. Pathlength equals ε × c; concentration is calculated from the slope using the appropriate extinction coefficient.
Reporting includes the slope (Abs/mm), calculated concentration, and R² of the linear fit.
Explainer Videos
This video from Repligen Process Analytical Solutions discusses the advantages of using Slope Spectroscopy and the SoloVPE system for gene therapy applications. Here are the key points:
- Introduction to Slope Spectroscopy and SoloVPE: Paul Mania explains that slope spectroscopy is a major improvement over traditional UV-Vis methods for gene therapy. The SoloVPE system measures absorbance at different path lengths, so samples don’t need to be diluted [05:45].
- Challenges in Gene Therapy Analytics: Mania discusses the difficulties with current gene therapy analytics, such as high development costs, complex regulations, inconsistent supply, and the absence of fast analytical methods. These problems lead to a high tolerance for error and slow processes [02:50].
- Gene Therapy Applications: The SoloVPE system offers benefits in several gene therapy applications:
- Culture Density (OD600): It effectively monitors cell growth in E. coli fermentation in real time and without dilution, which reduces errors and saves time [01:29].
- Antisense Oligonucleotides (ASO): For ASO drug manufacturing, SoloVPE is a simpler, faster, and more reliable method than HPLC, saving 67% of the time and improving precision [14:17].
- Oligonucleotide Determination: The SoloVPE system is more linear and consistent across a wide range of oligonucleotide concentrations than traditional methods like NanoDrop, which require dilutions for higher concentrations [16:05].
- Plasmid DNA Purity: SoloVPE allows for precise measurement of plasmid DNA purity, even in complex samples, by removing the need for dilution and improving data reliability [17:01].
- Customer Success and Support: Repligen offers full support for the SoloVPE system, including on-site installation, training, computer system validation documents, and SOP protocols, to ensure easy integration and regulatory compliance [18:06].
- Benefits of SoloVPE System: The SoloVPE system:
- Global Support Network: Since Repligen acquired C Technologies, a global support network has been created to help customers in Europe, Asia, and North America [19:37].
Common Instrumentation & Software
Data Output & Interpretation
Output: slope (Abs/mm), concentration; R² must meet method acceptance (commonly ≥ 0.999). Minimum sample volumes as low as ~50 µL are typical, though specialized low-volume configurations may allow <20 µL depending on matrix.
Ratios: MultiQ enables simultaneous wavelength collection (e.g., 260/280; optionally include 230 nm when required).
Strengths
No dilutions, minimal prep; fast (<30–60 s).
wide range: Proteins typically measurable in the ~0.05–200 mg/mL range (matrix- and extinction coefficient–dependent)
Small volume: tens of microliters typical.
Limitations
Matrix/scatter effects: absorbing or scattering excipients can bias 260/280 readings; While optical scatter can be minimized through method setup and blanking strategies, matrix effects must be carefully assessed during qualification.
Bulk measure: not impurity-resolving—cannot separate fragments/aggregates that co-absorb.
Extinction coefficient accuracy: sequence- or analyte-specific ε must be correct for accurate concentration.
Key Validation Considerations
For IPC/at-line use, labs typically qualify the SoloVPE method (fit-for-purpose), including linearity/range, accuracy (vs. known standards), precision, matrix/scatter checks, and R² criteria.
For potential release or stability use, methods should be validated per ICH Q2(R2)/Q14 and instruments qualified per USP <857>, although SoloVPE is more commonly applied in IPC, comparability, and process development settings.
Method Standardization & Reference Materials
Performance verification: use ConfiRM Slope Reference Materials (NIST-traceable certified slope values) to verify system health/suitability for VPT devices.
No-baseline requirement: slope methods generally do not require buffer/baseline correction when the buffer slope is ~0.
Use in Specific Modalities
Proteins/mAbs (A280), plasmid DNA, mRNA (A260), and published workflows for mRNA-LNP (with scatter correction as needed).