2.2 Components of a Flame Atomic Absorption/Emission Spectrometer System
2.2.11 Data Processing
Data collection has greatly advanced with the aid of computer technology that has replaced the strip charts of the decades before. But with this change other adjustments have been necessary. For example, almost all instrumentation signals are analog in nature (a continuous stream of data), while computers require data to be in a digital form (segmented). One of the best analogies used to explain the difference between analog and digital comes from the music recording industry. Vinyl records from the pre-1985 era were analog recordings, the music (groves in the vinyl) was a continuous source of sound just as a guitar string continuously vibrates. Digital recording takes the analog input and breaks it into very short bits (segments), so short that most human ears cannot detect the individual segments (others argue, such as Neal Young, that compact discs (CDs) are the worst thing to happen to the recording industry due to a decrease in sound quality). This same analog to digital (A to D) conversion must take place before a computer can record and process data from an analytical instrument.
Computers have unquestionably allowed more control and extended capabilities of analytical instrumentation. Examples include the automated nature of the instrument, operation of automatic samplers, automatic collection and processing of data including automatically generating calibration lines and reporting concentrations of analytes in samples as opposed to detector response. However, in some ways computers have decreased operator knowledge of the instrument; two examples are the lack of knowledge concerning the “nulling or zeroing” an instrument and “balancing” electronics of a system to optimum values. Prior to the addition of computers to instrumentation, these functions were manually adjusted, while today they are part of the automatic setup.
As an example, the operation of a typical FAAS blank and sample measurement will be used. Recall that in this system, a source lamp is split into two beams, one reference and one passing through the sample. Both beams go through the monochromator and to the detection where the reference and sample signal is processed separately. Finally an absorbance reading is displayed and recorded for each blank and sample. But notice, the blank and reference readings allow all of the source radiation to pass through the instrument unhindered (defined as 100 percent transmission), thus, generating a maximum response from the PMT detector, yet the reading displayed on the instrument panel is 0.000 absorbance units. Hence, there are unnoticed calculations and corrections going on “behind the scenes” in the instrument. What actually happens is the instrument sets the reference or blank sample to read a maximum signal (maximum number of electrons being generated by the PMT). Recall that the signal is rapidly chopped into reference and sample readings. Next, the instrument measures the signal for the sample, subtracts the reading of the blank from the sample, and the difference is the absorbed signal. Thus, readings go from low to high absorbance values.
It is also important to note that for systems such as FAAS units, electrical components that interact with each other must be “balanced” where signals, voltages, and currents must be optimized and matched between components. For example, when a hollow cathode lamp is turned on, the current placed on the electrodes is set to give out the maximum radiant output that the other instrument components can take but not damage the lamp by over heating or removing too much of the cathode material. The intensity of this radiant signal is adjusted further by increasing or decreasing the size of the entrance and exit slit on the monochromator. Finally the “gain”, the electrical potential applied to the PMT dynodes, is adjusted as to provide maximum amplification but not overload the PMT. Today this is all adjusted automatically, but the process should still be understood.
Most computer-controlled systems operate the instrument using two computer programs: a method and a sequence. The method program controls physical conditions of the instrument such as lamp current, fuel and oxidant flows, and electronic gain placed on the PMT. Once the instrument is operating at its maximum performance, a sequence is started. A sequence is a program that tells the instrument what samples to run, where they are located in the automatic sampler, and what order to run the samples. The sequence also collects and stores the data for each sample in a separate file. Finally, the method is used again to calibrate the instrument using the data from the reference standards and to calculate the concentration of analytes in the samples (convert detector response to concentration).
Overall, computers are a significant asset to analytical instrumentation. They have increased the capabilities of instruments and significantly decreased the cost of analysis through the operation of automatic samplers and through advanced data processing. An additional aspect of data processing is the elimination of the need to re-type data into a spreadsheet or report form that reduces typographical errors and other mistakes.
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