Chapter 10 - Introduction - Page 365

Liquid chromatography/mass spectrometry (LC-MS) has become one of the primary and most authoritative tools for analysis of most classes of molecules, including lipids. As high-resolution accurate-mass (HRAM) mass spectrometers become ever more affordable, their use is becoming more widespread and routine. When confronted with the dizzying array of options for available instruments, it can seem overwhelming to select the one best method for any analysis. One has options of whether to use a classic single quadrupole mass spectrometer or tandem sector quadrupole (TSQ) instrument, a time-of-flight mass detector, a classic ion-trap instrument (either a hyperbolic ion trap or linear ion trap), a HRAM Orbitrap instrument, or a magnetic sector mass analyzer, as well as hybrid instruments that incorporate combinations of these mass filters or others.
In addition to the type of mass filter, the type of ionization also plays a crucial role in any successful analytical strategy. Because this chapter is focused on MS coupled to LC, it focuses on the three most popular types of atmospheric pressure ionization (API) sources used to couple LC to MS. These are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI). Of course, there are numerous other ionization approaches that have been used for a wide range of applications, ranging from relatively recently developed ambient sampling techniques such as desorption ESI (Takats et al., 2004) or direct analysis in real time (DART) (Cody et al., 2005) to well-established techniques such as matrix assisted laser desorption ionization. Atmospheric pressure, ambient, and desorption techniques have been reviewed extensively in recent years (Covey et al., 2009; Huang et al., 2011; Monge et al., 2013).
Our ongoing interest has been analysis of lipids, primarily triacylglycerols (TAG), using high performance liquid chromatography (HPLC) coupled to MS. We initially employed APCI-MS and ESI-MS for lipid analysis, but as APPI-MS became commercially available, this was incorporated into our analytical methodologies.

Click on the thumbnail graphics below to access the original full-size figure.

Figure 10.1

Structure of dilinoleoyl-oleoyl-glycerol (LLO) and diacylglycerol-like fragment ions, [M + H - R_nCOOH]⁺, or [DAG]⁺, formed from loss of a fatty acyl chain. Location of protons, H⁺, is not limited to the positions shown as examples.

Figure 10.2

Atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) spectra of triacylglycerols in rice bran oil from AB Sciex QTrap 4000 hybrid (A-D) and TSQ 7000 (E-F) mass spectrometers. Fatty acid abbreviations: [DAG]⁺, diacylglycerol-like fragment; L, linoleic acid; [M + 29]⁺, acetonitrile-derived adduct; [M + 89]⁺, acetonitrile + dichloromethane-derived adduct; [M + H]⁺, protonated molecule; O, oleic acid; S, stearic acid. Red (gray in print versions) line indicates statistically expected amount of [AA]⁺ fragment compared with [AB]⁺ fragment (=1/2) from AAB/BAA/ABA triacylglycerols. TSQ, tandem sector quadrupole.

Figure 10.3

Wireless communication contact closure system (WCCCS). (A) Two-channel voltage-to-relay converter connected to 16-channel wireless sending unit #1 for high performance liquid chromatography (HPLC) autosampler; (B) two-channel voltage-to-relay converter connected to wireless sending unit #2 for ultra-high performance liquid chromatography (UHPLC) autosampler; (C) wireless receiving modules attached to wire distribution trace with switches mounteddleft two receiving modules for UHPLC, right two receiving modules for HPLC; (D) WCCCS receiving module and switching system installed and operational. Switches in UP position select contact closure from HPLC, in DOWN position select contact closure from UHPLC.

Figure 10.4

Arrangement of instruments for the first report of LC1/MS4, quadruple parallel mass spectrometry. Effluent after the UV detector (nondestructive) is split via Valco tees and fused-silica capillary tubing to go to various mass spectrometers and other detectors (ELSD and CAD). Electrolyte is supplied via syringe pump for electrospray ionization mass spectrometry and acetone dopant supplied via by an Agilent 1290 for APPI-MS. High- and lowsensitivity atmospheric pressure chemical ionization mass spectrometry used for vitamin D and triacylglycerols, respectively. APPI, atmospheric pressure photoionization; CAD, charged aerosol detector; ELSD, evaporative light scattering detector.

