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Gas Chromatography in Capillaries
34
Ernst Kenndler: Gas Chromatography 1 GAS CHROMATOGRAPHY ERNST KENNDLER Institute for Analytical Chemistry, University of Vienna 1 THEORETICAL ASPECTS OF GAS CHROMATOGRAPHY ............................................................ 2 1.1 DISTRIBUTION CONSTANT, SEPARATION SELECTIVITY............................................................................... 2 1.1.1 Temperature Dependence of Distribution Constant ........................................................................ 4 1.1.2 Retention index I R ............................................................................................................................ 5 1.2 DISPERSION IN CAPILLARY GC .................................................................................................................. 7 1.2.1 Golay equation .............................................................................................................................. 11 1.2.2 Plate height vs. retention factor. ................................................................................................... 13 2 PRACTISE OF GAS CHROMATOGRAPHY....................................................................................... 15 2.1 CARRIER GAS........................................................................................................................................... 15 2.2 SAMPLE INLET ......................................................................................................................................... 17 2.2.1 Split and splitless injector ............................................................................................................. 18 2.2.2 On-column injection ...................................................................................................................... 20 2.3 COLUMNS ................................................................................................................................................ 21 2.3.1 Stationary phases .......................................................................................................................... 22 2.3.2 Rohrschneider / McReynolds index ............................................................................................... 24 2.4 COLUMN OVEN ........................................................................................................................................ 26 2.5 SPECIAL DETECTORS ............................................................................................................................... 26 2.5.1 Flame ionisation detector.............................................................................................................. 26 2.5.2 Electron capture detector .............................................................................................................. 28 2.5.3 Alkali flame ionisation detector .................................................................................................... 31 3 FURTHER READINGS............................................................................................................................ 34 © Ernst Kenndler Version 19/01/2004
Transcript
  • Ernst Kenndler: Gas Chromatography

    1

    GAS CHROMATOGRAPHY

    ERNST KENNDLER

    Institute for Analytical Chemistry, University of Vienna

    1 THEORETICAL ASPECTS OF GAS CHROMATOGRAPHY ............................................................ 2

    1.1 DISTRIBUTION CONSTANT, SEPARATION SELECTIVITY............................................................................... 2 1.1.1 Temperature Dependence of Distribution Constant........................................................................ 4 1.1.2 Retention index IR............................................................................................................................ 5

    1.2 DISPERSION IN CAPILLARY GC .................................................................................................................. 7 1.2.1 Golay equation .............................................................................................................................. 11 1.2.2 Plate height vs. retention factor. ................................................................................................... 13

    2 PRACTISE OF GAS CHROMATOGRAPHY....................................................................................... 15

    2.1 CARRIER GAS........................................................................................................................................... 15 2.2 SAMPLE INLET ......................................................................................................................................... 17

    2.2.1 Split and splitless injector ............................................................................................................. 18 2.2.2 On-column injection...................................................................................................................... 20

    2.3 COLUMNS ................................................................................................................................................ 21 2.3.1 Stationary phases .......................................................................................................................... 22 2.3.2 Rohrschneider / McReynolds index............................................................................................... 24

    2.4 COLUMN OVEN ........................................................................................................................................ 26 2.5 SPECIAL DETECTORS ............................................................................................................................... 26

    2.5.1 Flame ionisation detector.............................................................................................................. 26 2.5.2 Electron capture detector.............................................................................................................. 28 2.5.3 Alkali flame ionisation detector .................................................................................................... 31

    3 FURTHER READINGS............................................................................................................................ 34

    Ernst Kenndler Version 19/01/2004

  • Ernst Kenndler: Gas Chromatography

    2

    1 THEORETICAL ASPECTS OF GAS CHROMATOGRAPHY This text should be read in context with Introduction in Chromatography, where a

    fundamental discussion of the migration and dispersion phenomena occurring in the

    chromatographic separation system is given in a general manner. In the present contribution

    the theoretical discussion is mainly directed towards gas liquid chromatography in capillary

    columns. This chapter about theory of gas chromatography is followed by a considerably

    detailed presentation of practical aspects of this method.

    1.1 Distribution constant, separation selectivity

    In Gas Liquid Chromatography the analytes are distributed between a liquid stationary phase

    and an ideal gas as the mobile phase as schematically shown in Figure 1.

    cim (t) GAS cis (t) LIQUID

    Figure 1 Schematic presentation of a gas-liquid chromatographic system. is the average

    linear velocity of the mobile phase. cim (t) and cis (t) are the concentrations of the analyte, i, in

    the mobile and the stationary phase, respectively. Both are functions of time.

    This distribution is determined by the partition constant, given as usual by

    (1)

    However, for practical reasons the concentration in the liquid is expressed by the mole

    fraction, xi, and that in the gas phase by its partial pressure, pi.

    For the concentration in the liquid we express the vapour pressure of the solute, i, over

    the binary mixture consisting of liquid phase and analyte by Henry`s law

    mi

    li

    i ccK =

  • Ernst Kenndler: Gas Chromatography

    3

    0i

    (2)

    Where H is the Henry constant, xil the mole fraction of analyte, i, in the liquid phase, i0 is the activity coefficient (at infinite dilution), pi0 is the vapor pressure of the analyte (as pure

    compound) at the given temperature.

    The partial pressure, pi, of the analyte in the ideal gas phase is given by Dalton`s law

    (3)

    T is the absolute temperature and R is the gas constant.

    Substitution of the volume concentrations in eq. 1 (for infinite dilution) gives the

    expression for the partition constant in GLC:

    molsoi

    oi

    oi Vp

    RTK,= (4)

    where Vs,mol is the mean molar volume of the stationary phase.

    Two most important parameters occur in this expression:

    oip , the vapour pressure of the analyte as pure compound at temperature T the activity coefficient of the analyte in the stationary liquid (at infinite

    dilution).

    Separation selectivity of two consecutively eluting components, i and j, is defined in

    chromatography by the selectivity factor, ji, the ratio of the distribution constants, K. However, measurement of the values of K is complicated compared to k` values. As in a

    certain chromatographic system the phase ratio is the same for all components, the selectivity

    coefficients can be expressed by the ratio of the k` values of the pair of separands

    ji ji

    k

    k=

    ,

    , (5)

    The selectivity coefficient for GLC using eq. 4 is thus

    jiio

    jo

    io

    jo

    pp

    = (6)

    It can be seen that selectivity in GLC is determined by two ratios:

    the ratio of the vapour pressures of the analytes as pure compounds (at working temperature)

    000i

    liiiHi pxpp ==

    RTVnp

    g

    gi

    i =

  • Ernst Kenndler: Gas Chromatography

    4

    the ratio of the activity coefficients the in stationary phase (at infinite dilution).

