APR January/February 2022 - 54

» SPECTROSCOPY
»
SERS Analysis
Standards Preparation
Lidocaine HCl and the lidocaine free base standards were weighed
using a calibrated analytical balance and an appropriate amount of
diluent was added to each using a calibrated electronic pipette. Stock
solutions of 1 mg/mL of each individual standard were prepared
in a 10% methanol solution in deionized water and diluted to a
concentration of 100 µg/mL. A SERS signature spectrum of lidocaine
was collected using the procedure below.
Ag colloidal solution (250 µL) was transferred to a 4 mL glass vial
followed by the addition of 250 µL of lidocaine (100 µg/mL) in 10%
MeOH in H2
O and 5 µL KBr (3 M) aqueous solution. The glass vial was
capped and vortexed for 30 seconds. The sample equilibrated for 30
seconds prior to conducting Raman analysis.
Sample Preparation
Suspect liquids (0.3 mL) were placed in separate 4 mL glass scintillation
vials. To each vial was added 0.5 mL of 10% MeOH in deionized water
and the contents were vortexed for 30 seconds. Ag colloidal solution
(500 µL) was transferred to each vial and was mixed for 30 seconds
using a vortex mixer. Finally, 5 µL KBr (3 M) solution was transferred to
the vial followed by vortexing for an additional 30 seconds. The sample
equilibrated for 30 seconds prior to conducting Raman analysis.
Results and Discussion
FT-IR: The FT-IR signature spectra of the reference standards for FDAapproved
filler materials are presented in Figure 1. The standards
exhibit characteristic spectral data sufficient for use as signature
spectra and can be easily distinguished from each other.
All suspect dermal filler gels were first analyzed neat (no sample
preparation) and spectrally searched using all standard spectral
libraries (in-house and commercial). Using FT-IR spectroscopy, the neat
gels were determined to contain water based on the OH stretching
and bending absorptions (Figures 2A and 2B). The presence of PDMS
was detected after spectral subtraction (Figure 2C); the suspect
spectrum exhibited nearly a peak-for-peak match with the standard
across the entire spectral range (Figure 2D) with exception of the
broad OH absorptions.
Since the broad OH absorptions assigned to water obscure those
characteristic peaks of other analytes in the neat sample, the gels were
placed in an oven to evaporate the water. The clear gel presented in
Figure 2A was dehydrated in an oven and the FT-IR analysis of the oven
dried sample (Figure 3A) revealed absorptions consistent with those
of sodium hyaluronate (Figure 3B). The sodium hyaluronate standard
spectrum was then spectrally subtracted from the sample spectrum,
which yielded the spectrum shown in Figure 3C that exhibited
peaks characteristic of PDMS (Figure 3D).8,9
Finally, the sample was
further examined using a micro-liquid-liquid (water-chloroform)
extraction procedure. The FT-IR spectrum of the residue obtained from
evaporation of the chloroform layer (Figure 3E) yielded a library search
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| January/February 2022
Figure 2. FT-IR spectrum of neat gel from suspect sample (A),
FT-IR spectrum of water reference standard (B), spectrum after
spectral subtraction of water from neat gel sample A (C), PDMS
reference standard (D). An expanded FT-IR spectrum of the neat
gel between 600-1500 cm-1
is shown on the right.
Figure 1. FT-IR spectra of sodium-HA (A), PLLA (B), PMMA (C),
calcium hydroxylapatite (D) and PDMS (E).
Figure 3. FT-IR spectrum of oven dried sample (A), FT-IR spectrum
of sodium hyaluronate reference standard (B), spectrum
after spectral subtraction of sodium hyaluronate (C), PDMS
reference standard (D), spectrum of oily residue after waterchloroform
extraction of gel (E) and lidocaine reference standard
recrystallized in chloroform (F).

APR January/February 2022

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