- Email: [email protected]
Chapter 3 Gas Chromatography in the Analysis of Air Pollutants
Chapter 3
Gas Chromatography in the Analysis of Air Pollutants
The analysis of a complex mixture of air pollutants requires the technique employed to be highly efficient, highly selective and highly sensitive. In our opinion, gas chromatography meets these requirements most completely. Let us consider this technique in more detail.
3.1.
General considerations
Chromatography is a field of science that studies the motion of a substance (or a group of substances) in a flow of one or several phases moving relative to another one (or other ones)
111. The mobile phase employed in gas chromatography is a gas (vapour). Gas chromatography permits the separation of volatile substances and the determination of their physico-chemical parameters. Its practical application is based on the different retention and band broadening patterns exerted by the substances moving in the gas-phase flow relative to a film of the stationary phase, with the solutes distributed between the stationary (solid or liquid) and gas phases. Gas chromatography is subdivided into two types, depending on the aggregate state of the stationary phase. In “gas-solid” or “gas adsorption” chromatography the latter is a solid, whereas in “gas-liquid” or “gas-liquid-solid’’ chromatography it is a liquid applied as a thin film on a solid support. This chapter is confined to the principles of gas chromatography. For more detailed information the reader is referred to Refs. [2-71. The advantages of gas chromatography are that the separation capacity of the sorbent can easily be changed by choosing the optimum stationary liquid phase (SLP), the symmetry of solute bands and a better reproducibility of sorbent properties. Compounds of different types are used as SLPs, such as hydroxydipropionitrile, diglycerol, squalane, tricresyl phosphate and eutectics [e.g., sodium nitrate (18.2%)-potassium nitrate (54.5%)-1ithium nitrate (27.3%)]. Especially high selectivity is exhibited by complex-forming phases. Thus, silver solution in ehtylene glycol is efficient for the separation of unsaturated compounds including cis and transisomers. SLPs must be thermally and chemically stable and possess low viscosity. Thermally stable selective SLPs include polyethylene glycols (upper temperature limit 225”C), cyanoethylmethylsilicones (275”C), Apiezone (300°C),methylsilicones (350°C), methylphenylsilicones (375°C) and carboranemethylsilicones (400°C). Basic parameters used in gas chromatography to characterize retention and band broadening include the retention volume, VR, the number of theoretical plates (TP) per metre of column length ( N L ) and the height equivalent to a theoretical plate (HETP) (H). The retention volume VR is that of the carrier gas passed through a column from the moment of sample injection to the output of the maximum peak solute concentrhtion. It can be expressed as VR = F C t R ,where tR is the retention time and Fc is the carrier gas velocity at a given temperature and gas pressure in the column. A more invariant measure of retention is the adjusted retention volume, V k , representing the retention volume of a solute minus that of a non-sorbed gas: V ; = ( t R - fM) F, = t R F , (3.1) where fk = tR - fM is the adjusted retention time and tM is the retention time of a component not sorbed by the stationary phase. This parameter ranges from fractions of a millilitre to
3 . 1 . General considerations
11
several litres, depending on the experimental conditions and the column diameter. Another parameter widely used in analytical gas chromatography is the relative retention value, rlJ (relative retention volume or relative retention time),
where V R ,and VKj are the adjusted retention volumes of compounds i and j , respectively, t k , - t R , - t M , t k , = tR, - tM and tk, and fk, are the adjusted retention times of compounds i and j , respectively. The extent of solute band broadening occurring as the solute traverses the column is characterized by the number of theoretical plates defined as N = 5.545(tRlWJ2, where W , is the peak width at half-height (in time units). Other parameters widely used to characterize a column include the specific efficiency, the number of theoretical plates per metre of column length ( N J , and the height equivalent to a theoretical plate (H):
N , = NIL, H = WN, H.N=1
(3.3) (3.4) (3.5)
where L is the column length. These parameters range from 1000 to 20 000 TP/m, depending on the experimental conditions (carrier gas velocity, diameter of sorbent particles). As a chromatographic band moves in the carrier gas flow, its broadening (swamping) occurs. The three basic processes causing band broadening are eddy diffusion, diffusion in the gas phase and interphase mass transfer. Eddy diffusion is independent of the carrier gas velocity and results from the path lengths for individual solute molecules being different owing to a non-uniform sorbent packing and sorbent particles being non-uniform in size. Longitudinal diffusion occurs in the gas phase and is caused by the concentration gradient associated with each chromatographic band. The finiteness of the mass transfer rate leads to the actual solute distribution being different to the equilibrium value, which also contributes to band broadening. The relative contribution of the above factors depends on the carrier gas velocity. Normally, longitudinal diffusion and the finite interphase mass transfer rate are the major factors of broadening at low and high flow-rates, respectively. Variation of the HETP with the linear carrier gas velocity is roughly described by the Van Deemter equation:
or
H
=A
+ B / u + Cu
(3.6a) (3.6b)
where A is the eddy diffusion coefficient including multi-flow-path factors, B is the longitudinal diffusion coefficient resulting from solute diffusion in the gas phase along a column, C is the mass transfer coefficient arising from the finiteness of the interphase mass transfer rate, u is the linear carrier gas velocity, 1 and y are constants, d , is the sorbent particle diameter, D, is the solute diffusion coefficient in the gas phase, k ist the capacity ratio (or partition ratio), defined as k = V',JVM, where V M is the dead volume, and dl is the effective thickness of the liquid phase film on the solid support surface. DIrepresents the solute diffusion coefficient in the liquid phase. The Van Deemter equation is helpful in optimizing the separation conditions. Figure 3.1 shows the dependence of the HETP and the role of different band broadening factors on the linear carrier gas velocity.
12
3. Gas chromatography in the analysis of air pollutants
I
_ _ _ - - - - -3 /
I
--
Fig. 3.1. Dependence of HETP on carrier gas velocity and the effect of the main band broadening factors. I Contribution of eddy diffusion; 2 contribution of longitudinal diffusion; 3 contribution of interphase mass transfer
As the solute bands are moved by the carrier gas along the column, two opposing processes occur. The distance between the concentration maxima of successive components governed by the column selectivity increases, which improves the separation, whereas the bands are broadened depending on the column efficiency, which impairs the separation. The separating ability of a column depends mainly on its efficiency and the selectivity of the sorbent used. A quantitative measure of separation of two successive components is the resolution, R , representing a dimensionless value equal to the difference in the retention volumes, AYj = VRi- VRj, divided by the sum of the peak widths at the baseline. The peak resolution is determined by the sorbent selectivity, a(= rij), the column efficiency, N , and the mass distribution ratio, D, (capacity factor, k ) .
R=-.-.-1. ( a - 1 ) 4
01
1+D,
(3.7)
Figure 3.2 gives a graphical description of the relationship between the major physical processes that occur in a capillary column and its chromatographic characteristics, such as selectivity, efficiency and capacity. It illustrates in more detail the role of the individual factors in eqn. (3.7). The carrier gases conventionally used include nitrogen, air, argon, carbon dioxide, hydrogen and helium. The carrier gas must be chemically inert with respect to the solutes and the stationary phase. It must match the detector employed, i.e., cause no decrease in the de-
t
kinetics
compound and
Fig. 3.2. Relationship between column selectivity, efficiency and capacity with the major processes occuring in the column in the course of separation
3.1. General considerations
13
tection sensitivity, and possess a low viscosity and be non-explosive (which is of particular importance for industrial use), sufficiently pure and cheap. The mobile phase in GC is compressible. A pressure drop in a column leads to the expansion of the carrier gas and increases its velocity, which affects V Rand VR.This is why in precise measurements the actual (pure) retention volume, V,, is used, which is calculated allowing for the pressure drop and is thus independent of the latter. VN = jVX (P,IPO)Z - 1 2 (Pi/P0)31
j = - 3.
