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Environmental Article

Monitoring Petroleum Hydrocarbons by Solid Phase Extraction/GC

Improved SPE Cartridges for Massachusetts EPH Analysis

By Lydia Nolan, Innovations Chemist
  • New SPE cartridges reduce extractables and assure greater fractionation reliability.
  • Large uniform lots of cartridges reduce frequency of verifying fractionation results.
  • New packaging reduces coextractables, provides better protection from humidity.

Concern for the environmental and health effects of exposure to materials from leaking underground petroleum storage tanks has grown over the last ten years. The US Environmental Protection Agency and international groups have addressed some of the analytical and sample preparation techniques needed for both on-site and environmental laboratory assessment of possible water and soil contamination. Several US states also have developed their own methods to meet the urgent or specific needs for analyzing samples from their own geographical areas. One of the earliest methods was developed by the Massachusetts Department of Environmental Protection. Introduced in 1998, their Method for Determination of Extractable Petroleum Hydrocarbons (EPH) was updated to Revision 1.1 in May 2004. This method is intended for either qualitative identification of total petroleum hydrocarbons (TPH) or for fractionation, detailed analysis, and quantification of both aliphatic and aromatic fractions from site samples of water or soil/sediment matrices.(1) This detailed quantitative analysis ensures optimal identification of suspected contaminants and supports toxicological assessment for human exposure. Because of the broad and thorough nature of the quantitative information generated, many site managers and engineering firms request this method to be used, even for their out of state samples. Some other states, including Texas, have adopted similarly detailed methods.(2)

Although these quantitative methods are useful, they present additional demands to the laboratories, especially at the sample preparation and fractionation steps. Heavily contaminated samples require initial liquid/liquid extraction into methylene chloride, drying with sodium sulfate, solvent exchange into hexane, and concentration, then fractionation and clean-up using solid phase extraction (SPE) cartridges containing large beds (5g) of silica gel. Although these cartridges are now commercially available from many suppliers, the quality of these products must be assured for each lot, and, sometimes, even within lots. The activity level and capacity of the silica used in the cartridge, the compression of the bed, and the quality of the constituents and packaging used in the product all are critical toward good results. Because the SPE cartridges have a large diameter, voiding can occur near the upper retaining frit and within the silica bed, due to settling during shipment/transport of the cartridges. Voids can be eliminated by tapping the tip of the cartridge against a hard surface, to resettle and level the bed, then inserting a clean, flat-ended rod into the cartridge and pushing firmly, to reposition the frit into contact with the silica bed.

Determining the activity level of the silica is more difficult. The volume of hexane required to fractionate the aliphatic portion of the sample, without allowing any aromatic analytes to break through into the second methylene chloride fraction, can vary, typically from 17mL to 22mL ± 0.5mL, and must be determined for every lot of cartridges. Slight variations in the silica material, or in humidity levels during manufacture of the cartridges, can have a dramatic effect on the hexane fractionation results. Exposure in the lab also can affect results, so all cartridges should be left in their original packaging until needed, and unused cartridges should be re-sealed immediately and stored in a desiccator.

Most frustrating for labs using this method has been the level of extractables in commercial SPE cartridges. Trace levels of phthalates and other contaminants from cartridges, frits, and packaging, are easily extracted along with the desired analytes, complicating low level quantification.

We have always specially treated our Massachusetts TPH SPE cartridges (cat.# 26065) to ensure minimum background extractables and maximum silica activity. Now, we have completed a new study that has allowed us to reduce extractables even further, and assure greater reliability of fractionation. Larger, uniform lots of silica for cartridges will reduce the frequency with which a lab will need to verify fractionation results. New packaging ensures reduced levels of coextractables and better protection from environmental humidity.

Figure 1C shows the background level of a typical previous lot of cartridges, compared to the significantly lower background from the new product, in Figure 1B. The cartridges were extracted with 15mL of hexane, with no prior conditioning. The hexane was evaporated, o-terphenyl and 1-chlorooctadecane were added, and samples were reconstituted to 1mL for analysis by GC-FID. Fractionation, extraction efficiency, and reproducibility also are excellent, as shown by the summary in Table I. Details of the extraction method, based on the Massachusetts procedure, also are presented in Table I.

If you are conducting Massachusetts EPH analyses, or similar analyses, and have been concerned about the quality and uniformity of the SPE cartridges you have been using, we think you will be as impressed as we are with the quality of our new product.


Figure 1  Low levels of background extractables in Restek's new Massachusetts TPH SPE cartridges.

  1. o-terphenyl (20ng on column)
  2. 1-chlorooctadecane (20ng on column)



Analytical Conditions

Column:

Rtx®-5 30m, 0.32mm ID, 0.25µm (cat. #10224)

Sample:

100µL Mass EPH Surrogate Spike Mix (cat.# 31479) diluted to 400µg/mL

Inj.:

0.5µL splitless (hold 0.75 min.), Precision™ split inlet liner with wool (cat.# 21027)

Inj. temp.:

290°C

Inj. press. prog.:

pressure pulse to 50cm/sec. @ -0.71min.
pressure to 35cm/sec. @ 0.8 min.

Carrier gas:

helium

Linear velocity:

35cm/sec., constant velocity

Oven temp.:

40°C (hold 1 min.) to 310°C @ 15°C/min.

