ASTM D7455 Standard Practice for Sample Preparation of Petroleum and Lubricant Products for Elemental Analysis
7. Methods Requiring No Sample Preparation
7.1 There are a number of test methods that essentially require no sample preparation. Mostly these test methods are X-ray fluorescence (XRF) test methods, and they include Test Methods D2622, D4294, D6334, D6445, D7039, D7212, and D7220 for sulfur; Test Methods D3348 and D5059 for lead; Test Method D3605 for trace metals; and Test Methods D6443 and D6481 for metals, and Test Method D7751 for additive elements in lubricating oils.
7.1.1 However, in some procedures such as D2622, dilution is needed for samples containing >4.6 m% sulfur or for samples containing >5 m% oxygen or other metals. Similarly, in Test Method D4294, it may be necessary to remove certain interfering elements from the matrix before measurements.
7.1.2 A monochromatic wavelength dispersive XRF method has been developed for the determination of trace amounts of silicon in gasoline and related products. This Test Method D7757 was in response to an industry need to counter the silicon contamination of gasolines.
7.2 For some X-ray fluorescence test methods given above, mixing with the internal standard is necessary before the analysis (for example, Test Method D5059 for lead).
7.3 Other non-XRF test methods that do not need special sample treatment include Test Method D3230 for salt in crude oil; Test Methods D3701, D4808, and D7171 for hydrogen by NMR; Test Method D5291 for carbon-hydrogen-nitrogen by combustion; Test Methods D6595 and D6728 for contaminants by rotrode atomic emission spectrometry; Test Method D6732 for copper by graphite furnace atomic absorption spectrometry, and Test Methods D7318, D7319, and D7328 for chloride and sulfate by potentiometry and ion chromatography.
8. Methods Requiring Solvent Dilution
8.1 For several test methods, the only sample preparation required is a simple dilution with appropriate organic solvent. Most of these test methods are based on atomic absorption spectrometry (AAS) or inductively coupled plasma atomic emission spectrometry (ICP-AES) measurement of analytes of interest. The reason for this step could be that either the sample is too viscous to flow through the nebulizer or the viscosity of the sample needs to match that of the calibration standards. In both cases this is achieved by appropriate dilution with an organic solvent or a solvent mixture.
8.2 A variety of organic solvents have been used for this purpose: methyl iso-butyl ketone, heavy distillate, kerosene, xylenes, acetone, etc.
8.3 The test methods that require such solvent dilutions include:
8.3.1 Atomic absorption spectrometry Test Methods D3237 and D3341 for lead, Test Method D3831 for manganese, Test Method D4628 for additive metals, and Test Method D5863 B for nickel, vanadium, iron, and sodium in crude oils.
8.3.2 ICP-AES Test Methods D4951 for additive elements, Test Method D5185 for additive elements and wear metals, Test Method D5708 for nickel, vanadium, and iron in crude oils, Test Method D7040 for phosphorus, Test Method D7111 for trace metals, and Test Method D7691 for multielement analysis of crude oils.
8.3.3 XRF Test Methods D4927 for additive elements.
8.3.4 Miscellaneous Test Method D5384 for chloride by coulometric titration, and Test Method D6470 for salt in crude oil by potentiometric titration.
9. Ash and Sulfated Ash Procedures
9.1 The fundamental and empirical procedures used for destroying organic materials and converting all inorganic components to oxides or sulfates are a cornerstone of many analyses.
9.1.1 In ash Test Method D482, a sample is ignited and later heated in a muffle furnace at 775°C to constant weight. All nonvolatile inorganic species are converted to oxides. The resultant ash can be used for the dissolution in acid(s) and the determination of metals by spectroscopy or other means.
9.1.1.1 Examples of procedures using preliminary treatment by ashing step include Test Methods D1318 for sodium, Test Method D5056 for trace metals, Test Method D5184 for aluminum and silicon, and Test Method D5600 for metals in coke.
