Electrospray interface sensitivity

A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

For HPLC/MS/MS analysis, a triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and speciflcity in the quantitative determination of oxime carbamates and their metabolites. This allows... 

Confirmation of FLU in catfish muscle by electrospray LC/MS was done (199). Residues of CIPRO, ENRO, SARA, and DIFLX were positively identified at 10-80 /ug/kg. The extraction procedure was based on LLE with acidic ethanol. Extracts were cleaned up on a PRS SPE cartridge. Analytes were eluted with 30% ammonium hydroxide-MeOH (1 4). Chromatographic conditions were optimized to be compatible with the electrospray interface. To obtain maximum sensitivity, separate MS acquisition programs were developed for CIPRO/ENRO and SARA/ DIFLX pairs. This method was used to confirm residues in tissues fortified in the 10-80-ppb range. All relative abundances were within 10% of the value calculated for standard compounds. 

The evolution of the ESI source has been marked by the use of electrospray devices as interfaces between the separation systems such as HPLC or CE and MS detectors, the earliest instances of which were reported by Yamashita and Fenn [59] and Aleksandrov et al. [60] in the mid-1980s. Because ESI-MS is used in many areas of chemistry, a vast number of articles reporting specific modifications of the electrospray interface has been published so far. Also, instrument manufacturers have provided innovative solutions for more sensitive and reliable mass spectrometers. 

Whilst the object of this chapter has been to show the extent and type of HPLC technique that is used today in today s environmental laboratories, there are a number of less routine techniques that may or may not have an impact on routine environmental monitoring. One of the most potentially important of these is the use of LC-MS. The problems associated with using LC-MS for trace analysis are twofold one is the usual LC-MS problem of interfacing the second is that of sensitivity of detector. The interfacing problem may well continue to have partial (compared with GC-MS interfacing) solutions such as FAB, and thermospray, etc. However, even given the advances arising from electrospray interfaces the answer may well be to move away from LC-MS to supercritical fluids and SFC-MS. 

A breakthrough was achieved with the advent of electrospray interfaces for the coupling of HPLC to mass spectrometers (MS). We discuss MS in detail in a following section. In conjunction with chromatography, MS provides subpicomole sensitivity and high-resolution m/z-based separation of chromatographically unresolved analytes... 

Capillary IC features a flow rate in the range of 10-30 pF/min, thus requiring modifications and reoptimization of existing electrospray interfaces that are usually optimized for either analytical flow (100 pF/min to several milliliters/minute) or nanoflow (<1 pF/min) rates. The optimization of interface parameters such as probe temperature, nebulizer gas, needle voltage, type of desolvation solvent, and the flow rate plays a critical role in establishing instrument sensitivity. When capillary IC is operated at a flow rate of 10-20 pF/min, a probe temperature of 300 °C, a needle voltage of 3 kV, a nebulizer gas pressure of 65 psi, and the use of... 

Because modem anion exchangers are solvent-compatible, solvents can also be used to alter selectivity. Figure 9-17 illustrates the gradient elution of chloro-and bromo-acetic acids on lonPac AS 11 with a constant content of methanol in the mobile phase. Even though separations of this kind are promising, it is questionable whether the necessary sensitivity for water analysis can be achieved with suppressed conductivity detection. Thus, the method of IC-MS via an electrospray interface introduced by Hashimoto and Otsuki [57] creates a lot of interest. With this method the authors analyzed nine different chloro- and bromo-acetic acids at trace levels. However, the method requires analyte extraction with methyl-tert.-butyl ether. Based on a 200-mL water sample, detection... 

In this study, the effect of mobile-phase flow rate, or more accurately, the rate of flow of liquid into the LC-MS interface, was not considered but as has been pointed out earlier in Sections 4.7 and 4.8, this is of great importance. In particular, it determines whether electrospray ionization functions as a concentration-or mass-flow-sensitive detector and may have a significant effect on the overall sensitivity obtained. Both of these are of great importance when considering the development of a quantitative analytical method. 

The method for chloroacetanilide soil metabolites in water determines concentrations of ethanesulfonic acid (ESA) and oxanilic acid (OXA) metabolites of alachlor, acetochlor, and metolachlor in surface water and groundwater samples by direct aqueous injection LC/MS/MS. After injection, compounds are separated by reversed-phase HPLC and introduced into the mass spectrometer with a TurboIonSpray atmospheric pressure ionization (API) interface. Using direct aqueous injection without prior SPE and/or concentration minimizes losses and greatly simplifies the analytical procedure. Standard addition experiments can be used to check for matrix effects. With multiple-reaction monitoring in the negative electrospray ionization mode, LC/MS/MS provides superior specificity and sensitivity compared with conventional liquid chromatography/mass spectrometry (LC/MS) or liquid chromatography/ultraviolet detection (LC/UV), and the need for a confirmatory method is eliminated. In summary,... 

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