RESPONSE FACTOR

1
RESPONSE FACTOR The response factor is
conventionally measured with standard mixtures.
The response factor F, can be defined as
2
AX area of analyte signal AS area of internal
standard X concentration of analyte S
concentration of standard X and S are the
concentrations of analyte and standard after they
have been mixed together.
3
The response factor measures the relative
response of the detector (e.g. FID, TCD) to
different compounds. For each type of detector,
there is a range of solute concentrations in
which the detector response is linear.
For quantitative analysis, it is important to
be aware of this range and stay within it.
4
If the detector responds equally to standard and
analyte, F 1. If the detector responds twice
as much to analyte as to standard, F 2. If
the detector responds half as much to analyte as
standard, F 0.5.
5
The relative areas of the signals from X and S
allow for the determination of the amount of X in
the mixture. It is necessary first to measure
the relative response of the detector to
each compound.
6
QUANTITATIVE ANALYSIS QA is based on the area of
a chromatographic peak. In the linear response
concentration range, the area of a peak is
proportional to the quantity of that component.
(For narrow peaks, height is often used instead
of area). In most chromatographs peak area
is measured by computer.
7
If peak area must be measured by hand, and if the
peak has a Gaussian shape, then the area
is AREA OF GAUSSIAN PEAK 1.064 x Peak Height
x W1/2 (W1/2 width of peak at half-height)
8
QUESTION The ratio of peak areas of a
solution containing A (an internal standard RMM
95) and B (an analyte RMM 120), was 1.001.75.
If the solution contained 0.5 mg of A, and 0.75
mg of B, what was the response factor? (Hint In
this example you need only calculate the number
of moles of A and B, as you are not given any
information about the volume in which they are
diluted before injection.)
9
SOLUTION
AX 1.75 AS 1.00 X 6.25 x 10-6 S
5.26 x 10-6.
ANSWER 1.47
10
CAFFEINE ANALYSIS Caffeine is traditionally
classed as a CNS stimulant although it bears
few similarities with other more
recognisable drugs in this category. Caffeine is
the most widely used drug in the world. In the
UK 90 of the adult population consume caffeine
in their daily tea and coffee.
11
Only two species of coffee are extensively cultiva
ted Coffea arabica and C.canephora, each
comprising a large number of varieties, including
Robusta, the most important variety of
the C.canephora species. The Arabica class is
the most valuable as it produces a better
tasting beverage and as such, it is subject to a
greater risk of adulteration.
12
Caffeine is also present in fizzy soft drinks and
chocolate bars consumed by children each day.
Caffeine also occurs in many OTC medicines (e.g.
analgesics). regular caffeine users display a
brief withdrawal syndrome if deprived of
their usual supplies. Also it is unclear
whether caffeine generates real alertness gains
or just allows normal alertness levels to
be maintained.