Spectra Acquisition

There are three modes of analysis commonly used spectrum acquisition spatial distribution, or dot, mapping of the elements and elemental line scans. 

In the spectrum acquisition mode the probe is either fixed in the spot mode or raster scanned over a small area at high magnification and a complete spectrum acquired. A typical spectrum is shown in Figure 2. 

MS/MS Duty Cycle Typical MS/MS analysis is a serial process, relying on the selection of precursors (peptides) in MS mode, followed by high-energy fragmentation in MS/MS. This process is termed data dependent acquisition (DDA). The duty cycle for the completion of MS and MS/MS cycles (the time necessary for MS/MS spectrum acquisition) is of primary importance. When the separation performance is viewed from the mass spectrometry perspective, the peak capacity can be characterized by the number of MS/MS scans, yielding successful... 

The pulser pulses are generated at a constant frequency, so that the variance of the pulser peak area that results from the statistical nature of spectrum acquisition is approximated at counting losses well below 50% by... 

Methods of Data Acquisition. Any of three methods of data acquisition may be selected. The first method is the single spectrum acquisition. The mass storage devices will be zeroed and a hardware stopwatch is started. 

The signal to noise ratio for an individual spectrum is given in Table II. The methods of acquisition were either A (single spectrum acquisition) or B (ensemble averaged spectrum). 

Fig. 2. a) Axial view through the liver of a Cafe diet mouse showing the position of the 3x3x3 mm localised MRS voxel in the right lateral lobe of the liver, and b) the resultant in vivo localized PRESS spectrum (acquisition parameters Broker Biospec 4.7T 40 cm magnet repetition time approx. 3000 ms (dependant on respiration rate) echo time 6.8 ms number of averages 64) (Abdel Bidar, AstraZeneca personal communication). 

Principle of TOF mass spectrum acquisition with ITR in analogue mode. Data from JEOL documentation. 

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. 

Saeki A, Kozawa T, Tagawa S. (2006) Picosecond pulse radiolysis using femtosecond white light with a high S/N spectrum acquisition system in one beam shot. Nucl Instrum Meth A 556 391-396. 

Energy analysis of the photoelectrons is carried out by a spectrometer of which there are two types cylindrical and spherical sector. In both cases, they are in the form of a capacitor that focuses electrons with the same selected energy level to a detection point, spectrum acquisition is carried out sequentially by scanning the selected energy. 

Repetition of the spectrum acquisition sequence at different probe-pulse arrival times provides a series of spectra delayed with respect to each other. Thus, time resolution of the reaction is achieved in a point-by-point manner, from which the intensity-time-wavelength surface can be produced. 

Fast spectrum-acquisition capabilities Unlike quadrapole or ion-trap instruments, the acquisition of a mass spectrum is not based on scanning. Typical pulse rates of TOF instrument are 20-50 kHz. Spectra from different ion-introduction events are accumulated, resulting in improved S/N due to the averaging of random noise. The resulting mass spectra are stored at 4-10 spectra/s. The TOF is an integrating rather than a scanning instrument. 

Commercial ESI LC-TOF-MS systems are available from several instrument manufacturers. They are applied in two main areas on-line accurate-mass determination for identification and confirmation of identity, and fast separations, which can be monitored due to the high spectrum-acquisition rates. The four- and eight-channel MUX source (Figure 5.12) t es full advantage of the high spectrum acquisition rates and provides accurate-mass determination of the analytes [63]. 

A number of selection criteria and data manipulation techniques can be applied prior to analysis to improve the quality of the analytical result. These include setting different spectrum acquisition modes, background corrections, normalization, and threshold settings. 

Determination of standard deviation for 10 successive spectra of the secondary standard, without adjustment of sample position. This result indicates short-term repeatability of spectrum acquisition. 

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