GPC Molecular Weight Calculator
Calculate Mn, Mw, and PDI using Standards Calibration Curve
| Retention Vol (mL) | Molecular Weight (Da) |
|---|---|
| Retention Vol (mL) | Detector Signal (hi) |
|---|---|
User Guide: GPC Molecular Weight Calculator
This advanced tool helps pharmaceutical scientists and quality control analysts precisely determine the molecular weight distribution of polymers (like excipients such as PEG or Povidone) using Gel Permeation Chromatography (GPC) data.
How it Works: The Calibration Method
- Calibration Curve Construction: You first establish a relationship between the retention volume (elution time) and the molecular weight using a series of narrow-dispersity polymer standards (typically polystyrene). This tool fits a 3rd-order polynomial regression to your calibration data: $$ \small \log(M) = A + B(V_R) + C(V_R)^2 + D(V_R)^3 $$
- Sample Chromatogram Integration: Once the calibration curve is established, the tool takes your sample's raw chromatogram data (detector signal vs. retention volume). For each data slice, it uses the calibration curve to determine the molecular weight ($M_i$) at that specific retention volume ($V_R$).
- Molecular Weight Averages: Finally, it calculates the Number-Average ($M_n$), Weight-Average ($M_w$), and Polydispersity Index (PDI) based on these individual slices.
Step-by-Step Instructions
Input Calibration Standards Data
Enter your calibration standards data (Retention Volume vs. known Molecular Weight) into the first table. Ensure you have at least 4 points for a reliable 3rd-order polynomial fit.
Tip: Use the "Paste Data from Excel" feature for quick import (e.g., paste two columns: `Volume (tab) MW`).
Input Sample Chromatogram Data
Enter your unknown sample's chromatogram data (Retention Volume vs. Detector Signal, e.g., height or area) into the second table. This data represents the elution profile of your sample.
Tip: Directly paste data from your chromatography software's integration report (e.g., Waters Empower, Agilent ChemStation) for convenience.
Calculate Distribution
Click "Calculate Distribution". The tool will first generate the calibration curve equation and then apply it to your sample data to determine Mn, Mw, Mz, and PDI.
Key Terms Explained
What is Number-Average Molecular Weight (Mn)?
Mn is the total weight of all polymer molecules divided by the total number of polymer molecules. It gives equal weight to each molecule regardless of its size. Mn is often determined by colligative properties like osmometry.
What is Weight-Average Molecular Weight (Mw)?
Mw accounts for the molecular weight of each molecule in determining the average. Larger molecules contribute more to Mw. It is more sensitive to high molecular weight components and is typically determined by light scattering or GPC.
What is Polydispersity Index (PDI)?
PDI (Mw/Mn) is a measure of the molecular weight distribution's breadth. A PDI of 1.0 indicates a perfectly monodisperse polymer (all molecules have identical weight). Values >1 indicate polydispersity, with higher values meaning a broader distribution.
What is Z-Average Molecular Weight (Mz)?
Mz is even more sensitive to high molecular weight components than Mw. It is particularly useful for detecting the presence of very large molecules or aggregates in a polymer sample.
Labels: GPC Molecular Weight Calculator