Collision-Gas Pressure

Similar to the collision energy, variation of collision-gas pressure has a large impact on ion fragmentation. Collision-gas pressure mainly affects the collision pathway 

Similar to the variation of collision energy, the 2D mass spectrometric analysis by varying collision-gas pressure can be explored to identify the location of the double bonds of the unsaturated fatty acids based on the intensity changes of the fragment ion 

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Fig. 2.27. Total collision probability and fractions of single, double, triple and quadruple collisions versus collision gas pressure. The transmission of the main beam /(is given on the right ordinate. Values are for an ion of collision cross section 5 x 10 cm and 1 cm collision path 10 Torr= 1.33 Pa. Reproduced from Ref. [9] with permission. John Wiley Sons, 1985.

Bradley, C.D. Derrick, P.J. CID of Peptides. An Investigation into the Effect of Collision Gas Pressure on Translational Energy Losses. Org. Mass Spectrom. 1991,26,395-401. 

Protonated isomeric molecules, such as testosterone and dehydroepiandrosterone, are characterized in the MIKE/CAD spectra by peak widths, but also by intensities, which vary as a function of collision gas pressure. 

The El source precedes the FD source and is used as a collision cell. The gas has a major role in collisions collision gas pressure required to attenuate the main beam by 2/3 varies as a function of the products, their molecular weights and the type of gas chosen. 

The effect of collision gas pressure on the sensitivity of the peaks obtained was demonstrated. The relative abundance of these ions can indeed vary by a factor of two, and the pressure chosen was 2.10 Torr. In the most recent version of their instrument, the authors applied a voltage on the order of 20 kV, thus leading to a much better resolution of the energy spectrum (spherical electric field). 

The superposition of several pure MIKE/CAD spectra on the same spectrum may indeed cloud the issue . Nevertheless, we believe that by changing either collision gas pressure or type, it is possible to obtain sufficient information to resolve the ambiguities. Those which remain are due (relatively rarely) to the partial isomerization of molecular ions and fragment ions which are most often encountered. In the latter case, the examination of the MIKE/CAD spectra of other fragment ions belonging to the conventional mass spectrum may be useful for determining these structures. 

The collision gas pressure can influence the observed cross sections because an ion that is not sufficiently energized by one collision with the target gas may gain enough energy in a second collision to be above the dissociation threshold. Such collisions can lead to a measured threshold that is too low. This is accounted for by linearly extrapolating data taken at several pressures to a zero pressure cross section, which is then fit with the method described above. 

Quadrupole mass analysers provide good reproducibility and represent a relatively small and low-cost system. Low-energy collision-induced dissociation (CID) MS/MS experiments are enabled in triple quadrupole and hybrid mass spectrometers and have efficient conversion of precursor to product. These spectra depend strongly on energy, collision gas, pressure, and other factors. Quadmpole mass... 

TOF/TOF is the most restrictive of the MS/MS formats because neither analyzer can be scanned. However, these instruments combine the high sensitivity associated with MALDl, with CID conducted at keV energies. In TOF/TOF instruments the residence time of the ions in the CID cell is shorter and lower collision gas pressures are used than in other MS/MS systems. CID takes place at 1-2 keV to provide sufficient fragmentation, and this has the added benefit of providing fragmentation that is less structure dependent and more comprehensive than that obtained with the 10-100 eV CID used in most MS/MS systems. 

Ins trumcnt Hlectron Energy Emisson Current Collision Gas Collision Gas Pressure Collision Energy ... 

ESI source negative ionization mode source voltage 3 kV needle temperature 350 °C collision gas argon collision gas pressure 1.5 mTorr operating mode SRM (185 -> 41) and (185 123) for endothaii and SRM (137 74) for glutaric acid-d samples 5 pg/L of endothaii spiked into Lake Tahoe water (a) and 50 pg/L endothaii spiked into salty lake water with 10-fold dilution (b). 

