Biomedical Engineering Reference
In-Depth Information
[
139,
141
] also combined with higher temperature [
134,
139,
141,
143,
148
] . It has
been shown that higher temperature also increases the rate of degradation of some
substances: above 65 °C a significant increase in the conversion of cocaine into
benzoylecgonine occurs [
151
] .
Furthermore, to increase the efficiency of extraction of basic molecules such as
amphetamine, methamphetamine and cocaine, methanol can be acidified with HCl
[
151
]. Methanol is a good extraction solvent that allows the simultaneous extraction
of compounds sensitive to hydrolysis, such as cocaine and heroin, and very lipo-
philic compounds such as THC [
148
]. A disadvantage of using methanol is an
extract with a high level of impurities, which must necessarily be subjected to a
subsequent cleanup [
42
] .
Another important factor is the pH of the extractive solution for not promoting
the hydrolysis of certain substances such as cocaine and 6-MAM. In some methods,
the extraction solvent is acetonitrile with addition of formic acid [
135
] or in mix-
tures with methanol, with extraction times of 18 h [
145,
147
] .
The optimization of the extraction phase from hair with dichloromethane and
methanol as modifier has recently been published by Fernandez et al. [
140
] . In this
case MAE has been used, obtaining a very rapid extraction procedure and allowing
the chlorinated solvent to permeate into the structure of the hair keratin.
The use of dilute aqueous solutions of HCl [
142,
152,
153
] or phosphate buffer
(pH 6.5-5) [
133
] are very appropriate for the extraction of amphetamines and other
compounds in basic character. In this case, the extraction times are about 20 h.
When supported by increased temperature, the extraction lasts only for 2 h, but it is
not effective for the extraction of lipophilic compounds such as THC. The use of
aqueous solutions of HCl causes partial or complete hydrolysis of some substances,
so phosphate buffer at pH 5 is preferred for the determination of cocaine and
6-MAM; in these conditions these two molecules appear to have increased stability
[
154
]. In addition to acidic aqueous solutions, formic acid instead of hydrochloric
acid solutions are also used in organic solvents [
136
] .
Supercritical fluid extraction (SFE) has been used for the recovery of analytes
from hair: this is a technique that offers several advantages due to the characteristics
of low viscosity of supercritical fluid: the speed of extraction, high extraction
efficiency, and ultimately the ability to easily remove the extraction solvent. Also
there is the possibility of working with automated systems that also allow the recy-
cling of the solvent. The major limitation of this technique is the cost of the instru-
mentation [
42
] .
Matrix Dissolution
These processes provide a complete destruction of the structure of the hair keratin
by the complete hydrolysis of proteins that compose it. Hydrolysis can occur enzy-
matically or chemically.
