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

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232

Chapter

present day capability substantially. Optimization of the separation by exchang-

ing a single column for one

a different polarity or by variation of the parame-

ters of a temperature program is also limited

these circumstances since an im-

provement

the resolution of some compounds in the mixture may be accom-

panied by the overlapping of other, previously separated, compounds.

But there is still another problem related

the use of single-column systems.

Even

cases where the performance of a single column is good enough

sepa-

rate

two

components with adequate concentration levels completely, it may be

impossible to detect a late eluting trace component in a large amount of the early

eluting major component. This is due to the fact that Raoult’s law is only true for

very dilute solutions. In the following, it will be seen that the use of multi-

column systems can solve problems caused by very complex mixtures as well as

the problems related to the analysis

very different concentration levels in real

samples.

9.1.2

Multi-column systems

There are

two

general ways of tackling the problems listed above. The first is

to introduce a second dimension of separation leading to a dramatically im-

proved peak capacity (at least for the peaks of interest). The second is to influ-

ence the elution order in such a way that the peaks of interest are eluted at “free”

places in the chromatogram. That means that a multi-column system allows

either to find optimal separation conditions by selectivity tuning (described in

Section

9.2)

or to reduce the complexity of the sample by the use of column

switching techniques (described in Section

9.3).

When using a heart-cutting

technique, the latter is identical to the introduction of a “second dimension” in

separation.

Today multi-column systems are mainly used for improving separating power,

shortening analysis time and for trace analysis. Some general advantages of

multi-column systems are listed below

[

121:

(a)

Increase

resolution:

the resolution within a particular group

over-

lapping peaks can be improved either by transferring only this group from

the first column to a second column with higher efficiency or another sta-

tionary phase or by a pre-calculable change of the overall selectivity of

the system.

(b)

Shorter

analysis

time:

when a particular analysis of a complex mixture is

to be performed, it is necessary to remove the components that are not of

interest in order to condition the system for the next analysis. By revers-

ing the flow of the pre-column, a fast removal

late eluting components

can be achieved.

Multi-column systems in

gas

chromatography

233

(c)

Extended column and detector life:

sample components which may con-

taminate the main column can be prevented from entering the column af-

ter

separation in a pre-column. In the same way, a contamination of

specific detectors can be avoided (e.g. chlorinated solvents in a ECD) by

cutting these components out of the system after the first column.

(d)

Decrease in detection limits:

overlapping by long-tailing solvent peaks

or major peaks can be avoided by “heartcutting”.

These techniques also offer enhanced possibilities for qualitative analysis in

two ways. Firstly, the availability of two more or less independent sets of reten-

tion data allows a greater reliability of the identification of individual compo-

nents in complex mixtures by chromatographic means

[2].

Secondly, one has to

take into account that the modern “hyphenated” techniques such as GC-MS and

GC-IR give the most reliable identification only if they are dealing with pure

substances. This means that the preceding chromatographic process should be a

multi-column system to avoid peak overlapping.

Still another problem

the many types of samples which cannot be analyzed

on a single column without a prior clean-up procedure. The use of a multi-

column system, designed as a combination of a pre-column for an on-line sample

clean-up and a second analytical column can be extremely useful in such cases.

Some examples are discussed in Section

9.4.

This chapter provides only a general introduction to the use of multi-column

systems in GC. More detailed information can be found in the papers of Bertsch,

Ettre, Giddings, Guiochon, Huber, Purnell, Sandra, Schomburg and others [3-

111. Some interesting examples for the use of column switching in gas analysis

are also given in Chapter

of this book. The reader is referred to the literature

cited at the end of the chapter for a comprehensive coverage of the technique and

its applications.

9.1.3

Multi-column chromatography

9.1.3.

DeJinition

In chromatography, multi-column systems can be assembled in a number of

different ways. This large number of variations has led to some confusion in the

literature about the definition of such multi-dimensional GC (MDGC) systems.

