Absorption instruments

Beeause of its high ehemieal reaetivity, aeetylene has found wide use in synthesis of vinyl ehloride, vinyl aeetate, aerylonitrile, vinyl ethers, vinyl aeetylene, triehloro- and tetraehloro-ethylene ete., in oxyaeetylene eutting and welding, and as a fuel for atomie absorption instruments. 

E. Photoelectric photometer method (Section 17.6). In this method the human eye is replaced by a suitable photoelectric cell the latter is employed to afford a direct measure of the light intensity, and hence of the absorption. Instruments incorporating photoelectric cells measure the light absorption and not the colour of the substance for this reason the term photoelectric colorimeters is a misnomer better names are photoelectric comparators, photometers, or, best, absorptiometers. 

A large number of commercial instruments are now available and are based either on a single- or double-beam design. Important instrumental features of a modem atomic absorption instrument include the following facilities. 

Some costs are easy to estimate, such as wages and salaries or utilities. The cost of equipment service is often overlooked, as is the cost of replacement parts. Lamps for atomic absorption instruments, for example, have finite lives and are quite costly to replace. Electrodes for pH meters and other instrument components all need periodic replacement. 

At present, calcium and magnesium are estimated almost exclusively by atomic absorption (36). Present instrumentation permits the dilution of the specimen to approximately 1 - 100 for calcium and even higher for magnesium. For many instruments, the two elements are not read out simultaneously such as is practicable for sodium and potassium with the flame photometer. The lower limit of serum volime at present, for the practical assay for calciim and magnesiim in the laboratory of Neonatology, is approximately 10 ul The instruments are very readily automated, and it is not uncommon for results to be available at the rate of 240 per hour in the routine laboratory, where a typical atomic absorption instrument such as a Perkin-Elmer has been attached to an automatic feed system. 

There are available also several kits for the assay of calcium, in 10 or 20 microliter samples by chelate formation colorimetrically or fluorimetrically. (Pierce Chem. Co., Rockford, 111.). These are read either with the spectrophotometer or by spectrofluorometry. In our experience, while these systems can be used for approximate results, the plot of concentration versus reading curves are rather flat and only an approximation of the values can be obtained. This may be very important late at night or at times when the atomic absorption machine is down, but if the atomic absorption instrument is available it should be used in preference to these procedures. 

Principles and Characteristics Flame emission instruments are similar to flame absorption instruments, except that the flame is the excitation source. Many modem instruments are adaptable for either emission or absorption measurements. Graphite furnaces are in use as excitation sources for AES, giving rise to a technique called electrothermal atomisation atomic emission spectrometry (ETA AES) or graphite furnace atomic emission spectrometry (GFAES). In flame emission spectrometry, the same kind of interferences are encountered as in atomic absorption methods. As flame emission spectra are simple, interferences between overlapping lines occur only occasionally. 

Modem absorption instruments can usually display the data as transmittance, %-transmittance, or absorbance. An unknown concentration of an analyte can be determined by measuring the amount of light that a sample absorbs and applying Beer s law. If the absorptivity coefficient is not known, the unknown concentration can be determined using a working curve of absorbance versus concentration derived from standards. 

Olsen et al. [660] used a simple flow injection system, the FIAstar unit, to inject samples of seawater into a flame atomic absorption instrument, allowing the determination of cadmium, lead, copper, and zinc at the parts per million level at a rate of 180-250 samples per hour. Further, online flow injection analysis preconcentration methods were developed using a microcolumn of Chelex 100 resin, allowing the determination of lead at concentrations as low as 10 pg/1, and of cadmium and zinc at 1 pg/1. The sampling rate was between 30 and 60 samples per hour, and the readout was available within 60-100 seconds after sample injection. The sampling frequency depended on the preconcentration required. 

Spectral line sources are used as light sources in atomic absorption instruments rather than the continuum sources used for UV-VIS molecular absorption instruments, and several atomic emission techniques require no light source at all apart from the thermal energy source. 

As mentioned in item 5 of Section 9.1, the light sources used in atomic absorption instruments are sources that emit spectral lines. Specifically, the spectral lines used are the lines in the line spectrum of the analyte being measured. These lines are preferred because they represent the precise wavelengths that are needed for the absorption in the flame, since the flame contains this analyte. Spectral line sources emit these wavelengths because they themselves contain the analyte to be measured, and when the lamp is on, these internal atoms are raised to the excited state and emit their line spectrum when they return... 

The sources of acetylene, nitrous oxide, and sometimes air are usually steel cylinders of the compressed gases purchased from specialty gas or welders gas suppliers. Thus, several compressed gas cylinders are usually found next to atomic absorption instrumentation and the analyst becomes involved in replacing empty cylinders with full ones periodically. Safety issues relating to storage, transportation, and use of these cylinders will be addressed in Section 9.3.7. The acetylene required for atomic absorption is a purer grade of acetylene than that which welders use. 

Mercury is the only metal that is a liquid at ordinary temperatures. It is therefore also the only metal that has a significant vapor pressure at ordinary temperatures. For this reason, it is possible to obtain mercury atoms in the gas phase for measurement by atomic absorption without the use of thermal energy. It is a matter of chemically converting mercury ions in the sample into elemental mercury, getting it in the gas phase, and channeling it into the path of the light of an atomic absorption instrument. 

