Sample insertion

To insert the sample, first push the sample tube gently into the spinner turbine and adjust the vertical position of the tube using a gauge to assure that the actual sample solution will be centered in the probe inside the RF coils. Place the sample on the air cushion at the top of the magnet bore and deactivate the eject air, and the sample will gently descend into the bore until the spinner rests on the probe with the bottom of the NMR tube inserted into the probe. 

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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. ... 

W. E. Petit and G. Horlick. Spect. Acta. 41B, 699, 1986. Describes an automated system for direct sample-insertion introduction of 10-pL liquid samples or small amounts (10 mg) of powder samples. 

The mass spectrometer should provide structural information that should be reproducible, interpretable and amenable to library matching. Ideally, an electron ionization (El) (see Chapter 3) spectrum should be generated. An interface that fulfils both this requirement and/or the production of molecular weight information, immediately lends itself to use as a more convenient alternative to the conventional solid-sample insertion probe of the mass spectrometer and some of the interfaces which have been developed have been used in this way. 

Labour intensive (no automatic weighing, problematic solid sample insertion)... 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

One of the disadvantages concerns the fact that in axial magnets it is rather difficult to use probes with solenoid RF coils. The difficulties are related to sample insertion/removal complications and to numerous spatial constraints, exacerbated by the presence of a glass dewar for sample-temperature control (see Section VI). This is unfortunate because the alternative saddle coils are substantially less efficient, especially at the relatively low excitation/detection frequencies used in FFC NMR. 

An alternative to a saddle coil would be a solenoid coil which, however, would have to be oriented perpendicularly to the magnet bore and thus to the physical axis of the probe assembly. Due to spatial constraints, such an arrangement complicates considerably sample insertion, especially when the sample temperature has to be controlled and the assembly has to include an enveloping dewar for temperature control of the sample. ... 

Key A, cylindrical microwave cavity 6, cylindrical tube for sample insertion C, rectangular waveguide D, nut for adjustment of sample position E, thermocouple ports. 

Fig. 4.9 FESEM images of a sample annealed at 400°C for 30 minutes showing (a) the uniformity of the pore formation across the sample (insert shows the pore diameter after annealing) (b) cross sectional image showing the pore depth as 383 nm, pore diameter as about 100 nm, and the barrier oxide thickness as approximately 600 nm.

Doty Scientific, Inc. (Columbia, SC, USA) has been relatively successful in incorporating high resolution capabilities into their more standard solid-state MAS probes,32 where, in a traditional application, the extra care may not ever be noticed, but it provides the ability to use one MAS probe for both solid and gel-phase samples. They have introduced a sample insert that permits ready preparation of a number of samples that can be inserted into a MAS rotor for spectral acquisition, then preserved (or discarded) without requiring the dedication of a more expensive rotor. 

Sample insertion and removal is facilitated by permitting horizontal movement... 

A simple example of the use of LD/FTMS for surface analysis may be found in the analysis of blue stain on the surface of a copper part. In order to determine the nature of the stain, we obtained LD/FTMS spectra for samples of both stained and unstained copper. Both samples were mounted on the automatically rotated sample insertion probe on the FTMS-2000. The samples were rotated after each laser shot to expose a fresh surface at the laser focus. Approximately twenty-five laser shots were signal averaged for each spectrum, in order to increase signal-to-noise, and to provide a spectrum which would represent the averaged composition of the surface. 

After sample insertion, the cavity is tuned to achieve critical coupling, so that the microwave power stored in the cavity is maximized while its dissipation is minimized. Tuning generally involves two steps roughtuning to approximately match the microwave frequency to the cavity resonance frequency followed by fine-tuning to establish critical coupling. 

For special applications direct current plasma (DCP) (Leis et al., 1989) and micro-wave-induced plasma (MIP) may be used. The MIP first became widely used as a spectroscopic radiation source after a stable discharge at atmospheric pressure had been obtained (Beenakker, 1977 Beenakker et al., 1978). The MIP is not capable of taking up wet aerosols, but is useful for the excitation of dry aerosols, produced by electrothermal evaporation from a graphite furnace (Aziz et a ., 1982). Direct sample insertion has been discussed recently by Blain and Savin (1992). 

Direct Sample Insertion. In direct sample insertion (DSI) [82], the sample is placed on a rod, metal loop, or cup on a rod. After desolvation (by inductive heating of the rod or use of a heat gun), the sample is inserted into the plasma. The advantages of the DSI system include nearly 100% sample transport efficiency into the ICP and use of a single power source. The most exciting capability of DSI is preconcentration using aerosol deposition that can provide two orders of magnitude of improvement in ICP-MS detection limits [83]. Detection limits as low as 0.06 parts per trillion were obtained. 

Figure 2 Schematic diagram illustrating the configuration of the quadrupole ion trap for laser ablation inside the ion trap. Samples inserted through the ring electrode are atomized and ionized in the trapping volume by the laser beam, entering the ring electrode from 180° relative to the sample. (From Ref. 30.)...

The G-BASE project collects samples in random number order (Plant, 1973), as this helps identify any correctable systematic errors introduced during sample preparation and analysis, processes in which the samples are handled in numeric order. For every block of one hundred numbers, five numbers are reserved for control samples so when they are submitted within a batch of samples they are blind to the analyst. The control samples inserted are one duplicate sample, two replicate samples, two blanks, and two secondary reference materials (SRM) used to monitor accuracy and precision as well as to level data between different field campaigns (see Johnson et al, 2008). Along with the original sample ofthe duplicate pair, this means 8% of samples submitted are control samples, a point not to be overlooked in setting the budget for analyses. 

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