Spectrum Acquisition Speed

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. 

Spectrum acquisition rates are a major concern in high-speed GC-MS where total separation times in seconds and peak widths typically less than 200 ms are common. The separation conditions are relatively easy to achieve with short, wide bore, open tubular columns operated at low pressures [131-136]. The ion source of the mass spectrometer provides the vacuum source for the column. Time-of-flight mass analyzers are most suitable for these applications. Time-of-flight mass spectrometers in production are capable of recording up to 500 full-mass spectra per second based on averaging 10 transient spectra per single spectrum [135-138]. At present, this is a unique... 

The speed of the analysis is a parameter inversely correlated with the detection limits required because an increased statistical accuracy can be achieved by a longer time of spectrum acquisition. A couple of thousand second long measuring time is a typical compromise in most cases. 

In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

Phosphorus-31 MRS is continuing to advance and see wider application. The capability to perform P MRS on both clinical and high-held animal scanners is steadily becoming more widespread. Advances such as high magnetic fields and improved RF hardware, data-acquisition approaches, spectrum analysis automation, and pulse sequences promise a technique of increased speed and spectral and spatial resolution, with a widening range of potential applicahons. 

Irrespective of the method chosen, meaningful data can only be obtained if the appropriate level of signal to noise (S/N) is reached in the spectrum of each analyte. This has been achieved for Raman measurements through short data acquisition times (<1 s) and application of mathematical approaches such as If-harmonic means clustering (KHMC), factor analysis [57] and principal component analysis (PCA) [58] to the data set. Ultimately the sample response to the excitation energy determines the speed that a measurement can be made. 

The line-shape of a true (undistorted) EPR spectrum should be independent of the acquisition parameters, and therefore to assess spectral distortion one can compare spectra acquired with different parameters. Figure 15.6 illustrates the effect of modulation amplitude on EPR line-shape. The central line-width (peak-to-peak width AHpp = 1.6 G) remains unchanged when the modulation amplitude is increased from 0.5 to 1 G while at a modulation amplitude of 10 G, distortion and line-broadening (AHpp = 6.4 G) can be clearly observed. The main sources of spectral distortions are modulation amplitude, microwave power, and scanning rate (speed). These are discussed in the following sections. 

They are still the workhorses of coupled mass spectrometric applications, as they are relatively simple to run and service, relatively inexpensive (for a mass spectrometer), and provide unit mass resolution and scanning speeds up to approximately 10,000 amu/s. This even allows for simultaneous scan/ selected ion monitoring (SIM) operation, in which one part of the data acquisition time is used to scan an entire spectrum, whereas the other part is used to record the intensities of selected ions, thus providing both qualitative information and sensitive quantitation. They are thus suitable for many GC-MS and liquid chromatography-mass spectrometry (LC-MS) applications. In contrast to GC-MS with electron impact (El) ionization, however, LC-MS provides only limited structural information as a consequence of the soft ionization techniques commonly used with LC-MS instruments [electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)]. Because of this limitation, other types of mass spectrometers are increasingly gaining in importance for LC-MS. 

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