Aspiration rate

The total consumption type of burner consists of three concentric tubes as shown in Fig. 21.5. The sample solution is carried by a fine capillary tube A directly into the flame. The fuel gas and the oxidant gas are carried along separate tubes so that they only mix at the tip of the burner. Since all the liquid sample which is aspirated by the capillary tube reaches the flame, it would appear that this type of burner should be more efficient that the pre-mix type of burner. However, the total consumption burner gives a flame of relatively short path length, and hence such burners are predominantly used for flame emission studies. This type of burner has the advantages that (1) it is simple to manufacture, (2) it allows a totally representative sample to reach the flame, and (3) it is free from explosion hazards arising from unbumt gas mixtures. Its disadvantages are that (1) the aspiration rate varies with different solvents, and (2) there is a tendency for incrustations to form at the tip of the burner which can lead to variations in the signal recorded. 

Chemical interferences are the result of problems with the sample matrix. For example, viscosity and surface tension affect the aspiration rate and the nebulized droplet size, which, in turn, affect the measured absorbance. The most useful solution to the problem is matrix matching, matching the matrix... 

A number of instrument variables need to be set prior to making measurements. These include slit, wavelength, lamp current, lamp alignment, amplifier gain, aspiration rate, burner head position, acetylene pressure, air pressure, acetylene flow rate, and air flow rate. Some instruments are rather automated in the setup process, while others are not. Your instructor will provide detailed instructions for the particular instrument you are using. Be sure to turn on the fume hood above the flame. 

Controls that need to be optimized are wavelength, slit width, lamp current, lamp alignment, aspiration rate, burner head position, and fuel and oxidant flow rates. See Section 9.3.5 for details. 

Since AAS is a ratio method, many instrumental errors (e.g. long-term source drift, small monochromator drifts) should cancel out, as 7 is ratioed to I . However, a stable uptake rate, or aspiration rate, is required. This falls as the viscosity of the solution sprayed is increased. Nebulizer uptake interferences can be minimized if the dissolved salts content of samples and standards is approximately matched. For example, when determining pg cm sodium levels in 2 M phosphoric acid, ensure that the standards are also dissolved in 2 M phosphoric acid, using a blank to check for contamination. 

Physical interferences in FLAA analysis are of the same nature as the ones in ICP-AES analysis high dissolved solids contents in samples change the sample viscosity and alter the aspiration rate. 

As can be seen from the results in Table 3.1, the analyte transport efficiency is similar for both conventional and micro- or high-efficiency nebulizers when compared under identical flow rates. The increase in analyte transport efficiency with decrease in the sample uptake rate (sometimes called starving the nebulizer because uptake rates less than the natural aspiration rate are used) was reported long ago [21,22]. So the main advantage of the newer micronebulizers is that their internal volume is small, a feature that becomes more important as the uptake rate is reduced. A capillary can also be inserted into a conventional concentric, pneumatic nebulizer to decrease its internal dead volume [23,24]. 

Over the years many analytical spectroscopists have attempted to improve upon this situation, but the only reliable way to improve transport efficiency with pneumatic nebulizers is apparently to restrict the aspiration rate.17,18 Reduced aspiration rate means that the nebulizer energy is distributed to less aerosol per unit time, resulting in a finer droplet size distribution finer droplets (e.g. < 2 pm in diameter) are more likely to be transported through the spray chamber. Alternatively, the determinant may be introduced to the flame in gaseous form, or in a small cup. Such approaches are discussed in Chapter 6. However often the approach taken is to use electrothermal atomization rather than a flame,6,19 but this is outwith the scope of the present small volume. 

In flame spectrometry, physical interferences are related to transport of determinant from sample solution to the flame. The pneumatic nebulizer functions not only as a spray generator, but also as a pump.1,2 Anything which influences the pumping rate will influence the size of the absorbance signal obtained. The pumping, or aspiration, rate is most sensitive to changes in viscosity of the sample solutions. 

Aspiration rate is inversely proportional to viscosity. It is important therefore that sample and standard solutions have identical viscosities. This is best achieved by careful matrix matching. 

Aspiration rate is only a small part of the overall transport process in flame spectrometry. The production of aerosol and its transport through the spray chamber are also of great importance. The size distribution of aerosol produced depends upon the surface tension, density, and viscosity of the sample solution. An empirical equation relating aerosol size distribution to these parameters and to nebulizer gas and solution flow rates was first worked out by Nukiyama and Tanasawa,5 who were interested in the size distributions in fuel sprays for rocket motors. Their equation has been extensively exploited in analytical flame spectrometry.2,6-7 Careful matrix matching is therefore essential not only for matching aspiration rates of samples and standards, but also for matching the size distributions of their respective aerosols. Samples and standards with identical size distributions will be transported to the flame with identical efficiencies, a key requirement in analytical flame spectrometry. 

