Analysis of Dimethylsiloxanes in Hydrocarbon Matrices like Marine Fuels Applying a Novel Enrichment Procedure and a Two-Column Gas Chromatography – Mass Spectrometry Measurement

Figures & data

Figure 1. Nomenclature of cyclic and linear dimethylsiloxanes.

Table 1. GC-FID settings for ‘fingerprint-analyses’ with marine fuel samples.

GC-column	High temperature metal column VF-5ht UltiMetal, (Agilent Technologies, Santa Clara, USA) length: 30 m, i. d.: 0.25 mm, film thickness: 0.10 µm
Detector temperature	320 °C
Detector gases	Hydrogen (30 mL/min), synthetic air (300 mL/min)
Carrier gas	Helium (1 mL/min)
Make up gas	Nitrogen (25 mL/min)
Injector temperature	380 °C
Injection mode	Splitless
Injection volume	1 µL
Oven temperature program	50 °C − 400 °C (5 °C/min)

Table 2. Settings for analysis of the fuel’s total Si-contents with ICP-OES.

RF power	1.40 kW
Plasma gas flow rate	15.0 L Ar/min
Auxillary gas flow rate	1.50 L Ar/min
Nebulizer gas pressure	140 – 180 kPa
Measurement time	30 s
Pump speed	20 rpm
Measurement repeats	3
Emission line for Si	251.611 nm

Table 3. Chemicals that were used for the analysis of dimethylsiloxanes.

Chemical	Purity [%]	Supplier
Octamethyltrisiloxane (L3)	98	Sigma-Aldrich
Decamethyltetrasiloxane (L4)	97	Aldrich Chemical Company Inc.
Dodecamethylpentasiloxane (L5)	97	Sigma-Aldrich
Tetradecamethylhexasiloxane (L6)	>95	Key Organics Ltd.
Hexadecamethylheptasiloxane (L7)	–	Apollo Scientific Ltd.
Octadecamethyloctasiloxane (L8)	95	AstaTech Inc.
Eicosamethylnonasiloxane (L9)	–	AstaTech Inc.
Hexamethylcyclotrisiloxane (D3)	98	Aldrich Chemical Company Inc.
Octamethylcyclotetrasiloxane (D4)	98	Sigma-Aldrich
Decamethylcyclopentasiloxane (D5)	97	Aldrich Chemical Company Inc.
Dodecamethylcyclohexasiloxane (D6)	≥97	Sigma-Aldrich
Dichloromethane (Rotisolv HPLC)	≥99.9	Carl Roth GmbH + Co. KG
n-Pentane (Rotisolv Pestilyse plus)	≥99	Carl Roth GmbH + Co. KG

– : no information was given by the supplier.

Table 4. Instrument settings of the heart-cutting gas chromatography with FID/MSD.

GC: 7890B GC system (Agilent Technologies) with autosampler
Injection volume	1 µL
Injector temperature	250 °C
Injection mode	Splitless
Gas chromatographic columns	1. Agilent HP-5ms UI, 30 m x 250 µm x 0.25 µm, −60 to 325 °C  2. Agilent VF-5ms, 30 m x 250 µm x 0.25 µm, −60 to 325 °C
Restrictor capillary	Agilent, 65 cm x 100 µm x 0 µm, 0 bis 325 °C
Carrier gases	Helium (30 psi; constant pressure) Helium (22 psi; constant pressure)
Temperature program	40 °C (2 min), Ramp 1: heating to 136 °C with 20 °C/min (4.8 min) Ramp 2: heating to 139 °C with 0.5 °C/min (6.0 min) Ramp 3: heating to 260 °C with 20 °C/min (6.0 min) 260 °C, 30 min (total time: 48.85 min)
FID
Detector temperature	300 °C
Detector gases	Hydrogen (combustion gas, 30 mL/min) Synthetic air (400 mL/min) Nitrogen (make-up gas, 25 mL/min)
MS: 5977 A MSD (Agilent Technologies)
Ionization	EI, 70 eV
Ion source	230 °C
Quadrupole	150 °C
Transfer line	250 °C
Mode	Scan/SIM
Scan range (m/z)	33 - 700

Table 5. SIM-masses and heart-cutting time windows for the analytes D₃ - D₆ and L₃ – L₉.

