Sample Flow-rate

Within the time available, the amount of sample which can be loaded (processed in the precipitation) depends on the maximum applicable sample flow-rate, which in turn is determined by other factors discussed in the next section. Although sample volumes ranging from 2.5 [21] to 250 ml [5] have been reported for preconcentration purposes, the routinely applicable range would be in a much narrower range of about 2-10 ml. 

The impedance of the on-line precipitate collector. This is particularly im] when the collection of a relatively large amount of precipitate is inteh befoit dissolution, or when a small capacity Altering device is used to limit dispersion. Thus, with a nylon membrane filter of 0.45-/xm pore and 5 mm diameter the highest tolerable flow-rate was found to be only 0.3 ml min in the collection [ of a calcium oxalate precipitate [10]. 

The form and amount of the precipitate formed. Crystalline precipitates, being more compact, produce more impedance in the filters. Large amounts of precipitate formed in coprecipitation processes also create impedance which limits the upper range of the flow-rate. 

Tbg speed of the precipitation reaction. With slow reactions sample flow-rates have to be decreased in order to achieve a reasonable degree of equilibrium. In extreme cases, as in the on-line formation of calcium oxalate, the reaction mixture even had to be stopped in the coil to wait for the precipitation to proceed 14]. 

For indirect determinations without precipitate dissolution, the sample and reagent flows are transferred directly to the detector. For atomic spectromet-ric detectors which require an optimum uptake, the combined flows from the sample and reagent (ani ometimes the diluent) should be such that the detector will not be excessively stan ed as to deteriorate its performance. For photometric detectors, the flow-rates are more flexible. 

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The volume of the manifold and the sampling flow rate determine the time required for the gas to move from the inlet to the collection medium. This residence time can be minimized to decrease the loss of reactive species in the manifold by keeping the manifold as short as possible. 

The sensor is the element of an instrument directly influenced by the measured quantity. In temperature measurement the thermal mass (capacity) of the sensor usually determines the meter s dynamics. The same applies to thermal anemometers. In IR analyzers used for concentration measurement, the volume of the flow cell and the sample flow rate are the critical factors. Some instruments, like sound-level meters, respond very fast, and follow the pressure changes up to several kHz. 

In the case of a temperature probe, the capacity is a heat capacity C == me, where m is the mass and c the material heat capacity, and the resistance is a thermal resistance R = l/(hA), where h is the heat transfer coefficient and A is the sensor surface area. Thus the time constant of a temperature probe is T = mc/ hA). Note that the time constant depends not only on the probe, but also on the environment in which the probe is located. According to the same principle, the time constant, for example, of the flow cell of a gas analyzer is r = Vwhere V is the volume of the cell and the sample flow rate. 

Similar considerations apply to best volume flow rates for samples of different molar mass. For high molar mass samples, flow rates should be reduced to avoid shearing the macromolecule in the column. Moreover, a reduced flow rate is necessary because the diffusion coefficients of large molecules will get pretty small. This means that the macromolecule will pass by a pore in the packing material without having the time to enter it, if the linear flow rate is too high. 

These workers found that the efficiency of oxidation was a function of the residence time of the sample in the reactor and the flow rate of the carrier gas. A high precision of carbon dioxide determination was achieved at a sample flow rate of 50 ml/h and a carrier gas flow rate of 621/h, of which 401/h passes through the shielded zone. 

In contrast to this, sample presentation is thoroughly investigated. On behalf of the Ministry of Housing, Physical Planning and Environment, TNO investigated the influence of flow rate in relation to dilution factor, both analytically and sensorically. The flow rate varied from 6 to 35 1/min. Propane concentrations were measured at the back of the nose of an artificial head through which 20 1/min. was sucked continuously. Sensory measurements were carried out with butanol and ethylbutyrate. The results are summarized in figure 1. Based on this research 16 1/min. was recommended as the minimal sample flow rate. 

Fig. 1 Dilution factor as a function of sample flow rate...

Sample flow rates and dilution factors when olfactometer runs correctly and percentage deviation when galvanometer reads full scale deflection... 

