Ionization methods atmospheric-pressure chemical

Recent advances in electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), thermospray, and particle beam LC-MS have advanced the analyst toward the universal HPLC detector, but price and complexity are still the primary stumbling blocks. Thus, HPLC-MS remains expensive and the technology has only recently been described. Early commercial LC-MS uses particle beam and thermospray sources, but ESI and APCI interfaces now dominate. Liquid chromatography MS can represent a fast and reliable method for structural analyses of nonvolatile compounds such as phenolic compounds (36,37), especially for low-molecular-weight plant phenolics (38), but the limited resolving power of LC hinders the widespread use of its application for phenolics as compared to GC-MS. 

El and Cl methods can be used if the compound to be studied is sufficiently volatile and stable to be vaporized intact. However, only 20% of the organics found in surface water are volatile enough to be amenable to GC-EI-MS or GC-CI-MS. Today, there are a variety of other ionization techniques available electrospray ionization (ESI), atmospheric pressure chemical ionization, matrix-assisted laser desorption ionization, and fast atom bombardment. Each of these has its advantages and disadvantages. A simple guideline to the most likely optimum ionization technique for a given class of substance is given in Table 1. 

In terms of the hardware, TRMS methods described in this book use most common types of ion sources and analyzers. Electrospray ionization (ESI), electron ionization (El), atmospheric pressure chemical ionization (APCI), or photoionization systems, and their modified versions, are all widely used in TRMS measurements. The newly developed atmospheric pressure ionization schemes such as desorption electrospray ionization (DESI) and Venturi easy ambient sonic-spray ionization (V-EASI) have already found applications in this area. Mass analyzers constitute the biggest and the most costly part of MS hardware. Few laboratories can afford purchasing different types of mass spectrometers for use in diverse applications. Therefore, the choice of mass spectrometer for TRMS is not always dictated by the optimum specifications of the instrument but its availability. Fortunately, many real-time measurements can be conducted using different mass analyzers equipped with atmospheric pressure inlets - with better or worse results. For example, triple quadrupole mass spectrometers excel at quantitative capabilities however, in many cases, popular ion trap (IT)-MS instruments can be used instead. On the other hand, applications of TRMS in fundamental studies often require a particular type of instrument (e.g., Fourier transform ion cyclotron resonance mass spectrometer for photodissociation studies on trapped ions). 

Chapter 6, titled Selection of Ionization Methods of Analytes in the TLC-MS Techniques provides an overview of mass spectrometric techniques that can be coupled with TLC and act as specific detectors in this hyphenated approach. The mass spectrometric techniques discussed in this chapter are secondary mass spectrometry (SIMS), liquid secondary ion mass spectrometry (LSIMS), fast atom bombardment (FAB), matrix-assisted laser desorption/ionization (MALDI), atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI), electrospray ionization (ESI), desorption electrospray ionization (DESI), electrospry-assisted laser desorption/ionization (ELDI), easy ambient sonic spray ionization (EASI), direct analysis in real time (DART), laser-induced acoustic desorption/electrospray ionization (LIAD/ESI), plasma-assisted multiwavelength laser desorption/ionization (PAMLDI), atmospheric-pressure chemical ionization (APCI), and dielectric barrier discharge ionization (DBDI). For the sake of illustration, the authors introduce practical examples of implementing TLC separations with detection carried out by means of individual mass spectrometric techniques for the systematically arranged compounds belonging to different chemical classes. 

A liquid chromatography-mass spectrometry (LC-MS) method that can quantitatively analyze urinar y normal and modified nucleosides in less than 30 min with a good resolution and sufficient sensitivity has been developed. Nineteen kinds of normal and modified nucleosides were determined in urine samples from 10 healthy persons and 18 breast cancer patients. Compounds were separ ated on a reverse phase Kromasil C18 column (2.1 mm I.D.) by isocratic elution mode using 20 mg/1 ammonium acetate - acetonitrile (97 3 % v/v) at 200 p.l/min. A higher sensitivity was obtained in positive atmospheric pressure chemical ionization mode APCI(-i-). 

