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An important tool for hydrogen content measurement in fuels is nuclear magnetic resonance (NMR). However, high-field frequency domain NMR instruments are highly expensive and time domain NMR instruments come with little flexibility. For hydrogen content measurement, the Thermo Scientific™ picoSpin™ 80 Series II NMR spectrometer is both flexible and cost-effective compared to time-domain and high-field NMR instruments.

This article discusses the analysis of a range of fuel standards and reference compounds using a picoSpin 80 Series II NMR spectrometer and the comparison of the results obtained against their theoretical values and against the values collected experimentally using a high-field NMR. The results reveal that the picoSpin 80 Series II NMR spectrometer provides accurate and precise hydrogen content measurements comparable to the high-field NMR measurements reported in the literature.

The Thermo Scientific picoSpin 80 Series II spectrometer

Hydrocarbon containing fossil fuels are used the main source of energy all across the world. The more number of hydrogen atoms attached to each carbon atom in the hydrocarbon, the lower the oxidation state of that carbon and the higher the energy generated during combustion. Hence, the hydrogen content is a key parameter for petroleum distillate products.1

A number of methods are available for hydrogen content determination in petroleum products. ASTM D5291 is a combustion method for measuring the hydrogen content in petroleum fuels, but cannot be used for samples with a low boiling range. Moreover, repeatability studies are not possible with this method due to its destructive nature.2 NMR has been used for hydrogen content determination in petroleum hydrocarbons for the past several years and many different ASTM methods have been reported in the literature. Earlier NMR methods (ASTM D3701 and ASTM D4808) involved the use of obsolete continuous-wave spectrometers.3-4 Later, an updated method was published (ASTM D7171)  for hydrogen content determination in middle distillates using a pulsed timedomain (TD) NMR spectrometer.5 TD-NMR uses the different relaxation properties of materials for their quantification. It is used in QC labs in various industries, but is not suitable for acquiring frequency spectra, making it incapable of structural analysis like frequency-domain NMR spectrometers.

Methods for hydrogen content determination in petroleum products are also available for high-field NMR spectrometers, but these spectrometers are expensive to buy and maintain, and need a specific NMR facility in addition to the factory or testing laboratory.1,6

This article discusses how hydrogen content is determined in a variety of reference materials and petroleum fuel standards using the picoSpin 80 Series II NMR spectrometer. The results conclude that the picoSpin NMR spectrometer is an effective, low-cost option to high-field NMR for hydrogen content determination.

Here, a picoSpin 80 Series II benchtop NMR spectrometer, which is an 82 MHz, pulsed, Fourier transform 1H NMR, was employed to collect spectra for hydrogen content determination. The spectrometer features a 2 Tesla temperature controlled permanent magnet heated to 36 °C and a 40-microliter capillary cartridge for introducing sample into the instrument. All samples were bought from Sigma Aldrich® and were used in as received condition.

To determine hydrogen content, a vial was positioned on an analytical balance and ~45mg of hexamethyldisiloxane (HMDSO) was introduced. After recording the mass, the balance was tared. The next step was to add ~300mg of the sample to the same vial and record the mass.

The vial was capped and shaken until achieving adequate mixing. For each sample, five runs were prepared. 1 mL sliptip polypropylene syringes and 22 gauge blunt-tipped needles were used to inject the samples into the capillary cartridge. All spectra were collected using 16 scans, 4000 acquisition points and a recycle delay of 40 seconds.

For hydrogen content determination using the pulsed picoSpin 80 NMR spectrometer, the first step was to select an appropriate internal standard. For this application, hexamethyldisiloxane (HMDSO) was selected for the following reasons:

After obtaining the spectrum of the mixture consisting of the HMDSO and target sample, the hydrogen content of the sample was determined using the equation given below:

As shown in the equation, the percent hydrogen by weight was calculated using the recorded weight of the sample and HMDSO as well as the integration of the peak area of the HMDSO singlet and the combined peak areas of the sample signals. HMSDO has 11.17% of hydrogen by weight and this value was applied in the above equation.

Figure 1 illustrates a 1H NMR spectrum used for determining percent hydrogen content in this study, with the integrations labeled.

Figure 1. 1H NMR spectrum of diethyl malonate with HMDSO internal standard

To evaluate the suitability of the picoSpin 80 NMR spectrometer for hydrogen content determination over a variety of samples, several published results obtained from other NMR instruments, and reference compounds with known hydrogen content were analyzed.1,5,6 The precision of the hydrogen content determination method was first examined using the reference compounds. The six reference compounds chosen have a hydrogen content range from ~7% to 15% (Table 1). The reported “Ave %H” is an average of five runs and the percent relative standard deviation (%RSD) is presented for each compound. The %RSD was less than 1% in all cases. The average %RSD of the six compounds studied was 0.44%, agreeing well with the results obtained using a high-field instrument.1

Table 1. Average hydrogen content and % relative standard deviation of five runs per reference sample determined using picoSpin 80 NMR.

The relative percent error was calculated for each sample using the theoretical % hydrogen content in order to examine the accuracy of this method for hydrogen content determination. The six reference samples analyzed with the picoSpin spectrometer provided a range of relative error from 0.56% to 2.59%.

Table 2. Accuracy of picoSpin 80 NMR spectrometer for determining hydrogen content using five runs per reference sample.

Generally, these relative error values track the values obtained using a high-field NMR spectrometer, which analyzed 25 reference samples and gave a range of relative error from -10.58% to 2.65%.1 On an average, the picoSpin method reported an average value of 1.58% compared to 2.75% relative error reported by the high-field instrument. The results conclude the capability of the low-field picoSpin NMR spectrometer to determine hydrogen content of chosen reference compounds with an accuracy similar to a high-field instrument.1

Three ASTM fuel standards were then examined and the results obtained are presented in Table 3.

ASTM D5307 Crude Oil Internal Standard is composed of a mixture of C14-C17 hydrocarbons, having a calculated theoretical hydrogen content of 15.18%. This standard is comparable to diesel fuel which is a mixture of C10-C19 hydrocarbons. The average percent hydrogen determined using the picoSpin spectrometer was 15.33%, which gave 0.98% of relative error compared to the theoretical value. The ASTM D5580 Calibration Mix 4 is made primarily up of 2,2,4-trimethylpentane, a key component in gasoline. The picoSpin method reported a hydrogen content of 14.40%, with -1.43% relative error. The hydrogen content of the third standard ASTM D5134 Splitter Linearity Check Mix determined using the picoSpin method was 14.32%, which also agreed well with the theoretical value of 14.38% with a relative error of -0.39%. ASTM D5580 is composed of a mixture of C6-C8 hydrocarbons, and ASTM 5134 is composed of a mixture of C6-C9 hydrocarbons, both of them are within the typical C4-C12 hydrocarbon range of gasoline.

Table 3. Hydrogen content determination using fuel standards.

Although existing ASTM methods for hydrogen content determination in petroleum and fuels using NMR spectroscopy are available, the instruments used are either obsolete, or have limited applications. It has been already demonstrated that high-field NMR can determine the hydrogen content of fuel samples.1,6 Here, 6 reference compounds representing a variety of hydrogen contents were examined using the picoSpin 80 benchtop NMR spectrometer. The precision and accuracy of results reported by the picoSpin method were shown to be similar to the results reported by high field instruments.

Three fuel standards representing actual fuels were also examined, and their hydrogen content show good agreement with the theoretical and high-field NMR measurements. These results conclude the ability of the picoSpin 80 spectrometer to provide hydrogen content measurements with an accuracy and precision similar to high-field instruments NMR but in a cost-effective manner.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.

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