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Absorption of 193- and 213-nm Laser Wavelengths in Sodium Chloride Solution and Balanced Salt Solution
Geoffrey T. Dair, BSc;
Ross A. Ashman, BSc;
Robert H. Eikelboom, PhD;
Fred Reinholz, PhD;
Paul P. van Saarloos, PhD
Arch Ophthalmol. 2001;119:533-537.
ABSTRACT
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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.
INTRODUCTION
EXCIMER LASERS operating with a wavelength of 193 nm have been used
to correct refractive errors in the cornea since the mid-1980s.1-3
In recent years, a solid-state laser with an output wavelength of 213 nm has
been investigated as an alternative to excimer lasers.4-7
These investigations have focused largely on establishing the ablation rate
behavior with fluence for this longer wavelength. Although the ablation rate
has been found to be similar to that achieved with excimer lasers, other factors
that may affect ablation rate are yet to be investigated.
The ablation rate of corneal tissue depends primarily on the fluence
of the laser. The fluence determines the precision and efficiency with which
tissue is removed. Two other related factors that have an effect on the ablation
rate (at 193 nm) are corneal hydration8-9
and the presence of surface fluids. Fluid on the cornea can result in a reduction
of the ablation rate during refractive surgery procedures with 193-nm laser
pulses.10 This reduction in ablation rate is
evidenced by the high degree of absorption of 193-nm light in balanced salt
solution (BSS).11 Underlying tissue, therefore,
is effectively masked from incident radiation.12
An unknown reduction of the ablation rate can result in undercorrection of
refractive errors or irregular ablations, such as the formation of corneal
islands.
During initial patient trials with the 213-nm wavelength solid-state
laser in our laboratory, it was observed that the presence of fluid on the
corneal surface had little effect on the ablation rate. This suggests that
while the fluid is a strong absorber for 193-nm radiation, it may be a weak
absorber for 213-nm radiation. The implication is that fluid on the corneal
surface during surgery may not have a masking effect for underlying tissue
using the 213-nm wavelength.
The purpose of this study was to quantify the absorption at the 193-
and 213-nm laser wavelengths in 2 solutions commonly used during refractive
surgery. The transmission characteristics of each wavelength are investigated
through the constituent components of BSS and 0.9% sodium chloride solution.
Absorption coefficients are then derived from these data.
MATERIALS AND METHODS
The transmission characteristics for each component of BSS were investigated
for the ultraviolet wavelengths of 193 and 213 nm. Two refractive lasers in
clinical use were used for the study. An excimer laser (Telco Medical Technologies,
Perth, Australia) with a pulse duration of 20 nanoseconds and repetition rate
of 10 Hz was used to supply the 193-nm wavelength. The 213-nm radiation was
supplied from the fifth harmonic wavelength of an Nd:YAG laser (Telco Medical
Technologies). The pulse duration was 6 nanoseconds and the repetition rate
was 20 Hz. The pulse energy of each laser was measured with an energy detector
(model ED200; Gentec, Sainte-Foy, Qiebec) and monitored on a storage oscilloscope.
The energy was obtained from the amplitude of the voltage signal delivered
from the detector. To reduce the error arising from pulse-to-pulse variations,
5 pulses obtained randomly were averaged for each measurement.
Figure 1 shows the experimental
setup used for obtaining the measurements for each wavelength. Solutions were
held in a vessel with a fused silica window at the base to maximize transmission.
Above the vessel, a fused silica beam splitter was used to allow simultaneous
measurement of the initial and transmitted pulse energies. The splitting ratio
of the beam splitter was measured at each wavelength to allow calculation
of the pulse energy incident on the solution. The reflection losses at each
interface through the vessel were also calculated. This included the air-solution
and solution-fused silica and fused silica-air boundaries that are encountered
during a single pass through the vessel. For 193 and 213 nm, the refractive
index values of the solution were taken to be 1.436 and 1.409, respectively.13 The overall reflection losses were 8.0% for 193 nm
and 7.4% for 213 nm.
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Figure 1. Experimental setup for measuring
transmission of 193-nm excimer and 213-nm solid-state laser pulses through
various solutions. Reflection losses through the vessel were 8.0% for 193-nm
pulses and 7.4% for 213-nm pulses. CRO indicates cathode ray oscilloscope.
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Solutions containing various concentrations of each species were tested
for transmission. The depth of solution was maintained at a fixed level of
17.5 mm by placing an equal volume of solution in the vessel for each concentration.
This corresponded to the use of 10 mL of solution. Species investigated were
based on the recipe used for BSS (Alcon, New South Wales, Australia) and included
sodium chloride, potassium chloride, calcium chloride, magnesium chloride,
sodium acetate, and sodium citrate. Each solution was prepared by diluting
with deionized water. The range of concentrations used varied between the
concentration found in BSS for each species down to a dilution factor of 1/200.
