Peak data, converting digital

In principle one could monitor a GC effluent with a nondestructive detector like a TCD, pass it on through a suitable interface, and acquire and print out the mass spectra of peaks as they are eluting. With fast digital data converters and the speed and power of modern desktop PCs, it is better to simply acquire mass spectral data as a continuous sequence of spectra or selected mass fragment signals as the GC run proceeds. A data file in computer memory consists of a sequence of MS scans ordered by retention time. There are three main modes of acquiring and employing such data files ... 

Both gas chromatographic instruments were connected with a PDP 11/45 computer via an analog-to-digital converter. The peak areas were calculated from the digitalized chromatographic data by means of software developed at Delft University of Technology. 

The most popular method of measuring peak area is by integration. Integration is a method in which the series of digital values acquired by the data system as the peak is being traced are summed. The sum is thus a number generated and presented by the data system and is taken to be the peak area. See Figure 11.20. We will discuss in Chapters 12 and 13 exactly how this area is converted to the quantity of analyte in GC and HPLC and the issues involved. See Workplace Scene 11.7. 

Fig. 4.12. The molecular ion peak of [60]fuUerene at different settings of the dwell time per data point of the analog-to-digital converter. At 1 ns per datapoint (1 GHz) the peaks are well resolved and resolution is limited by the analyzer, at 2 ns (500 MHz) some broadening occurs and at 10 ns (100 MHz) peak shapes are reduced to triangles.

Modern HPLC systems equipped with analog-to-digital converters (ADCs) and initial data acquisition and signal analysis are performed by either built-in microprocessor or attached computer [38,39]. Depending on the data acquisition rate and initial analysis criterion (peak height, peak area, or sensitivity thresholds, etc.), the integration limits or area calculation could be erroneous, and in some cases the whole peaks could be missed. 

The hardware used consists of an AD converter connected to an amplifier, a relay switch (operated by the microprocessor) for tltrant portioning and a DAC unit for decoding of data obtained and stored by the microprocessor. The microcomputer assembled by Betteridge et aL for this purpose consisted of 1 kb ROM, 2kb RAM and a digital I/O unit. The main functions of the software are data collection and display, derivation, curve smoothing, peak detection, Interpolation (determination of the maximum slope of the titration curve by calculation) and result delivery (display). 

The Fourier transform is a mathematical operation that converts amplitude as a function of time to amplitude as a function of frequency. An inverse Fourier transform can be used to go from the frequency domain back to the time domain. To use an inverse Fourier transform to generate a (possibly damped) data array whose amplitude varies sinusoidally and then abruptly drops to 0, we must sum up many different frequencies. The corduroy pattern we observe in the vicinity of a narrow peak in the frequency spectrum reflects the wide range of nearby frequencies required to make our digitized fid s intensity drop abruptly to 0 in the time domain. 

---