(3.8) (3.9)
where j is the pressure gradient factor and Piand Po are the inlet and outlet pressures, respectively. Another parameter used in gas chromatography is the specific retention volume, V , , representing the retention volume per unit mass of the stationary phase: (3.10) where Wl is the mass of the stationary phase and T is the absolute temperature in the column. The specific retention volume is used to determine certain physico-chemical parameters such as the activity coefficient. The main purpose of the solid support in gas chromatography is to provide a thin (fractions of a micrometre or less) film of SLP to improve the mass transfer between the mobile and stationary phases. Solid supports include diatomites subjected to special treatment in order to decrease their adsorptive activity, polymeric supports based on polytetrafluoroethylene and inorganic salts. Normally, their specific surface area varies from 0.1 to 1.5 m2/g. In capillary chromatography, the inner walls of a capillary column serve as a solid support. The role played by the solid support in gas-liquid chromatography is not less important than that of the SLP. Retention in GLC is due to the absorption of solutes in the SLP and their adsorption on the SLP/solid support and SLP/carrier gas interfaces [8, 91. The actual retention volume, V N , is governed by the following parameters of the sorbent [9]: VN=K~V~+KglS~+K~K~S~
(3.11)
where K I , KgI and K , are the solute distribution constants in the systems SLP-gas, SLP surface-gas and solid support surface-SLP, respectively; V, is the column volume occupied by SLP and S, and S, are the areas of the gas/SLP and SLP/solid support interfaces, respectively. The contribution of interface adsorption to the retention value varies from 0 to 100%. Therefore, in studying solute interactions with SLPs it is essential that the fraction of the retention volume corresponding to the solvation of a component in the SLP be extracted from the overall value. Procedures have been developed that permit the quantitative characterization of all the main types of solute interactions with the stationary phase, including adsorption on the gas/SLP interface ( K g lin eqn. (3.11)) and the SLP/sblid support interface (K,) and solvation in the SLP ( K J . Gas-liquid chromatography is the basic chromatographic technique used successfully in analysing composite mixtures. Band identification is based on the retention data. The retention of a component in a mixture is compared with that of a standard. Note that given a particular sorbent and identifying a substance by means of chromatography alone one should consider the sum of retention data obtained on columns with SLPs of different types. For chromatographic identification relative retention values are used, which can be evaluated much more precisely (the error decreases by a factor of two or more). At present, the most
3. Gas chromatography in the analysis of air pollutants
14
widely accepted parameters used in analysis and identification are Kovhts retention indices. The retention index system describes the retention behaviour of a compound relative to that of n-alkanes. As the number of carbon atoms in the n-alkane molecule increases by one, the retention index, Zi, is incremented by 100 units:
(3.12) where z and z + 1 are the carbon numbers of n-alkanes eluted before and after the substance i , respectively. V k , , VX,, VR(,+ are the adjusted retention volumes of the substance i and n-alkanes of carbon number z and z + 1 , respectively. The values of retention indices depend markedly on the type of sorbent used. Thus, for example, the retention of ethyl formate on silicone SLPs is characterized by the following retention indices: 487 (OV-1), 605 (OV-17) and 766 (XE-60); the same parameters for ethyl acetate are 506 (OV-l), 632 (OV-17) and 741 (XE-60). These data indicate that the retention index is governed by the natures of both the solute and SLP employed. Parameters other than retention index and relative retention value also used in chromatography. It has been shown that the known relative values can be considered as particular cases of the following general expression [8]:
(3.13) In developing a procedure for the determination of impurities it sometimes seems desirable to calculate the required efficiency of a column when advantage is taken of the stationary phase for which the retention values of analytes are known from the literature. Such a calculation can be accomplished using eqn. (3.7). However, in the last decade, experimental data have been presented in the literature in the form of the retention index rather than separation factor (see eqn. (3.7)). In this instance the calculation should utilize the following equation [lo]: 104(i0ge)2
n = 16R2
b;(AZ)*
+I-
10Zloge b,(AZ)
(3.14)
where n is the number of effective theoretical plates, AZ = Z2- Il is the difference in the retention indices of two sorbates, R is resolution, e is the basis of natural logarithms, b, = log(t:+ l / f L ) and t : + , and t i are the adjusted retention times of two elements of a homologous series of standard compounds of carbon number z + 1 and z, respectively. The efficiency of a chromatographic column depends markedly on its type. For classical packed columns (3-5 mm I.D., 100-200 cm long, sorbent particle diameter 0.1-0.3 mm) it is normally in the range 1000-3000 theoretical plates. Capillary columns are 10-100 times more efficient, the efficiency of the open type being 30 000-100000 theoretical plates [ll-131. At present, the predominant gas chromatographic mode is capillary chromatography. In 1986 ca. 75% of the papers published on gas chromatography in leading journals such as Analytical Chemistry, Journal of High Resolution Chromatography and Chromatography Communications and Journal of Chromatography utilized capillary chromatography. The use of capillary gas chromatography is distinguished by a number of features that are responsible for its rapid development in comparison with packed column chromatography: (1) higher efficiency; (2) speed of determination; (3) high sensitivity (when using a splitless sample injection technique.and the desired components being concentrated at a reduced column temperature or using the “Grob solvent effect” [13-151; (4) better reproducibility of the thermal conditions utilizing temperature programming as a result of a smaller column diameter and weight; and (5) lower sorbent and carrier gas consumption.
15
3.1. General considerations
Table 3.1. Compounds identified in the PAH fraction of street dust from Giza Square, Egypt. Peak numbers refer to Fig.3.3 [17] Peak
number 1
2 3
4
5 6 7 8 9 10 11
12 13 14 1s 16 17 18 19 20 21 22 23 24 25
Compound
Peak number
Compound
Naphthalene Dimethylnaphthalene Trimethylnaphthalene Trimethylnaphthalene Dibenzothiophene Phenanthrene Anthracene C2-9H-Fluorene C2-9H-Fluorene Methyldibenzothiophene Methyldibenzothiophene Methylphenanthrene Methylphenanthrene Methylphenanthrene Methylphenanthrene C,-Dibenzothiophene C2-Dibenzothiophene Dibutyl phthalate C2-Dibenzothiophene Dimethylphenanthrene Dimethylphenanthrene Fluoranthene Pyrene C,-Phenanthrene C,-Phenanthrene
26 27 28 29 30 31 32 33 34 3s 36
C,-Phenanthrene C,-Phenanthrene C,-Phenanthrene C,-Phenanthrene Me thylfluoranthene Benzonaphthothiophene Benzo[ghi]fluoranthene Benzo[c]phenanthrene Benz[a]anthracene Chrysene/triphenylene Methylbenz[a]anthracene or methylchrysene Phthalate ester Phthalate ester Benzo[b]fluoranthene and benzoblfluoranthene Benzofluroanthene Benzo(e1pyrene Benzo[a]pyrene Perylene Indeno[l,2,3-cd]pyrene Dibenzanthracene Benzo[ghi]perylene Anthanthrene Dibenzopyrene
37 38 39 40
41
42 43 44 4s 46 47 48
Capillary chromatography is very popular for air pollutant determinations and readers are referred to specialized books (11-131. Currently, capillary columns rely mainly on bonded and immobilized phases. The use of immobilized liquid stationary phases (1) enhances the lifetime of the columns because chemically immobilized phases will be incapable of curling into large drops, which usually results in a sharp decrease in column efficiency and (2) increases the detection sensitivity because the immobilized phases permit the utilization of a concentration procedure o n sample/ solvent injection, e.g., using the “Grob solvent effect”. The capillary column materials and the coating materials are equally important, for the following reasons: (1) the surface of the internal column walls which serve as a solid carrier in capillary chromatography can frequently produce a detrimental effect (especially in pollutant analysis) on the chromatographic process as a result of irreversible and catalytic conversions of the compounds treated on the internal capillary surface (see, e.g., ref. [16]); and (2) the polymeric coating of fused-silica columns has a limited temperature stability. Urban air contains a variety of harmful aromatic hydrocarbons. Figure 3.3 [17] shows a chromatogram for the fraction of PAHs in urban dust (Egypt). It is natural that such a complex mixture can only be separated by the use of high-efficiency capillary gas chromatograPhY.
