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

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312

Chapter

fully with the FID

[22,23].

Alternative detectors must be found when other or-

ganic modifiers (e.g. methanol) are used.

(ii)

Photo-ionization detector (PID).

The PID is a non-destructive

detector

often placed

series with an FID and with which it shares a number of useful

characteristics (responsive to most organic compounds, high sensitivity and wide

linear dynamic range). In other respects, however, the PID (concentration- and

compound-sensitive response) and FID (mass-sensitive and uniform hydrocarbon

response) are complementary to each other.

the PID, a sealed excitation source emits photons in the far UV which

ionizes analytes having an ionization potential

(IP)

less than the energy of the

emitted photons. The IPS of carbon dioxide, nitrous oxide and sulphur hexafluo-

ride are 13.79 eV, 12.89 eV and 19.3 eV, respectively,

that these fluids should

be transparent to the PID when using the 10.2 eV excitation source. Some

the common

SFC

modifiers also have high IPS, e.g. methanol (10.85 eV)

dichloromethane

1.35 eV) and acetonitrile (12.22 eV), suggesting the

possi-

bility of using mixed fluids with this detector. In practice, the addition of

even low concentrations

methanol modifier significantly reduces the sensitiv-

ity of the PID and there have been few published applications of its use in

SFC.

(iii)

Light scattering detector (LSD).

If the

LSD

had not originally been devel-

oped for

LC,

then it would almost certainly have been developed for packed col-

umn

SFC.

The detection mechanism is complex but, in simple terms, relies on

the nebulization of the column effluent to form droplets and particles which

cause light to be scattered as they pass through a light beam. The amount of

scattered light is measured and

a non-linear function of the analyte mass

passing through the detector.

For

LC,

an inert gas must be used to nebulize the liquid eluent whereas for

most

SFC

applications the natural expansion of the supercritical fluid provides

efficient nebulization

[24,25].

In the nebulization process, volatile components

(including organic modifiers) do not form droplets or particles and are therefore

not detected by the LSD. The LSD is therefore unaffected by composition and

pressureldensity gradient programming and makes

excellent universal detec-

tor suitable for use with all common

SFC

mobile phases. The sensitivity of the

detector (low ng) is adequate for most purposes. The only drawback

that the

detector response,

and

SFC

modes,

compound-dependent and non-

linear.

For

the analysis of homologues (e.g. polymers) or individual species, this

drawback is easily overcome by appropriate calibration of the detector. How-

ever, many petroleum analyses determine hydrocarbon types or compound

classes for which there are no standards available for calibration. The composi-

Supercritical

fluid

chromatography

313

90°C

135

285

bar

2.27 bar/min

minutes

80%

160

250 bar

5.5

bar/min

;f,

minutes

Fig. 11.2.

SFC

anslysis

a diesel fuel with (a)

FID

and

(b)

UVD

at 254nm detection. Mi-

cropacked fused silica column (300

0.32pm

ID),

C02

mobile phase. Peak identification: (1)

naphthalene;

(2)

methylnaphthalene; (3) fluorene;

(4)

phenanthene;

(5)

methylphenmthrene

(reproduced from ref. 25 with permission

Preston Publications).

References

pp.

343-345

Chapter

tional variation within a hydrocarbon-type fraction (e.g. aromatics) even pre-

cludes the use of such fractions themselves as calibration standards.

11.2.4.2

Spectroscopic detectors

Compounds possessing certain structural features (e.g. an aromatic ring or car-

bony1 group) can be selectively identified in the column effluent using spectro-

scopic detectors. The ubiquitous UV/visible detector (UVD) currently vies with

the FID as the most popular general-purpose SFC detector, although this position

largely due to its use on packed column

SFC

systems and non-petroleum ap-

plications.

UV/visible detector (UVD).

Many compounds possess a W-absorbing func-

tional group in the 200-800 nm spectral region and are readily detected with

high sensitivity by the UVD. The majority of compounds found in petroleum are,

however, saturated hydrocarbons which do not respond to the UVD. Since the

UVD is non-destructive, this limitation can be turned to advantage by using the

selectivity of the UVD to monitor olefins and aromatics in combination with the

universal detection capabilities of the FID. Figure

1.2 illustrates the potential of

selective UVD detection for the SFC analysis of diesel fuel. The SFC-FID chro-

matogram (Fig.

