The Thermo Scientific ice 3500 AA Spectrometer - a True Dual Atomizer AAS

Transcription

1 Technical Note: The Thermo Scientific ice 3500 AA Spectrometer - a True Dual Atomizer AAS This technical note provides an insight into the design concepts of the ice 3500 dual atomizer instrument. Key Words Absorption Dual Atomizer FAAS GFAAS Graphite Furnace Quadline D 2 Background Correction Zeeman Background Correction Introduction Dual atomizer atomic absorption spectrometers are, as the name suggests, simply those that include both a flame and graphite furnace atomizer in one instrument. The benefits of such instruments are numerous and this technical article discusses the design merits of the Thermo Scientific ice 3500 dual atomizer spectrometer, and contrasts them against other optical designs. The benefits of dual atomizer instruments in the modern analytical laboratory will be explored, with a simple comparison between a single and a dual atomizer method being presented. The advantages of a dual atomizer instrument over what can be regarded as the more simple approach of buying two separate single atomizer AA instruments is also discussed. Atomic Absorption Spectrometry Atomization Sources The technique of Atomic Absorption Spectrometry requires an atomization source. This is typically either a flame (FAAS) or a graphite furnace (GFAAS) depending on the sensitivity required and the sample matrix. FAAS - Flame Atomization The sample is normally presented to the instrument as a liquid. When it is passed through the nebulizer, it forms a fine mist or spray, which is mixed with the oxidant and fuel gases in the spray chamber before being passed into the flame. Once in the flame, the sample undergoes evaporation of solvent, vaporization of salts and finally atomization. The flame fuel gas is usually acetylene, and either air or nitrous oxide is used as the oxidant gas. Nitrous oxide supported flames burn at much higher temperatures than air supported flames. Flame temperature has a strong influence on the atomization process, because hotter flames are required to vaporize some salts and to decompose certain compounds. Stoichiometry also affects flame temperatures and consequently has to be optimized for each sample and element required. Flame atomization systems are simple and relatively easy to use but do have several disadvantages: They require fairly large sample volumes They have limited sensitivity GFAAS - Electrothermal (Graphite Furnace) Atomization In this technique, an electrically heated graphite furnace is used as the atomizer. GFAAS can be used to measure most of the same elements as the flame but with typically 1,000 times the sensitivity and a fraction of the sample size. A discrete volume, typically a few micro-litres, of the sample is placed inside an small graphite cuvette, and heated using the following general procedure: i. Drying phase, where the sample is warmed to remove the solvent ii. Ashing phase, where as much of the sample matrix as possible is removed iii. Atomization or measurement phase iv. Cleaning phase, where the cuvette is heated to a high temperature to remove any previous sample The analytical signal is measured as a transient peak instead of the continuous signal used in flame work. The residence time of analyte atoms in the light path is increased with this technique and the total volume of the sample used contributes to the final signal. Both of these effects result in much improved analytical sensitivity. A dual atomizer instrument includes both the flame and graphite furnace atomizers in one instrument. Why would a Modern Analytical Laboratory want a Dual Atomizer AAS? Many laboratories are required to carry out analyses that necessitate the application of both FAAS and GFAAS techniques. This requires one of three generic instrument package options (Figure 1), and the optimum solution depends largely on the throughput and frequency of measurements requiring the two techniques. The possible generic solutions are: Option 1: Option 2: Two separate, dedicated instruments - one FAAS instrument one GFAAS instrument (e.g. A Thermo Scientific ice ) A FAAS instrument with a GFAAS atomizer option (Thermo Scientific ice GFS33) Option 3: A Dual Atomizer Instrument (Thermo Scientific ice 3500)

