Chemistry Reference
In-Depth Information
the mass analysis capabilities of mass spectrometry-MS. The UPLC-MS is a
sensitive and selective device which enables chemists to identify unknown com-
pounds or to determine the structure of a compound by observing its fragmentation
afforded by
the apparatus. It is also possible to include UV-VIS or NMR detector
into the system in order to achieve a parts/whole analysis which depends on the
associated milieu.
Chemists well know that the study by UPLC-SM of a mixture of antibiotics
contained in a sample of water varies according to: (i) the original matrix (waste-
water, particular place from which sample was taken along the river), (ii) the
associated milieu - the mobile phase which is a mixture of polar and non-polar
liquid solvents whose respective concentrations are varied depending on the com-
position of the sample, and (iii) the nature and the quantity of the antibiotics present
in the mixture. The results of the study also depend on internal elements related to
the apparatus itself: (1) the pressure, (2) the flow rate fluctuations, (3) the chemical
nature and characteristics of the separating column-length, porosity, polarity,
specific surface, diameter, use of capillary tubes, (4) the temperature along the
process, (5) and the type of detectors and their limits of use (Snyder et al.
2010
).
In addition, both the parameterization and use of this apparatus depend on the
choice of a quantitative method, and the determination of attenuation factors in
order to ensure that the value of surface of the peaks corresponds to the concentra-
tion of the products. Last but not least, chemists must use a wide range of statistical
methods and tests so as to shape their quantitative results in the due form before
deciding as to whether a result is validated or not.
In the most recent methodologies, an analysis consists of at least three steps:
taking the sample, preparing the sample, and carrying out the physicochemical
analysis. If many factors change at the same time, at the end of the process, the
concentration of the product must nonetheless belong to the confidence interval
defined by a particular standard such as the norm ISO 5725.
3
The confidence
interval characterizes the confidence or the credibility which can be ascribed to
the result of a particular quantitative determination. Confidence intervals consist of
a range of values that act as good estimates of the unknown population parameter.
However, it is not unusual that none of these values covers the value of the
concentration that must be quantified. A level of confidence of the confidence
interval is chosen using chemical knowledge and know-how which are already at
the disposal of chemists. This level of confidence indicates the probability with
which the confidence range captures the “true” value of the quantity under consid-
eration. This interval is calculated from the standard deviation characterizing
the dispersion of the operational process. The statistical modeling of sets of data
becomes of crucial importance for chemical inferences and should not be left aside
in a philosophical study concerned with the
ceteris paribus
clause in chemistry.
3
The ISO 5725 provides a procedure for obtaining intermediate measures of precision, basic
methods for the determination of the trueness of a measurement method, the determination of
repeatability and reproducibility of a standard measurement method.
