ASTM D7343 Standard Practice for Optimization, Sample Handling, Calibration, and Validation of X-ray Fluorescence Spectrometry Methods for Elemental Analysis of Petroleum Products and Lubricants
4. Sample Handling
4.1 It is necessary to use precautions to minimize the possibility of contamination of trace elemental analysis samples. Good laboratory practices in this area include:
4.1.1 Samples received by the laboratory and required for trace element analysis should be stored in a designated specific location for storage while awaiting analysis. This area, whenever possible, should not contain samples that could contaminate those requiring trace element analysis.

4.1.2 All laboratory equipment used specifically for trace element analysis should be free of any source of contamination. This may require that specific equipment be used only for trace element analysis.

4.1.3 Analyses of blank samples are highly recommended.

4.1.4 Sample preparation should be carried out in a clean area. This area should use surfaces that can be decontaminated easily if a spillage occurs.

4.1.5 Operators should wear clean, fresh, protective gloves for sample preparation for trace element analysis. Tests should be run to confirm that the gloves do not contain interfering elements or elements of interest, since they may cause contamination. The development of clean area sample handling protocols is encouraged.

5. Sample Preparation
5.1 Choice of Sample Carrier - XRF testing requires a sample cell and a support film to hold the liquid sample in place during analysis. The choice of the sample cell or cup, the material in which it is held, and the type of support film used can all influence the result.
5.1.1 Sample Cell - The most common cell is a plastic cup, of which various designs are available. These designs allow for a variety of sample types to be measured either in a liquid or powder form. It is important to check that the cup type used is best suited for the compositions of samples to be analyzed. Liquid sample cups usually have a seal that ensures the film is sealed to a level above that of the liquid in the cell and that the film is taut with no wrinkles.
5.1.1.1 Within XRF spectrometers heat is produced, both from the spectrometer components themselves and from the interaction of X-rays with the sample. Petroleum products that are not stable due to volatility should only be placed into vented sample cups or special sealed sample cups specifically designed for volatile samples (see 8.3).

5.1.1.2 The cup size may be important. Depending on the film type used to support the liquid, different films will sag due to the weight of sample and relax due to chemical interaction, or heat, or both. To reduce this sagging effect, the smallest diameter sample cups should be used. Cups with diameters well in excess of the area detected by the spectrometer are likely to increase errors due to sagging.

5.1.1.3 A number of petroleum products require heating to ensure homogenization prior to analysis or to enable transfer to the sample cell; examples include fuel oils and wax products. The sample cup should be able to withstand the temperature used in this process. In general, most plastic sample cells should withstand temperatures up to 70 °C.

5.1.2 Sample Cell Holder - Many manufacturers recommend metal holders to hold sample cups while they are transferred into the XRF instrument. These holders can be made from aluminum, stainless steel, or other materials. It is important to recognize that these represent a potential spectral contamination to the analysis either if the spectrometer is to determine an analyte that the holder is made from or if the material from the holder causes an interference with the analyte. Generally, this is not a problem for elements with atomic number <30. For elements with atomic number >30 it is advisable to check the potential contamination from the sample cup holder using a blank.

5.1.3 Sample Support Films - Many support films are available from both XRF instrument manufacturers and accessory suppliers. It is important to examine the film types specified in any method being used. There are four criteria that should be considered when selecting a X-ray transmission sample support film:
(1) Thickness of film,
(2) Composition of film,
(3) Chemical and physical resistance of film to the liquid intend for analysis, and
(4) Element contaminants contained within the film.
5.1.3.1 Film thickness typically ranges from 2 µ to 6 µ for most applications. Consideration should be given to the variations in thickness from batch to batch of films. For thinner films, the relative variance in film thickness is often higher than that of the thicker films, thus precision of analysis can be affected more if thinner films are used. One way to avoid this is to recalibrate or adjust calibrations using monitors each time a new batch of film is used.

5.1.3.2 Film types are composed of different polymer materials. Those containing oxygen or nitrogen will absorb lighter elements more than those that do not. Examples of oxygen and nitrogen containing polymers are polyester and polyamide. For the determination of elements lighter than sulfur, these films should be avoided in favor of polymers containing only carbon and hydrogen, provided that the film is not attacked by the sample.

5.1.3.3 Chemical resistance is often a compromise with film type. Often, the best resistance is offered by polymers containing oxygen or nitrogen. Physical aspects such as temperature will also be an issue especially if hot liquids are to be measured. Most film types will withstand temperatures up to 80 °C, but relaxation of the polymer, especially in wide cups, will affect accuracy and precision.

5.1.3.4 All films contain element contaminants. Before using any film, blanks should be run to ensure that the backgrounds are not elevated by the existence of a contaminant element present in the film. These contaminant elements will affect detection limits if they correspond to, or interfere with, the analyte element(s).

