Improving Label-Free Quantification of Plasma and Serum Proteins Using a High-Resolution Hybrid Orbitrap Mass Spectrometer

Significance of the topic

The ability to quantify proteins in plasma and serum without labels is critical for high-throughput biomarker discovery workflows. Label-free relative quantification simplifies sample handling and reduces complexity compared to isotopic labeling while maintaining high resolution and mass accuracy requirements for accurate peptide identification and quantification in complex biological samples. The implementation of real-time targeted acquisition strategies promises to improve sensitivity and confidence in quantitative proteomics at low abundance levels in challenging matrices such as human plasma.

Objectives and study overview

This study compares traditional MS1-based extracted ion chromatography quantification with a novel real-time intelligent MS2 acquisition scheme on a hybrid Orbitrap mass spectrometer. A spectral library was generated from data-dependent acquisition of eight non-human standard proteins spiked into human plasma, and the performance of full-scan MS1 and targeted MS2 quantification was evaluated across protein loads ranging from 0.5 to 500 femtomoles on column. The goal was to assess quantitative confidence, sensitivity, and false-positive rates in a label-free workflow.

Methodology and instrumentation

Sample preparation included digestion of equimolar mixtures of eight standard proteins (cytochrome c, α-lactalbumin, serum albumin, carbonic anhydrase, ovalbumin, α-S1-casein, α-S2-casein, β-casein) spiked into 1 ug of human plasma at concentrations from 0.5 to 500 fmol on column. Analyses were performed on a Thermo Scientific Q Exactive mass spectrometer equipped with a Nanospray Flex Ion Source. Initial unbiased data-dependent MS/MS runs generated a spectral library of 170 peptides with precursor m/z, product ions, relative abundance distribution, and retention time. Real-time targeted acquisition used this library via an inclusion list to trigger MS2 scans based on isotope patterns and retention windows. Data processing and quantification utilized Thermo Scientific Proteome Discoverer 1.3 and Pinpoint 1.3 software.

Main results and discussion

The spectral library look-up table enabled real-time state-modeled acquisition of targeted peptides, enhancing selectivity by matching both precursor isotopes and product ions during elution. MS2-based quantification demonstrated equivalent performance to MS1 XIC above 2 fmol loads but provided substantially higher confidence below this threshold. At 0.5–1 fmol levels, MS2 acquisition reduced false positives by 25–50% compared to MS1. Reproducibility assessments showed lower variance in MS2 peak areas across replicate injections. Detailed peak profile comparisons illustrated clearer signal definition and reduced interference in MS2-targeted scans.

Benefits and practical applications

Real-time intelligent MS2 acquisition offers improved sensitivity and accuracy for low-abundance peptides in complex matrices, enabling more reliable biomarker discovery and verification assays. Automated method building and dynamic inclusion lists streamline workflows, reduce manual intervention, and increase throughput. This strategy is directly applicable to targeted proteomics studies requiring high multiplexing and quantitative precision without the need for isotopic labels.

Future trends and possibilities

Advancements may include integration of machine learning algorithms for automated method optimization, expansion of spectral libraries to encompass broader proteomes and post-translational modifications, and cloud-based data processing to further accelerate discovery pipelines. Combining real-time acquisition with novel ion mobility separations and advanced fragmentation techniques could enhance selectivity and depth of coverage in next-generation proteomic analyses.

Conclusion

The implementation of real-time state-modeled MS2 acquisition on a high-resolution Orbitrap platform significantly enhances label-free quantification sensitivity and specificity, particularly at low femtomole levels. Compared to traditional MS1 XIC approaches, this intelligent targeted strategy reduces false positives and improves reproducibility, offering a powerful tool for biomarker research and quantitative proteomics in complex biological samples.

References

1. Prakash A.; Peterman S.; Frewen B.; et al. Improving throughput for highly multiplexed targeted quantification methods using novel API-remote instrument control and state-model data acquisition schemes. 61st ASMS Conference on Mass Spectrometry and Allied Topics, Minneapolis, MN, June 9–13, 2013. Poster TP08 – Peptides: Quantitative Analysis I, poster number 131.