2 FAAS GFAAS FAAS GFAAS FAAS GFAAS Dual Atomizer Figure 1: The Three Options for Flame and Furnace Atomizer Capability The benefits and disadvantages of each option are explained as follows: Option 1: Two separate, dedicated instruments The first package is predominately used by extremely high throughput laboratories with roughly equal, constant and very high numbers of FAAS and GFAAS samples. If it is established that the throughput of the laboratory is higher than a single dual atomizer instrument can cope with (then the laboratory really does run a lot of samples!), Thermo Scientific can of course provide two AA systems. It is sometimes suggested that cost can be saved, and convenience improved, by running both instruments with a single Data Station. However, Thermo Fisher Scientific does not recommend this approach. Running two instruments from a single Data Station is relatively easy to accomplish, but we know that the problems introduced for the analyst far outweigh the perceived benefits of such an approach. There are a number of problems that can be encountered when using two AAS instruments running off one Data Station. The user can find that rather than doubling their productivity they actually cause themselves 'double the trouble'. The potential exists that a user wishing to run the same element by both FAAS and GFAAS in the same analysis would require two sets of hollow cathode lamps (HCL) - doubling the cost Two separate instruments running off one PC will require two independent instances of the instrument software to be running on one PC. Imagine trying to find your way around two versions running separately and with no doubt cluttered screens - very confusing, how can you tell which is the flame version and which is the furnace? Two databases on one PC Two sets of instrument spares Two separate service contracts - one for each instrument would be necessary Two AA's have twice the footprint, bench space is precious in most labs, is there room for 2 AA's? The potential exists for 2 operators to be using the software but only one can be securely logged on, surely there are compliance issues with this approach? Option 2: A single atomizer instrument with a GFAAS accessory The second option, is to buy a FAAS with a separate GFAAS accessory. This is sometimes chosen as a cost effective solution when the number of flame samples to be analyzed greatly exceeds the number of furnace samples, so that GFAAS capability is required only rarely. For example, for infrequent, unknown or emergency samples. Initially only a flame system may be necessary but over time the analytical needs of the laboratory can grow, so that a GFAAS capability is required. Hence when making the purchasing decision on flame-only systems it is important to ensure the chosen system is adequately 'futureproof' i.e. the system is capable of GFAAS upgrade. The disadvantage of this approach is the time and complexity involved in changing over the atomizers in the instrument. Different vendors have alternate solutions for this operation but the generic process of changing over atomizers is illustrated in Figure 2. At least two critical alignment procedures are required; if these are not performed correctly for any reason the quality of the data generated will deteriorate. Switch on HCL s and allow stabilzation Light flame Load Flame/Dual method Make flame measurements Extinguish the flame Dual Atomizer Make furnace measurements Single Atomizer Load furnace method Allow burner to cool Align furnace autosampler Remove flame autosampler Remove burner assembly Fit furnace assembly and autosampler Align furnace assembly Figure 2: Flowchart showing the additional steps required if using a single Option 3: A dual atomizer instrument A dual atomizer instrument will have both atomizers permanently mounted so there is no requirement to spend time realigning atomizers prior to an analysis. Both flame and furnace auto-samplers can be left in place, and the furnace auto-sampler in particular should retain its alignment. With properly designed software, a dual atomizer instrument can be easily controlled from a single Data Station without any of the problems encountered with the two AA's approach. An additional benefit is that flexible combinations of atomizers are possible on dual atomizer instruments which provide further flexibility to enable the optimum configuration to be set-up. Examples of typical combinations: FAAS and GFAAS (ice 3500) FAAS and Electrically Heated Vapor Generation (ice EC90 + VP100)

3 Electrically Heated Vapor Generation and Zeeman GFAAS (ice EC90 + VP100) Flame heated vapor generation and Zeeman/D2 only GFAAS (ice VP100) Since the switch between analyzers is automatic, dual analysis sequences can be set up such that the flame analyses are completed then the instrument switches analyzers and runs the furnace analyses. The benefits and disadvantages of each of these approaches are summarized in Table 1. For the vast majority of laboratories where sample throughput is medium to high and the use of GFAAS in addition to FAAS is regular and/or frequent the table indicates that the third option of a dual atomizer is the optimum configuration. The ice 3500 dual atomizer instrument has been designed based on these considerations. FEATURE OPTION 1. OPTION 2. OPTION 3. TWO SEPARATE SINGLE ATOMIZER DUAL ATOMIZER DEDICATED AA S WITH GFAAS AAS ACCESSORY Flame High Medium/High High Throughput Furnace Medium/High Low Medium/High Throughput Frequency High Low High of GFAAS Samples Hollow Cathode 2 Sets 1 Set 1 Set Lamps Software 2 Instances 1 Instance 1 Instance Atomizer Change Manual Software changeover Instruments Changeover controlled, automatic Ease of Atomizer switch N/A Poor Excellent Instrument Service Contracts Spares 2 Sets 1 Set 1 Set Table 1. Instrument Option Comparison. Do all Dual Atomizer Instruments use the Same Arrangement of Atomizers? There are a number of possible options for arranging the dual atomizers. An understanding of the other optomechanical arrangements for the switch between atomizers is useful to put into perspective the elegance and simplicity of the ice 3500 design. Such options are illustrated in Figure 3 include: Fixed atomizers, where the two atomizers are positioned in a serial arrangement, with the light beam always passing through both atomizers. The major disadvantage of this approach is that the instrument suffers from greatly reduced light throughput because extra interatomizer optics are required. This results in degraded detection limits. The physical positioning of the atomizers also impacts on the size of the instrument required to house the optics, leading to large, long instruments. Additionally, perfect alignment of two serial atomizers is very difficult to achieve. a) Burner b) Burner c) Burner Burner Furnace Furnace Furnace Figure 3: Other Opto-mechanical arrangements for dual atomizer instruments. a) Serial b) Parallel c) Moving. Moving atomizers, which shuttle or swing into position in a single sample compartment create various problems. The major disadvantage of this approach is that the instruments rely on positioning complex precision mechanisms in chemically vulnerable areas prone to corrosion. Atomizer changeover can be quite slow in operation, as the movement of one atomizer out of position must take place before the new one can be moved in. Poor repeatability of the repositioning can result in inadequate optical reproducibility, degrading the quality of the analytical results. The mechanics and engineering required with such designs result in large instrument footprint sizes. Designs based on this principle cannot be considered to be truly automatic as the changeover usually involves some sort of manual intervention. For example, it is often necessary to remove and re-fit auto-samplers and burner heads. This means that the changeover is not truly automated and cannot be carried out automatically under software control.