An on-line chromatography/atmospheric pressure chemical ionization tandem mass spectrometry (LC-APCI/MS/MS) methods was developed for rapid screen of pharmacokinetics of different drugs, including 5 (98RCM1216). The electron impact mass spectrum of 5 and ethyl 9,10-difluoro-3-methyl-7-oxo-2,3-dihydro-7Ff-pyrido[l,2,3- fe]-l,4-benzoxazine-6-carboxylate was reported (97MI28). Electron impact/Fourier transform... 

Ionization methods that may be utihzed in LC-MS include electron ionization (El), chemical ionization (Cl), fast-atom bombardment (FAB), thermospray (TSP), electrospray (ESI) and atmospheric-pressure chemical ionization (APCI). 

Atmospheric-pressure chemical ionization (APCI) is another of the techniques in which the stream of liquid emerging from an HPLC column is dispersed into small droplets, in this case by the combination of heat and a nebulizing gas, as shown in Figure 4.21. As such, APCI shares many common features with ESI and thermospray which have been discussed previously. The differences between the techniques are the methods used for droplet generation and the mechanism of subsequent ion formation. These differences affect the analytical capabilities, in particular the range of polarity of analyte which may be ionized and the liquid flow rates that may be accommodated. 

If a high -molecular-weight compound is being studied by LC-MS, the analyst has little choice in the ionization method to use, with atmospheric-pressure chemical ionization (APCl) being wholly inappropriate. However, when low -molecular-weight componnds are involved, both electrospray ionization and APCl are potentially of value. 

The most recent progress in MS analysis of chlorophylls has been obtained with the development of atmospheric ionization methods such as atmospheric pressure chemical ionization (APCl) and electrospray ionization (ESI). These techniques have demonstrated much more sensitivity than thermospray ionization, detecting chloro-... 

Crescenzi et al. developed a multi-residue method for pesticides including propanil in drinking water, river water and groundwater based on SPE and LC/MS detection. The recoveries of the pesticides by this method were >80%. Santos etal. developed an on-line SPE method followed by LC/PAD and LC/MS detection in a simultaneous method for anilides and two degradation products (4-chloro-2-methylphenol and 2,4-dichlorophenol) of acidic herbicides in estuarine water samples. To determine the major degradation product of propanil, 3,4-dichloroaniline, the positive ion mode is needed for atmospheric pressure chemical ionization mass spectrometry (APCI/MS) detection. The LOD of 3,4-dichloroaniline by APCI/MS was 0.1-0.02 ng mL for 50-mL water samples. 

Electron impact ionization, also known as particle beam ionization, has been applied to the online determination of steroids such as hydrocortisone, cortisone, prednisolone and prednisone. Polymer additives such as NC-4, Irga-nox 1076,1-octadecanol and Naugard -XL were identified and quantitated online by electron impact and, separately, by atmospheric pressure chemical ionization methods.78... 

For confirmatory assay, liquid chromatography-tandem mass spectrometry (LC-MS/MS) is becoming more frequently used in the analysis of OTC owing to its high sensitivity and ability. Electrospray ionization (ESI) [55-57] and atmospheric pressure chemical ionization (APCI) [41] methods combined with tandem mass spectrometry are favored because of their higher sensitivity and better reproducibility. Hamscher et al. [58] developed a method for the determination of persistent TC residues in soil fertilized with manure by HPLC tandem mass spectrometry, MS-MS, and confirmation by MS-MS-MS. Zhu et al. [59] developed an LC-tandem mass spectrometry for the analysis of common tetracyclines in water. The detection limit for oxytetracycline was 0.21 pg/L. Lykkeberg et al. [60] used LC-MS/MS for determination of oxytetracycline and its impurities EOTC, TC, ETC, ADOTC, oc-AOTC, and /i-AOTC. 

Currently no comprehensive libraries have been developed for methods, such as ESI (see Section 2.1.15) and atmospheric pressure chemical ionization (APCI see Section 2.1.8), used to ionize compounds after LC separation. Generally these types of library are user generated for a specific purpose. 

---