Adlard E.R. (ed.) Chromatography in the Petroleum Industry

292

Supercritical

Fluld (1.e.

CO,)

I

Chapter

10

Extraction Vessel

GC

Split Injector

Split

Vent

Capillary GC Column

Fig.

10.9. Schematic representation

of

an on-line SFE-GC interface with a split injector. (Courtesy

of'

Suprex Corporation.)

The most generally applicable method

of

coupling the SFE and

GC

has been

by inserting the restrictor from the SFE system directly into the injector of the

gas chromatograph. Both on-column and split-splitless injectors have been suc-

cessfully employed with the latter being the most flexible and widely applicable.

A

schematic representation

of

an SFE-GC system is shown in Fig.

10.8

and a

more detailed schematic of the interface using a split injector

is

shown in Fig.

10.9.

In this approach, the supercritical state

is

maintained until it reaches the tip

of the transfer line and then decompresses directly inside the heated injection

port.

The heat

of

the injection port aids in minimizing the Joule Thomson ex-

pansive cooling effect and also vaporizes the analytes which are then carried

Supercritical

fluid

extraction

293

%

Peak area

v

16

14

12

10

8

6

4

2

0

-

I

nC5

nC6

nC7

nC8

nC9

nClO

nC11 nC12

Alkanes

-

Direct inlection

6

min extraction

0

15

min extraction

!ES

30

min extraction

Fig.

10.10.

Recoveries of

nC5

to

nC12

standards

by

coupled

SFE-GC.

The extraction was inter-

faced to the chromatograph via expansion of the extract into the split injector. The

GC

oven was

maintained at

40°C

by

cryogenic cooling. Results for direct injection without

SFE

are presented

together with results for three different extraction times for each component. (Reproduced courtesy

of

British Petroleum.)

onto the GC column where they are trapped by the GC stationary phase. The GC

oven is often cooled cryogenically to aid the focusing of the analytes on the sta-

tionary phase. The capabilities of such a system is illustrated in Fig. 10.10 where

the recoveries for a mixture of nC5 to nC12 hydrocarbons are shown. The stan-

dard mixture was spiked on sand and then extracted with carbon dioxide. The

extract was expanded directly inside a split injection port and trapped on the GC

capillary column by cryogenically cooling the

GC

oven to

40°C.

The GC was

then temperature programmed and the area of the eluting peaks measured. The

normalized area percent for each peak is represented by a bar and data resulting

from experiments carried out at three different extraction times is compared with

the results obtained by direct injection

of

the standard solution without

SFE.

The

results show that in each case, the recoveries were very good with the lowest

being nC5 at

-80%.

The results were largely independent of extraction time and

this indicates that the method could be applicable for the analysis of real sample

matrices where the time required to achieve quantitative extraction

is

generally

longer due to the effects of the sample matrix.

In many cases it is difficult to identify a11 the components which are extracted

from samples by the flame ionization detector response alone. This is a particu-

lar problem when the sample history, and/or the identity of the compounds of

interest, are not known, e.g. when examining a catalyst to try and identify poi-

References pp. 301-303

294

Chapter

I0

100

1

90

-

80

-

8

70-

2

50-

-

*

40-

7

30-

20

-

h

v

-

a,

60-

u.

sons. It is in these cases that multi-hyphenated techniques such

as

SFE-GC-MS,

and SFE-GC-FTIR-MS can be particularly powerful tools.

SFE-GC

with spectroscopic detection

for

the identification

of

pollutants in soil.

The examination of soil samples for pollution

is

important in many sectors of the

oil industry. For example, it is important to ensure that land which has,

or

has

had, oil installations sited on it

is

free from toxic chemicals such

as

PAHs

to

minimize the risk

to

workers during excavations. It is also useful during clean up

operations to be able to determine the extent of the contamination.

Conventional methods for the extraction and analysis of such samples are

time-consuming and expensive but SFE-GC combined with spectroscopic detec-

lo

0

j

I I

I

I I

I

I I

I

600

800

1000 1200 1400

1600

1800

2000

2200

Scan number

b

100

1573

I

1603

1;

I

80

-

70-

50-

-

cn

40-

30-

20

-

10

-

0-

v

-

a,

60-

I

1200

1300 1400

1500

1600

1700

1800

Scan number

Fig.

10.1

I.

(a)

Total

ion chromatogram obtained from

SFE-GC-MS

of

a soil sample showing the

presence

of

polycyclic aromatic hydrocarbons

(PAHs).

(b) Expanded portion

of

chromatogram

(a)

with library spectral matches for

PAHs.