4 How was the ice 3500 Dual Atomizer Instrument Designed? The ice 3500 dual atomizer instrument was designed with very specific optical requirements. The following factors were considered crucial to the instrument design: Dual-atomization - with the atomizers permanently in position, such that they would require no re-alignment The switch between atomizers should be instantaneous and the changeover capable of occurring whilst the instrument is unattended No compromise in instrument performance should be sacrificed to fulfil the dual atomizer increased functionality The instrument must present continuum source background correction as standard and should encompass a Zeeman furnace option The lamp carousel should house six data-coded hollow cathode lamps all being capable of auto-alignment and with separate power supplies The optics of the instrument should incorporate Stockdale double beam optics, and include the option for a Furnace Vision System' The instrument should provide full accessory compatibility A footprint of less 0.5 m 2 was desired as a response to the reality that in today's extremely cost-conscious, commercial, environment laboratory space is a valuable commodity The first major conceptual breakthrough with the design of the ice 3500 was the realization that in a double beam system the function of reference and sample beam are interchangeable. Therefore an atomizer could be placed in each beam and the function of the beams changed by a simple software command. The rest of the instrument design was based around optical components that optimize the optical signal, combined with highly functional and ergonomic user access. The ice 3500 Optical Concept In the ice 3500 optical system the lamp turret is mounted at the front of the instrument in the centre, enabling easy user access. The turret is mounted vertically, optimizing bench-space and ensures that no forces exist that would act to misalign the lamps. Radiation from the HCL passes vertically upwards towards a motorized deflecting mirror which in tandem with microrotation of the HCL carousel achieves fast, automatic lamp selection and alignment. From this first lamp selector mirror the optical beam passes through a speckled beam combiner. This component combines the light beam from the HCL with the light emitted by the QuadLine deuterium arc lamp, which is required to implement the QuadLine background correction system. The next step is that both HCL and D 2 beams meet the front beam selector mirror, which directs the combined beam either to the left, to the flame atomizer or to the right to the furnace atomizer. After passing through the atomizer, a plane mirror then deflects each beam to the rear of the instrument. Toroidal focusing mirrors at the rear corners of the instrument deflect and focus the beams onto the rear beam selector mirror, which works in concert with the front beam selector mirror, and determines whether the beam which has passed through the flame or the furnace atomizer is directed to the monochromator (see Figure 4). It is inevitable that the design of a dual atomizer will involve a small increase in reflective surfaces in comparison with a simple single beam system. It is important that these small losses are more than compensated for by the quality of the remaining optics. It was determined that the optical system should be designed around the requirements for a graphite furnace analysis and the furnace cuvette profile since this is the more demanding application. Longer light paths necessitate the use of a pencil beam optical path (Figure 5) through the instrument, to reduce divergence. This has the additional benefit that the measurement beam only passes through the hottest region of the atomizer, resulting in reduced tube wall emission effects (breakthrough) and improved sensitivity for furnace measurements and less curvature and improved sensitivity for flame measurements. Figure 4: The Optical design of the ice 3500 Dual Atomizer AAS. a) b) Figure 5: 'Pencil beam' optics passing through a) furnace cuvette b) flame atomizer.

