HPLC-MS-analysis

The recommended technique for the determination of oxime carbamates and their metabolites by HPLC/MS and HPLC/MS/MS is positive ESI. Electrospray is a soft ionization technique and is suitable for thermally labile compounds. Ions are produced in the liquid phase at quasi-ambient temperature and atmospheric pressure, thus leaving the fragile pesticides intact. For oxime carbamates, the molecular adducts that can be monitored during HPLC/MS analysis with electrospray in positive mode are [M- -H]+, [M- -Na]+, or [M- -NH4]+, depending on the nature of mobile phase used. ... 

Currently, HPLC/fiuorescence is still the most common technique for the determination of residues of oxime carbamates. With the introduction of ESI and APCI MS interfaces, HPLC/MS analysis for oxime carbamates in various sample matrices has become widespread. However, for a rapid, sensitive, and specific analysis of biological and environmental samples, HPLC/MS/MS is preferred to HPLC/MS and HPLC/fiuorescence. With time, improved and affordable triple-quadrupole mass spectrometers will be available in more analytical laboratories. With stricter regulatory requirements, e.g., highly specific and conclusive methods with lower LOQ, HPLC/MS/MS will be a method of choice for oxime carbamates and their metabolites. 

SUMMARY OF PIGMENT CHARACTERISTICS FROM HPLC-DAD AND HPLC-MS ANALYSIS... 

A second approach to on-line SPE is to use an SPE extraction column that can be used for hundreds of samples. In the simplest of systems, two pumps (either HPLC systems or stand-alone pumps) are connected to an extraction column and an analytical column via 6 or 10 ports, and these are further linked to an MS system. The pump that is connected in-line with the autosampler loads the sample under high flow rate (3 to 5mL/min). The large molecules from the matrix are not retained by the SPE sorbent and are diverted to waste. The analytes of interest are retained by the sorbent. The valve then switches so that the second pump with the elution solvent is now in-line with the SPE column and elutes the analytes onto the analytical column for HPLC/MS analysis. This type of system has proved useful for the analysis of small molecules in a variety of sample matrices such as plasma and urine. While it is relatively straightforward to plumb this type of system with components already in the laboratory, commercial systems are available from such companies... 

The challenges of this method have kept the majority of DCLs relatively small. However, Ludlow and Otto recently demonstrated that, in some cases at least, direct HPLC-MS analysis of large libraries is possible [22]. 

As shown in Fig. 1.21, a series of di- and tri-thiols were mixed under conditions suitahle for disulfide formation and exchange, and allowed to evolve in the presence of an ephedrine template. HPLC-MS analysis of the library mixture after equilihrium had been reached allowed the identification of two heterotetrameric receptors with high (K = ICfi) affinity for ephedrine in borate buffer, although it is not clear whether these were in fact the best binders in the library. 

HPLC methods can be ntilized for the pre-concentration of aromatic amines from polluted waters on silica gel or octadecyl silica (ODS) colnmns [55], The determination is then performed by RP HPLC using ODS packings as the stationary phases and a mixture of methanol, isopropanol, and water as the mobile phase [55], RP HPLC with diode array detector (DAD) methods coupled on-line with a continnons seqnential anaerobic/aerobic reactor system have been employed in wastewaters treatments [56], A continnons monitoring of the possible presence of aromatic amines in azo dyes wastes is based on indncing in the waste, the reaction of a reduction of the dye, followed by HPLC/ UV or HPLC/MS analysis [57-59], The redncing agent solutions are sodium dithionite or tin(II) chloride in an aqneons acidic medinm at 70°C, followed by SPE [58,59], LLE [60,61], or SEE [60-62],... 

MS techniques, mainly because of its ease of operation, the commercial availability of advanced instrumentation, and the high resolving power of capillary column GC that allows one to carry out specific high-resolution measurement at extremely low levels. The GC-MS techniques are, however, not applicable to all compounds, especially the thermolabile compounds, such as N-nitrosamides, or the nonvolatile NOC. Furthermore, direct HPLC-MS analysis of nonvolatile NOC eliminates the need for derivatization. 

Scheme 3.11. Combinatorial MCR searching according to Mironov et al. A combinatorial set of different starting materials displaying multiple functional groups (left panel) was subject to HPLC-MS analysis (right panel).

Thurman, E.M., I. Ferrer, and D. Barcelo. 2001. Choosing between atmospheric pressure chemical ionization and electrospray ionization interfaces for the HPLC/MS analysis of pesticides. Anal. Chem. 73 5441-5449. 

M. Grieg, Use of automated HPLC-MS analysis for monitoring and improving the purity of combinatorial libraries. Am. Lab. 31 (1999) 28-32. 

An essential feature of this approach is that well-to-well cross-contamination with reagent solution or resin is avoided by the fact that the plates are tilted, while the direction of centrifugation is horizontal. Consequently, any liquid or resin expelled from the wells is either captured in the inter-well space of the plate or collected on the wall of the centrifugal drum. HPLC/MS analysis of all products prepared on the microtiter plate proved the fact that cross-contamination is not an issue. 

The choice of a HPLC/MS analysis method depends greatly on the characteristics of the sample (i.e. proton affinity, polarity, thermal stability and volatility) as well as the structural information and sensitivity required. The use of various techniques including enzymatic hydrolysis and physical-chemical reactions can assist in achieving the analysis goals for certain compounds. Alternatively, the use of complimentary HPLC/MS techniques such as thermospray and particle beam can be useful for the analysis of a variety of compounds, as demonstrated in this paper. Employing less commonly available instrumentation, such as tandem MS, with thermospray or particle beam can prove valuable in determining structure when other methods are unsuccessful. The further development of existing HPLC/MS techniques and the implantation of new HPLC/MS methods will continue to increase the variety of compound classes that can be routinely monitored with adequate sensitivity and specificity. 

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