Figure 10.5

Plumbing diagram for electrospray ionization (ESI) source rinsing between runs using deionized water supplied by an HP 1050 pump. The outlet from the valve goes as an input to the perpendicular arm of a tee attached to the ESI source grounding nut. Ammonium formate supplied during runs, water supplied between runs.

Figure 10.6

APCI-MS(/MS) of soybean oil in dietary supplement containing vitamin D₃ from fish oil, from TSQ Vantage EMR mass spectrometer. (A) Total ion current chromatogram (TIC); (B) extracted ion chromatogram (EIC) of full scans; (C) EIC of vitamins D₂ and D₃; (D) mass spectrum of CyCyCa; (E) CaCaCy and vitamin D₂ (internal standard); (F) LnLP; (G) OLO; (H) OLP; and (I) SLS + PPP + PLA. A, arachidic, 20:0; Ca, capric, 10:0; Cy, caprylic, 8:0; L, linoleic, 18:2; Ln, linolenic, 18:3; O, oleic, 18:1; P, palmitic, 16:0; S, stearic, 18:0. Labels are regiospecific at sn-2. APCI-MS, atmospheric pressure chemical ionization mass spectrometry; TSQ, tandem sector quadrupole.

Figure 10.7

APPI-MS of soybean oil in dietary supplement containing vitamin D₃ from fish oil, on QTrap 4000 hybrid mass spectrometer. (A) Total ion current chromatogram (TIC); (B) selected ion monitoring (SIM) of vitamins D₂ and D₃; (C) mass spectrum of CyCyCa; (D) CaCaCy and vitamin D₂ (internal standard); (E) LnLP; (F) OLO; (G) OLP; and (H) SLS + PPP + PLA. DBP, dibutyl phthalate; DOP, dioctyl phthalate; NDP, nonyl, decyl phthalate; other abbreviations are given in Fig. 10.6. APPI-MS, atmospheric pressure photoionization mass spectrometry. Phthalates from new Photospray source.

Figure 10.8

NARP-HPLCeESI-MS of soybean-oil-based dietary supplement containing 2000 IU vitamin D₃ on LCQ Deca XP ion-trap mass spectrometer. Mass spectra show ammonium adducts, [M + NH₄]⁺. (A) Total ion current chromatogram (TIC); (B) extracted ion chromatogram (EIC) of full scans; (C) LnLLn; (D) LnLP; (E) MS/MS spectrum of [LnLP + NH₄]⁺ m/z 870.7; (F) OLP; (G) OOP; and (H) MS/MS spectrum of [OOP + NH₄]⁺ m/z 876.7. Vitamin D₃ from distilled fish oil included three short-chain triacylglycerols (TAGs), CyCyCy, CyCyCa, and CaCaCy. TAGs labeled with most abundant regioisomer identified by APCI-MS. Fatty acid abbreviations are given in Fig. 10.6. APCI-MS, atmospheric pressure chemical ionization mass spectrometry; ESI-MS, electrospray ionization mass spectrometry; NARP-HPLC, nonaqueous reversed-phase high performance liquid chromatography.

Overview of the Contents:

The Handbook of Advanced Chromatography /Mass Spectrometry Techniques is a compendium of new and advanced analytical techniques that have been developed in recent years for analysis of all types of molecules in a variety of complex matrices, from foods to fuel to pharmaceuticals and more. Focusing on areas that are becoming widely used or growing rapidly, this is a comprehensive volume that describes both theoretical and practical aspects of advanced methods for analysis. Written by authors who have published the foundational works in the field, the chapters have an emphasis on lipids, but reach a broader audience by including advanced analytical techniques applied to a variety of fields.

Chapter 10 - Introduction - Page 365

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Overview of the Contents:

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