Quadrupole choosing

Some Factors Important in Choosing between Quadrupole and Magnetic-Sector Mass Spectrometers... 

Remarkably, only one nuclear constant, Q, is needed in (4.17) to describe the quadrupole moment of the nucleus, whereas the full quadrupole tensor Q has five independent invariants. The simplification is possible because the nucleus has a definite angular momentum (7) which, in classical terms, imposes cylindrical symmetry of the charge distribution. Choosing x, = z as symmetry axis, the off-diagonal elements Qij are zero and the energy change caused by nuclear... 

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

Personnel Choosing an appropriate technique sometimes comes down to having the right person on your staff to perform the technique. If you, or no one in the lab, had had experience with the hydroxyl value titration, it might not have been an option in the Bulging Drum Problem. Certainly, if the operator of your triple quadrupole mass spectrometer is on vacation and there is no back-up operator, another method, or sending the samples to another lab, may be necessary. Should the method need to be transferred to another lab, does that lab have the requisite people for the job ... 

Criteria for choosing specific models. We have seen above that spectral profiles of binary complexes can be computed from a rigorous quantum formalism. These describe the measurements well. Long profiles, i.e., profiles with a peak-to-wing intensity of several orders of magnitude, can be readily obtained. As an illustration of how well the various available model profiles approximate the exact computations, we show results for one basic profile type characteristic of absorption by H2-H2 pairs [69], a pure quadrupole-induced profile which accounts for roughly 90% of the total intensity of the rototranslational H2-H2 collision-induced absorption spectra at 77 K. 

Normally full valence MCSCF calculations (choosing all valence orbitals as active) represent a balanced treatment of correlation. However this is not always the case, especially not in systems containing lone-pair electrons. For example, a full valence MCSCF calculation for the water molecule yields less accurate values for the bond distance, the bond angle, and the dipole and quadrupole moment than an SCF calculation. The reason is that there are only two orbitals available for correlating the eight valence electrons (the 4ax and the 2b2 orbitals). Thus correlation is only introduced into the lone-pair orbital... 

On his initiative, A. G. Doroshkevich and I. D. Novikov [56] constructed a global spectrum of the electromagnetic radiation in the Universe and showed that relic radiation in thermodynamic equilibrium can be found in the centimeter region. The discovery of relic radiation answered the question of what model to choose for the Universe. Ya.B. became an ardent proponent of the theory of a hot Universe (see the 1966 review [26 ]). He was one of the first in the world to understand what a powerful tool relic radiation represented for discovery of the Universe s past. His reviews of 1962-1966, which became the basis for excellent books written later with I. D. Novikov [57-59], contain practically all the ideas which have now become the methods for studying the large-scale structure of the Universe. These include the question of dipole and quadrupole anisotropy, and of angular fluctuations of relic radiation, the problem of nuclear synthesis reactions in the hot Universe, and the quark problem, first raised by Ya.B. together with L. B. Okun and S. B. Pikelner (1965) [11 ]. 

A 30-lb field-portable quadrupole ion trap TOF (QitTof) mass spectrometer with an atmospheric photoionization source was constructed [17]. The photoionization source has the ability to choose a narrow-band ionization energy that is sufficiently high to ionize and detect most compounds of interest but low enough to avoid ionization of most common air constituents, such as N2, 02, H20, C02, CO, Ar, etc. Fragmentation is minimal because ionization occurs just above the ionization thresholds, with very little excess energy. The ion trap stores ions from a continuous ionization source followed by pulsed extraction into the TOF mass analyzer. 

As in similar cases, where different nuclides of the same element are discussed, the chemical shifts of Li and Li (in ppm) are identical because primary isotope effects can safely be neglected, scalar spin-spin coupling constants are related by the factor y( Li)/y( Li) = 2.64, and relaxation mechanisms as well as NMR linewidths differ. Nuclear properties which are important for Li NMR experiments are collected in Table 2, where data for the widely used nuclei H and as well as N and P, which are of interest in the present context, are included for comparison. Both Li and Li possess a quadrupole moment, Q, but that of Li is the smallest one known for any nucleus. Li ium-6 NMR is, therefore, not dominated by the quadrupole moment and Li has been termed an honorary spin-1/2 nucleus [11]. Long relaxation times may sometimes cause difficulties for Li NMR measurements and this has to be taken into account by choosing a sufficiently long relaxation delay between individual... 

Connect to http //chemistry.brookscole.com/skoogfac/. From the Chapter Resources menu, choose Web Works and locate the Chapter 32 section. Find the link to LC-GC magazine. From the LC-GC home page, search for articles on LC/MS. Find an article, written in 2001, that compares mass analyzers for LC/MS applications. What are the most common ionization sources used for LC/MS Describe any differences in mass range and mass resolution between quadrupole, time-of-flight, and ion-trap (Fourier transform) mass analyzers. Do these three mass analyzers show any differences in qualitative and quantitative analysis ... 

The MS/MS measurements were based on collision-induced dissociation (CID) occurring in the collision cell (Q2) of the triple quadrupole at a collision energy of 49 eV. Argon was used as the target gas at a target thickness of 1.5 x 10 " molecules/cm. Each 100-200 pmol PA, PA2, PAS, and PB2 (5 jL in MeOH) was introduced into the mass spectrometer by flow injection with MeOH at a flow rate of 0.1 ml/min. The fragment ion spectra of the MNa" ions for standard PA, PA2, PAS, and PB2 were measured by choosing the MNa ions at m/z 847, 833, 861 and 821, respectively, as the... 

Now we are confronting a serious problem (which we always encounter in the multipole expansion) what is R1 We are forced to choose the two local coordinate systems in A and B. We arbitrarily decide here to locate these origins in the middle of each dipolar system, and therefore R = 10. It looks like a reasonable choice, and as will be shown later on, it really is. We are aU set to compute the dipole-dipole interaction —2 10 (—1)(—1) = —0.0020000. Close The computed exact interaction energy is —0.0020202. Where is the rest Is there any error in our dipole-dipole interaction formula We simply foigot that our dipolar systems represent not only the dipole moments, but also have nonzero octupole moments (the quadrupoles equal zero) and nonzero higher odd-order multipoles, and we did not take them into account. If somebody computed all the interactions of such multipoles, then we would recover the correct interaction energy with any desired accuracy. But why does such a simple dipolar system also have a nonzero octupole moment The answer is simple it is because the dipole is not pointlike." The conclusion from this story is that the reader has to pay attention whether we are dealing with pointlike or non-pointlike multipole moments. 

Tbe reader new to mass spectrometry is advised to consult an appropriate introductory text [2-9]. A few mass spectrometric terms will be explained here by way of background and to outline the principles of choosing a flame ionization detector, ions are produced in the mass spectrometric detector, but the mass spectrometer is able to analyze these ions further according to their molecular weights or rather, mass-to-charge ratios (m/z, see below) to provide a mass spectrum. Different principles are employed to achieve this in a variety of types of mass spectrometer. The instruments most commonly used in GC—MS are known as magnetic sector, quadrupole and ion trap mass spectrometers. Their differences are not further described here. Bench-top systems are of the quadrupole or ion trap type. 

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