297 IN SITU ANALYSIS OF GLUCOSE IN COSMETIC FORMULATIONS
surfactant concentration of up to ∼20% w/w, urea concentration of 15% w/w, ethanol of
62% w/w (Table I)). It might be possible that the presence of polymers in the formulations
offers a certain stability to the enzymatic system, despite the presence of denaturants.21 The
above provide an indication of the applicability potential of the assay to a good variety of
cosmetic matrices.
ACCURACY UPON ANALYSIS OF COMMERCIAL PRODUCTS
Mock glucose-containing formulations were then prepared in glucose-free commercial
creams (emulsion B, emulsion C (O/W, brown-colored hand and foot cream) and emulsion
D (O/W, blue-colored body butter)). The concentration of spiked glucose in each matrix,
as well as the color/transparency of each matrix, is indicated in Table VI. All mock
glucose-containing formulations were then treated as “unknown” samples. The mock
unknown samples were either analyzed directly, or were diluted appropriately, to lower
glucose levels within the linear region of the method (dilution indicated in Table VI).
As diluent, untreated emulsion B (glucose-free) was used. They were then submitted
to the standard addition protocol adapted to the proposed analysis format. For spiked
concentration calculation with the standard addition method, the blue channel output
of emulsion B treated with the WR was taken as zero-analyte signal, in the case of the
1:1,000 and 1:10,000 dilutions.
For the 1:40 and 1:10 dilutions, emulsion B treated with the WR did not provide a suitable
“zero signal” since there was a significant effect of the colored matrix. Instead, extrapolation
to nonzero-analyte concentration (a standard containing 0.001 – 0.005% w/w glucose in
the corresponding matrix) was used. A good linear relationship was obtained between light
intensity and glucose concentration in emulsion, as indicated by the R2 values displayed in
Table VI. Our results also demonstrate an acceptable bias. An image of the colored wells
upon application of the standard addition method is given in Figure 3, as well as of the
initial emulsion.
These data provide an initial indication in favor of the validity of the approach for
quantifying glucose content in commercial cosmetic matrices within the frame of QC.
They also demonstrate that the proposed analysis format successfully takes into account
Table VI
Technical Parameters for the Quantitative Determination of Glucose in Commercial Products Using the
Standard Addition Procedure
Matrix Color Transparency Dilution applied Spiked [Glu], %w/w R2 Bias %
emulsion B (sensitive
skin cream)
White opaque Nondiluted 0.0091 0.993 −15.3
emulsion C (hand and
foot cream)
Brown opaque 1:1,000 3.12 0.997 +4.9
emulsion C (hand and
foot cream)
Brown opaque 1:40 0.08 0.991 +3.2
emulsion C (hand and
foot cream)
Brown opaque 1:10 0.02 0.999 +13.12
emulsion D (body
butter)
Blue opaque 1:10,000 8.26 0.995 +15.9
R2: correlation coefficient.

298 JOURNAL OF COSMETIC SCIENCE
any matrix effects in O/W emulsions, even in colored emulsions (commonly encountered
in the cosmetics industry).
Glucose oxidase upon which this method relies, is a highly specific enzyme for glucose.22
This is a prerequisite for the specificity of the proposed analysis method. Even if however,
a matrix component is unspecifically generating color upon the proposed treatment, this is
expected to be taken into account by the standard addition procedure.
The proposed analysis format inside the actual cosmetic formulation presents the advantage
of no use of organic solvents or pretreatment procedures, or of analytical instrumentation.
Those advantages may render it the method of choice in applications where the determined
analytical figures of merit are sufficient for the analysis purpose.
CONCLUSION
We here provide evidence of the validity of a simple, extraction-free methodology for
quantification of glucose in various cosmetic formulations. Quantification is based on
enzymatic conversion of glucose by the POD/GOD system and is accomplished by digital
image colorimetry. The favorable technical characteristics of the methodology in terms
of intermediate precision and accuracy were demonstrated in an O/W emulsion. Glucose
was moreover sufficiently accurately quantified upon application of the standard addition
method on emulsions, even when colored, at concentrations addressing quantification needs
of commercial glucose-containing cosmetic products. We demonstrated the sufficient
robustness of the enzymatic system and the applicability of the proposed methodology in
cosmetic matrices of different viscosity, color, transparency, pH and composition (including
rather extreme concentrations of protein denaturants (ethanol, surfactants)). Abolishing
the need for analyte extraction, for use of organic solvents and for instrumental-based
quantification in emulsion analysis are great advantages, ensuring a straightforward,
and low-cost/low-waste methodology. This study may open the route for routine glucose
quantification in cosmetic, galenic, or food formulations.
Figure 3. Images of microstrips loaded with each matrix of Table VI, upon application of the standard
addition procedure in conjunction with the proposed assay format. Exogenously added glucose concentration
is indicated (in 10−3 %w/w). B: emulsion B, C: emulsion C (1:1,000 dilution), D: emulsion D (1:10,000
dilution). BEF DIL indicates the cosmetic product before any dilution or other treatment.