From MALDI and ESI to MAI

The achievable sensitivity and specificity of mass spectrometers makes them the premier analytical technology for both targeted and non-targeted analyses of minute amounts of material.  The high resolving power, along with the ability to obtain mass measurements that are frequently accurate enough to determine elemental composition, allows analysis of complex mixtures, especially when interfacing with chromatographic separations. And yet, despite huge successes, mass spectrometry (MS) still has ample room to grow.

Successful MS analysis begins with the ionization step, which converts molecules into gas-phase ions. In the early days of MS, compounds were vaporized and subjected to an energetic event, such as electron ionization of the gas-phase molecules, which made analysis of most biological compounds inaccessible. A great deal of research went into developing methods capable of converting nonvolatile compounds into (quasi) molecular gas-phase ions, culminating in the development of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) in the 1980s. These are now the most widely used ionization methods in MS, and both are capable of converting a wide range of (intact) compounds – regardless of volatility – efficiently into gas-phase ions.

Recently, these ionization methods have been supplemented by new ionization processes. One of these methods, termed matrix-assisted ionization (MAI), uses a matrix, similar to MALDI, but does not require a laser and produces multiply-charged ions nearly identical to ESI, without the use of high voltages. Astonishingly, MAI requires no external energy input to convert solid- or liquid-phase compounds to gas-phase ions; the energy that drives the ionization process is contained in the matrix and released when exposed to the vacuum of the mass spectrometer.  Simply inserting a small quantity of the matrix:analyte sample into the atmospheric pressure inlet designed for ESI produces abundant protonated (and deprotonated in the negative mode) ions from low femtomoles (ca. 10-14 moles) of analyte. Alternatively, the sample can be introduced directly into the vacuum region to achieve ionization – similar to MALDI but without the laser.  Using MAI, the expense of an ion source is eliminated while bewildering simplicity and competitive sensitivity is achieved – using the very same mass spectrometers optimized over the past 25 years for ESI or intermediate pressure MALDI.

Multiply-charged ions produced by MAI allow use of mass spectrometers that have a limited mass-to-charge (m/z) range, high mass resolution, and/or high performance fragmentation technology. For example, the singly-charged molecular ions of a small protein, such as insulin (5730 Da), produced by MALDI will have an m/z value that is outside the mass range of most of these mass spectrometers. For this reason, MALDI requires a laser and a specialized mass spectrometer for high-mass compounds, and is also poorly applicable to small compounds because of the high chemical background. In contrast, with MAI, even the 66,000 Da bovine serum albumin protein forms highly-charged molecular ions that fall within the range of mass spectrometers commonly used with ESI. Meanwhile, the same instruments are used with MAI to analyze and – if desired – quantify, for example, metabolites from undiluted smokers’ urine, illicit drug-addicted infant urine, and a schizophrenia drug from tissue surfaces.

MAI also has advantages relative to ESI in terms of reducing energy requirements and consumables, because it eliminates the need for a hot inlet and nebulizing/desolvation gases, while notably increasing the speed of analyses and simplifying field portable mass measurements. Examples include the membrane protein bacteriorhodopsin, protein toxins, and Ebola protein, all measured within a few seconds. Selective use of MAI matrices has allowed successful analysis of even a monolayer of compound on a surface where both ESI and MALDI have failed. This new ionization discovery is successful with applications that are difficult or impossible using the traditional ionization methods, and the simplicity diminishes the requirement for highly trained personnel. Thus, MAI is especially promising for applications outside of analytical laboratories, such as field portable MS, homeland security applications, as well as in clinical and bedside diagnostics.