5 The ice 3500 Monochromator These design considerations led to the specification of a monochromator capable of covering the normal range of AA wavelengths, from 180 to 900 nm, with high light throughput, a small optical aperture, high dispersion, and a small focal length, and able to provide conventional AA bandpasses. The option of a conventional monochromator was ruled out since a 40 x 60 nm grating with 1800 l/mm and a 500 nm focal length would be required to provide the dispersion needed. The long focal length could cause more thermal/mechanical drift and would not meet the desired footprint requirement. Increasing the number of lines on the grating and/or the size of the grating would restrict the wavelength range, and reducing the slit widths would lead to reduced light throughput and poorer detection limits. The only real alternative was to move to the use of an Echelle monochromator. An Echelle monochromator contains two dispersing elements (typically a grating and a prism), and provides excellent optical performance combined with very small space requirements. Other benefits of this design include: High dispersion even with short focal lengths Relatively small physical size Very rapid wavelength selection - selected by moving grating & order sorter simultaneously Monochromator Very low stray light, as a result of the double monochromator design, giving improved AA sensitivity and less calibration Camera curvature Greater stability - less demanding mechanics and assembly lead to greater reliability Physically wider slits can be used for a given bandpass, giving greater light throughput, and better detection limits HCL The Echelle system used on the ice 3500 has a reciprocal linear dispersion (RLD) of ~0.5 nm/mm at 200 nm. It is desirable to have as small a reciprocal linear dispersion as possible since this improves the amount of radiation passing the monochromator and reaching the detector even for small bandpass widths, improving sensitivity. To put the performance of the echelle into perspective it is approximately as good as a half metre focal length traditional monochromator. Field lens Additional Design Features The unique Thermo Fisher Scientific Stockdale double beam optics are effectively included for free in the ice 3500 optical system. The traditional 'leaping' mirrors are now replaced by the front and rear beam selectors. This makes all systems automatically double beam, with the added benefit that the Stockdale method ensures that during the measurement period, the system effectively operates in the optimum single beam mode, whilst reference bracketing measurements before and after the key measuring period ensure perfect drift correction. The back of the rear beam selector has also been designed to be reflective and is cleverly used to deflect the furnace optical beam below the optical base and into a small additional optical system that terminates in the optional 'Furnace Vision' GFTV camera (Figure 6). GFTV provides high definition images of events inside the furnace cuvette, this allows monitoring of the sample injection, dry and ash phases of the furnace program (see Figure 7). Another additional component to the furnace optical system is the polarizer, which is only fitted to Zeeman systems. This component is motorized in order to simply flip into the beam when the Zeeman background correction is being employed and to flip out of the beam when a Quadline only analysis is being carried out. Leaping mirror Cuvette GFTV Off Monochromator Camera HCL Field lens Leaping mirror Cuvette GFTV On Figure 6: Schematic showing the deflection of the furnace optical beam into the GFTV camera The entire optical system is mounted in an aluminum precision casting to provide it with mechanical and thermal stability and the system is built into a dust sealed structure to ensure long-term optical cleanliness and stability. The ice 3500 was designed such that the instrument depth fits a standard laboratory benchtop, with adequate space for access, ventilation, extraction and supply lines, the width was then minimized as much as the instrument optics would allow resulting in the ice 3500 final dimensions being the smallest footprint of any dual atomizer instrument on the market.

