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Atomic absorption spectroscopy: simple and effective.

Atomic Absorption Spectroscopy: Simple and Effective

It is interesting to note, as we start the final decade of the twentieth century, that the prognosticators of our industry continue to predict the demise of AAS (atomic absorption spectroscopy) as a mainstay fo elemental analysis. This prediction has been made ever since the introduction of competitive techniques such as ICP-AES (inductively coupled plasma-atomic emission spectrometry) and DCPAES (direct current plasma-atomic emission spectrometry) Now with the market acceptance of ICP-MS (inductively coupled plasma-mass spectrometry) for trace element work the doomsayers are once again predicting its end.

The truth is that the technique is now stronger than ever and there have never been more competitive product offerings from manufacturers than are available in today's marketplace. The reason the technique has kept its place in laboratories around the world is easy to understand. It is a technique which offers its users simplicity of operation, a very good cost/performance ratio and uncompromising accuracy and precision. The main problems associated with analysis by flame atomic absorption, ie. interferences and ionization, were documented years ago and are well understood. The addition of alternate atomization sources has also contributed to the popularity and versatility of the technique. These include vapour generation (VG-AAS) and graphite tube furnace (GFAA) pushing the detection limits for many elements down to the ppb (ug/L) level. Both these atomisation sources can be automated for maximum productivity.

During the past decade manufacturers have placed great emphasis on safety aspects of flame atomic absorption. Many interlocks are now built into the base system preventing accidental operator error from producing hazardous situations. In the Varian SpectrAA, interlocks are provided to ensure correct gas pressures and flows as well as correct burner, burner shield and liquid seal. The computer constantly monitors each of these parameters to ensure safe operating conditions are maintained during operation.

Computers and AAS

Perhaps the most significant changes have been in the application of computers to the control of AAS and the various atomization sources. Today the main differentiater on product price is the degree of automation available. A good example of the application of automation to AAS is the Varian SpectrAA series, first introduced in 1985 as the SpectrAA 40. The product had over 40 man-years of software development associated with it and, because it was designed from the beginning to be computer-controlled, is able to take advantage of newer and more powerful computers as they reach the market place. This design philosophy has also allowed an unprecedented number of software enhancements to be added to the base system. The software is offered to all customers so the value of the initial investment can be maintained as these product offerings become available. The application of automation has become most cost effective in GFAA; this technique can be very time consuming, and although it offers trace level capability (ug/L), it often suffers from chemical matrix effects. A great deal of time and effort has been expended to overcome these effects and coverage in the literature is extensive. Most often matrix interference effects can be overcome by the use of a graphite platform or a matrix modifier. By keeping the instrument control central and, basically having dumb peripherals slave to the central controller, it is possible to use software to program the furnace sample dispenser to pick up and add matrix modifier in a variety of ways. Sometimes it is necessary to add modifier to the graphite tube prior to the sample being added, at other times it is necessary to mix modifier with the sample in the graphite tube or to add modifier in the prsence of a reducing atmosphere.

Central control allows all of these parameters to be programmed into the computer. The sample dispenser simply follows its instructions. In old system designs, where each part of the system had some built-in intelligence, this was not possible. Central control has allowed unprecedented flexibility and, unlike the old systems with distributed intelligence, has allowed existing customers to take advantage of new developments as they become available.

Quality Control for Laboratories

One of the primary concerns of environmental and other regulatory agencies is the validity of the data being produced by a host of private and specific company run laboratories. To overcome this problem, the EPA (Environmental Protection Agency) in the US has introduced very specific guidelines on how samples should be run on a GFAA. These guidelines lend themselves very nicely to software control, which greatly simplifies the laboratory procedures. The software offered by Varian is of course flexible enough to fit in with a wide range of quality control procedures such as good laboratory practice (GLP). Named QC Protocol Software, it can be loaded onto new or existing Varian 300/400 systems to provide automated analysis using some or all of the following criteria; QC standards, blanks and spikes, overrange volume reduction, time and date stamping, duplicates, matrix spikes, replicate % RSD test and goodness of fit parameters for GFAA analysis.

As an example of how this would work, imagine a typical laboratory set up to do waste water analysis by GFAA. The regulatory agency has specified that each calibration shall be validated by running a QC check standard and that after each group of ten samples the QC check standard must be run again. The QC check standard must fall within a given range of values to be valid, say from 90% to 110% of its concentration value. If the result falls outside this value, restandardization is required. With QC Protocol Software the sample dispenser will automatically go back and restandardize, recheck the QC check standard and, provided the results now fall within limits restart the analysis rerunning all samples following the last valid QC check standard.

