Analytical mass scan

The DIT scan function differs substantially from that used in the quadrupole IT experiments. Time is reported on the abscissa, while, on the ordinate, contrary to what was discussed for QIT (for which the Vrf voltage is reported), for DIT the period (T) of the square wave is reported. The DIT scan function used to obtain a complete mass spectrum is based on four separate steps (1) a standby time at high T values, followed by (2) a stage devoted to ion introduction inside the trap, (3) a field adjusting phase, and (4) the analytical mass scan. 

An unknown analytical response in lettuce was detected61 and thought to contain P and S. The initial GC/MS Cl CH4 data were dominated by ions at m/z 121, 197 and 291 (equation 40). The GC/MS Cl NH3 data, however, indicated that perhaps the molecule had a higher molecular weight of 306 (MH+ equal to m/z 307, cf. equation 40). Comparison with the Cl CH4 and Cl NH3 spectra of demeton-S- (199) and demeton-O-sulfones showed, however, that the retention time and mass spectra of the former only matched the unknown (except the omission of the ion at m/z 307). On another stationary phase (OV-101) limited mass scanning of the ion at m/z 121 revealed a second unknown (200) eluting at 4.2 min (the major unknown, i.e. demeton-S-sulfone at 3.0 min). The molecular weight of 200 was believed to be 306. Because of the presence of ions at m/z 121,... 

The analytes are determined by acquiring a full mass scan and obtaining the extracted ion current profiles (EICP) for the primary mass-to-charge ratio and at least two secondary masses of each analyte. Ions recommended for this purpose are listed in the EMSL methods. 

Using SIFT-MS it is possible to record mass spectra in the full-scan mode (FS) of quadrupole MS or in multiple ion monitoring mode (MIM) when ions are passed thorough the quadrupole with a certain m/z ratio. FS mode allows the full mass spectrum of analytes in a defined range of m/z values to be registered within a specified time period. In the case of MIM, precursor ions and selected ions, formed during reaction of the reagent ion and the analyte, are scanned. 

In our methodology, the mass scans were, by necessity, limited to at most five ions in order to meet the milestone for sensitivity. This greatly reduces the informing power of the GC/MS portion of the analytical procedure however, this is more than compensated for by the sample preparation steps in the... 

Similar to a quadrupole, an ion trap is constructed. However, the ions are collected in the trap, and then, either a mass scan or single to multiple fragmentation of the target analyte can be performed. Modern ion-trap MS systems are characterized by a very good linearity and sensitivity and a fast data acquisition (e.g., 20 Hz) and thus can even be coupled with UHPLC. They are particularly suitable for structure determination of biomolecules (carbohydrates, peptides, etc.). 

A set of DC-RF voltage on the rods will steer the analyte ions of interest electrostatically through the center of the quadrupole mass filter until the exit and they will be converted to an electrical pulse by the detector while other ions of different mass-to-charge ratios will stop in the quadrupole. In a multielemental analysis, the mass scan process is repeated one after another for all analyte ions of different mass-to-charge ratio until all the analytes in a multielement analysis have been measured. Quadrupole scan rates are typically on the order of 2,500 atomic mass units (amu) per second and can cover the entire mass range of 0-300 amu in about 0.1 s. However, real-world analysis speeds are much slower than the above. 

Stored and counted by a multichannel analyzer. This multichannel data acquisition system typically has 20 channels per mass and as the electrical pulses are counted in each channel, a profile of the mass is built up over the 20 channels, corresponding to the spectral peak of Cu. In a multielement run, repeated scans are made over the entire suite of analyte masses, as opposed to just one mass represented in this example. The principles of multielement peak acquisition are shown in Figure 12.3. In this example, signal pulses for two masses are continually collected as the quadrupole is swept across the mass spectrum, shown by sweeps 1-3. After a fixed number of sweeps (determined by the user), the total number of signal pulses in each channel is obtained, resulting in the final spectral peak. ... 

However, some systems allow you to change resolution settings on the fly on individual masses during a multielanent analysis. Under normal analytical scenarios, this is rarely required, but at times it can be advantageous to improve the resolution for an analyte mass, particularly if it is close to a large interference and there is no other mass or isotope available for quantitation. This can be seen in Figure 21.6, which shows a spectral scan of 10 ppb Mn+, which is monoisotopic, and 100 ppm... 

The major benefit of this approach is that no scanning of the magnet is required. For this reason, it is ideal for any applications that benefit from a rapid simultaneous measurement of the analyte masses. Some of these application benefits include ... 

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