Nuclear Magnetic Resonance (NMR) is the youngest of spectroscopic techniques. Even though all other forms of atomic and molecular spectroscopy were theoretically predicted and experimentally proven already in the 19th or very early 20 th century,  it was only in 1924 that Pauli predicted the possibility of nuclear spin. This was still only theory and mostly an extension to the theory about the electronic spin. In 1933 Stern discovered and measured the magnetic moment of the electron thus showing that the theory was correct Stern was awarded the Nobel prize in physics in 1943 for this discovery. The next step was in 1938 when Rabi measured proof of nuclear magnetism in molecular beams thus providing even more confidence to the theory about nuclear spin.

The real breakthrough came in 1946 when independently Felix Bloch in Stanford University and Edward Purcell in Harvard University measured actual NMR signals in liquids and solids. The lines observed were very broad with today's standards but still proved that the theory laid down more than 20 years ago was valid. For this discovery Bloch and Purcell shared the Nobel Prize in physics in 1953.

In the very beginning NMR was a physics curiosity. It was regarded as a new tool for measuring magnetic fields. It was on the basis of this discovery and the invention of the klystron microwave tube that Varian Associates was formed in 1947 by the brothers Russel and Sigurd Varian.

The breakthrough for chemistry came in 1950 when Warren Proctor discovered that on the 14N NMR spectrum of NH4NO3 there were two lines (or bands as they were referred to back then) observed.

This was totally unexpected as everyone thought that there should be a single signal. This experimental discovery was quickly attributed by the physicist doing the study as an 'annoying chemical affect which could terribly impede our progress in trying to measure the magnitude of nuclear magnetic moments'. It was attributed to the different amount of shielding of the 14N nuclei by the electrons surrounding them. NMR instrumentation kept improving at a very rapid pace and already in 1951 Proctor was able to see the first spin-spin couplings while measuring the magnetic moments of the antimony isotopes in NaSbF5. Again a single line was expected but instead 5 were observed. A bit of improvement of the experimental conditions showed that the lines were actually 7, resulting from the coupling with the 6 19F nuclei. In 1951 Packard, Arnold and Dharmatti saw chemical shift effect in hydrogen spectra thus paving the way for NMR to be used for chemical structure elucidation.

The first commercial NMR spectrometer was produced in 1953 by Varian Associates. It was a 30 MHz system with a permanent magnet. The main market for the product back then was for the petroleum industry and indeed the first one was sold to Standard Oil. It was then that the first NMR applications laboratory was opened and James Schoolery was the first ever NMR applications scientist employed by Varian.

In 1961 Varian introduced the A60 NMR spectra meter. The A60 was a 60 MHz spectrometer with a permanent magnet. It had a transmitter and a receiver working at a matched frequency equal to the Larmor frequency of the proton. The spectrum was obtained by sweeping the magnetic field in a slow way and recording the signal detected at the receiver. A single spectrum took quite a few minutes and these type of instruments were called Carrier Wave, CW-NMR, There were hundreds of A60s sold around the world and it was the NMR spectrometer that brought NMR to the routine organic chemist. The A60 was designated a National Historic Chemical Landmark by the American Chemical Society in 2011.

The next important milestone for NMR came in 1965  when Wes Anderson and Richard Ernst, while working  at the Varian Applications Laboratory in Palo Alto,  developed the Fourier Transform method for recording  NMR spectra. They were trying to solve the problem that plagued all CW-NMR instruments, that of very  low sensitivity. Ernst and Anderson realised that one  source of the problem was the fact that at any given : moment on a CW instrument one was observing only a single frequency. They thought that if they somehow managed to excite all frequencies at the same time and, then detect the signal emitted by the nuclei as they  relax back to the ground state they will be able to have all the information in one go after applying a Fourier Transformation to the detected signal.

The initial idea was to use a multichromatic RF source generated by a  frequency generator coupled to a noise source. They quickly realised that the same effect could be achieved by means of a radiofrequency pulse of high intensity. After detecting several such Free Induction Decays,  FIDs, and spending hours waiting for the central Varian Corporate computer to finish payroll processing so that it could perform the overnight Fourier Transformation the first FT-NMR spectra were recorded. The introduction of FT-NMR offered an instant factor of 10 improvement in NMR sensitivity and paved the way for the developments that followed.

