Handbook of Food Analysis Instruments

7
CONTENTS
Gas Chromatography
in Food Analysis
Jana Hajslova and Tomas Cajka
7.1 Introduction.......................................................................................................................... 119
7.2 Sample Introduction............................................................................................................. 120
7.2.1 Split=Splitless Injection............................................................................................ 120
7.2.2 Cold On-Column Injection ...................................................................................... 122
7.2.3 Programmable Temperature Vaporization Injection................................................ 123
7.2.4 Direct Sample Introduction=Difficult Matrix Introduction...................................... 125
7.2.5 Solid-Phase Microextraction.................................................................................... 125
7.3 Sample Separation................................................................................................................ 126
7.3.1 Capillary Columns for GC....................................................................................... 126
7.3.2 Fast Gas Chromatography ....................................................................................... 126
7.3.3 Comprehensive Two-Dimensional Gas Chromatography....................................... 130
7.3.3.1 GC GC Setup ........................................................................................ 131
7.3.3.2 Optimization of Operation Conditions and Instrumental
Requirements in GC GC ....................................................................... 131
7.3.3.3 Advantages of GC GC .......................................................................... 133
7.4 Sample Detection ................................................................................................................. 134
7.4.1 Flame Ionization Detector ....................................................................................... 135
7.4.2 Thermal Conductivity Detector ............................................................................... 136
7.4.3 Electron Capture Detector ....................................................................................... 136
7.4.4 Nitrogen–Phosphorus Detector................................................................................ 136
7.4.5 Flame Photometric Detector and Pulsed Flame Photometric Detector ................... 136
7.4.6 Photo-Ionization Detector........................................................................................ 136
7.4.7 Electrolytic Conductivity Detector .......................................................................... 136
7.4.8 Atomic-Emission Detector....................................................................................... 136
7.4.9 Mass Spectrometric Detector................................................................................... 137
7.5 Matrix Effects ...................................................................................................................... 137
7.6 Food Analysis Applications................................................................................................. 140
7.7 Conclusion and Future Trends............................................................................................. 142
Acknowledgments......................................................................................................................... 142
References..................................................................................................................................... 142
7.1 INTRODUCTION
In food analysis, gas chromatography (GC) represents one of the key separation techniques for
many groups of (semi)volatile compounds. The high separation power of GC in a combination with
a wide range of the detectors makes GC an important tool in the determination of various
components that may occur in such complex matrices as food crops and products.
ß 2008 by Taylor & Francis Group, LLC.

Conventional
Advanced
Split
Sample
preparation
Sample
introduction
Classical splitless
Pulsed splitless
Cold on-column
Programmable temperature
vaporiser
Direct sample introduction/
Difficult matrix introduction
Solid-phase microextraction
In practice, a GC-ba sed method consi sts typicall y of the follow ing steps : (1) isol ation of
analyt es from a representat ive sample (extr action); (2) separa tion of co-extract ed mat rix componen ts
(clea nup); (3) ident i fication and quanti fica tion of targe t analyt es (dete rminativ e step) , and if the
need is imp ortan t enough, this is foll owed by (4) con firmation of results by an addit ional analys is
(Figur e 7.1). In any case, the sample prepar atio n pract ice plays a cruci al role for obtaining required
param eters of a parti cular analytical method. Under some cond itions, especi ally when polar analytes
are to be analyze d, derivatiz ation is ca rried out prior to the GC step to avoid hydroge n bondin g,
hence incre asing the a nalyte volatil ity and reducing interacti on wi th acti ve sit es in the syst em.
In Figure 7.2, the interrel ationshi p between solut e amoun ts and inst rumental options (inlet,
colum n, and detector) is illust rated. GC users shoul d examine the relations hip of analyz ed samp les
to the operat ing range of the instrum ent syst em. If the analyt e concent ration lies outside this range, a
diff erent injectio n technique, column dim ension, or detector may be appropr iate.
7.2 SAMPLE INTRODUCTION
The re are a numbe r of options avail able for GC inlet syst ems; the most common (charac terized
below) being spli t=splitless, progra mmed temperat ure vap orizer, and cold on-column (COC)
inje ctor. The choice of an optimum samp le introduct ion strategy depend s mainly on the c oncentration
range of targe t analyt es, thei r p hysico-chem ical proper ties, and the amount and natur e of mat rix
co-ext racts presen t in the samp le.
7.2.1 S PLIT=SPLITLESS INJECTION
1D-GC
2D-GC
Data analysis
Separation Detection
Conventional GC
Fast GC
Very fast GC
Ultra-fast GC
Heart-cut GC
Comprehensive twodimensional
GC
Split=splitl ess inje ction remains the main samp le introduct ion techniq ue in the analysis of
GC-am enable food compo nents mai nly due to its easy operat ions.
Conventional
Mass spec.
Flame ionisation
Thermal conductivity
Electron capture
Nitrogen–phosphorus
(Pulsed) flame photometric
Photo-ionisation
Electrolytic conductivity
Atomic-emission
Quadrupole
Quadrupole ion trap
Magnetic sector
Time-of-flight
Hybrid instruments
FIGURE 7.1 Basic steps typically involved in the determinative step of gas chromatographic analysis of
organic food compounds.
ß 2008 by Taylor & Francis Group, LLC.

Mass and
concentration
Splitless, direct,
on-column Split
Column diameter
and film thickness
Detector minimum detection
limit and dynamic range
Femtograms
parts per trillion
Picograms
parts per billion
Nanograms
parts per million
Micrograms
parts per thousand
1 10 100 1 10 100 1 10 100 1 10 100 1000
10 –15 10 –14
10 –15 10 –14
10 –13 10 –12
10 –13 10 –12
10 –11 10 –10
10
Splitless
–11 10 –10
Solute mass (g)
10 –9 10 –8
Percentage:
Split 100:1
dc df 530 µm 0.1 µm 5.0 µm
320 µm
0.1 µm 5.0 µm
250 µm
0.1 µm 1.0 µm
180 µm
0.1 µm 0.5 µm
100 µm
0.1 µm 0.5 µm
Flame ionization
Nitrogen–phosphorus
Electron–capture
MS (scan)
MS (single-ion monitoring)
Thermal conductivity
10
Solute mass (g)
–15 10 –14 10 –13 10 –12 10 –11 10 –10 10 –9 10 –8 10 –7 10 –6 10 –5 10 –4 10 –3
In a split inje ction mode, typically small volum e of samp le extra ct (0.1 –2 m L) is rapid ly
delivered into a heated glass line r foll owed by its splitting into tw o streams: the large r part is
vented , while the smal ler part is trans ferred onto the column. Conside ring that the most of injected
samp le is lost, this techniqu e is obviou sly not suitable for trace analys is, where very low detection
limits are requi red. Anot her problem associated with split injectio n is a potent ial discrim ination due
to the heati ng of the syri nge during its introduct ion into a hot inje ctor resul ting in a change of
relative abund ances of samp le compo nents when a mixture of analytes large ly diff ering in boiling
points is analyz ed. Another advers e phenomen on relat ed to this technique is n onlinear splitting due
to adsorp tion of samp le compo nents on liner surfa ces or deposi ted mat rix ‘‘dirt. ’’
Nowaday s, hot splitl ess injectio n repres ents the most comm only used injection techniq ue in
trace quanti tative ana lysis since e ntire inje cted samp le is intr oduced onto the GC capillary . The
major limitati on of this inlet is that it suffe rs from the potential therm al degrada tion and adsorption
of susceptible analytes that may result either in matrix-induced response enhancement or its
diminishment. In addition, the volume of injected sample=solvent is typically limited to 1 mL (for
some solvents even less) due to the expansi on volume of solve nt used (T able 7.1), since the total
liner volume is in a range of 150–1000 mL and the safety limit is typically 75% in maximum of the
total liner volume.
10 –9
10 –8
10 –7
10 –7
0.1% 1% 10% 100%
FIGURE 7.2 GC dynamic range nomogram. Concentrations expressed in grams per microliter (g=mL).
(Reproduced from Hinshaw, J.V., LC GC Eur., 20, 138, 2007. With permission.)
ß 2008 by Taylor & Francis Group, LLC.
10 –6
10 –6
10 –5
10 –5
10 –4
10 –4
10 –3
10 –3