5.1.3.5 It is necessary to verify that the sample does not dissolve the film or permeate through it. This is especially important for gasoline-range samples, when a new product is to be analyzed, or when a new kind of film is used for a sample type. This verification can be done as follows:
(1) Prepare a specimen cup and fill it with a typical specimen.
(2) Place the cell on a clean tissue and wait for 30 min to 60 min.
(3) Remove the cell, and inspect the tissue and the underside of the film. Both should be dry.
(4) This test does not need to be repeated for every measurement when the analyst is certain that the film and the material to be analyzed are compatible.

5.1.3.6 Use of Multiple Films - A common method of ensuring that spectrometers are not contaminated by leaking films is to use a second film in the sample cup holder of the instrument. This provides a high level of security, and for many systems is essential to avoiding costly down times if a sample should leak. The use of this second film will increase both the detection limits as well as the errors of measurement. Some petroleum products can permeate through polymer films and, while this may not be a problem for any single analysis, the buildup on a second protective film in some cases may cause drift of analysis results. When trace level determinations are required and the optimum performance in both precision and detection limit are required, the use of secondary films should be given careful consideration. If they are considered essential, they should be inspected or replaced for every analysis as part of standard operating procedures.

6. Sample Stability
6.1 Sample stability during measurement is essential for accurate determinations. Pay particular care, since a sample can undergo physical change during analysis. An example of this is catalyst residues in fuel oils that can settle during measurement. When this type of situation can occur or is suspected, maintaining constant masses, heating times (in the case of fuel oils), mixing times, transfer time from preparation to measurement, and the determination of low atomic number elements first in a sequential analysis scheme should be used.

6.2 This procedure will not eliminate the particulate settling problem; filtering of such samples may be needed before analysis.

7. Instrument Set-Up Specific to Technique
7.1 Wavelength Dispersive X-Ray Fluorescence (WDXRF):
7.1.1 Before using any WDXRF spectrometer, it is essential that the instrument is performing to the manufacturer's specifications. Consult with the manufacturer on how to perform spectrometer quality control checks.

7.1.2 Pay particular attention to the goniometer settings for sequential instruments, and ensure the goniometer positions are set correctly. Before performing a calibration of the goniometer angles, it is highly recommended that pulse height discriminator settings (PHDs) be carried out for each element and background being used.

7.1.3 Account for observation of known instrument interferences. These include crystal fluorescence, tube line overlaps, and any element spectral contamination from the metals within the instrument construction. A number of these interferences can be avoided by careful selection of window settings during PHD set-up. For element interference, the selection of an alternative line or minimizing overlap using higher resolution collimators and crystals with higher resolving power (or smaller 2d spacing) can be used.

7.1.4 When carrying out analysis for trace elements, it is important to monitor the bias in the measurement using a blank specimen. Improved consistency in analysis can be achieved by subtracting the measured concentration of the analyte in the blank from the determined concentration of the analyte in the recently measured samples.

7.1.5 Even though minimizing the background using a fine collimator will improve the theoretical detection limit, it does so only by increasing the analysis time. Many types of samples do not remain stable during exposure to X-ray radiation. Therefore the analyst is responsible for establishing the benefits of lower detection limits at the expense of increasing the time the sample is exposed to X-ray radiation. Generally, the selection of a mid-range collimator balances a quality detection limit with an effective analysis time. However, contact the manufacturer for the best settings to achieve this for each element of interest or determine the settings experimentally.

7.2 Energy Dispersive X-Ray Fluorescence, EDXRF:
7.2.1 Before using any EDXRF spectrometer, it is essential that the instrument is operating at the manufacturer's specifications. Consult with the manufacturer on how to perform spectrometer quality control checks.

7.2.2 Pay particular attention to detector resolution. The manufacturer will provide the detector specifications that need to be met for different detector types.

7.2.3 Account for observation of known instrument interferences. These include the normal artifacts seen in EDXRF: escape peaks, pile-up or summation peaks, tube line overlaps, diffraction peaks, and any element spectral contamination from the metals within the instrument construction. A number of these interferences can be avoided by careful selection of voltage, primary beam filter, choice of secondary target (if provided in the instrument), and pulse processor settings. For element interference, consider selecting an alternative line or minimizing overlap using optimal settings for a particular element.

7.2.4 Accurate deconvolution of elements in EDXRF is very important in providing the most precise intensities for any analysis. Minimizing the number of deconvolution components overlapping for any analyte element will provide the most reliable data. The use of regions of interest (ROI), where appropriate, will produce data with the minimum of statistical error.

7.2.5 When carrying out analysis for trace elements, it is important to minimize background while still obtaining the maximum sensitivity possible. Generally, the selection of the highest resolution settings with narrow excitation conditions will achieve the best limits of detection. Contact the manufacturer for the best settings to achieve this for each element of interest or determine the settings experimentally.