    Whereas the temperature only influences the first, the second reflects the difference in the

    chemical interactions of the two separands in the stationary liquid. Insofar it is in fact a kind

    of molecular recognition, which determines selectivity. This is exactly what makes the use of

    different stationary phases meaningful.

    1.1.1 Temperature Dependence of Distribution Constant

    Eq. 4 might lead to the erroneous conclusion that the distribution constant in GLC increases

    with increasing temperature. In fact the contrary is the case, because this linear dependence of

    K on T is more than overcompensated by the exponential increase of the vapour pressure, pio ,

    with temperature, according to the Clausius-Clapeyron equation. For this reason the

    distribution constant, and the capacity factor as well, decreases strongly with increasing

    temperature. In fact the following linear approximation can be found

    TkrespK i

    oi

    1log.log ' (7) The experimental dependence of the capacity factor on the temperature is shown in

    Figure 2 for two analytes.

    Figure 2 Relation of the logarithm of ethylbenzoate and 1-heptanol on the reciprocal of the

    absolute temperature.

    0,0023 0,0024 0,0025 0,0026 0,0027 0,0028 0,0029

    1

    10

    etbenzoat

    C7ol

    lok

    k

    1 /T

  • Ernst Kenndler: Gas Chromatography

    5

    1.1.2 Retention index IR

    In contrast to HPLC, gas chromatography possesses a useful and generally accepted

    parameter for the characterisation and identification of analytes. This parameter is based on

    the finding that log k` values of the members of a homologous series of organic compounds

    are linearly depending on the number of carbon atoms, n, in their molecules. Applied to the

    homologous series of n-alkanes this means that

    log .,k A B nn = + (8) This relation can be graphically represented by

    Figure 3 Logarithm of the retention (capacity) factor, k`, of the homologues series of the n-

    alkanes as function of the number of carbon atoms

    For each analyte with a certain capacity factor a pair of n-alkanes exists, between that the

    analyte is eluted in the chromatogram. This analyte is considered as a fictive n-alkane with a

    hypothetical number of C-atoms, in the example in Figure 3 with 11.63 C-atoms. This number

    (and the number of C-atoms of the n-alkanes as well), multiplied by 100, is the retention

    index. The analyte in the example given consequently has the retention index 1163. It is

    obvious that n-alkanes have always retention indices with full hundreds, n-hexane e.g. 600, n-

    undecane 1100, n-eicosane 2000, etc.

    Obviously the retention index of analyte i is normally determined with higher

    accuracy than it would be possible by graphical interpolation. For this reason the following

    equation is used that can be derived simply from Figure 3 by comparison of similar triangles:

    nkk

    kkzI

    nzn

    niRi 100loglog

    loglog100 ,,

    ,,

    +=

    + (9)

    Substitution of the capacity factors by the net retention times

    4 6 8 10 12 14 16 184

    6

    8

    10

    12

    14

    16

    18

    "n" = 11,63

    log

    k`

    number of C atoms

  • Ernst Kenndler: Gas Chromatography

    6

    0RRiNRi ttt = (10)

    leads to an expression convenient for measurement and determination of the retention index

    n

    tt

    tt

    zI

    NRn

    NznR

    NRn

    NRi

    Ri 100log

    log100

    )(

    +=+

    (11)

    Here (n+z) and n are the numbers of C-atoms of the n-alkanes eluting after and before the

    analyte. Usually, but not necessarily, z is 1. If e.g. due to economic reasons only the (cheaper)

    even-numbered higher n-alkanes are used (e.g. C26 and C28), z is 2 in that case.

    It is the advantage of the retention index that it is independent of certain, often varying

    experimental parameters: velocity of the mobile phase phase ratio length of the column.

    It depends on: kind of the stationary phase (column temperature)

    It is therefore a very well suited number for the characterization of an analyte on a certain

    stationary phase. Retention indices are used for identification of an analyte by comparison of

    the value of the index with either that found in tables, or determined previously with the help

    of reference compounds. Retention indices of an unknown compound on different stationary

    phases allow further conclusions about the polarity of the analyte.

    Table 1 Retention indices and IR values of differently polar C8 compounds on apolar polydimethyl siloxane (OV1) and polar polyethylenglycol (PEG) as stationary phase

    Compound IR (OV1) IR (PEG) IR =IRpolar IRunpolar n-octane 800 800 0

    n-dibutylether 864 966 102

    n-hexylacetate 963 1101 138

    n-octanon-2 957 1295 338

    n-octanol-1 1038 1545 507

  • Ernst Kenndler: Gas Chromatography

    7

    For this purpose the indices are determined at two columns with different polarity, e.g. with

    methylsiloxane and polyethylenglycol as stationary liquids. Obviously n-alkanes have always

    a IR -value of zero. The more polar the functional group is, the larger is the IR value. IR value values of selected reference compounds are used to describe the polarity of stationary phases as well (see concept of the Rohrschneider / McReynolds index, chapter

    2.3.2.).

    Conclusion for the use of retention indices:

    Retention indices at one phase

    n-alkanes always full hundreds, n-hexane 600, etc. homologues differ by 100 at one stationary phase

    Retention indices at two phases

    IR =IRpolar IRunpolar zero for n-alkane IR the larger, the more polar functional group IR values of selected reference compounds are used to describe the polarity of

    stationary phase

    1.2 Dispersion in capillary GC

    Peak dispersion in chromatography is discussed in general in Introduction to

    chromatography. We will here concentrate on dispersion in capillary GC.

    Peak broadening is described by the model of the theoretical plate height, H. Note that

    4 processes were found to contribute to the total plate height in chromatography.

    Hdiff describes the contribution from longitudinal diffusion Hconv that from convective mixing Hex,m that stemming from the kinetics of mass exchange from the mobile phase to the

    interface between mobile and stationary phase

    Hex,s that from the kinetics of mass exchange from the stationary phase.

    Consequently the total plate height is the sum of the four contributions:

  • Ernst Kenndler: Gas Chromatography

    8

    H H H H Hdiff conv ex m ex s= + + +, , (12)

    (ad Hdiff )

    The contribution of longitudinal diffusion, that in direction of zone migration, is caused by the

    concentration gradient occurring between the sample and its surrounding in this direction.

    According to the Einstein equation the resulting peak variance in the space domain is given by

    (13)

    Broadening of the peak with time according to eq. 13 is shown in Figure 4.

