Mass analyzers data acquisition rate

One of the latest mass analyzer is the linear-trap quadrupole (LTQ) Orbitrap mass spectrometer. In this, the commercial LTQ is coupled with an ion trap, developed by Makarov [73, 74]. Due to the resolving power (between 70000 and 800000) and the high mass accuracy (2-5 ppm), Orbitrap mass analyzers, for example, cab be used for the identification of peptides in protein analysis or for metabolomic studies. In addition, the selectivity of MS/MS experiments can be greatly improved. However, the coupling is not useful with UHPLC for rapid chromatographic pre-separation, as the data acquisition rate is too low for a reproducible integration of the narrow signals produced with UHPLC. 

From the standpoint of resolution and reproducibility, instruments equipped with TOF mass analyzers are not as satisfactory as those with magnetic or quadru-pole analyzers. ICP-TOF mass spectrometers, for example, typically are an order of magnitude poorer in sensitivity and in detection limits than comparable quadrupole systems.Several advantages partially offset these limitations, however, including simplicity and ruggedness, ease of accessibility to the ion source, virtually unlimited mass range, and rapid data-acquisition rate. Several instrument manufacturers now offer TOF instruments, but they arc less widely used than are quadrupole mass spectrometers. 

In both electron post-ionization techniques mass analysis is performed by means of a quadrupole mass analyzer (Sect. 3.1.2.2), and pulse counting by means of a dynode multiplier. In contrast with a magnetic sector field, a quadrupole enables swift switching between mass settings, thus enabling continuous data acquisition for many elements even at high sputter rates within thin layers. 

Finally, one concept that must be included in assessing quantitation by HRMS is the effective scan rate of the system. Quadrupole and time of flight mass analyzer are capable of rapid scan rates for SRM-type quantitation, with individual dwell times (quad) or scans (TOF) at 10-50 milliseconds possible. This permits acquisition of numerous data points across a chromatographic peak, which is critical for accurate and precise quantitation. Mass resolution is unaffected by changes in dwell time/scan... 

Selective NO reduction with NH3 a mixture of NO (500 ppm), NH3 (550 iq>m), O2 (2 vol %) and He (balance) flowed through the sample (corre nding to 1.2 mg Mn in the catalyst bed) at 10 Pa with a flow rate of 50 cm min, a space velocity of 24 000 h" and in a temperature range 110 - 300°C. The product stream is analyzed on line by using a computer-monitored mass spectrometo allowing automated data acquisition and process control [6],... 

Nowadays, pulsed, frequency tripled Nd YAG UV lasers (355 nm) are usually employed for MALDI experiments with a repetition rate of 1000 Hz in commercial instruments for sufficient data acquisition. In MALDI-IMS, the resolving power for application strongly depends on the sample preparation step (e.g., matrix crystal size), stepper motor accuracy, and laser spot sizes. To achieve MALDI-IMS to a practical resolution, the laser spot size of 20 pm is usually used. Therefore, the time needed to obtain images from a sample depends on the number of analyzed spots, the repetition rate of the laser (Hz), and the data collecting and processing speed of computers. For example, imaging a whole-body mouse or rat section with current commercially available MALDI mass spectrometers equipped with lasers operating at 1 kHz would take 2-4 h. 

Either the information obtained during the data-dependent acquisition is sufficient or a fraction of interest can be re-analyzed by chip-based infusion at a flow rate ca. 200 nl min. Due to the miniaturization sample consumption is very low (typically 1-3 pi) and acquisition time is no longer critical. Therefore various MS experiments can be performed on various instruments, including MS and accurate mass measurements. An additional advantage is that the eluent can be removed and the infusion solvent can be optimized for positive or negative ion detection or for deuterium exchange measurements. 

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