A further range was used in the case of the chlorides for 213 nm, which was
in excess of the molarities found in BSS. This was necessary for 213 nm because
none of these species produced a detectable change in transmission for the
concentration range used at 193 nm. Higher concentration ranges were used
for sodium chloride (1-5 mol/L), potassium chloride (0.5-4 mol/L), and calcium
and magnesium chlorides (0.5-2 mol/L) to produce an appreciable attenuation.
The method of varying concentration was used due to the large degree
of absorption observed at 193 nm for sodium chloride solution and BSS. This
large absorption made the more conventional method of measuring transmission
through varying distances of solution of fixed concentration difficult. By
varying the concentration, the requirement of precise and accurate measurement
of small distances is circumvented. The method allows calculation of the absorption
coefficient for a particular species at other concentrations. Also, insight
is provided into the degree of absorption from each species in a mixture such
as BSS.
With increasing concentration, incident laser pulses are attenuated
in accordance with the Beer-Lambert law for absorption: Ef = Eie- MCd, where m is the molar
absorption coefficient in moles per liter per centimeter (M-1cm-1); C, the concentration in moles per
liter; d, distance in centimeters; and Ef and Ei, the final and initial pulse energies
(in millijoules), respectively.
By plotting transmission against concentration in a log-linear plot
yielding a linear relationship, the gradient of the plot was used to obtain
the molar absorption coefficient of each species. Multiplication of the molar
absorption coefficient (M, M-1cm-1) with concentration produces the absorption coefficient for a given
molarity (, cm-1). The distance over which 63% of
the incident energy is absorbed is defined as the penetration depth and is
obtained from the reciprocal of the absorption coefficient.
The absorption coefficient for each species at the molarity occurring
in BSS was calculated for each wavelength. By summing the absorption coefficients
from each species, an overall absorption coefficient for BSS was obtained.
In this way, the penetration depth for each species and BSS was found. The
absorption coefficient and penetration depth was also found for 0.9% sodium
chloride solution.
RESULTS
Transmission was found to decrease exponentially with increasing concentration
for all species tested. Figure 2
shows a log-linear plot of the transmission data for sodium chloride depicting
a linear relationship at 193 nm. Similar plots were created for each species.
These data obtained at 213 nm for this range of concentrations are also shown.
No attenuation was detected over this range for this wavelength. Data obtained
at 213 nm for sodium chloride at the higher concentration range of 1.0 to
5.0 mol/L are shown in Figure 3.
The gradient of each plot was used to calculate the molar absorption coefficient. Table 1 presents the results obtained for
each species and wavelength. The largest molar absorption coefficient was
found to occur in sodium citrate for each of the wavelengths tested. However,
for the 193-nm wavelength, the coefficient was larger by an order of magnitude.
Similarly, sodium chloride showed the lowest coefficient for each wavelength,
although the value for 193 nm was 4 orders of magnitude larger.
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Figure 2. Log-linear plot of transmission
in sodium chloride solution vs concentration for 193 nm and 213 nm. Solution
path length is 17.5 mm.
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Figure 3. Log-linear plot of transmission
in sodium chloride solution vs concentration for 213 nm. Solution path length
is 17.5 mm. Note the different units for concentration from that of Figure
2.
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Table 1. Molar Absorption Coefficient (M) Results
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Absorption coefficients and molarity for each substance found in BSS
are shown in Table 2. The penetration
depth calculated for each component is also presented in Table 2. While sodium chloride has the lowest molar absorption coefficient
for 193 nm, the relatively large concentration used in BSS means it has the
shortest penetration depth (173 µm). Conversely, magnesium chloride
exhibited the longest penetration depth (5010 µm) due to the low value
of concentration and despite a molar absorption coefficient comparable to
calcium chloride and sodium acetate.
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Table 2. Absorption Coefficients () and Penetration Depths for
Components of Balanced Salt Solution (BSS)
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For 213 nm, sodium acetate and in particular sodium citrate were responsible
for the observed absorption at this wavelength. The contribution to the absorption
from the 4 chloride solutions was negligible by comparison. The absorption
due to sodium citrate is relatively weak compared with that of 193 nm, exhibiting
a penetration depth of almost 2 mm compared with approximately 0.3 mm for
193 nm.
Table 3 displays results
for absorption coefficient and penetration depth in BSS, 0.9% sodium chloride
solution, and water. The results for water were included to provide a basis
for comparison and were taken from the literature.14
The molarity of 0.9% sodium chloride solution (0.154 mol/L) is approximately
40% higher than the molarity of sodium chloride in BSS (0.11 mol/L). The results
show that the penetration depth in BSS for the 213-nm wavelength is longer
than 193 nm by a factor of 20. In the 0.9% sodium chloride solution, the penetration
depth for 213 nm is 3 orders of magnitude longer than that obtained for 193
nm.
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Table 3. Absorption Coefficients () and Penetration Depths for
Various Fluids*
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COMMENT
By obtaining the molar absorption coefficient for the various components
used in the production of BSS, the penetration depth can be calculated for
any concentration of a particular species or for a mixture of species. For
sodium chloride, this allows the comparison of penetration depths for solutions
of varying molarity. Balanced salt solution contains 0.64% sodium chloride
(0.11 mol/L), which is a lower concentration than that of 0.9% sodium chloride
solution (0.154 mol/L).