16
3. Gas chromatography in the analysis of air pollutants
37
39
42
41
'
I
35f80
Fig. 3.3.
~71.
120
1
160
I
200
I
24 0
I
I
280
315
L
"C
Chromatogram of the fraction of polycyclic aromatic hydrocarbons contained in street dust
Column: 21 m x 0.32mm I.D.; SLP, SE-52; temperature programme, 35 to 80°C (temperature balistically programmed), 80 to 315°C at 5 K/min. Compounds identified as in Table 3.1
Capillary columns are equally efficient in determinations of nitroaromatic compounds, which is a difficult problem. Table 3.2 shows the concentrations of this important class of organic compounds which were detected in atmospheric dust in Tokyo [18]. The use of capillary columns allows the applicability of gas chromatographic techniques to be greatly extended for environmental control purposes. The adsorption of solutes on the interfaces results in the relative retention value in the general case depending not only on the ratio of distribution constants of the solute and standard, but also on the adsorptive properties of the SLPisolid support and gadliquid interfaces. Therefore, the relative retention time and retention index are not constants for a compound when adsorption occurs. The chromatographic constant of compound i is the invariant retention index, Ioi, corresponding to the solute interaction with the SLP only, which can be evaluated in terms of the following equation [9, 191:
I, = lor + aik,,
(3.15) (3.16)
where a is a constant defining the adsorption of the solute on the sorbent used, k,,is the capacity ratio of a standard, k,, = VX,,/V,, Vk,, is the adjusted retention volume of a standard, V , is the retention volume of a non-sorbed component, Kli is the distribution constant of
17
3.1. General considerations Table 3.2. Nitroarenes identified in airborne particulates in Tokyo air [18] Compound
1 2 3 4 5
6 7 8 9 10 11 12 13 14 15 16 17 18 19
o-Ethylnitrobenzene pEthylnitrobenzene pDinitrobenzene 2,6-Dinitrotoluene 2,4-Dinitrotoluene 1-Nitro-2methylnaphthalene 4,6-Dinitro-rn-xylene 3,s-Dinitro-o-xylene 1,5-Dinitronaphthalene 1,3-Dinitronaphthalene 5-Nitroacenaphthene 2-Nitrofluorene 9-Nitroanthracene 4,4’-Dinitrodiphenyl 2,s-Dinitrofluorene 1-Nitropyrene 2,7-Dinitrofluorene 4-Nitro-pterphenyl 6-Nitrochrysene
Relative retention.) Phenylmethylsilicone
Carbowax 20M
Reference
Sample
Reference
Sample
0.340 0.432 0.564 0.611 0.696 0.795 0.796 0.841 1.13 1.16 1.18 1.29 1.30 1.51 1.61 1.68 1.75 1.77 1.90
0.346 0.439 0.568 0.606 0.690 0.792 0.798 0.841 1.13 1.16 1.17 1.28 1.30 1.51 1.60 1.68 1.75 1.78 1.90
0.391 0.470 0.745 0.727 0.773 0.783 0.811 0.847 1.46 1.49
0.388 0.474 0.739 0.727 0.780 0.786 0.817 0.843 1.46 1.50
Concentration (ng/m3)
Mutagenicity
0.056 0.009 0.017 0.006 0.024 0.005 0.011 0.078 0.057 0.12 0.11 0.050 0.27 0.071 0.19 0.14 1.5 0.16 0.27
-
+ + + k + f + + + + + + i+ + + +
*) Internal standard: Benzo[qquinoline (retention time = 16.61 min for 5 % phenylmethylsilicone column and 27.46 min for Carbowax 20M column). Analysis of nitroarenes in airborne particulates was carried out under the following conditions. Neutral organic fraction was prepared from the particulates collected from 7 500 m3 air by a high-volume sampler. After adding 1.0 pg of benzo[flquinoline to a benzene solution of the neutral organic fraction (0.5 d ) , 3 pl auf the solution were injected on to the capillary gas chromatograph (HP 5880A) by a splitless injection mode. Identification was made by comparison of relative retentions of sample peaks with those of 40 reference substances. Quantitative determination was performed using calibration graphs for each nitroarene identified. Concentrations indicate the values obtained with the phenylmethyl silicone capillary column. The values obtained with the Carbowax 20M capillary column were as follows; 0.047, 0.009, 0.027, 0.005, 0.021, 0.006, 0.014, 0.083,0.045 and 0.083 ng/m3 for the compounds numbered 1,2,3,4,5,6, 7, 8 , 9 and 10, respectively.
compound i between the SLP and the gas and K I ,and Klc,+1) are the distribution constants of two standard compounds of carbon number z and z + 1, respectively, with K , , 5 K l i < Klc,+l). To evaluate Zoimeasurements are performed on several columns differing in SLP content on the solid support. The contribution of adsorption to the retention index value may be appreciable (several to a few tens percent). In capillary chromatography, adsorption also affects the retention values [16]. At the usual low pressures, the role of the carrier gas lies mainly in transferring solutes along the column. At higher pressures, however, the non-ideality of the gas phase begins to count, which alters the distribution of the solute between the mobile and stationary phases. The chromatographic mobility of many substances increases. 3
Berezkin, Gas Chrom-BE
18
3. Gas chromatography in the analysis of air pollutants
Fig. 3.4. Schematic diagram of a gas chromatograph. 1 High-pressure carrier gas source; 2 carrier gas preparation unit; 3 sample injection unit; 4 chromatographic column; 5 thermostat; 6 detector; 7 recorder; 8 minicomputer for instrument control and data processing
High-pressure gas chromatography (HPGC) is a technique intermediate between gas and liquid chromatography. HPGC has the following advantages over the latter: (1) the possibility of directed variation of the solute retention volumes by varying the pressure over a wide range, (2) rapidity of the analysis owing to the lower viscosity of the mobile phase and higher diffusion coefficients, (3) the possible application of universal highly sensitive detectors employed in gas chromatography. However, the HPGC instrumentation and experimental procedures remaining fairly complicated, which restricts the large scale application of this technique. A particularly interesting variant is chromatographic separation with the gaseous mobile phase in the supercritical state. Supercritical fluid chromatography (SFC) has made it possible to separate porphyrins, which could not be analysed at high temperatures owing to their thermal instability. The use of carbon dioxide and ammonia in the supercritical state proved efficient in separating compounds with molecular weights as high as 40000. The unusual characteristics of HPGC prompted the following classification of gas chromatography according to the state of the mobile phase: (1) GC at mobile phase pressures of 1-25 atm and (2) GC with the mobile phase at high pressures (100-500 atm), including the supercritical state. A schematic diagram of a gas chromatograph is shown in Fig. 3.4. The carrier gas is continuously supplied from the high-pressure cylinder I to the carrier gas preparation unit 2, where it is further purified and the required parameters of the mobile phase (pressure, velocity) are maintained throughout the experiment. The carrier gas then passes to the sample input system 3, which normally in laboratory chromatographs is an independently thermostated cylindrical flow chamber provided with a self-sealing thermostable rubber septum through which a sample (1-10 pl) is injected from, e.g., a syringe (9) into the carrier gas flow at high temperature. In the input system the liquid sample rapidly vaporises and the carrier gas flow transfers it to the chromatographic column 4 located in a thermostat. The separation is usually run at 20-400°C. Special thermostats allow the separation to be performed at temperatures as low as liquid nitrogen boiling point, which is mainly used in separating the isotopes of low-boiling gases. Analytical separations are usually performed on 0.5-5 m X 0.5-4 m m I.D. packed columns and 10-100 m x 0.2-0.5 mm I.D. open capillary columns. The columns are packed with sorbents consisting either of a non-volatile liquid (stationary phase) supported as a thin coating on a solid macroporous carrier with low specific surface area (0.5-2 m2/g) or of a solid with a developed surface (50-500 m2/g). The weight of the stationary phase is normally 2-20% of that of the solid support. The mean diameter of the sorbent particles is usually 0.1-0.4 mm, but fine fractions are used when filling the column. The separation of the components of a mixture occurs in a chromatographic column as a result of the solute bands moving at different velocities. The bands are passed by the carrier gas flow to a detector 6 whose signal, which is proportional to the analyte masses, is recorded
3.1. General considerations
19
