10 Analytical Instrumentation

Less than 90 Minutes Turnaround

Time for Trace Metal Analysis on
Petroleum Products

Dr. Linda Kuenstl, Product Management Sample Preparation, Anton Paar GmbH
Graz/Austria
Email: asc@anton-paar.com • Web: www.anton-paar.com • Tel: +43 316 257-0

Conventional methods for the sample preparation of petroleum products and lubricants like dilution or ashing suffer from severe drawbacks.
Microwave-assisted acid digestion according to ASTM D7876 is a beneficial approach which achieves reliable results in a short time frame.

Petroleum products and lubricants are routinely analyzed for their elemental content for various Proof of Principle
reasons. In crude and residual oils the concentrations of silicon, aluminum, vanadium, nickel,
In order to compare conventional methods such as ASTM D5708, D5863, D4951, D5185 and IP
iron and sodium are used to define their quality and value. Nickel and vanadium in crude oil can
501 with microwave-assisted closed-vessel digestion an engine oil lubricant, a crude oil sample
deactivate catalysts during processing, but also initiate corrosion in motors and boilers during the
and a “NIST standard reference material 1634c – trace elements in fuel oil” were investigated.
combustion when present in fuels. The presence of silicon and aluminum in residual fuel oils causes The engine oil lubricant and the crude oil sample were part of an ASTM interlaboratory cross-
abrasion within the combustion engine. Before use, products like lubricating oils are tested to check program. Anton Paar’s Multiwave PRO with Rotor 8NXF100 was used to digest the samples.
determine the concentration of additives (which contain metals such as calcium, copper, magnesium, Approximately 0.5 g of the respective samples were digested under stirring for 65 minutes with
phosphorus, sulphur and zinc) as this is an important quality control parameter. a temperature limit of 260 °C and a pressure limit of 60 bar using a mixture of HNO3 (65 %) and
H2O2 (30 %) as reagents (see Fig. 1). To investigate the sample homogeneity, even when small
sample amounts are used, all samples were processed in triplicates. For a complete dissolution
Standard Methods
of silicon, the addition of hydrofluoric acid (or a fluoride salt like ammonium fluoride) might be
There are several standard methods available which describe the sample preparation prior to required. For this reason a second set of digestions was performed after adding a 50 % NH4F
elemental analysis in petroleum products. These methods mainly apply two different approaches. solution to the reagent mixture. In this way HF is generated in situ, while direct handling of
The first method incorporates the dilution of the oil with an organic solvent and direct introduction concentrated hydrofluoric acid is avoided. The samples without NH4F were measured on ICP-
into an AAS or ICP-OES (ASTM D4951, D5708 – A, D5863 – B, D5185). This method is fast but not OES only, whereas the samples with NH4F were analysed on both ICP-OES and ICP-MS. Prior to
applicable for samples containing larger particles. Typical drawbacks are clogged nebulisers, instable measuring with ICP-MS the samples were diluted 1 + 9.
plasma conditions and measurement interferences. The second frequently used method is dry
ashing (with subsequent acid digestion); dry ashing burns off the organic matrix of the sample. The
inorganic residues, the ashes, are dissolved with acids in an open digestion system and subsequently
analysed as an aqueous solution (IP 501, ASTM D5708 – B, or D5863 – A). This method allows for
processing of large sample quantities (> 10 g) but suffers from significant errors related to loss of
volatile elements and results in turnaround times in the range of 10 hours.

Current Change of Technology

More and more laboratories are changing their analytical measuring equipment from FAAS to
modern measuring techniques such as ICP-OES or ICP-MS. There are several advantages provided by
these techniques: they are rapid, provide multi-elemental capability and, especially for ICP-MS, have
a wide dynamic range. Additionally, they have much lower limits of determination and therefore do
not require several grams of sample to obtain accurate analytical data. This fact opens the door for
modern sample preparation techniques like microwave-assisted closed-vessel digestion. The release
of the new standard practice ASTM D7876 (which covers sample decomposition by using microwave
heating) confirms the increased importance of this sample preparation method in the petrochemical
industry.

Microwave-Assisted Closed Vessel Digestion

Using microwave-assisted closed-vessel digestion, the sample is decomposed with concentrated acids
Fig. 1: Digestion program of crude oil samples. The introduced microwave power (blue line)
under pressure resulting in a clear aqueous solution with low residual carbon content. Due to the
follows the preset power program until the pressure limit of 60 bar (green line) is reached in any
application of closed vessels which withstand an operating pressure of up to 80 bar, the temperature
of the monitored vessels. If this is the case, the power is reduced accordingly. The three crude oil
is not restricted to the boiling point of the acids. According to the Arrhenius Law, a 10 °C increase samples reach a vessel surface temperature of about 200 °C; the blank vessel temperature (yellow
in temperature means a 2-fold acceleration of the reaction and halving of the reaction time. As a line) is lower as no reaction happens inside. As the vessels are continuously cooled to remove
result of the achievable high temperature the time required for complete digestion is significantly overheat and to minimise material stress, the surface temperatures are lower compared to the
reduced. Microwaves are the perfect heating source as they heat the liquid bulk directly while the internal temperatures (digestion temperatures). If the inner temperature is requested, an immersion
surroundings, such as vessels, remain comparably cool. As a consequence, microwave heating is temperature probe can be used for the digestion. It can be clearly seen that reactive samples such as
fast and energy-efficient, saving additional process time. Further advantages of this modern sample oil create a high pressure and high temperature during digestion. Therefore, a prerequisite for results
preparation technique are no loss of volatiles and a minimised risk of contaminations. Based on the is a microwave instrument which withstands both high temperature and high pressure at the
precise documentation, the sample preparation procedure is highly reproducible. same time.

