Instrumentation and Sample Handling

Beginning in 1953 with the first commercial NMR spectrometer, the early instruments used permanent magnets or electromagnets with fields of 1.41, 1.87, 2.20, or 2.35 T corresponding to 60, 80, 90, or 100 MHz, respectively, for proton resonance (the usual way of describing an instrument). 

The horsepower race, driven by the need for higher resolution and sensitivity, has resulted in wide use of 300-600 MHz instruments. All of the instru- 

The sample (routinely a solution in a deuterated solvent in a 5-mm o.d. glass tube) is placed in the probe, which contains the transmitter and receiver coils and a spinner to spin the tube about its vertical axis in order to average out field inhomogeneities. 

The proton spectrum obtained either by CW scan or pulse FT at constant magnetic field is shown as a series of peaks whose areas are proportional to the number of protons they represent. Peak areas are measured by an 

FIGURE 3.14 Schematic diagram of a CW NMR spectrometer. The tube is perpendicular to the z axis of the magnet A, sample tube B, transmitter coil C, sweep coils D, receiver coil E, magnet 

The second problem is pictorial. How can these phenomena be presented in the conventional, static, Cartesian frame of reference We avoid the complexities by using a rotating frame of reference. 

The horsepower race, driven by the need for higher resolution and sensitivity, has resulted in wide use of 200-500 MHz instruments and in the production of 800-MHz instruments. All of the instruments above 100 MHz are based on helium-cooled superconducting magnets (solenoids) and operate in the pulsed FT mode. The other basic requirements besides high field are fre- 

The spectrum obtained either by CW scan or pulse FT at constant magnetic field is shown as a series of peaks whose areas are proportional to the number of protons they represent. Peak areas are measured by an electronic integrator that traces a series of steps with heights proportional to the peak areas (see Fig. 4.22). A proton count from the integration is useful to determine or confirm molecular formulas, detect hidden peaks, determine sample purity, and do quantitative analysis. Peak positions (chemical shifts, Section 4.7) are measured in frequency units from a reference peak. 

A routine sample for proton NMR on a 300-MHz instrument consists of about 2 mg of the compound in about 0.4 mL of solvent in a 5-mm o.d. glass tube. Under favorable conditions, it is possible to obtain a spectrum on 1 /Ltg of a compound of modest molecular weight in a microtube (volume 185 /j ) in a 300-MHz pulsed instrument. Microprobes that accept a 2.5 mm or 3-mm o.d. tube are convenient and provide high sensitivity. A capillary microprobe that accepts a few nanograms of material in a few nanoliters of solvent is under development.  

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Sample preparation and measurement procedures are very important, especially for infrared methods of analysis. A brief discussion of instrumentation and sample handling accessories, along with a summary of the most common sample handling methods, is provided in Sec. 5. Raman spectroscopy is quite diflerent from infrared spectroscopy, insofar as there is... 

The LB films were prepared by the previously reported technique [4, 5]. The instruments and sample-handling technique adopted in this work for reeording the IR transmission and reflection-absorption spectra were the same as those described previously [5]. 

John Wiley Sons, Inc., New York (1963). Discusses theory, instrumentation, and sample-handling techniques. 

Handbook of Industrial Infrared Analysis, R. G. White, Plenum Press, New York (1964). A discussion of instrumentation and sample handling, plus spectrogram interpretation. 

Laboratory Methods in Infrared Spectroscopy, R. G. J. Miller, Heyden and Son, London (1965). Deals with infrared instrumentation and sample-handling techniques. 

Instrumentation and sample handling considerations are reflected in the applications of IR and Raman spectroscopy. Raman spectroscopy is effective for aqueous solutions and in low-frequency ranges. 

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