The Determination of Benzene and Toluene in Finished Gasolines

application Note

Gas Chromatography

Author A. Tipler PerkinElmer, Inc. Shelton, CT 06484 USA

The Determination of Benzene and Toluene in Finished Gasolines Introduction ASTM® Test Method, D-3606, is designed Containing Ethanol Using the to determine the benzene and toluene content in gasolines using packed columns PerkinElmer Clarus 680 GC in a 2-column backflush configuration. This is an established method that was with Swafer Technology originally developed to analyze gasoline that did not contain ethanol. Ethanol is a biofuel that is added to modern gasolines to improve combustion efficiency. Significant quantities of this additive may be present – for instance 10% in the USA (E10) and 25% (E25) in Brazil. The presence of large amounts of ethanol in samples caused problems with chromatographic co-elution with benzene when using the D-3606 method. The method was revised (D-3606-07) to include an alternative column set designed to handle the presence of the ethanol but there are still reports of problems with co-elution and further column sets are under consideration.

This application note describes a method that is based on the original ASTM® D-3606 method with the main difference being that capillary columns are used. This approach completely eliminated all chromatographic interference from the ethanol (even solutions made up in pure ethanol could be run), improved the quality of the chromatography in general and reduced the analysis time significantly (by 50% or 75% depending on the column set). Analytical approach Initial Setup Procedure The traditional D-3606 packed column method uses a 2-column The Swafer Utility Software was used to establish the carrier backflush configuration. The first column (precolumn) gas pressures needed for this analysis. Figure 1 shows a was packed with 10% polydimethylsiloxane (PDMS) on a screen shot taken from this software showing the configuration Chromosorb®-W support. The second column (analytical and suggested pressure settings used for the initial work. column) contained a packing of 20% 1,2,3-tris(cyanoethoxy) propane (TCEP) on a Chromosorb®-P support. To begin, 0.3 µL of a solution of ethanol, benzene and toluene in iso-octane was injected into the system using the In this application, these columns were replaced with narrow- conditions shown in Figure 2. Figure 3 shows the resultant bore capillary columns containing the same stationary chromatography from this injection. All components have phases as used in the packed columns. A split/splitless injector eluted from the precolumn (and into the TCEP column) was used to introduce the sample into the precolumn. A within 1.61 minutes and so this time was adopted as the Swafer™ system was used to manage the backflush process. backflush point. Although D-3606 specifies the use of a thermal conductivity detector (TCD), in this work a flame ionization detector (FID) 100mL/min Injector at 200°C was used as it was more suited to high-resolution capillary S-Swafer chromatography and did not give baseline drift as column 60cm x 0.100µm fused silica restrictor pressures were changed during the backflushing process. FID at 200°C 30m x 0.250mm x 0.25µm Elite-1

Experimental conditions 30psig 20psig hydrogen hydrogen A PerkinElmer® Clarus® 680 gas chromatograph with a programmable split/splitless injector (PSS), flame ionization detection (FID) and Swafer system was used to perform this 50m x 0.250mm x 0.4µm GC oven = 100°C isothermal analysis. Further details on the Swafer technology may be TCEP found on the PerkinElmer website (www.PerkinElmer.com/ Figure 2. System configured to monitor the precolumn chromatography to Swafer.com). In this instance, the Swafer S6 configuration was establish the backflush point. used with a fused silica restrictor tube on one of the outlet ports to enable the chromatography on the precolumn to be directly monitored by a detector. This enables the backflush Ethanol

point to be easily and accurately established. Is o- Benzen octane

Note that hydrogen is used as the carrier gas. This enables e

the run time to be reduced to increase sample throughput To luen

and eliminates the need for increasingly expensive helium e as world stocks are depleting.

Backﬂush Point

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70 1.80 1.90

Figure 3. Precolumn chromatography of mixture of ethanol, benzene and toluene in iso-octane.

To check the separation of the mixture on the complete system, the restrictor is then disconnected from the FID detector and the TCEP column connected to this detector, as shown in Figure 4.

Figure 1. Screen shot taken from the Swafer Utility Software showing the S6 configuration used in this method and gas pressures used for the initial work.

2 100mL/min

Injector at 200°C octane S-Swafer - Is

60cm x 0.100µm fused o silica restrictor Ethanol

30m x 0.250mm x 0.25µm Elite-1 Precolumn Precolumn

30psig 20psig

hydrogen hydrogen FID at 200°C Benzen chromatography e To luen e 50m x 0.250mm x 0.4µm GC oven = 100°C isothermal TCEP

Figure 4. System configured to monitor the TCEP column chromatography to check the final chromatographic separation. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8

Figure 6. Combined precolumn and TCEP column chromatography of The resultant chromatogram is shown in Figure 5. Good mixture. separation of all of the components is evident.

