Sample probes

The sample chamber into which the sample holder is placed is called the probe in an NMR spectrometer. The probe holds the sample fixed in the magnetic field, contains an air turbine to spin the sample holder while the spectrum is collected and houses the coil(s) for transmitting and detecting NMR signals. A schematic of a probe is presented in Fig. 3.20(b). 

The probe is the heart of the NMR system. The most essential component is the RF transmitting and receiving coil, which is arranged to surround the sample holder and is tuned to the precession frequency of the nucleus to be measured. Modern NMR probes use a single wire coil to both excite the sample and detect the signal. The coil transmits 

For maximum sensitivity, a fixed frequency probe is needed. This means that a separate probe is required for each nucleus to be studied H, F, and so on. A 

Probes usually have variable temperature control to run experiments at temperatures selected by the analyst. Cryogenically cooled probes can improve the resolution of a system, so that a 600 MHz spectrometer equipped with such a probe can provide resolution equivalent to a 700-800 MHz instrument. New probe designs with flow-through sample holders are commercially available, for use in coupled HPLC-NMR instruments and HPLC-NMR-MS instruments. These hyphenated instruments are discussed under applications later in the chapter. 

The probe is installed in the spectrometer magnet so that the cods are centered in the magnet. The sample tube is inserted into the top of the probe and is moved by an air column through the magnet bore and centered among the probe coils. The tube exits the spectrometer at the top of the probe, again moved by an air column from the bore. 

For maximum sensitivity, a fixed-frequency probe is needed. This means that a separate probe is required for each nucleus to be studied H, C, F, and so on. A high-end probe costs on the order of 120,000. Much time can be lost in changing probes, which must be retuned before use, if one has to switch between nuclei frequently. Some variable frequency probe designs are available but have decreased power, sensitivity, and spectral quality compared with fixed-frequency probes. Probes for double-resonance experiments require two concentric coils for the two RF sources. Tripleresonance probes with many gradient options for liquids, solids, and flow experiments are available. 

The bore also contains air conduits for pneumatic sample changing and spinning of the sample holder in the magnetic field. The size of the bore determines how large a sample can be introduced into the magnetic field. Conventional analytical NMRs generally have bore diameters of 5-10 cm. 

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Fig. 11. In-pipe sampling probe having 0.635 cm dia sampling ports.

A sample is taken at several designated points in the stack, which represent equal areas. The sampling probe is placed at each samphng... 

When the operating conditions are uniform and steady (there are no fluctuations in flow rate or in concentration of CO in the gas stream), the continuous sampling method can be used. A sampling probe is placed in the stack at any location, preferably near the center. The sample is extracted at a constant sampling rate. As the gas stream passes through the sampling apparatus, any moisture or carbon dioxide in the sample gas stream is removed. The CO concentration is then measured by a nondispersive infrared analyzer, which gives direct readouts of CO concentrations. 

In the direct insertion technique, the sample (liquid or powder) is inserted into the plasma in a graphite, tantalum, or tungsten probe. If the sample is a liquid, the probe is raised to a location just below the bottom of the plasma, until it is dry. Then the probe is moved upward into the plasma. Emission intensities must be measured with time resolution because the signal is transient and its time dependence is element dependent, due to selective volatilization of the sample. The intensity-time behavior depends on the sample, probe material, and the shape and location of the probe. The main limitations of this technique are a time-dependent background and sample heterogeneity-limited precision. Currently, no commercial instruments using direct sample insertion are available, although both manual and h ly automated systems have been described. ... 

TTie system can be used for continuous measurement of the mass concentration at a single point for up to 12 hours, for traverse measurements of stack particulate mass concentrations using sample probe extensions, with the mass transducer up to 6 m in the stack, or for intermittendy measuring particulate mass concentrations of emission gases for long-term readings (e.g.. 30-sec samples every 60 minutes. ... 

Isokinetic A process in which the velocity at the entrance to the sample probe in a gas stream is the same as the velocity at a given point in the duct or stack at a given time. 

Pyrolyzer CDS Model Pyroprobe 2000 with coil sample probe and quartz boat sample holder Pyrolysis temperature about 650°C for 20 sec Pyrolysis sample size 0.2-0.3 mg GC Hewlett Packard Model 5890... 

Hand-lupe, /. hand lens, magnifying glass, -mulde,/. hand basin, hand bowl, -muster, n. hand specimen, small sample, -probe, /, hand (or small) test or sample. 

In large boiler plants, carryover is measured by employing a singleport sampling nozzle connected to a steam supply line between the top drum and the superheater. Sampling from superheaters is difficult, however, because a pump is needed to inject cool condensate water into a double-walled sample probe (via an attemperating nozzle). This is to remove the degrees of superheat and thus reduce the tendency for any contaminants to deposit in or on the sample probe, rather than be collected with the steam. 

From slope log nm x vs. log nhydruoirbon added uncertainty in slopes 0.1. 6 From small sampling probes... 

The main features of f.a.b.-m.s. are shown schematically in Fig. 1. The hardware consists of (i) an atom gun (or ion gun, see later) which is either mounted on the source housing of the mass spectrometer or, if small enough, inside the housing on the source itself, (it) a sample probe to the end of which is attached a small metal target onto which the sample is loaded, and (Hi) suitable source-optics for the efficient extraction of ions into the analyzer of the mass spectrometer. 

ESCA Sample Pretreatment. Samples were pelleted and cut to fit into a rectangular depression in an ESCA sample probe similar in design to one used by Hercules (16). The portion of the probe holding the catalyst sample could be withdrawn into an outer cylinder and sealed under an atmosphere of the pretreatment gas. For pretreatment the calcined samples were exposed to a hydrogen flow at one atmosphere and heated to AOO C. After this pretreatment the sanqile was withdrawn into the insertion tube, sealed in the pretreatment gas, inserted into the ESCA, evacuated, and then the ESCA spectra were recorded. A similar procedure was followed for the uncalcined catalysts except that the temperature was first increased in hydrogen flow to SOO C and held at this temperature for 3 to 4 hours the sample was then heated to 400 C and held at this temperature for 18 hours. 

In addition to being smooth, it is preferable that the soil surface be firmly packed. This is because loose soil is not always retained in large-diameter sampling probes. Firming of the soil surface may be accomplished using a turf roller or equivalent. Alternatively, the soil surface may be prepared in advance of study initiation to allow rainfall or irrigation to settle and firm the soil. This latter approach also allows soil surface depressions to be observed and avoided when laying out the test plots. 

Breath Connect Teflon sampling probe to analyzer and syringe through a sampling valve and loop insert probe 4 cm into mouth between closed lips withdraw 20 mL over 6 seconds into syringe flush and fill the sample loop with 10 mL mouth air carry sample to analysis in nitrogen gas. GC/FID 7ppb NR Blanchette and Cooper 1976... 

Gill, L. E., G. F. Hewitt, and P. M. C. Lacey, 1963, Sampling Probe Studies of the Gas Core in Annular Two Phase Flow, Part II Studies of Flow Rates on Phase and Velocity Distributions, UK Rep. AERE-R-3955, Harwell, England. (3)... 

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