1.2 (a)) is dominated by the saturated hydrocarbons (especially

the normal paraffins) yet it is still possible to identify the underlying polycyclic

aromatic hydrocarbons (PAHs) in a single run using the

UVD

(Fig. 1

1.2(b)).

The UVD is easily interfaced to packed column SFC instruments; installation

of a high pressure flow cell is usually the only modification required. Capillary

SFC places stringent limitations on the detector cell volume which are usually

met using on-column or pseudo on-column flow cells [27]. The use of Z-shaped

flow cells (Fig. 11.3), now commercially available, allows the cell path length

(and therefore UV response) to be increased dramatically although at the ex-

pense of higher noise levels [28]. Photodiode array detectors have been used

with both packed [29] and capillary columns

[30],

although the latter suffers

from

excessively high noise levels; rapid scanning

WDs

offer improved signal

to noise ratios for capillary systems

11.

The common supercritical fluids are useable over the full W/visible spec-

trum, as are non-W absorbing modifiers (e.g. methanol, acetonitrile). High pu-

rity carbon dioxide can be used below 200 nm which enables its use as an olefin-

selective detector for hydrocarbon-type analysis. Baseline drift observed with

pressure/density programming has been attributed to density-related refractive

index changes; this effect

minimized if detection is performed under subcriti-

cal (i.e. liquid state) conditions.

(ii)

Fluorescence detector (FLD).

For certain compounds (e.g. PAHs), consid-

erably lower limits of detection can be achieved using a FLD in place of a UVD.

Supercritical fluid chromatography

lamp

tubing

photo

diode

315

On-column: path length

Z-shape: path length up to

Fig. 11.3. Comparison

on-column and Z-shaped

flow

cell design

for

UV/visible detectors

(reproduced

with

permission

LC Packings).

The extra sensitivity, coupled with a high degree of selectivity, makes fluores-

cence detection a particularly attractive option for capillary SFC where the flow

cell is often an integral part of the column itself

[32].

more recent develop-

ment is laser-induced fluorescence/supersonic jet spectroscopy (LIFEJS) where

the effluent from the column expands into a low pressure region to produce a

narrow beam of ‘cold’ gas phase molecules. The very high resolution spectra

obtained under these conditions enables compounds, even geometric isomers, to

be accurately identified. At present the sensitivity

the technique, even with

LIF, is insufficient for real time identification of chromatographic peaks

[33].

Virtually all supercritical fluids are compatible with the FLD as are density

programming and compositional mobile phase gradients. The following mobile

phases have been used: carbon dioxide (with and without modifiers), alkanes

(propane, butane, pentane), dichlorofluoromethane, and ammonia. Fluorescence

detection, with the exception of SJS, is non-destructive and hence may be used

in conjunction with other detection methods.

References pp. 343-345

316

Chapter

(iii)

Mass

spectrometry detector

(MSD).

MSD is the most popular detection

method after FID and

UVD

for the following reasons: high sensitivity (low pg)

and selectivity (especially with MS-MS techniques), plus the ability to identify

confirm unknown analytes. For routine oil laboratories, the costs of purchas-

ing and maintaining an SFC-MSD system may not be justifiable. Nevertheless

this

an area of considerable interest to the petroleum analyst because of the

largely unknown composition of petroleum. The sensitivity/selectivity combina-

tion of mass spectrometry enables more complex mixtures to be analysed thus

reducing

or minimizing sample preparation times. Higher selectivities (with

lower sensitivities) are possible with high resolution magnetic sector instruments

MS-MS techniques. Cheaper benchtop quadrupole instruments are, however,

often adequate for most analyses where the SFC separation has removed possible

interferents. Facilities for both electron impact (for structural identification) and

chemical ionization (molecular mass information) are desirable but not always

attainable with the same SFC-MSD interface.

Prior developments in GC-MS and LC-MS interfaces have greatly accelerated

the introduction of commercial SFC-MSD systems. Since mass spectrometers

operate under high vacuum, they can only accept small gas flows such as those

generated by open tubular capillary columns. SFC-MSD interfaces for capillary

columns are therefore the simplest and use some form

direct fluid injection

(DFI) where the commonest problems are restrictor blockage and

loss

in sensi-

tivity with density programming (due to increased ion-source pressures). The

higher flow rates generated by packed columns can still be accommodated by a

DFI interface if only a portion of the column effluent is sent to the mass spec-

trometer. This strategy, however, gives rise to a large loss

sensitivity and there

are a number of alternative interfaces (e.g. thermospray, moving belt, particle

beam and atmospheric pressure ionization

(MI))

which can take the full flow

(1-2

ml/min) from a packed SFC column. SFC-MSD interfaces have been re-

viewed by Lee and Markides

[

151.

