Absorption of 193- and 213-nm laser wavelengths in sodium chloride solution and balanced salt solution.

OBJECTIVE To determine absorption coefficients for sodium chloride solution (saline) and balanced salt solution at the 193- and 213-nm laser wavelengths. METHODS Absorption coefficients were obtained for each of the component species found in balanced salt solution. This was achieved by measuring laser pulse transmission through solutions of varying concentration. The experiments were repeated using the 193-nm excimer and 213-nm solid-state laser wavelengths. Results for each species were then used to obtain an overall absorption coefficient and penetration depth for balanced salt solution and 0.9% sodium chloride solution. RESULTS Absorption coefficients in balanced salt solution for the 193- and 213-nm wavelengths were found to be 140 and 6.9 cm(-1), respectively. In 0.9% sodium chloride solution, the absorption coefficient was 81 cm(-1) at 193 nm and 0.05 cm(-1) at 213 nm. At 193 nm, absorption in balanced salt solution was dominated by sodium chloride. Sodium citrate emerged as the dominant species of absorption at 213 nm. CONCLUSIONS For the species investigated, we found reduced absorption for the longer wavelength of 213 nm. While the difference in wavelength between 193 and 213 nm is within about 10%, the respective molar absorption coefficients varied by 1 to 4 orders of magnitude. This indicates that predictions for the wavelength-dependent changes of absorption coefficients of other solutions are unreliable. CLINICAL RELEVANCE Fluid placed on the surface of the cornea during keratorefractive surgery has proved to be a barrier to ablation for the 193-nm wavelength. The increased penetration depth through sodium chloride solution and balanced salt solution for the longer 213-nm laser wavelength may mean that these solutions cannot be used as a masking agent for keratorefractive procedures performed with this wavelength.