Ionization parameter

MA Moseley, JW Jorgenson, J Shabanowitz, DF Hunt, KB Tomer. Optimization of capillary zone electrophoresis/electrospray ionization parameters for the mass ppectrometry and tandem mass spectrometry analysis of peptides. J Am Soc Mass Spectrom 3 289-300, 1992. 

There is an ongoing controversy about whether there is any stabilization of the transition state for nucleophilic substitution at tertiary aliphatic carbon from interaction with nucleophilic solvent." ° This controversy has developed with the increasing sophistication of experiments to characterize solvent effects on the rate constants for solvolysis reactions. Grunwald and Winstein determined rate constants for solvolysis of tert-butyl chloride in a wide variety of solvents and used these data to define the solvent ionizing parameter T (Eq. 3). They next found that rate constants for solvolysis of primary and secondary aliphatic carbon show a smaller sensitivity (m) to changes in Y than those for the parent solvolysis reaction of tert-butyl chloride (for which m = 1 by definition). A second term was added ( N) to account for the effect of changes in solvent nucleophilicity on obsd that result from transition state stabilization by a nucleophilic interaction between solvent and substrate. It was first assumed that there is no significant stabilization of the transition state for solvolysis of tert-butyl chloride from such a nucleophilic interaction. However, a close examination of extensive rate data revealed, in some cases, a correlation between rate constants for solvolysis of fert-butyl derivatives and solvent nucleophicity. " ... 

We fitted the combined data with model spectra based on the atomic data compiled by Raymond and Smith (1977). The model spectra employed here are both for collisional ionization equilibrium (CIE) and non-equilibrium ionization (NEI) models with cosmic abundances (Allen, 1977). Single Te spectrum for both models can not fit the data. Two components of different Te models can reproduce the data well for both models. The physical parameters obtained with CIE models are self inconsistent because the ionization parameter r ( the electron density n X the elapsed time t the after shock heating ) is about 1011cm 3sec which is too short by an order of magnitude for the CIE condition to be reached. 

This function increases monotonically with network density < ) and has a large Maxwell s loop for the large ionization parameter f. The swelling curves of N-isopropylacrylamide gels at low ionization were essentially well described by this equation [10]. 

Carballeira, J. L., Antelo, J. M., Rey, F., and Arce, F. (1999). Modeling the effects of ionic strength on ionization parameters for a soil fulvic acid at low concentrations. Anal. Chim. Acta 401, 243-249. 

Figure 3.57. The ion survival probability as a function of time at To = 0.5 ns with a great excess of acceptors. In line with UT and IET (above) and Markovian theory (below) (dashed curve), the contact approximation (dashed-dotted line in the middle) and exponential model with fcjep = A et = 1.0 ns-1 (dotted line) are also shown. The horizontal thick lines indicate the free-ion quantum yield ((). The concentrations and ionization parameters are the same as in Figure 3.56, while wy = 3.4ns-1, D = D = 1.2 X 10-6 cm2/s, k1 — 7S4 A3/ns, and kr — 4S6 A3/ns. (From Ref. 195.)...

However, photoionization models show that such simple relations do not necessarily hold. For example, the charge transfer reaction 0++ + H° —s- 0+ + H+ being much more efficient than the Ne++ + H° —> Ne+ + H+ one, Ne++ is more recombined than 0++ in the outer parts of nebulae and in zones of low ionization parameter. 

Moseley, M.A. Jorgenson, J.W. Shabanowitz, J. Hunt, D.F. Tomer, K.B. Optimization of Capillary Zone Electrophoresis/Electrospray Ionization Parameters for the Mass Spectrometry and Tandem Mass Spectrometry Analysis of Peptides, J. Am. Soc. Mass Spectrom. 3,289-300 (1992). 

Huorescence Excitation 250 nm, emission 395 nm Positive ion mode, mass range m/z 100-2000. Ionization parameters capillary temperature 300 1°C, spray voltage... 

Note that the ionization occurs analogously to HPLC-MS and the amounts detected on the plate are comparable to HPLC-MS when using the same MS system. One can start with applications in the range of 1-100 ng/ band. If the standard solution is not available for direct flow injection analysis (FIA) and if information about the capability of ionization and its ionization parameters are not available, higher amounts (500 ng/band) and more tracks are also recommended for application. Typical application volumes were between 2 and 10 pL/ band. Drying of the start zones followed, for example, on a TLC plate heater set at 60°C for 1 min or in a homogeneous stream of warm air. 

For MS recording, a run was started in the MS software. Ionization parameters were the same as for HPLC-MS recording at the given flow rate and eluent (same principle). The MS parameters (negative/positive mode, mass-to-charge ratio [m/z range, etc.) depended on the compounds of interest. 

If information about the capability of ionization, its ionization parameters, and standard solutions for testing are not available, higher amounts (1 pg/band) and more tracks are reconunended for application. Typical appli-... 

Here the terms (AF ) , (A(S i)°, and (A//J)° are the standard free energy, entropy, and enthalpy changes for ionization of a nm-hydrogen xmded donor group on the protein at a pH where the protein has zero net chai, while AF , AS , and AHi are the ionization parameters at any other pH under consideration. From Eq. (11-13) we obtain for the hydrogen-bonded donor group... 

Amphetamines have also been extensively analyzed by GC-MS [2,32-36]. Recent work has focused on improvements in derivatization procedures [13], extraction techniques [1,33,35,36], and optimizing ionization parameters [14]. Solid-phase microextraction was utilized for headspace analysis of urine to determine both amphetamine and methamphetamine [32], In conjunction with the use of pentadeuterated methamphetamine as an internal standard, LODs were obtained as low as 0.1 llg/ml for both compounds monitored in the SIM mode and using isobutane CL This method was found to be 20 times more sensitive than traditional headspace analysis of urine without SPME extraction [32]. Even lower limits of detection were obtained utilizing derivatization of amphetamine and methamphetamine with propylchloroformate and LLE of the propylcarbamate derivatives from urine [13]. When deuterated amphetamine and methamphetamine were used as internal standards, the LODs were 25 ng/ml for both drugs the LOD improved to 5 ng/ml when Wpropylamphetamine was used as the internal standard. In this case, ions were formed via El and SIM was used (Fig. 4) [13]. This method was shown to be rugged and relatively free from potential interferences from other related drugs. 

The SMP system was chosen as it is a real food ingredient that is involved in the Maillard reaction but is relatively simple (compared to that of coffee, for instance). Thus it represents the lower end of the scale of complexity of food Maillard reactions. The headspace from the heated (and cooled) SMP sample was analyzed by direct API monitoring (Fig. 1). Visual inspection of Fig. 1 reveals about 20 compounds, but the only information on their identity is provided by the ion mass. Since this trace was obtained in scan mode using a general set of ionization parameters, there may also be some fragment ions present. For known compounds, molecular ion formation is favored by using specific conditions for each compound [2] when the APIMS is operated in selected ion mode. 

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