Instrumentation & Measurement Magazine 25-1 - 66

extraction and classification
to Raman spectra of
lysed blood using a partial
least squares discriminant
analysis (PLS-DA) model.
PLS-DA works by finding
new variables, called latent
variables (LVs), that
maximize separation between
classes along their
axes. These new LVs are
linear combinations of the
original variables (Raman
shifts), and the exact contribution
of each original
variable to a certain LV is
given by loadings vectors.
Within the dataset, there
Fig. 3. Representative mean Raman spectra (n = 500 spectra) along with 95% confidence intervals measured from control
lysed blood and blood components.
blood plasma, and a pellet of control white blood cells (WBC).
This is representative of spectra measured using the flow-cell
based technique and preprocessed following the steps described
above. As seen in Fig. 3, peaks in the Raman spectra
are associated with vibrational frequencies of specific chemical
bonds of molecules that are unique to the sample. Some of
these Raman peaks are useful for feature extraction for the purpose
of building discrimination models.
Many machine learning methods exist for class prediction,
such as linear discriminant analysis (LDA), k-nearest neighbours
(KNN), support vector machines (SVM), and random
forests [4], [15]. Fig. 4 shows an example of applying feature
were two sets of control
blood lysate spectra and
two sets of dosed blood lysate
spectra (20 mM and
100 mM of hydrogen peroxide)
with ~500 spectra
per set. After spectra were preprocessed using aforementioned
techniques, 70% of the set was used to train the model, then the
remaining 30% used to test it in order to assess model generalizability.
Fig. 4a shows the resulting scores plot along the first
two LVs. Each point represents the projection of a spectrum on
the plane in original variable space defined by LV1 and LV2.
An increasing trend is present, as control spectra are found to
cluster in the bottom left, while dosed spectra are found along
the upwards diagonal to the right. According to this model,
LV1 contains the most power when it comes to discriminating
between control and dosed samples. Looking at the loadings
vector plot for LV1 in Fig. 4b, one can identify Raman peaks
Fig. 4. (a) Scores plot and (b) loadings vector plot from a PLS-DA model comparing control blood and blood treated with two different concentrations (20 mM and
100 mM) of hydrogen peroxide to induce oxidative stress.
66
IEEE Instrumentation & Measurement Magazine
February 2022

Instrumentation & Measurement Magazine 25-1

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