ASTM D7111 Standard Test Method for Determination of Trace Elements in Middle Distillate Fuels by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
11. Preparation of Apparatus
11.1 Spectrometer - Prepare the ICP spectrometer according to the manufacturer's instructions and parameter settings for organic materials and the elements of interest. At least three integrations should be made for all samples (standards, blank, fuels) run. Table 1 provides recommended element wavelengths for fuels; however, other wavelengths may be used due to possible instrument variations or spectral interferences. The optical path can be purged with argon or another high purity gas (for example, nitrogen) recommended by the manufacturer. Before igniting the plasma, inspect the quartz torch to make sure that it is clean. If carbon build-up is observed, replace the torch and make the manufacturer's recommended adjustments for this problem. Warm up the instrument while purging the optics for the time period recommended by the ICP manufacturer. If necessary, replace the peristaltic pump tubing and adjust the solution uptake to the desired rate. Ignite the torch, then begin aspirating kerosine through the nebulizer and into the spray chamber. Continue plasma warm-up/stabilization for the duration specified by the ICP manufacturer.
11.2 Glassware, Plasticware - Acid clean glassware and plasticware with 10 % nitric acid (trace metal analysis grade) followed by several distilled water rinses. Do not use glassware and plasticware that has previously contained solutions with high concentrations of the element(s) of interest.
12. Preparation of Standards and Test Specimens
12.1 Purity ofKerosine - Sources of satisfactory high purity kerosine are commercially available. For ICP instruments which provide a visual profile of emission peaks, a check may be made of the kerosine purity by aspirating the kerosine and viewing the spectral regions where the element emissions of interest are to be found. The absence ofemission peaks in these regions is evidence that the purity is satisfactory.
12.2 Internal Standard Stock Solution:
12.2.1 The analyst's selection of the single element internal standard may be influenced by the capabilities (wavelength availability, sensitivity) of the ICP instrument available. The single element chosen for the internal standard should not be a component of the fuel test specimen or calibration standard. Organometallic yttrium has performed well as an internal standard for this test method and is recommended. Table 2 lists internal standards, their recommended wavelengths, and their approximate use concentrations for this test method.
12.2.2 Prepare a stock solution of the internal standard by weight from a 5000 mg/kg single element organometallic standard material and kerosine. Prepare a concentration that is approximately 50 times the concentration required in the fuel test specimen and working standard. Prepare a minimum of 50 grams of internal standard stock solution. Prepare fresh internal standard stock solution weekly.
12.2.3 The following is an example for preparing a nominal 50 mg/kg yttrium internal standard stock solution: Tare on an analytical balance a clean glass or HDPE plastic container (for example, 125 mL bottle, use HDPE for sodium analysis) sized for the following procedure. Weigh a nominal 0.5 g (to the nearest 0.001 g) of the 5000 mg/kg yttrium organometallic internal standard into the container. Add kerosine to bring the solution mass to a nominal 50.0 g. Determine the solution mass to the nearest 0.001 g. Seal the container and mix well. The internal standard stock solution concentration is determined in the same manner as described for the calibration standard in 16.1.
12.3 Working Standard:
12.3.1 Preparation of a nominal 2.0 mg/kg elemental working standard is described in this test method as an example. Higher or lower working standard concentrations may be prepared depending on the sensitivity of the ICP spectrometer (for example, radial or axial viewing, detector type, age ofICP) and the elements of interest in the fuel sample. Determine the suitability of the working standard concentration after calibration by analyzing a kerosine sample prepared with a known concentration (for example, 1.0 mg/kg) of the elements of interest. Measured values within 5 % of the prepared concentration are acceptable. Commercially available organometallic standards with certified element concentrations are suitable for this test method.
12.3.2 Calibration Standard Solution (nominal 2.0 mg/kg) - Tare on an analytical balance a clean glass or HDPE plastic container (for example, 125 mL bottle, use HDPE for sodium analysis) sized for the following procedure: Weigh a nominal 1.0 g (to the nearest 0.0001 g) of the nominal 100 mg/kg organometallic standard (for all elements of interest) into the container. Add kerosine to bring the solution mass to a nominal 50.0 g. Determine the solution mass to the nearest 0.0001 g. Seal the container and mix the solution well. Calculate the element concentrations as shown in 16.1. Use these values for establishing the calibration lines (see Section 14). The calibration standard solution is to be prepared daily when samples are to be analyzed.
