Analyte polarity

Figure 1 illustrates a typical, good quality, analytical polarizing microscope. Polarizing microscopes are extraordinarily versatile instruments that enable the trained microscopist to characterize materials rapidly and accurately. 

Table 7.53 shows the main characteristics of LC-PB-MS. Of all LC-MS interface methods, LC-PB-MS comes closest to GC-MS (Scheme 7.7). The particle beam is an acceptable choice in cases where sensitivity, volatility and analyte polarity are not an issue. Usually, the function of UV is added to LC-PB-MS this allows the investigation of peak homogeneity. Drawbacks of LC-PB-MS are the low sensitivity and the nonlinearity... 

Wide range of analyte polarity (even with NPLC)... 

Analytes must be liberated from their associated solvent molecules as well as be ionized to allow mass separation. Several ionization methods enable ion production from the condensed phase and have been used for the coupling of CE to MS. Among them, atmospheric pressure ionization (API) methods, matrix-assisted laser desorption/ionization (MALDI), and inductively coupled plasma (ICP) ionization are mainly used. API techniques are undoubtedly the most widespread ionization sources and cover different analyte polarity ranges. 

Ideal for gaseous/volatile analytes, high retention for trace analysis For polar analytes, especially for alcohols Developed for high-performance liquid chro-matograpy applications, e.g. surfactants Ideal for a broad range of analyte polarities, good for C3-C20 range... 

At present, the most powerful and promising interfaces for drug residue analysis are die particle-beam (PB) interface that provides online EI mass spectra, the thermospray (TSP) interface diat works well with substances of medium polarity, and more recently the atmospheric pressure ionization (API) interfaces that have opened up important application areas of LC to LC-MS for ionizable compounds. Among die API interfaces, ESP and ISP appear to be the most versatile since diey are suitable for substances ranging from polar to ionic and from low to high molecular mass. ISP, in particular, is compatible with the flow rates used with conventional LC columns (70). In addition, both ESP and ISP appear to be valuable in terms of analyte detectability. These interfaces can further be supplemented by preanalyzer collision-induced dissociation (CID) or tandem MS as realized with the use of triple quadrupole systems. Complementary to ESP and ISP interfaces with respect to the analyte polarity is APCI with a heated nebulizer interface. This is a powerful interface for both structural confirmation and quantitative analysis. 

Here P(r, t) is the analytic polarization, which is defined in terms of the real polarization P(r)(r, t) in the same way as the analytic signal was defined in terms of the real signal [Eq. (Bl) is usually written for the real field and polarization however, it follows from their definition that the analytic quantifies satisfy the same equation]. 

When the analytic polarization P(r, t) is given, the wave equation is linear and inhomogeneous and can be solved exactly in a closed form. The general solution of Eq. Bl is [17]... 

Different reversed phase [195,239,240], mixed mode (ion exchange and reversed phase) SPE cartridges [173,218] and online SPE column [193, 238] have been also reported for samples preparation and extraction. Some of these assays combined both PP and SPE in order to achieve an extensive sample cleanup [193, 195, 238-240], Likewise SPE, LLE provides cleaner plasma extracts than PP. Nevertheless, LLE procedure does not always provide satisfactory results with regard to extraction recovery and selectivity, especially with polar analytes and particularly in the case of multicomponent analysis such as in drug-metabolism studies, where analytes polarity varies widely. This issue was addressed by Zweigenbaum J and Henion J [235] and extraction solvent optimization, using isoamyl alcohol, to achieve acceptable extraction selectivity and recovery for polar analytes has been discussed. 

For the case of three coupled spins 1 /2 with planar coupling tensors (C = P), analytical polarization-transfer functions have been reported. 

Molecular polarity of analytes is difficult to quantify unequivocally. The descriptors of polarity are expected to account for differences among the analytes regarding their dipole-dipole, dipole-induced dipole, hydrogen bonding and electron pair donor-electron pair acceptor (EPD-EPA) interactions. To find good descriptors of these chemically specific interactions is difficult, particularly since changes in analyte polarity also affect analyte geometry and its ability to take part in bulkiness-related interactions 7,l2j. 