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 Analytical Instrumentation 11
The measured values after microwave digestion are in good agreement with the data obtained from
Results conventional methods. Additional elements (which have not been considered in the interlaboratory
The measured element contents of engine oil lubricants, shown in Table 1, and of crude oil, shown cross-checks or the NIST certificate) have been measured for comparison purposes. For most of
in Table 2, are compared with the values obtained during the ASTM interlaboratory cross-check. The
these elements the values from ICP-OES and ICP-MS are comparable, indicating a reliable sample
element contents of NIST fuel oil, shown in Table 3, are compared with the certified value.
preparation procedure. Results for ICP-OES measurement were combined from both acid mixtures
(mean value of digestions with and without NH4F) as no significant differences could be observed.
Table 1: Results for Engine Oil Lubricants
The low standard deviations observed for the 6 independently digested samples indicate a
D7876 D7876 D4951 D5185 homogenous sample and shows that 0.5 g of sample, instead of the >10 g conventionally used, are
ICP-OES ICP-MS ICP-OES ICP-OES
[µg·g-1] [µg·g-1] [µg·g-1] [µg·g-1] sufficient to achieve representative results.
Al 4.1 ± 1.1 Silicon could be determined only from the solutions containing NH4F and this determination only
B 249 ± 1 229 ± 26 worked on the ICP-OES due to its HF-resistant layout. Generally, it depends on the amount and type
Ca 2130 ± 70 2170 ± 10 2400 ± 100 2390 ± 200 of silicon present in the sample whether NH4F (HF) has to be used or not.

Fe 0.7 ± 0.1 < 3.5

Mg 9.0 ± 0.5 Why Use Anton Paar’s Multiwave PRO Plus Rotor 8NXF100?
Mo 79.9 ± 1.5 78.5 ± 0.4 77.0 ± 6.0 74.3 ± 10.1 Multiwave PRO combined with Rotor 8NXF100 can hold a temperature of 260 °C and simultaneously
Na < 10 < 10 withstands a pressure of 60 bar for a time frame of more than 2 hours. These combined
Ni 0.49 ± 0.12 <5 specifications are required to obtain complete digestions for petroleum products. Petroleum products
P 831 ± 16 761 ± 38 743 ± 66 are highly reactive; they create a lot of pressure and heat during the digestion. To prevent damage
to the equipment it is essential to have the temperature and pressure under full control. The Rotor
S 2830 ± 90 2950 ± 220 2830 ± 370
8NXF100 provides wireless pressure (Fig. 2) and temperature control of each vessel. The pressure is
Ti 94.5 ± 1.8
measured 50 times per second and is simultaneously monitored in all reaction vessels. If the pressure
V 1.1 ± 0.1
or the pressure increase rates exceed the preset limits the microwave power is immediately shut off
Zn 690 ± 20 837 ± 7 837 ± 42 825 ± 76 and, if necessary, the vessels are cooled via a guided airflow along the vessel surface.

ICP-OES: n = 6 (mean of digestions with and without NH4F)

ICP-MS: n = 3 (only digestions with NH4F)

Table 2: Results for Crude Oil

D7876 D7876 D5708 (A) D5708 (B)

ICP-OES ICP-MS ICP-OES ICP-OES
[µg·g-1] [µg·g-1] [µg·g-1] [µg·g-1]
Fe 6.7 ± 0.6 8.35 ± 0.27 5.4 ± 0.6 6.2 ± 1.2
Mg 2.3 ± 0.3
Mo 1.1 ± 0.1 0.98 ± 0.02
Na 60 ± 13 54 ± 3
Ni 57.8 ± 0.3 65.2 ± 0.9 63.6 ± 7.4 59.8 ± 6.0
S 2.71 ± 0.03 [%]
Si 81 ± 6*)
Ti 2.08 ± 0.05
V 263 ± 1 255 ± 4 267 ± 30 255 ± 27

ICP-OES: n = 6 (mean of digestions with and without NH4F)

ICP-MS: n = 3 (only digestions with NH4F)
*) n = 3 (only digestions with NH4F)

Table 3: Results for Residual Fuel Oil (NIST SRM 1634c)

Fig. 2: Schematic of the pressure measurement of Rotor 8N
D7865 D7865 Certified Value
ICP-OES ICP-MS [µg·g-1]
[µg·g-1] [µg·g-1]
Conclusion
Al 3.6 ± 0.6
Multiwave PRO with Rotor 8NXF100 is the ideal solution when it comes to microwave-assisted
Co 0.14 ± 0.05 0.1510 ± 0.0051
digestion of various petroleum products and lubricants according to ASTM D7876. The unique
Fe 49.5 ± 1.5 47.8 ± 2.9 simultaneous reaction control in all vessels allows for digestion of comparably high sample weights.
Mg 2.0 ± 0.1 The high digestion temperature provides reproducible digestion conditions and therefore accurate
Na 42 ± 2 33 ± 2 (37)* results, within a reasonable time frame (less than 1.5 hours incl. cooling time). The closed-vessel
Ni 15.7 ± 0.2 17.2 ± 0.3 17.54 ± 0.21 design minimizes the risk of contamination and loss of volatile analytes, and requires small amounts
V 27.1 ± 0.4 28.6 ± 0.2 28.19 ± 0.40 of reagents only. This reduces both the consumption of expensive reagents and the costly disposal
of hazardous waste. To avoid handling concentrated hydrofluoric acid, the use of solid NH4F instead

NICP-OES: n = 6 (mean of digestions with and without NH4F) provides a convenient and safer workaround. Low sample amounts of 0.5 g are enough to obtain
representative results. Due to its robustness this system can be applied to a variety of petrochemical
ICP-MS: n = 3 (only digestions with NH4F)
samples and facilitates demanding sample preparation for subsequent routine analysis.
*information value

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