12.3.3 Working Standard - The working standard is prepared with the calibration standard solution and the internal standard stock solution as follows: To a 50 mL volumetric flask, pipette 1000 µL of the internal standard stock solution. Fill the volumetric flask to the volume mark with the calibration standard solution prepared in 12.3.2. Seal the volumetric flask and mix well. Working standards are to be prepared daily when samples are to be analyzed.
12.3.3.1 Use of the yttrium internal standard stock solution described in 12.2.3 will provide a nominal 1.0 mg/kg internal standard in the working standard.
12.4 Check Standard - Prepare an instrument check standard in the same manner as the working standard (see 12.3) at element concentrations that are anticipated for the fuel samples to be analyzed. It is advisable to prepare the check standard from an alternative source of certified organometallic standard.
12.5 Test Specimens - To a 50 mL volumetric flask, add 1000 µL of the internal standard stock solution. Fill the flask to the mark with the fuel to be analyzed. This provides a fuel test specimen with an internal standard at the same concentration as provided in the working standard. If insufficient fuel sample is available, the flask volume and added internal standard stock solution volume may be proportionally reduced. Since the same amount of internal standard stock solution has been added to the working standard and the fuel test specimens, no dilution factor correction is needed.
13. Wavelength Selection and Background Correction
13.1 Recommended wavelengths for each element to be determined and for internal standards are given in Table 1 and Table 2, respectively. To accommodate different ICP instruments and their performances, other wavelengths not shown in these tables may be used. Select wavelengths with best intensity, peak shape, and lack of interferences. Since analyses are for trace levels of elements, background correction is required. Thus, for all elements possible, the baseline for the emission peak should be set with points as close to both sides of the peak without measuring the element wavelength intensity (see Fig. 1). After these baselines are set, a check standard is used to test the system response and calibration as described in the next section.
13.2 Appropriate selection of wavelengths for background corrections is extremely critical for the determination of sodium since the predominate sodium emission line (588.995 nm) resides near a significant argon emission interference. During method development it is recommended, if possible, that the spectra of samples and standards be compared at the sodium emission wavelength to ensure that signal integration occurs accurately.
NOTE 1 - Some emission wavelengths occur on a highly structured background (for example, sodium emission at 588.995 nm); hence a single off-peak background measurement may provide inaccurate results. For emission wavelengths on a structured background, background correction is recommended at both lower and higher wavelengths from the emission wavelength. In addition, some low-resolution, photomultiplier tube-based instruments may require a comprised selection of background points, which could also provide inaccurate results.
14. Calibration
14.1 A two-point calibration, consisting of the kerosine blank and the working standard, of the instrument is conducted within the linear range of the spectrometer. Calibration shall be performed each time a new batch of fuel samples is to be analyzed.
14.2 Analyze the check standard to determine if all elements are in calibration. Each element must be within 5 % of its prepared concentration in order to proceed to testing of fuel samples. If not, make necessary instrument adjustments and recalibrate until all elements are within 5 % of check standard values.
14.3 Aspirate kerosine between standard (and fuel test specimen) runs to purge the system of elements prior to the next run. If high element concentrations have been run, check the element signal intensity after the kerosine purge to ensure that it has been removed.
14.4 Most ICP spectrometers have software that automatically performs the calculations to establish the calibration curve when using an internal standard. Element emission intensities are ratioed to the internal standard emission intensities. Subsequent references to emission intensities in this section and Section 15 pertain to baseline corrected peak areas. The calibration curve is a plot of the intensity ratio for an element e in the working standard (Rws) versus the concentration of element e in the calibration standard (Ccs), and
Rws = (Iws - Ib)/Iints
where:
Iws = emission intensity for element e in the working standard,
Ib = emission intensity for element e in the kerosine blank, and
Iints = emission intensity of the internal standard in the working standard solution.