    The equivalent expression for the relation between plate height and variance is

    (14)

    Figure 4 Development of peak broadening with increasing time due to diffusion. The

    concentration zone was infinitely narrow at time 0 ( - or Dirac function)

    This contribution is as more pronounced as larger the diffusion coefficient, Dm,i , of the

    analyte in the mobile phase is, and as longer the time is available for diffusion. Consequently,

    0,0

    0,2

    0,4

    0,6

    0,8

    1,0

    1,2

    t3

    t2

    t1

    conc

    entra

    tion

    z

    Hzz =2

    tDmiz 22 =

    100 150 200

    t0 function

    lim z = 0

    conc

    entr

    atio

    n

    z

  • Ernst Kenndler: Gas Chromatography

    9

    this increment increases with decreasing velocity of the mobile phase; it is proportional to 1/v.

    It follows that the plate height contribution due to longitudinal diffusion is

    HDvdiff

    m i= 2 , (15) It should be mentioned that longitudinal diffusion in the stationary phase does not

    significantly contribute to peak broadening, because the diffusion coefficients in the

    stationary phase are 5 to 6 orders of magnitude smaller than those in the gas phase.

    (ad Hconv )

    The radial velocity profile of the mobile phase flowing through the column introduces an

    effect to peak broadening due to convective mixing. In case of an open tube (as it is in

    capillary GC) the flow profile can be analytically described in a simple way, in contrast to

    packed beds. It has a parabolic shape with zero velocity at the capillary wall, and maximum

    velocity at the centre of the tube with radius rC. For a non-retained component the

    contribution to the plate height is described by the so-called Taylor-dispersion

    vDrH

    im

    cconv

    ,

    2

    24= (16)

    Figure 5 Profile of the velocity, v, of the mobile phase in an open tube as function of radius, r

    The resulting dispersion due to the parabolic flow profile and the radial diffusion would give

    the following peak for a non-retained component

    30000 25000 20000 15000 10000 5000 0 -5000 -10000

    vmax

    radius, r

    velocity

  • Ernst Kenndler: Gas Chromatography

    10

    Figure 6 Peak profile resulting from parabolic flow and radial diffusion

    (ad Hex,m )

    Both mass exchange terms have their origin in the finite rate of mass transport from the inner

    part of the mobile or stationary phase, respectively, to the interface between these two phases.

    Roughly it can be said that at the front of the peak in the mobile phase a higher concentration

    exists than the equilibrium concentration. At the rear side the opposite situation occurs. From

    these kinetic reasons peak dispersion occurs, which increases with increasing velocity of the

    mobile phase. The effect in the mobile phase is connected to the flow situation. Therefore the

    contribution of the finite mass transfer in the mobile phase is combined with that stemming

    from the flow profile, which enlarges eq. 16. The combination of both effects leads to

    Hk k

    krD

    vconv ex mi i

    i

    c

    m i+ = + ++,

    , ,

    ,,

    ( )( )

    1 6 111 24

    2

    2

    2

    (17)

    (ad Hex,s )

    The term which stems from the finite kinetics of mass transport in the stationary phase is

    given by

    Hkk

    d

    Dvex s

    i

    i

    f

    s i,

    ,

    ,,( )

    = +23 1 2

    2

    (18)

    20 10 0 -10 -20-0,2

    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

    parabolic

    flow profile

    flow profile

    radial diffusionconc

    entra

    tion

    z

  • Ernst Kenndler: Gas Chromatography

    11

    where df is the thickness of the stationary phase layer, and Ds,i is the diffusion coefficient in

    the stationary phase.

    1.2.1 Golay equation

    The Golay equation describes the total plate height, given by the sum of the particular

    increments that contribute to peak dispersion:

    HDv

    k kk

    rD

    vkk

    d

    Dvm i i i

    i

    c

    m i

    i

    i

    f

    s i

    = + + ++ + +2 1 6 11

    1 2423 1

    2

    2

    2

    2

    2,

    , ,

    ,,

    ,

    ,,

    ( )( ) ( )

    (19)

    This equation shows the dependence of the particular increments, and thus the total plate

    height, as a function of the velocity of the mobile phase, apparently one of the most important

    experimental variables to influence peak dispersion. Eq. 19 can be rewritten in a simplified

    form as

    H Bv

    C C v Bv

    Cvm s= + + = +( ). (20)

    0 20 40 60 80 1000,0

    0,2

    0,4

    0,6

    0,8

    1,0

    C.v

    B/v

    H

    v

    Figure 7 Plot of the plate height as a function of the mobile phase velocity (Golay equation).

    The depiction of the plate height in relation to the mobile phase velocity is given in Figure 7.

    It results in the summation of the hyperbolic B-term of eq. 15, that depends on 1/v, with the

    C-term, which increases linearly with v. From Figure 7 it can be seen that a singular velocity

    exists where the plate height exhibits a minimum value. Here peak dispersion is smallest. If

    minimum dispersion (highest efficiency) is necessary for sufficient separation, this particular

    velocity has to be selected. On the left-hand side of the H vs. v curve the plate height steeply

  • Ernst Kenndler: Gas Chromatography

    12

    increases due to pronounced diffusion and the time of analysis increases as well. Insofar there

    is no cause to select such velocities in practice. On the right hand side of the minimum, in

    contrast, the plate height increases considerably slowly, and approaches the C-term

    asymptotically at high velocity. In this range the working conditions are selected favourably

    when the separation is large enough due to sufficient selectivity. In this case the time of

    analysis, which is often an important analysis parameter, will be reduced with not too high

    loss in efficiency.

    1.2.1.1 Gas as compressible fluidum

    It was pointed out that in GC there is an average linear velocity, , which differs from the

    local velocity, v, at a certain position at the longitudinal coordinate of the column due to the

    compressibility of the gaseous mobile phase. The particular velocity depends on the pressure

    drop across the column.

    v

    p p0

    Figure 8 Schematic drawing of the local flow velocity, v, (indicated by arrows) of a

    compressible fluidum across the capillary. p pressure, p0 pressure at the column outlet

    The deviation of the volume flow velocity can be derived by the correction factor according to

    Martin and Synge

    (21)

    where p and p0 are the pressures at the top and the end of the capillary.

    Consequently the minimum plate height is observable only in a small part of the

    separation capillary, where the particular mobile phase velocity is optimal. All other velocities

    ( )( ) 1

    123

    30

    20

    =

    pp

    ppj

  • Ernst Kenndler: Gas Chromatography

    13

    deviate in principle from this value for minimum plate height. However, under usual

    conditions in capillary GC (for columns not too narrow and long) p does not exceed p0 by

    more than 1.5 fold. In this case the difference between the average linear velocity (which is

    measured by = L/tR0) and the actual velocity at any section inside the capillary is

    negligible, and the plate height does not change significantly across the capillary. For this

    reason the variation of the velocity due to the compressibility of the gaseous mobile phase

    normally is not a matter for consideration.

