Matrix-Free Approaches

200 fmol small molecule mix on perfluorophenyl derivatized pSi surface 

This property was exploited in the chromatographic separation and subsequent M S analysis of endogenous analytes from human semm and small drug molecules in biofluids (see Section 9.6). 

The applicability of ordered nanocavity arrays as SALDI substrates in the analysis of peptides and plant metabolites has been reported [50]. These Si nanocavities are about 200 nm in diameter and 200 nm deep, and are fabricated by nanosphere lithography coupled with reactive ion etching. The desorption/ionization characteristics of the arrays were found to depend heavily on the surface morphology. 

An analysis of small peptides (m/z 500) from this arrayed surface shows analyte peaks with SNRs comparable to those for pSi surfaces. However, for large peptides more background ions were observed due to the higher laser fluence needed for desorption. Plant metabolites from Arahidopsis ihaliana root extract were successfully detected from this arrayed surface with minimal background ions. 

The evaporation of the solvent allows for cocrystallization of the analyte and matrix this crystallization process usually results in the heterogeneous distribution of analyte within the sample spot, which in turn leads to poor shot-to-shot and sample-to-sample reproducibility. Several sample preparation methods are discussed in the following section to address this issue. 

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Surface-assisted laser desorption/ionization (SALDI). SALDI is a matrix-free approach for the analysis of low-mass molecules. In this innovative approach, the sample solution is placed directly onto a solid surface prepared by depositing an active material, such as powdered graphite, active carbon, carbon nanotubes, or silica sol-gel, onto a suitable substrate (e.g., A1 foil or Cu tape) and bombarded with a laser beam [47,48,60]. 

Desorption/ionization on silicon. Another matrix-free approach for the analysis of small molecules is to use modified porous silicon surfaces [61]. A separate name, desorption/ionization on silicon (DIOS), has been coined for this technique [62]. In this approach, the sample is dispensed directly onto a modified porous silicon surface (e.g., silylated porous silicon) and laser-desorbed from the surface. The silicon surface is prepared from flat crystalline silicon by galvanostatic etching. 

DESORPTION/IONIZATION ON SILICON (DIOS) AND OTHER MATRIX-FREE APPROACHES... 

A matrix-free approach through desorption-ionization on silicon was used for IMS of lipids [67, 68]. In this method, the physical properties of the silicon material (high area surface, UV absorption) are cmcial for the desorp-tion/ionization process. The method requires the transfer of analytes to the silicon surface by direct contact with the tissue samples. IMS analysis can be performed with the silicon surface after removal of the tissue. 

It is evident that the matrix elements of the Hamiltonian and overlap are independent of the index r of BT in Eq. (13) and only the first diagonal element of the irreducible representation matrix, D P), is required, which has been well discussed [31,33,42,43], and is easily determined. It is worthwhile to emphasize that Eqs. (21) and (22) are the unique formulas of the matrix elements in the spin-free approach, even though one can take some other forms of VB functions. For example, it is possible to construct VB functions by Young operator [2], but the forms of the matrix elements are identical to Eqs. (21) and (22) [44],... 

This mechanistic proposal in turn prompts the suggestion that the function of the superoxide dismutase proteins is to prevent free HOO- from coming into contact with allylic C-H bonds in the biological matrix. One approach is to minimize the lifetime of O2 -/HOO-, which is in addition to the radical-radical coupling proposition to deactivate HOO-. For a steady-state flux of 30 X 10 M O2 -/HOO- at pH 5 (1) without superoxide dismutase (SOD) the approximate half-life of O2 -/HOO- is about 30 ms... 

The use of DOE in the pharmaceutical industry began early. For example, S. M. Free, while head of statistics at Smith Kline French in 1957, wrote an internal statistics booklet with F. A. Oyer (14). It showed how DOE could be used in formulation development and stability studies. Now called matrixing, this approach is now referenced in International Conference Harmonisation (ICH) Guideline Q1 (15). Many articles on DOE have appeared in the pharmaceutical literature during the past 25 years (16-31). Currently, the routine use of DOE is sporadic. The key to its success is an... 