TABLE 7.1
Expan sion Vol ume of So lvents Used in GC
Solvent
1 mL at 2508C
and 69 kPa (10 psig)
To overcome, or at least partly compensate for these problems, pulsed splitless injection can be
applied. Increased column head pressure for a short period during the sample injection (usually 1–2min)
leads to a higher carrier gas flow rate through the injector (8–9 versus0.5–1 mL=min during classical
splitless injection), thus faster transport of sample vapors onto the GC column. In this way, the residence
time of analytes and, consequently, their interaction with active sites in the GC inlet is fairly reduced [3].
The detection limits of troublesome compounds obtained with pulsed splitless injection are thus lower
and their further improvement can be obtained by injection of higher sample volumes (for most liners up
to 5 mL) without the risk of backflash (Table 7.1) [4]. It should be noted that for injections > 1–2 mL, a
retention gap prior to the analytical column is generally required to avoid excessive contamination of
separation column and peak distortion (Figure 7.3).
7.2.2 C OLD ON-COLUMN INJECTION
Expansion Volume (mL)
1 mL at 2508C
and 345 kPa (50 psig)
5 mL at 2508C
and 345 kPa (50 psig)
Water 1414 540 2700
Methanol 631 241 1205
Acetonitrile 487 186 929
Acetone 347 133 663
Ethyl acetate 261 100 498
Toluene 241 92 460
Hexane 195 75 373
Isooctane 155 59 295
Source: From Hewlett-Packard FlowCalc 2.0 software. Available at http:==www.chem.agilent.
com=cag=servsup=usersoft=files=GCFC.htm via the Internet. Accessed July 1, 2007.
Note: Calculated using Hewlett-Packard FlowCalc 2.0 software.
In COC injectio n, a sample aliq uot is directly introduced by a speci al syri nge onto the analyt ical
colum n or a reten tion gap at tem peratures lower (608 C –80 8C) than those typicall y used in hot
Pulsed splitless Pulsed splitless
3 µL
4 µL
2 µL
1 µL
Splitless 1 µL
(A) (B)
FIGURE 7.3 Peak shapes obtained by pulsed splitless injections of different volumes of standard solution
onto the GC column (A) without a retention gap; (B) with an installed retention gap. (Reproduced from Godula,
M., Hajslova, J., and Alterova, K., J. High Resolut. Chromatogr., 22, 395, 1999. With permission.)
ß 2008 by Taylor & Francis Group, LLC.
5 µL
3 µL
2 µL
1 µL

split=splitless mode (2008C–3008C). COC is therefore expected to cause less thermal stress
on analytes during the injection process. This low-temperature injection eliminates both syringe
needle and inlet discrimination and is suitable namely for high-boiling analytes. On the other hand,
the introduction of the entire sample (both analytes and matrix components co-isolated from food
matrix) into the GC system is associated with increased demands for cleaning and maintenance
when such complex samples as food is analyzed [5].
There are two alternative approaches available to perform on-column injection.
1. Small volume on-column injection: In this approach, a small volume of the sample (up to
1–2 mL) is injected onto the separation column, or preferably, in the case of dirty samples,
onto a retention gap [6].
2. Large volume on-column injection: In this mode, a large volume of the sample (up to
1000 mL) can be introduced into the GC system [7]. The bulk of solvent is usually
eliminated via a special solvent vapor exit. Once the venting is finished, the solvent
vapor exit is closed and analytes, together with remaining traces of solvent, are transferred
onto the analytical column. However, a modification of the GC system is required in this
case to include a large diameter retention gap (10–15 m 0.53 mm), connected to a
retaining precolumn (3–5 m 0.32 mm) assisting in retention of volatile analytes. In
addition, injection speed has to be slowed down to prevent flooding of the system during
large volume injections (LVI) with injection speeds in LVI-COC of 20–300 mL=min as
compared to LVI-PTV at 50–1500 mL=min [8].
7.2.3 PROGRAMMABLE TEMPERATURE VAPORIZATION INJECTION
A programmable temperature vaporization (PTV) injector represents the most versatile GC inlet
offering significant reduction of most problems typically present when using hot vaporizing devices
(splitless and=or cool on-column inlets) in trace analysis. The most important fact is that a PTV
injector chamber is cool at the moment of injection. A rapid temperature increase, following
withdrawal of the syringe from the inlet, allows efficient transfer of the volatile analytes onto the
GC column while leaving behind nonvolatiles in the injection liner. With regard to these operational
features, PTV is ideally suited for thermally labile analytes and samples with a wide boiling range
(when needed, PTV operating temperature can be programmed even higher than the usual column
temperature allowing injection of analytes that would not be vaporized through a classic split=splitless
inlet). In addition eliminating a discrimination phenomenon and diminishing adverse affects of
nonvolatile matrix deposits on the recovery of injected analytes, PTV enables to introduce large
sample volumes (up to hundreds of microliters) into the GC system. No retention gaps or precolumns
are needed for this purpose; instead of that, the liner size is increased. This feature makes
PTV particularly suitable for trace analysis and also enables its online coupling with various
enrichment and cleanup techniques such as automated solid-phase extraction (SPE) approaches.
From practical point of view, PTV is compatible with any capillary GC column diameter including
microbore columns. However, to attain optimal PTV performance in particular application, many
parameters have to be optimized (e.g., initial and final injector temperature, inlet heating rate,
venting time, flow and pressure, transfer time, injection volume, type of liner). Due to the inherent
complexity of this inlet, method development might become on some occasions a rather demanding
task. Despite that, the use of PTV in food analysis is rapidly growing. The paragraphs below
describe two most commonly used PTV operation modes.
1. PTV splitless injection. The sample is introduced at a temperature below or close to the
boiling point of the solvent. A split exit is closed during the sample evaporation and
solvent vapors are vented via a GC column. PTV splitless injection can be employed for
both LVI of up to 20 mL of sample and for conventional small volume injections [9]. The
ß 2008 by Taylor & Francis Group, LLC.

advantage of this technique is that no losses of volatile analytes occur. Operating parameters
have to be carefully optimized to avoid inlet overflow by sample vapors (losses of
volatile compounds) as well as column flooding by excessive solvent (poor peak shapes of
more volatile analytes). It has been reported that for some analytes the PTV splitless
injection may produce better stability of responses and less matrix influence [10].
2. PTV solvent vent injection. When employing this technique, a sample is injected at temperatures
well below the boiling point of the solvent, holding the temperature of the injection
port at low a value, thus enabling elimination of solvent vapors via a split exit. After the
venting step, the inlet is rapidly heated and analytes are transferred onto the front part of a
GC column. In this way, sample volumes of up to hundreds of microliters can be injected
[11,12]. For injection of large volumes, injector liner is often packed with various sorbents
(e.g., Tennax, polyimide, Chromosorb, glass wool, glass beads, PTFE, Dexsil) in order
to protect solvent from reaching bottom of injector what may lead to column flooding
with liquid sample [13]. However, some labile compounds can be prone to degradation=
rearrangement due to the catalytic effects of the sorbent; alternatively, strong binding onto
the packing material causes poor desorption (Figure 7.4). If selection of a suitable inactive
sorbent fails to prevent these adverse effects, then the only viable solution is the use of an
empty or open liner. Under these conditions, rather smaller volumes (in maximum about
50 mL) of sample can be injected, typically employing a concept of multiple injections to
get a larger injection volume. Obtaining good performance of the PTV injector in solvent
vent mode requires thorough experimental optimization of all relevant parameters as
described above.
pA
1400
1200
1000
800
600
400
200
0
(A)
(B)
pA
1400
1200
1000
800
600
400
200
0
Methamidophos
12.255 12.328
10 12.5 15 17.5 20 22.5 25 27.5 30 32.5 min
12.041
Acephate
13.975
14.194
14.205
Omethoate
15.750
Dimethoate
17.742
17.739
19.182
19.188
20.906 Carbaryl
21.156
21.656
22.185
22.592
20.915
21.664
22.600
10 12.5 15 17.5 20 22.5 25 27.5 30 32.5 min
FIGURE 7.4 Chromatograms obtained by programmable temperature vaporization (PTV) injection into
(A) empty liner and (B) liner packed with glass wool plug. The differences in responses of sensitive analytes
when injections were carried out into empty multibaffle liner and into single-baffle liner packed with glass
wool. (Reproduced from Godula, M. et al., J. Sep. Sci., 24, 355, 2001. With permission.)
ß 2008 by Taylor & Francis Group, LLC.
24.749
25.803
25.801
31.943 Phosalone
31.950