9.1.2 In the sulfated ash (SASH) Test Method D874, a sample is ignited and the residue is treated with concentrated sulfuric acid and finally heated in a muffle furnace at 775°C to a constant weight. Although it is presumed that all metals are converted to sulfates in this procedure, experimental evidence definitively shows that the resultant residue contains a number of sub-stoichiometric compounds, phosphates and pyrophosphates in addition to sulfates. As a result, the theoretical sum of total sulfates calculated from metal concentrations in the sample does not match the experimental weights of residues obtained by this procedure.
9.1.2.1 There are a number of notes in Test Method D874 that caution about the interaction of various metal species during this procedure.
9.1.3 In both ash and SASH procedures, platinum, quartz or high purity silica crucibles are preferable. However, most of the time many laboratories use porcelain or quartz crucibles. Platinum ware should not be used if the sample contains phosphorus, since it can react with the platinum and contaminate the ash residue. Similarly, alkalies present in crude oil during field treatment can react with platinum or silica crucibles.
9.1.3.1 Platinum, silica, quartz, porcelain or other crucibles being used should be placed on silica plates or silica triangles on the floor of the muffle furnace.
9.1.4 Microwave ovens have also been used for dry ashing of petroleum products. However, all volatile organic-based products need to be handled in this step with extreme care to prevent fire hazards. See extensive warning notes given in Test Method D7303 regarding the use of microwave ovens/furnaces/digestion devices.
9.1.5 Most metals will be converted to oxides and sulfates, respectively, in these ash and SASH procedures. The sulfates are preferable because metal sulfates are generally non-volatile. The residue obtained by this procedure can be used for dissolution in acid(s) such as nitric and hydrochloric acids with water, and the determination ofmetals by spectroscopy or other means.
9.1.6 Examples ofthe procedure using sulfated ashing as the preliminary sample treating step include Test Method D3340 for lithium and sodium in greases, Test Method D5708 B for nickel, vanadium, and iron in crude oil, and Test Method D7303 for metals in lubricating greases.
9.1.7 During the initial ignition period, the muffle door should be carefully adjusted so that too great an air flow into the muffle will not cause the ash to be blown from the beaker or to be lost because of too rapid burning of the carbonaceous material.
9.1.8 Contamination from the muffle furnace should be prevented by properly covering the specimen container with a lid of the same type of material as the crucible, to prevent contamination of the samples with particles from the roof, walls, and the door of the furnace. The heating element or furnace walls should be replaced if they show flaking.
10. Test Methods Requiring Decomposition With Acid(s) or Other Reagents
10.1 A variety ofdecomposing agents have been used based on the analysis requirements for the specific sample types. The most commonly used acids are sulfuric and nitric acids or their mixtures, sometimes supplemented by hydrogen peroxide for faster oxidation of organic materials (see Test Method D1091 for phosphorus). (Warning - Poison. Causes severe burn. Harmful or fatal if swallowed or inhaled).
10.2 Other inorganic decomposing agents may have to be used in specific instances, such as zinc oxide in Test Methods D3231 and D4047 for phosphorus, and catalyst mixtures of potassium sulfate + mercuric oxide + copper sulfate for Kjeldahl nitrogen determination in Test Method D3228.
10.2.1 The Kjeldahl Test Method D3228 for nitrogen is not applicable to materials containing N-O or N-N linkage resulting in low results if such compounds are present in the sample. For such materials, alternate nitrogen Test Methods D4629, D5291, or D5762 may be used for obtaining the true estimate of total nitrogen content.
10.3 Silica, ifpresent in the residue, will not be dissolved by acid digestion. If required, silica can be dissolved with a few drops of hydrofluoric acid; the excess can be neutralized or complexed with dilute boric acid solution. However, this may cause later interference in the determination of other metals in the solution.