    1.2.2 Plate height vs. retention factor.

    Another important fact can be seen from the Golay equation as well: the plate height is

    dependent on the capacity factor. It follows that for given instrumental conditions H may

    change drastically for different components of a sample mixture, given that these components

    have considerably different k values. For a capillary column, e.g., with a very thin film of

    stationary phase, for which the third term in eq. 19 can be neglected, the dependence of the

    plate height from the k` values of the sample components can be expressed by Figure 9.

    Figure 9 Dependence of the second term of the Golay equation (eq. 19) on the capacity

    factor.

    This term reaches values between 1 (for k` of zero; here peak broadening is given by Taylor

    dispersion) and approximately 11 at large values for k`. This means that for otherwise equal

    experimental conditions the plate height of the peaks within one chromatogram can vary by as

    much as one order of magnitude.

    If the stationary phase of the capillary column has not such a thin film, the Cs-term

    may play a role as well. For such cases the dependence of this term on the k` of the analytes is

    shown in Figure 10.

    0 2 4 6 8 100

    2

    4

    6

    8

    10

    12

    (1+6

    k,+1

    1k,2)/(

    1+k,

    )2

    k,

  • Ernst Kenndler: Gas Chromatography

    14

    0 2 4 6 8 100,00

    0,05

    0,10

    0,15

    0,20

    0,25

    0,30

    k`/(1

    +k`)2

    k`

    Figure 10 Cs-term as function of the retention factor (eqs. 18 and 19)

    A closer analysis of the dependence of the plate height on the capacity factor of the solutes

    enables conclusions on the properties of the separation column concerning the significance of

    the particular terms. According to the weight of the two terms different relations of H as

    function of k` are found for the individual columns. The theory of chromatography in

    capillary column delivers the instrument to evaluate the occurring effects.

    Note that it is the plate number, N, rather than the plate height, which is decisive for

    the separation of two analytes:

    N LH

    = (22) The plate number of the particular components of a sample can simply be calculated from

    their retention time, and the corresponding standard deviation (or the half peak width w), all parameters taken in the time domain, and in the same units by

    Nt t

    wiR i

    t i

    R i

    t i

    =

    =

    ,

    ,

    ,

    ,

    * .2 2

    554 (23)

  • Ernst Kenndler: Gas Chromatography

    15

    2 PRACTISE OF GAS CHROMATOGRAPHY

    A gas chromatograph consists of several parts, which are described in the following in more

    detail (cf. e.g. refs. 1-13

    ) and can indeed often be handled as modules in instrumental practice.

    They are schematically shown in Figure 11. Roughly, a chromatograph is composed from the

    carrier gas supply, the sample inlet, the column, positioned in a column oven, the detector(s)

    and a device for data collection, acquisition and processing.

    Figure 11 Scheme of a gas chromatograph

    2.1 Carrier gas

    As mobile phase an inert gas is used, which is delivered by a gas generator, or a gas cylinder.

    The most common carrier gases are H2, N2, He. They must be of very high purity, because

    traces of water or oxygen may decompose the stationary phase, which leads to column

    bleeding and finally destruction of the column. Therefore special devices for gas purification

    are installed often prior to the sample inlet.

    The choice of the carrier gas depends on several demands, e.g. the appropriate

    operation of the detector (for the combination of GC with MS e.g., He is needed), on safety

    reasons (H2 is explosive), or on the price (N2 is the cheapest gas), but also on demands on

    separation efficiency and speed. Due to its lowest viscosity of all gases, H2 e.g. allows to

    operate the column with the highest mobile phase velocity - and therefore lowest analysis

    time - at comparable efficiency (see below).

    Summary of the demands on the carrier gas:

    Chemically inert High purity (water, oxygen)

    Carrier Gas Supply

    Sample Inlet

    Column Oven

    Detector Data Collect. Acquis. Process.

  • Ernst Kenndler: Gas Chromatography

    16

    Detector compatibility Economic / Safety reasons Efficiency / Speed

    Concerning efficiency and speed depending on the kind of carrier gas we ask for the condition

    of plate height minimum. This is

    (24)

    With the simplified Golay equation (eq. 20) we obtain

    (25)

    It can be seen that this plate height is independent on the kind of mobile phase (see Figure

    12). It is depending on the radius of the capillary only. This plate height is reached at the

    mobile phase velocity

    (26)

    The velocity where minimum plate height is reached depends on the gas used as mobile

    phase, because the diffusion coefficient is related to the gas viscosity. Therefore hydrogen as

    mobile phase reaches minimum plate height at higher mobile phase velocity than e.g.

    nitrogen. Faster analyses can be achieved therefore.

    Figure 12 Peak height vs. mobile phase velocity for hydrogen and nitrogen as carrier gases

    0 5 10 15 200

    2

    4

    6

    8

    10

    H2

    N2

    plat

    e he

    ight

    , H

    mobile phase velocity, v

    0=dudH

    cmi rDCBu 7min =

    32min crBCH =

  • Ernst Kenndler: Gas Chromatography

    17

    2.2 Sample inlet

    In GC the sample is normally brought into the separation system in liquid solution (sampling

    techniques for vapours, e.g. head space or adsorption / thermodesorption injection, or for solid

    samples, e.g. pyrolysis injection, are not discussed here). The sample is dissolved in an

    organic solvent, which is normally injected into the carrier gas flow by the aid of a syringe or

    a valve. Indeed the quantitative and non-discriminating introduction of the sample into the

    column is the most critical part of all experimental steps in capillary gas chromatography.

    Although GC is a well-developed and established method, sample introduction in capillary

    GC is still a non-trivial task due to practical limitation. This results in a number of different

    injector types, which should be selected in practice according to the nature of the sample and

    the demands in accuracy and reproducibility of the analysis.

    In contrast to capillary GC, sample introduction in packed column GC is not a

    problem. This is due to the fact that in packed bed GC the column volume is large, and the

    phase ratio is considerably large, too. Here the sample, dissolved in a volatile organic solvent,

    is simply injected into a heated and thermostatted injector cell, where it is evaporated, and the

    vapour is transported by the carrier gas into the column. Note that 1 l of liquid sample

    delivers several hundred L vapour after evaporation. It is clear that such a large volume

    would overflow the entire column in case of a capillary column. As an example: a capillary

    with 0.2 mm I.D. and 25 m length has a total volume of less than 800 L. It is obvious that it

    is not possible to introduce the entire evaporated sample directly into the column in capillary

    GC. Therefore mainly two possibilities are proposed to overcome this problem in practise:

    (i) In case of the evaporating injectors the sample is inserted by a syringe into the

    heated injector and evaporated. Either only a part of the evaporated sample is allowed

    to enter the capillary this is realised with the split injector; or the main part of the

    solvent (and a small part of the sample as well) is separated in the injector from the

    sample components. In this splitless mode the sample components normally are

    recondensed at the top of the column either by cold trapping or by solvent

    trapping.

