154
M. Talebi et al.
certain challenges with each approach. RI measurement requires a thermal pretreatment step for the sample that leads to some loss of moisture content and therefore
inaccurate results [133]. Employing the LOD technique usually generates numbers
that are lower than the true water content of the samples [134]. While KFT has a
broad dynamic range, it can be troublesome for samples with low levels of water
[135, 136]. Expensive consumables, limited solubility of samples, reactive impurities, side reactions, and other matrix effects are frequent challenges associated with
using the KFT approach [137–140].
Gas chromatography is another approach for the determination of water content
in different matrices [24, 25, 130–132, 135, 141–145]. Traditionally, the combination of a packed column with a thermal conductivity detector (TCD) was used for
GC analysis of moisture. However, there were several problems, such as non-ideal
adsorption isotherms of water to the diatomaceous earth and various other supports
(e.g., molecular sieves) resulting in unsymmetrical poor peak shapes, bad response,
and even interference with other common analytes [146–150]. Also, employing capillary columns coated with conventional stationary phases leads to film degradation
by repeated high temperature exposure to water [131]. The convenient quantitation
of water can be performed using water-compatible columns that eliminate all of the
concerns mentioned above.
The three triflate-based IL columns were recently made available commercially
by MilliporeSigma, under the trade name Watercol
TM . The Watercol
TM 1460 column is coated with tri-(tripropylphosphoniumhexanamido)trimethylamine trifluoromethanesulfonate, a trigonal phosphonium-based ionic liquid. The Watercol
TM
1900 and Watercol
TM 1910 columns are composed of dicationic imidazolium-based
ILs, connected with a polyethylene glycol spacer chain [24, 131]. The chemical
structures of the three IL stationary phases that were specifically designed for water
analysis are shown in Table 6.1.
The Watercol columns provide optimal selectivity in differentiating water from
a vast variety of polar and nonpolar compounds. They also produce water peaks of
good efficiency and symmetry to allow for proper integration and subsequent quantitation. The retention of water is lowest on the tricationic Watercol 1460 column
with the lowest polarity among water columns. This column is excellent for separation of water in matrices with high-boiling point components. On the other hand,
while Watercol 1900 and Watercol 1910 columns have very close Kovats retention
indices for water, they provide slightly different selectivities in separating other compounds. This selectivity difference primarily results from different substituents on
the imidazolium ring of the dicationic IL moieties [24].
The Watercol 1910 column was used for determination of water in active pharmaceutical ingredients (APIs) as well as solid pharmaceutical products [25, 141]. Monitoring water content in pharmaceutical products is important due to microbial growth
in the formulation of drugs. In addition, physical and chemical stability of APIs is
significantly correlated to the water content present in these compounds. The 1-ethyl3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([C 2 C 1 im][FAP]) IL
M. Talebi et al.
certain challenges with each approach. RI measurement requires a thermal pretreatment step for the sample that leads to some loss of moisture content and therefore
inaccurate results [133]. Employing the LOD technique usually generates numbers
that are lower than the true water content of the samples [134]. While KFT has a
broad dynamic range, it can be troublesome for samples with low levels of water
[135, 136]. Expensive consumables, limited solubility of samples, reactive impurities, side reactions, and other matrix effects are frequent challenges associated with
using the KFT approach [137–140].
Gas chromatography is another approach for the determination of water content
in different matrices [24, 25, 130–132, 135, 141–145]. Traditionally, the combination of a packed column with a thermal conductivity detector (TCD) was used for
GC analysis of moisture. However, there were several problems, such as non-ideal
adsorption isotherms of water to the diatomaceous earth and various other supports
(e.g., molecular sieves) resulting in unsymmetrical poor peak shapes, bad response,
and even interference with other common analytes [146–150]. Also, employing capillary columns coated with conventional stationary phases leads to film degradation
by repeated high temperature exposure to water [131]. The convenient quantitation
of water can be performed using water-compatible columns that eliminate all of the
concerns mentioned above.
The three triflate-based IL columns were recently made available commercially
by MilliporeSigma, under the trade name Watercol
TM . The Watercol
TM 1460 column is coated with tri-(tripropylphosphoniumhexanamido)trimethylamine trifluoromethanesulfonate, a trigonal phosphonium-based ionic liquid. The Watercol
TM
1900 and Watercol
TM 1910 columns are composed of dicationic imidazolium-based
ILs, connected with a polyethylene glycol spacer chain [24, 131]. The chemical
structures of the three IL stationary phases that were specifically designed for water
analysis are shown in Table 6.1.
The Watercol columns provide optimal selectivity in differentiating water from
a vast variety of polar and nonpolar compounds. They also produce water peaks of
good efficiency and symmetry to allow for proper integration and subsequent quantitation. The retention of water is lowest on the tricationic Watercol 1460 column
with the lowest polarity among water columns. This column is excellent for separation of water in matrices with high-boiling point components. On the other hand,
while Watercol 1900 and Watercol 1910 columns have very close Kovats retention
indices for water, they provide slightly different selectivities in separating other compounds. This selectivity difference primarily results from different substituents on
the imidazolium ring of the dicationic IL moieties [24].
The Watercol 1910 column was used for determination of water in active pharmaceutical ingredients (APIs) as well as solid pharmaceutical products [25, 141]. Monitoring water content in pharmaceutical products is important due to microbial growth
in the formulation of drugs. In addition, physical and chemical stability of APIs is
significantly correlated to the water content present in these compounds. The 1-ethyl3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([C 2 C 1 im][FAP]) IL