Heparan Sulfate (HS) and heparin are linear, heterogeneous carbohydrates of the glycosaminoglycan (GAG) family that are modified by N-acetylation, N-sulfation, O-sulfation, and uronic acid epimerization. HS interacts with growth factors in the extracellular matrix, thereby modulating signaling pathways that govern cell growth, development, differentiation, proliferation, and adhesion. HPLC-chip-based hydrophilic interaction liquid chromatography/mass spectrometry has emerged as a method for analyzing the domain structure of GAGs. However, analysis of highly sulfated GAG structures decasaccharide or larger in size has been limited by spray instability in the negative-ion mode. This report demonstrates that addition of post-column make-up flow to the amide-HPLC-chip configuration permits robust and reproducible analysis of extended GAG domains (up to degree of polymerization 18) from HS and heparin. This platform provides quantitative information regarding oligosaccharide profile, degree of sulfation, and non-reducing chain termini. It is expected that this technology will enable quantitative, comparative glycomics profiling of extended GAG oligosaccharide domains of functional interest.

The study of protein phosphorylation events is one of the most important challenges in proteome analysis. Despite the importance of phosphorylation for many regulatory processes in cells and many years of phosphoprotein and phosphopeptide research, the identification and characterization of phosphorylation by mass spectrometry is still a challenging task. Recently, we introduced an approach that facilitates the analysis of phosphopeptides by performing automated, online, TiO(2) enrichment of phosphopeptides prior to mass spectrometry (MS) analysis. The implementation of that method on a "plug-and-play" microfluidic high-performance liquid chromatography (HPLC) chip design will potentially open up efficient phosphopeptide enrichment methods enabling phosphoproteomics analyses by a broader research community. Following our initial proof of principle, whereby the device was coupled to an ion trap, we now show that this so-called phosphochip is capable of the enrichment of large numbers of phosphopeptides from complex cellular lysates, which can be more readily identified when coupled to a higher resolution quadrupole time-of-flight (Q-TOF) mass spectrometer. We use the phosphochip-Q-TOF setup to explore the phosphoproteome of nonstimulated primary human leukocytes where we identify 1012 unique phosphopeptides corresponding to 960 different phosphorylation sites providing for the first time an overview of the phosphoproteome of these important circulating white blood cells.

Protein phosphorylation may be the most widespread and possibly most important post-translational modification (PTM). Considering such a claim, it should be no surprise that huge efforts have been made to improve methods to allow comprehensive study of cellular phosphorylation events. Nevertheless, comprehensive identification of sites of protein phosphorylation is still a challenge, best left to experienced proteomics experts. Recent advances in HPLC chip manufacturing have created an environment to allow automation of popular techniques in the bioanalytical world. One such tool that would benefit from the increased ease and confidence brought by automated 'nanoflow' analysis is phosphopeptide enrichment. To this end, we have developed a reusable HPLC nanoflow rate chip using TiO 2 particles for selective phosphopeptide enrichment. Such a design proved robust, easy to use, and was capable of consistent performance over tens of analyses including minute amounts of complex cellular lysates.

We report an experimental study of separation efficiency in microchip high-performance liquid chromatography (HPLC). For this study, prototype HPLC microchips were developed that are characterized by minimal dead volume, a separation channel with trapezoidal cross section, and on-chip UV detection. A custom-built stainless steel holder enabled microchip packing under pressures of up to 400 bar and ultrasonication. Bed densities were investigated with respect to the packing conditions and consistently related to pressure drop over the packed microchannels and separation efficiency under isocratic elution conditions. The derived plate height curves show a decrease of mobile phase mass transfer resistance with increasing bed density. High bed densities are critical to separation performance in noncylindrical packed beds, because only at low bed porosities does hydrodynamic dispersion in noncylindrical packings come close to that of cylindrical packings. At higher bed porosities, the presence of fluid channels of advanced flow velocity in the corners of noncylindrical packings affects hydrodynamic dispersion strongly. We demonstrate that the separation channels of HPLC microchips can be packed as densely as the cylindrical fused-silica capillaries used in nano-HPLC and that consequently microchip-HPLC separation efficiencies comparable to those of nano-HPLC can be achieved.

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The concept and performance of the first multiwavelength deep UV light-emitting-diode-based high-performance liquid chromatography (HPLC) absorbance detector are presented. In single-wavelength mode and with optical reference, the limit of detection (LOD) is comparable to conventional state-of-the-art HPLC absorbance detectors. In multiwavelength mode--at present up to eight wavelengths without optical reference--the LOD is about 10 times higher than in single-wavelength mode. Multiplexing and demultiplexing methods are used to separate chromatographic signals in multiwavelength mode and keeps the detector configuration simple and yet flexible. Depending on the operation mode, stray light is either totally negligible or controlled electronically and digitally.

Absorption spectrophotometry has been and still is the industry standard for detection in HPLC. Limit of detection (LOD) and linear dynamic range (LDR) are the primary performance requirements and have driven continuous improvement of spectrophotometric HPLC detectors. Recent advances in HPLC column technology have led to low flow-rate HPLC such as capillary HPLC and nanoflow HPLC and put higher demands on optical HPLC signal detection. However, fundamental principles in spectrophotometric HPLC detection have not been reviewed for many years. In particular the relationship between the detector's signal-to-noise ratio (SNR) and band broadening needs to be re-evaluated. In this work, a new quantitative model is presented which allows the calculation of the trade-off made between chromatographic resolution and SNR in spectrophotometric HPLC detection. Modern optics flow cells based on total internal reflection are included and compared to conventional flow cells.

Height- and area-based quantitation reduce two-dimensional data to a single value. For a calibration set, there is a single height- or area-based quantitation equation. High-speed high-resolution data acquisition now permits rapid measurement of the width of a peak (W), at any height h (a fixed height, not a fixed fraction of the peak maximum) leading to any number of calibration curves. We propose a width-based quantitation (WBQ) paradigm complementing height or area based approaches. When the analyte response across the measurement range is not strictly linear, WBQ can offer superior overall performance (lower root-mean-square relative error over the entire range) compared to area- or height-based linear regression methods, rivaling weighted linear regression, provided that response is uniform near the height used for width measurement. To express concentration as an explicit function of width, chromatographic peaks are modeled as two different independent generalized Gaussian distribution functions, representing, respectively, the leading/trailing halves of the peak. The simple generalized equation can be expressed as W = p(ln h̅), where h̅ is h/h, h being the peak amplitude, and p and q being constants. This fits actual chromatographic peaks well, allowing explicit expressions for W. We consider the optimum height for quantitation. The width-concentration relationship is given as ln C = aW + b, where a, b, and n are constants. WBQ ultimately performs quantitation by projecting h from the width, provided that width is measured at a fixed height in the linear response domain. A companion paper discusses several other utilitarian attributes of width measurement.