Page 58 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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over the last 10 years. Virtually all modern instruments use a personal computer to store analytical
parameters, procedures and individual results, and plot calibration curves. Most will also make
statistical analyses of the data if required. Hard copies of the data may be obtained by using printers.
Most instruments also allow transient signals, such as those obtained from flow injection or
chromatography, to be obtained by using time-resolved or continuous graphics software and will
integrate the area under each peak. Previously, either a chart recorder was used to measure peak
heights (or, if peak areas were required, the peaks could be cut out and weighed) or an integrator could
be attached.

Many instruments have the facility of a digital meter. This may be used either with or without the
computer since many instruments provide the option to be operated manually if required. It is well to
remember that such a meter can only display a single reading and, rather than display instantaneous
readings at the moment the display is updated, it is possible to use integration. The signal is
continuously stored, or integrated, in a condenser for a fixed (usually selectable) period of time. After
normalization, the integrated signal is displayed.

The lamp, electronics and flicker of the flame may all contribute to short-term, irregular fluctuations
in the signal, i.e. noise. The meter will follow this to some extent. It is usual to include some electronic
damping to reduce the noise, i.e. to average it out to some extent.

The photomultiplier output is proportional to the transmission of the flame, yet concentration is
proportional to absorbance. A logarithmic readout is preferred as being linear in absorbance This
can be achieved by using an appropriate logarithmic amplifier, or when normalizing the output from the
integrating condenser.

Many instruments offer scale expansion facilities which are essential for accurate work at low or high
concentrations. The signal can be electrically expanded (as is the noise) to a fixed factor, or
continuously, so that the reading of a chosen standard comes to a desired figure (e.g. 10 µg ml reads
-1
100). This latter option is referred to as 'concentration read-out'. Care must be taken when using this
at high concentrations, as few calibration curves are still linear above 0.5 absorbance. Curves tend to
bend towards the concentration axis, principally because stray light (i.e. unabsorbable light, for
example from nearby lines in the lamp) become an important contribution to the total light falling on
the detector. Controls for 'curve correction' may be supplied which enable the calibration to be
linearized by extra expansion on standards of high concentration. Many modern instruments provide an
autosampler, and if this is in use then, for samples whose concentration exceeds the linear part of the
calibration curve, the analytical procedure may be programmed so that automatic dilution is performed.