Sometimes systems involving multi-dimensional detection such as the hyphen-

ated techniques have been incorrectly considered to be MDGC, although only a

single column one-dimensional separation is used.

In many papers, definitions

MDGC are given [e.g. 13-15], especially

“two-dimensional” GC. This term “two-dimensional” has its origin in thin-layer

References pp.

266268

234

Chapter

chromatography where it has been defined as chromatographic migration in one

direction that is followed by a second development in the perpendicular direction

1161.

But today this original definition is not transferable to other chroma-

tographic methods like gas chromatography and is limited to only

two

spatial

dimensions.

A more general definition has been proposed by Snyder

[17]

as follows.

Multi-dimensional chromatography is a separation process in which a single

sample

subjected to

sequence of chromatographic separations, each of

which:

acts upon all or part of the separated components from a previous chroma-

tographic step and

differs in its relative selectivity and/or capacity.

This definition includes all types of chromatographic separations, HPLC

(high performance liquid chromatography,

SFC

(supercritical fluid chroma-

tography),

TLC

(thin layer chromatography),

(capillary electrophoresis) etc.

Multiple separations, where only the column capacity is changed, are also in-

cluded.

For example, the use of

two

columns with different polarities coupled in se-

ries can be called

MDGC,

but the term should not be used if the

two

columns

differing in selectivity are coupled in parallel and lead to the same detector.

However, in some cases, multi-column systems which are not multi-dimensional

can also have a useful application in chromatography.

the following section, the numerous variants of MDGC are categorized by

different criteria to provide an overview of the many possibilities of this tech-

nique.

9.1.3.2

Variants

column

switching

There are many ways in which a chromatographer can vary a multi-column

system. Depending on the analytical problem and financial requirements, the

system can be optimized for the method

coupling, the operation mode, the

type of column and its working parameters. The various possible ways of cou-

pling columns are listed in Table

9.1

[

181.

Coupling

modes and hardware.

Routine analysis requires reproducible condi-

tions. Basic functions, e.g. heartcutting, foreflushing, backflushing, distribution

and intermediate trapping, should be time programmed and controlled by the

microprocessor of the

GC.

All these requirements can only be fulfilled by on-

line column switching. Off-line methods often work quite well but are hardly

feasible for automated routine analysis

[

191.

Multi-column

systems

gas chromatography

235

TABLE

9.1

VARIANTS

COLUMN

SWITCHING

~ ~~

Criterion Variant

Mode of coupling On-lineloff-line

Coupling hardware Mechanical (valves)/pneumatic (valveless)

Operation mode

Seriedparallelheries-parallel

Complete

partial eluate transfer

WiWwithout intermediate trapping

WitWwithout monitor detector after the first column

Packed, micropacked, capillary

Length, inside diameter

Type (selectivity) and amount (capacity)

stationary phase

Sequence of

columns

(column switching)

Column temperature

Type

column

Working parameters

isothermal (TI

T2, T1

T2)

temperature programmedisothermal

temperature programmedtemp. prog.

for each column

Variants

flow rates (either in

the

total system

sequentially

Generally

two

distinct forms of on-line coupling have been recognized:

(a) A sample is vaporized in an injector and split into separate fractions. All

of these are simultaneously analyzed on columns with different polarity

connected in parallel

[20,21].

(b) A sample is injected into the chromatographic system. After the pre-

column, one or more fractions of the complex mixture are directed

straight to the second column connected in series with or without inter-

mediate trapping. Two or more columns can be combined.

These forms can be obtained by connection with mechanical valves (e.g. ro-

tary, slider, piston or membrane valves

[22]) or by a pneumatic switch first de-

scribed by Deans

[23] (see Section 9.4).

Especially in valve-switching systems with packed columns, many combina-

tions

of these

two

basic forms (serial + parallel) are found in practice. In capil-

lary

column switching, the main technique is serial coupling of two columns.