In the early 1970's several people thought that NMR  had achieved what it could and that there would be  nothing more important developed. This proved to  be totally wrong as the 70s saw probably some of the most important developments in modern NMR that established the technique.

To begin with, in 1971 Jean Jeener came up with the idea of two dimensional NMR. He explained the principle at   the Ampere Summer School in BaskoPolje, Yugoslavia  and a very careful Richard Ernst was in the audience taking detailed notes of everything. Back in his lab in   ETH Zurich, Ernst and his group, comprising of Walter Aue and Anil Kumar, succeeded in recording the first 2D   experiment, a Correlation Spectroscopy, COSY spectrum.

Very soon afterwards and with the help of improved computers more 2D NMR experiments came to life like the homonuclear 2D-J resolved spectrum, the Exchange Spectroscopy (EXSY), the Nuclear Overhauser Effect Spectroscopy (NOESY) and so on. Richard Ernst was awarded the Nobel Prize in Chemistry in 1991 for his contribution to NMR Spectroscopy.

In 1972 Alexander Pines, while working in the lab of John Waugh at MIT, developed the method of Cross Polarization which, together with magic angle spinning made NMR in solid materials possible. Previously NMR of solids consisted of very broad and weak signals. The properties of liquids spectra, like chemical shifts and spin-spin couplings were not realty visible and solids NMR was still just a physics curiosity. It was discovered that spinning the samples at a very high rate while being at an angle of 56.4 degrees to the magnetic field (the magic angle) made the signals significantly narrower and chemical shifts and couplings could be observed. Sensitivity, though, was quite low, mostly because people were observing the 13C nucleus which provided more information because its chemical shift range is far larger than that of proton. Pines thought that if he could transfer some magnetization from the highly abundant protons to the carbons the sensitivity would improve. Hence Cross Polarization-Magic Angle Spinning, CP-MAS was born and NMR became practical for solid materials as well.

In 1973 Peter Mansfield and Paul Lauterbur came with the idea of recording NMR spectra in inhomogeneous magnetic fields. This was quite ironic as a considerable effort was made to have very homogeneous magnets for NMR. They proved that by having a magnetic field gradient of a specific shape one is able to generate images of living organisms containing water. Thus the most widely known application of NMR was developed, Magnetic Resonance Imaging. MRI is one of the most widely used medical imaging techniques being advantageous to any other because it is non-invasive and does not involve any form of harmful radiation. The annual MRI market is estimated to be around 5-6 billion dollars today thus making MRI by far the most widely used form of NMR. Mansfield and Lauterbur were awarded the Nobel Prize in Physiology or Medicine in 2003.

In 1979 Gareth Morris and Ray Freeman devised a technique which they named INEPT that allowed them to observe insensitive nuclei in liquids and, consequently, obtain correlation information between two different nuclei. This was enhanced later on with the addition of indirect experiments, by Luciano Muelter, where it was shown that one can obtain information about insensitive nuclei by looking at sensitive ones. This way some of the limitations of NMR for interesting but 'difficult' nuclei were lifted and experiments using biologically important nuclei like 15N became possible.

Towards the late 1970s Kurt Wuetrich started experimenting with the NMR spectra of proteins and established methods for assigning the spectra and solving the structures of them. This was the first time that the accurate structure of a protein in solution was possible to be observed and gave insight to the mechanisms of protein actions and interactions with substrates. To this date NMR is the only physical method that can give the protein structure down to the atomic level in solution.

On the instrumentation front the 1970s saw the introduction of the superconducting NMR magnets that provided a huge leap in magnetic field strength. A higher magnetic field offers more sensitivity and spectral dispersion and it is always something desirable by NMR spectroscopists. Superconducting magnets provided a jump from the then 60 MHz permanent magnets and 100 MHz electromagnets to the 200 MHz magnets, a jump by a factor of 2-3 which completely transformed the field. Today NMR instruments with superconducting magnets upto 1 GHz field strength exist.