continuously by a recorder 7. A microcomputer 8 serves for instrument control and data processing. Gas chromatography is a hybrid technique: separation of a mixture into components occurs in the chromatographic column, and quantitative analysis of the eluate is performed by the detector. Hence separation and quantitative (and, frequently, qualitative) analysis are separated in time and space. Therefore, the final result of the analysis is determined by the characteristics of both the column and the detector. The detector plays a very important role in gas chromatography. Now mostly differential detectors are used whose signal is proportional to the instantaneous value of the solute concentration or mass flow. The most useful types are the katharometer (thermal conductivity detector) and flame ionization, electron-capture and flame photometric detectors. The highly sensitive ionization and flame photometric detectors are employed in the analysis of trace impurities and very small samples and in capillary chromatography. The analytical potential of gas chromatography can be substantially expanded by employing several detectors. This makes it possible to determine quantitatively and qualitatively the composition of unresolved bands containing two or more compounds, provided that the selectivity of their detection by the detectors employed is different. The use of the mass spectrometer for detection has led to the development of highly e f i cient combined gas chromatography-mass spectrometry (GC-MS). A gas chromatograph with a packed or capillary column is connected to a mass spectrometer via a molecular separator serving to concentrate the separated bands and partially remove the carrier gas. The ion sources of a number of mass spectrometers equipped with powerful high-vacuum pumps sometimes allow the direct connection of a chromatographic column to a mass spectrometer without a molecular separator, with the solutes passing directly from the column to the ion source for ionization. The flow of charged particles is then supplied to a mass analyser where separation according to mass-to-charge ratio ( m l z ) occurs. Each substance is characterized by its specific mass spectrum, reflecting its structure. The analyte is identified by comparing its mass spectrum with standard spectra measured on pure samples or available in the literature. The mass spectrometer can be used first as a highly selective detector for taking chromatograms at a fixed mass-to-charge ratio and second for the rapid monitoring of a full mass spectrum during chromatographic analysis. Computers are widely used for data processing. The sensitivity of a mass spectrometer is l O - ” - l O - ” mg/s. GC-MS is currently one of the most universal and informative instruments and it is widely used in the analysis of air pollutants. Figure 3.5 demonstrates the relationship between the various parameters and the criteria of chromatographic separation for gas chromatography. It is made in the form of two quasicircumferences one of them includes criteria of chromatographic separation, another one units the parameters of chromatographic experiment. A similar approach was described by BERRIDGE for HPLC [20]. The inner circumference indicates the following major criteria: R,, peak resolution; t , , time of separation; AP, pressure difference on the column (the pressure drop); S, response factor (peak height) for a bulky (1) and a smaller sample (2). Intermediate parameters (dotted circumference) are k’, capacity factor; a,selectivity factor; N , column efficiency (number of theoretical plates); 7 , carrier gas viscosity; ps, volume fraction of the carrier gas component with displacing properties; d , , column diameter; T, column temperature; u , carrier gas linear velocity; d , , size of solid support (adsorbent) particles; L, column length; P,,content of SLP on the solid support; MCS, weight ratio between the mixed stationary phase components. The data in Fig. 3.5 indicate a complex relationship between the various parameters and the criteria of the chromatographic method. It is recommended that the relationships are used in the compilation of the programme for the optimization of a chromatographic separation.
20
3. Gas chromatography in the analysis of air pollutants
Fig. 3.5. Relationship between the various characteristics of chromatographic separation
Gas chromatography allows one to carry out qualitative and quantitative analyses of organic and inorganic compounds whose vapour pressures at the temperature of the column exceed 0.001-1 mm Hg and which are thermally stable at the decomposition temperature. This method is widely used for determination of compounds present in samples in trace amounts (10-4-10-8%).
3.2.
Peculiarities of the gas chromatographic analysis of impurities
The difficulties associated with the gas chromatographic determination of impurities are mainly due to the fact that when in very low concentrations the latter show a behaviour different to that exerted by macroscopic amounts of the same substances. A detailed discussion of this problem has been given elsewhere [21-231. Organic pollutants are often preconcentrated at reduced temperatures in order to analyse them in air (see, e.g., ref.24). An interesting example of determining Arctic hydrocarbon air pollutants at ppt levels was described by Norwegian researchers [25]. The detection limit was 1-5 ppt. The Arctic air was shown to contain various gases of this type (C2-C4). The 'main problem to be faced in the determination of trace impurities (10-4-10-8%)in air is potential analyte losses that may accompany any step of the gas chromatographic analysis. The disappearance of trace impurities can occur when taking a sample (sorption by container walls, incomplete consumption and chemical reactions in the concentrator, etc.), during the desorption of analytes from the trap (inefficient desorption), when injecting the samples into the chromatograph (e.g., sample decomposition in the vaporizer at high temperature) and owing to sorption on or chemical reactions with the sorbents and the inner surfaces of the
3.2. Peculiarities of the gas chromatographic analysis of impurities
21
chromatographic equipment. In the latter instance we are dealing with phenomena similar to the adsorption of traces of isotopes in radiochemistry. The concentration of the active sites in the sorbent becomes comparable to that of the analytes and therefore adsorption, which is insignificant at normal concentrations, is important at micro concentrations. Overestimated results may arise from analyte vapour adsorption on the walls of the sampling valve. There are various ways of preventing such losses, but the most useful involves continuous conditioning of the column and the whole system with analytes in order to saturate active sorbent sites and the equipment surfaces. Such conditioning may take a long time (several hours), especially when analysing reactive inorganic gases or unstable and readily hydrolysed substances (nitrogen and sulphur oxides, ozone, halogens, mixed halogen compounds, hydrides, etc.) even if all the parts of the chromatograph are made of materials resistant to corrosion (nickel, PTFE, glass, etc.), and inert sorbents (PTFE, graphite, polymers based on polytrifluoromonochloroethylene) are utilized for separation [26]. Frequently, the preliminary sorbent treatment takes about 20-30 h (e.g., for nitrogen dioxide) and must be repeated (1-2 h) before each set of analyses. It should always be borne in mind that the absence of peaks of the impurities being analysed (especially when analysing reactive, unstable and readily hydrolysed substances) by no means indicates their absence in the sample or an undetectable concentration. This may be due to irreversible sorption or chemical reactions undergone by the impurities (hydrolysis, pyrolysis, reactions with sorbents, etc.). Such interactions can be minimized by taking advantage of cryogenic gas chromatography. By analysing corrosive and reactive compounds (ozone, nitrogen, halogen and sulphur oxides, fluorides, etc.) at low temperature (below - 20°C) or with a programmed increase from - 100°C until the elution of one of the components (e.g., sulphur or nitrogen dioxide) one can successfully separate the impurities of corrosive substances in the presence of moisture and low-boiling compounds (CO, C 0 2 , N2, 0 2 , etc.) without losses. One of the problems attending the analysis of impurities is the drying of air dehumidification, which is particularly important when dealing with large samples. As the amount of moisture accumulated in traps is several orders of magnitude greater than that of the concentrated impurities, analyte losses often result which requires the solutions being analysed to be strongly diluted. A widely used means of drying air consists in passing it through a dehydration plug filled with absorbents or chemicals that selectively absorb water. One of the best absorbents of this type are molecular sieves 3A, which absorb water but allow organic and inorganic impurities except ammonia and methanol to pass through [27]. The interaction of analytes with the chromatographic equipment and sorbents results in insufficient resolution when analysing trace amounts of flavours and many other compounds of natural and biological origin. One of the means of increasing the sensitivity of such analyses is to utilize packed or capillary columns and vaporizers made of fused silica, which decreases the possibility of irreversible adsorption. Another helpful approach consists in injecting a sample of thermally unstable compounds or large volume of diluted solution directly into the chromatographic column. The purity of the solvent used for the extraction of impurities form the sorbent following sample concentration is of great importance. The solvent must not contain impurities at concentration comparable to and eluted within a short time interval of the analyte substances. It is desirable that these sorbents (e.g., chlorobenzene) should be purified of impurities by ordinary distillation. Of no less importance is the purity of the carrier and diluent gases employed for the preparation of standard mixtures and the calibration of detectors. The thorough purification of cylinder nitrogen used for sorbent vaporization and trace compound analysis by GC and GC-MS involves five traps connected in series [28]. Two of them are filled with a mixture of Carbopak, activated carbon and Chromosorb W, the latter being coated with Apiezon. The
