FAQ: "Our customer is a user of a ColorQuest XE, which they use to measure color of its pharmaceutical containers they produce for this industry (syrup bottles, droppers, etc.). Right now they have come with a new need. They want to measure the light transmittance of some transparent bottles as requested in USP 671. In this, it is asked to measure the transmittance in an spectrophotometer “capable of measuring and recording light transmitted in diffuse as well as parallel rays” in the range of 290-450 nm, at intervals of 20 nm. Despite the CQXE only is able to measure in the part of 400-450 nm they need, they have shown me a graph of a measurement in a service lab, showing a symmetrical curve, with the peak at 400 nm, so, we wonder if this behavior (symmetry) happens with all their samples, the CQXE would help as an internal quality test verifying the right part of the graph. Can you let me know your thoughts on this matter?"
From your question, I can tell you have given this some thought.
Yes, our instruments can help as an internal quality test doing about 80% of what would be needed to meet USP 671, but does not completely conform. Unfortunately our best instrument would be the UltraScan VIS rather than the ColorQuest
To measure the transmittance in an spectrophotometer “capable of measuring and recording light transmitted in diffuse as well as parallel rays” in the range of 290-450 nm, at intervals of 20 nm" requires a sphere based spectrophotometer with UV range capability. A sphere attachment on a UV/VIS analytical spectrophotometer is typically found with higher priced instruments.
A ColorQuest XE only measures to 400 nm but the UltraScan VIS measures down to 350 nm. Typically the presence of an encapsulated UV absorber in the bottle side wall can be seen in the spectral measurement where full transmittance occurs down to 400 nm after which there is a rapid fall-off to near 0 at 370 nm or 380 nm.
As you mention, if the curve is symmetrical with full transmittance at 400 nm and falling off rapidly, then the UV absorbance should continue until some point, say 320 nm, where it will rise again.
The UltraScan VIS does not strictly conform to USP 671. However, it is a sphere spectrophotometer able to verify the rapid fall-off below 400 nm to a defined level at 370 nm (typically under 10% transmittance), indicating that there is a UV absorber present and in sufficient amount to be effective in protecting the bottle contents.
If the customer is prepared to accept that compromise, the UltraScan VIS could be used for rapid internal quality lab testing of single-wall bottle cut-outs on these bottles.
When they need to officially report a 290 nm to 450 nm transmittance measurement in conformance with USP 671, they could make a measurement on a conforming UV/VIS sphere spectrophotometer.
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