Spectrometer selection

Hewlett-Packard 5973 mass spectrometer Selected ion monitoring (SIM)... 

Atoms with odd numbers of protons and/or neutrons are nmr-active. The inactive atoms are "C (6p, bn), 0(8/J, 8rt), and S(16/ , Ibn). To detect the nmr activity of atoms other than H requires alteration of the nmr spectrometer. The ordinary spectrometer selects the range of radiowave frequency that excites only H. 

The SRM acquisition mode allows one to obtain a sensitivity and selectivity gain with respect to SIM. The detection of selected reactions, based on the decomposition reactions of ions that are characteristic of the compounds to be analysed, requires the use of a tandem mass spectrometric instrument. In order to carry out this type of analysis, the spectrometer is set so as to let through only the ions produced by a decomposition reaction in the chosen reaction region for example, the first spectrometer selects the precursor ion with an mp+/z ratio that is characteristic of the compound to be detected, while the second spectrometer selects the fragment ion with an ntf+/z ratio resulting from the characteristic decomposition reaction of the compound to be analysed, mp 1 —> ni(+ + mn, that occurs between the two analysers. 

Mass spectra of short-lived, unstable episelenides were obtained by taking advantage of a mass spectrometric technique developed for the time-resolved detection of transient intermediates in flash-photolyzed systems <66JA4277>. Detection does not depend on the electronic absorption characteristics of the transient, and, in combination with kinetic absorption spectroscopy, the technique assumes great flexibility. The apparatus consists, essentially, of a photolysis cell attached to a small leak into the ion source of an Atlas CH4 mass spectrometer. Selected mass peaks can be studied with a response time of a few milliseconds, and thereafter at times limited by bleeding of the photolyzed mixture into the ion chamber. Typical photolytic flash energies were 480 calories, passed into a reaction volume of 5 ml. 

In some portable spectrometers selective filtration entirely replaces pulse-height analysis as a means of wavelength separation, and only a simple counting system is needed. 

The more universal and less sensitive flame-ionization detector (EID) is used much less often than the electron capture detector (ECD), which has exceptional sensitivity to multiply chlorinated compounds. The mass spectrometer-selected-ion-monitoring (MS-SIM) or ion-trap mass spectrometer (ITMS)... 

The resolution afforded with CE is a good match for the duty cycle times of present day tandemMS instruments, particularly in the area of proteomics. In the past, the speed at which mass spectrometers selected, isolated, fragmented, and detected peptide ions did not warrant extremely high resolution... 

Tandem MS is used to provide more information than can be afforded by a single mass spectrometer and is widely used for screening complex matrices such as blood and urine. Analysis is achieved, in effect, by performing two stages of SIM. The first mass spectrometer is set to transmit the precursor ion of interest into a region where fragmentation occurs. One of the product ions is monitored by a second mass spectrometer. Selection of an appropriate internal... 

Because it uses two consecutive mass spectrometers, selected reaction monitoring is also called tandem mass spectrometry or mass spectrometry-mass spectrometry, or just MS-MS. 

Transfer 1 ml of the cell suspension to a cuvette and place this in the spectrometer, select the appropriate wavelength(s) and then zero the absorbance/fluorescence reading. 

Rg. 5-33 Experimental apparatus with mass spectrometer (selective detector) for gas chromatography with volatile modifier [122]. 

A powerful tool to enhance the analytical potential of combined chromatography-MS especially in LC-MS, is tandem MS (MS/MS). MS/MS was first introduced as a way to induce fragmentation of ions generated in the ion source, e.g. by soft ionization methods, and after their mass selection. In such an experiment, the first mass spectrometer selects a particular precursor ion, which is (collisionally) dissociated, and the product ions are analysed with the second mass spectrometer. This is the product-ion scan mode which is primarily used to elucidate the structures of ions. 

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