    (ii) the total liquid volume is brought into the cold injector by the aid of a syringe, and

    solely the solvent is evaporated carefully first and usually recondensed either at the top

    of the column, or in the retention gap. Due to careful selection of the temperature

    conditions the sample remains at the top of the separation system. Then the sample is

    evaporated, too, and introduced into the separation capillary.

  • Ernst Kenndler: Gas Chromatography

    18

    2.2.1 Split and splitless injector

    In the split mode (see Figure 13A) the sample is rapidly injected and evaporated in the liner of

    the heated injector. The gas flow (vaporised sample mixed homogeneously with the carrier

    gas) is divided at the top of the column by the aid of a needle valve (which enables to adjust

    different split ratios). The main part of the gas mixture is flushed out, and only a small part is

    allowed to stream into the separation capillary. The injector has the advantage that the

    injected zone is narrow, and the small sample aliquot entering the capillary avoids

    overloading of the column.

    Although very flexible in practice, this injector has a number of disadvantages. In

    many cases mass discrimination of sample components is observed, especially when the range

    of their volatility differs much. Therefore systematic errors for quantitative analysis may

    occur. Another disadvantage, especially in trace analysis, is the fact that only a part of the

    analytes is transferred into the capillary, and reaches the detector. The main part of the sample

    is deleted via the split and therefore lost for detection.

    To overcome such problems the injector can be operated in the splitless mode. Here

    the capillary is first run in the split mode. Directly prior to injection the split is closed. The

    sample is then slowly injected into the heated injector, and sample and solvent are evaporated.

    It is most important that in this mode the column is kept at relatively low temperature, lower

    than the boiling point of the solvent. Therefore the volatile solvent condenses at the top of the

    column, and forms a kind of a stationary phase here. Volatile sample components, which

    evaporate in the injector, too, are dissolved again in the liquid formed, and are therefore

    refocused (solvent trapping). Less volatile sample components, which were also evaporated

    in the hot injector, are recondensed on the top of the colder column, and focused, too (cold

    trapping).

    After these two processes have taken place, the split is opened (after about 30 90 s)

    and the rest of the solvent is flushed via the split valve. The solvent that initially forms a

    liquid sheath at the top of the column evaporates gradually, with progressive evaporation

    from the injector to the detector side. This effect supports the refocusing of the sample

    components. Finally the sample is evaporated by the application of a temperature program,

    which is obligatory for this injection technique. The entire procedure avoids the large tailing

    of the solvent peak observed otherwise, and allows the transfer of the main part of the sample

    components into the column and, finally, into the detector. It is therefore a favourable

    technique for the insertion of diluted samples in trace analysis.

  • Ernst Kenndler: Gas Chromatography

    19

    A) B)

    Figure 13 (A) Schematic drawing of a split-splitless injector (from ref. 10

    with permission).

    (B) Schematic drawing of an on-column injector. 1 Carrier gas inlet, 2 Sealing; 3 Capillary

    column, 4 Cooling gas (from ref. 14

    with permission).

    The recondensation of the solvent at the top of the capillary can be critical by damaging the

    stationary phase (because it might be partially dissolved by the condensed liquid), and

    therefore only columns with chemically bonded phases should be used. Another limitation is

    the necessary wettability of the stationary phase by the condensed solvent, otherwise droplets

    are formed rather than a liquid layer. These problems may be overcome by the use of an

    empty (widebore) capillary without stationary phase, mounted between injector and

    separation column. In this capillary indeed both refocusing processes solvent trapping and

    cold trapping as described above can be established. This construction is named retention

    gap, and is used in the splitless mode and, more common, with the on-column injection

    technique.

  • Ernst Kenndler: Gas Chromatography

    20

    It should be mentioned that the discriminating evaporation of volatile sample

    components (according to their boiling points) when injected into the hot injector can be

    diminished by a modification called programmed temperature vaporiser (PTV injector). The

    PTV device can be used for the split and the splitless mode as well. Here injection is carried

    out into the cooled injector. When operated initially at a temperature slightly above the

    boiling point of the solvent (but below those of the most volatile sample components), the

    main part of the solvent can be separated via the open split (solvent purge). After flushing

    the solvent the PTV injector is heated up rapidly, and the sample is transferred to the top of

    the column as a more or less narrow band.

    Summary of the advantages and disadvantages of split and splitless injectors:

    SPLIT INJECTOR Advantage Limitations Injected zone narrow Small sample aliquot avoids overloading

    Mass discrimination of sample components (different range of volatility) Systematic errors for quantitative analysis In trace analysis: only part of analytes to detector

    SLITLESS INJECTOR Advantage Limitations Avoids large tailing of solvent peak Allows transfer of main part of sample

    components into detector Trace analysis: favourable technique for insertion of diluted samples

    Recondensation of solvent at top of capillary: possible damaging stationary phase

    Only columns with chemically bonded phases Necessary wettability of stationary phase for

    condensed solvent (droplets)

    2.2.2 On-column injection

    With the on-column technique the sample solution is directly inserted into the column with

    the aid of a syringe with a long, narrow needle, whereby the injector (Figure 13B) is

    maintained at low temperature. Due to the restricted mechanical stability of the thin syringe

    needle a normal septum cannot be used, and is replaced by special sealing. In most cases a

    piece of a wide bore capillary (retention gap) is connected to the thinner separation capillary

    to avoid column flooding by the large volume of sample vapour.

    Retention gap: empty capillary - fused silica, widebore (0.5 mm i.d.), 20 200 cm L - without stationary phase mounted between injector and separation column

  • Ernst Kenndler: Gas Chromatography

    21

    The capillary for the retention gap is mounted in the column oven, whose temperature must be

    adjusted to the boiling point of the solvent. If the temperature is below the boiling point,

    solvent trapping takes place. If it is selected slightly above the boiling point, cold trapping of

    the sample components occurs. In both cases the chromatogram must be developed with an

    adequate temperature program of the column, which is an essential step when using this

    injector type.

    Advantage of on-column injector:

    avoids mass discrimination effects trace analysis: enables quantitative insertion of sample into column (and detector) labile components not stressed thermally

    2.3 Columns

    In gas chromatography packed bed columns and capillary column are used. Packed columns

    are tubes made of glass or metal with 2-4mm I.D. and 1-6 m length. They are filled with

    porous particles, which act as support of the stationary liquid phase, which is coated on the

    porous material.

    Capillary columns are open tubes with 0.1 to 0.5 mm I.D. and 5 to 100 m lengths.

    Most common dimension, however, are 0.3 mm I.D. and 25 m length. Originally the

    capillaries were made from metal or glass; in the last decade fused silica replaced all other

    materials. Fused silica has the advantage of a very inactive inner surface, which avoids

    adsorptive interactions between analytes (especially when they are polar) and adsorption

    centres, leading otherwise to tailing peaks or even loss of material due to irreversible

    adsorption (see note below on Grob`s test mixture). It has the further advantage of extremely

    high mechanical stability that reduces breakage of the columns.