Before measurements of sample flow rates were made, the air flows through the fans were set to 240 10 1/min. The potentiometers that adjust motor speeds were too coarse to permit more accurate adjustment. The sample flows were set to the flow rates specified and galvanometer was set to zero by adjusting the relevant potentiometer in turn. The transfer flow entering fan 1 from fan 2 for two-stage dilution was set to 2.41/min. 

During operation, it was difficult to obtain steady null readings on the galvanometer, particularly when the sample flow rates were small. In some positions, small movements of the control valve caused a full scale deflection (FSD) of the galvanometer while in other areas, movement of the valve in the same direction caused the flow to increase and decrease alternately. Fig. 3 shows the variation of flow rate with valve position, within... 

Fig. 3 Variation in the sample flow rate with the angle of rotation of the sample control valve. The integers within the graph refer to the number of rotations. The y axis is shown on a log scale. The parallel, dashed lines, show the eleven flow rates corresponding to the eleven dilutions...

A number of different types of ESI sources, known as nanospray sources, have been designed that can operate at lower sample flow rates (10-200 nL min ). These generate smaller droplets and improve the signal intensity of the protein-ligand noncovalent complexes further, with the added benefit of reducing protein consumption up to 100-fold compared to standard ESI flow rates. Nanospray has also been reported to be more tolerant to nonvolatile cations in solution [37]. Recently, an automated fabricated chip nanospray source has been developed. This chip-based device has improved the ease-of-use for nanospray, while the design eliminates carryover effects as the spray is produced directly from an orifice on each sample well of the chip [38]. 

Several physical parameters may interfere with analytical accuracy. High sampling flow rates and high temperature and humidity may cause decreased adsorption of 1,4-dichlorobenzene vapor on the solid sorbent (APHA 1995a). Interference by other VOCs with similar retention times may be resolved by using different GC column materials and temperatures or be using MS techniques. 

The NO concentration measurements were made using a chemiluminescence analyzer calibrated with 89 ppm standard mixture of NO in N2. A choked flow orifice controls the sample flow rate through the analyzer and therefore the probe is not choked during sampling for NO measurements. The pressure drop across the analyzer is approximately 80 kPa and the exit of the analyzer is operated at 10 kPa absolute pressure. 

Sample Flow rate Pore or UVA isible Peak start... 

As is the case for LIF, calibration to obtain absolute concentrations is a challenge. In the instrument shown in Fig. 11.45, a calibration source based on the photolysis of water at 185 nm is installed in the inlet. From the absorption cross section of HzO gas at 185 nm, its concentration, the light intensity, and the sample flow rate, the concentration of OH generated by the photolysis can be calculated. However, not only is there significant uncertainty in the absorption cross section for HzO at 185 nm (e.g., see Lazendorf et al., 1997 Hofzumahaus et al., 1997, 1998 and Tanner et al., 1997), but the measured calibration factor was highly variable from day to day, by as much as a factor of two (Tanner et al., 1997). 

Aerosol photometer — light-scattering type with a threshold sensitivity of at least 10 mg/1. Capable of measuring concentrations in the range of 80 to 120 mg/1, and with air sample flow rate of 1 fC + 10%/min. 

The chamber is being used to produce replicate samples of Pb, , and Se at several loading levels for the analytical studies. Filter sampling and collection efficiencies are also being explored using Pb aerosols and by varying the sample flow rate through the 37 mm filter cassettes from 0.1 to 4.0 Lpm. 

Development of a Diffusion Head Sensor Cell. The use of air sampling pumps in portable electrochemical gas detection apparatus introduces potential problems into the instrument. First, the sensor cell response is dependent on gas flow rate. The sample flow rate, therefore, must be accurately controlled to obtain reproducible results, or the sample flow rate must be set high enough to insure a flow independent response. Secondly, failure of the pump itself could prevent a sample from reaching the sensor cell. Thirdly, the pumps are usually one of the largest users of current in a portable instrument and thereby limit usable battery life. 

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