Ionization enhancement,

Liang, H.R. Foltz, R.L. Meng, M. Bennett, P. Ionization enhancement in atmospheric pressure chemical ionization and suppression in electrospray ionization between target drugs and stable-isotope-labeled internal standards in quantitative liquid chromatography/tandem mass spectrometry. Rapid Commun. Mass Spectrom. 2003, 17, 2815—2821. 

Work by Catanzaro et al. in 1968 (6) led to a new analytical procedure permitting the measurement of isotopic ratios to about 0.05% (95% L.E.) this resulted in the availability of three standard reference materials, so that results could be placed on an absolute basis. This procedure, still the most precise and accurate one available, requires about 1 mg of lead for an analysis. A second procedure (7) has been developed which utilizes silica gel as an ionization enhancer. This method permits the measurement of isotopic ratios to about 0.1% (95% L.E.), but it requires only 0.1 /xg of lead per analysis. In addition, the instrumentation and data handling have been vastly improved so that many samples can be studied quickly and conveniently. 

Acceptance. The RSD of the matrix factor for all lots must not deviate <15.0 %. If MF= 1, it indicates that there is no matrix effect. If MF >1, it indicates that there is ionization enhancement. If MF <1, it indicates that there is ionization suppression. 

A fundamental requirement for LC-MS/MS calibration materials is that matrix effects exerted by these materials are most similar to the matrix effects exerted by actual patients sample materials. Lyophilisation, virus inactivation and other procedures applied during the industrial production of calibration and control materials, may notably impact the ionization behaviour of extracts from such samples and can result in differential matrix effects in calibrators and actual patients samples. If the internal standard peak areas found for calibration samples systematically differ from those found in patients samples, inappropriateness of the calibration materials should be suspected. However, we have previously observed that calibration materials from different commercial sources lead to inaccurate tacrolimus results in an instrument specific manner, without showing deviations in the internal standard peak area. This effect was most likely related to ionization enhancement affecting the target analyte but not the homologue internal standard (ascomycin) ionization and being restricted to calibrator samples. This resulted in systematically low tacrolimus results of clinical samples in one instrument for one specific calibrator lot [52],... 

Strontium. Strontium like Rb and Pd was included in anticipation of a standard. Strontium may be subject to more ionization and chemical interferences than other alkaline earths. Thorough studies of interfernces from mineral acids, HC1, HNO, and l SO are reported (3) as well as from Al, Si, and other ionization enhancement elements. Sr is strongly ionized, 84%, in nitrous oxide/acetylene the addition of 1000 ppm Cs is very important in suppressing Al, Si, and other interferences that cause enhancement. The 460.7 nm line is significant for AAS analysis of Sr. 

Peak identification and interpretation of mass spectra Mobile phases for proanthocyanidins contain 2% (v/v) of acetic acid, and it suppresses electrospray ionization at negative mode. This can be overcome by adding ammonia acetate as an ionization enhancer. The ammonia acetate (lOmM in methanol, 0.1 mL/min flow rate) can be added into the flow via a three-way micro-splitter ( P-445, Upchurch Scientific, WA) just before the mass spectrometry. It can be delivered by a separate HPLC pump or by a syringe pump. 

H. R. Liang, R. L. Foltz, M. Meng, P. Bennett, Ionization enhancement in APCI and suppression in ESI between target drugs and ILIS in quantitative LC-MS-MS, Rapid Commun. Mass Spectrom., 17 (2003) 2815. 

The first substantial 70% ionization suppression and population trapping in relation to LlCS has been observed in atomic helium [69, 101]. Earlier experiments on LlCS have observed dominantly ionization enhancement, with only a few percent recognizable ionization suppression [50-54,56,57,62,65]. The LlCS in the flat photo-ionization continuum of helium is a strong and spectrally sharp resonance showing both enhancement and diminished ionization. 

The mechanism of suppression/enhancement of ionization in APCI has received much less attention than that of the much more serious effect in electrospray ionization, and is currently not well understood. It is likely that a major source of these effects is the complex ion-molecule chemistry established in the APCI plasma (Figure 5.16). The example shown in Figure 5.19 is consistent with this idea, particularly the ionization enhancement observed on HPLC injection of a pure water sample into the mobile phase that was 80 % acetonitrile. 

However, methods have been developed to test for the presence of ionization suppression or enhancement of an analyte extracted from a complex matrix, to take its effects into account and thus hopefully alleviate the problem or even eliminate it altogether. An early systematic approach (Buhrman 1996), directed at detection and quantification of ionization suppression and other parameters, was later modified (Matuszewski 2003) to take account of the possibihty of ionization enhancement, and this modification is adopted here. The approach compares the results obtained for the mass spectrometric responses (peak areas) in three different LC/MS experiments for a target analyte, using the same known amount of analytical standard at the same final concentration in each case ... 

Photoelectron spectroscopy is a powerful technique to study ionic and electronically excited levels of atoms and molecules. In the case of single photon excitation of cold molecules the photoelectron spectrum reflects the internal energy levels of the ionic system. Many experiments are performed via two photon ionization enhanced by a one-photon resonance (R2PE spectroscopy) in which transitions to intermediate electronic levels are accessed which strongly enhance the ion yield. Photoelectron spectroscopy of molecules inside superfluid helium droplets is of particular interest since the interaction of free electrons with liquid helium is known to be highly repulsive, so much so that the electrons form bubbles of about 34 A diameter. In this section, three recent photoelectron spectra will be discussed those of bare helium droplets, of Ags clusters and of single aniline molecules in helium droplets. 

Han, X., Yang, K. and Gross, R.W. (2008) Microfluidics-based electrospray ionization enhances intrasource separation of lipid classes and extends identification of individual molecular species through multi-dimensional mass spectrometry Development of an automated high throughput platform for shotgun lipidomics. Rapid Commun. Mass Spectrom. 22, 2115-2124. 

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