	SIM-masses (dwell time 55 ms)
Substance	Quantifier (m/z)	Qualifier 1 (m/z)	Qualifier 2 (m/z)	Heart-cutting time window [min]
D3	207.00	190.95	208.00	6.27 − 6.73
L3	221.05	205.05	222.10	7.07 − 7.48
D4	281.05	265.00	282.05	8.57 − 8.98
L4	207.00	191.00	295.10	9.57 − 9.93
D5	267.00	268.00	355.05	10.97 − 11.38
L5	281.00	207.00	282.05	12.87 − 13.28
D6	341.00	324.95	341.95	14.65 − 15.03
L6	221.05	266.95	281.00	16.10 − 16.58
L7	221.00	222.05	295.05	18.39 − 18.81
L8	221.00	206.95	147.00	20.20 − 20.57
L9	221.00	206.95	147.00	21.60 − 21.94

D₃ – L₉ identified in Table 3.

Figure 2. Kinematic viscosities of marine fuel samples (mm²/s; 40–50 °C).

Figure 3. HT GC-FID chromatograms of different marine fuel samples (A, B, C), diluted 1:100 in hexane.

Figure 4. GC-FID chromatogram of a marine fuel oil diluted 1:500 with n-pentane.

Figure 5. GC-FID chromatogram of fraction 3, diluted 1:50 with n-pentane after marine fuel oil SPE clean-up.

Table 6. Recovery rates of siloxanes D₄ – D₆ and L₃ – L₉ determined by standard addition method.

Siloxane compound	Kerosene recovery rate [%]	Marine fuel recovery rate [%]
L3	106.4	72.4
D4	106.8	96.9
L4	105.2	94.6
D5	111.4	103.0
L5	109.2	83.0
D6	108.0	90.7
L6	93.4	94.6
L7	88.6	94.2
L8	104.6	71.2
L9	87.8	72.4

L₃ – L₉ identified in Table 3. Relative standard deviations (n = 5): Kerosene -between 2.1% (D₅) and 5.0% (D₄); Marine fuel – between 1.0% (L₄) and 3.6% (D₄).

Table 7. Limits of detection and quantitation determined for D₄ – D₆ and L₃ – L₉ for matrix-free standard solutions.

Siloxane	Limit of detection (S/N 3:1) [µg/L]	Limit of quantification (S/N 10:1) [µg/L]
L3	0.11	0.71
D4	0.06	0.73
L4	0.61	1.94
D5	0.55	1.44
L5	2.82	7.27
D6	1.44	3.93
L6	2.11	6.37
L7	3.48	13.9
L8	4.08	23.6
L9	5.68	20.4

S: signal; N: noise; solvent: n-pentane.

L₃ – L₉ identified in Table 3.

Table 8. Properties of six marine fuel samples to be analyzed for dimethylsiloxanes.

	Density [g/cm^3]	Si-content [mg/L]	Colour	Consistency at 21 °C
Fuel A	0.943	4.6	black	pasty
Fuel B	0.948	12	black	pasty
Fuel C	0.938	34	black	liquid
Fuel D	0.859	21	clear brown	liquid
Fuel E	0.901	12	black	liquid
Fuel F	0.927	13	black	pasty

Table 9. Siloxane contents in [μg/L] of the real marine fuels A to F.

	Fuel A	Fuel B	Fuel C	Fuel D	Fuel E	Fuel F
L3	8.48	8.93	1420	193	<LOD	<LOD
D4	2210	2000	11400	3030	<LOD	<LOD
L4	6.91	6.88	1780	589	<LOD	<LOD
D5	515	475	5410	2160	<LOD	<LOD
L5	<LOD	<LOD	2040	490	<LOD	<LOD
D6	214	177	1020	1020	<LOD	<LOD
L6	25.5	37.9	768	544	<LOD	<LOD
L7	143	144	718	534	<LOD	<LOD
L8	<LOQ	<LOQ	691	621	<LOD	<LOD
L9	<LOD	<LOQ	333	420	<LOD	<LOD
Sum	3120	2850	25600	9600

L₃ – L₉ identified in Table 3. LOD: Limit of detection; LOQ: Limit of quantification. Values rounded to three significant digits. Relative standard deviations (n = 5) between 2.3% (D₄) and 5.4% (L₄).

Table 10. Comparison of multiple measurements of a dimethylsiloxane mixed standard.

	Retention time [min]			Signal [pAs]
	D4	L5	L9	D4	L5	L9
Measurement 1	10.083	14.750	23.381	9990	3236	616
Measurement 2	10.085	14.750	23.384	10073	3048	601
Measurement 3	10.085	14.753	23.381	9382	2868	558
Average	10.084	14.751	23.382	9815	3051	591.7
Standard deviation	<0.001	0.0014	0.0014	308.0	150.2	24.58

D₄, L₅, L₉ identified in Table 3. Heart-cutting GC/MS, SIM, 100 pg/µL each substance.

Availability of data and materials

All data generated during this work are available from the corresponding author on reasonable request.