7.2.4 DIRECT SAMPLE INTRODUCTION=DIFFICULT MATRIX INTRODUCTION
Direct sample introduction (DSI) or its fully automated version, difficult matrix introduction (DMI),
represents a novel LVI technique. The DSI approach involves adding up to 30 mL of the extract to a
microvial that is placed in the adapted GC liner. The solvent is evaporated and vented at a relatively
low temperature. After that, the injector is ballistically heated to volatilize the GC-amenable
compounds, which are then focused at the front of a relatively cold GC column. The column then
undergoes normal temperature programming to separate the analytes and cool to initial conditions,
at which time the microvial is removed and discarded along with the nonvolatile matrix components
that it contains. Only those compounds with the volatility range of the analytes enter the
column [14]. In the commercial DMI approach, the entire liner along with the microvial is replaced
after each injection [15].
In this way, time-consuming and expensive purification steps can be omitted=significantly
reduced for some matrices [15,16]. Since the bulk (semi)volatile matrix components introduced
from the sample into the injector may influence the quantitative aspects of the injection process and
interfere in analytes detection, instruments with MS analyzers (single or tandem) providing more
accurate results should be preferably used [17]. In Figure 7.5 the distinct improvement obtained by
sample cleanup is illustrated. Regardless sample preparation strategy, reduced demands for the GC
system maintenance represents a positive feature of this technique.
7.2.5 SOLID-PHASE MICROEXTRACTION
Solid-phase microextraction (SPME) represents a solvent-free sampling technique employing a
fused-silica fibre that is coated on the outside with an appropriate stationary phase. Volatile analytes
emitted from the analyzed sample are isolated from the headspace or by direct immersion into the
liquid sample and concentrated in fibre coating. After the extraction, thermal desorption in the hot
GC injection port follows [18]. The main features of SPME include unattended operation via
robotics (if a fully automated option is available) and the elimination of maintenance of the liner
and column. However, this sample introduction technique is associated with strong matrix effects,
1 2
Metalcap (septum inside)
O-rings
Needle guide
Microvial
Liner
FIGURE 7.5 Difficult matrix introduction (DMI) liner after injection of (1) purified baby-food extract,
(2) crude extract, and (3) detail of microvials used for introduction of crude extracts. (Reproduced from
Cajka, T. et al., J. Sep. Sci., 28, 1048, 2005. With permission.)
ß 2008 by Taylor & Francis Group, LLC.
3

thus compl ications in quanti fi cation. In addit ion, varia bility of limit of de tections for different
analyt es depends on the equil ibrium b etween the coati ng mat erial and the matrix.
7.3 SAMPLE SEPARATION
To be amena ble for the GC analys is, an analyt e shoul d posses s not only appreci able volatil ity at
tem peratures below 350 8C –400 8 C, but also must be able to wi thstand relative high tem peratures
without degrada tion and reaction with other compo unds presen t in the GC syste m.
Wit h regard to a typicall y co mplex mixture of matrix components occurring in food extrac ts
(oft en even after its puri fication ), the optimizati on of GC separa tion requires careful attentio n to a
numbe r of importan t varia bles and their interaction. Both physical (column length, internal diamet
er, and stationar y phase incl uding its film thickness) , and param etric (temperat ure an d flow
veloci ty) column varia bles affect the separa tion proces s.
7.3.1 C APILLARY COLUMNS FOR GC
To illustrate a wide range of combinations to be considered when selecting capillary GC column, the
overview of commonly available internal diameters is shown in Table 7.2.
Tab le 7. 3 shows relative polariti es of comm ercially available stationar y phases . The range of
stationary phases including also those dedicated for specific applications (e.g., volatiles, fatty acids,
dioxins) is growing and also their quality characterized by reduced bleed and increased upper
temperature limit is improving.
In addition, the limits of inlet pressure, sampling system, and mass spectrometric detector
(MSD) parameters have to be involved into consideration.
7.3.2 FAST GAS CHROMATOGRAPHY
A higher sample throughput together with the need to reduce laboratory operating costs has brought
attention of many laboratories to the implementation of high-speed GC (HSGC) systems. Although
the basic principles and theory of HSGC were formulated as early as in the 1960s, its practical
development occurred at the end of last century from introduction of novel technologies such as new
methods of fast and reproducible column heating, inlet devices allowing large sample volume
introduction, and MS detectors with fast acquisition rates. It should be noted that full exploitation
of the potential of this technique in routine practice is conditioned by reduction of sample
TABLE 7.2
Classification of Capillary Column
Category
Column Diameter
Range (mm)
Standard Commercial
Column Diameters (mm)
Max Flow Rate
(mL=min) a
Megabore 0.5 0.53 660
Wide bore 0.3 to

TABLE 7.3
Characterization of Stationary Phases Used in GC Analysis
Polarity Phase Composition Commercial Description
Nonpolar 100% Dimethylpolysiloxane DB-1, DB-1 ms, HP-1, HP-1 ms, Ultra 1, DB-1ht,
Equity-1, SPB-1, AT-1, AT-1MS, Optima 1, Optima-
1 ms, BP-1, VF-1MS, CP Sil 5 CB, CP Sil 5 CB MS,
ZB-1, 007-1, Elite-1, Rxi-1 ms, Rtx-1, Rtx-1MS
5% Diphenyl–95% dimethylpolysiloxane DB-5, HP-5, DB-5 ms, HP-5 ms, Ultra 2, DB-5ht,
Equity-5, SPB-5, AT-5, AT-5MS, Optima 5, Optima-5
ms, BP-5, BPX-5, VF-5MS, CP Sil 8 CB, CP Sil 8 CB
MS, ZB-5, 007-2, PE-2, Rxi-5 ms, Rtx-5, Rtx-5 ms,
Rtx-5Sil MS
20% Diphenyl–80% dimethylpolysiloxane Rtx-20, SPB-20, At-20, 007-7
6% Cyanopropyl-phenyl–94%
DB-1301, HP-1301, Rtx-1301, SPB-1301, AT-624,
dimethylpolysiloxane
Optima 1301, 007-1301
35% Diphenyl–65% dimethylpolysiloxane DB-35, HP-35, DB-35 ms, Rtx-35, Rtx-35MS, SPB-35,
AT-35, AT-35MS, BPX-35, VF-35MS, ZB-35, 007-11,
PE-11
Moderately polar 50% Diphenyl–50% dimethylpolysiloxane DB-17, DB-17 ms, HP-50 þ , DB-17ht, Rtx-17,
VF-17MS, SPB-50, AT-50, AT-50MS, Optima 17,
BPX-50, CP Sil 24 CB, ZB-50, 007-17, PE-17
14% Cyanopropyl-phenyl–86%
DB-1701, HP-1701, SPB-1701, AT-1701, Optima 1701,
dimethylpolysiloxane
BP-10, CP Sil 19 CB, ZB-1701, 007-1701, PE-1701
50% Cyanopropyl-phenyl–50%
DB-23, DB-225, DB-225 ms, Rtx-225, AT-225,
dimethylpolysiloxane
Optima 225, BP-225, CP Sil 43 CB, 007-225, PE-225
Polar Polyethylene glycol DB-WAX, HP-INNOWax, Rtx-WAX, Stabilwax,
Supelcowax-10, AT-Wax, Optima WAX, BP-20, CP
Wax 52 CB, ZB-WAX, 007-CW, PE-CW
Highly polar 70% Cyanopropyl-phenyl–30%
dimethylpolysiloxane
BPX-70
100% Cyanopropylsiloxane SP-2340
preparation time and other operations limiting laboratory throughput. Using approximate terms, the
classification of GC analyses on the basis of their speed is summarized in Table 7.4.
Alike in conventional GC, the separation time is defined as the retention time (t R) for the last
target component peak eluting from the column:
TABLE 7.4
Classification of GC Analyses Based on Speed of Sample
Separation
Type of
GC Analysis
ß 2008 by Taylor & Francis Group, LLC.
Typical Separation
Time (min)
Full Width at
Half-Maximum
Conventional >10 >1 s
Fast 1–10 200–1000 ms
Very fast 0.1–1 30–200 ms
Ultra-fast