(iv)

Infrared detector

(IRD).

The strength

infrared spectroscopy lies in its

ability to characterize functional groups (e.g. carbonyl groups in acids, esters and

amides) and differentiate between isomeric compounds. Its major weakness,

particularly with respect to petroleum analysis, is that it is poor at identifying

homologues; fortunately, information on homologous compounds can be readily

obtained via the chromatographic separation andor by interfacing to an MSD.

Conventional dispersive infrared spectrophotometers can only be used as chro-

matography detectors if fixed at a single wavelength. To obtain spectral infor-

mation on-the-fly it is necessary to use Fourier transform IR (FTIR) instruments.

SFC-IRD interfaces may be of the flow cell type (on-line detection)

or use

solvent elimination techniques (off-line detection). Early designs used high pres-

Supercritical fluid chromatography

317

sure flow cells to enable real time non-destructive monitoring of the column elu-

ent. Chromatograms are reconstructed using the Gram-Schmidt algorithm which

has the added benefits of removing the baseline drift caused by density/pressure

programming and increasing signal intensity. The

spectra for each chroma-

tographic peak can be extracted for identification purposes; exact matches with

library spectra may not always be possible due to changes in the position and

intensity of the

absorption bands arising from solute-solvent interactions.

Both packed and capillary

SFC

systems can be interfaced to the

FTIR

spectro-

photometer although the difficulties are considerably greater for capillary sys-

tems where small cell volumes dramatically reduce sensitivity (stopped flow

techniques can be used to increase sensitivity). The choice of mobile phase is

fairly restrictive for on-line detection because very few solvents are transparent

over the whole infrared region. Carbon dioxide is, fortunately, one of the better

fluids although it is only transparent over the 800-2100 cm-l and 2500-

3500 cm-1 region of the

spectrum; organic modifiers cannot be used. Super-

critical xenon is an excellent mobile phase and completely transparent over the

whole

spectrum but is very expensive. The separation

a synthetic mixture

Time/ min

Pressure/atm

310

4000

2000

700

cm-1

Fig.

11.4.

Capillary SFC-FTIR chromatogram of model

PAHs

using xenon mobile phase: (a) re-

constructed chromatogram from total IR signal;

(b)

IR spectra of peaks

{

chrysene) and

(mixture

9,10-diphenylanthracene,

2a,

and perylene,

2b)

(reproduced from ref.

with permission of

Elsevier Science Publishers).

References pp. 343-345

318

Chapter

PAHs by capillary SFC-IRD using xenon mobile phase and a low volume IR

flow cell (sub-microlitre)

illustrated in Fig.

11.4

along with examples of the

high quality IR spectra which can be obtained

[34].

Eliminating the solvent removes any limitations arising from the choice of

mobile phase and can also provide a large enhancement in sensitivity. The col-

umn restrictor is placed close to a moving window of IR transparent material,

KBr or ZnSe for transmission mode or aluminium for reflectance mode

[35],

and

the analytes deposited as a discrete spot

the mobile phase evaporates. The

moving window

cooled to minimize spot size and improve recovery of volatile

components. FTIR measurements can be made on-line (real time chroma-

tograms) or off-line (highest sensitivity by repeatedly analysing the spot) using

an FTIR microscope. A more detailed review

SFC-IRD interfaces can be

found elsewhere

[36].

(19

Nuclear magnetic resonance

i”.R)

detection.

Allen and co-workers have

reported the direct coupling of SFC with a proton

(‘H)

nuclear magnetic reso-

nance spectrometer

[37].

Using a packed column

(250

4.6

mm ID) and

carbon dioxide modified with

deuteroacetonitrile as mobile phase, these

workers were able to obtain reasonable on-line proton spectra for a series of

model hydrocarbons. Proton

NMR

is widely used throughout the petroleum in-

dustry for characterizing saturated and aromatic hydrocarbons after chroma-

tographic separation.