Because of their high separation power, capillary systems with three and more

columns are unnecessary.

Operation

mode

serial coupling.

coupled system can work in the three basic

operational modes: complete/partial eluate transfer, witWwithout intermediate

trapping and with/without monitor detection.

References pp.

266-268

236

Chapter

In chromatographs with only packed

capillary columns, the complete eluate

is transferred from one column to the other. Disadvantages of splitting such as

discrimination and

loss

in detection sensitivity are excluded.

On the other hand, if the coupling piece

only used to permanently split the

eluate of the pre-column to a

called “monitor” detector and the second col-

umn,

two

complete sets of retention indices and response data

for

characteriza-

tion of the sample can be determined.

coupled system where a part of the eluate

split

waste is used if packed

and capillary columns are combined (pre-column

packed, main column

capillary) and the eluate will be trapped after the pre-column. In most cases the

split is positioned in the coupling piece between the

two

columns. The only in-

termediate trap without splitting

the total transfer technique (see Section

9.4.3).

routine and on-line analysis, the advantages of the direct combination of

columns without an intermediate trap (a simple, inexpensive and easily main-

tained instrument) predominate. In most cases problems caused by merging re-

solved peaks in the second column can be avoided.

Systems with intermediate traps

[24,25]

are relatively complex and expensive.

By multiple enrichment of components, this method

suited for trace analysis.

The connection of a high capacity packed pre-column to

selective capillary

column

of advantage for many applications.

By refocussing the bands after the pre-column, an exact starting point for each

fraction in the second column is obtained,

that retention data from each col-

umn are available.

If the eliminated components of a mixture are also of interest, the fractions

which have been cut out

the chromatographic system after the pre-column can

be measured by a second detector. The selection of this monitor detector de-

pends on the eliminated substances. In laboratory

GCs,

such a detector is in-

stalled basically

the use of serial-coupled capillary columns. In on-line pro-

cess

GCs,

which are applied to only one separation problem,

second detector is

frequently dispensed with because of the cost. Nevertheless, if the cut compo-

nents are

interest, the cut outlet can be connected to the main detector.

Type

colurnn.

Many applications are to be found in the literature, either with

packed and micropacked

with capillary columns in multi-column systems.

The variation in length, inside diameter, type and amount of stationary phase

depends on the requirements of the analytical problem. Either gas-solid

gas-

liquid columns can be used and sometimes both together.

contrast to packed columns, most separation problems can be solved with a

capillary system with a comparatively small number of stationary phases because

its higher separating power. Except for the separation

permanent gases

Multi-column systems in gas chromatography

237

which is a domain of the PLOT (porous layer open tubular) columns,

WCOT

(wall coated open tubular) columns predominate.

SCOT

(support coated open

tubular) columns find relatively little use.

Working

parameters. The sequence of columns is controlled by mechanical or

pneumatic switching devices and can be changed by the operation software of

the chromatograph.

typical time events program for multi-column-GC

soft-

ware is shown in Fig.

9.1.

Laboratory analyses are either isothermal or temperature programmed. Tem-

perature programming is still unusual for process

GCs;

for an on-line analysis

method like process gas chromatography constant instrument conditions, e.g.

oven temperature, carrier gas pressure or switching events, are extremely impor-

tant for reproducible cyclic runs under rigorous conditions and an oven con-

struction with high thermal mass guarantees reliable isothermal analysis.

single oven systems, the maximum temperature that can be applied is lim-

ited by the column of least thermal stability. By the use

double oven systems,

TIHE

EVENTS

PROGRAH

ChannelA

Command valid for streau ‘n’=1

Time

Function Action

1.,4

5..8

9.12

13..

21.. 25.. 29..