22
3. Gas chromatography in the analysis of a h pollutants
third, fourth and fifth traps are filled with Zeolyte 3A, silica gel impregnated with 20% sulphuric acid and Carbosieve S , respectively. Neglect of the above requirements may cause appreciable errors and make the results of the analysis useless. One of the significant sources of errors can be the glass-wool used as an insert or plug in chromatographic columns and splitters. The treatment of used glass-wool with n-pentane, dichloromethane, gaseous hydrogen chloride, and carbon disulphide followed by gas chromatography recovered traces of previously analysed hydrocarbons, phthalates and organic acid salts at levels of 0.3-0.005 mg/g. It is advisable for the glass-wool to be pretreated with gaseous hydrogen chloride and then extracted for 24 h with dichloromethane in a Soxhlet apparatus. It is also recommended that its surface be silylated. Another frequently occurring source of errors is gases evolved from the septa and vaporizers of chromatographs, which give rise to ghost peaks that distorts the results. The composition and concentration of these gases have been established to depend on the septum material, temperature and the period during which the septum was used in the chromatograph. The best properties are exhibited by septa made of perfluorinated elastomers and some silicone rubbers. At any step in the analysis of impurities one should take into account the possibility of thermal and exclusive desorption of the components previously adsorbed on the column or their exclusion from subsequent samples by more polar compounds ( e g , water and acids). Competitive sorption is also observed when concentrating impurities from air, the possible exclusion of one compound by others creating conditions that are unfavourable for sorption of the former, as occurs with non-polar impurities in the presence of polar substances. When the concentration of impurities in a trap containing a sorbent increases by a factor of 102-105and the molecules are within the range of the action of the sorbent, the possibility of chemical (catalytic, heterogeneous, etc.) transformation of the sample increases sharply. Sample heating during thermal desorption (150-250°C) further increases this possibility owing to potential pyrolysis and other interactions of the concentrated compounds. This substantially distorts the results of analysis and frequently makes them useless under these conditions. Catalytic reactions lead to losses of styrene when it is being concentrated on charcoal, and the analysis of atmospheric air containing alkenes and halogens (chlorine and bromine) performed in the presence of ozone results in the formation of halogenated hydrocarbons on the column sorbent. Thus, the presence of butenes and chlorine gives rise to 2,3-dichlorobutene and such reactions can occur on both porous polymers and carbon-containing sorbents. To avoid false peaks of halogenated hydrocarbons, glass-fibre impregnated with 10% sodium thiosulphate solution is installed before the sorbent. The false peaks of halogenated hydrocarbons change with the ozone and nitrogen oxide concentrations in the atmosphere. When concentrating toxic impurities from flue gas on Tenax, the sorbent undergoes partial transformation into 2,6-biphenyl-p-quinone under the action of sulphur dioxide and nitrogen oxide. The analysis of sulphur and nitrogen oxides contained in industrial emuents is accompanied by the sulphonation and nitration of porous polymeric sorbents (Tenax and XAD 21, which affects their capacity and selectivity (especially with polar sorbents). Gas chromatography has shown ihat extremely toxic N-nitrosodimethylamines may be formed in a Tenax concentration trap during sorption from air through the interaction of dimethylamine present in the air with nitrogen oxides and ozone in the presence of traces of moisture [29]. This is a serious obstacle to the concentration and quantitation of N-nitroso compounds. The false peaks of the latter are also formed when impurities are concentrated on other sorbents (charcoal, silica gel, alumina, Florisil and the sorbents treated with aqueous KOH solution, ascorbate or phosphate-nitrate buffer). The formation of “false” N-nitrosamines is particularly intensive with activated carbon and silica gel.
References
23
Account should also be taken of the direct interaction of the sorbent in the trap with the impurities being analysed, which leads to losses of the latter. This can be exemplified by the absorption of nitrogen dioxide impurities by Porapak Q (accompanied by sorbent nitration) or irreversible interaction between sulphur-containing compounds and activated carbon..The concentration of ozone and chlorine dioxide impurities is not safe because when concentrated these compounds are explosive [26]. When employing the thermal desorption of impurities from sorbent-containing traps it is essential that the temperature applied is not too high (150°Cis the optimal value), as certain compounds undergo isomerization at 250°C to yield “false” compounds of higher molecular mass. Thus, C1-C3 hydrocarbons give cis-2-butene and cis-2-pentene under these conditions 1301. It should be noted that the adsorptive activity of the inner walls of capillary columns, including the quartz type, may cause irreversible adsorption of polar analytes [31, 321. The above peculiarities in the determination of impurities should be taken into account when analysing air pollutants.
References (Chapter 3) BEREZKIN, V. G. ; GAVRICHEV, V. S.; KOLOMNETS, L. N.; KOROLEV, A. A,; LIPAVSKII, V. N.; NIKITINA, N. S.; TATARINSKII, V. S.: Gazovaya Khromatografiya v Neftekhimii (Gas Chromatography in Petrochemistry), Moscow: Nauka 1975, p. 17. GROB,R. L. (Ed.): Modem Practice of Gas Chromatography. New York: John Wiley 1985. KATZ,E. (Ed.): Quantitative Analysis Using Chromatographic Techniques. Chichester: John Wiley 1987. SUPINA,W. R.: The Packed Column in Gas Chromatography. Bellefonte, Pennsylvania: Supelco 1974. POOLE,C. F.; SCHUETI’E, S. A,: Contemporary Practice of Chromatography. Amsterdam: Elsevier 1984. LEIBNITZ, E.; STRUPPE, H. G. (Ed.): Handbuch der Gaschromatographie. Leipzig: Akademische Verlagsgesellschaft Geest & Portig K.-G. 1984. KISELEV,A. V.; YASHIN,Y. I.: Gas-Adsorption Chromatography. New York: Plenum Press 1969. BEREZKIN, V. G.: J. Chromatogr. 98 (1974) 477. BEREZKIN,V. G. In: GIDDINGS, J. C.; GRUSHKA, E.; BROWN,P. R. (Eds.): Advances in Chromatography, vol. 27. New York: M. Dekker 1987, pp. 1-35. BEREZKIN,V. G.; RETUNSKY, V.N.: J. Chromatography 330 (1985) 71. JENNINGS, W.: Gas Chromatography with Glass Capillary Columns. New York: Academic Press 1980. JENNINGS, W.: Comparisons of Fused Silica and Other Glass Columns in Gas Chromatography. Heidelberg: Huthig 1981. LEE,M.; YANG,F.; BARTLE, K.: Open Tubular Column Gas Chromatography. New York: Wiley-Interscience 1984. GROB,K.; GROB,G.: J.Chromatogr. Sci. 7 (1969) 584. GROB,K.; GROB,G.: J. Chromatogr. Sci. 7 (1969) 587. BEREZKIN, V. G.: Gas-Liquid-Solid Chromatography (in Russian). Moscow: Khimiya 1985. MASHALY, M.; SANDRA, P. In: VIIIth Intern. Symp. on Capillary Chromatography. Vol. 1. Riva del Garda, May 19-21, 1987. SANDRA, 0. (Ed.): Heidelberg: Huthig 1987, p. 476. MATSUSHITA, H.; IIDA,Y.: J. HRC and CC. 9 (1986) 710. BEREZKIN, V. G.: J. Chromatogr. 159 (1978) 359. BERRIDGE, J. C.: Techniques for the Aromated Optimization of HPLC Separations. Chichester: John Wiley 1986. BEREZKIN, V. G.; TATARINSKII, V. S.: Gas-Chromatographic Analysis of Trace Impurities. New York: Consultants Bureau, 1973.