11.2.4.3 Element specific detection

Element specific detectors are widely used in the petroleum industry for the GC

analysis of volatile non-hydrocarbons, particularly those containing nitrogen and

sulphur. The FID is often described as a carbon detector. Element and analyte

specific detection is very useful for trace analysis and can eliminate the need for

extensive sample clean-up.

11.2

4.3.1 Thermionic detectors.

The thermionic detector (TID) is used for the

selective determination of nitrogen- and phosphorus-containing compounds and

is often referred to as a nitrogen-phosphorus detector (NPD). Organo-nitrogen

and -phosphorus compounds are combusted in a hydrogenlair flame to form

electronegative specieshons which react with hot alkali metals (rubidium or

caesium), supported on a glass or ceramic bead, to produce a current which is

amplified to give the TID signal. When operated in an inert atmosphere (i.e.

ni-

trogen rather than hydrogenlair) the TID exhibits high selectivity for nitro-

compounds

[38].

Both carbon dioxide and nitrous oxide have been used as mobile phases

[38,39]

although there may be some loss in nitrogen selectivity and sensitivity

Supercritical fluid chromatography

319

when nitrous oxide is used. Best results are obtained by optimizing the detector

parameters (e.g. gas flows, bias voltage) for the chosen mobile phase and type of

analyte; even

the detection limit and specificity are likely to be less than that

achievable by GC. The influence of organic modifiers on the TID response ap-

pears to be variable; the TID operated successfully with

methanol in carbon

dioxide

[23]

but not with methanol concentrations higher than 0.8% in nitrous

oxide [40].

11.2.4.3.2 Flame photometric detection.

The flame photometric detector (FPD)

offers selective detection for organosulphur and organophosphorus compounds

by measuring the chemiluminescence emitted as excited

dimers and

HPO

species, generated in the hydrogedair (or hydrogedoxygen)

ame, return to the

ground state. Quantification with the FPD, in GC and SFC, is not trivial because

of its non-linear response, which is compound dependent, and quenching of the

sulphur signal by co-eluting hydrocarbons. The dual flame photometric detector

(DFPD)

better but still far from ideal.

Carbon dioxide is the only reported mobile phase used with the FPD.

Markides et al. found detection limits for sulphur (as benzo[b]thiophene) in the

low ng range and a rapidly increasing baseline as the mobile phase pressure was

increased [41]. Olesik et al. studied the flame gas chemistry in SFC, using ex-

perimental design algorithms, and found that the optimum gas flow rates for sul-

phur detection were very different from those used in GC [20]. Detection limits

under these optimum conditions approached those obtained in GC and baseline

drift with increasing pressure was not observed when a tapered restrictor, rather

the linear restrictor used by Markides et al., was employed.

11.2.4.3.3 Sulphur chemiluminescence detection

(SCD). There are three types of

SCDs: fluorine-induced SCD, redox CD and the ozone-induced SCD. These de-

tectors work by first oxidizing the organosulphur compound to give a species

which may either react with fluorine or ozone to form a chemiluminescence

species

(HF*

and SO2*, respectively) or in the case of the redox CD it is the NO,

produced from the oxidation of the sulphur compound by NO2, which is reacted

with ozone to produce the chemiluminescent NO2*.

The FSCD, like the FPD, is compound sensitive and uses toxic fluorine gas.

The RCD responds to oxidizable species and not selectively to

The 0-SCD is

by far the best option since it is selective for

to which it responds quantita-

tively (irrespective of the compound) and is far less susceptible to quenching or

interference and hence should be satisfactory with modified SFC. Its use with

open tubular capillary, packed capillary and packed microbore columns has been

demonstrated with C02 as mobile phase [42]. Earlier thoughts that large quanti-

ties of C02 would quench the chemiluminescence and make packed column un-

References pp.

343-345

320 Chapter

usable have proved unfounded. On-column detection limits are very similar for

capillary and packed columns. The

0-SCD

could be used with

MeOH modi-

fied

C02

[42c].

11.2.4.3.4

Other element selective detectors.

Other selective detectors that have

been used for

SFC

include inductively coupled plasma emission detectors and

electron capture detectors.