10.4 Examples of acid decomposition of ash residues of petroleum products include Test Methods D1548 (vanadium in crude oil), D5863 A (nickel, vanadium, iron, and sodium in crude oils), and D7303 (trace metals in lubricating greases).
10.5 Heating in an acid mixture in a microwave oven can be used for effective decomposition such as in Test Method D7303 for trace metals in lubricating grease samples. See the detailed caution statements given in Test Method D7303 in this regard.
10.5.1 A standard practice for sample decomposition using microwave heating, with or without prior ashing, has been described in Practice D7876. The metal determination is completed using atomic spectroscopic techniques.
10.6 A standard practice for preparation of oils and oily waste samples by high-pressure, high-temperature digestion followed by atomic spectroscopic determination is described in Practice C1234.
11. Test Methods Requiring Bomb or Lamp Decomposition
11.1 In several cases, the complete decomposition of organic material is achieved by combusting a small amount of sample in an oxygen atmosphere in a closed vessel. The sample burns in contact with pressurized oxygen converting the organic material into carbon dioxide and water, and the metals to oxides. After the combustion is complete, the residue can be dissolved in suitable acid(s) and the metals determined by various means.
11.2 Examples of such combustion procedures include Test Methods D129 for sulfur and D808 for chlorine, both determined gravimetrically.
11.2.1 Instead of using a pressurized bomb, the following test methods use a lamp for combustion: Test Method D1018 for hydrogen (determined gravimetrically), Test Method D1266 for sulfur (determined gravimetrically), and Test Method D2784 for sulfur in LPG determined by titration or turbidimetry.
11.3 For the determination of sulfur in heavier products that cannot be burned effectively in a lamp, other test methods such as the bomb Test Method D129 or the high temperature Test Method D1552 can be used.
11.4 Strict adherence to all of the provisions prescribed in these test methods ensures against explosive rupture of the bomb, or a blow-out, provided the bomb is of proper design and in good mechanical condition. It is desirable, however, that the bomb be enclosed in a shield of steel plate at least 13-mm thick, or that equivalent protection be provided against unforeseeable contingencies.
11.5 Most of these test methods have been largely supplanted by more modern techniques in the industry laboratories.
12. Test Methods Using Combustion - Adsorbant Trains
12.1 In addition to simple combustion followed by the detection of the analyte of interest described in Section 11, several other test methods use separation of combustion products that may require adsorption on specific reagents for specific gaseous oxides or gas chromatographic separation.
12.2 On combustion in oxygen, all nonmetallic constituents of the sample are converted to gaseous compounds such as carbon dioxide, chlorine, nitrogen, water vapor, sulfur oxides, etc. A variety of adsorbant columns have been used for the separation of these gaseous products.
12.3 A majority of these test methods are used for the determination ofsulfur in a variety ofproducts. In all cases, the separated analyte gas of interest is quantitatively measured by a variety of means of detection such as iodate color or infrared spectrometry (Test Method D1552 for sulfur), UV-fluorescence (Test Methods D5453, D6667, and D7620 for sulfur), chemiluminescence (Test Methods D4629 and D5762 for nitrogen), microcoulometry (Test Methods D3120 and D3246 for sulfur), thermal conductivity (Test Method D5622 for oxygen), rateometric colorimetry (Test Method D4045 for sulfur), electrochemistry (Test Method D6920 for sulfur), and flame photometry (Test Method D7041 for sulfur).
12.4 Various adsorption trains can be used for the removal of undesirable oxidation products. These include removal of water vapor using phosphoric acid dehydration cell (Test Method D3120 for sulfur), anhydrous magnesium perchlorate (Test Methods D4629 and D5762 for nitrogen, and D6920 for sulfur), and membrane drying cell or a permeation dryer tube (D5453, D6667, D6920, and D7620 for sulfur). Test Method D5622 for oxygen determination uses a train of molecular sieve for absorbing carbon dioxide, heated copper column for absorbing sulfur oxides, and a scrubber for absorbing acidic gases. Gas chromatographic separation columns are used to separate carbon dioxide and water vapor for the determination of sulfur as sulfur dioxide in Test Method D7041.