tR ¼ L
( k þ 1) (7:1)
u
wher e
k is the solut e c apacity ratio (capacity facto r, reten tion facto r) for the last compo und
L is the column length
u is the average linear carrier gas velocity
On the b asis of this equation, the faster GC separa tion can be achiev ed by follow ing ways
[19,20] :
. Red uced c olumn length ( # L)
. Decr eased retention factor ( # k): (1) increased isotherm al temperat ure, (2) faster temperature
programm ing, (3) alte red stat ionary phase to imp rove selectivity, (4) thinner film of
the stationar y phase, (5) large r diameter c apillary colum n (for fixed length)
. Increas ed carrier gas veloci ty ( " u ): (1) higher than opti mum carrier gas velocity,
(2) increased optimum carrier gas veloci ty (hydro gen as a carrier gas and vac uum outlet
operat ion)
The increase in separa tion speed generally requi res a compromi se in terms of reduced resoluti on ( R)
and=or samp le capacity ( Qc ). The accept ability of these losses has to be considered for each
particu lar case separa tely . Availab ility of compa tible samp le introduct ion technique and detection
param eters play an imp ortant role in selection of the analyt ical strategy.
Red uction of column length repres ents a very sim ple approac h to decreas e time of GC analys is.
In practice, almo st all fast GC analys es are perfor med wi th short colum ns (usually 10 m) in a
combi nation with other approac hes (Figur e 7.6). On this accou nt, reduct ion of the length of a given
colum n results in reduced resol ution ( R ~ p L), which can be compe nsated to some extent by
suitable MS detector (spectral resolutio n).
Use of a colum n with a small internal diam eter is anothe r attracti ve way tow ards faster GC
analys is. How ever, the inst rumental requireme nts especiall y the dif fi culties with the samp le introducti
on of large r sample volumes and also the lower samp le capaci ty limit their applicati on in many
real- world analyses.
Use of a column with a thin fi lm of stationar y phase results in the decreas e of the capacity
(ret ention) factor and thus in the faster GC analysis. In addit ion, due to the decreas ed contribut ion of
mass trans fer in the stationar y phase, separa tion ef ficiency is incre ased. On the other hand, reduced
ruggedn ess and samp le capacity are the fees for analys is speed.
Fa st temperat ure progra mming is the most popula r approac h in applicati on of fast GC in food
analys is. Ei ther convect ion heating facil itated by a convent ional GC ov en or resistive heati ng can be
empl oyed. If ‘‘fast ’’ separa tion in terms of class ifi cation show n in Tab le 7.4 is required, a convent ional
GC oven can be used. At fast er progra mming rates , heat losses from the oven to the surrounding may
cause poor oven tempe rature pro file, hence lower reprod ucibili ty of analyt e elution.
Oper ation of colum n outlet at low p ressure (low-pr essure GC) is anothe r fast GC alte rnative that
may find a wide use in routine labor atories concern ed wi th food analysis. Because o f o perating a
megab ore separa tion colum n (typicall y 10 m length 0.53 mm inte rnal diameter 0.25 –1 mm
phase) at low pressure, optimum carrier gas linear velocity is attained at higher value because of
increased diffusivity of the solute in the gas phase. Consequently, faster GC separations can be
achieved with a disproportionately smaller loss of separation power [22]. The main attractive
featu res of LP- GC –MS invol ve (1) reduced peak tailing and width (F igure 7.7) thus their improved
detection limits, (2) increased sample capacity of megabore column allowing injection of higher
sample volume resulting in lower detection limits for compounds not limited by matrix interferences,
and (3) reduced thermal degradation of thermally labile analytes [23,24].
ß 2008 by Taylor & Francis Group, LLC.

Intensity
(A)
Intensity
(B)
50000
40000
30000
20000
10000
80000
60000
40000
20000
1
0 10 20 30 40 50 min
1
2
2
0.0 0.5
3 4
4
5
6
6
7
7
Hydrogen can be used as a carrier gas because with the highest diffusion coefficient it is
obviously the best carrier gas for fast GC. Its low viscosity also results in lower inlet pressure
requirements. In practice, however, helium is usually preferred as a carrier gas flow for safety and
inertness reasons.
8
8
9
10 11 12 13
14
14
20
27
28
16
15 18 19 22 25 30
2123
26 29
31
20
9
10 11 12 13 24
16 22
15 18 2123
25
19
27
28
30
29 31
1.0 1.5 2.0 min
FIGURE 7.6 GC–FID chromatograms of fatty acid methyl esters obtained under conditions of (A) conventional
(column: Rtx-WAX, 30 m 0.25 mm 0.25 mm; injection: split 1:100; oven temperature
program: 508C, 38C=min to 2808C; acquisition rate: 12.5 Hz) and (B) fast GC (column: Supelcowax,
10 m 0.10 mm 0.10 mm; injection: split 1:200; oven temperature program: 508C, 808C=min to 1508C,
708C=min to 2508C, 508C=min to 2808C (1 min); acquisition rate: 50 Hz). (Reproduced from Mondello, L.
et al., J. Chromatogr A., 1035, 237, 2004. With permission.)
ß 2008 by Taylor & Francis Group, LLC.

Abundance Abundance
3000
2000
1000
m/z 201 m/z 201
3000
0 0
Time−> 9.50
10.50 min Time−> 3.00 4.00 min
Abundance
Abundance
4000
3000 m/z 283 m/z 283
3000
2000
2000
1000
(A) Conventional GC–MS (B) LP-GC–MS
7.3.3 COMPREHENSIVE TWO-DIMENSIONAL GAS CHROMATOGRAPHY
In the analysis of complex mixtures, such as food extracts, by one-dimensional chromatography
(1D-GC), overlap of some sample components unavoidably occurs. To achieve a considerable
increase in peak capacity, two independent separation processes with peak capacities n 1 and n 2 can
be employed in the sample analysis. Supposing that separations are based on two different
mechanisms (orthogonality criterion), the maximum peak capacity calculated as n1 n2 is typically
enhanced by at least one order of magnitude.
Most of the successful applications reported in food analysis since 1960 up to 1990 employed
so-called heart-cut mode, in which only narrow fraction(s) containing analytes of interest is (are)
transported for further separation onto the second column.
However, this approach has limitations. Increasing the width of the first column fraction or
isolating too many parts of 1D-GC analysis to subject them to two-dimensional gas chromatography
(2D-GC) separation becomes troublesome. Also, time-demanding reconstruction of
generated chromatograms may become a serious problem. The introduction of systems that
allow the entire sample from the first column to be analyzed on the second column has enabled
and improved both target and nontarget screening of food components in a wide range of matrices.
This approach, called comprehensive two-dimensional (GC GC), is introduced in the following
sections in a greater detail.
4000
2000
1000
1000
0
0
Time−> 9.50 10.50 min Time−>3.00
4.00 min
FIGURE 7.7 Comparison of peak shapes of thiabendazole (m=z 201) and procymidone (m=z 283) obtained by
(A) conventional GC–MS (column: Rtx-5MS, 30 m 0.25 mm 0.25 mm; injection: splitless, 1 mL; oven
temperature program: 908C (0.5 min), 208C=min to 2208C, 58C=min to 2408C, 208C=min to 2908C (6.5 min))
and (B) LP-GC–MS (column: Rtx-5Sil MS, 10 m 0.53 mm 1.0 mm coupled to 3 m 0.15 mm restriction
column; injection: splitless, 1 mL; oven temperature program: 908C (0.5 min), 608C=min to 2908C (3.0 min)).
(Reproduced from Mastovska, K., Lehotay, S.J., Hajslova, J., J. Chromatogr. A, 926, 291, 2001. With
permission.)
ß 2008 by Taylor & Francis Group, LLC.

