Protective Properties of Colored Glass Containers

A stimuli article on the determination of the protective properties of colored glass containers has been published in USP´s Pharmacopeial Forum (PF) 47(4). The deadline for comments is September 30, 2021.

Proposal for replacement of spectral transmission with spectral absorbance

The proposal outlined by USP´s General Chapters–Packaging and Distribution Expert Committee (GCPD EC) describes an approach to replace spectral transmission with spectral absorbance for amber glass containers based upon wall thickness. According to the GCPD EC, the purpose of the article is to provide a rationale for the adoption of new limits for amber glass containers used to contain light-sensitive drug products. Currently, USP chapter <660> requires that a glass container’s size and nominal fill volume is used to determine light transmission. However, the GCPD EC believes that a container’s wall thickness should be used because wall thickness is the main influencing factor on light transmission.

Spectral Transmission Tests in USP <660> and Ph. Eur. 3.2.1

The current spectral transmission requirements in USP Chapter <660> Containers—Glass and in Ph. Eur. Chapter 3.2.1 Glass Containers for Pharmaceutical Use differentiate between parenteral and nonparenteral drug products. The acceptance limit for nonparenteral drug products is maximum 10%. The limits for parenteral products are given stepwise according to the filling volume (for flame-sealed containers / containers with closures). The general chapters <660> and Ph. Eur. 3.2.1 require to measure light transmission in the range of 290–450 nm. However, currently, the Ph. Eur. and USP requirements are not yet harmonized with 7.01 Test for Glass Containers for Injection in the Japanese Pharmacopoeia (JP). 

The stimuli article states that the amount of light that passes through the glass wall depends on

• the composition of the glass,
• the annealing time and temperature,
• the wall thickness.

It is emphasized that currently marketed colored Type I glass is typically composed of iron and titanium, and colored Type III glass is composed of iron and manganese. The annealing time and temperature are usually set to specific parameters and generally remain constant. However, "as the market does not use standardized containers, a correlation between the filling volume and the wall thickness cannot reliably be made". Thus, it is proposed to correlate spectral transmission to the wall thickness rather than to the filling volume.  

Rationale for using Spectral Absorbance instead of Spectral Transmission

According to the article, the transmittance will decrease exponentially with increasing wall thickness, whereas the absorbance will increase linearly. Thus, a single limit can be assigned over the whole absorbance range and different wall thicknesses. The authors therefore propose to use spectral absorbance instead of spectral transmission.

Proposed Acceptance Criteria

The authors propose three different minimum allowed absorbance values for tubular and molded colored glass containers:

• For Type I amber tubular glass containers: 0.75
• For Type II and III amber tubular glass containers: 0.40
• For all molded amber glass containers: 1.0

The article says that molded glass containers are produced by either blow-and-blow or press-and-blow processes. In both methods the wall thicknesses of the side wall and base vary for an individual container. Thus, the approach of using a precise wall thickness is not appropriate. Therefore, a single minimum allowed absorbance value was assigned for all molded amber glass containers.

The calculation of the minimum spectral absorbance for colored tubular glass containers is based on the nominal wall thickness obtained either from the supplier or by measuring the wall thickness. The spectral absorbance of a representative section of glass from the side wall of the tubular container is measured with a spectrophotometer. The minimum absorbance between 290–450 nm is then divided by the wall thickness to calculate the minimum absorbance value. The resulting value is not lower than that given above.

For more information please read the stimuli article "Measurement of the Protective Properties of Amber Glass Containers" published online in PF 47(4).