By utilizing delayed pulsed ion extraction of ions generated via the matrix-assisted laser desorption/ionization (MALDI) technique, fast (< 320 ns) metastable ion fragmentation is observed for both peptide and protein analytes in the ion source of a linear time-of-flight mass spectrometer. Small peptides such as the oxidized B chain of bovine insulin exhibit fragmentation at the amide linking bond between peptide residues. Overlapping sequence information is provided by fragmentation from both the C- and N-terminal ends of the peptide (cn-, yn-, and z*n-type fragment ions). Larger proteins can also exhibit a wealth of sequence specific fragment ions in favorable cases. One example is cytochrome c, which undergoes substantial (approximately 80%) fast fragmentation at the amide bonds along the amino acid backbone of the protein. Only amide bond cleavages initiating from the C-terminal end (cn fragments) are observed. The observed fragmentation pattern provides a significant amount of potential sequence information for these molecules. External mass calibration of the intact protonated molecular ions is demonstrated with mass accuracies typically around 100 ppm. Mass accuracies for the observed fragment ions ranged from +/- 0.20 Da for the smaller peptides studied (i.e., oxidized B chain of bovine insulin) to +/- 0.38 Da for the largest protein studied (cytochrome c), based upon the known sequences.

A linear time-of-flight mass spectrometer has been modified to incorporate pulsed ion extraction of matrix-assisted laser desorption/ionization (MALDI) generated ions. A unique aspect of the experiments presented is the combination of pulsed extraction with very high source potentials (up to 25 kV) which allows improved mass resolution while maintaining excellent sensitivity for the large m/z ions generated by the MALDI technique. Mass resolution in excess of 1000 (fwhm) is demonstrated for cytochrome c (12,361.1 Da) with the pulsed ion extraction linear time-of-flight mass spectrometer described. The influence on obtainable mass resolution of experimental variables such as delay time between laser ionization and ion extraction, amplitude of the pulsed voltage employed, and the source bias voltage are presented. It is shown that, for any given source potential, the optimum pulsed extraction voltage is a linear function of the mass of the analyte. This is consistent with the observation that the initial ion velocity distribution for MALDI-generated ions is independent of mass.

Background The application of whole-exome sequencing for the diagnosis of genetic disease has paved the way for systems-based approaches in the clinical laboratory. Here, we describe a clinical metabolomics method for the screening of metabolic diseases through the analysis of a multi-pronged mass spectrometry platform. By simultaneously measuring hundreds of metabolites in a single sample, clinical metabolomics offers a comprehensive approach to identify metabolic perturbations across multiple biochemical pathways. Methods We conducted a single- and multi-day precision study on hundreds of metabolites in human plasma on 4, multi-arm, high-throughput metabolomics platforms. Results The average laboratory coefficient of variation (CV) on the 4 platforms was between 9.3 and 11.5% (median, 6.5–8.4%), average inter-assay CV on the 4 platforms ranged from 9.9 to 12.6% (median, 7.0–8.3%) and average intra-assay CV on the 4 platforms ranged from 5.7 to 6.9% (median, 3.5–4.4%). In relation to patient sample testing, the precision of multiple biomarkers associated with IEM disorders showed CVs that ranged from 0.2 to 11.0% across 4 analytical batches. Conclusions This evaluation describes single and multi-day precision across 4 identical metabolomics platforms, comprised each of 4 independent method arms, and reproducibility of the method for the measurement of key IEM metabolites in patient samples across multiple analytical batches, providing evidence that the method is robust and reproducible for the screening of patients with inborn errors of metabolism.

Fragmentation processes that occur very early during matrix-assisted laser desorption ionization (MALDI) of peptides are examined by utilization of delayed pulsed ion extraction with a linear time-of-flight mass spectrometer. The oxidized B chain of bovine insulin (MW=3495. 95 u), which produces a wide range of fragment ions, is utilized as a probe to examine the effects of several experimental parameters on this process. Experimental evidence suggests that this MALDI process is not prompt fragmentation and involves metastable ion decay that is quite different from that which is observed with postsource decay experiments. This conclusion is based upon the significant differences observed in the fragmentation products produced by the two techniques. This metastable ion decay process also appears to be over within the minimum pulse delay period (320 ns) that is possible with the current pulsed ion extraction hardware. These two observations suggest that either different activation processes are involved in the two techniques or that the much different time frame of the methods influences the observed ion decay pathways. This fast MALDI metastable ion fragmentation also is shown to be influenced by both the MALDI matrix and the laser fluence.

A technique is described for identifying and locating posttranslational modifications (PTMs) in peptides and proteins of known sequence by interpretation of c(n) ion signals generated by in-source decay during delayed ion extraction in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Sites of phosphorylation in seven synthetic peptides were determined, as was the location of both the heme group and N,N,N-trimethyllysine in yeast cytochrome c. A semi-automated data analysis process facilitates the identification of segments of the sequence on each side of the PTM, permitting its placement at the junction of the segments and definition of the added mass. A graphical display facilitates illustration of both the location and mass of the PTM.