Application Note · multi EA® 5000

Fast and Reliable TS Determination in
Liquefied Pressurized Gases (LPG) by Meansof Elemental Analysis
Introduction
This application note describes different application methods for the fastand reliable determination of widely varying sulfur contents by means of UV
fluorescence with a minimum effort for calibration and sample preparation.

Challenge
Fast, safe and reliable matrix
independent analysis of LPG
samples of a wide concentration
range and pressure spectra.

Solution
Direct sample dosing from liquid
phase, controlled complete
combustion and result evaluation
using liquids calibration mode.

Gaseous hydrocarbons are valuable source materials for synthesis processes,fuels, energy carriers or auxiliary materials in pharmaceutics and cosmetics. Theirquality and composition, especially their sulfur content, play an important role.
To use them as propellant gas, they need to be free from sulfur (c < 100 ppb).
The same applies to their use in the synthesis of high molecular compounds(e.g., polyethylene), since sulfur compounds may cause catalyst contamination,
undesirable byproducts and increased production costs. A reliable sulfur trace
detection in such matrices requires precise and safe metering technology,
quantitative combustion and highly sensitive detectors.
For the use as combustible or fuel (LPG, CNG), different requirements andstandards apply. Here emission limits for sulfur must not be exceeded and thesulfurous odorants added for safety reasons must be at the appropriate level.
To guarantee optimum product quality and instant process optimization, therapid detection of ultra-low and high sulfur contents is required in LPG analysis.
LPG analysis confronts the operator with two main challenges: 1) the sample
digestion, including introduction and combustion quality and 2) the analysis
evaluation, including calibration strategies and quality assurance of results.

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Fast and Reliable TS Determination in LPG by Means of Elemental Analysis

Due to widely varying pressures, safe and controlled handling and dosing of LPG into the analysis system is challenging.
Samples tend to evaporate too early, affecting analysis quality and operating safety due to deflagration, explosion, sootformation or incomplete combustion. When expanding the LPG to allow a more controlled handling, the samples undergochanges due to adsorption loss, condensation, etc., which clearly influences the analysis quality. Direct LPG analysis out of
the liquefied phase followed by stepwise evaporation in controllable safe small volumes is one approach to overcome thesechallenges. Inert Restek™ coating of relevant parts thereby supports the detection of the tiniest TS impurities without
falsifying component losses. Still, a quantitative sample conversion does not necessarily lead to correct analysis results.
These strongly depend on a suitable and correct calibration strategy. In general, the system is calibrated either by just one
certified standard using a single point calibration or by doing an error-prone and laborious dilution line for one standard
material. The first approach can deliver suitable results when high element contents must be analyzed and the blank neednot be considered. For analysis in the trace and ultra-trace range, where there are no suitable calibration gases available,
this strategy will deliver results that are false, because they are too low. Here, the use of a dilution line is recommended
but requires a dedicated and skilled operator as it bears a high risk of error (changing blank, adsorption loss, cross-contamination, pressure effects, etc.).
The problems discussed here can be easily avoided by applying alternative calibration strategies. These simplify lab workand make it possible to obtain reliable results independent of the operator‘s skills and experience. System calibration canbe achieved using a calibration gas with variable injection volumes, as well as calibration kits of liquid standards at different concentrations and constant injection volume. This is possible because the multi EA® 5000 provides a quantitative
detection. Detection of the SO2 is the same originating from the combustion of a liquid, solid or gaseous sample. Furthermore, the daily factor function helps to ensure the long-term stability of the respective calibration resulting in best results
without laborious re-calibration of the system.
Materials and Methods
Samples and Reagents■■ 3.70 mg/L TS standard (dimethyl sulfide in propane)■■ LPG (butane/propane mix)

Sample Preparation
The samples were contained in LPG cylinders. The sample pressure varied within a wide range. The LPG samples andstandards were analyzed directly without sample pretreatment.
Calibration
The analysis system was calibrated in advance applying two different strategies to confirm the suitability of alternative
calibration strategies compared to the common method.
The calibration of the system using one calibration gas with variable injection volumes is shown in Figure 1 a. The calibration
by means of liquid standards at different concentrations (here dibenzothiophene in iso-octane) is shown in Figure 1 b. The
received calibration functions already indicate the comparability of both strategies.

Figure 1 a: Calibration using a LPG standardFigure 1 b: Calibration using liquid standards

Fast and Reliable TS Determination in LPG by Means of Elemental Analysis

Instrumentation
The analysis was performed on the elemental analyzer multi EA® 5000 S in vertical operation mode. The system wasequipped with an LPG 2.0 high pressure LPG sampling system for the direct introduction of liquefied pressurized gases up to34 bar, compliant with ASTM D 6667 and D 7551 and DIN EN 17178.