The next step was to check the chromatography of a gasoline Is

o- sample. For these samples, backflushing of the precolumn octane was important to keep the bulk out of the gasoline material off the sensitive TCEP column. To perform the backflush Ethanol technique, the inlet pressure was reduced and simultaneously, the midpoint pressure at the S-Swafer was increased slightly.

Benzen The effect of both lowering the inlet pressure and raising

e the midpoint pressure to initiate backflushing provides a To luen cleaner separation between the ethanol and benzene peaks e on the TCEP column chromatography as shown in Figure 7. This approach also shortens the chromatographic run time – the full separation is complete within just 4 minutes!

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 To luen Figure 5. Chromatography of mixture eluting from TCEP column. Ethanol Benzen e e Because the precolumn chromatography is completed before the TCEP column chromatography starts, the restrictor and TCEP column can be connected simultaneously to the same detector. This means that both chromatograms may be monitored in the same run as shown in Figure 6. This approach makes It very easy to establish and maintain the backflush point and it provides a visible record of the chromatography on the precolumn for QC purposes. This information is not available from the original ASTM D-3606

method. 2.80 2.85 2.90 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60 3.65 3.70 3.75 3.80 3.85 3.90 3.95

Figure 7. Chromatography eluting from the TCEP column for a typical gasoline sample with a timed event to reduce the injector pressure and increase the midpoint pressure at 1.61 minutes.

The Swafer Utility Software was further used to model backflush conditions as shown in Figure 8.

3 ASTM® method D-3606 specifies the use of an internal standard. In the original method this was 2-butanol and in the updated method for gasoline samples that may contain ethanol, methyl ethyl ketone (MEK) was specified.

On this system, it was found that MEK eluted after toluene and so would extend the run time. 2-Butanol eluted between benzene and toluene as shown in Figure 9 so this compound was chosen as the internal standard to keep the chromatography to 4 minutes.

The final method for the determination of benzene and toluene in finished gasoline containing ethanol, as used to produce the chromatography in Figure 9, is listed in Table 1.

Under these conditions, the chromatography took just 4 minutes to complete. A short equilibration time is added to stabilize the carrier gas pressures before the start of each Figure 8. Screen shot taken from the Swafer Utility Software showing the run. The total cycle time for each analysis is just 5.4 minutes system in backflush mode (note that in practice, a single detector is used). which enables 11 samples to be run each hour or 88 samples over an 8-hour shift. Ethanol To luen e 2- Butanol (I.S.) Benzen e

3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60 3.65 3.70 3.75 3.80 3.85 3.90 3.95

Figure 9. Chromatography of a typical gasoline sample with 2-butanol added as an internal standard.

4 Tolerance to ethanol Table 1. Full Experimental Conditions. Since there were concerns with the original ASTM® D-3606 Gas Chromatograph: PerkinElmer Clarus 680 method when used with gasoline samples containing sig- Oven: 100 °C isothermal for 4 min. 0.5-min nificant quantities of ethanol, a solution containing a low equilibration time concentration of benzene in ethanol was injected. The result Injector: Programmable Split/Splitless Injector (PSS) is shown in Figure 10. This clearly shows that this method 100 mL/min. split at 200 °C has good tolerance to ethanol even when present at ~100% Quartz Liner Part No. N6121001 levels when determining low levels of benzene. A Split/Splitless (capillary) injector may also be used. Detector: Flame Ionization at 200 °C Linearity Air 450 mL/min. hydrogen 45 mL/min. ASTM® Method, D-3606 specifies that the system should be range x1, attenuation x8 calibrated using 7 standard solutions as listed in Table 2. Backflush Device: S-Swafer in S6 configuration S-Swafer Kit part no. N6520272 Table 2. Calibration Solutions for ASTM® D-3606. Swafer Kit part no. N6520270 and N9306262 Concentration (%v/v)* for existing Clarus GCs with PPC Solution Benzene Toluene Precolumn: 30 m x 0.25 mm x 0.25 µm Elite-1 Part No. N9316010 1 5 20 Analytical Column: 50 m x 0.25 mm x 0.4 µm TCEP 2 2.5 15 Part No. NR210048 3 1.25 10 Midpoint Restrictor: Fused silica, 60 cm x 0.100 mm 4 0.67 5 Part No. N9316601 5 0.33 2.5 Carrier Gas: Hydrogen 6 0.12 1 Carrier Gas Pressure 30 psig for 1.62 min., then 5 psig Inlet: programming by timed event until end of run 7 0.06 0.5 Midpoint: 20 psig for 1.61 min. then 25 psig by timed *Prior to addition of internal standard event until end of run Injection: 0.3 µL by autosampler in fast mode A set of standard solutions was used to check linearity and 4 solvent washes following injection calibrate the system being used for this application work. Sample Preparation: 1 mL 2-butanol (I.S.) added to 25-mL Some examples of the chromatography are given in Figure 11. volumetric flask and then gasoline sample added to bring total volume to 25 mL. 2- To0.5% = luene Butanol (I.S.)

