GPC/SEC–MALLS analysis of Hydroxyethyl Starch (HES)

Introduction

Hydroxyethyl starch (HES) is a nonionic starch derivative. Special HES-formulations are the most frequently used blood plasma substitutes. An intravenous application of this plasma expander is used to prevent shock following severe blood loss. The blood volume is increased immediately and red blood cells continue to deliver oxygen to the body.

Different types of hydroxyethyl starches are described by their average molecular weight, by their degree of substitution and the C2/C6 ratio. To measure the average molecular weight and the molecular weight distribution, GPC/SEC with RI and light scattering detection is applied.

Experimental Conditions

Eluent: acetate buffer acc. to Pharmacopoeia¹

Columns: PSS SUPREMA Lux 10 μm 125 Å, 200 Å and 17 μm 1000 Å, 3000 Å (7.5 × 300 mm) + precolumn

Data acquisition: PSS WinGPC Unity with Compliance Pack

Detectors: SECcurity GPC1200 RI, MALLS SLD7000

Flow rate: 1.0 mL/min

Concentration: 0.5 g/L

Injection volume: 50 μL

Results and Conclusions

The buffer solution, the test solution, the CRS and the samples were prepared as described in the Pharmacopoeia. The GPC/SEC system was equipped with an multi angle laser light scattering detector that measures simultaneously at seven different scattering angles and an RI detector in series. Figure 1 shows the raw data for a HES standard with narrow molar mass distribution obtained with such a setup. The seven signals for the seven different angles of the light scattering detector are displayed as well as the signal of the refractive index detector.

Figure 1: Raw data example for a HES standard measured using the SLD7000 and an RI detector (red).

PSS WinGPC Unity software was used to determine the mean Mw and the Mw for the lowest and highest 10 % fractions from these raw data. Values for low Mw, medium Mw and high Mw HES are given in the Pharmacopoeia.

Although light scattering is an absolute method several remarks should be considered:

• The refractive index increment, dn/dc, is an important parameter needed for proper light scattering data evaluation. A wrong dn/dc leads to a wrong Mw. The dn/dc can be determined in an additional experiment using dedicated instrumentation, it can be measured on-line using a feature in WinGPC Unity and it can be taken from literature. For this work the dn/dc was determined off-line using dedicated instrumentation, since this method is the most precise.

• For validation reasons the HES samples were dried before use. They contained between 1–4% of water. This step is important and allows WinGPC Unity to calculate the mass recovery of the samples.

Figure 2 shows the results for a HES 200 with a broad molar mass distribution. The results highlighted in green are the results required by Pharmacopoeia.

Figure 2: Results for a HES 200 with broad molar mass distribution. The results in green are required by Pharmaeuropa.

References

1. European Pharmacopoeia 7.0, 01/2011:1785, p.984.

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