0.050

0.300

0.200

0.500

5 0.050

2.150

2.600

6,200

9 6.200

9.900

9.950

12 9.950

10,000

Backf

lush

Sample change

Liquid injection

Cut

cut

Cut

Backf

lush

Report

Basic-Exec

End

run

off

ANALoG_A

PRINTW

1111

1110

0000

0000 0000

1111

1110

0000

0000 0000

0000

1111

1110

0000

1111

1110

0000

1111

1110

0000 0000

0000 0000 0000

1111

1110

0000

1111

1110

0000 0000

0000

1111

1110

0000

1111

1110

0000

0000 0000

0000

1111

1110

0000

1111

1110

0000

1111

1110

0000

1111

1110

0000

0000 0000

Fig.

9.1.

Typical time events program

a multi-column process application (time events for the

analysis shown in Fig.

9.7).

In process

GC,

backflush valves are inversely configurated. The valve

position “backflush

off

means reversed flow through the precolum, so-called backflush.

References

pp.

266268

238

Chapter

an optimal individual temperature, isothermal or temperature programmed, can

be selected for each column. The advantages of double oven systems are listed

later

Section 9.3.1. Finally, a less obvious use of dual oven systems is “multi-

chromatography” (see Section 9.2). This technique is based on the possibility of

controlling the carrier gas pressure over the complete system, either before the

pre-column or before the main column.

9.2

SELECTIVITY TUNING

SERIES-COUPLED COLUMNS

High-resolution gas chromatography implies the optimization of column effi-

ciency, selectivity and capacity ratio. With the introduction of capillary columns,

the main emphasis was laid on their high efficiency. The optimization of the se-

lectivity was not of particular interest. Due to the demands for analyses in very

low concentration ranges it was recognized that many samples were more com-

plex than had been originally thought and the necessity arose for capillary

with high selectivity. Such a system can be obtained by selectivity tuning. This

means that the selectivity of a system

adapted to the analytical need by creat-

ing a selectivity between

two

more or less extreme selectivities. Sandra

al.

[lo]

suggested that in capillary

GC,

selectivity tuning can be accomplished in

one

three ways: by using tailor-made stationary phases, by using mixed

phases, or by serial coupling of columns with different stationary phases. The

last variant is the most elegant method for producing a tuneable selectivity and it

offers the chance of using standardized stationary phases with known retention

properties.

The theory of series-coupled columns is discussed in detail in some papers of

Buys, Ettre and Pumell and their co-workers [5,9,26,27]. We only wish to re-

mind the reader that the retention time

a solute in a system of

two

coupled

columns

and

can be expressed as

where

tR’

the retention time (system),

is the capacity ratio,

is the column

length and

is the average carrier gas velocity.

From this expression one can derive four principal ways for selectivity tuning:

variation of

values by changing the stationary phases (chemical nature

and/or film thickness);

variation of

values by changing the column temperatures;

variation of the column lengths (segmented column method);

variation

the average carrier gas velocities of the columns (multi-

chromatography).

Multi-column systems

gas chromatography

239

quite clear that the use of other stationary phases or the variation of the

column lengths in a system consisting of very different single columns will lead

to dramatic changes in the selectivity of the system. However, the disadvantage

of these

two

methods is that a new column system is necessary for an optimiza-

tion procedure after every optimization step. This would be a very time- and

material-consuming process which would only be useful for the development of

a routine analytical method.

For the optimization of frequently changing analytical problems, selectivity

tuning by variation of column temperatures or flow rates is more attractive. Gen-

erally, one has to consider that the limits for selectivity tuning are given by the

selectivities of the individual columns. That means that small differences in the

selectivity of the individual columns allow only a small range for selectivity

tuning. Therefore, it is useful to combine columns which are as different as pos-

sible in their retention characteristics. To demonstrate the effect of selectivity

tuning by variation of temperature even for very similar compounds, we show in

Fig.

9.2

the dependence of the retention indices of the

octane isomers listed in

Table

9.2

on the temperature of the first column (for more details see

[28]).

