Interface atmospheric pressure

Several other interface designs were introduced over this period, including continuous flow fast atom bombardment (CFFAB)" and the particle beam interface (PBI)," but it was not until the introduction of the API source that LC/MS applications really came to the forefront for quantitative analysis. Early work by Muck and Henion proved the utility of an atmospheric pressure interface using a tandem quadrupole mass spectrometer. 

The performance of atmospheric pressure interfaces appears to vary widely from instrument to instrument. A variety of interface designs are available from the various manufacturers. Optimisation of operating parameters, such as cone voltage, temperature, and mobile-phase composition is always necessary prior to actual analysis of samples. A given optimised set of parameters is likely to change with changing matrices, and may also vary with local conditions, such as the alkaline metal content of water or tubing used. 

LC/MS is the ultimate analytical technique, which combines the versatility of HPLC with the identification power of MS. The weak link in LC/MS has always been the interface which connects the liquid stream at atmospheric pressure to the high vacuum present inside the mass spectrometer. The development of several atmospheric pressure interfaces, electrospray and atmospheric pressure chemical ionization (APCI), has contributed to the tremendous success and popularity of LC/MS and LC/MS/MS in bioresearch, drug discovery, combinatorial analysis and pharmacokinetic assays. This topic is covered in more depth in a later chapter. 

Diagram of an AP-MALDI source. Ions are transferred into the mass analyser using the atmospheric pressure interface. 

Such sources ionize the sample at atmospheric pressure and then transfer the ions into the mass spectrometer. An atmospheric pressure interface is then used to transfer ions into the high vacuum of the mass analyser. The problem lies in coupling an atmospheric pressure source compartment with an analyser compartment that must be kept at a very low pressure or at a very high vacuum (10 5 Torr). 

A new ionization method called desorption electrospray ionization (DESI) was described by Cooks and his co-workers in 2004 [86]. This direct probe exposure method based on ESI can be used on samples under ambient conditions with no preparation. The principle is illustrated in Figure 1.36. An ionized stream of solvent that is produced by an ESI source is sprayed on the surface of the analysed sample. The exact mechanism is not yet established, but it seems that the charged droplets and ions of solvent desorb and extract some sample material and bounce to the inlet capillary of an atmospheric pressure interface of a mass spectrometer. The fact is that samples of peptides or proteins produce multiply charged ions, strongly suggesting dissolution of the analyte in the charged droplet. Furthermore, the solution that is sprayed can be selected to optimize the signal or selectively to ionize particular compounds. 

AcCOOH = acetic acid, ACN = acetonitrile, DCM = dichloromethane, MeOH = methanol, IPA = isopropanol, APl-ES = atmospheric pressure interface electrospray... 

Song, O., Kothari, S., Senko, M.A., Schwartz, J.C., Amy, J.W., Stafford, G.S., Cooks, R.G., Ouyang, Z., Rectilinear ion trap mass spectrometer with atmospheric pressure interface and electrospray ionization source. Anal. Chem. 2006, 78, 718. 

MALDI ionization is mainly performed in the vacuum region of the mass spectrometer however, it can also be performed at atmospheric pressure (Moyer and Cotter, 2002 Schneider et al., 2005). One of the major advantages of MALDI is that in can be adapted very easily, using the atmospheric pressure interface on virtually any type of mass spectrometer. 

LC-MS has been used for organotin analysis, though its application is much less wide spread than ICP-MS. It offers the advantage of providing structural information because of the soft ionization processes employed in the atmospheric pressure interfaces, and molecular ions (or their adducts) are generally observed for organotin species. 

The AP MALDI PDF+ ion source can be flanged to the atmospheric pressure interface (API) of ion trap-, ion trap-Orbitrap-, or bench-top Orbitrap instruments. The ion source is equipped with a frequency-tripled Nd YAG laser operating at 355 nm with 3 ns pulse duration, while the laser beam is typically coupled into a 400 pm core diameter fiber forming a spot size of 500 x 600 pm on the plate (a smaller-diameter fiber can also be applied). The ease-of-use of this ion source for regular MALDI applications, and the possibility ofa rapid source exchange between MALDI and ESI, have led to this source becoming an excellent tool for today s analytical studies. Details on the coupling of this source with the Exactive series instrumentation are available from the manufacturer. 

The electrocharged aerosol particles interact with the surface, and as a result of their impact, the analyte is desorbed from the surface and ionized. Then, ions are transferred to the mass spectrometer by an ambient pressure sample transfer line, and mass spectra are recorded using an unmodified commercial mass spectrometer equipped with an atmospheric pressure interface (see Fig. 2). DESI ionize both small and large molecules (up to small proteins), and the efficiency of the desorption/ionization depends mainly on three factors (i) physicochemical properties of the analytes (ii) the spray mixture, i.e., the solvent composition, pH, viscosity, volatility, etc. and finally (iii) DESI surface on which the analyte is deposited. Indeed, the chemical composition, roughness, surface energy, and conductivity of the surface are all factors that determine the behavior of a DESI surface. 

Misharin A, Novoselov K, Laiko V, Doroshenko VM. Development and characterization of a field-deployable ion-trap mass spectrometer with an atmospheric pressure interface. Anal Chem. 2012 84 10105-12. 

Grimalt S, Pozo OJ, Marin JM, Sancho JV, Hernandez F. Evaluation of different quantitative approaches for the determination of noneasily ionizable molecules by different atmospheric pressure interfaces used in liquid chromatography tandem mass spectrometry abamectin as case of study. J Am Soc Mass Spectrom. 2005 16 1619-30. 

Fig. 13.1. Schematic of a DESI interface. A jet of gas and charged microdroplets is created by means of a standard pneumatic ESI sprayer and directed onto a sample surface at angle a. As a result, charged microdroplets containing ions of the surface material are created and transported away due to the action of the reflected gas stream and electric repulsion at angle p. A portion of the secondary ESI spray may be taken up by the atmospheric pressure interface of the mass spectrometer. Although at the expense of optimum sensitivity, an extended ion transfer line is normally employed to bridge the gap from surface to interface sampling orifice [1,12].

Figure 4 Atmospheric-pressure interface for LC-MS with electrospray and heated nebulizer APCI spray probes.

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