Chemical Structural Analysis of Bone by Spectroscopy - Vibrational Spectroscopy for Tissue Analysis - page 207

Phosphate

Theoretically, there are four vibrational modes present for phosphate ions, ν 1 ,
ν 2 , ν 3 , and ν 4 . All these modes are Raman and infrared active and are observed
for all the spectra of carbonated apatite and hydroxyapatite powders.
In the carbonated spectrum, a single intense ν 3 band is present at 1046 cm −1 ,
whereas, in the hydroxyapatite spectra, the ν 3 band has three different sites
present at 1096, 1085, and 1056 cm −1 (Figure 7.16 and Table 7.1). The intense
ν 3 band is thought to be responsible for totally obscuring the ν 1 carbonate
bands [20,24,29,30].
The peak area of the ν 3 band was calculated and used to determine the
phosphate-to-carbonate ions ratio. The peak areas of the ν 3 band of carbon-
ated and hydroxyl apatite powders are given in Table 7.2. This ratio increases
with an increase in the phosphate content.
Phosphate ν 1 band is present at 961 cm −1 and can be observed in all the
spectra of hydroxyapatite and carbonated apatites (Figures 7.7-7.12 and
Tables 7.1 and 7.2).
The phosphate ν 4 band is present in the region of 660 and 520 cm −1 and is a
well-defined and sharp band, observed in the carbonated and hydroxylapatites.
It has two sites in the case of carbonated apatite, centred at 603 and 567 cm −1 ,
and hydroxyapatite spectra have three sites observed at 633, 602, and 566 cm −1
(see Figure 7.16 and Table 7.1). This splitting of the ν 4 vibrational band indicates
the low site symmetry of molecules, as two and three observed bands confirm
the presence of more than one distinction site for the phosphate group [31].
The phosphate ν 1 band is observed in the region of 475 and 440 cm −1 and
has two sites. These are weak bands, not as strong as the ν 3 and ν 4 bands.

(b)

(a)

1223 1135 1047 959 871 783 95 607 19 431

Wavenumber (cm -1 )

Figure 7.16
FTIR spectra of carbonated (a) and hydroxyl (b) apatites, phosphate (v 3 ) and (v 4 ) bands. (With
permission.)