    The stationary phase is coated as a thin layer (with 0.1 to 5 m film thickness) onto

    the inner wall of the open tube. Normally this phase is a liquid. Due to modern column

    technology, which enables cross-linking of the polymer molecules of the liquid, and even

    attachment of the phase at the silica surface due to chemical bonding, the initially liquid phase

    might behave as large, single, polymeric molecule. Interestingly these cross-linked phases

    thermodynamically behave very similar to the initial liquid.

  • Ernst Kenndler: Gas Chromatography

    22

    PACKED COLUMN CAPILLARY COLUMN

    Stationary phase

    Packing

    Figure 14 Schematic drawing of packed and capillary columns

    It should be mentioned that a mixture of solutes, which allow conclusions about specific

    adsorptive sites, could test the adsorptivity of columns. The most common is the so-called

    Grob test mixture, which consists of octan-2-on, octanol-1, 2,6-dimethyl phenol, 2,4-dimethyl

    anilin, naphthalin, tridecan and tetradecan.

    2.3.1 Stationary phases

    Stationary phases must cover a wide range of polarity as indicated in the following Table.

    Apolar phase Polar phase vapour pressure intermolecular forces

    dispersion polarisation dipole-dipole hydrogen bonding

    Besides being the source for separation selectivity in GC, the demand on stationary phases is

    thermal stability. It is clear that polymeric compounds best fulfil the latter restrictions.

    Especially siloxane polymers have a high thermal stability, caused by the Si-O backbone of

    the silicon chain. The type of the substituents attached at this chain implements selectivity.

    CH3 groups as substituents give the lowest polarity, resulting in a polymer that is nearly as

  • Ernst Kenndler: Gas Chromatography

    23

    SiCH3

    CH3

    OSiCH3

    CH3

    O SiO

    R2R1

    O

    R1Si

    O

    R2

    A)

    B)

    Figure 15 Structural formulae of stationary phases:

    A) Polysiloxane-based phases.

    Polydimethyl siloxane: R1, R2: CH3

    Methylphenyl polysiloxanes: different ratio R2/R1;

    R1, R2 = CH3,

    Or R1, R2 = CH3

    Methyl cyanopropylphenyl polysiloxanes:

    R1, R2 = CH3

    Or R1, R2=

    B) Polyethyleneglycol

    High Temperature Phases

    a) Carboran modified polysiloxane

    b) Silarylen polymer

    OO

    OO n

    NCor

  • Ernst Kenndler: Gas Chromatography

    24

    apolar as a hydrocarbon. Gradual substitution of the CH3 groups by polarizable phenyl

    moieties increasingly changes polarity, and enables therefore separation of moderately polar

    analytes. Introduction of cyanopropyl substituents partially replacing phenyl groups in the

    silicon chain leads to a phase with highest polarity amongst the siloxanes. Polyethylenglycol,

    finally, allows interactions based on hydrogen bonds, and is therefore best suited for the

    separation of analytes that are H-donors, e.g. alcohols. These phases are depicted in Figure 15.

    Two examples for high temperature phases are also given. The one consists of

    dimethyl polysiloxane polymers, which determine the selectivity. These polymeric chains are

    connected with carborane (carbon-boron compounds) anchors, which are responsible for the

    usability at temperatures as high as about 400C. The second phase in the example given

    introduces temperature stability by implementation of phenyl groups directly into the polymer

    chain.

    2.3.2 Rohrschneider / McReynolds index

    A concept to characterise the polarity of stationary phases was introduced by Rohrschneider

    and McReynolds. It is based on the finding that the interaction of polar functional groups of a

    solute is reflected by the difference of its retention indices on a polar and a nonpolar

    stationary phase, respectively. Taken a very apolar phase as a reference (for this purpose

    squalane is used, a branched C30 alkane, hexamethyl tetracosane), the retention index

    difference on a certain phase relative to squalane is a measure for the polarity of this phase.

    If e.g. the solute is an alcohol, it can be formally divided into the alkyl rest (which interacts

    only by weak dispersion forces with all phases), and the OH group that is able to donate

    hydrogen bonds. Interaction with stationary phase molecules, able also for hydrogen bonding,

    will lead to stronger retention on the polar phase than on an apolar phase, and therefore a

    large increase in retention index , IR, will result. According to the concept of Rohrschneider / McReynolds, the increase of the retention index for the alcohol on the polar stationary phase

    is given by a certain constant, characteristic for the given stationary phase. Measured for

    butanol-1 as reference solute it is

    (27)

    To characterise the stationary phase polarity in a more general manner, the concept uses 5

    special solutes, which are considered to represent typical chemical interactions. For each of

    them, the constants x, y, z, u and s are defined accordingly; they are shown in Table 2.

    squalaneolbu

    polarolbu IIy 1tan1tan

    , =

  • Ernst Kenndler: Gas Chromatography

    25

    The polarity of the stationary phase is expressed by the sum of the constants, the

    Rohrschneider / McReynolds index, (28)

    Examples for stationary phases most common in practice are given in Table 3. If two phases

    do not differ in their indices by more than about 200, their polarity can be considered as

    nearly equal. In general it is not meaningful to use two such phases. This does not mean,

    however, that probably their selectivities concerning one special type of interaction are

    negligible.

    Table 2 Rohrschneider / McReynolds index, 5 special solutes represent typical chemical interactions

    Type of interactions Reference

    solute Rohrschneider/McReynolds constant Dipole -complex H-bond

    Typical for

    benzene x - donor - olefines, aromatic compounds

    butanol-1 y 9 - donor alcohols, phenols, acids, amides

    2-pentanon z 9 acceptor - aldehydes, ketones, esters, ethers

    1-nitropropane u 9 acceptor - nitro-, nitrilo compounds pyridine s 9 donor - Amines, aromatics

    Table 3 Rohrschneider / McReynolds constants and indices for characterisation of stationary

    phase polarity

    Stationary phase Composition x y z u s Index Squalane 0 0 0 0 0 0 Dimethyl silicon 100 % methyl 17 57 45 67 43 229 Phenyl methyl silicon 5 % phenyl 32 72 65 98 67 334 50 % phenyl 119 158 162 243 202 884 75 % phenyl 178 204 208 305 208 1103 Cyanopropylphenyl (cpph) dimethy silicon

    6 % cpph 50 115 107 164 103 539

    50 % cpph 227 373 336 489 398 1823 100 % cpph 523 757 659 942 801 3682 PEG 20 M 322 536 368 572 510 2308

    suzyx ++++=

    suzyx ++++=

  • Ernst Kenndler: Gas Chromatography

    26

    2.4 Column oven

    The column oven has the function to adjust the column temperature to an accurate and

    reproducible value. It is prerequisite in practice to establish these temperature conditions for

    the column over the entire length, not only where the temperature sensor is located in the

    oven. The column oven should not only meet these demands in the isocratic manner; it must

    also enable the application of appropriate gradients, either a single linear or ballistic, or

    multiple gradients by temperature programming. It should be noted that after such

    programming a very important but often underestimated aspect is the establishment of the

    exact initial column temperature after cooling. The inappropriate re-establishment often is the

    source of systematic measuring errors.