7.3.3. 1 GC GC Setup
The hea rt of the GC GC syst em is a modul ator that connect s the fi rst-dimensi on convent ional-si ze
column with a short microbore column in the second dimensi on (Figure 7.8).
There are three fundamental functions of this interface: (1) trapping of small adjacent fractions
(typically 2–10 s) of the effluent from the first separation column, (2) refocusing these fractions (either in
time or in space), and (3) injection of the refocused fractions as narrow pulses into the second-dimension
column. The separation on the latter column is extremely fast and takes only 2–10 s versus 20–120 min
for the first dimension, and is therefore performed under essentially isothermal conditions.
A large seri es of high-speed chromatogr ams as the outcome of the transfer of chromatogr aphic
band from the first to the second dim ension are generated durin g the GC GC run. As shown in
Figure 7.9, these adjace nt pulse s are usual ly stack ed side- by-side by a special soft ware to form a 2D
chromatogram with one dimension representing the retention time on the first column (tR1) and the
other, the retention time on the second column (tR2). The most convenient way to visualize GC GC
data is as contour plots representing the bird’s eye view, where peaks are displayed as spots on a
plane using colors and shading to indicate the signal intensity (Figure 7.9).
7.3.3.2 Optimization of Operation Conditions and Instrumental
Requirements in GC GC
Compared to conventional 1D-GC, the optimization of GC GC analysis requires a more complex
approach. The changes in operational parameters such as oven temperature or carrier gas flow rate
have different impacts on the performance of separation columns since these differ both in their
geometry and separation mechanism. Furthermore, new parameters such as modulation frequency
and modulator temperature have to be optimized [25].
Conventional columns, typically 15–30 m length 0.25–0.32 mm internal diameter 0.1–1 mm
film thickness, are used in the first dimension. This allows application of virtually all sample
introduction techniques (split=splitless, on column, LVI-PTV, DMI=DSI, and=or SPME). Stationary
phases commonly used in first-dimension columns are typically 100% dimethylpolysiloxane or (5%
phenylene)-dimethylpolysiloxane. The separation on these nonpolar columns is governed mainly by
analyte volatility. The size of columns for second dimension is commonly in a range of 0.5–2 m
length 0.1 mm internal diameter 0.1 mm film thickness. More polar stationary phases such
as 35%–50% phenylene–65%–50% dimethylpolysiloxane, polyethylene glycol, carborane, and=or
cyanopropyl–phenyl–dimethylpolysiloxane are employed. Analytes interact with these mediumpolar=polar
phases via various mechanisms such as p–p interactions, hydrogen bonding, etc.,
hence the requirement for different, independent separation principle is met. In most applications,
orthogonality is achieved using nonpolar polar separation mechanisms.
To obtain acceptable separation in both dimensions, a compromise has to be made with regard
to both columns. The linear velocity of the carrier gas in the (narrow bore) first-dimension column is
usually rather lower than optimal (about 30 cm=s) while, at the same time, the linear velocity in the
(microbore) second-dimension capillary is relatively high, typically exceeding 100 cm=s. Also when
setting the temperature programming rate, the requirement for obtaining at least four modulations
Injector
Modulator
First column Second column
FIGURE 7.8 GC GC instrument configuration.
ß 2008 by Taylor & Francis Group, LLC.
Detector

Second dimension
First dimension
Modulation
Transformation
Visualization
Second dimension
2D plot 3D plot
First dimension
First dimension
1D chromatogram
(first column outlet)
Raw 2D chromatogram
(second column outlet)
Second dimensional
chromatograms
FIGURE 7.9 Generation and visualization of a GC GC chromatogram. (Reproduced from Zrostlikova, J.,
Hajslova, J., and Cajka, T., J. Chromatogr. A, 1019, 173, 2003. With permission.)
over each first-dimension peak (so-called modulation criterion) has to be taken into account. In most
analyses, this is achieved by using programming rates as low as 0.58C–58C=min, which is less than
in conventional 1D-GC [26]. It should be noted, however, that even steeper programming rates (thus
faster GC separation) can be employed (108C–208C=min) in GC GC, which typically results in two
modulations over each first-dimension peak. Under these conditions, the separation accomplished in
the first column might be lost. However, because of different activity coefficients on the second
column, the analytes can be completely separated (with higher chromatographic resolution than in
the case of 1D-GC) in the second dimension [27,28]. For better tuning of the GC GC setup,
systems with a programmable second oven are preferred.
Effective and robust modulation is a key process in the GC GC analysis. Thermal modulation in
a capillary GC can be performed by both heating and cooling. While heated modulators use a thickfilm
modulation capillary to trap subsequent sample fractions eluting from the first column by means
ß 2008 by Taylor & Francis Group, LLC.
Second dimension

of stationar y phase focusing (com pounds are released by the temperat ure incre ase), the cryogen ically
cooled modul ators do not use a modulati on capil lary. Inste ad, they trap and focus the sample fractions
eluting from the first column at the front part of the seconda ry colum n itself. Initially, moving
heated=cooled modulato rs were used but they exhibi ted relativel y low robustness (frag ile capillary
can be easily broken) . The se shortcomi ngs of movi ng modulato rs have bee n overcom e by two-st age
jet modul ators that use a stream of nitrogen or carbon dioxide for cooling a short secti on of the second
column for trapp ing=focusing of the analytes elut ing from the fi rst colum n [29,30] .
In practice, fixed modul ation freque ncy, typically in a range of 0.1 –10 Hz is employ ed durin g
the analys is. Under ideal experi ment al condition s, the reten tion time of the most retaine d compo und
in the second d imensio n is shorter than a modul ation time. If this is not the case, i.e., analyt es do not
elute in their modul ation cycle, so-called wrap-arou nd, which may cause co-elutio ns, occurs.
Avoidin g this phenom enon can be achiev ed, e.g., by an incre ase of the second- dimensi on colum n
temperat ure (if a n indepe ndent ove n is avail able). In any case, optimal funct ion of modulato r is
essential for the q uality of separa tion and detection proces s.
The fast separa tion on a short and mic robore second- dime nsion column results in very n arrow
peaks with widths of 50 –1000 ms at the ba seline. Althoug h fast analogue detectors such as a flame
ionizati on d etector o r electron captur e detect or (E CD) are full y compa tible with fast chromatography
and provi de reliable peak recogni tion, they do not provi de stru ctural informat ion. Coupli ng
GC GC separation with MS detect or resul ts into the three-dim ensional syst em that may contribut e
to the identi fication of 2D separa ted peaks and brings a dee per unders tanding of stru ctured
chrom atograms [26]. However , convent ional scanning MS detectors are typi cally too slow and do
not provi de reliable spectra an d peak reprod ucti on. At presen t, only time-of - flight mass spect rometers
(see Cha pter 10) can acquir e the 50 or more mass spectra per second, whi ch are requi red for
the proper recons truction of the chromatogr am and for quanti ficati on in GC GC [35].
7.3.3. 3 Adva ntages of GC GC
A nu mber of charact eristics of GC GC have been reported that documen ts superi ority of this
techniqu e over convent ional 1D-GC [26].
High peak capaci ty. The peak c apacity, characteri zed as a maximal numbe r of chromatogr aphic
peaks that can be placed side by side into the available separa tion space (chromato gram ), is
signi fica ntly en hanced. Unde r the real-world condition s, the total peak capaci ty in GC GC is rathe r
lower than the calcul ated value due to the imperfect ions in the sample transfer betw een the two
columns; however, it still great ly exc eeds the limits of convent ional GC. As an examp le, the meri t in
pesticide residue analys is resultin g from the separa tion powe r is shown in Figure 7.10.
Enh anced sensi tivity . Compar ed to 1D- GC separa tion, pronoun ced imp rovement of detection
limits in GC GC syst em is o btained; thanks to compr essing the peak in the modulation capillary
and front part of the second colum n (following fast chromatogr aphy avoids band broaden ing of
focuse d peak s). Furthe rmore, thanks to imp roved separa tion of analytes and mat rix interfere nces
(chem ical noise ) in the GC GC system, the signa l to noise ratio is also improved. An e xample is
given in Figure 7 .11 that ill ustrates diff erences in 1D-GC versus GC GC a nalysis of limonene.
Struc tured chrom atogra ms. Thanks to compleme ntary separa tion mecha nisms occurr ing in both
columns, the chromatogr ams resultin g from particula r GC GC setup are ordered, i.e., molecules
have thei r de finite locations in the reten tion space based on their stru cture. In the recons tructed 2D
contou r plots, characteri stic patt erns are obtained, in which the members of homologi cal seri es
differing in thei r volatility are ordere d along the first-dim ension axis (nonpol ar capillary is typicall y
employe d in first dim ension), whereas the compounds differing by polarity are spread along the
second-dimension axis. The formation of clusters of the various subgroups of compounds in a
GC GC contour plot may be useful for the group type analysis.
Improved identification of unknowns. Nontarget screening allows obtaining of overview of the
sample constituents. This approach consists from: (1) peak finding and deconvolution (algorithm for
ß 2008 by Taylor & Francis Group, LLC.