Set up the atomic absorption instrument and obtain absorbance readings for all solutions using the extracting solution for the blank. Follow the instructions provided for instrument shutdown. 

Prepare the atomic absorption instrument for iron analysis, and measure all standards, the control, and the water sample. Use iron-free hard water for a blank... 

Obtain absorbance values for all standards, samples, and the control using an atomic absorption instrument. 

What purpose does the light chopper serve in an atomic absorption instrument ... 

How does a single-beam atomic absorption instrument differ from a double-beam instrument What advantages does one offer over the other ... 

Since the sample cuvette is the flame located in an open area of the atomic absorption instrument, rather than a glass container held in the light tight box in the case of molecular instruments, how is room light prevented from reaching the detector and causing an interference ... 

The reference beam in the atomic absorption instrument does not pass through the blank, but merely bypasses the flame. Thus the fluctuations in light intensity are accounted for, but the blank adjustment must be made at a separate time. 

Double-beam atomic absorption spectrophotometers are designed to control variations which may occur in the radiation source but they are not as effective as double-beam molecular absorption instruments in reducing variation because there is no blank sample in flame techniques. 

Lester et al. [24] have described a robotic system for the analysis of arsenic and selenium in human urine samples which demonstrates how robotics has been used to integrate sample preparations and instrument analysis of a biological matrix for trace elements. The robot is used to control the ashing, digestion, sample injection and operation of a hydride system and atomic absorption instrument, including the instrument calibration. The system, which routinely analyses both As and Se at ppb levels, is estimated to require only... 

When chemists talk about an analytical method or when instrument vendors tout their products, they often quote the standard deviation that is achievable with the method or instrument as a measure of quality. For example, the manufacturer of an HPLC pump may declare that the digital flow control for the pump, with flow rates from 0.01 to 9.99 mL per minute, has a RSD less than 0.5%, or a chemist declares that her atomic absorption instrument gives results within 0.5% RSD. The most fundamental point about standard deviation is that the smaller it is, the better, because the smaller it is, the more precise the data (the more tightly bunched the data are around the mean) and, if free of bias, the greater the chance that the data are more accurate. Chemists have come to know through experience that a 0.5% RSD for the flow controller and, under the best of circumstances, a 0.5% RSD for atomic absorption results are favorable RSD values compared to other comparable instruments or methods. 

Intensity of absorption Energy level of absorption Instrumentation Instrument calibration Sample preparation... 

Dorn, H.-P., U. Brandenburger, T. Brauers, and M. Hausmann, A New In-Situ Laser Long-Path Absorption Instrument for the Measurement of Tropospheric OH Radicals, J. Atmos. Sci., 52, 3373-3380 (1995a). 

Flame AAS (often abbreviated FAAS) was until recently the most widely used method for trace metal analysis. However, it has now largely been superseded by inductively coupled plasma atomic emission spectrometry (see Chapter 4). It is particularly applicable where the sample is in solution or readily solubilized. It is very simple to use and, as we shall see, remarkably free from interferences. Its growth in popularity has been so rapid that on two occasions, the mid-1960s and the early 1970s, the growth in sales of atomic absorption instruments has exceeded that necessary to ensure that the whole face of the globe would be covered by atomic absorption instruments before the end of the century. 

Double-beam atomic absorption instrumentation a = rotating half-silvered mirror b = front surface mirror. 

In the past, much atomic emission work has been performed on atomic absorption instruments which use a flame as the excitation source. However, these have been surpassed by instruments which utilise a high-temperature plasma as the excitation source, owing to their high sensitivity and increased linear dynamic range. 

Figure 14.8—Burner for an atomic absorption instrument. This type of burner is used in models 3100 3300 from Perkin-Elmer. (Reproduced by permission of Perkin Elmer.)...

Application of this principle is used in two types of background absorption correction set-up. Single beam atomic absorption instruments have an electromagnet at the level of the graphite furnace (or flame) and a polariser in the optical path (Fig. 14.14). However, this accessory is quite expensive. 

An A AS method is employed for the determination of lead (Pb) in a sample of adulterated paprika by the introduction of lead oxide (of the same colour). An electrothermal atomic absorption instrument that provides a background correction based upon the Zeeman effect is used. 

A mid-infrared absorption instrument generally consists of a Fourier transform design with the same basic components as noted above for the Fourier transform near-infrared spectrometers (broadband light source, Michelson interferometer, and detector optimized for the mid-infrared spectral region.)... 

Comparisons with other systems. Data presented in Table VI provide a comparison of results obtained with the image dissector with results reported by others with other systems. Results in the second column represent multielement detection limits observed in this work. Results in the third and fourth columns represent detection limits reported for single element determinations with conventional optics and a silicon vidicon (12J and a commercial atomic absorption instrument (33). 

The multielement detection limits with the echelle/image dissector are comparable to, or better than, single element detection limits reported for a silicon vidicon and conventional optics. Detection limits for Cr, Cu, and Mn with the echelle/ image dissector compare favorably with single element data reported for a conventional atomic absorption instrument with a photomultiplier detector, but detection limits obtained here for Ni and Co are higher by factors of 10 or more than for the conventional instrument. The echelle/image dissector system should be adaptable to a so-called flameless atomizer and be subject to the same improvements in sensitivities and detection limits as conventional detector systems. 

Comparison of detection limits for different atomic absorption instruments.9... 

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