(Reproduced courtesy

of

British Petroleum.)

Supercritical jluid extraction

295

Naphthalene

th

128

I

I I I

I I

I

800

900 1000

1100 1200

1300

1400

1500

Scan number

5

ring

b

eg

benzo

pyrenes

228

202

AI

4

ring

eg

chrysenes

t

E

I

9

t

17%

4

ring

eg

pyrenes,

fluoranthenes

3

ring

eg

anthracenes,

phenanthrenes

.-

v)

,h

A

A,,

,.._.

I I

I

800

900

1000 1100 1200

1300

1400

1500

Scan number

Fig.

10.12.

(a) Chromatograms obtained by single ion monitoring

for

substituted naphthalenes in a

soil sample by on-line SFE-GC-MS. (b) Chromatograms obtained by single ion monitoring

for

3-,

4-

and 5-ring

PAHs

in a soil sample by on-line SFE-GC-MS. (Reproduced courtesy

of

British

Pe-

troleum.)

tion can be a powerful tool for screening samples to decide whether they require

the full analysis. For example, Fig. 10.1 l(a) shows the total ion chromatogram

obtained by SFE-GC-MS

of

a

soil

sample from

a

petroleum installation. Exami-

nation

of

the spectra for each peak has identified the presence of polycyclic aro-

matic hydrocarbons as shown in the expanded portion of the chromatogram in

Fig. lO.ll(b). The technique can be further refined by performing single ion

scans for ions which are characteristic of particular PAHs. Examples from the

examination of

a

soil sample are shown in Fig. 10.12(a) where ions typical of

substituted naphthalenes are monitored and in Fig. 10.12(b) where ions charac-

teristic of 3-5-ring

PAHs

are monitored.

References pp.

301-303

296 Chapter

I0

TABLE 10.6

SINGLE

ION COUNTS FOR NAPHTHALENE (PARENT) AND C1, C2 AND C3

SAMPLES TAKEN FROM THE SAME SITE

SUBSTITUTED NAPHTHALENES OBTAINED

BY

ON-LINE SFE-GC-MS FOR FOUR

SOIL

~

Sample

Parent ion Cl

c2

c3

no.

(M/W

128)

(MIW 142) (MIW 156) (M/W 170)

i

1000

109 66

0

2

787 73

0

3

120

0 0

0

4

0

Although this has not yet been shown to be fully quantitative, it can be em-

ployed as a rapid screening process to determine the extent of contamination and

to

give a guide as to whether samples should be submitted for more detailed

quantitative analysis. The results listed in Tables

10.6

and

10.7

show the ion

counts for single ion monitoring for selected

PAHs

for

a

number of soil samples

taken from the same site. These results show that the technique is at least semi-

quantitative and can be employed to determine the extent of contamination on a

site, or

in

cases of dispute, identify the source of the contamination. Each analy-

sis

can be run

in

under

2

h

although data analysis may take longer if a lot of

component spectra have to be examined. Figure 10.13(a) shows the total ion

chromatogram from another soil sample. Examination of the spectrum of the

peak

at

scan number

1054

and a library search reveals the presence of tetraethyl

lead

in

this particular sample (Fig. 10.13(b)). The examination of samples for

alkyl lead compounds can also be accomplished using SFE-GC with an atomic

emission detector (AED). This detector has the advantage that the detector can

selectively monitor for lead compounds in the chromatogram. This means that

the lead compounds can be detected without any interferences from hydrocar-

bons etc. which have been co-extracted. This together with the inherent sensitiv-

ity

of

the detector and the high concentration factors which can be achieved by

'IABLE

10.7

SINGLE ION COUNTS

FOR

3-

TO 6-RING

PAHS

OBTANED BY ON-LINE SFE-GC-MS OF

FOliR

SOIL

SAMPLES TAKEN FROM THE SAME SITE

Sample

3-ring 4-ring 4-ring 5-ring 6-ring

110.

(M/W

178) (MIW 202)

(MIW 228) (M/W 252)

(M/W 276)

1

1000

2144

444 304

63

-

595

1000

158 89

0

3

516 1373 333 345 96

4

20

32

0

3

Supercritical ji'uid extraction

1099

1188

1054

1048

s

297

a

569

563

l0j

uu

0

I

600

800

1000 1200 1400 1600 1800 2000 2200 2400 2600

Scan

number

b

1065

100

1

I

- -

3

LL

50

0

Scan

1054

7

105

'p

.

.I

n

I

..

