combination of liquid chromatography (LC) and nuclear magnetic resonance
(NMR) offers the potential of unparalleled chemical information from analysts
separated from complex mixtures.
magnetic resonance (NMR) detection coupled with liquid chromatography (LC)
offers great promise in combining the ability to separate complex mixtures
into individual components with one of the most structurally rich detection
schemes available. In 1978, Watanabe reported the coupling of LC effluent to
NMR using a stopped flow approach, and within 1 year, an on-line system had
been reported. The major advantages of on-line as opposed to off-line NMR
detection of LC are improved chromatographic resolution, consistent
response, on-line data analysis, and rapid data acquisition. The drawbacks
of continuous flow NMR include poorer sensitivity due to the limited time
available to measure each analyze and the flow rate dependence of the NMR
line width. Over the past 15 years, numerous groups have reported improved
LC-NMR hyphenation methods, improved NMR plus sequences (e.g., to remove the
effects of strong solvent resonance and hence alleviate the need for
deuterated solvents), and increased chromatographic resolution.
The principal drawback with these approaches has been the relatively
poor mass sensitivity of the NMR detection system, especially when the
observation time is limited for each analyte peak. Recent trends in LC
separations have been toward smaller diameter columns, smaller injections,
and faster separations, all of which make LC-NMR even more difficult.
Previous flow cells for NMR detection were in the range of 25-200 mL,
which the smallest detector cells greater than 20 mL.
Also problematic, the mass detection limits of previous on-line work appear
to be greater than 10 mg. Thus,
reduction in detector cell volume and improvements in mass sensitivity are
needed for improved smaller volume LC columns.
Recently we are planning to use radio frequency (RF) microcoils for NMR spectroscopy to create 5 nL to 1- mL volume detection cells. The noise in an NMR experiment is predominantly thermal noise and originates primarily within the conducting sample and rf detection coil. When microcoils (i.e., rf coils less than ~ 1mm in diameter) are used to examine small amounts of an aqueous or organic sample with high-field NMR spectrometers, the resistance of the coil dominates the resistance of the sample. Furthermore, when solenoidal coils are used, the resistance is relatively independent of coil diameter. Hence, as a coil is reduced in size, the mass sensitivity improves because of the increase in the strength of the rf field per unit current. Peck et al. studied this in detail, and found over 20-fold improvements in mass limits of detection as the coil diameter is scaled from 1 mm to 50 mm. This corresponds to a 400-fold increase in measurement time for the larger coil to obtain the same signal-to-noise ratio (SNR) as the smaller coil. Therefore, microcoils offer substantial advantages in NMR detectability for mass-limited samples such as the effluent form an LC column. Wrapping the microcoil directly around a section of capillary allows easy connection to the LC tubing and provides a high filling factor for enhanced NMR sensitivity.