The 1980s, 1990s and 2000s saw also quite a bit of development in the NMR methodology but nothing as what was discovered in the 1970s. Developments included the wider use of field gradients in NMR Spectroscopy which allowed a greater variety of much 'cleaner' experiments, the introduction of the fast NMR techniques (non-linear sampling, projection reconstruction, ultra-fast NMR etc) that allowed the easy acquisition of multidimensional spectra as well as the huge development of NMR pulse sequences. To date there are probably more than 5000 different NMR pulse sequences, experiments, that can be performed giving all sorts of varying information. The sensitivity of the method has improved by almost another two orders of magnitude by the development of the actual detectors, the NMR probes, and the introduction of the new generation cryogenically cooled probes. These use a helium cooling system to lower the temperature of the electronics and reduce the thermal noise. The signal to noise ratio is increased not only by increasing the signal but also by decreasing the noise. NMR has expanded from the routine organic chemistry laboratory to natural products, materials science, metabonomics, polymer analysis, protein structure and dynamics, food analysis and so on. NMR instruments range from multimillion dollar ultra high field systems for work in biology to desktop instruments used for food analysis. The samples studied can be liquids, solids, gels, pastes etc.

The type of experiments performed has also changed dramatically. In the first 3 decades of the technique the spectra recorded were predominantly one dimensional proton observe spectra. Some advanced instruments could also perform homonuclear decoupling experiments, a technique that helped a lot in structure determination. In the 1970s with the evolution of FT-NMR 13C spectra started being popular but still not that sensitive.

The two dimensional experiments that were invented in the 1970s found their way to the chemistry labs in the 1980s. By the 1990s all commercially available spectrometers could perform the basic and advanced 2D experiments that had been invented and the interest of the NMR spectroscopists shifted towards making the technique easier to use and faster. Techniques like gradient shimming were invented that made the procedure of optimising the homogeneity of the magnetic field much easier and reliable even for beginning users.

Protein NMR became really commonplace and specialised techniques using isotopically enriched techniques were invented that made possible the complete elucidation of the structure of a protein in solution. Towards the end of last century techniques like TROSY were invented that allowed the recording of spectra of very large proteins thus extending the useful range of molecular weights that could be studied. Also a new application appeared, that of protein screening. This involved the examination of whether a substrate interacts with a given protein. NMR proved to be very powerful in this as it can give an answer in a few minutes while any other technique requires at least hours. In this way very large libraries of potential active compounds could be screened in a few days.

The focus of NMR researchers in the 21st century is towards faster techniques. There have been reported methods that can record a full two dimensional experiment from a time of a few seconds down to a few milliseconds. Applying the same principles three dimensional spectra that would have normally required hours or even days can be recorded in minutes. This applies to both small molecules and biological samples.

The biggest development in pre-clinical MRI was probably the invention of functional MRI. fMRI is a technique that is used for brain imaging that allows to see which parts of the brain react to specific stimuli.

Also MRI started looking at other nuclei. It was the 1H nucleus that was traditionally observed as it is by far the most sensitive. The development of sensitive hardware allowed the detection of nuclei as 31P and 13C to be possible thus opening new pathways for the study of metabolism.

Thus, the NMR spectroscopy has become an indispensable tool across all segments. Currently, NMR spectrometers are deployed towards studies of organic and bioorganic molecules, natural products, and biological macromolecules by way of both solution and solid state NMR methods and such studies have more or less become routine. NMR spectroscopy has found its greater utilization and widespread usage in India in both government and private segments.

With reference to government segments, the usage of NMR in research institutions and laboratories has become very vital. With reference to private segments, pharmaceutical companies and their associated research and development centres are purchasing NMR spectrometers to augment their research in the areas of analytical, drug discovery, and new chemical entities. Contract research organizations and custom manufacturing facilities are purchasing NMR spectrometers to supplement the results of NMR along with results from other analytical techniques. While the government funded institutions and labo­ratories are purchasing NMR spectrometers upto 800 MHz, the private companies are routinely buying 400 MHz and 500 MHz NMR spectrometers.

The overall market scenario for NMR spectrometers in India is growing at a sustained phase. On an average, 18 to 20 systems are being purchased every year. While the 400 MHz system along with sample automation to achieve high throughput is the most purchased system, followed by 500 MHz, purchase of ultra high field systems such as 700 MHz and 800 MHz systems has started happening with an estimated purchase of two or three systems in a calendar year. As the collaboration between institutions in India and abroad is growing at a good pace by way of exchange programs and other academic programs, the purchase of 900 MHz systems and beyond can soon become a practice in India too. The promising future, by way of having more and more NMR systems installed, is placing a greater demand on manufacturers of NMR systems to offer systems with exceptional quality.                  

The author is  the European NMR Applications Laboratory manager responsible for NMR applications in the Europe, Middle East, Africa and India regions for Agilent