Figure 2: LPG 2.0 module

The LPG 2.0 module can be used very flexibly for the automatic metering of liquefied gases at different pressures. The sample
volume can be varied by the user within the range of 1–50 µL (liquid phase).During the analysis 10 µL sample volume (liquidphase) were injected for each LPG measurement.
LPGs can expand even with a minimum of pressure reduction (e.g., sampling). This results in a nonreproducible analysisof the sample at excessive speeds which leads to strongly scattering results and sooting of the analysis system. To prevent
this, the sample pressure is normally increased using an auxiliary gas. Thanks to the cooled sampling valve of the LPG 2.0,
this is no longer required. Even liquefied gases at lowest pressure (e.g., butadiene at 2.1 bar) can be analyzed with goodreproducibility.
Prior to the transfer to the combustion module, the samples pass through a heated evaporation chamber where allcomponents (inhibitors etc.) are quantitatively converted into the gaseous state. This prevents memory effects and hence
incorrect measurements. The quantitative transfer of the gaseous samples to the combustion system is done using PermaPurge, a powerful inert gas flushing. The complete oxidation takes place in a two-phase process at 1050 degrees Celsius. Thegenerated SO2 is introduced to the HiPerSens UV-fluorescence detector after drying the measuring gas. The sensitivity of thedetector enables a detection limit of 5 ppb S = 0.2 ng S without time-consuming enrichment procedures (trap and release).
To check the system performance, a fivefold analysis of a standard LPG was performed 10 times over a period of 24 hours.
The deviation of the single analysis is less than 2 %. Thanks to this excellent reproducibility, replicate measurements can be
reduced to a minimum.

Table 1: Reproducibility test of LPG analysis
Measurement12345678910c S in mg/l

3.78

3.60

3.69

3.71

3.68

3.69

3.61

3.66

3.69

3.56

Average value

3.67 mg/L

SD

0.06 mg/L

RSD

1.77%

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Fast and Reliable TS Determination in LPG by Means of Elemental Analysis

Method parameters
Standard method settings for vertical operation stored in the method library are applied. The parameter settings for thecombustion process are summarized in the following table. The quartz combustion tube was equipped with a quartz wool
plug in the inner tube.
Table 2: Process parameters
Parameter

Specification for liquid calibration

Specification for LPG analysis
and calibration

Furnace temperature

1050 °C

1050 °C

Second combustion

60 s

60 s

Ar flow (first phase)

150 mL/min

100 mL/min

O2 main flow

300 mL/min

300 mL/min

O2 flow (second phase)

150 mL/min

100 mL/min

Draw up

2 µL/sautomatically by LPG 2.0

Injection

0.5 µL/s

automatically by LPG 2.0

Evaluation parameters
Standard method settings from the method library are applied. They are summarized in the following table.
Table 3: S detection parameters
Parameter

Specification SMax. integration time

300 s

Start

0.1 ppb

Threshold0.11 ppb

Stability7

Results and Discussion
The results of the analysis of two different LPG samples and one standard material are summarized in table 1, 4, 5. They are
either averages of five (table 1) or three replicate analyses (table 4–5). Due to the matrix-optimized combustion, a threefold
determination is sufficient to achieve results within 3 % RSD. This is remarkably affecting the sample processing time andtherefore generates a higher sample throughput. The analysis results and their reproducibility depict the high quality of thedigestion process. The proper performance of the analysis system and suitability of the applied alternative calibration strategy(adaptation of liquids calibration) was confirmed by analyzing a TS standard material with known concentration (see table 4).
Table 4: Comparison of different calibration strategies
SamplecS [mg/L]
LPG calibration

SD [mg/L]
Liquid calibration

3.70 mg/l TS standard(dimethyl sulfide in propane)

3.73 ± 0.03 mg/L

3.73 ± 0.03 mg/L

LPG (butane/propane mix)

579.59 ± 5.10µg/L

571.62 ± 5.55 µg/L

Fast and Reliable TS Determination in LPG by Means of Elemental Analysis

For the liquefied gases (2) and (3) a volume of 20 µL was dosed. With regards to the high purity of the sample (1), thevolume was increased to 50 µL in order to achieve the best possible results.
To optimize the analysis process the sample volume can be adjusted flexibly to the expected sulfur contents in order toachieve the optimal result in the shortest time. Thanks to the high precision of the metering technology, the next sample canbe analyzed after a minimum of replicate measurements resulting in high sample throughput.
Table 5: Sulfur contents of the examined gas and liquefied gas samples
SamplecS [mg/L]

SD [mg/L]

Butadien (1)0.02< 0.01

6.56 mg/L S in butane (2)

6.56

0.11

3.70 mg/L S in propane (3)

3.73

0.03

Figure 3 shows a typical TS analysis curve. The duration of the measurement depends on the sulfur content and metering
volume and is between 150 and 420 seconds.

Figure 3: TS analysis curve for propane (LPG)

Conclusion
By applying liquids calibration, a high quality of the analysis results can be achieved in less time with minimum effort andcosts compared to the classical calibration methods (one-point LPG or LPG dilution line). The results achieved using bothstrategies are equivalent. Especially in trace and ultra-trace analysis, the use of liquids calibration is superior, as it allows asufficient blank correction, and reliable liquid standards are available in concentration levels as low as ppb-level.
With the LPG 2.0 and the analysis system multi EA® 5000, liquefied gases can be analyzed independent of their matrix
properties and pressure in the shortest possible time without any sample pretreatment. High-precision metering technology
and intelligent safety functions integrated into the instrument, such as the Self Check System, ensure complete sample
digestion and thus a high quality of the results.
Additional functions, such as the integrated filter system and the purge tool for preventative care, significantly minimize themaintenance effort. The easy-to-operate hardware and top safety standards permit use in shift operation and in analysis
centers with the highest possible sample throughput.

This document is true and correct at the time of publication; the information within is subject to change. Other documents may supersede this document, including technical modifications andcorrections.

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