Time (seconds)
spectrum #
(A)
1st Time (seconds)
2nd Time (seconds)
spectrum #
(B)
250000
200000
150000
100000
50000
30000
25000
20000
15000
10000
5000
640
697
recogni zing of partly co-el uting peaks in the GC –MS chrom atogram and obtai ning thei r ‘‘ pure ’’
mass spect ra), (2) library searching, and (3) furt her post- processing. Since a large amoun t of data
have to be proces sed, automated data proces sing is employed.
7.4 SAMPLE DETECTION
650 660
747 797
2
2
670
847
79 109 185
20000
18000
16000
14000
12000
10000
8000
6000
4000
2000
Time (seconds)
spectrum #
79
71 87
128
1
47
60
662 662 662 662 662 662
0.6 0.8 1 1.2 1.4 1.6
145
500
185
220
60 80 100 120 140 160 180 200 220 240
1000
Library Hit - similarity 727, “Phosphoric acid,
2,2-dichlorovinyldimethylester”
109
500
60
79
93 145 185
220
80 100 120 140 160 180 200 220 240
662 662
1.8 2
32520 32560 32600 32640 32680 32720 32760 32800
79 109 185
Depe nding upon the type of food compo unds being measured severa l different detectors are
avail able for this purpose (T able 7 .5), e ach with its own advantages and drawbacks. The following
sections briefly introduce various GC detectors most commonly in use today [32,33].
1000
680
897
650
747
690
947
660
797
1
670
847
Peak true–sample
109
680
897
79 109 186
FIGURE 7.10 Separation of dichlorvos (1) in apple extract at 0.01 mg=kg from matrix co-extract
5-(hydroxymethyl)-5-furancarboxaldehyde (2). Plotted are three most abundant ions in the mass spectrum of
dichlorvos (79, 109, and 185). Chromatogram of (A) 1D-GC analysis of zoomed section shows the peak
of dichlorvos (m=z 185) and matrix interference (m=z 79 and 109); and (B) GC GC analysis (DB-XLB DB-17
columns); matrix interference resolved on medium polar DB-17 column. Data acquired by TOFMS at
acquisition rates 5 spectra=s and 250 spectra=s, respectively.
ß 2008 by Taylor & Francis Group, LLC.
700
997
690
947

Abundance
(A)
410 4
310 4
210 4
110 4
570 590
Retention time (s)
7.4.1 FLAME IONIZATION DETECTOR
S/N = 639 S/N = 19570
610
Flame ionization detector (FID) represents one of the most widely used detectors. The effluent from
an analytical column is mixed with hydrogen and air, and is directed into a flame, which breaks
down organic molecules and produces ions. A voltage potential is applied across the gap between
the burner tip and an electrode located just above the flame. The resulting current is then measured
and is proportional to the concentration of the components present.
Abundance
(B)
310 4
210 4
110 4
590.8
0.22
593.8
0.22
596.8 1tR (s)
0.22 2tR (s)
FIGURE 7.11 Improvement of detectability of limonene (m=z 93) isolated from honey headspace by SPME
under the conditions of (A) 1D-GC and (B) GC GC (DB-5 ms Supelcowax-10 columns). Data acquired by
TOFMS at acquisition rates of 10 spectra=s and 300 spectra=s, respectively. (Cajka, T. et al., J. Sep. Sci., 30,
534, 2007.)
TABLE 7.5
Overview of GC Detectors Applicable for the Determination of Food Components
Detector Selectivity Detectability Linearity
Flame ionization detector (FID) No 2 pg C=s 10 7
Thermal conductivity detector (TCD) No 300 pg=mL 10 4–6
Electron capture detector (ECD) Halogens fg=s 10 4
Nitrogen–phosphorus detector (NPD) N, P fg–pg N, P=s 10 4–7
Halogen-specific detector (XSD) Halogens pg Cl=s 10 4
Thermionic ionization detector (TID) N, P 100 fg N=s, N: 10 5 ,P:10 4
100 fg P=s
Photoionization detector (PID) Aromatics pg 10 6
Flame photometric detector (FPD) S, P pg a
S: 10 3 ,P:10 5
Pulsed flame photometric detector (PFPD) Tuneable for 28 elements pg S=s, S, P: 10 3
100 pg P=s a
Atomic-emission detector (AED) Tuneable for any element pg=s a
10 3–4
Electrolytic conductivity detector (ELCD)
or Hall electrolytic conductivity detector
S, N, halogens pg 10 6
Mass spectrometric detector (MSD) Yes fg–pg 10 4–7
Fourier transform infrared (FTIR) Yes pg 10 2
a The detectability considerably varies among particular elements.
pg, picogram; fg, femtogram.
ß 2008 by Taylor & Francis Group, LLC.