24
3. Gas chromatography in the analysis of air pollutants
Yu.S.; BEREZKIN, V. G.: Gas Chromatographic Analysis of Pollution Air. (in Russian). DRUGOV, Moscow: Khirniya 1981. JENNINGS, W.G.; UP, A,: Sample Preparation for Gas Chromatographic Analysis. Heidelberg: HUthig 1983. BRETIZLL, T.A.; GROB,R. L.: Intern. Lab., Apr. 1985, p. 30. SCHIDBAUER, N. S.; OEHME,M.: Journal HRC and CC. 8 (1985) 404. ANVAER,B. I.; DRUGOV,Yu.S.: Gazovaya Khromatografiya Neorganicheskikh Veshchestv (Gas chromatography of inorganic compounds). Moscow: Khimiya 1976. BEREZKIN, V. G.; DRUGOV,Yu.S.; GORYACHEV, N. S.: Zhurnal analyticheskoi khirnii. 37 (1981) 319. WESTRICK, T. J.; LAMPARSKI, L. L.: Anal. Chem. 53 (1981) 22. BERKLEY, R.E.; PELLIZZARI, E.D.: Anal. Lett. A l l (1978) 327. CRIP, S.: Ann. Occup. Hyg. 23 (1980) 47. PURCELL, J . E.: Chrornatographia 15 (1982) 546. BLOMBERG, L.G.: Journal HRC and CC 7 (1984) 232.
Gas Chromatography in the Analysis of Air Pollutants
The analysis of a complex mixture of air pollutants requires the technique employed to be highly efficient, highly selective and highly sensitive. In our opinion, gas chromatography meets these requirements most completely. Let us consider this technique in more detail.
3.1.
General considerations
Chromatography is a field of science that studies the motion of a substance (or a group of substances) in a flow of one or several phases moving relative to another one (or other ones)
111. The mobile phase employed in gas chromatography is a gas (vapour). Gas chromatography permits the separation of volatile substances and the determination of their physico-chemical parameters. Its practical application is based on the different retention and band broadening patterns exerted by the substances moving in the gas-phase flow relative to a film of the stationary phase, with the solutes distributed between the stationary (solid or liquid) and gas phases. Gas chromatography is subdivided into two types, depending on the aggregate state of the stationary phase. In “gas-solid” or “gas adsorption” chromatography the latter is a solid, whereas in “gas-liquid” or “gas-liquid-solid’’ chromatography it is a liquid applied as a thin film on a solid support. This chapter is confined to the principles of gas chromatography. For more detailed information the reader is referred to Refs. [2-71. The advantages of gas chromatography are that the separation capacity of the sorbent can easily be changed by choosing the optimum stationary liquid phase (SLP), the symmetry of solute bands and a better reproducibility of sorbent properties. Compounds of different types are used as SLPs, such as hydroxydipropionitrile, diglycerol, squalane, tricresyl phosphate and eutectics [e.g., sodium nitrate (18.2%)-potassium nitrate (54.5%)-1ithium nitrate (27.3%)]. Especially high selectivity is exhibited by complex-forming phases. Thus, silver solution in ehtylene glycol is efficient for the separation of unsaturated compounds including cis and transisomers. SLPs must be thermally and chemically stable and possess low viscosity. Thermally stable selective SLPs include polyethylene glycols (upper temperature limit 225”C), cyanoethylmethylsilicones (275”C), Apiezone (300°C),methylsilicones (350°C), methylphenylsilicones (375°C) and carboranemethylsilicones (400°C). Basic parameters used in gas chromatography to characterize retention and band broadening include the retention volume, VR, the number of theoretical plates (TP) per metre of column length ( N L ) and the height equivalent to a theoretical plate (HETP) (H). The retention volume VR is that of the carrier gas passed through a column from the moment of sample injection to the output of the maximum peak solute concentrhtion. It can be expressed as VR = F C t R ,where tR is the retention time and Fc is the carrier gas velocity at a given temperature and gas pressure in the column. A more invariant measure of retention is the adjusted retention volume, V k , representing the retention volume of a solute minus that of a non-sorbed gas: V ; = ( t R - fM) F, = t R F , (3.1) where fk = tR - fM is the adjusted retention time and tM is the retention time of a component not sorbed by the stationary phase. This parameter ranges from fractions of a millilitre to
3 . 1 . General considerations
11
several litres, depending on the experimental conditions and the column diameter. Another parameter widely used in analytical gas chromatography is the relative retention value, rlJ (relative retention volume or relative retention time),
where V R ,and VKj are the adjusted retention volumes of compounds i and j , respectively, t k , - t R , - t M , t k , = tR, - tM and tk, and fk, are the adjusted retention times of compounds i and j , respectively. The extent of solute band broadening occurring as the solute traverses the column is characterized by the number of theoretical plates defined as N = 5.545(tRlWJ2, where W , is the peak width at half-height (in time units). Other parameters widely used to characterize a column include the specific efficiency, the number of theoretical plates per metre of column length ( N J , and the height equivalent to a theoretical plate (H):
N , = NIL, H = WN, H.N=1
(3.3) (3.4) (3.5)
where L is the column length. These parameters range from 1000 to 20 000 TP/m, depending on the experimental conditions (carrier gas velocity, diameter of sorbent particles). As a chromatographic band moves in the carrier gas flow, its broadening (swamping) occurs. The three basic processes causing band broadening are eddy diffusion, diffusion in the gas phase and interphase mass transfer. Eddy diffusion is independent of the carrier gas velocity and results from the path lengths for individual solute molecules being different owing to a non-uniform sorbent packing and sorbent particles being non-uniform in size. Longitudinal diffusion occurs in the gas phase and is caused by the concentration gradient associated with each chromatographic band. The finiteness of the mass transfer rate leads to the actual solute distribution being different to the equilibrium value, which also contributes to band broadening. The relative contribution of the above factors depends on the carrier gas velocity. Normally, longitudinal diffusion and the finite interphase mass transfer rate are the major factors of broadening at low and high flow-rates, respectively. Variation of the HETP with the linear carrier gas velocity is roughly described by the Van Deemter equation:
or
H
=A
+ B / u + Cu
(3.6a) (3.6b)
where A is the eddy diffusion coefficient including multi-flow-path factors, B is the longitudinal diffusion coefficient resulting from solute diffusion in the gas phase along a column, C is the mass transfer coefficient arising from the finiteness of the interphase mass transfer rate, u is the linear carrier gas velocity, 1 and y are constants, d , is the sorbent particle diameter, D, is the solute diffusion coefficient in the gas phase, k ist the capacity ratio (or partition ratio), defined as k = V',JVM, where V M is the dead volume, and dl is the effective thickness of the liquid phase film on the solid support surface. DIrepresents the solute diffusion coefficient in the liquid phase. The Van Deemter equation is helpful in optimizing the separation conditions. Figure 3.1 shows the dependence of the HETP and the role of different band broadening factors on the linear carrier gas velocity.
12
3. Gas chromatography in the analysis of air pollutants
I
_ _ _ - - - - -3 /
I
--
Fig. 3.1. Dependence of HETP on carrier gas velocity and the effect of the main band broadening factors. I Contribution of eddy diffusion; 2 contribution of longitudinal diffusion; 3 contribution of interphase mass transfer
As the solute bands are moved by the carrier gas along the column, two opposing processes occur. The distance between the concentration maxima of successive components governed by the column selectivity increases, which improves the separation, whereas the bands are broadened depending on the column efficiency, which impairs the separation. The separating ability of a column depends mainly on its efficiency and the selectivity of the sorbent used. A quantitative measure of separation of two successive components is the resolution, R , representing a dimensionless value equal to the difference in the retention volumes, AYj = VRi- VRj, divided by the sum of the peak widths at the baseline. The peak resolution is determined by the sorbent selectivity, a(= rij), the column efficiency, N , and the mass distribution ratio, D, (capacity factor, k ) .
