Evaporation/ionization step

An on-line supercritical fluid chromatography-capillary gas chromatography (SFC-GC) technique has been demonstrated for the direct transfer of SFC fractions from a packed column SFC system to a GC system. This technique has been applied in the analysis of industrial samples such as aviation fuel (24). This type of coupled technique is sometimes more advantageous than the traditional LC-GC coupled technique since SFC is compatible with GC, because most supercritical fluids decompress into gases at GC conditions and are not detected by flame-ionization detection. The use of solvent evaporation techniques are not necessary. SFC, in the same way as LC, can be used to preseparate a sample into classes of compounds where the individual components can then be analyzed and quantified by GC. The supercritical fluid sample effluent is decompressed through a restrictor directly into a capillary GC injection port. In addition, this technique allows selective or multi-step heart-cutting of various sample peaks as they elute from the supercritical fluid... 

Because plasma and urine are both aqueous matrixes, reverse-phase or polar organic mode enantiomeric separations are usually preferred as these approaches usually requires less elaborate sample preparation. Protein-, cyclodextrin-, and macrocyclic glycopeptide-based chiral stationary phases are the most commonly employed CSPs in the reverse phase mode. Also reverse phase and polar organic mode are more compatible mobile phases for mass spectrometers using electrospray ionization. Normal phase enantiomeric separations require more sample preparation (usually with at least one evaporation-to-dryness step). Therefore, normal phase CSPs are only used when a satisfactory enantiomeric separation cannot be obtained in reverse phase or polar organic mode. 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Application of LC-MS/MS techniques to the analysis of phthalate ester metabolites in urine have also been developed. For example, Blount et al. (2000b) have developed an assay to quantify the monoester metabolites (including MEHP) of eight phthalate diesters in urine, utilizing HPLC coupled with atmospheric pressure chemical ionization and tandem mass spectrometric (APCI-MS/MS) detection techniques. Urine samples were treated with -glucuronidase to release the free phthalate monoesters followed by a two-step solid phase extraction procedure. After evaporative concentration of the eluant, the analytes in the purified samples are further separated on a phenyl reverse phase HPLC column and quantified by APCI-MS/MS, following careful optizimation of the APCI-MS/MS instrument. The limits of detection for MEHP were determined to be 1.2 ng/ml urine with recovery efficiencies of between 78 and 91%. 

Hydrochloric acid is the most frequently used titrant in analytical acid-base work. According to Kolthoff and Stenger, 0.1 Af solutions of hydrochloric acid can be boiled for 1 h without loss of acid if the evaporated water is replaced. Even 0.5 M hydrochloric acid can be boiled for 10 min without appreciable loss. Sulfuric acid has the disadvantage of a relatively weak second step of ionization (pAT 2.0). Moreover, a number of metallic and basic sulfates are sparingly soluble. Nitric acid is relatively unstable, though useful in special procedures such as the alkalimetric method for phosphorus. 

With the development of the TSP interface for LC-MS (Ch. 4.7), Vestal et al. [4, 16-18] also introduced a new ioiuzation technique. While the analyte ionization in their first experiments was initiated by electrons from a filament, they subsequently demonstrated that collision of the vapour-droplet beam from the TSP nebulizer with a nickel-plated copper plate leads to soft ioiuzation of analytes. Next, the collision was found not to be a vital step in the process [18]. The presence of a volatile buffer or acid in the mobile phase appeared more important in TSP, i.e., in charging the droplets generated by TSP, and in generation of preformed ions in solution. The ionization phenomena were explained in terms of the ion evaporation (lEV) model [4]. 

The process involves two steps an atomic beam of uranium vapor produced by electron-beam evaporation is excited by light of wavelength 4266.266 A from a pulsed laser and is subsequently ionized by light of wavelength 3609 A from a second laser. The ions produced are detected by a mass spectrometer. To achieve selectivity for the excitation process the incident wavelength must coincide exactly with one of many absorption lines of atoms. The isotope shift of the absorption lines between and near 4266 A is about 0.06 A or about 0.32 cm". Hence, the width of the laser line must be less than 0.32 cm". ... 

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