11.2.5

Restrictors

Some form of post-column-restriction is required to maintain the pressure of the

mobile phase above its critical pressure throughout the whole separation process

(i.e.

the column). The performance characteristics of the restrictor, which may

be of the non-variable, fixed

variable type, are extremely important especially

capillary

SFC

systems. Variable restrictors can be operated manually

pro-

grammed during the course of the

SFC

analysis.

For

high pressure liquid phase

detectors (e.g.

WD) the design of the restrictor

not as important provided it

maintains a stable pulse-free flow and does not plug easily.

For

gas phase detec-

tors the design requirements are more demanding because of the need to quanti-

tatively transfer all the analytes (especially non-volatile and low solubility com-

pounds) to the detector. Ideally the length of the pressure reduction zone be-

tween the column and the detector should be short enough that the fluid main-

tains its density (high solvating power) right

to the entrance to the detector.

2.5.

Fixed restrictors

Simple fixed restrictors, made out of fused silica or metal capillary tubing, are

used

most

SFC

systems and particularly in those designed for use with capil-

lary

small diameter packed columns. The major drawback with these restric-

tors

is that the mass flow of a fluid through a fixed restrictor increases rapidly

with pressure/density programming, resulting

higher linear mobile phase ve-

locities and lower column efficiencies.

The simplest restrictor, known as a linear restrictor, is made from a short

length

(3-15

cm) of fused silica capillary tubing (5-25pm). The geometry of

these restrictors is far from ideal and the long pressure reduction zone results in

poor

transfer of analytes with low solubility. The relatively large

IDS,

however,

make these restrictors less prone to plugging when used with high flow rate sys-

tems (packed columns and

SFE).

The tapered and integral restrictors represent

attempts to approach the ideal geometry; comparative tests indicate that these

designs are more efficient

transporting the higher molecular mass and less

volatile compounds to the detector [43]. The smaller orifice

(1-2

pm) of the ta-

Supercritical fluid chromatography

321

pered and integral restrictors makes them more prone to plugging than either the

linear or frit restrictor. The frit restrictor contains a porous ceramic frit which is

not easily plugged because of its multichannel pore structure. Despite some evi-

dence suggesting poorer transmission of non-volatile solutes, this robust device

is now widely used in many

SFC

systems.

Metal restrictors (platinum, platinum-iridium and stainless steel) are prepared

by crimping the end of the tubing until the desired flow rate is obtained. Their

performance is comparable to the tapered and integral restrictors although they

work best with moderate to high flow rates

(0.25

ml/min upwards). In the

author’s experience they are less likely to plug than tapered or integral restrictors

due to the excellent heat-conducting properties of the metal and because manual

crimping often produces more than one orifice. The main disadvantages are a

tendency for the restrictor to open up over a period of time and the irreproduci-

ble nature of the crimping process.

the supercritical fluid expands from the supercritical to the gaseous state,

severe cooling occurs as a result of the Joule-Thompson effect resulting in the

build-up of solid carbon dioxide or other supercritical fluid at the restrictor tip.

Some heat input is therefore required to prevent the fluid from freezing. In capil-

lary

SFC,

where column temperatures are often above

100°C,

the mobile phase

will provide some of the necessary heat which can be supplemented by heaters in

the detector. Packed column

SFC

is frequently carried out at much lower tem-

peratures (e.g.

40°C)

and higher flow rates and

must rely entirely on the de-

tector for heating the restrictor.

11.2.5.2

Variable restrictors

The first commercial

SFC

instrument for packed columns was equipped with a

manually adjustable needle valve (back pressure regulator) which allowed inde-

pendent control over the mobile phase linear velocity but only under isobaric

conditions

[44].

An electronically controlled version of the back pressure regula-

tor, in which the needle valve is rapidly pulsed under the control

a solenoid

valve, was described by Saito et al.

[45].

This variable restrictor, now commer-

cially available, allows flow and modifier gradient programming for isobaric

packed column

SFC.

1991,

Morrissey et al. described a computer-controlled pressure valve unit

which was capable of simultaneously generating both pressure and solvent

modifier gradients for packed column

SFC

[46].

This design was developed fur-

ther into a commercial system with microprocessor-control gradient program-

ming facilities for mobile phase pressure (density), composition and flow. The

only non-progammable parameter is temperature which

of only minor incon-

venience since most packed column

SFC

carried out under isothermal condi-

tions.

References pp. 343-345