    2.5 Special Detectors

    For GC a number of detectors have been developed. For trace analysis, however, not all of

    them have increased importance. Most important detectors in this area are (beside the mass

    spectrometer)

    Flame ionisation detector (FID) Electron capture detector (ECD) Alkali flame ionisation detector (NPD) These three detectors will be discussed in more detail in the following.

    2.5.1 Flame ionisation detector

    The FID is a mass flow sensitive detector. It is based on the measurement of the electric

    charges, which are produced in a small hydrogen flame. In the absence of organic molecules

    in the carrier gas, this flame is very poor in charged particles, because the combustion of

    hydrogen with oxygen delivers only a very small number of ions or electrons. Indeed the

    residual current is in the range of only 10-12 A (under normal working conditions, i.e. when a

    voltage of about 200-300 V is applied between the flame and the collector electrode). This

    extremely small residual current is amplified and represents the background signal (the

    blank).

    Organic molecules, which possess CH-groups, form CHn-radicals (n=0-3) at the

    periphery of the hydrogen flame, where also excited O2* and OH* molecules are formed.

  • Ernst Kenndler: Gas Chromatography

    27

    Reaction of the radicals with the excited molecules leads to the formation of positively

    charged ions and electrons, e.g. according to

    (29)

    Consequently the positively charged molecular ions (e.g. CHO+) and the electrons formed

    increase the current when organic molecules are entering the detector flame, and the analytes

    are detected in this way (note that there is not full combustion to CO2).

    Figure 16 Schematic drawing of a flame ionisation detector (from ref. 15

    with permission).

    The more CH-groups a molecule contains, the larger is the detector response.

    Heteroatoms in the molecule lead to a smaller response. Molecules without CH-groups do not

    deliver a signal, except due to overloading effects of the detector. The FID is therefore not

    universal, as e.g. water, CO2, NOx , CS2, CCl4, etc. are not or only poorly detected. The same

    is the case for the lower alcohols or substances with many heteroatoms.

    Beside the analyte properties, the detector response is also dependent on its geometry,

    and on the flow rate of the burning gases. Favourable flow rates for H2 are in the range of 25-

    30 mL/min, those for the air are by a factor of 10 higher. Especially the H2 flow must be

    selected carefully, because its optimal range is narrow (see Figure 16). The use of so-called

    make-up gas is normally advisable in capillary GC for the improvement of the detector

    response. It is also favourable to avoid loss in separation efficiency due to extra-column

    + +=+ eCHOOHC

  • Ernst Kenndler: Gas Chromatography

    28

    effects caused by the dead volume of the detector. Because the FID is mass flow dependent,

    the make-up gas does not negatively influence the detector sensitivity.

    Figure 17 Dependence of the sensitivity of the FID on the flow rates of H2 and N2,

    respectively. N2 is added as make-up gas (from ref. 14

    with permission).

    Performance data FID

    sensitivity ~ 0.015 As/g

    limit of detection ~ 10-11 to 10-12 g carbon/s

    linearity 7 orders of magnitude

    Time constant ~2 ms

    2.5.2 Electron capture detector

    In principle the ECD is an ionisation chamber. It has a very high specifity and sensitivity for

    compounds that have atoms in their molecules with high electron affinity. Especially halogens

    exhibit this property, but also for oxygen containing groups or nitro-groups this detector

    responds very well (see the following Table). It will be, however, discussed below, that it is

    not sufficient to take only the electron affinity into account to interpret the detector response.

    X ElectronAffinity [eV] X Electron Affinity [eV] H 0.72 J 3.12 C 1.2 Br 3.52 O 2.34 Cl 3.78 CN 3 F 4.1 NO2 3.9

  • Ernst Kenndler: Gas Chromatography

    29

    energyAXeAX +=+

    + +

    )(thermecgcg

    +=+ XAeAX

    Roughly, the detector principle is based on the property of analytes, AX, containing

    such atoms or groups, X, to attract electrons according to

    (30)

    Electrons are generated by a radioactive -source like 63Ni. However, these primary electrons exhibit a too high energy (and therefore too high speed) to be captured from the

    affine groups in the analytes. They interact first with carrier gas, which is present in very large

    excess compared to the trace analytes; thus the chance for interaction with the -particles is much greater for the former than for the latter molecules. As carrier gases (cg) those with

    large mass are used, e.g. N2 or Ar (mixed with 5% CH4). According to the reaction scheme

    given in eq. 31 the -particles generate thermal, secondary electrons by interaction with the carrier gas molecules (these electrons have much lower, namely only thermal energy).

    (31)

    The current produced from these low energy electrons in the detector cell is measured and

    amplified. As long as there is no analyte with electron affine groups in the carrier gas, the

    background current (the blank) with about 10-8 A is delivered. When analytes with e.g.

    halogen substitution are eluted from the column and enter the detector, electrons from the

    basic current are captured according to eq. 30, and the current is decreased, detecting the

    analytes in this way. In principle the number of charged particles is not changed in this

    reaction, because instead of an electron a negatively charged sample ion is generated.

    However, the velocity of these molecular ions (10 cm/s) is many orders of magnitude smaller

    than that of the electrons (105 cm/s). For this reason the ions do not reach the collector

    electrode; they recombine faster with positively charged carrier gas molecules under

    formation of the neutral molecules according to

    (32)

    However, under such conditions no signal would be obtained. For favourable measuring

    conditions a pulsed voltage is applied, as indicated in Figure 18.

    For a closer insight into the signal generation of the ECD we have to differentiate two

    processes: the one is the simple addition of an electron, as expressed by eq. 30. The more

    common process, on the other hand, includes a dissociative reaction of the product formed

    after electron addition according to

    (33)

    cgAXcgAX +=+ +

  • Ernst Kenndler: Gas Chromatography

    30

    Figure 18 Schematic drawing of the ECD with pulsed polarisation voltage. b pulse width, e.g.

    3s; D pulse distance, e.g. 10-200 s; h pulse height, e.g. 50 V (from ref. 15

    with permission).

