Interface APCI

The pump must provide stable flow rates from between 10 ttlmin and 2 mlmin with the LC-MS requirement dependent upon the interface being used and the diameter of the HPLC column. For example, the electrospray interface, when used with a microbore HPLC column, operates at the bottom end of this range, while with a conventional 4.6 mm column such an interface usually operates towards the top end of the range, as does the atmospheric-pressure chemical ionization (APCI) interface. The flow rate requirements of the different interfaces are discussed in the appropriate section of Chapter 4. 

In this book, a number of different LC-MS interfaces have been described, where some of these have been included primarily from an historical standpoint. Currently, the most widely used interfaces are, undoubtedly, the electrospray and APCI interfaces and it is these that will be concentrated upon (a search of the Science Direct database [1] for 2001 nsing the term thermospray , previously the most widely used interface, yielded only one paper). 

Reversed-phase Cig chromatography column. Keystone Scientific Betasil, 100 x 2.0-mm i.d., 5-pm particle size, 100 A, Part No. 105-701-2-CPF TSQ 7000 LC/MS/MS system with electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) interface and gradient high-performance liquid chromatography (HPLC) unit, or equivalent Vacuum manifold for use with SPE cartridges (Varian Vac Elut 10 or equivalent)... 

The APCI interface uses pneumatic nebulization in an atmospheric pressure region to form fine spray droplets. Typically, these systems use a heated nebulizer (300-650 °C)... 

In an attempt to minimize matrix effects, the APCI interface was used to analyze samples using the same purification procedure (PLE followed by liquid-liquid partitioning). In APCI, the method generated acceptable recoveries (70-120%) at the 20 and40ngg levels. 

Table 6.22 summarises the main characteristics of APCI-MS. Sometimes the heat burden of the APCI interface causes thermal decomposition, which is unwelcome if the requested information is only the molecular weight. On the other hand, studying the thermal fragmentation can provide additional data about the sample. Since the thermal and collisional (CID) fragmentation do not necessarily follow the same pathway, the two methods do complement each other. Therefore, even good use can be made of the thermal decomposition for structure elucidation during APCI experiments [145],... 

LC-APCI-MS is a derivative of discharge-assisted thermospray, where the eluent is ionised at atmospheric pressure. In an atmospheric pressure chemical ionisation (APCI) interface, the column effluent is nebulised, e.g. by pneumatic or thermospray nebulisation, into a heated tube, which vaporises nearly all of the solvent. The solvent vapour acts as a reagent gas and enters the APCI source, where ions are generated with the help of electrons from a corona discharge source. The analytes are ionised by common gas-phase ion-molecule reactions, such as proton transfer. This is the second-most common LC-MS interface in use today (despite its recent introduction) and most manufacturers offer a combined ESI/APCI source. LC-APCI-MS interfaces are easy to operate, robust and do not require extensive optimisation of experimental parameters. They can be used with a wide variety of solvent compositions, including pure aqueous solvents, and with liquid flow-rates up to 2mLmin-1. 

For sensitive quantification in LC-MS analysis of non-ionic surfactants, selection of suitable masses for ion monitoring is important. The nonionic surfactants easily form adducts with alkaline and other impurities present in, e.g. solvents. This may result in highly complicated mass spectra, such as shown in Fig. 4.3.1(A) (obtained with an atmospheric pressure chemical ionisation (APCI) interface) and Fig. 4.3.2 (obtained with an ESI interface). 

Six sets of results from five laboratories were obtained for the analyses of NPEO in three cartridges. All laboratories used MS for quantitative analysis, except laboratory 5, which used LC-FL. Laboratory 1 used an LC with an APCI interface laboratories 2 and 4 used LC-ESI-MS and laboratory 3 used FIA-MS analysis. All laboratories performed three independent replicate analyses, i.e. analysed three replicate cartridges of each type of spike. 

However, large between-laboratory variabilities occurred that are indicative of poor reproducibility of methods. In so far as systematic trends could be observed, FIA-MS resulted more often in significantly lower concentrations than LC-MS or LC-FL methods. Within the LC-MS methods, the APCI interface yielded quantitative results which were invariably less than those obtained with LC-ESI-MS. This aspect is discussed in more detail in Chapter 4.3. LC-FL data were generally in fair-to-good agreement with LC-ESI-MS data. This held true for both LAS and NPEO analyses. 

Corona discharge Occurs when the field at the tip of the electrode is sufficiently high to ionize the gas surrounding it but insufficiently high to cause a spark. An integral part of the APCI interface. 

The APCI method has the potential drawback that it is designed for higher flow rates (optimum between 0.2 and 1 mL/min). Micro-APCI approaches may reduce this problem (one so-called semimicro-APCI interface... 

Atmospheric pressure chemical ionization (APCI) interface for LC-MS became a reality at the end of the 1980s [42]. Its sensitivity and robustness were immediately impressive in comparison with all the other interface mechanisms present on the market. 

An atmospheric pressure chemical ionization (APCI) interface is generally considered extremely easy to optimize and operate. This is perhaps best proven by the fact that hardly any optimization of the interface parameters is reported... 

The applicability of the APCI interface is restricted to the analysis of compounds with lower polarity and lower molecular mass compared with ESP and ISP. An early demonstration of the potential of the APCI interface is the LC-APCI-MS-MS analysis of phenylbutazone and two of its metabolites in plasma and urine (128). Other applications include the LC-APCI-MS analysis of steroids in equine and human urine and plasma (129-131), the determination of six sulfonamides in milk samples after a simple solid-phase extraction and LC separation (132), of tetracyclines in muscle at the 100 ppb level (133), of fenbendazole, oxfendazole, and the sulfone metabolite in muscle at the 10 ppb level, and of five thyreostats in thyroid tissue at the 1 ppm level (134). 

The APCI interface uses a heated nebulizer to form a fine spray of the HPLC eluate and to facilitate solvent evaporation. In addition, a cross-flow of heated nitrogen gas is used to... 

The APCI interface uses a heated nebulizer to form a fine spray of the HPLC eluate, which is much finer than the particle beam system but similar to that formed during thermospray. A cross-flow of heated nitrogen gas is used to facilitate the evaporation of solvent from the droplets. The resulting gas-phase sample molecules are ionized by collisions with solvent ions, which are formed by a corona discharge in the atmospheric pressure chamber. Molecular ions, M+ or M , and/or protonated or de-protonated molecules can be formed. The relative abundance of each type of ion depends upon the sample itself, the HPLC solvent, and the ion source parameters. Next, ions are drawn into the mass spectrometer analyzer for measurement through a narrow opening or skimmer, which helps the vacuum pumps to maintain very low pressure inside the analyzer while the APCI source remains at atmospheric pressure. 

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. 

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