The

temperature of the second column was kept constant.

this case, not only dif-

TABLE 9.2

COMPOSITION

THE TEST MIXTURE

FOR

SELECTIVITY TUNING

Peak Component

n-Hexane

2 2,2,4-Trimethylpentane

3 n-Heptane

2,2-Dimethylhexane

5 2,5-Dimethyihexane

2,4-Dimethylhexane

2,2,3-Trimethylpentane

2,3,4-Trimethylpentane

9 2 3,3-Trimethylpentane

2,3-Dimethylhexane

2-Methyl-3-ethylpentane

2-Methylheptane

13 4-Methylheptane

3,4-Dimethylhexane

15 3 -Methyl3 -ethylheptane

3-Methylheptane

3 -Ethylhexane

n-Octane

GC-conditions: instrument; Sichromat 2 (SIEMENS); columns

(A)

0.32

id.

OV-1,

fs;

(B)

1.5

i.d. graphitized carbon black (micropacked),

ss;

carrier

gas,

4.3/4.0 bar hydro-

gen.

References pp.

266-2158

240

16Q'C

Chapter

800

75c

70(

650

fcl

Fig.

9.2.

Selectivity tuning by variation

temperature

(for

substances, see Table

9.2).

condi-

tions: column

0.32

OV-1;

column

1.5

m micropacked graphitized thermal

carbon

black

(GTCB), carrier hydrogen, temperature

column

160°C.

ferent polarities but also a combination

two

different chromatographic

mechanisms (partition and adsorption) were used to optimize the separation. It

can

seen that for selectivity tuning by variation

column temperatures, a

double oven chromatograph or at least

two

chromatographs with a connection

line are necessary to operate both columns independently at optimum tempera-

ture. The advantages

a double oven chromatograph are discussed in more de-

tail

Section

9.3.1.

Multi-column

systems

gas

chromatography

Variation of flow-rates in a system of series coupled columns (see Kaiser and

co-workers

[29])

is a very special way of selectivity tuning. In practice, carrier

gas is added at the connection point between the first and second column

(leading to a higher

ow-rate in the second column and an increasing influence

the selectivity of the first column on the system selectivity), or carrier gas and

consequently a part of the sample, are vented at the connection point (leading to

an increasing influence of the second column on the system selectivity). With

this method, impressive changes in the system selectivity can be achieved with a

very simple apparatus; the selectivity can also be easily pre-calculated. The

problem is that at least one column is not operated at the optimum flow rate.

This means that the range of possible flow rates is limited by the effect of a re-

duced number

theoretical plates according to the van Deemter equation. Con-

sequently, the use of hydrogen as carrier gas for selectivity tuning is recom-

mended because of its broader van Deemter optimum.

9.3 COLUMN SWITCHING TECHNIQUES

9.3.1

General

Column switching was first developed for process control

[32].

Some indus-

trial processes require the analysis of only a single compound or a limited group

substances which may be in complex matrices or emerge after main compo-

nents which are not of analytical interest. Under these circumstances, it is desir-

able to develop a chromatographic system which allows the fractionation

the

sample and the removal of fractions of no interest. Analyses using temperature

programming in laboratory gas chromatography, especially the removal of high

boiling compounds from the system, can be efficiently solved in an isothermal

mode by the use of a column switching system. One of the first applications of a

column switching system in process control was published by Villalobos and co-

workers

[30]

1961.

Since then, the advantages of column switching have also

been used in laboratory applications. Figure

9.3

shows in a simplified form, what

effects can be achieved by the application of the special column switching tech-

niques, described later. Figures

9.4

and

9.5

illustrate the schematic flow paths of

typical column switching systems for valveless as well as for valve switching

systems.

A very helpful tool for the development of column-switching methods is

the availability of two independently controlled ovens. Only a few manufac-

turers offer double oven chromatographs for laboratory analysis as well as for

process-control. The advantages of such instruments can be described as fol-

lows:

References

pp.

264-268