Direct liquid introduction, mass spectrometry

Even a technique as complicated as direct liquid-introduction mass spectrometry has been coupled with reactor systems to provide real-time compositional analysis, as described in a series of articles by Dell Orco and colleagues.32-34 In their work, these authors used a dynamic dilution interface to provide samples in real time to un-modified commercial ionization sources (electrospray (ESI) and atmospheric pressure chemical ionization (APCI)). Complete speciation was demonstrated due to the unambiguous assignment of molecular weights to reactants, intermediates, and products. 

Direct introduction of a sample, either in solid or liquid state, in the ion source of a mass spectrometer may be achieved through two procedures the first one is based on the use of a direct insertion probe (DIP) the second one necessitates a direct exposure probe (DEP). Direct introduction followed by heating of the sample in the ion source of the mass spectrometer is also known as direct temperature resolved mass spectrometry (DTMS). 

In addition to GC/MS, high performance liquid chromatography (HPLC/MS) has been used to analyse natural resins in ancient samples, particularly for paint varnishes containing mastic and dammar resins [34]. A partial limitation of chromatographic techniques is that they do not permit the analysis of the polymeric fraction or insoluble fraction that may be present in the native resins or formed in the course of ageing. Techniques based on the direct introduction of the sample in the mass spectrometer such as direct temperature resolved mass spectrometry (DTMS), direct exposure mass spectrometry (DE-MS) and direct inlet mass spectrometry (DI-MS), and on analytical pyrolysis (Py-GC/MS), have been employed as complementary techniques to obtain preliminary information on the... 

M.A. Baldwin and F.W. McLafferty, Liquid chromatography-mass spectrometry interface. I The direct introduction of liquid solutions into a chemical ionization mass spectrometer, Org. Mass Spectrom., 7 (1973) 1111-1112. 

The first approaches to the coupling of liquid-phase separation techniques with mass spectrometry were designed for HPLC needs, starting in the 1970s with since-forgotten techniques such as direct liquid introduction (DLI) and moving belt. In the 1980s, techniques such as thermospray, continuous-flow-fast atom bombardment (CF-FAB), and particle beam arose. 

J Apffel, U Brinkman, R Frei, E Evers. Gas-nebulized direct liquid introduction interface for liquid chromatography/mass spectrometry. Anal Chem 55 2280-2284, 1983. 

Lee, E. D. Henion, J. D. 1985. Micro-liquid chromatography/mass spectrometry with direct liquid introduction. /. Chromatogr. Sci., 23, 253-264. 

Over 30 years of liquid chromatography-mass spectrometry (LC-MS) research has resulted in a considerable number of different interfaces (Ch. 3.2). A variety of LC-MS interfaces have been proposed and built in the various research laboratories, and some of them have been adapted by instmment manufacturers and became commercially available. With the advent in the early 1990 s of interfaces based on atmospheric-pressure ionization (API), most of these interfaces have become obsolete. However, in order to appreciate LC-MS, one carmot simply ignore these earlier developments. This chapter is devoted to the older LC-MS interfaces, which is certainly important in understanding the histoiy and development of LC-MS. Attention is paid to principles, instrumentation, and application of the capillary inlet, pneumatic vacuum nebulizers, the moving-belt interface, direct liquid introduction, continuous-flow fast-atom bombardment interfaces, thermospray, and the particle-beam interface. More elaborate discussions on these interfaces can be found in previous editions of this book. 

The scientific curiosity to explore the utility of mass spectrometry to compounds that could not be analyzed by conventional GC/MS was supported by the need to extend the technique into the expanding field of biochemistry. While the development of LC/MS is still undergoing rapid evolution as evidenced by the number of reviews published at regular intervals, three main technological approaches have been constructed which continue to gain popular acceptance for practical use. These three introduction interfaces that are available commercially are the moving belt or transport interface (MB1), direct liquid introduction (DLI), and thermospray (TSP). This review will concentrate on these three interface types that are currently in widespread use. 

WMA Niessen. Review of direct liquid introduction interfacing for LC/MS part II mass spectrometry and applications. Chromatographia, 21 342, 1986. 

Bieri RH, Greaves J. 1987. Characterization of benzo[a]pyrene metabolites by high performance liquid chromatography-mass spectrometry with a direct liquid introduction interface and using negative chemical ionization. Biomed Environ Mass Spectrum 14 555-561. 

Eckers, C. Skrabalak, D.S. Henion, J. On-line direct liquid introduction interface for micro-liquid chro-matography/mass spectrometry application to drug analysis. Clin.Chem., 1982, 28, 1882-1886... 

Ranalder, U.B. Lausecker, B.B. Huselton, C. Micro liquid chromatography-mass spectrometry with direct Uquid introduction used for separation and quantitation of all-(rans- and 13-cis-retinoic acids and their 4-oxo metabolites in human plasma. J.Chromatogr., 1993, 617, 129-135 [LC-MS LOQ 0.3 ng/mL extracted metabolites, isotretinoin, tretinoin]... 

Several designs of a direct-liquid introduction probe, popular in the early days of LC/MS, have been used to introduce LC efQuent directly into the MS ion source [6,23], One version contains a diaphragm with a pinhole at the end of a capillary tube through which the LC effluent is sprayed into the desolvation chamber [24]. The solute molecules enter the mass spectrometry ion sonrce, where they are ionized via a solvent-mediated Cl process. A flow splitter is needed to divert most of the LC solvent from conventional columns because only 10 to 50 xLmin liquid flow rates can be accommodated without breakdown of the mass spectrometry vacuum. The microbore capillary columns are connected directly to the ion source. One of the practical operational difficulties of this probe is frequent clogging of the small orifice. 

Liquid chromatography-mass spectrometry The initial attempts to couple LC with MS lacked important attributes for trace analysis sensitivity, robustness, and reliable quantitation. Moreover, the cost of the early LC-MS instruments was prohibitive for most laboratories. The revolutionary introduction of atmospheric pressure ionization (API) techniques, mainly electrospray (ESI) and atmospheric pressure chemical ionization (APCI), resulted in greater applicability of LC-MS and manufacture of more reliable, affordable, and user-friendly instruments. Thus, LC-MS is now becoming an indispensable part of the analytical strategy in many routine laboratories, enabling direct, selective, and sensitive multiclass, multiresidue analysis of more polar, low volatile, and/or thermolabile pesticides, such as carbamates, phenylureas, sulfonylureas, imidazoles, triazoles, imidazolinones, chlorophenoxy acids, and many others. 

Owen, A.W., Nordon, A., Littlejohn, D., Lynch, T.P., Lancaster, J.S., Wright, R.G. (2014) Qn-line Detection and Quantification of Trace Impurities in Vaporisable Samples by Direct Liquid Introduction Process Mass Spectrometry. Anal. Meth. 6 8148-8153. 

A wide variety of liquid chromatography-mass spectrometry (LC-MS) techniques have been reported for retinoid analysis including direct liquid introduction with Cl (281,282,299-301), particle beam (302-304), thermospray (305-307), electrospray (308,309), and atmospheric pressure chemical ionization (APCI) (310). Because many of the early LC-MS applications to retinoids carried out hydrolysis of retinyl esters and then derivatization of retinoic acid and related retinoids, Wyss (141) predicted in a review of retinoid analysis that derivatization of retinoids would be necessary for all LC-MS techniques, even the anticipated application of atmospheric pressure chemical ionization (APCI). Recently, LC-MS analyses of retinoids have been carried out using APCI (310) and electrospray (308,309), and highly sensitive LC-MS analyses of retinoic acid and retinyl esters were achieved without hydrolysis or derivatization. 

---