Atmospheric-pressure chemical ionization advantages

Polymeric precolumns of styrene-divinylbenzene were used by Aguilar et al. to monitor pesticides in river water. Water samples (50 mL) were trace enriched on-line followed by analysis using LC combined with diode-array detection. LC atmospheric pressure chemical ionization (APCI) MS was used for confirmatory purposes. It was found that after the pesticides had been extracted from the water sample, they could be stored on the precartridges for up to 3 months without any detectable degradation. This work illustrates an advantage of SPE for water samples. Many pesticides which may not be stable when stored in water, even at low temperature, may be extracted and/or enriched on SPE media and stored under freezer conditions with no detectable degradation. This provides an excellent way to store samples for later analysis. 

For the last several years, mass spectrometry with atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) have determined the trends in the analysis of dyes. Since 1987, various variants of ESI have been used in which droplet formation was assisted by compressed air,[1,2] temperature (e.g. Turbo Ion Spray ) or ultrasound, and they were able to handle flow rates up to 1 2 ml min This made a combination of analytical RPLC and ESI easily and widely used. The reason why it often was (and is) used instead of a traditional UV-Vis detector is the better sensitivity and selectivity of MS in comparison with spectrophotometric detection. Apart from these advantages, MS offers easily interpretable structural information. However, various... 

One of the major advantages of LC-MS/MS over GC-MS or GC-MS/MS is that steroid hormones may be analyzed directly by LC-MS/MS without derivatization procedures, which are time-consuming and tedious [22,50-53], However, a number of studies demonstrated that the chemically derivatized steroid hormones were significantly more sensitive to LC-MS/MS detection than the underivatized hormones, because the neutral molecules of estrogens and metabolites might not be effectively ionized under electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) modes [4, 21, 25, 54, 55], In order to enhance the steroid hormone molecules sensitivity for LC-MS/MS analysis at pg/mL level, chemical derivatization is an effective technique for analysis of steroid hormones and metabolites. A list of derivatization reagents and application examples for steroid hormone analyses by LC-MS/MS and GC-MS are presented in Table 3. 

MAI,PI was introduced in the late 1980s and is one of the most successfully developed MS soft ionization techniques that uses the matrix assists laser ablation of sample-coated target to vaporize gas-phase ions for injection into a mass spectrometer. The advantage of MALDI is its gentleness compared with ESI and Atmospheric Pressure Chemical Ionization (APCI) and its ability to analyze the polar, nonvolatile, and large molecules. It has been very successfully used for the analysis of both biopolymers compounds and small molecular organic compounds (<1,500 Da). 

While the ESI technique has revolutionized the analysis of biomolecules such as peptides, proteins, polysaccharides, and nucleotides, the constraints imposed by its dependence on solution chemistry for ionization limit its applicability in the realm of small- to medium-sized nonpolar molecules (i.e., weakly basic or neutral compounds).120 Atmospheric pressure chemical ionization (APCI), on the other hand, takes advantage of gas-phase processes... 

Liquid chromatography coupled with mass spectrometry (LC-MS) has been applied in the determination of BPA, OP, and NP in the environment. The advantage of this analytical method is the capability of directly determining nonvolatile or polar compounds by using ESI or atmospheric pressure chemical ionization (APCI) techniques as an interface between LC and... 

One of the most powerful techniques used in Upid analysis today is HPLC coupled with mass spectrometry (HPLC/MS). Several mass spectrometric ionization techniques, such as fast atom bombardment (FAB) [23], electrospray ionization (ESI) [29,30], ionspray ionization (ISI) [31], and atmospheric pressure chemical ionization (APCI) [22,30,32] have been used. By using HPLC/MS, one can get information on the molecular structure of the intact lipids, which helps differentiate molecular species within different lipid classes. By using tandem mass spectrometry (MS/MS), identification of molecular species of different sphingolipids can be achieved in an easier and more sensitive way. There are many other advantages of using MS, such as small sample size, minimal sample preparation, and lack of need for derivatization, speeds, and sensitivity. In the literature, sphingolipids of both animal and plant origin were analyzed by 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. 

Nearly all available ionization techniques of MS have already been applied successfully to Upids, and their individual advantages and disadvantages discussed [25]. Nowadays, the overwhelming majority of lipidomic investigations makes use of electrospray ionization (ESI) MS [26], and useful protocols have been developed in this respect [27, 28]. In comparison to ESl-MS, Upid studies based on MALDl [29] or atmospheric-pressure chemical ionization (APCI) [30] are much less abun-danL and a major aim of this chapter will be to emphasize the benefits of MALDI-MS. 

Other API techniques, such as atmospheric pressure photoionization (APPI) and atmospheric pressure chemical ionization (APCI) have been marginally applied. Although providing advantages over ESI, such as reduced matrix effects, APCI has been rarely applied for PFC analysis. Analytes measured with APCI comprise various ethoxylated PFC [40, 55] and PFOA [56]. However, no investigations with respect to matrix effects were made in these articles. 

In one study, Clinton et al. [69] established a single-stage membrane-based interface (Figure 4.2), and implemented it in the real-time MS monitoring of a concentrated pharmaceutical process reaction mixture. In this case, an atmospheric pressure chemical ionization (APCI) source was used in conjunction with a quadrupole instmment. The advantages of that approach included minimal analyst intervention and short sample preparation/analysis time [69]. 

Today, electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) are considered the standard ionization techniques for LC-MS/MS due to their predominant advantages in quantitative analysis of drug molecules in various sample matrices with high sensitivity, selectivity, reliability, robustness, and ease of operation. Other techniques, for example, atmospheric pressure photoionization (APPI), electron capture atmospheric pressure chemical ionization (EC-APCI), and high-field asymmetric waveform ion mobility mass spectrometry (FAIMS) serve as complements to the established ESI and/or APCI technical platforms whenever necessary for an enhanced sensitivity and/or selectivity of a bioanalytical assay [4,5]. 

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