1 Selection of the best drying parameters for the dispensed sample volume is an
important criterion in achieving satisfactory precision. The droplet should dry smoothly without vigorous boiling.
With aqueous solutions best drying conditions have been achieved by ramping to a temperature of about 85 °C in 5 seconds and then applying a slow ramp to about 95 °C. It is possible to dry the aqueous sample smoothly with a maximum gas flow at a temperature below the boiling point with an extended dry time. For other solvents it is recommended that a 5 second ramp to a temperature of about 60% to 70% of the boiling point be followed by a slow ramp to about 90% of the boiling point and this temperature maintained with maximum gas flow until drying is completed. Alternatively, the solvent may be conveniently injected onto a pre-heated tube and dried virtually on contact.
2 Droplet drying should always take place on the internal section of the graphite
tube for best precision. If the liquid spills over the internal partitions, the resultant spread of the liquid will cause variability in the analytical signal. If the liquid is a corrosive acid and the dispensed volume is so great that the liquid spreads to the ends of the graphite tube, it may result in corrosion of nearby metal components.
If the pyrolytic platform is used it must be ensured that the droplet remains within the platform and does not spill over onto the graphite tube wall. The droplet should dry smoothly. Use of the mirror supplied will assist the correct choice of drying parameters to ensure this does not occur.
3 The dispensed volume itself must not be so great, or deposited in such a
manner, that the sampling hole is blocked. In this event the windows may become covered in condensate from the drying step. The gas should always be flowing during the drying stage, preferably at maximum flow, to remove the drying or ashing products.
4 20 seconds is automatically allocated for the cool down period after completion
of the atomization program. This will ensure cooling of the graphite to within 10 °C of the cooling water temperature, before the next sample is introduced.
5 Gas Flows: The flow should be kept high (3 L/min) during the drying period to
remove solvent etc. If air ashing is employed, the flow should be sufficient to remove ashing products but should not continue when the temperature approaches 500 °C.
The presence of air will result in excessive corrosion of the pyrolytic graphite at temperatures greater than about 500 °C. At higher temperatures the corrosion rate is greatly accelerated. It is recommended that the inert gas flow is not stopped during atomize for longer than about 6 - 7 seconds. This will normally allow sufficient time to measure the true peak.
The inert gas flow should always be recommenced after the peak has been completed if gas stop is used during atomization. This will help protect the graphite tube. Remember that the maximum gas flow automatically commences on completion of the program and extends for 20seconds, to ensure that the hot graphite is protected during cool down.
6 The quartz windows should be inspected at regular intervals to ensure that they
are clean. They may be cleaned with a tissue and alcohol, and wiped dry.
7 It is always good practice to choose an analytical program which minimizes the
amount of background (or non-atomic) absorption during atomization. It is always recommended that the background corrector be on for analytical measurements.
8 The use of chemical (or matrix) modifiers in modern graphite furnace analyses
is widespread (see also Chapter One). The signal graphics facility will be of great benefit in establishing the correct amount of modifier and most suitable furnace program for your analysis.
9 The most suitable analytical wavelength for the Zeeman graphite furnace
determination of each element has been given in Chapter Four. This has been based on the analytical sensitivity, MSR% and linearity of calibration (see also Calibration Procedures, Chapter Five).
If other wavelengths are used, it will be necessary to establish the maximum absorbance (MAX ABS). Above a defined maximum absorbance the calibration will either asymptote to a constant absorbance value or reflex to lower
absorbances. It is essential to establish this MAX ABS value for each
wavelength before proceeding with the calibration (see Calibration Procedures, Chapter Five).
10 Dynamic range of an analysis: By careful choice of certain parameters, the
analytical signal from any analysis can be varied considerably. For example, the signal may be increased by:
a Using argon rather than nitrogen as the inert gas
b Using gas stop during atomize, rather than gas flow (the gas flow can be
varied from 0-3 L/min.)
g Conversely, the signal may be reduced by the appropriate selection of
the above parameters.
11 With furnace atomization, three standards are normally sufficient to establish
the analytical curve. One or two standards are quite adequate for many analyses for which the highest absorbance is about 0.5. Most important is the correct selection of the standard values within the analytical range. By using five standards for calibration, the analysis time is extended, and the accuracy of the measurement will not always be improved.
12 For the graphite tube, the maximum recommended sample volume, consistent
with an acceptably short drying time, would be about 40 μL. Volumes in excess of this may take an unacceptably long drying time, and also show poorer precision. For the graphite platform, the maximum recommended sample volume would be about 20 μL (See Chapter Four.)
13 Programmable Sample Dispenser: The PTFE capillary should be treated
carefully - if it is bent it may take some time to return to its initial shape. When the multiple injection facility is used, it may be necessary to allow a cool down period before the next injection (from the same sample vial). This cool down period should be incorporated into the program, following the required dry and ash stages.
The tip should be cleanly cut at 90°. If variable volumes are automatically dispensed during an automated analysis, the capillary tip should be set for the minimum volume dispensed. In this case, the drying time should allow the maximum volume to dry.
After continuous operation some graphite may appear on the capillary tip. This should be carefully removed by wiping with a tissue. (The dispensing
characteristics may change in the presence of this graphite film.)
14 It is necessary that the rinse bottle of the autosampler contain about 0.01% by
volume of nitric acid for work with aqueous solutions. It may be beneficial to also add 0.005% by volume of Triton X-100 for some samples such as biological solutions or organic solvents.
15 The normal (inert) gas used in these studies was argon. It is used in preference
to nitrogen.
16 Graphite tube lifetime: There are many factors which affect the useful lifetime
of a pyrolytic coated graphite tube. Among these are the sample type, matrix composition, program temperature, total gas stop time and type of inert gas. It has been found that materials such as perchloric acid, perchlorates, sodium nitrate and ammonium nitrate adversely affect the coating, and high
concentrations of these materials tend to reduce tube lifetime. The higher the programmed temperature the shorter is the useful tube lifetime. Argon provides better protection for the graphite than nitrogen and allows a longer tube life. Argon is the recommended inert gas.