R=-.-.-1. ( a - 1 ) 4
01
1+D,
(3.7)
Figure 3.2 gives a graphical description of the relationship between the major physical processes that occur in a capillary column and its chromatographic characteristics, such as selectivity, efficiency and capacity. It illustrates in more detail the role of the individual factors in eqn. (3.7). The carrier gases conventionally used include nitrogen, air, argon, carbon dioxide, hydrogen and helium. The carrier gas must be chemically inert with respect to the solutes and the stationary phase. It must match the detector employed, i.e., cause no decrease in the de-
t
kinetics
compound and
Fig. 3.2. Relationship between column selectivity, efficiency and capacity with the major processes occuring in the column in the course of separation
3.1. General considerations
13
tection sensitivity, and possess a low viscosity and be non-explosive (which is of particular importance for industrial use), sufficiently pure and cheap. The mobile phase in GC is compressible. A pressure drop in a column leads to the expansion of the carrier gas and increases its velocity, which affects V Rand VR.This is why in precise measurements the actual (pure) retention volume, V,, is used, which is calculated allowing for the pressure drop and is thus independent of the latter. VN = jVX (P,IPO)Z - 1 2 (Pi/P0)31
j = - 3.
(3.8) (3.9)
where j is the pressure gradient factor and Piand Po are the inlet and outlet pressures, respectively. Another parameter used in gas chromatography is the specific retention volume, V , , representing the retention volume per unit mass of the stationary phase: (3.10) where Wl is the mass of the stationary phase and T is the absolute temperature in the column. The specific retention volume is used to determine certain physico-chemical parameters such as the activity coefficient. The main purpose of the solid support in gas chromatography is to provide a thin (fractions of a micrometre or less) film of SLP to improve the mass transfer between the mobile and stationary phases. Solid supports include diatomites subjected to special treatment in order to decrease their adsorptive activity, polymeric supports based on polytetrafluoroethylene and inorganic salts. Normally, their specific surface area varies from 0.1 to 1.5 m2/g. In capillary chromatography, the inner walls of a capillary column serve as a solid support. The role played by the solid support in gas-liquid chromatography is not less important than that of the SLP. Retention in GLC is due to the absorption of solutes in the SLP and their adsorption on the SLP/solid support and SLP/carrier gas interfaces [8, 91. The actual retention volume, V N , is governed by the following parameters of the sorbent [9]: VN=K~V~+KglS~+K~K~S~
(3.11)
where K I , KgI and K , are the solute distribution constants in the systems SLP-gas, SLP surface-gas and solid support surface-SLP, respectively; V, is the column volume occupied by SLP and S, and S, are the areas of the gas/SLP and SLP/solid support interfaces, respectively. The contribution of interface adsorption to the retention value varies from 0 to 100%. Therefore, in studying solute interactions with SLPs it is essential that the fraction of the retention volume corresponding to the solvation of a component in the SLP be extracted from the overall value. Procedures have been developed that permit the quantitative characterization of all the main types of solute interactions with the stationary phase, including adsorption on the gas/SLP interface ( K g lin eqn. (3.11)) and the SLP/sblid support interface (K,) and solvation in the SLP ( K J . Gas-liquid chromatography is the basic chromatographic technique used successfully in analysing composite mixtures. Band identification is based on the retention data. The retention of a component in a mixture is compared with that of a standard. Note that given a particular sorbent and identifying a substance by means of chromatography alone one should consider the sum of retention data obtained on columns with SLPs of different types. For chromatographic identification relative retention values are used, which can be evaluated much more precisely (the error decreases by a factor of two or more). At present, the most
3. Gas chromatography in the analysis of air pollutants
14
widely accepted parameters used in analysis and identification are Kovhts retention indices. The retention index system describes the retention behaviour of a compound relative to that of n-alkanes. As the number of carbon atoms in the n-alkane molecule increases by one, the retention index, Zi, is incremented by 100 units:
(3.12) where z and z + 1 are the carbon numbers of n-alkanes eluted before and after the substance i , respectively. V k , , VX,, VR(,+ are the adjusted retention volumes of the substance i and n-alkanes of carbon number z and z + 1 , respectively. The values of retention indices depend markedly on the type of sorbent used. Thus, for example, the retention of ethyl formate on silicone SLPs is characterized by the following retention indices: 487 (OV-1), 605 (OV-17) and 766 (XE-60); the same parameters for ethyl acetate are 506 (OV-l), 632 (OV-17) and 741 (XE-60). These data indicate that the retention index is governed by the natures of both the solute and SLP employed. Parameters other than retention index and relative retention value also used in chromatography. It has been shown that the known relative values can be considered as particular cases of the following general expression [8]:
(3.13) In developing a procedure for the determination of impurities it sometimes seems desirable to calculate the required efficiency of a column when advantage is taken of the stationary phase for which the retention values of analytes are known from the literature. Such a calculation can be accomplished using eqn. (3.7). However, in the last decade, experimental data have been presented in the literature in the form of the retention index rather than separation factor (see eqn. (3.7)). In this instance the calculation should utilize the following equation [lo]: 104(i0ge)2
n = 16R2
b;(AZ)*
+I-
10Zloge b,(AZ)
(3.14)
where n is the number of effective theoretical plates, AZ = Z2- Il is the difference in the retention indices of two sorbates, R is resolution, e is the basis of natural logarithms, b, = log(t:+ l / f L ) and t : + , and t i are the adjusted retention times of two elements of a homologous series of standard compounds of carbon number z + 1 and z, respectively. The efficiency of a chromatographic column depends markedly on its type. For classical packed columns (3-5 mm I.D., 100-200 cm long, sorbent particle diameter 0.1-0.3 mm) it is normally in the range 1000-3000 theoretical plates. Capillary columns are 10-100 times more efficient, the efficiency of the open type being 30 000-100000 theoretical plates [ll-131. At present, the predominant gas chromatographic mode is capillary chromatography. In 1986 ca. 75% of the papers published on gas chromatography in leading journals such as Analytical Chemistry, Journal of High Resolution Chromatography and Chromatography Communications and Journal of Chromatography utilized capillary chromatography. The use of capillary gas chromatography is distinguished by a number of features that are responsible for its rapid development in comparison with packed column chromatography: (1) higher efficiency; (2) speed of determination; (3) high sensitivity (when using a splitless sample injection technique.and the desired components being concentrated at a reduced column temperature or using the “Grob solvent effect” [13-151; (4) better reproducibility of the thermal conditions utilizing temperature programming as a result of a smaller column diameter and weight; and (5) lower sorbent and carrier gas consumption.