A study by Kornmehl and coworkers12 on
the masking properties of various fluids suggested that 0.9% sodium chloride
solution (saline) exhibits significant absorption at 193 nm and negligible
absorption for 213 nm. The study did not, however, calculate any absorption
coefficients. Absorbance was shown in relative terms over a spectrum of wavelengths
between 190 and 240 nm. This trend for the 2 wavelengths investigated in our
study is consistent with these results.
Another study by Keates et al11 investigated
the absorption coefficient of BSS for 193- and 308-nm wavelengths. Experiments
were performed by measuring the transmission through an increasing thickness
of solution. For 193 nm, the total thickness of solution was 200 µm.
These data yielded an absorption coefficient of 145 cm-1
for a penetration depth of 69 µm. This value agrees to within 5% for
our results in BSS at 193 nm.
For each trial, the transmission through pure water was measured initially
corresponding to a concentration of zero. At 193 nm, an average transmission
of 75% was observed; for 213 nm, the average transmission was 96%. These transmission
values are predicted in data by Hale and Querry,14
who tabulated absorption coefficients for water over a broad spectral range.
For 193 nm, an absorption coefficient of approximately 0.12 cm-1 was calculated. The depth of solution used in our study of 17.5 mm
would produce approximately 80% transmission through pure water based on this
value for the absorption coefficient. For 213 nm, Hale and Querry calculated
an absorption coefficient of 0.04 cm-1, resulting in approximately
93% transmission for our experiments. For each wavelength, our readings corresponded
to the findings of Hale and Querry.
Our findings may have implications for refractive laser surgery. Corneal
hydration has been shown to be an important factor in determining refractive
outcome following laser surgery at the 193-nm wavelength. Dougherty et al8 showed that ablations carried out on dehydrated cornea
resulted in overcorrection of the refractive error. Similarly, superhydration
of the cornea produced an undercorrection of the refractive error. More predictable
results were achieved with corneas at a physiological level of hydration.
In addition to this, work by Campos et al15
examined the effects of using a nitrogen gas blower to remove the ablation
plume. The study found that the gas acted to dehydrate the cornea and an irregular
ablated surface resulted. For ablations performed without the presence of
the nitrogen gas, the ablated surface appeared smooth. Subsequent work, however,
showed that corneal hydration could be better controlled by using humidified
gases to remove the ablation plume.16 These
points suggest that corneal hydration should be closely monitored during refractive
surgery. Corneal hydration is often controlled during surgery by the topical
application of BSS and removal of excess surface fluid. A possible consequence
of the long penetration depth of this fluid at 213 nm compared with 193 nm
could be a relaxation of the close monitoring requirement of corneal surface
fluid.
The use of BSS or sodium chloride solution also has application for
phototherapeutic keratectomy procedures as a masking agent for 193-nm lasers.
Phototherapeutic keratectomy is used to treat corneal irregularities and protuberances.
The role of a masking fluid is to shield deeper-lying areas of tissue while
exposing the corneal irregularities to the incident laser radiation. This
results in a smooth and regular postoperative corneal surface. A range of
masking fluids have been used for this procedure12, 17-18
in addition to BSS.19 Desirable properties
of a masking agent for phototherapeutic keratectomy include moderate viscosity
and high absorbance.12 The results of our study
show that the solid-state laser wavelength of 213 nm is weakly absorbed in
BSS and 0.9% sodium chloride solution compared with the 193-nm excimer laser
wavelength. The increased penetration depth of the 213-nm wavelength may affect
the suitability of these solutions as masking fluids for phototherapeutic
keratectomy procedures.
Results of this study reveal the difference in absorption coefficient
for BSS at the 193- and 213-nm laser wavelengths, which could have implications
for both refractive and therapeutic laser procedures. For phototherapeutic
keratectomy, other solutions used as masking agents at 193 nm should be investigated
for suitable absorption before being used at the longer solid-state laser
wavelength. In terms of refractive procedures, the possibility that BSS or
sodium chloride solution have little effect on the corneal ablation rate may
translate to a less stringent monitoring requirement of excess fluid on the
cornea. This should, however, be verified with an ablation rate study incorporating
known levels of surface fluid on the cornea.
AUTHOR INFORMATION
Accepted for publication September 8, 2000.
The 2 laser devices used in this study were supplied by Telco Medical
Technologies.
Corresponding author and reprints: Geoffrey T. Dair, BSc, Lions Eye
Institute, 2 Verdun St, Nedlands 6009, Perth, Australia (e-mail: gdair{at}cyllene.uwa.edu.au).
From the Lions Eye Institute, Centre for Ophthalmology and Visual Science,
University of Western Australia, Nedlands, Australia. Dr van Saarloos has
a financial interest in the company Telco Medical Technologies.
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