In theory, to fully investigate the effects of ionization conditions on ionization efficiency and/or the effects of collision conditions on fragmentation processes or other effects, a variety of ionization voltages, ionization temperatures, collision energies, collision gas pressures, etc., should be employed in an experiment (see Chapter 4). These variables can all be logically varied unit by unit within a certain range. Therefore, from a new set of spectra when an individual variable of MS is ramped, a new dimension is added to the basic 2D MS. All of these dimensions form the family of MDMS [11]. Specifically, MDMS is defined as the comprehensive MS analyses conducted under a variety of instrumental variables that collectively comprise an MDMS spectrum. 

Figure 4.7 Two-dimensional mass spectrometric analysis of polyunsaturated fatty acid fragmentation pattern with variation of collision-gas pressure. MS analysis was performed with a TSQ Quantum Ultra Plus triple-quadrupole mass spectrometer (Thermo Fisher Scientific, San Jose, CA) equipped with an automated nanospray apparatus (i.e.. Nanomate HD, Advion Bioscience Ltd., Ithaca, NY) and Xcalibur system software. Product-ion scan of 8,11,14-eicosatrienoic acid (20 3 FA) (5 pmol/pL) was performed after direct infusion in the negative-ion mode at the fixed collision energy of 16 eV and varied collision-gas pressures ranging from 0 to 3 mTorr as indicated. A 2-min period of signal averaging in the profile mode was employed for each scan. All the scans were automatically acquired with a customized sequence subroutine operated under Xcalibur software. All the scans are displayed after being amplified to the 5% of the base peak in each individual scan.

Figure 6.3 Representative two-dimensional mass spectrometric analyses of lysoPC molecular species in Upid extracts of mouse plasma in the positive-ion mode. A full MS scan in the mass range from m/z 460 to 610 was acquired first from a diluted hpid extract of mouse plasma in the positive-ion mode, which displayed the abundant sodium adducts of lysoPC species. Neutral loss scan (NLS) of 59.0 amu (i.e., trimethylamine) with a collision energy (CE) of 22 eV or 205 amu (i.e., sodium chohnephosphate, CE 34 eV), precursor-ion scan (PIS) of m/z 104.1 (i.e., choline, CE 34 eV), and PIS of m/z 147.0 (i.e., sodiated five-membered cyclophosphane, CE 34 eV) with collision-gas pressure at 1 mTorr were also acquired from the diluted hpid extract of mouse plasma in the positive-ion mode. All scans were displayed after normalization to the base peak in individual scan.

Figure 6.6 Product-ion ESI-MS mass spectra of sodiated triacylglycerol species after CID. Product-ion mass spectra of lithiated 16 0-18 1-20 4 (a) and 18 1-18 1-20 4 TAG (b) were acquired with collision energy at 32 eV and collision gas pressure at 1 mTorr. The tandem MS mass spectra displayed abundant fragment ions corresponding to the neutral losses of either fatty acids or fatty acyl lithium salts from TAG species.

Figure 6.7 Representative two-dimensional mass spectrometric analyses of triacylglycerol species in hepatic lipid extracts of mice. Neutral loss scans (NLS) of all naturally occurring fatty acyl chains (i.e., the building blocks of TAGs) of mouse liver lipid extract were acquired to determine the identities of individual lithiated TAG molecular ion, deconvolute isomeric species, and quantify individual TAG species by comparisons with a selected internal standard (i.e., T17 l TAG atm/z 849.6 as lithium adduct, shown in NLS268). Collision activation was performed with collision energy at 32 eV and collision gas pressure at 1 mTorr on a QqQ-type mass spectrometer (TSQ Vantage, Thermo Fisher Scientific, San Jose, CA, USA). All displayed mass spectral traces are normalized to the base peak in each trace.

Figure 10.1 Representative intensity distribntions of the fragment ions from different paired FA isomers as varied with collision energy. Prodnct-ion ESI analysis of individual FA isomer in the negative-ion mode was performed by varying collision energy at fixed collision gas pressure of 1 mTorr. The intensity distributions of the fragment ions resulting from the loss of COj from 22 5 (a), 20 4 (b), or 20 3 (c) FA isomers as varied with collision energy were displayed after normalization to the intensity of its corresponding molecular ion in the full mass spectrum (i.e., normalized absolute intensity).

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