13.3—
Instrument Computer Interfaces
The purpose of interfacing instruments with computers is to enable raw analytical data to be collected
as it is produced, then processed, stored and displayed or printed out. This may be accomplished as it is
gathered, i.e. in real-time, or at some later time, i.e. post-run. Complete chromatograms or spectra can
easily be stored in the main memory or RAM or transferred to disk. The immense storage capacity of
mainframe computers can be used to provide large libraries of data (data banks) for future reference.
Interfacing can be achieved in various ways depending on the nature of the detector signal produced by
the instrument. Many generate analogue signals, i.e. a continuously variable voltage or current related
to the concentration or nature of the analyte. As computers can accept only digital information,
analogue-to-digital converters (ADC) are required to facilitate the direct input of data. These devices
convert, for example, an input voltage which typically may vary between 0 and 1 V into integer
numbers within a specified range, e.g. 0 to 4095 on a linear scale, and represent them in binary form.
The resolution of such an ADC signal would be 1 in 4096, which allows discrimination between
analogue signals differing by as little as 2.44 × 10 –^4 V for a 0–1 V input range. This degree of
discrimination would be described as 12 - bit-resolution as the binary representation of numbers up to
4095 requires 12 bits. The resolution of commercial ADC units is usually 10, 12, 14 or 16 bits.
The frequency at which an ADC needs to sample an analogue signal and the total number of samples
required varies greatly and depends on the nature of the particular analytical technique and instrument.
In all cases, it is vital that the digitized version of an analogue signal, which is necessarily
discontinuous, represents the original signal as faithfully as possible. In theory, an infinite number of
samples of the analogue signal, called data points, would be required for a perfect digital representation
but this is impractical and a compromise has to be made. A limited number of data points are collected,
the precise number and frequency of sampling being determined primarily by the rate at which the
analogue signal changes and the time scale of the measurements. It may be sufficient to collect
individual data points or small groups of points intermittently and over comparatively long time
intervals if the analogue signal does not vary rapidly. This would be the case, for example, where
absorbance readings for a quantitative spectrophotometric UV analysis are to be taken. A faster and
perhaps variable sampling rate might be required during a potentiometric titration, whilst a rate of 10–
100 Hz would be necessary where the detector signal is varying widely in a much shorter time, such as
during a chromatographic run or the recording of an infrared spectrum. Extremely fast sampling rates,
sometimes as high as 50 kHz, are necessary in mass spectrometry where scans lasting only a few
seconds and covering a wide range of m/z values may be required. Similarly, sampling of the FID
signal in FT NMR (p. 413), where information over the complete spectral range is gathered in less than