Interferences and contaminants encountered in modern mass spectrometry

A large number of matrix substances have been used for various applications in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). The majority of matrices applied in ultraviolet-MALDI MS are crystalline, low molecular weight compounds. A problem encountered with many of these matrices is the formation of hot spots, which lead to inhomogeneous samples, thus leading to increased measurement times and hampering the application of MALDI MS for quantitative purposes. Recently, ionic (liquid) matrices (ILM or IM) have been introduced as a potential alternative to the classical crystalline matrices. ILM are equimolar mixtures of conventional MALDI matrix compounds such as 2,5-dihydroxybenzoic acid (DHB), α-cyano-4-hydroxycinnamic acid (CCA) or sinapinic acid (SA) together with organic bases [e.g., pyridine (Py), tributylamine (TBA) or N,N-dimethylethylenediamine (DMED)]. The present article presents a first overview of this new class of matrices. Characteristic properties of ILM, their influence on mass spectrometric parameters such as sensitivity, resolution and adduct formation and their application in the fields of proteome analysis, the measurement of low molecular weight compounds, the use of MALDI MS for quantitative purposes and in MALDI imaging will be presented. Scopes and limitations for the application of ILM are discussed.

Ionic (liquid) matrices for matrix-assisted laser desorption/ionization mass spectrometry—applications and perspectives

β-Amyloid Is Different in Normal Aging and in Alzheimer Disease

The mechanism of neurodegeneration caused by -amyloid inAlzheimer disease is controversial. Neuronal toxicity is exertedmostlybyvariousspeciesofsoluble -amyloidoligomersthatdifferintheirN-andC-terminaldomains.However,abundantaccumula-tion of -amyloid also occurs in the brains of cognitively normalelderlypeople,intheabsenceofobviousneuronaldysfunction.Wepostulatedthatneuronaltoxicitydependsonthemolecularcompo-sition,ratherthantheamount,ofthesoluble -amyloidoligomers.Hereweshowthatsoluble -amyloidaggregatesthataccumulateinAlzheimer disease are different from those of normal aging inregard to the composition as well as the aggregation and toxicityproperties.Aseriesofevidenceindicatesthatprogressivecerebralaccumulationof -amyloid (A ),

Amyloid Is Different in Normal Aging and in

Matrix-assisted laser desorption ionization (MALDI) has become an enabling technology for the fields of protein mass spectrometry (MS) and proteomics. Despite its widespread use, for example, in protein identification via peptide mass fingerprinting, a comprehensive model for the generation of free gas-phase ions has not yet been developed. All matrices in use today, such as α-cyano-4-hydroxycinnamic acid (CHCA), have been found empirically and stem from the early days of MALDI. By systematic and targeted variation of the functional groups of the α-cyanocinnamic acid core unit, 4-chloro-α-cyanocinnamic acid (Cl-CCA) was selected and synthesized, and it exhibited outstanding matrix properties. Key features are a substantial increase in sensitivity and a considerably enhanced peptide recovery in proteomic analyses because of a much more uniform response to peptides of different basicity. Using Cl-CCA as a matrix for a 1 fmol bovine serum albumin (BSA) in-solution digest, the sequence coverage is raised to 48%, compared with 4% for CHCA. For a gel band containing 25 fmol of BSA, unambiguous protein identification becomes possible with Cl-CCA. These findings also imply ion formation via a chemical ionization mechanism with proton transfer from a reactive protonated matrix species to the peptide analytes. The considerable increase in performance promises to have a strong impact on future analytical applications of MALDI, because current sensitivity limits are overcome and more comprehensive analyses come into reach.

https://www.pnas.org/content/pnas/105/34/12200.full.pdf

4-Chloro-α-cyanocinnamic acid is an advanced, rationally designed MALDI matrix

Progress in high-throughput MALDI-TOFMS analysis, especially in proteome applications, requires development of practical and efficient procedures for the preparation of proteins and peptides in a form suitable for high acquisition rates. These methods should improve successful identification of peptides, which depends on the signal intensity and the absence of interfering signals. Contamination of MALDI samples with alkali salts results in reduced MALDI peptide sensitivity and causes matrix cluster formation (widely reported for CHCA matrix) observed as signals dominating in the range below m/z 1200 in MALDI spectra. One way to remove these background signals, especially for concentrations of peptides lower than 10 fmol/microL, is to wash matrix/sample spots after peptide cocrystallization on the MALDI plate with deionized water prior to analysis. This method takes advantage of the low water solubility of the CHCA compared to its alkali salts. We report here that the application of some ammonium salt solutions, such as citrates and phosphates, instead of deionized water greatly improves the efficiency of this washing approach. Another way to reduce matrix cluster formation is to add ammonium salts as a part of the MALDI matrix. The best results were obtained with monoammonium phosphate, which successfully suppressed matrix clusters and improved sensitivity. Combining both of these approaches-the addition of ammonium salts in the CHCA matrix followed by one postcrystallization washing step with ammonium buffer-provided a substantial ( approximately 3-5-fold) improvement in the sensitivity of MALDI-MS detection compared to unwashed sample spots. This sample preparation method resulted in improved spectral quality and was essential for successful database searching for subnanomolar concentrations of protein digests.

I A Papayanopoulos

Papers

Suppression of alpha-cyano-4-hydroxycinnamic acid matrix clusters and reduction of chemical noise in MALDI-TOF mass spectrometry