The significance of deliberate or accidental changes of instrumental variables such as aspiration rate, current of the hollow cathode, wavelength, and slit width during an analysis was self-evident. The importance of background correction, especially for the low concentration levels, was another consideration. The importance of the other variables listed, such as the direct readout of concentration, was dependent upon the proper utilization of these instrumental features by the analyst. 

Solution aspiration rates, fuel and oxidant mixtures, gas flow rates, burner choice, matrix effects and interelement interferences must all be taken into account when using flame AAS. While optimal choices for the above parameters vary from instrument to instrument, recommendations which afford reasonable starting points for operation have been published by both the Intersociety Committee for Methods of Air Sampling and Analysis (ISC) [7] and the National Institute of Occupational Safety and Health (NIOSH) [8]. These recommendations were the result of ISC efforts supported by both the United States Environmental Protection Agency and NIOSH. 

While industrial or agricultural chemists for the most part have large samples available for analysis, the clinical chemist is faced with an ever increasing number of tests to be performed on a small single sample, for instance a few milliliters of serum. The sample size in atomic absorption spectroscopy depends on necessary sample dilutions, aspiration rate, and time of aspiration required to obtain one reading. These are a function of sensitivity and instrumental stability. Since a few seconds (usually 10-30) of sample aspiration suffice for one reading, the total... 

Fig. 2 Atomizer of IL 943 Flame Photometer a) sample orifice assembly b) air orifice c) gas tube assembly d) atomizer bowl drain e) atomizer thumb screws f) U-tube g) sample injection nozzle tubing h) ground fitting i) top atomizer assembly j) bottom atomizer assembly k) adjustment for aspiration rate setting. (Courtesy of Instrumentation Laboratory, Inc.)...

Instrument, Instrumentation Lab model 153 solvent, methyl isobutyl ketone aspiration rate, 3-4 ml min mode, 10 sec integration slit width 80 / ... 

Dilution with lanthanum-HCl reduces interference from protein, phosphate, citrate, sulfate, and other anions. Phosphate causes the greatest interference because calcium-phosphate complexes are not dissociated readily by the air-acetylene flame. Lanthanum-HCl dissociates complexes, ensuring that all fractions of calcium (free, protein-bound, and complexed) are measured. Dilution effectively reduces the viscosity, which can also interfere by reducing the aspiration rate and atomization of the specimen. 

Flow injection procedures are very useful for performing trace analyses in highly concentrated salt solutions. Fang and Welz [270] showed that the flow rate of the carrier solution can be significantly lower than the aspiration rate of the nebulizer. This allows even higher sensitivities than with normal sample delivery can be obtained. Despite the small volumes of sample solution, the precision and the detection limits are practically identical with the values obtained with continuous sample nebulization. The volume, the form of the loop (single loop, knotted reactor, etc.) and the type and length of the transfer line between the flow injection system and the nebulizer considerably influence the precision and detection limits that are attainable. 

Flow rates should always be given. For specific applications involving intermittent streams [151], sinusoidal flows [152], flow reversal [153] or variable flow rates, e.g., slow during sample transportation towards detection and fast after achievement of the analytical signal [80], the temporal variations in flow rates (see Fig. 3.4) should also be indicated. This is also true for sample stopping [122], It is advisable to indicate less critical flow rates, e.g., the sample aspiration rate in loop-based injections or the outlet stream flow rate in a de-bubbler unit. 

Time-based introduction is better implemented automatically, i.e., without human intervention. In this mode, the sampler arm selects the sample, the carrier/wash and eventually the reagent solutions (Fig. 6.8) to be directed towards the manifold. The sampling time is the main parameter governing the volumes of these solutions introduced, which can also be modified by changing the aspiration rate in the fluid propulsion unit. This is usually an option in modem instmments. 

As stream segmentation prior to stream splitting was not involved in either of these classical applications, the flow rates of the emergent streams were governed by the aspiration rates of a peristaltic pump. Although the splitting process is a simple concept, its initial development required much effort. After the experience with a two-channel system... 

Mercury lamp current Digital concentration readout Aspiration rate... 

The FCC was developed during a study of the influence of gas convection into the base of the arc column near the cathode of the arc (11). The conical tip of the cathode is surrounded by an annular nozzle which terminates upstream from the tip and which directs the gas in a converging high-speed layer into the column of the arc close to the point where it originates on the cathode. It was found that if this were done so that the gas impinged on the arc column in the contraction zone, the gas would preferentially enter the column. Further, the gas could be so injected at 10-20 times the natural aspiration rate. If an attempt is made to force the gas into the arc column elsewhere, the degree of penetration is far less and the injected gas tends to unstabilize and blow out the arc. 

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