6 Figure 7: Clear, high-resolution 'Furnace Vision' (GFTV) images of the interior of the cuvette. Indicating the position of capillary tip as: too high, too low, optimum and with a platform cuvette. Deuterium Background Correction The unique QuadLine D 2 arc source that provides the broad band radiation used to implement the QuadLine background correction system is much brighter than most HCL's. The 'speckle ratio' of the HCL/D 2 beam combiner can therefore be increased to an 80/20 split in favor of the HCL to allow more energy from the measuring HCL to reach the detector. The Deuterium arc lamp behaves as a point source, while the HCL s sources produce a broader, diffuse patch of light. The QuadLine background correction approach demands that the two optical beams have the same energy profile and image size. To achieve this, the ice 3500 optically magnifies the HCL beam by a factor of 2 at the Atomizer, and the D 2 beam by a factor of 3. The D 2 beam is deliberately de-focused, so that the resultant light patch at the atomizer matches the energy profile of the HCL beam. The imbalance between the intensities of the QuadLine and HCL lamps increases as the wavelength decreases. The QuadLine beam is therefore focused with a silica lens rather than a mirror. The focal length of a lens is dependent on the wavelength of the light passing throughout, so that the effect of this component is to progressively de-focus the QuadLine beam as the wavelength decreases, thus reducing the excessive energy of the QuadLine beam and enabling a better match with the HCL intensity to be achieved. This design, together with other unique attributes described in detail in reference 1, provide confidence in the accuracy of the QuadLine background correction, and enables Thermo Fisher Scientific to guarantee that the system can correct up to 2A of background signal with less than 2 % error. Zeeman Background Correction The AC inverse Zeeman system used on the ice 3500 and ice 3400 instruments is the only background correction technique available that corrects at the exact analyte wavelength, gives accurate correction of structured backgrounds and spectral overlaps (See Figure 8). This is vital if the sample background signal has significant fine structure or if there is a spectral interference present. The Thermo Scientific Zeeman system employs a very high field strength magnet (0.9 Tesla), which improves the magnitude of the Zeeman effect, improving sensitivity and detection limits, and reducing the calibration curvature traditionally associated with Zeeman effect based background correction. The magnet drive current is modulated at Hz with a trapezoidal waveform, so that fast furnace signals are accurately tracked, and the complete background correction measurement is taken at full field strength. Uniquely the ice 3500 allows a combined QuadLine/Zeeman mode to be run for method development work. Zeeman measurements are only made at the time of measurement, but QuadLine can be used to monitor background or interfering species in the dry and ash phases of a furnace measurement.

7 Absorption + π Absorption profile split into π and components by magnetic field around atomizer Emission Normal hollow cathode lamp radiation Magnet off: Normal AA system Signal measured = AA + Background Static Polarizer π: component removed by polarizer HCL measures background only Background Figure 8: The AC Inverse Zeeman Background Correction Magnet on: Signal measured = Background only Conclusions On the ice 3500 the switch between a flame analysis and a furnace analysis is simply achieved by the flip of a mirror, therefore, flame to furnace changeover can be totally automatic - no manual intervention is required. Alignment of both atomizers is perfect, productivity is significantly enhanced and performance guaranteed. No energy is wasted and single beam performance is achieved with double beam stability. The design of our dual atomizer means that there is no energy loss due to extra relay optics. References 1) Design Considerations for High Performance Background Correction Systems in Atomic Absorption Spectrometry - Thermo Scientific Technical Brief

8 Thermo Scientific ice 3000 Series Product Range In addition to these offices, Thermo Fisher Scientific maintains a network of representative organizations throughout the world. Thermo Scientific ice 3300 Single flame atomizer AAS with fully automatic gas box. Complete solution for laboratories with a main need to perform flame analysis but with occasional furnace samples. Simple flame system but with incredible versatility Six lamp auto-aligning carousel Double beam optics and self-calibrating Ebert monochromator Thermo Scientific ice 3400 Single furnace atomizer AAS with Zeeman and D2 background correction. When challenging detection limits are critical. Six lamp auto-aligning carousel Furnace vision Echelle dual prism and and grating monochromator Vapour system and electrically heated cell can be utilised in this instrument Thermo Scientific ice 3500 Dual flame and furnace AAS with standard or Zeeman furnace option. Essential furnace vision tool included as standard. Ideal for high throughput environments with a requirement for quick and regular flame and furnace analysis. Software-controlled changeover from flame to furnace analysis without the operator even being in the room! Six lamp auto-aligning carousel for maximum light throughput D2 background correction for flame and furnace analysis Zeeman background correction option available for furnace work Double beam optics with a dual monochromator consisting of an echelle prism and a grating Africa Australia Austria Belgium Canada China Denmark Europe-Other France Germany India Italy Japan Latin America Middle East Netherlands South Africa Spain Sweden / Norway / Finland Switzerland UK USA FM Thermo Fisher Scientific Inc. All rights reserved. All other trademarks are the property of Thermo Fisher Scientific Inc. and its subsidiaries. Specifications, terms and pricing are subject to change. Not all products are available in all countries. Please consult your local sales representative for details. Thermo Electron Manufacturing Ltd (Cambridge) is ISO Certified. TN40942_E 02/08C Part of Thermo Fisher Scientific