I

1

I I I

I I

I

50

100

150

200

250

300

50

100

150

200

250

300

Mass

Ich

arg

e

Mass

/charg

e

Fig.

10.13.

(a) Total ion chromatogram obtained from on-line

SFE-GC-MS

of

a contaminated soil

sample. (b) Portion of total ion chromatogram shown in (a) with mass spectrum and library match

for

scan number

1054

identifying the peak as tetra-ethyl lead. (Reproduced courtesy of British Pe-

troleum.)

chromatogram shows the response from the carbon emission line and the top one

the response from the lead emission line where peaks for tetramethyl and tetra-

ethyl lead can readily be detected.

In many cases mass spectrometry cannot give sufficient information to posi-

tively identify components of interest, particularly when looking for unknown

components, for example in problem-solving types of application. In these cases

"multi-hyphenated" techniques such as

SFE-GC-FTIR-MS

can be particularly

useful as both the infrared and mass spectrometer can be employed to give

References pp.

301-303

298

Chapter

I0

301

Lead

tEtPb

tMePb

1

h

-7

4

6

8

10

12

14 16 18

20

Time

(minutes)

Fig.

10.14.

Lead (top) and carbon (bottom) specific chromatograms obtained from on-line

SFE-

GC-AED

of a soil sample spiked with leaded gasoline. The lead concentration is equivalent

to

1

part million in the

soil.

(Reproduced courtesy

of

British Petroleum.)

structural information. This allows confirmation

of

identification

by

two inde-

pendent techniques in

a

single experiment

1571.

HPLC

~

I

'

PUMP

1

I

EXT::f,'oN/

+

,

J

-

SFE

OVEN

I

DETECTOR^

fig.

10.15.

Schematic representation

of

a

coupled SFE-HPLC system employing a recirculating

atraction manifold interfaced

to

the HPLC via a sample injection valve.

Supercritical

fluid

extraction

299

10.5.2.2

On-line

SFE-HPLC

There are many compounds which are not amenable to GC analysis and in

these cases SFE coupled on-line to liquid chromatography can be a useful tool.

The recirculating mode of SFE with

an

in-line sampling valve discussed earlier

provides an easy means of coupling SFE with HPLC.

A

schematic representa-

tion

of

such a coupling is shown in Fig.

10.15.

In

this system a small aliquot

of

extract is removed from the recirculating extraction system via a sample loop.

This loop

is

introduced directly into the HPLC eluent where the carbon di-

oxide extraction solvent expands and passes through the HPLC system unre-

tained. The extracted components are retained and then separated on the analyti-

cal column.

A

chromatogram obtained from this system for a phenol formaldehyde resin

sample

is

shown in Fig.

10.16.

One advantage

of

this configuration

is

that the

remaining extract in the system can be collected on depressurization and can

then be examined by other techniques.

In

this case, off-line GC-MS and HPLC-

MS identified the peaks as phenol and isomeric phenol formaldehyde condensa-

tion products. Dynamic extraction can also be linked with HPLC and this is par-

ticularly useful where maximum sensitivity

is

required. One way of achieving

this is via an intermediate trap linked between the SFE and HPLC system via a

switching valve. The

SF

extract is expanded into an HPLC pre-column which

has been packed with the same stationary phase as the analytical column. The

extracted analytes are trapped on this column and the gaseous carbon dioxide

vented. After the extraction is complete, the HPLC eluent is introduced to the

n

The

Ern

and

HPLCrn

results

from

the isolated extract, show that the

low mlecular weight material consists

of

phenol and isomeric phenol

formaldehyde condensation products of the type shown below

:

CHZOH

HO

,OH

11111,111,,,1,,11,,,

10

20

30

40

50

Time

(minutes1

Fig.

10.16.

Chromatogram obtained by SFE-HPLC

of

a phenol formaldehyde resin. The recirculat-

ing SFE system

was

coupled

to

the HPLC system

as

illustrated in Fig.

10.9.

(Reproduced by cour-

References

pp.

301-303

300

EXTRACTION

CELL

Chapter

I0

C02 PUMP

i

GRADIENT

HPLC

1

PUMPS

co=Q+

SmTCHlNG

/

VALMS(2)

ACCUUUUTOR

/

COLUMN

S

THERM0K)SPAAY

C

P

E

T

R

I

PARTICLE BEAM

0

M

Ii

R

Fig. 10.17. Schematic representation

of

possible coupled

SFE-HPLC-MS

systems which have

been etnployed

for

the examination

of

polymer additives and their breakdown products in polyeth-

ylene.

pre-column via a valve switch and the trapped analytes are introduced into the

HPLC system. In this case the whole extract is transferred to the HPLC system

and therefore off-line techniques for identification are not available.

