General implementation of the ERETIC™ method for pulsed field gradient probe heads

Abstract

A capacitive coupling between a secondary radiofrequency (rf) channel and the gradient coil of a standard commercially available high resolution NMR spectrometer and probe head is described and used to introduce a low level exponentially damped rf signal near the frequency of the primary rf channel to serve as an external concentration standard, in analogy to the so-called ERETIC™ method. The stability of this inexpensive and simple to implement method, here referred to as the Pulse Into the Gradient (PIG) approach, is superb over a 14-h period and both gradient tailored water suppression and one-dimensional imaging applications are provided. Since the low level signal is introduced via the pulsed field gradient coil, the coupling is identical to that for a free induction signal and thus the method proves to be immune (within 5%) to sample ionic strength effects up to the 2 M NaCl solutions explored here.

Introduction

It is well known that nuclear magnetic resonance (NMR) spectroscopy is quantitative, i.e. measured peak intensities directly correspond to nuclear spin concentrations [1]. This spectral intensity dependence combined with chemical shift and scalar J coupling resolution make NMR spectroscopy a powerful tool since the relative number of the different chemical environments and their geometric relationship in a molecule can be determined from the NMR spectrum [2]. The absolute nuclear spin concentration within a given molecular environment or at a given chemical shift cannot be determined directly from the spectrum without exhaustive spectrometer calibration or the inclusion of an internal standard with known concentration. Provided that a “friendly” internal standard can be found, i.e. a compound that does not react with, exists in the same phase as, and has a chemical shift different than the sample being studied or the analyte, the problem of absolute quantitation is solved. Here the ratio of the integrated peak intensity of the analyte to that for the standard multiplied by the standard concentration gives the analyte concentration. However, there are many situations where an internal standard cannot be used [3], [4], [5] and external referencing is required. One alternative approach that uses an electronic reference (ERETIC™) has become an increasingly popular option [6]. The ERETIC™ method uses a low voltage, exponentially damped, synthetic radiofrequency (rf) signal near the Larmor frequency of the analyte to effectively provide an external concentration standard with the main advantage being that the corresponding signal intensity is a faithful representation of electrical variations in the NMR detection circuitry. While all ERETIC™ implementations require either an additional spectrometer channel or a high frequency waveform generator clocked with the NMR instrument, two distinct modes of coupling the ERETIC™ signal into the NMR detection electronics have been presented in liquids and solids NMR as well as magnetic resonance imaging [6], [7], [8], [9], [10], [11], [12], [13]. However, both approaches have their limitations. Historically, the first ERETIC™ applications coupled the low voltage calibration signal into the NMR detection electronics with a broadband untuned coil, an approach most efficiently and reproducibly accomplished by mounting the ERETIC™ antenna inside of the NMR probe head close to the rf pulsing/receiving coil [6], [7], [10]. Drawbacks to this approach are obvious. The NMR probe head must be customized to accommodate the broadcast antenna, an option that is not viable for most commercial NMR instrumentation, and the antenna also behaves as a receiver picking up high power rf pulses, an effect that at best decreases probe head efficiency and at worst damages the ERETIC™ signal generator. Both of these problems have been circumvented in more recent ERETIC™ applications, which use a directional coupler on the second channel of a double resonance NMR spectrometer and probe head to present the low voltage ERETIC™ signal to the sample and primary detection channel [8], [9], [11]. In this way the tuned LC circuit for one frequency is used to broadcast the ERETIC™ signal at another frequency. Although no NMR probe modifications are required, provided that a double resonance probe is used, in practice the method is extremely sensitive to the mutual coupling between rf channels, an effect that is controlled by the respective tuning and matching capacitors on both channels. In addition, as the rf isolation between channels improves, the performance of ERETIC™ drops. In spite of the concentration quantitation offered by the ERETIC™ method the implementation awkwardness described above limits its broad application, and thus alternative external referencing methods have been proposed [14].

This work describes a straightforward implementation of the ERETIC™ method that can be quickly integrated into standard spectrometers with the minimal addition of extra hardware. This scheme combines the positive aspects of both approaches described above to present the ERETIC™ signal to the detection electronics with only the addition of a simple capacitor. The method is completely general and can be implemented on any probe head equipped with a pulsed field gradient (PFG) coil, either in high-resolution or imaging setups, allowing the untuned PFG coil available in virtually all modern liquids or solids NMR probe heads to serve as an ERETIC™ antenna. An rf directional coupler is not needed as the ERETIC™ signal is capacitively coupled into the cable connecting the gradient amplifier to the gradient channel in the NMR probe head as shown in Fig. 1. The capacitive coupling of this method, hereafter referred to as the Pulse Into the Gradient (PIG) approach, has the benefit of electrically isolating the gradient channel from the low voltage ERETIC™ signal source, i.e. the long time scale μs–ms high current DC field gradient pulses do not feed back into the ERETIC™ channel, but the higher frequency ns–μs ERETIC™ signal couples into the gradient cable. The use of the gradient circuit included in the probe head offers several advantages in comparison to existing ERETIC™ signal coupling strategies. Since the PFG coil is not tuned, a flat frequency response is observed and the extremely small mismatch between rf and gradient coil positions along with their close proximity is enough to inductively couple the ERETIC™ signal into the detection electronics. The use of the untuned PFG coil as an ERETIC™ antenna guarantees the possibility of extending the method to very high field and thus Larmor frequencies of up to a few GHz. An additional attractive feature of the PIG approach is the concomitant increase in ERETIC™ signal stability offered by the properly shielded PFG coil and cables.