Liquid matrices

Liquid chromatography is a separation method that is often applied to nonvolatile, thermally labile materials such as peptides, and, if their mass spectra are required after the separation step, then a mild method of ionization is needed. Since FAB/LSIMS is mild and works with a liquid matrix, it is not surprising that attempts were made to utilize this ionization source as both an inlet... 

Where the target is a liquid (matrix), the more descriptive term LSIMS should be used, as noted above. 

The two examples of sample preparation for the analysis of trace material in liquid matrixes are typical of those met in the analytical laboratory. They are dealt with in two quite different ways one uses the now well established cartridge extraction technique which is the most common the other uses a unique type of stationary phase which separates simultaneously on two different principles. Firstly, due to its design it can exclude large molecules from the interacting surface secondly, small molecules that can penetrate to the retentive surface can be separated by dispersive interactions. The two examples given will be the determination of trimethoprim in blood serum and the determination of herbicides in pond water. 

A major difference between MALDI and FAB is that a solid rather than a liquid matrix is used and a mixture of the analyte and matrix is dried on the laser target. For this reason, the effective combination of HPLC with MALDI is not as readily achieved although, since MALDI is largely free of the suppression effects experienced with FAB, it is able to provide useful analytical data directly from mixtures. 

FAB-MS is a renewed old technique, first described in 1966 as molecular beam for solid analysis (MBSA) [73], and later (1981) further extended with a liquid matrix [72,74], The FAB experiment is closely related to... 

Sample preparation for FAB is primarily dissolving the analyte in a liquid matrix. The matrix plays... 

As evident from Scheme 7.13, most modern ionisation techniques have been used for TLC-MS, and no single ionisation method is used exclusively with TLC-MS. Various ionisation methods may be applied that avoid the need to evaporate the sample into an El or Cl source these are based in particular on sputtering (FAB, SIMS) or laser desorption. Several sputtering methods of ionisation do not require the use of a liquid matrix, e.g. TLC-SIMS [797], Recent developments include the use of matrix-assisted laser desorption ionisation (MALDI) and surface-assisted laser desorption ionisation (SALDI). It is obvious that TLC-MS is complemented with TLC-MS11 [800] and TLC-HRMS techniques. Table 7.82 lists the general characteristics of TLC-MS. 

Direct analysis (no analyte removal from plate, no liquid matrix)... 

Chan, T.-W. D. Thomas, I. Colburn, A. W. Derrick, P. J. Initial velocities of positive and negative protein molecule-ions produced in matrix-assisted ultraviolet laser desorption using a liquid matrix. Chem. Phys. Lett. 1994,222,579-585. 

The addition of a liquid matrix, usually glycerol, improves the reproducibility of the mass spectra because inhomogeneities caused by analyte and matrix co-crystallisation are avoided. In some cases, it also enhances signal intensity and resolution. However, analytes can also be desorbed from dry surfaces [7 9]. 

The molar transfer rate coefficient kG (gas side) or kL (liquid side) (m s-1) can be defined as the ratio between the intrinsic molecular diffusivity of the solute gas A in the gas or liquid matrix and the diffusion lengths dG or dL (Eqs. (2) and (3)). The diffusion lengths depend on the reactor flow and mixing properties. 

C. Dass. The Role of a Liquid Matrix in Controlling FAB-Induced Fragmentation. J. Mass Spectrom., 31(1996) 77-82. 

Liquid-liquid extraction is a form of solvent extraction in which the solvents produce two immiscible liquid phases. The separation of analytes from the liquid matrix occurs when the analyte partitions from the matrix-liquid phase to the other. The partition of analytes between the two phases is based on their solubilities when equilibrium is reached. Usually, one of the phases is aqueous and the other is an immiscible organic solvent. Large, bulky hydrophobic molecules like to partition into an organic solvent, while polar and/or ionic compounds prefer the aqueous phase. 

What is a major advantage of using fast atoms rather than ions as bombarding particles in desorption ionization using a liquid matrix (i.e., FAB) (eliminates charging, which can lead to signal instability). 

Magnetic (iron oxide core) microspheres have been imprinted with S-pro-pranolol. The magnetism allowed the facile separation of the imprinted beads from the liquid matrix. The particles exhibited the expected affinity towards the template molecule. This technique was also proposed as a putative tool for cell sorting [81]. 

MALDI is a method that allows for vaporization and ionization of non-volatile biological molecules from a solid-state or liquid phase directly into the gas phase. The presence of a solid or liquid matrix (around 1 part in 100-50,000), which strongly absorbs the laser radiation, spares the smdied species from degradation, by protecting them from the harsh effects of the laser. It results in the detection of intact molecules... 

The analyte, either solid or admixed to some liquid matrix, is introduced into the FAB ion source by means of a probe bearing a sample holder or FAB target. The... 

The precursor model of FAB applies well to ionic analytes and samples that are easily converted to ionic species within the liquid matrix, e.g., by protonation or deprotonation or due to cationization. Those preformed ions would simply have to be desorbed into the gas phase (Fig. 9.6). The promoting effect of decreasing pH (added acid) on [M+H] ion yield of porphyrins and other analytes supports the precursor ion model. [55,56] The relative intensities of [Mh-H] ions in FAB spectra of aliphatic amine mixtures also do not depend on the partial pressure of the amines in the gas phase, but are sensitive on the acidity of the matrix. [57] Furthermore, incomplete desolvation of preformed ions nicely explains the observation of matrix (Ma) adducts such as [M+Ma+H] ions. The precursor model bears some similarities to ion evaporation in field desorption (Chap. 8.5.1). 

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