Sampling heads

Condensation of high-boiling-point components in the line between sample head and sensor Hostile environment... 

Sampling mode effects The goal was to be able to measure at-line, with no sample preparation at all, using a fiber-optic probe or a remote sampling head. However, the area sampled by the hber-optic probe is much smaller than for the sample transport module. It was found that the remote (probe) spectra were very similar to the static (sample transport) spectra, but the baselines were shifted significantly higher and the absorbance peaks consequently reduced in intensity as before, the characteristic peak positions were not affected. Calibration models developed using spectra obtained with the hber-optic probe performed equivalently to those developed with the sample transport module. 

The optimal derivatization time was also tested. The ethanol solution spiked with 5 ppb of methional was exposed for 15, 30, 60, 90, and 120 min at 50 °C. it was determined that the time to reach equilibrium between stationary phase and sample head-space was 90 min (Figure 3). A derivatization time of 60 min at 50 °C appeared to be a good compromise between the time of reaction and analyte response. 

The commercially available Fido and Fido XT explosives detection systems are both handheld devices as shown in Figure 9.5. The Fido incorporates the sampling head directly into the body of the device. The XT version includes a tethered extension for the inlet that allows the sampling head to be separated from the rest of the instrument. The sampling head is mounted on a pistol grip so that sampling can be performed more conveniently (Fig. 9.6). 

In addition to the handheld models, Fido has been adapted for integration with robotic platforms most popular with those in the explosives detection community. As examples, these include the Foster Miller Talon, the iRobot Packbot, and the USMC Dragon Runner. The Fido XT tethered sampling head can thus be mounted on a robotic arm to accommodate a variety of operational concepts as shown in Figure 9.7. 

Sampling mode effects The goal was to be able to measure at-line, with no sample preparation at all, using a fiber-optic probe or a remote sampling head. However, the area sampled by the fiber-optic probe is much smaller than for the sample transport... 

A fast FID consists of a main control unit and two remote sampling heads (which house the FIDs). The dual channel nature of the instrument enables simultaneous real-time measurement in two locations allowing, for example, evaluation of catalyst performance. 

SPME can be used to extract semivolatile organics from environmental waters and biological matrices as long as the sample is relatively clean. Extraction of semivolatile organic compounds by SPME from dirty matrices is more difficult [134], One strategy for analyzing semivolatiles from dirty matrices is to heat the sample to drive the compound into the sample head-space for SPME sampling another approach is to rinse the fiber to remove nonvolatile compounds before analysis [134],... 

Experimental. The experimental set up consisted of a -pum-ped-dye laser (Molectron UV-14, DL-400), spatial filters to isolate the central part of the dye laser beam, a H2 02-Ar or N2 flame supported by a capillary burner with Ar or N2 sheath, and a fluorescence detection system at right angles (a JY-H-10 monochromator, a photomultiplier, and a PAR 162-164 boxcar averager). All measurements were taken 1 cm above the burner top the concentration of Ca, Sr, In, and Na was low ( 1 yg/ml). The fluorescence waveform was monitored with a 75 ps sampling head (PAR 163). The laser spectral bandwidth was also measured with a JY-HR-1000 monochromator (6AS < 0.1 ). 

Fig. 3.5-8 i finally exhibits another universal sample head. It utilizes an optical fiber (Christie and McCreery, 1990 Schrader et al. 1988) to transport the exciting radiation to the sample. A bundle of fibers, usually with a larger diameter (Fig. 3.3-7), is arranged around the central fiber in order to transport Raman radiation from the sample to the spectrometer. The connection to the spectrometer is at its optimum if the optical conductance of the fiber bundle is as high as that of the spectrometer. The end of the fiber bundle is covered with a shield (NMR tube) to prevent sticking and pyrolysis of particles at the end of the laser fiber. It can be used to cool the head if the laser power... 

Samples can be extracted from the product stream by the projeetion of a sample tube, containing a nozzle or orifice, into the flow. The particles impact on the tube and fill the open eavity. The sampling head is out of the stream when not sampling. The snorkel type sampler (Figure 1.9) is available for vertical or inclined applieations and can be pre-programmed on sampling frequency. It is not possible to sample non-homogeneous streams representatively with this type of device. 

Through the sampling head, the pump unit feeds air to the detection head which contains a sequentially advanced filter cartridge that captures alpha and beta emitters for detection. The detection type is gross counting for beta, gamma and spectrometry for alpha particulates using dual passivated silicon detectors. 

Headspace SPME involves three phases (viz. coating, headspace and matrix), the chemical potential difference of the analytes between the phases being the driving force that moves them from the matrix to the fibre coating. With aqueous samples, head-space/water partition coefficients are directly related to the Henry constants for the analytes, which are in turn determined by their volatility and hydrophobicity. 

The hair blood concentration ratio for total mercury is frequently cited as 250. However, a precise basis for this particular value is unclear. Ratios reported in the literature range from 140 to 416, a difference of more than a factor of 2.5 (see Table 2-9). Differences in the location of hair sampled (head versus chest, distance of sample from head or skin) may contribute to differences in observed ratios between studies. 

Then, sample heads are taken randomly for pole-tip geome-try measurements after lapping and then good individual sliders are sorted out. 

Taken in bathroom/shower and run concurrently with number 4. Sample head was hung inside shower... 

Figure 1. DSC sample head with LEDs mounted in Teflon support block.

Fig. I. (a) Details of the sample mounting on the transfer rod for resistive heating (via Ta wires) and liquid nitrogen cooling (via conduction down the Cu feedthrough rods). From Ref. 10. (b) Sample head and sample mounting configuration. From Ref. 176.

A representative example of a flexible, single-parameter analyser with final transfer is the Vitatron Akes, depicted In Fig. 8.5. In the aspiration position, the sample meets the reagent or diluent stream and the reaction mixture is subsequently transferred to the measuring cuvette, from which it is flushed by the aspiration system after detection, the cuvette being suitably washed. The instrument includes a linear sample train, sample turntable, sampling head, dilutor, reagent dispenser, data-input keyboard, photometric detection system, computer, printer, evacuation pump and wash solution doser. It is prepared for kinetic measurements. 

Filtration Is the sampling method most frequently used in industrial hygiene work (personal monitors) on account of its operational simplicity. A filter assembly typically consists of a sampling head and a filter (passive atmospheric samplers) or a sampling head, filter and pump (active samplers), all of which are constructed In a material introducing no contamination In the samples. The different types of filters (depth, membrane) and materials (glass fibre, quartz, cellulose esters, PFTE, sliver) used can be readily adapted to the unknown analyte. 

The holders and sampling heads are made of different alloys or plastics. The basic requirements for the holder quality are as follows ... 

Figure 15.5. Segmented sample head, showing filter baffles, rolls of encapsulating polyester strips, and the wraparound path between the detector and lead shield. U.S. Patent 5,614,724. (By permission of Pacific Northwest National Laboratory)...

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