Determining Accurate Moisture Content of Soft Contact Lenses Using NIR Spectroscopy

Routinely measured products can be rapidly analyzed using NIR spectroscopy without any loss of product or time while improving quality control in real time. This minimal contact method is required in the analysis of delicate soft contact lenses to avoid any damage to the lens itself. This article covers the analysis of four sets of soft contact lenses with known moisture levels using NIR spectroscopy. NIR spectroscopy provides a rapid and damage-free solution.

Sample Presentation

NIR offers a range of unique advantages compared to traditional techniques including recovery of the intact sample, shortened analysis times, and no sample preparation. In addition, the NIR method is a non-destructive method and uses no solvents. Appropriate sample presentation is the first step of the NIR method development and is especially significant in the analysis of soft contact lenses to obtain good NIR spectra. This study observed the impact of three different sample presentations. Calibration was built using the three sampling methods and the results were compared for precision, prediction accuracy, ease of use, and any damage caused to the product.

Experiment

The analysis used four sets of lenses with varied embedded water content of 24%, 38%, 48%, and 58%. A spectral library was built for method development using 20+ spectra obtained from five to six samples from each of the four lots. Each spectrum comprised of 32 co-added scans of sample and reference in the 400-2500nm scan range. A Kim wipe was used to pat dry the lens. The lens was then centered on its respective lens mount and the NIR spectrum was acquired. From close monitoring, the time taken for the activities, starting from the pat dry to spectrum acquisition, was found to be 45-70s. The time from the beginning of the pat dry to spectrum acquisition was closely observed and ranged from 45-70 seconds. Timing remained consistent in order to reduce the effect from air drying. Each sample spectrum was collected in 16 seconds.  

The NIRS XDS SmartProbe Analyzer was used to measure the reflectance by applying an 80% reflectance standard as the background (Figure 1). The NIRS XDS Transmission OptiProbe Analyzer and NIRS XDS RapidLiquid Analyzer (RLA) were used to test the transmission sample presentation (Figures 2 and 3). Lenses were mounted in RLA using a special lens holder. The NIRS XDS RapidContent Analyzer was used to measure samples in Trans-reflectance mode (Figure 4). The contact lens were mounted on a gold-plated reflector and into a quartz cup for further analysis. The optical pathlength of the reflector was 2 mm.

Figure 1. Reflectance : NIRS XDS SmartProbe Analyzer

Figure 2. Transmission: NIRS XDS Transmission OptiProbe Analyzer

Figure 3. Transmission: NIRS XDS RapidLiquid Analyzer

Figure 4. Trans-Reflectance: NIRS XDS RapidContent Analyzer

Results and Discussion

The collected spectra were used to develop a quantitative model in the Vision® chemometric software, within the aforementioned region. The spectra profile was correlated with the embedded moisture percentage by developing a partial least squares regression model from 1272-1648nm, clearly differentiating the trends for the different concentrations. The scattering effects on the spectra in Figure 5 were reduced by applying the standard normal variate math treatment.

Figure 5. This image shows the water absorption region of the NIR absorption spectra of the contact lenses with various percentages of embedded water collected using RCA in trans-reflectance mode. The standard error of calibration using only two factors was 0.82 % with an R2 value of 0.9964.

Another water absorption band was observed near 1900nm, but was not added to the model as it appeared to overlap with the specific absorption features of the lens material. The variations in the lens material can be differentiated by the absorption features in other regions of the spectrum more accurately than the embedded water content. The following table summarizes the standard error of calibration, correlation value, and prediction for each type of sample presentation.

Sample Presentation Correlation Number of Factors Standard Error of Calibration Cross Validation Error Precision Simplicity Sample Damage
Transmission - Optiprobe 0.9882 2 1.48% 1.50% Good Good NO
Trans-reflectance - RCA with Liquid Kit 0.9964 2 0.82% 0.92% Very Good Very Good NO
Transmission - RLA sample mount 0.9931 3 1.14% 1.30% Very Good Very Good NO
Reflectance - SmartProbe 0.9186 3 0.92% 0.96% Good Good YES

The NIR spectra of lenses with the same concentrations were used to validate the models. These spectra are different from those used for building the model. The experimental values were then compared with the various known concentrations. Values acquired using the trans-reflectance mode showed a better correlation than those found using other sample presentation methods.

Conclusion

The results clearly demonstrate the suitability of NIR in monitoring the percentage of embedded water content of soft contact lenses. It took approximately 15 seconds to carry out the NIR measurements and provide various advantages over traditional methods. Advantages include; no sample preparation, rapid analysis times, and recovery of the intact sample. The reference values given were rounded numbers and more precise and accurate lab values will lead to more precise NIR predictions. The submitted values were considered as label claim or estimates used for feasibility. Easy sample presentation was offered by the trans-reflectance mode, with good prediction, intact sample recovery, and accuracy.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.

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