7.4.2 THERMAL CONDUCTIVITY DETECTOR
Thermal conductivity detector (TCD) consists of an electrically heated wire or thermistor. The
temperature of the sensing element depends on the thermal conductivity of the gas flowing around it.
Changes in thermal conductivity cause a temperature rise in the element, which is sensed as a
change in resistance.
7.4.3 ELECTRON CAPTURE DETECTOR
In ECD, the sample is introduced into the detector through an analytical column and passes over a
63 Ni radioactive source emitting b particles, which causes ionization of the carrier gas and the
subsequent release of electrons. When organic molecules containing electronegative functional
atoms or groups pass by the detector, they capture some of the electrons and reduce the current
measured between the electrodes.
7.4.4 NITROGEN–PHOSPHORUS DETECTOR
In nitrogen–phosphorus detector (NPD), a glass bead containing an alkali metal is electrically
heated until electrons are emitted. These electrons are then captured by stable intermediates to
form a hydrogen plasma, which ionizes compounds from the column effluent. A polarizing field
directs these ions to a collector anode creating a current.
7.4.5 FLAME PHOTOMETRIC DETECTOR AND PULSED FLAME PHOTOMETRIC DETECTOR
In flame photometric detector (FPD), a sample is burned in a hydrogen=air flame to produce molecular
products that emit light by means of chemiluminescent chemical reactions. The emitted light is then
isolated from background emissions by narrow bandpass wavelength-selective filters and is detected
by a photomultiplier and then amplified. Unfortunately, the detectability of the FPD is limited by light
emissions of the continuous flame burning products. This disadvantage is eliminated by pulsed flame
photometric detector (PFPD), where a hydrogen=air mixture flows into the FPD so low that a
continuous flame could not be sustained. By inserting a constant ignition source into the gas flow,
the hydrogen=air mixture would ignite, propagate back through a quartz combustor tube to a
constriction in the flow path, extinguish, then refill the detector, ignite, and repeat the cycle.
7.4.6 PHOTO-IONIZATION DETECTOR
In photo-ionization detector (PID), the column effluent is ionized by ultraviolet light and the current
(proportional to the concentrations of the ionized material) produced by the ion flow is measured.
7.4.7 ELECTROLYTIC CONDUCTIVITY DETECTOR
In electrolytic conductivity detector (ELCD), compounds eluting from an analytical column are
swept into a nickel reaction tube at the temperature up to 9008C. The components are stripped off
their halogenated atoms and these atoms are carried into a conductivity cell. As the concentrations of
the halogens change in this cell, the measured conductivity of a solution in the cell changes
proportionally.
7.4.8 ATOMIC-EMISSION DETECTOR
In atomic-emission detector (AED), eluted compounds from an analytical column are transported
into a microwave powered plasma (or discharge) cavity where those compounds are destroyed and
their atoms are excited by the energy of the plasma. The emitted light by the excited particles is
separated into individual lines via a photodiode array. The individual emission lines are then sorted
ß 2008 by Taylor & Francis Group, LLC.

and produce chromatogr ams co nsisting of peaks from eluant s that co ntain only a speci fic element. In
this way, elem ental compo sition can be estimat ed. It shoul d be noted that the inte nsity of signa l
large ly varies among the elements and it is relat ively low for oxygen, nitrogen, chlorine, brom ine
while higher sensi tivity is obtai ned for carbon, phospho rus, and sulph ur.
7.4.9 MASS S PECTROMETRIC DETECTOR
The mass spect rome ter (MS) is by far the most powerful an d fle xible of the detectors used in the
analys is of GC-am enable food compone nts today. The advantage over all GC detectors descri bed
above is a possibil ity to ob tain, in addit ion to selec tive de tection of analyt e elut ed at certa in retention
time, also stru ctural infor mation, enabli ng either con firmation of targe t compo und or identi ficati on
of unknow n speci es. The charact er of data obtained largely depends on the type of mass analyz er
employe d. The princ iple s of this type of detection are thoro ughly discussed in Cha pter 10.
7.5 MATRIX EFFECTS
Unde r the real- world condition s, some residues of mat rix co-ext ractives u navoidably remain in the
puri fied samp le prepar ed for examinati on by GC analysis. Inaccur ate quanti fication, decreas ed
method ruggedn ess, low analyte detectabil ity, and ev en report ing of false positive o r negati ve
results are the most serious matrix- associated probl ems, whic h c an b e e ncountered [3]. The extent
of these phenomena depend on a wide range facto rs incl uding samp le compo sition and injectio n
techniqu e empl oyed.
Matr ix-induced chromato graphi c respon se enhancem ent, first descri bed b y Erney et al., is
presuma bly the most discu ssed matrix effect adversely imp acting quanti ficati on accuracy of c ertain,
particula rly more polar analytes [34]. Its principle is as follow s: During inje ction of particula r
compo unds in neat solve nt, adsorp tion and=or therm o-degr adation of susceptibl e analytes on the
active sites (mainly free sil anol groups ) presen t in the inje ction port and in chromatogr aphic colum n
may occur. On this account, the number of analyte molecules reaching GC detector is reduced. This
is, however, not the case when a real-world sample is analyzed. Co-injected matrix components tend
to block the active sites in GC system thus reducing the analyte losses and, consequently, enhancing
their signa ls as co mpared to the injectio n in neat solve nt (Figur es 7.12 and 7.13). If these facts are
ignored and calibration standards in solvent only are used for calculation of target analytes concentration,
recoveries as high as even several hundred percent might be obtained [3]. It is worth
noticing that hydrophobic, nonpolar substances, such as persistent organochlorine contaminants
(with some exceptions such as DDT that may thermally degrade in a dirty hot injector), are not
prone to these hot injection-related problems.
Repeated injections of nonvolatile matrix components, which are gradually deposited in the GC
inlet and=or front part of the GC column, can give rise to successive formation of new active sites,
which might be responsible for the effect, sometimes called matrix-induced diminishment [36].
Gradual decrease in analyte responses associated with this phenomenon together with distorted peak
shapes (broadening, tailing) and shifting the retention times towards higher values negatively impact
ruggedness, i.e., long-term repeatability of analyte peak intensities, shapes, and retention times,
performance characteristic of high importance in routine trace analysis [24].
Three basic approaches and their combination should be considered as way to improved quality
assurance [3]: (1) elimination of primary causes, (2) optimization of calibration strategy enabling
compensation, and (3) optimization of injection and separation parameters.
Unfortunately, the first concept of the GC system free of active sites is in principle hardly
viable—not only because of commercial unavailability of virtually inert materials stable even under
long-term exposure to high temperatures that typically occur in a GC inlet port, but also due to
impossibility to control formation of new active sites from deposited nonvolatile matrix. In this
content, a more conceivable alternative might be based on avoiding sample matrix to be introduced
ß 2008 by Taylor & Francis Group, LLC.

Standard
Sample
C C
Injection
X Liner Y
Transfer onto
the GC column
C – X C – Y
FIGURE 7.12 Illustration of the cause of matrix-induced chromatographic enhancement effect; (C) number
of injected analyte molecules; (X, Y) number of free active sites for their adsorption in injector; (*) molecules
of analyte in injected sample; (.) portion of analyte molecules adsorbed in GC injector; (~)
molecules of matrix components in injected sample; (~) portion of matrix compounds adsorbed in GC liner;
(C X) < (C Y). (Reproduced from Hajslova, J. and Zrostlikova, J., J. Chromatogr. A, 1000, 181, 2003.
With permission.)
260
240
220
200
180
160
140
120
100
80
60
Time−> 12.20 12.30
(P)
(S)
12.40 12.50
FIGURE 7.13 Matrix-induced enhancement response effect: 1 pg of 2-nitronaphthalene (m=z 173) injected in
pure solvent (S) and in purified sample of pumpkin seed oil (P). (Reproduced from Dusek, B., Hajslova, J., and
Kocourek, V., J. Chromatogr. A, 982, 127, 2002. With permission.)
ß 2008 by Taylor & Francis Group, LLC.