In a typical unattended run some samples will be higher than the calibration standards. This software allows a volume reduction factor to be applied. As a result any sample which is over-range on the first analysis will be reanalyzed with the volume reduced. This procedure will continue until either the sample result falls within the calibration limits or the sampler volume has been reduced to 1uL. If the sample is still overrange the result will be flagged. When a result is obtained, after one or more volume reductions, the computer will calculate the actual concentration by multiplying the calculated concentration by the volume reduction factor. This facility is available with or without the QC standard testing. This is only one example of the application of software to a specific problem.

Other Central Control Applications

Another requirement in many laboratories is to import data into spread sheets or data management programs. These very often have file structures different from the raw data produced by the instrument and it is necessary to translate the raw data file into a file format recognizable by the software being used. One specific application might be the generation of trend reports using data generated over many months of operation. By using a combination of translation facilities and macros the data needed to generate the trend analysis could be imported automatically from the instrument operated in flame, vapour generation or graphite furnace modes.

Today's analytical balances can also be connected to the computer controlling the AAS. In this way sample weights can be entered directly into a data table in the operating software. The computer calculates the difference between the actual weight measured per sample versus a nominal weight selected by the operator and automatically performs a weight correction calculation on the result.

Sample labels can be read into a sample table directly using a bar code reader. This is basically the same device used at supermarket checkouts. The bar code reader scans a sample label and imports it into a sample label table which is part of the system operating software. This capability is ideal for busy laboratories processing large numbers of samples and for any SpectrAA user wanting to guarantee the validity of sample label information from collection, to analysis, to the final report.

Another exciting applicating of central control is the automation of hydride generators. Figure 1 shows the flow diagram of the Varian VGA 76. When the sample inlet is connected to the flame auto sampler and the flow cell is placed on the air acetylene burner of the SpectrAA 400 multielement hydride analysis can be accomplished in an automatic sequence. Reagents are pumped through separate channels and are brought together with the sample in a mixing coil. Inert gas is injected into the mixing coil to promote thorough mixing. The hydride generated is then swept through a gas-liquid separator where waste sample and reagents are separated from the hydride. A second channel of inert gas flushes the hydride out of the gas-liquid separator into the heated flow cell where absorption takes places. Typical single element analysis time is 45 seconds. When the generator is connected to an autosampler, standardization, reslope and QC checks can be analyzed in an automatic multi-element run whilst the operator is preparing the next batch of solutions for analysis. This same apparatus is also easily converted to run mercury by the cold vapour technique.

Flame Analysis

In the area of flame analysis one interesting development during the last couple of years is the application to commercial AAS of research work by Watling and Brown et al. There are analytical situations in flame AA where the elemental concentration in a sample is too low to measure by flame but too high to measure by furnace.

Researchers found that by placing a slotted quartz tube over an air acetylene flame an improvement in sensitivity could be achieved. The Varian ACT-80 is a commercial example of this development. The ACT-80 is a quartz tube with two longitudinal slots cut in it, one 100 mm long, the other 80 mm long. The tube is mounted in a cell holder such that the longer slot is positioned immediately above the air acetylene burner slot. The shorter slot faces to the rear and is angled at 120 degrees. In operation, when the solution is aspirated, the atoms produced will pass into the tube and are prevented from rapidly leaving the optical path. As a result an improvement in sensitivity over normal flame AA is achieved which is element dependent but typically is in the order of two to three times.

What's in the Future

What do the 1990s hold in store for AAS? The application of artificial intelligence to sample analysis by AAS will almost certainly be seen within the next decade. Expert systems will automate sample preparation through analysis using a knowledge base stored on a central computer. Robotics will be incorporated to provide automatic standard and sample preparation.

One thing is certain. Because AAS is so element specific and cost effective, it will continue tobe an important analytical technique. Perhaps a parallel can be drawn with a story centered around Mark Twain. While visiting London, somehow a rumour was spread back home that he had met an untimely end. Death notices appeared in US newspapers and eventually got back to him in London. He cabled his home office "The reports of my death of AAS are also exaggerated". The reports of the death of AAS are also greatly exaggerated.

References

Watling, R.J. Analytica Chimica Acta, 94, 181-6 (1977). Watling, R.J. Analytica Chimica Acta, 97, 395-8 (1978). Taylor, A., Brown, A.A., Analyst, 108, 1159-61 (1983). Brown, A.A. Taylor, A., Analyst, 109, 1455-9 (1984). Brown, A.A., Milner, B.A., Taylor A., Analyst, 110, 501-5 (1985).
COPYRIGHT 1990 Chemical Institute of Canada
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Author:Lewzey, John
Publication:Canadian Chemical News
Date:Jan 1, 1990
Words:1922
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