Sub-microlitre dialysis system to enable trace level peptide detection from volume-limited biological samples using MALDI-TOF-MS

The detection of peptides with mass spectrometry from volume-limited biological samples is a challenging task due to low sample volume, a broad range of peptide concentrations down to trace levels, endogenous high proteins and salt levels. Previously, a microspotting method was presented for trace-level peptide detection with MALDI-MS from sub-microlitre samples with biological salt levels. However, in the presence of proteins, peptide signals are significantly reduced. This paper presents a novel dialysis device for removal of proteins from sub-microlitre samples using a semipermeable hollow fiber membrane to enhance peptide detection. A dialysis device was constructed to perform sub-microlitre dialysis to remove proteins from complex samples. Angiotensin I was used as a model peptide in the presence of 350 mg L^-1 BSA prepared in physiological saline to mimic biological samples. In the absence of BSA, clear angiotensin I peaks were seen at 250 pM, yet in the presence of the BSA, 10 nM angiotensin I was barely detected. After dialysis, peak detection was improved to a 500 pM level. Protein removal and peptide recovery (approximately 66%) were determined using CE-LIF. Clinical vitreous samples as low as 200 nL were successfully dialyzed in 30 min and a 3-fold increase in peptide peaks were detected with greatly improved signals. This method is simple and can be a useful technique for trace level peptide detection from volume-limited biological samples.