into the GC syst em. Unfortu nately, again, none of the comm on isolati on and=or cleanu p techni ques
are selec tive eno ugh (mainly in the case of broad scope methods) to avoid the presen ce of residual
samp le compo nents in the analyt ical sample.
Since an effect ive elimin ation of the source s of the matrix effects is not like ly to occur in
pract ice, their compen sation by using alte rnative cali bration methods is obviously the most feasible
option. Se veral stra tegies are concei vable for this purpos e: (1) addition of isotopic ally labeled
internal standards, (2) the use of stand ards addition method, (3) the use of mat rix-mat ched
stand ards, an d (4) the use of analyt e protectant s (introduce d only recent ly). The main disad vantages=requi
rements of these methods are summ arized in Tab le 7.6 [37].
As regards analyt e protec tants , these compound s are capabl e to strongly inte ract wi th acti ve
sites in the GC syst em, thus decreas ing de gradation an d adsorp tion of targe t analyt es [37]. The same
amoun t of the analyte protectant s is added to both samp le extra cts and mat rix-free standards, which
results in maximi zation and equ alization of the matrix- induced respon se enh ancement effect and
avoids overes timati on of results, which can occur with stand ards in neat solve nt [38]. A wide range
of compo unds containing mul tiple polar=ionizable groups such as vario us po lyols and their deriv atives,
carboxy lic a cids, amino acids , and deriv atives of basic nitrogen contai ning h eterocycles have
been exp erimenta lly evalua ted as analyte prote ctant s. In a study con cerned with the analys is
of mul tiple pesticide resi dues using hot splitless injection , a mixture of 3-ethoxyp ropane-1,2-di ol,
L-gulo nic acid g-lact one, and D-gluc itol (in aceton itrile extra cts) was found to most effect ively cover
a wide volatility range of GC-amenable analytes [38]. This analyte protectant mixture worked also
very well in the multi residue GC analysis of pesticides using DMI [15], which has more active glass
surfa ces that need effective deacti vation d uring each injectio n. Figure 7.14 show s chrom atograms
TABLE 7.6
Quantification Strategies and Their Critical Assessment
Method Comments
Standard additions Extra labor effort required for preparation
Inaccuracies may occur because the matrix effect is concentration dependent
Isotopically labeled standards Only a limited number of certified isotopically labeled standards is currently
commercially available; not available in wide scope methods
Restriction in the use of detection techniques other than mass spectrometry
Additional labor=time burden of developing analytical conditions for so many
more compounds
Matrix-matched standards Need for enough blank matrix (ideally identical as the samples) and its longterm
storage
Extra time, labor, and expense for preparing the blank extracts for calibration
standards needed
Greater amount of matrix material injected onto the column in a sequence,
which leads to greater requirements for GC maintenance
Greater potential for analyte degradation in the matrix solution
Analyte protectants Following criteria have to be met for analyte protectants:
Unreactiveness with analytes in solution and the GC system
Sufficient stability under the GC conditions (no thermodegradation,
re-arrangement, etc.)
No deterioration of the GC column or detector performance (e.g., due to
accumulation)
No interference with the detection process of analytes (i.e., low intensity, low
mass ions in its MS spectra)
Good availability, low cost, no toxicity
Good solubility in the solvent of interest
ß 2008 by Taylor & Francis Group, LLC.

CI
CI
CI
CI
CI
(A) Lindane
(B) Phosalone
OH
(C) o-Phenylphenol
A) Without analyte protectants
CI
CI
O
O O
O
N
S
S P
1.12x
3.98x
10.6x
for three pesticide s lindane, phosal one, and o-phenyl phenol obtained by hot splitl ess inje ction in
solve nt a nd matrix-mat ched stand ards wi thout and with the above mixture of analyte prote ctants,
demon strating dram atic improvem ent in peak shapes an d intensit ies wi th the use of analyt e
prote ctants [38].
Bes ides the above compe nsation approac hes, also careful optimizati on of injectio n and separa tion
param eters (includin g the choice of suitable inje ction technique, temperat ure, and volume; liner size
and its desig n; solvent expansi on volum e; column fl ow rate; colum n dimensi ons) can reduce to some
extent the numbe r of active sites avail able for interacti on (lower surface area) and its durat ion [37].
7.6 FOOD ANALYSIS APPLICATIONS
Injection in:
Matrix
Solvent
Since a large range of food co mpounds are (semi)vo latile co mpounds, the GC is widely used for their
deter minati on. The ch oice of an optimal GC setup depends on the requireme nts for the performanc e
charact eristics of methods used, cost, speed, and several other factors. In Tab le 7.7, the curren t GC
methods for several groups of food constituents are summarized with special attention paid to
applicability of recent advances in the field of this technique for their analysis [39–43].
2.16x
B) With analyte protectants
FIGURE 7.14 Comparison of peak shapes and intensities of 100 ng=mL lindane (m=z 219), phosalone (m=z
182), and o-phenylphenol (m=z 170) obtained by injection in matrix (mixed fruit extract) and solvent (MeCN)
solutions (A) without and (B) with the addition of analyte protectants (3-ethoxypropane-1,2-diol, L-gulonic acid
g-lactone, and D-glucitol at 10, 1, and 1 mg=mL in the injected sample, respectively). (Reproduced from
Mastovska, K., Lehotay, S.J., and Anastassiades M., Anal. Chem., 77, 8129, 2005. With permission.)
ß 2008 by Taylor & Francis Group, LLC.

TABLE 7.7
Overview of Typical Conditions of Most Common Applications Employing GC
for Separation
Natural
substances
Food
contaminants
Lipids (fatty acids,
mostly derivatized)
Aroma and flavor
compounds
ß 2008 by Taylor & Francis Group, LLC.
Injection Typical GC Column Phase Detection
Split Polyethylene glycol
70% Cyanopropyl-phenyl–30%
dimethylpolysiloxane
Split, splitless, SPME 5% Diphenyl–95%
dimethylpolysiloxane
Polyethylene glycol
FID, MS
MS, FID
Modern pesticides Splitless, PTV, 5% Diphenyl–95%
MS, ECD,
DSI=DMI, SPME dimethylpolysiloxane
NPD, FPD,
50% Diphenyl–50%
PFPD, AED,
dimethylpolysiloxane
6% Cyanopropyl-phenyl–94%
dimethylpolysiloxane
35% Diphenyl–65%
dimethylpolysiloxane
PID, ELCD
Polychlorinated Splitless, PTV 5% Diphenyl–95%
ECD, MS
biphenyls
dimethylpolysiloxane
50% Diphenyl–50%
dimethylpolysiloxane
Polychlorinated Splitless, PTV 5% Diphenyl–95%
MS
dibenzo-p-dioxins
dimethylpolysiloxane
and dibenzofurans
50% Cyanopropyl-phenyl–50%
dimethylpolysiloxane
other special phases
Brominated flame Splitless, PTV 100% Dimethylpolysiloxane MS, ECD
retardants
5% Diphenyl–95%
dimethylpolysiloxane
14% Cyanopropyl-phenyl–86%
dimethylpolysiloxane
Polycyclic aromatic Splitless, PTV 5% Diphenyl–95%
MS, PID
hydrocarbons
dimethylpolysiloxane
50% Diphenyl–50%
dimethylpolysiloxane
Veterinary drugs Splitless 100% Dimethylpolysiloxane MS
(derivatized)
5% Diphenyl–95%
dimethylpolysiloxane
Mycotoxins Splitless 5% Diphenyl–95%
MS, ECD
(derivatized)
dimethylpolysiloxane
Acrylamide Splitless, PTV, DSI 5% Diphenyl–95%
MS (both
dimethylpolysiloxane
forms), ECD
(derivatized form)
(derivatized
Polyethylene glycol
(nonderivatized form)
form)
Chloropropanols Splitless, PTV 5% Diphenyl–95%
MS
(derivatized)
dimethylpolysiloxane
Heterocyclic amines Splitless 5% Diphenyl–95%
MS
(derivatized)
dimethylpolysiloxane
50% diphenyl–50%
dimethylpolysiloxane
(continued )

TABLE 7.7 (continued)
Overview of Typical Conditions of Most Common Applications Employing GC
for Separation
Food
contaminants
7.7 CONCLUSION AND FUTURE TRENDS
After severa l decades of GC on the mark et, the techno logy and its appli cations have improved
signi fi cantly. Despit e that they have no t reached an end to the p ossibilities, which are con ceivable.
The re are alwa ys new challenges for further imp rovements of performanc e and extend ing the scope
of applicati ons. Con sidering future uses o f GC in food analys is, the mai n trend fores een is
succes sive replacemen t of conventional detection approac hes by MSD s employin g vario us types
of mass analyz ers. Fast GC –MS can be introdu ced in many applicati ons; thanks to the spectral
resol ution of co-el uting compo unds that can compe nsate for low er GC resolution obtai ned in h ighspeed
separa tions.
ACKNOWLEDGMENTS
This chapte r was financia lly suppor ted by the Mini stry of Edu cation, Youth and Sp orts of the Czech
Rep ublic (project MSM 604 6137305) .
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Phthalate and
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Splitless, SPME 5% Diphenyl–95%
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