Signal Enhancement in HPLC/Microcoil NMR Using Automated Column Trapping

Department of Chemistry, Purdue University, ウェストラファイエット, Indiana, United States
Analytical Chemistry (Impact Factor: 5.64). 11/2006; 78(20):7154-60. DOI: 10.1021/ac0605748
Source: PubMed


A new HPLC NMR system is described that performs analytical separation, preconcentration, and NMR spectroscopy in rapid succession. The central component of our method is the online preconcentration sequence that improves the match between postcolumn analyte peak volume and microcoil NMR detection volume. Separated samples are collected on to a C18 guard column with a mobile phase composed of 90% D2O/10% acetonitrile-D3 and back-flushed to the NMR microcoil probe with 90% acetonitrile-D3/10% D2O. To assess the performance of our unit, we separated a standard mixture of 1 mM ibuprofen, naproxen, and phenylbutazone using a commercially available C18 analytical column. The S/N measurements from the NMR acquisitions indicated that we achieved signal enhancement factors up to 10.4 (+/-1.2)-fold. Furthermore, we observed that preconcentration factors increased as the injected amount of analyte decreased. The highest concentration enrichment of 14.7 (+/-2.2)-fold was attained injecting 100 microL of solution of 0.2 mM (approximately 4 microg) ibuprofen.

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    • "When samples are mass-limited, reducing the detection volume to match the sample size offers enhanced signal-to-noise ratio (SNR) performance, and significant efforts have been undertaken to perfect high-resolution spectroscopy in very small coils [1] [2] [3] [4] [5] [6] [7] [8] [9]. The integration of NMR with separation techniques such as liquid chromatography (e.g., [10]) or capillary electrophoresis [11] [12] proceeds more naturally when the NMR detection volume can be made compatible with the very small sample volumes and fluid handling tubing typical of the separation step. "
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    ABSTRACT: Microcoil probes enclosing sample volumes of 1.2, 3.3, 7.0, and 81 nanoliters are constructed as nuclear magnetic resonance (NMR) detectors for operation in a 1 tesla permanent magnet. The probes for the three smallest volumes utilize a novel auxiliary tuning inductor for which the design criteria are given. The signal-to-noise ratio (SNR) and line width of water samples are measured. Based on the measured DC resistance of the microcoils, together with the calculated radio frequency (RF) resistance of the tuning inductor, the SNR is calculated and shown to agree with the measured values. The details of the calculations indicate that the auxiliary inductor does not degrade the NMR probe performance. The diameter of the wire used to construct the microcoils is shown to affect the signal line widths.
    Journal of Magnetic Resonance 10/2007; 188(1):74-82. DOI:10.1016/j.jmr.2007.06.008 · 2.51 Impact Factor
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    ABSTRACT: Recent advances in microcoil NMR have provided commercially available, robust methodologies for analyzing mass and volume limited samples in the low microliter regime, and the technology has been applied in a number of areas. Unfortunately, due to constraints on sample size and the limited solubility of some compounds of interest, the application of this approach to certain areas of development, such as the structural analysis of chromatography eluates, is restricted. A current challenge is to provide an option within a previously unexplored sample size regime (tens of microliters) while still taking advantage of the increase in mass sensitivity afforded by solenoidal microcoil NMR. In this article, we present the design and construction of a microcoil NMR probe with a custom detection cell for the routine analysis of 20-μL samples. The detection cell is comprised of a CO2-laser-heated HF-etched borosilicate active volume with fused silica transfer lines added to provide sample input and output. This setup produces an enlarged sample bubble within the detection coil and provides easy connection with 1/16 in. standard LC connections, lending itself to applications with HPLC-NMR, online SPE and similar separation techniques, as well as higher-throughput robotic automation. NMR performance characteristics determined using standard compounds showed the probe exhibited reasonable resolution (<0.01 ppm), although sensitivity was less than optimal due to tuning constraints. Future improvements and opportunities are also discussed. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 33B: 1–8, 2008
    Concepts in Magnetic Resonance Part B Magnetic Resonance Engineering 02/2008; 33B(1):1 - 8. DOI:10.1002/cmr.b.20101 · 0.69 Impact Factor
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    ABSTRACT: We demonstrate remote detection of nuclear magnetic resonance (NMR) with a microchip sensor consisting of a microfluidic channel and a microfabricated vapor cell (the heart of an atomic magnetometer). Detection occurs at zero magnetic field, which allows operation of the magnetometer in the spin-exchange relaxation-free (SERF) regime and increases the proximity of sensor and sample by eliminating the need for a solenoid to create a leading field. We achieve pulsed NMR linewidths of 26 Hz, limited, we believe, by the residence time and flow dispersion in the encoding region. In a fully optimized system, we estimate that for 1 s of integration, 7 x 10(13) protons in a volume of 1 mm(3), prepolarized in a 10-kG field, can be detected with a signal-to-noise ratio of approximately 3. This level of sensitivity is competitive with that demonstrated by microcoils in 100-kG magnetic fields, without requiring superconducting magnets.
    Proceedings of the National Academy of Sciences 03/2008; 105(7):2286-90. DOI:10.1073/pnas.0711505105 · 9.67 Impact Factor
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