Acquisition rate

The minimum sampling rate of the ADC is critical to faithful representation of the signal frequencies. This rate, the number of measurements per second, is often also expressed as the dwell time of the ADC, that is, the time interval between measurements, At=t —, . It is well established in information theory that 

As we saw in Section 3.4, quadrature phase detection discriminates between frequencies higher and lower than the pulse frequency, but it does not prevent foldover from frequencies higher than the Nyquist frequency. For a desired spectral width FT, there are two common methods for carrying out quadrature phase detection, as was indicated in Section 3.4. One method uses two detectors and samples each detector at FT points per second, thus acquiring 2 FT data in the form of FT complex numbers. The other (commonly called the Redfield method ) requires only a single detector and samples at 2 FT points per second while incrementing the phase of the receiver by 90° after each measurement. (In two-dimensional NMR studies, a variant of this method is usually called the rime-proportional phase incrementation, or TPPI, method.) Because these methods result in quite different treatment of folded resonances, we now consider these approaches in more detail. 

With two detectors we make simultaneous measurements in quadrature, which we may take to be along the x and y axes in the rotating frame. Because we can 

FIGURE 3.5 (a) Depiction of magnetization M precessing in the rotating frame at the frequencies 

FIGURE 3.6 (a) Depiction of magnetization M precessing in the rotating frame at the frequencies indicated. W/2 is the Nyquist frequency, and sampling of both phase-sensitive detectors is illustrated at t- /W and 2/ H7to obtain projections along both x and y.  

Mass spectrometers do not continuously record the substance stream arriving in the detector (as, for example, with FID, ECD, etc.). The chromatogram is comprised of a series of measurement points which are represented by mass spectra. The scan rate chosen by the user establishes the time interval between the data points. The maximum possible scan rate depends on the scan speed of the spectrometer. It is determined by the width of the mass range to be acquired and the necessity of achieving an analytical detection capacity of the instrument which is as high as possible. For routine measurements, scan rates below 1.0 s/scan are usually chosen. Compared with a sharp concentration change in the slope of a GC peak, these scan rates are only slow. 

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This means an acquisition rate of 64 channels simultaneously, and 128 data channels as 2 frequencies are used. The acquisition speed is 140 points per second for each channel, meaning practically 6 to 12 points per rod mm. Since the method was developed 10 years ago, the processing and analysis time of this amount of information (20 Mbytes average) was long and difficult. 

The data acquisition rate is generally set so that the sample spacing of the sonic log (the distance between two acquired data points) ranges from 6 in. to 1 ft based on the anticipated drilling rate of penetration (ROP). 

The full-scan mode is needed to achieve completely the full potential of fast GC/MS. Software programs, such as the automated mass deconvolution and identification system (AMDIS), have been developed to utilize the orthogonal nature of GC and MS separations to provide automatically chromatographic peaks with background-subtracted mass spectra despite an incomplete separation of a complex mixture. Such programs in combination with fast MS data acquisition rates have led to very fast GC/MS analyses. 

Applications The diversity of MS/MS instrumentation offers considerable opportunities for polymer/addit-ive analysis. The best geometry for a particular application depends on a number of factors, including mass resolution in the first and third stages, mass range, sensitivity, available collision energy, the type of information required, data acquisition rate, etc. Polymer science applications of MS/MS comprise ... 

ToF-MS Simple operation high acquisition rate (>100 spectra s 1) high m/z accuracy ( 0.01 units) m/z range > 10000 high resolving power (>5000)... 

Low resolving power (up to 4000) low m/z accuracy ( 0.1 units) slow acquisition rate (1 spectrum s-1) maximum m/z range 3000... 

Very high cost complex operation and maintenance low acquisition rate (1-10 spectra s-1)... 

If the LC part is optimized to deliver peaks in a shorter time or more peaks in the same time when compared to a conventional method, we must consider the system s ability to handle data. Because the speed optimization described above will produce much narrower peaks, widths below 1 sec can be achieved easily. However, the data acquisition rate and data filtering steps must be considered. 

To identify a compound, five data points per peak may be sufficient. Quantitation may require at least 10 data points across a peak. Many of today s laboratories still house standard detectors (UV, ELSD, fluorescence, etc.) with maximum data acquisition rates at or below 20 Hz. Many conventional LC/MS methods acquire data at rates of 5 Hz or less. As shown in Figure 3.8, this is not sufficient for modem speed optimized chromatography. Obviously, selecting the wrong data acquisition rate will nullify all attempts to optimize chromatography. 

Careful setting of the scan range is recommended. The narrower the scan range, the higher the number of data points per peak. This is not necessarily a linear relation it can depend on the MS. If a large mass range and fast data acquisition rate are required, a time-of-flight (ToF) MS would be... 

MS operated at 20 to 40 Hz data acquisition rates in centroid or profile mode and at a mass range of 900 amu. The resulting cycle times varied from 49 to 62.22 The longest system overhead time was 21 sec and the shortest was 8 sec—a difference of 260 percent Translated into throughput, the difference equals almost 400 samples per day ... 

For non-simultaneously operating multi-sources it is necessary to check whether the data quality achieved is sufficient, especially for fast LC systems with very narrow peaks. A switching source splits the available acquisition time for the offered ionization types and therefore reduces the true acquisition rate of the mass spectrometer. However, simultaneously operating sources do not explicitly show which compound ionized with what technique. If this information is really necessary, two separate runs must be performed. 

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