Transient signals data acquisition

Longerich, H.P, Jackson, S.E., Gunther, D. (1996) Interlaboratory note. Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation. Journal of Analytical Atomic Spectrometry, 11, 899-904. 

For some mass spectrometers, such as TOF mass spectrometers, for reasons of data transfer speed and data storage capacity, the data acquisition system needs to accumulate data for a period of time in the summing memory, and forward the accumulated data to the data system. Each spectrum is added to the sum of the previous spectra so that a continuous summation process takes place. This type of data acquisition system is called digital signal averaging (DSA) or integrating transient recorder (ITR). Figure 3.8 illustrates the principle of mass spectrum acquisition with this type of system. 

Which brings us to a related point. Modern computer-based data acquisition methods sometimes show a low-frequency oscillation superimposed on the signal. Say that we have a transient signal decaying with a time constant of 0.1 s, and sampled with a 20 ms. Any 60 Hz pick-up will then show as a 10 Hz oscillation, i.e., as the beat frequency between the sampling rate of 50 Hz and the 60 Hz pick-up. Fig. 7.4-9a illustrates this. 

Fourier transformation of these two transients gives the distribution of harmonics that comprise the signals. One therefore knows the amplitude of excitation and the corresponding amplitude and phase angle for current flow at each frequency in the Fourier distributions. In other words, one has the faradaic impedance as a function of o) for the potential dc- All of this can be obtained with a 100-ms period of data acquisition even on a single drop at a DME hence it is feasible to repeat the whole procedure, perhaps on subsequent drops, so that more precise ensemble-averaged results are obtained. Changing E for each complete set of measurements then provides the potential distribution, that is, E E, 0)). 

The transient signals provided by the detectors were formerly registered with a strip-chart recorder, which required human participation in the final stage of the analytical process the operator had to measure signals, contrast samples with standards and match samples and results. The later use of microcomputers for data acquisition and treatment allows the easy delivery of results, expressed in the preselected units, through a printer. Technicon market hardware and software suited to their simpler AutoAnalyzers, which can also be adapted for this purpose with the interesting innovations reported recently [22-25]. Multi-channel models (e.g. SMAC) feature a built-in central computer which, in addition to serving this function, controls the analyser operation. 

To facilitate data recording and storage, oscilloscopes with storage capabilities have been replaced by personal computers with A/D conversion-based data acquisition cards to record all transient measurement signals. 

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