    Therefore not only the electron affinity, but also the binding energy between the carbon atom

    and the electron affine atom or group decides over the extent of reaction. This fact explains

    the result of the relative sensitivity, rel. Si, of different compounds as given in the following

    Table.

    Detector Response and Sensitivity

    Substance Rel. Si Binding energy [kJ/mol] Electron Affinity [eV]

    Fluorobenzene 1 C-F 538 F 4.1 Chlorobenzene 100 C-Cl 391 Cl 3.78 Bromobenzene 600 C-Br 281 Br 3.52 Iodobenzene 37 000 C-I 210 I 3.12

    Performance data ECD

    several 10 fg of analytes (e.g. lindan) per injection

    linearity 3 to 4 orders of magnitude

  • Ernst Kenndler: Gas Chromatography

    31

    2.5.3 Alkali flame ionisation detector

    The alkali flame ionisation detector, also named thermionic detector, belongs to the group of

    ionisation detectors in which thermal energy is used as source for ionisation. From the

    construction point of view it is a modification of the FID, with a pearl of alkali salt (Rb, Cs)

    located between the flame and the collector electrode. Heating of the alkali salt pearl leads to

    the emission of alkali atoms. The detector is responding especially to analytes containing

    nitrogen or phosphorus atoms. According to the special conditions the detector can be run

    either in the P- or in the N- and P- specific mode. The detailed mechanism of detection is still

    a matter of question. Here we follow the discussion given by Kolb et al. (cf. e.g. ref. 15).

    2.5.3.1 P mode

    At elevated temperature alkali ions A+ in the (silicate) pearl are neutralised by electrons

    delivered from the electrical source applied (A+ + e- = A). The neutral alkali atoms, A,

    evaporate. These atoms are thermally excited giving A*. In the hydrogen flame H and OH

    radicals are formed. However, these two radicals can only recombine to H2O when a partner

    is present that is able to overtake their energy of formation. This may occur with a triple

    collision, with excited A* as partner. As a result A* is ionised according to the reaction scheme

    (34)

    The alkali cation A+ formed is collected at the negatively charged pearl, whereas the electron

    is migrating to the collector electrode. In this way the background current of the detector is

    formed in the P mode.

    Phosphorus containing molecules, when present, are transformed to phosphorus oxide

    radicals, R, in the flame of the detector. These radicals react in a double collision (which is

    more probable than a triple collision with H and OH) with excited A*

    (35)

    e.g.

    (36)

    The P-containing anion R- reacts further with an OH radical to

    (37)

    e.g.

    + ++=++ eAOHAOHH 2

    + +=+ ARAR

    [ ] + +===+== AOPOAOPO

    +=+ eROHOHR

  • Ernst Kenndler: Gas Chromatography

    32

    (38)

    This reaction delivers the electron for the signal current specific for P-containing analytes.

    Figure 19 Schematic presentation of the thermionic detector in the P mode (from ref. 15

    with

    permission).

    Note that the flame jet is grounded, and has therefore a positive potential. For this reason the

    electrons formed by reaction of CH- containing molecules due to the same processes as with

    the FID are not interfering the detector signal, because they are conducted to ground, and do

    not reach the collector anode.

    2.5.3.2 NP mode

    The N analogous oxides formed from N-containing molecules would decompose rapidly in

    the flame of the detector, and would therefore not react with alkali. For this reason the

    detector is run in the NP mode under reducing conditions, leading to CN radicals instead.

    These conditions are established by decreasing the flow rates of hydrogen to about 1-3

    mL/min, and that of air to less than 100 mL/min. The flame goes out then, but the remaining

    free hydrogen is ignited at the electrically heated pearl. It forms a kind of plasma around the

    pearl, where a CN radical formed adds an electron taken from excited A*, forming a cyanide

    ion and alkali cation (see P-mode, eq. 35)

    (39)

    + +=+ ACNACN

    [ ] +=+== eHPOOHOPO 3

  • Ernst Kenndler: Gas Chromatography

    33

    The CN anion formed finally reacts either with H or OH radicals to HCN or HCNO,

    respectively.

    Figure 20 Schematic representation of the thermionic detector in the NP mode (from ref. 15

    with permission).

    This reaction delivers the electron for the signal current analogous to eq. 37. As the

    formation of CN radicals is essential, such a structure must be present initially in the analytes

    to enable detection. Therefore e.g. organic nitro compounds are detectable, but not nitrogen

    oxides, or nitrate esters.

    In the following table some data for the performance of the detector in both modes are

    given.

    Typical performance data for the NPD in P- and NP- mode, respectively.

    P mode NP mode Parameter

    P N

    Sensitivity 1 C/g P 5 C/g P 0.5 C/g N

    Limit of detection 5.10-14 g P/s 10-14 g P/s 10-13 g N/s

    Selectivity (vs. carbon) 106 105 104

    Linearity 105 105 105

  • Ernst Kenndler: Gas Chromatography

    34

    3 FURTHER READINGS

    (1) Scott, R. P. W. Introduction to Analytical Gas Chromatography; 2nd ed.;

    Marcel Dekker, 1998.

    (2) Jennings, W. G.; Mittlefehldt, E.; Stremple, P. Analytical Gas

    Chromatography; 2nd ed.; Academic Press, 1997.

    (3) McNair, H. M.; Miller, J. M. Basic Gas Chromatography; Wiley, 1997.

    (4) Grant, D. W. Capillary Gas Chromatography; Wiley, 1996.

    (5) Fowlis, I. Gas Chromatography; 2nd ed.; Wiley, 1995.

    (6) Scott, R. P. W. Techniques and Practices of Chromatography; 2nd ed.; Marcel

    Dekker, 1995.

    (7) Grob, R. L. Modern Practice of Gas Chromatography; 3rd ed.; Wiley, 1995.

    (8) Baugh, P. E. Gas Chromatography: A Practical Approach; Oxford, 1994.

    (9) Hinshaw, J. V.; Ettre, L. S. Introduction to Open Tubular Column Gas

    Chromatography; Advanstar, 1994.

    (10) Grob, K. Split and Splitless Injection in Capillary Gas Chromatography; 3rd

    ed.; Hthig, 1993.

    (11) Hill, H. H.; McMinn, D. G. Detectors for Capillary Chromatography; Wiley,

    1992.

    (12) Grob, K. On-Column Injection in Capillary Gas Chromatography; 2nd ed.;

    Hthig, 1991.

    (13) Poole, C. F.; Poole, S. K. Chromatography Today; Elsevier, 1991.

    (14) Baars, B.; Schaller, H. Fehlersuche in der Gaschromatographie; VCH, 1994.

    (15) Kolb, B. Gaschromatographie in Bildern; Wiley-VCH, New York, 1999.

    (16) Kenndler, E.; Huber, J. F. K. In Analytiker Taschenbuch; Springer, 1989.


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