A method for quantifying the percent sulfate associated with sodium lauryl sulfate (also referred to as sodium dodecyl sulfate)
and sodium laureth sulfate (also referred to as sodium lauryl ether sulfate) found in commonly used commercial body care products
using FT-IR attenuated total reflectance (ATR) spectroscopy is outlined.
Sodium lauryl sulfate (SLS) and sodium laureth sulfate (SLES) are used as surfactants in a wide range of body care and household
products. SLS content is predominantly determined using techniques such as UV-vis spectrophotometry, ion selective electrodes
or turbidity measurements (1). FT-IR-ATR spectroscopy, specifically using an FT-IR horizontal attenuated total reflectance
(HATR) accessory, offers an alternative method requiring minimal sample preparation prior to SLS and SLES concentration measurement.
In this application note, % SLS or SLES is determined using the peak height of the sulfate band centered at 1210 cm-1 (2). There is minimal overlap of the sulfate band in this region with peaks from other ingredients in the body care products,
allowing accurate calculation of peak heights. Due to a total overlap of the peak location and intensity of the sulfate band
for SLS and SLES, the concentration is reported as a percent total sulfate value for commercial products where both are present.
Since the intensity of both sulfate band for SLS and SLES is nearly equal in magnitude, sulfate concentration can be accurately
determined in body care products containing either SLS or SLES or a mixture of both by using SLS or SLES standards interchangeably
The Shimadzu IRTracer-100 FT-IR spectrophotometer and PIKE Technologies ZnSe HATR™ accessory with a trough were used for analysis.
Measurement and data analysis were performed using the Shimadzu LabSolutions IR software. FT-IR spectra for unadulterated
body care products collected at a resolution of 4 cm-1 with 16 scans averaged for both background and sample.
A calibration curve was created using the peak height of the sulfate peak for a series of standards prepared from a 20% SLS
solution (Sigma Aldrich, BioUltra, CAS #151-21-3) and then used to determine the SLS or SLES concentration in commercial body
Figure 1 shows an example of a calibration curve and calculated % sulfate for a commercial body care product (sample A). Other
commercial products were tested as follows:
Figure 1: Calibration curve and calculation of % SLS in a commercial body care product using LabSolutions IR.
Sample B (body wash containing SLS only); total sulfate = 5.5%, RSD ±3.2%.
Sample C (shampoo and conditioner containing SLS and SLES); total sulfate = 13%, RSD ± 0.34%.
FT-IR-ATR spectroscopy provides an easy method for determining % SLS or SLES in commercial body care products with good reproducibility
between standard and sample trials.
(1) M.L. Fielden and P.M. Claesson, J. Colloid Interface Sci.
198, 261–265 (1998).
(2) M. Sabo, J. Gross, and I.E. Rosenberg, J. Soc. Cosmet. Chem.
35, 207–220 (1984).
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