However. recent developments in HPLC-MS mean that instruments are now

commercial Iy available at reasonable cost.

SFE

has been successfully coupled in

this way to commercial HPLC-MS systems and has been shown to give unique

information for the examination of polymer additives and their breakdown prod-

ucts

[57,58].

A

schematic diagram of the possible systems is shown in Fig.

10.17.

Three HPLC-MS interfaces were studied namely a moving belt, a particle

beam interface, and a thermospray interface. The particle beam/thermospray

combination was particularly useful as

it

was available on the same instrument

and

it

was a simple operation to select either as required. This gave excellent

flexibility as the particle beam gives EI-type spectra and therefore fragmentation

data, and the thermospray ionization mode provides molecular weight data.

Therefore a lot of structural information on unknowns can be collected

in

a short

time. This system has been shown to give less degradation in polymer additives

than when an off-line

SF

extraction with collection

in

solvent was employed and

this could provide a powerful tool to study the fate of reactive compounds

in

their natural environment.

Supercritical

fluid

extraction

301

10.6

CONCLUSION

The range of applications cited in this chapter has demonstrated that, even

though the analytical application of supercritical fluid extraction is a relatively

new technique when compared to the more mature areas of chromatography like

GC

and

LC,

it is a potentially powerful tool in

the

armoury

of

the analyst work-

ing in the petroleum industry. It has advantages (sometimes unique) in terms

of

both technical and environmental grounds over more conventional solvent-based

extraction processes and as the drive to remove environmentally unfriendly

sol-

vents increases, the use of supercritical carbon dioxide is likely to become the

technique of choice, particularly by the Environmental Protection Agency

(EPA)

in the United States.

In addition, the technology is still relatively new and is therefore currently the

subject

of

a great degree of research and development by instrument manufac-

turers and end users. It is likely therefore that, even in the time between writing

and publication of this chapter, many more significant developments will have

occurred in both the instrumentation and applications available to the analyst.

10.7

REFERENCES

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

R.C. Reid, J.M. Prausnitz and B.E. Poling, The Properties

of

Gases and Liquids, 4th edition,

McGraw-Hill; New York

(1987).

M.A. McHugh and

V.

J.

Krukonis, Supercritical Fluid Extraction: Principles and Practice

Buttenvorths, Boston,

(1986)

ISBN

0-409-9001 5-X.

T.J. Bruno and J.F. Ely (Eds.), Supercritical Fluid Technology: Reviews in Modern Theory

and Applications, CRC Press, Boca Raton, FL

(1991)

ISBN

0-8493-6847-2.

M.L. Lee and K.E. Markides (Eds.), Analytical Supercritical Fluid Chromatography and

Ex-

traction, Chromatography Conferences

(1990)

ISBN

0-8425-2394-4.

F.V. Bright and M.E.P. McNally (Eds.), Supercritical Fluid Technology: Theoretical and

Applied Approaches in Analytical Chemistry. ACS Symposium Series

488 (1992)

ISBN

0-

R.M. Smith (Ed.), Supercritical Fluid Chromatography. RSC Chromatography Monographs

E. Klesper, A.H. Convin and D.A. Turner, J.

Org.

Chem.,

27 (1962) 700.

S.T. Sie, W. van Beersum and G.W.A. Rijnders, Separation Sci.,

1 (1966) 469.

S.T. Sie and G.W.A. Rijnders, Separation Sci.,

2

(1967) 699.

Sie,

S

T; Rijnders, G W A. Sep Sci,

Vol2, 729, 1967

Sie,

S

T;

Rijnders, G W A. Sep Sci,

Vol2, 755, 1967

Sie,

S

T;

Rijnders, G W A. Anal Chim Acta,

Vol38, 31, 1967.

R.E. Jentoft and

T.H.

Gouw,

J.

Chromatogr. Sci.,

8

(1970)

138.

R.E. Jentoft and T.H. Gouw,

J.

Polym. Sci., Polym. Lett.,

7 (1969) 81 1.

J.R. Combes, K.P. Johnston, K.E. O’Shea and M.A. Fox, Supercritical Fluid Technology:

Theoretical and Applied Approaches in Analytical Chemistry, Chapter

3,

ACS Symposium

Series

488 (1992)

ISBN

0-8412-2220-7.

8412-2220-7.

ISBN

0-85186-577-1.

Adlard E.R. (ed.) Chromatography in the Petroleum Industry

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