13. Miscellaneous Test Methods
13.1 Organic chloride in crude oil is determined by Test Method D4929 either by sodium biphenyl reduction and potentiometry (Procedure A) or by combustion and microcoulometry (Procedure B). However, before these steps, the crude oil sample is distilled using Test Method D86 and a naphtha cut at 204°C is collected. This naphtha cut is washed with 1M KOH solution to remove hydrogen sulfide, and next with water to remove the inorganic chloride salts.
13.2 Mercury in Crude Oil - Determination of mercury in crude oils poses a particular problem in terms of sensitivity for ppb level measurements required, and the volatility of mercury species from crude oils during storage and handling. Mercury is present in various molecular forms in crude oils as metallic Hg(0) and other molecular species including alkyl mercury. Multiphase of analyte and matrix mixtures homogeneity needs to be considered.
13.2.1 The most promising storage containers appear to be quartz, borosilicate glass, titanium, sulfinet-coated cans, epoxy lined cans, and glassy carbon. The most likely preservative for speciation would be freezing in liquid nitrogen. The best preservative for keeping total mercury in solution appears to be dithizone, which forms colored stable complexes with many transition metals.
13.2.2 Many high purity solvents have a significant degree ofoxidizing capacity (as defined by the ability to oxidize Hg(0) to Hg+2). Thus, before using any solvent to dilute a hydrocarbon for mercury analysis, the solvent should be "redox-neutralized". See Practice D7482 for details.
13.2.3 While bulk mercury droplets are not highly volatile (vapor pressure <1 mm Hg at 60°F), dissolved mercury readily evaporates with an apparent vapor pressure similar to that of butane or pentane. Also, mercury can easily adsorb on to many metal surfaces. To avoid loss of mercury vapor from the samples, vials should be filled as quickly as possible, and to their total capacity or with only a small (~1 mL) headspace remaining. The vials should be capped immediately.
13.2.4 In no case should the hydrocarbon samples be taken directly or sub-sampled from tin-lined cans. Metal containers in any step of the sampling process are discouraged. Glass is preferred. See Practice D7482 for more details.
13.2.5 Organo-mercury species have been shown to be stable for at least 30 days in glass, aluminum, steel, or PTFE containers, while mercuric oxide was stable in all of these plus stainless steel and polyethylene containers. Mercury (II) was rapidly lost from all containers except those made from aluminum. Total mercury, mercury (II), and mercury methyl chloride in crude oil were stable over the course of more than 3 months in glass containers, but mercury (0) decreased with a half life of about 40 days. These storage experiments indicate that while total mercury, methyl mercury chloride, and dimethyl mercury are quite stable in glass and Teflon containers, for complete speciation, the samples must be analyzed as soon as possible. The metal and polyethylene containers are unsuitable for the collection and storage of mercury in petroleum due either to loss of mercury or species inter-conversion.
13.2.6 The volatile species of mercury (0) and dimethyl mercury partition to sample bottle headspace. The volatile forms of mercury can be lost from sample containers when they are opened, lost from aliquots that are removed from the containers, and lost in aliquot processing steps prior to element detection.
13.2.7 Contamination from improper sample containers should be guarded against. In general, the sample containers for mercury analysis should not be reused unless specially cleaned and tested as a blank prior to sampling. Uncoated metal containers shall not be used for any sampling step. Stainless steel containers may be used but must be cleaned and rinsed with a mild acid rinse and thoroughly dried before use. Epoxy-lined steel containers may also be used, and are preferred for larger volume (>1 L).
13.2.8 For further discussion on storage and analysis of crude oils for mercury, see Practice D7482.
13.2.9 Methods based on cold vapor atomic absorption spectrometry for determination of mercury in crude oils are available in Test Methods D7622 and D7623.