Det.:

PerkinElmer AutoSYS™ GC-FID @ 330°C
Table 1  Recovery and reproducibility for aliphatics and aromatics via Massachusetts TPH SPE fractionation, using new Restek SPE cartridges.
 
Aliphatic Fraction
Aromatic Fraction
 
% Recov.
Std. Dev.
RSD
% Recov.
Std. Dev.
RSD
  1. nonane (C9)
86.4
9.11
10.5
 
 
 
  2. decane (C10)
84.7
7.17
8.5
 
 
 
  3. naphthalene
 
 
 
82.3
6.09
7.4
  4. dodecane (C12)
83.8
8.33
9.9
 
 
 
  5. 2-methylnaphthalene
 
 
 
89.1
5.50
6.2
      2-fluorobiphenyl
      (fractionation surrogate)
 
 
 
92.1
6.70
7.3
  6. tetradecane (C14)
90.7
6.29
6.9
 
 
 
  7. acenaphthylene
 
 
 
91.6
7.63
8.3
      2-bromonaphthalene
      (fractionation surrogate)
 
 
 
84.9
6.82
8.0
  8. acenaphthene
 
 
 
93.4
6.32
6.8
  9. fluorene
 
 
 
92.4
6.19
6.7
10. hexadecane (C16)
90.9
4.37
4.8
 
 
 
11. phenanthrene
 
 
 
90.4
5.55
6.1
12. octadecane (C18)
94.9
3.45
3.6
 
 
 
13. anthracene
 
 
 
91.5
5.29
5.8
14. nonadecane (C19)
91.1
3.63
4.0
 
 
 
      o-terphenyl (surrogate)
 
 
 
96.4
3.43
3.6
15. eicosane (C20)
89.8
2.64
2.9
 
 
 
16. fluoranthene
 
 
 
93.4
3.16
3.4
     1-chlorooctadecane (surrogate)
83.1
5.02
6.0
 
 
 
17. pyrene
 
 
 
95.1
3.84
4.0
18. docosane (C22)
85.2
3.97
4.7
 
 
 
19. tetracosane (C24)
85.0
3.23
3.8
 
 
 
20. benzo(a)anthracene
 
 
 
91.2
2.38
2.6
21. chrysene
 
 
 
90.9
2.56
2.8
22. hexacosane (C26)
85.8
2.97
3.5
 
 
 
23. octacosane (C28)
85.7
2.51
2.9
 
 
 
24. benzo(b)fluoranthene
 
 
 
91.3
2.23
2.4
25. benzo(k)fluoranthene
 
 
 
90.8
2.10
2.3
26. benzo(a)pyrene
 
 
 
91.0
2.67
2.9
27. triacontane (C30)
86.0
2.49
2.9
 
 
 
28. dibenzo(a,h)anthracene
 
 
 
90.9
1.78
2.0
29. indeno(1,2,3-cd)pyrene
 
 
 
91.4
1.48
1.6
30. benzo(ghi)perylene
 
 
 
90.7
2.21
2.4
31. hexatriacontane (C36)
78.6
3.95
5.0
 
 
 
n=4 (2 analyses on each of 2 lots of SPE cartridges)

Analytical Conditions

Column:

Rtx®-5 30m, 0.32mm ID, 0.25µm (cat. #10224)

Sample.:

100µL Mass EPH Surrogate Spike Mix (cat.# 31479) diluted to 400µg/mL
1mL MA Fractionation Check Mix (cat.# 31481), 25µg/mL in hexane
1mL MA Fractionation Surrogate Spike Mix (cat.# 31480), diluted to 40µg/mL in hexane

Inj.:

0.5µL splitless (hold 0.75 min.), Precision™ split inlet liner with wool (cat.# 21027)

Inj. temp.:

290°C

Inj. press. prog.:

pressure pulse to 50cm/sec. @ -0.71min.
pressure to 35cm/sec. @ 0.8 min.

Carrier gas:

helium

Linear velocity:

35cm/sec., constant velocity

Oven temp.:

40°C (hold 1 min.) to 310°C @ 15°C/min.

Det.:

PerkinElmer AutoSYS™ GC-FID @ 330°C

SPE Method

Tube:

Massachusetts TPH 20mL/5g, cat.# 26065

Tube conditioning:

30mL hexane; do not allow top frit or bed to dry.

Sample:

Add 1mL EPH sample in hexane.

Elution #1:

Using gravity or very low vacuum, pass 18mL hexane through tube.*
Do not allow top frit or bed to dry; collect this aliphatic fraction in a clean sample container.
Reduce eluate to 1mL under gentle nitrogen purge or other concentration technique.
Do not concentrate to less than 1mL or allow eluate to dry before analysis.

Elution #2:

Using gravity or low vacuum, pass 20mL methylene chloride through tube.
Do not allow top frit or bed to dry; collect this aromatic fraction in a clean sample container.
Reduce to 1mL (see above) and analyze.

*Note that the volume of hexane will vary, and should be verified in each laboratory.
For details concerning the SPE method, refer to the original method in Reference 1.

References

  1. Method for the Determination of Extractable Petroleum Hydrocarbons (EPH) Massachusetts Department of Environmental Protection, Division of Environmental Analysis, Office of Research and Standards, Bureau of Waste Site Cleanup, Revision 1.1, May 2004.
  2. Total Petroleum Hydrocarbons, TNRCC Method 1005, Revision 03 (June 1, 2001); Draft TNRCC Method 1006 (May 2000) Texas Natural Resource Conservation Commission.