Biomedical Engineering Reference
In-Depth Information
7.2.9
Quality Management and Accreditation
Quality management and accreditation have become matters of increasing relevance
in analytical toxicology in recent years. Forensic laboratory accreditation is based
upon international standards (ISO/IEC 17025:2005 [
68
] ) which include require-
ments for method validation and allows to control and monitor overtime the labora-
tory performance and its analysts by proficiency tests [
69
] .
8
Mass Spectrometry
The quadrupole is the most common mass analyzers due to its cheapness and per-
formance, given by high efficiency transmission. The first implementation of a two-
dimensional mass spectrometry (MS
2
) was obtained with the quadrupole (the triple
quadrupole mass analyzer) that is now the most common. The first and third (Q1
and Q3) quadrupoles are separative, and the second one (Q2) is a collision cell, in
which the ions can be fragmented. MS
2
enables a reduction of background noise and
an increase in selectivity and also allows different types of experiments: Full Scan
(FS), Selected Ion Monitoring (SIM), Product Ion Scan (PIS), Precursor Ion Scan,
Selected Reaction Monitoring or Multi Reaction Monitoring (SRM or MRM). The
last one is the most widely used mode for the confirmatory analysis on targeted
molecules.
Also ion trap allows MS
2
experiments and since, separation occurs over time and
not over space, MS
n
experiments are possible; however, for increasing value of
n
there is a proportional loss in intensity.
Tandem mass spectrometry is considered as necessary to provide unequivocal
identification and quantification of analytes, as pointed out by Maralikova et al.
[
70
]. It is also clear that monitoring of a single transition could lead to false com-
pound identi fi cations [
71
]. This had been previously predicted in the 2002 docu-
ment issued by the European Union Commission that contains detailed information
about mass spectrometric identification of drug residues in foodstuffs [
57
] , where a
minimum of three so-called Identification Points (IPs) is required for compound
identi fi cation.
8.1
An Odd Coupled
The interfacing HPLC with MS is problematic, as pointed out by the image pro-
posed by Arpino of the coupling between HPLC-fish and MS-bird [
72
] :one pro-
vides for the separation of neutral molecules in solution with high operating
pressures, while the other responds to ions in the gas phase and requires high vac-
uum. The most widely used technique is electrospray ionization (ESI). It is suitable
for analysis of highly polar compounds which are ionized in solution, such as
