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Home > News & Events > Optical pH Measurement of Contact Lens Solutions

Optical pH Measurement of Contact Lens Solutions

Affirming Why Product Expiration Dates Matter

By various estimates, more than 125 million people worldwide are contact lens wearers, with about one-quarter of the total in the U.S. Contact lens popularity has grown considerably as lenses have become safer and more effective.

Contact lens safety comes with a caveat: to avoid eye irritation and infection, lens wearers must exercise proper care and handling of lenses and lens accessories. Yet in a 2014 study of contact lens wearers in the U.S., the Centers for Disease Control and Prevention reported that a remarkable 99% of lens wearers engaged in at least one hygiene-related risk behavior, from sleeping overnight in their lenses to not replacing contact lens cases frequently enough.

In this application note, we focus on the risky practice of using contact lens solution past its expiration date, utilizing optical pH measurements to demonstrate how the effectiveness of solutions changes over time and falls below healthy levels.

Background

Contact Lens SolutionContact lens solutions provide various lens care functions – cleaning, lubricating, disinfecting and so on – and use buffers to keep the pH of the solution close to the pH of natural tears, which varies from 6.5-7.6 depending on numerous factors.

Over time, and with exposure to sunlight or heat, the lens care solution will experience a decline in pH, a condition that can cause irritation in the eyes and lead to infection. Although commercial lens solutions with different formulas are available, all of them are subject to the same chemical effects over time that can alter pH levels.

Experimental Conditions

Figure 1: Spark is a compact spectral sensor available with attachable accessories like this LED-cuvette prototype.

Figure 1: Spark is a compact spectral sensor available with attachable accessories like this LED-cuvette prototype.

Contact lens solutions carry expiration dates of several years from date of manufacture, although some suppliers may suggest a “discard date” much sooner than that, with the clock ticking once the bottle is opened. For our experiment, we collected five contact lens solution samples – most of them soft-lens, multipurpose solutions — with various expiration dates ranging from June 2015 to December 2017. Samples ranged from US $3.50 for a 12-oz. drugstore brand to more than US $10 for a 16-oz. name brand.

Our pH measurement setup consisted of a Spark-VIS spectral sensor (380-700 nm) with attachable LED-cuvette holder accessory (Figure 1); a Smart pH Cuvette for transmissive pH sensing; and OceanView spectroscopy software.

Smart pH Cuvettes are embedded with a pH indicator dye. For our experiment, we combined the pH cuvettes with the Spark and its accessories to measure transmissive pH in the lens solutions. The cuvettes are designed for biological pH range (5-9) samples and have a 10 mm pathlength.

Measurement Results

We first measured the absorbance values of pH buffers in the Smart pH Cuvette, with our focus on the peak at 620 nm (the analytical wavelength for the pH cuvette). The results demonstrated the correlation between pH level and absorbance peaks (Figure 2).

Figure 2: Absorbance peaks of pH buffers were measured in the Smart pH Cuvette.

Figure 2: Absorbance peaks of pH buffers were measured in the Smart pH Cuvette.

We used our absorbance results to plot a full-range pH calibration profile for the Smart pH Cuvette. Since our focus was on the biological pH range, we adjusted our profile accordingly (Figure 3).

Figure 3: Creating a calibration profile for the Smart pH Cuvette is part of the process in later calculating pH values for the samples to be tested.

Figure 3: Creating a calibration profile for the Smart pH Cuvette is part of the process in later calculating pH values for the samples to be tested.

Taking care to rinse the cuvettes with fresh buffer between samples — contamination would affect results — we measured the absorbance of the five lens solution samples. All the samples exhibited similar spectral shapes, yet the expired sample had noticeably lower absorbance than the other samples (Figure 4).

Figure 4: The absorbance peak for the oldest (past expiration) lens solution sample correlated to the lowest pH value among the samples tested.

Figure 4: The absorbance peak for the oldest (past expiration) lens solution sample correlated to the lowest pH value among the samples tested.

With this data, we then could calculate the pH values for each sample from the active absorbance acquisitions. These values are shown in Table 1.

Table 1. pH Levels of Contact Lens Solution Samples by Expiration Date

Sample: pH 7 Reference OptiFree Artificial Tears BioTrue OptiFree Rexall
Expiration date: NA October 2017 December 2017 March 2017 September 2017 June 2015
Solution pH: 7.00 7.10 7.02 7.32 7.15 6.66

Summary

As our measurements demonstrated, the pH level of contact lens solutions varied according to the lens solution expiration date, with a considerable difference between the most recently expired sample and the sample with the longest remaining shelf life. Although additional data would be necessary to validate our results, the data we observed in our experiment suggested that contact lens wearers should follow recommended standards for lens solution use.

In addition, our experiment made good use of the Spark spectral sensor, a novel solid state optical sensor that provides users with the size and cost advantages typical of less robust RGB filter diode devices, but with the benefits of full spectral measurements. This accessibility makes possible a wide range of spectral applications, with Spark used as a standalone instrument for pH measurements or as an affordable, compact component integrated into other devices for absorbance, color, fluorescence and more.

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