The ability of an analyte to take part in polar interactions is normally difficult to characterize by means of a single descriptor. The importance of analyte polarity for retention is clearly demonstrated for isomers in 1956, James [53] related the retention of a series of isomeric xylidines to their dipole moment. However, simple QSRR involving dipole moments and other polarity descriptors are rare. 

Complementary to ESP and ISP interfaces, with respect to the analyte polarity, is the atmospheric pressure chemical ionization (APCI) interface equipped with a heated nebulizer. This is a powerful interface for both structural confirmation and quantitative analysis. 

Solid-phase extraction (SPE) is based on low-pressure liquid chromatography, where a short column is filled with an adsorbent. The separation mechanisms are based on the intermolecular interactions among analyte molecules and functional groups of sorbent. The choice of eluent is made by the relationship between the eleutropic value (2°) and the analyte polarity. SPE is fast, selective, and economical if compared with the extraction methods described previously. It can be applicable to both nonpolar and polar analytes, but both matrix and analyte must be in the liquid state. 

TABLE 4 Suggested Ionization and Detection Methods Based on Analyte Polarity... 

Figure 9 Approximate ranges of analyte polarity and size that may be suited to different ionization techniques. With respect to the surface desorption techniques, DESI and DART, they are comparable in their range of application to ESI and APCI, respectively.

Strong interactions between the polar matrix and polar analytes may lead to extremely long equilibrium times and errors in quantitation even when the MHS technique is used. In these cases, a displacer may be added to break the interactions between the matrix and analyte. Polar 2-cyclopentyl-cyclopentanone could be quantitatively determined in polar polyamide 6.6 by MHS-SPME if water was added as a displacer to break the hydrogen bonding between 2-cyclopentyl-cyclopentanone and polyamide. The addition of water also significantly reduced the equilibrium time. A correlation was found between the amount of 2-cyclopentyl-cyclopentanone emitted from polyamide 6.6 and the total amount of 2-cyclopentyl-cyclopentanone in the material. This correlation enables rapid assessment of the 2-cyclopentyl-cy-clopentanone content using headspace techniques under non-equilibrium conditions. The analysis time is significantly reduced if the polymer samples are milled to a powder prior to extraction. 

For libraries with an upper mass limit of approximately 500 amu, GC-MS can prove advantageous. Capillary-GC on fused silica capillaries is characterized by high separation power and the ability to analyze relatively polar substances. In many cases, the problems of tailing, thermal instability and volatility associated with excessive analyte polarity can be overcome by the use of derivatization techniques. 

The interface must be able to get rid of the liquid aqueous buffer, convert the relatively involatile and/or thermally labile analytes into a gas and transfer the gas from atmospheric conditions to a high vacuum. In most cases, the capillary is inserted directly into the ion source and in this case the ionisation source is the interface suitable ones are described below. Different ionisation methods must be used for these kinds of analytes (polar, non-volatile, thermolabile) as El and Cl, which are used in the GC-MS instrument interface, are not suitable here. Those used for LC-MS are often compatible with CE-MS. 

Figure 4.1 Degree of analyte volatility versus degree of analyte polarity.

A nonpolar mobile phase passing through a packed column that contains a polar stationary phase defines normal-phase HPLC (NP-HPLC). For example, if -hexane comprises the mobile-phase and silica gel is used for the stationary phase, separations of nonpolar organic analytes as shown in Fig. 4.1 is accomplished. With respect to neutral organic compounds, the polar and ionic domains cannot be reached by NP- HPLC. NP-HPLC was the first high-pressure form of liquid chromatography to be developed. If the stationary phase could be made hydrophobic by chemical treatment and the mobile phase made more polar, a reversal of mobile/stationary-phase polarities could be achieved. Like it or not, we are stuck with this nomenclature RP-HPLC has certainly extended the range of analyte polarity that... 

Organic Modifies Nonpolar Analytes Polar Analytes... 

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