When discussing matrix-free LDI strategies, we should also mention one more related approach that resonated in the specialist literature over the past two decades. In desorption/ ionization on silicon (DIOS), a nanostructured silicon chip is utilized as the SALDI-assisting material [81]. After depositing sample solution on such a chip, the sample is ready for LDI-MS analysis. Furthermore, DIOS chips are compatible with microfluidic and microreactor systems [92, 93]. Thus, DIOS-MS has occasionally been implemented in TRMS-related measurements (see also Chapters 7 and 13). 

If it is not possible to obtain a matrix free from chlorinated compounds, the standard addition approach should be used (see Section 29.3.1). 

Recently it was shown that matrix-free material enhanced laser desorption/ion-ization mass spectrometry (mf-MELDI-MS) is a powerful method to study the carbohydrate compositions of medicinal plants [23]. Using this approach, common silica gel is converted into 4,4 -azodianiline modified silica, which helps to generate a significant quantity of carbohydrate ions. 

A universal way of tackling the problem of molecular flexibility was suggested in paper [65] for kernel-based methods. It consists in averaging kernels over all conformations for each molecule. This approach is however computationally feasible only for veiy small number of conformations per molecule. Indeed, consideration of only 20 conformations for each molecule results in the necessity to consider 400 pairs of conformations in order to fill each cell in kernel matrix. In addition to huge computational burden, this approach does not solve the problem of ahgnment and therefore applicable only for alignment-free approaches. 

Standards are prepared by adding known weights of materials to a volumetric flask and then diluting to volume with the solvent or matrix. The approach is best illustrated with an example of the analysis of benzene in toluene at the 0.01% (weight) level. A standard is prepared by weighing 100 mg of benzene into a 10-mL volumetric flask. This is diluted to the mark with benzene-free toluene. This can be used as a master standard. Each milliliter of solution contains 10 mg of benzene. The master standard is then used to prepare several additional standards as follows ... 

C. Alternative matrix application Solvent-free and matrix-free methods. Sublimation of matrix was successfully developed for matrix application [23, 61, 62], This approach provided a homogeneous coating of matrix for high-resolution IMS of GPL species from tissue sections. The apparatus used for the method is relatively simple and commercially available. The advantages of sublimation include the elimination of diffusion of the lipid molecules because no solvent is used during the matrix application, the increased purity of the matrix, and the reduction of the crystal size. 

A laser-induced acoustic desorption (LIAD) device combined with a chemical ionization source was employed for the analysis of crude oil distillates under atmospheric pressure. In general, LIAD, a matrix-free and laser-based approach, is usually performed under vacuum conditions. The desorption process in LIAD is induced by the action of a shockwave that is generated as a pulsed laser irradiated on the backside of a metal foil. As the energy is transferred from the metal foil to the sample, which is deposited on another side of the foil, it induces the desorption of analytes. By the interaction of the analyte with an ion cloud generated by a chemical ionization (Cl) process, analytes with a wide range of polarity are successfully ionized. Marshall et ah have combined an atmospheric pressure AP-LIAD/ Cl with a 9.4 T FT-ICR/MS to perform high resolution chemical analyses imder ambient conditions. It was demonstrated that not only polar but also non-polar compounds in the crude oil distillates could be successfully characterized by this AP-LIAD/Cl/FT-ICR/MS approach. 

When considering the construction of exactly symmetric schemes, we are obstructed by the requirement to find exactly symmetric approximations to exp(—ir/f/(2fi,)). But it is known [10], that the usual stepsize control mechanism destroys the reversibility of the discrete solution. Since we are applying this mechanism, we now may use approximations to exp —iTH/ 2h)) which are not precisely symmetric, i.e., we are free to take advantage of the superior efficiency of iterative methods for evaluating the matrix exponential. In the following, we will compare three different approaches. 

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