5.2 The Binary Response
209
tion (or in an amount) below the limit of detection of the CMP concerned will
never be detected or identified. However, low and very low analyte levels can be
made detectable by using an appropriate separation technique for preconcentration.
Box 5.5
Practical expressions of sensitivity in classical qualitative analysis
In Classical Qualitative Analysis, sensitivity is not expressed in terms of the limit of detection
(CLOD) described in Sect. 5.2.2; this would entail measuring n blanks, which makes no
sense with visual (human) detection as the responses can never be the quantitative data
required for statistical processing. Rather, the sensitivity is expressed in terms of other parameters such as the lowest amount or concentration of analyte that can be detected taking
into account the results of the blank test. The following are the two most commonly used
alternatives:
The limit of perceptibility, LP. which is the smallest amount of analyte, in nanog rams, that
can be identified in a qualitative test.
The limit of dilution or limiting dilution, D, which is the smallest amount of analyte, in
nanograms, that can be identified per unit volume (in millilitres). Therefo re,
LP(ng) . 10- 6
D= - -- -
mL sample
The limit of dilution is usually expressed as pD = - log D or 1: l/D. Thus, for a limit of 1 ng in
1 mL of sample, the sensitivity can be expressed as
D= 10- 6
pD=6
1:1000000
Selectivity. This property is defined differently depending on the particular goal
of the analysis required to solve the analytical problem concerned. Thus, it may
refer to a single analyte (e. g. the absence of interferences from other sample
components). Also, it may refer to a compound "family" (e.g. organochlorine
pesticides, hydrocarbons, heavy metals) and the absence of interferences from
components other than the target congener(s). This property depends strongly
on the relationship between the signal(s) produced by the analyte(s) and their
properties; as such, it can differ markedly among analytes and may be zero in the
presence of potential interferents. Analytical techniques for the separation of
species, both individually and in groups, are commonplace in CMPs for qualitative purposes; in this context, selectivity refers to the separation proper.
Expeditiousness. As a rule, meeting the demand for (bio )chemical information in the industrial, ecological and clinical fields entails making timely
decisions, which relies on a reasonably high sample throughput. The growing use
of sensors and screening systems is one currently favoured trend with a view to
achieving expeditiousness.
Cost-effectiveness. Obtaining the high sensitivity and selectivity required to
assure reliable binary responses usually entails using analytical tools of high
209
tion (or in an amount) below the limit of detection of the CMP concerned will
never be detected or identified. However, low and very low analyte levels can be
made detectable by using an appropriate separation technique for preconcentration.
Box 5.5
Practical expressions of sensitivity in classical qualitative analysis
In Classical Qualitative Analysis, sensitivity is not expressed in terms of the limit of detection
(CLOD) described in Sect. 5.2.2; this would entail measuring n blanks, which makes no
sense with visual (human) detection as the responses can never be the quantitative data
required for statistical processing. Rather, the sensitivity is expressed in terms of other parameters such as the lowest amount or concentration of analyte that can be detected taking
into account the results of the blank test. The following are the two most commonly used
alternatives:
The limit of perceptibility, LP. which is the smallest amount of analyte, in nanog rams, that
can be identified in a qualitative test.
The limit of dilution or limiting dilution, D, which is the smallest amount of analyte, in
nanograms, that can be identified per unit volume (in millilitres). Therefo re,
LP(ng) . 10- 6
D= - -- -
mL sample
The limit of dilution is usually expressed as pD = - log D or 1: l/D. Thus, for a limit of 1 ng in
1 mL of sample, the sensitivity can be expressed as
D= 10- 6
pD=6
1:1000000
Selectivity. This property is defined differently depending on the particular goal
of the analysis required to solve the analytical problem concerned. Thus, it may
refer to a single analyte (e. g. the absence of interferences from other sample
components). Also, it may refer to a compound "family" (e.g. organochlorine
pesticides, hydrocarbons, heavy metals) and the absence of interferences from
components other than the target congener(s). This property depends strongly
on the relationship between the signal(s) produced by the analyte(s) and their
properties; as such, it can differ markedly among analytes and may be zero in the
presence of potential interferents. Analytical techniques for the separation of
species, both individually and in groups, are commonplace in CMPs for qualitative purposes; in this context, selectivity refers to the separation proper.
Expeditiousness. As a rule, meeting the demand for (bio )chemical information in the industrial, ecological and clinical fields entails making timely
decisions, which relies on a reasonably high sample throughput. The growing use
of sensors and screening systems is one currently favoured trend with a view to
achieving expeditiousness.
Cost-effectiveness. Obtaining the high sensitivity and selectivity required to
assure reliable binary responses usually entails using analytical tools of high