Standard 7 Benzene

28 v/ v Ethanol

- 2 24 = 0.06%

Butanol (I.S.) 20

16 To luen

12 v/ v

e 8

4 3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60 3.65 3.70 3.75 3.80 3.85 3.90 3.95

550 500

450 Standard 4 To5% = luene 400 Benzene Benzene 350

300 2- Butanol (I.S.) 250 v/ = 0.67%

200 v 150

100 v/ 50 v 0 3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60 3.65 3.70 3.75 3.80 3.85 3.90 3.95 Benzen

900 e 800 Standard 1 To20 = luene 700 Benzene Benzene 3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60 3.65 3.70 3.75 3.80 3.85 3.90 3.95 600

500 Butanol (I.S.) % v/ 400 = 5%

Figure 10. Chromatogram of standard mixture containing 0.05% v/v benzene v 300 - and 0.5% v/v toluene in pure ethanol with added internal standard. v/ v

Figure 11. Example calibration solution chromatograms.

5 2.5 Retention time precision Retention time precision is a good indicator of a robust

2.0 method. Table 4 shows excellent relative standard deviations y = 0.4096x obtained for the three components from the 100 gasoline R² = 0.9995 �o 1.5 sample runs. Ra

on Table 4. Retention Time Precision Obtained from 100 sp 1.0

Re Injections of a Single Gasoline Sample.

0.5 Retention Time Component Mean (min) RSD (%)

0.0 Benzene 3.478 0.020 0.01.0 2.03.0 4.05.0 6.0 2-Butanol (I.S.) 3.563 0.021 Concentra� on (% v/v) Toluene 3.887 0.019 Figure 12. Calibration plot for benzene.

Analyte carry-over 7.0 This method is designed to monitor levels of benzene and 6.0 toluene over a wide range of concentrations and so it is

5.0 y = 0.2935x important that cross-contamination between successive R² = 0.9992 sample injections is minimized. To check the levels of carry- �o 4.0 Ra over from one injection to the next, Calibration Solution 1 se

on 3.0 (see Table 2) was run and then immediately followed by sp

Re an injection of iso-octane solvent. Figure 14 shows the 2.0 chromatograms. The carry-over value of 0.013% from an

1.0 injection of toluene at a 20% v/v concentration represents a potential interference of 0.0026% v/v in the determined 0.0 concentration which is far below the 0.5% v/v minimum 0.05.0 10.0 15.0 20.0 25.0 calibration level. Concentra� on (% v/v)

Figure 13. Calibration plot for toluene.

900

800 Standard 1

700 To luen

Quantitative precision 600 2- Butanol (I.S.) 500 e

To check the quantitative precision, a gasoline sample was 400 Benzen

300

run 100 times and the standard deviations were calculated e 200 for the quantitative results. These data are shown in Table 3. 100 0 For each analyte, the quantitative repeatability requirements 3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60 3.65 3.70 3.75 3.80 3.85 3.90 3.95

of D-3606 are easily met. 6.70 6.60 Iso-Octane Carry Carry 6.50 -O

6.40 -O ve Non Detected Non

Table 3. Quantitative Precision Obtained from 100 ve 6.30 0.013% = r r = 0.016% = r Injections of a Single Gasoline Sample. 6.20 6.10

6.00

Concentration (% v/v) 5.90

5.80 Component Mean Std Dev D-3606-07 Requirement 3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60 3.65 3.70 3.75 3.80 3.85 3.90 3.95 Benzene 0.723 0.003 0.032 Figure 14. Carry-over study showing chromatogram of high concentration Toluene 5.713 0.092 0.191 standard mixture followed by injection of solvent.

6 Conclusions • The combination of modern capillary columns with the Swafer technology has taken a mature method and improved the quality of the data and reduced the run time.

• Baseline separation of ethanol, benzene, toluene and the 2-butanol internal standard has been demonstrated.

• Even though the method relies on significant carrier gas pressure and flow rate changes, the quantitative and peak retention time precisions are excellent.

• The chromatographic run time has been reduced from about 8 minutes for the original method (or 16 minutes for some revised column sets) to just 4 minutes. The total cycle time of the chromatographic analysis (including pressure equilibration) is 5.4 minutes enabling 88 samples to be analyzed during an 8-hour working shift.

• The method is able to analyze samples with low levels of benzene in the presence of high levels of ethanol.

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