15
3.1. General considerations
Table 3.1. Compounds identified in the PAH fraction of street dust from Giza Square, Egypt. Peak numbers refer to Fig.3.3 [17] Peak
number 1
2 3
4
5 6 7 8 9 10 11
12 13 14 1s 16 17 18 19 20 21 22 23 24 25
Compound
Peak number
Compound
Naphthalene Dimethylnaphthalene Trimethylnaphthalene Trimethylnaphthalene Dibenzothiophene Phenanthrene Anthracene C2-9H-Fluorene C2-9H-Fluorene Methyldibenzothiophene Methyldibenzothiophene Methylphenanthrene Methylphenanthrene Methylphenanthrene Methylphenanthrene C,-Dibenzothiophene C2-Dibenzothiophene Dibutyl phthalate C2-Dibenzothiophene Dimethylphenanthrene Dimethylphenanthrene Fluoranthene Pyrene C,-Phenanthrene C,-Phenanthrene
26 27 28 29 30 31 32 33 34 3s 36
C,-Phenanthrene C,-Phenanthrene C,-Phenanthrene C,-Phenanthrene Me thylfluoranthene Benzonaphthothiophene Benzo[ghi]fluoranthene Benzo[c]phenanthrene Benz[a]anthracene Chrysene/triphenylene Methylbenz[a]anthracene or methylchrysene Phthalate ester Phthalate ester Benzo[b]fluoranthene and benzoblfluoranthene Benzofluroanthene Benzo(e1pyrene Benzo[a]pyrene Perylene Indeno[l,2,3-cd]pyrene Dibenzanthracene Benzo[ghi]perylene Anthanthrene Dibenzopyrene
37 38 39 40
41
42 43 44 4s 46 47 48
Capillary chromatography is very popular for air pollutant determinations and readers are referred to specialized books (11-131. Currently, capillary columns rely mainly on bonded and immobilized phases. The use of immobilized liquid stationary phases (1) enhances the lifetime of the columns because chemically immobilized phases will be incapable of curling into large drops, which usually results in a sharp decrease in column efficiency and (2) increases the detection sensitivity because the immobilized phases permit the utilization of a concentration procedure o n sample/ solvent injection, e.g., using the “Grob solvent effect”. The capillary column materials and the coating materials are equally important, for the following reasons: (1) the surface of the internal column walls which serve as a solid carrier in capillary chromatography can frequently produce a detrimental effect (especially in pollutant analysis) on the chromatographic process as a result of irreversible and catalytic conversions of the compounds treated on the internal capillary surface (see, e.g., ref. [16]); and (2) the polymeric coating of fused-silica columns has a limited temperature stability. Urban air contains a variety of harmful aromatic hydrocarbons. Figure 3.3 [17] shows a chromatogram for the fraction of PAHs in urban dust (Egypt). It is natural that such a complex mixture can only be separated by the use of high-efficiency capillary gas chromatograPhY.
16
3. Gas chromatography in the analysis of air pollutants
37
39
42
41
'
I
35f80
Fig. 3.3.
~71.
120
1
160
I
200
I
24 0
I
I
280
315
L
"C
Chromatogram of the fraction of polycyclic aromatic hydrocarbons contained in street dust
Column: 21 m x 0.32mm I.D.; SLP, SE-52; temperature programme, 35 to 80°C (temperature balistically programmed), 80 to 315°C at 5 K/min. Compounds identified as in Table 3.1
Capillary columns are equally efficient in determinations of nitroaromatic compounds, which is a difficult problem. Table 3.2 shows the concentrations of this important class of organic compounds which were detected in atmospheric dust in Tokyo [18]. The use of capillary columns allows the applicability of gas chromatographic techniques to be greatly extended for environmental control purposes. The adsorption of solutes on the interfaces results in the relative retention value in the general case depending not only on the ratio of distribution constants of the solute and standard, but also on the adsorptive properties of the SLPisolid support and gadliquid interfaces. Therefore, the relative retention time and retention index are not constants for a compound when adsorption occurs. The chromatographic constant of compound i is the invariant retention index, Ioi, corresponding to the solute interaction with the SLP only, which can be evaluated in terms of the following equation [9, 191:
I, = lor + aik,,
(3.15) (3.16)
where a is a constant defining the adsorption of the solute on the sorbent used, k,,is the capacity ratio of a standard, k,, = VX,,/V,, Vk,, is the adjusted retention volume of a standard, V , is the retention volume of a non-sorbed component, Kli is the distribution constant of
17
3.1. General considerations Table 3.2. Nitroarenes identified in airborne particulates in Tokyo air [18] Compound
1 2 3 4 5
6 7 8 9 10 11 12 13 14 15 16 17 18 19
o-Ethylnitrobenzene pEthylnitrobenzene pDinitrobenzene 2,6-Dinitrotoluene 2,4-Dinitrotoluene 1-Nitro-2methylnaphthalene 4,6-Dinitro-rn-xylene 3,s-Dinitro-o-xylene 1,5-Dinitronaphthalene 1,3-Dinitronaphthalene 5-Nitroacenaphthene 2-Nitrofluorene 9-Nitroanthracene 4,4’-Dinitrodiphenyl 2,s-Dinitrofluorene 1-Nitropyrene 2,7-Dinitrofluorene 4-Nitro-pterphenyl 6-Nitrochrysene
Relative retention.) Phenylmethylsilicone
Carbowax 20M
Reference
Sample
Reference
Sample
0.340 0.432 0.564 0.611 0.696 0.795 0.796 0.841 1.13 1.16 1.18 1.29 1.30 1.51 1.61 1.68 1.75 1.77 1.90
0.346 0.439 0.568 0.606 0.690 0.792 0.798 0.841 1.13 1.16 1.17 1.28 1.30 1.51 1.60 1.68 1.75 1.78 1.90
0.391 0.470 0.745 0.727 0.773 0.783 0.811 0.847 1.46 1.49
0.388 0.474 0.739 0.727 0.780 0.786 0.817 0.843 1.46 1.50
Concentration (ng/m3)
Mutagenicity
0.056 0.009 0.017 0.006 0.024 0.005 0.011 0.078 0.057 0.12 0.11 0.050 0.27 0.071 0.19 0.14 1.5 0.16 0.27
-
+ + + k + f + + + + + + i+ + + +
*) Internal standard: Benzo[qquinoline (retention time = 16.61 min for 5 % phenylmethylsilicone column and 27.46 min for Carbowax 20M column). Analysis of nitroarenes in airborne particulates was carried out under the following conditions. Neutral organic fraction was prepared from the particulates collected from 7 500 m3 air by a high-volume sampler. After adding 1.0 pg of benzo[flquinoline to a benzene solution of the neutral organic fraction (0.5 d ) , 3 pl auf the solution were injected on to the capillary gas chromatograph (HP 5880A) by a splitless injection mode. Identification was made by comparison of relative retentions of sample peaks with those of 40 reference substances. Quantitative determination was performed using calibration graphs for each nitroarene identified. Concentrations indicate the values obtained with the phenylmethyl silicone capillary column. The values obtained with the Carbowax 20M capillary column were as follows; 0.047, 0.009, 0.027, 0.005, 0.021, 0.006, 0.014, 0.083,0.045 and 0.083 ng/m3 for the compounds numbered 1,2,3,4,5,6, 7, 8 , 9 and 10, respectively.
compound i between the SLP and the gas and K I ,and Klc,+1) are the distribution constants of two standard compounds of carbon number z and z + 1, respectively, with K , , 5 K l i < Klc,+l). To evaluate Zoimeasurements are performed on several columns differing in SLP content on the solid support. The contribution of adsorption to the retention index value may be appreciable (several to a few tens percent). In capillary chromatography, adsorption also affects the retention values [16]. At the usual low pressures, the role of the carrier gas lies mainly in transferring solutes along the column. At higher pressures, however, the non-ideality of the gas phase begins to count, which alters the distribution of the solute between the mobile and stationary phases. The chromatographic mobility of many substances increases. 3
Berezkin, Gas Chrom-BE
18
3. Gas chromatography in the analysis of air pollutants
Fig. 3.4. Schematic diagram of a gas chromatograph. 1 High-pressure carrier gas source; 2 carrier gas preparation unit; 3 sample injection unit; 4 chromatographic column; 5 thermostat; 6 detector; 7 recorder; 8 minicomputer for instrument control and data processing
High-pressure gas chromatography (HPGC) is a technique intermediate between gas and liquid chromatography. HPGC has the following advantages over the latter: (1) the possibility of directed variation of the solute retention volumes by varying the pressure over a wide range, (2) rapidity of the analysis owing to the lower viscosity of the mobile phase and higher diffusion coefficients, (3) the possible application of universal highly sensitive detectors employed in gas chromatography. However, the HPGC instrumentation and experimental procedures remaining fairly complicated, which restricts the large scale application of this technique. A particularly interesting variant is chromatographic separation with the gaseous mobile phase in the supercritical state. Supercritical fluid chromatography (SFC) has made it possible to separate porphyrins, which could not be analysed at high temperatures owing to their thermal instability. The use of carbon dioxide and ammonia in the supercritical state proved efficient in separating compounds with molecular weights as high as 40000. The unusual characteristics of HPGC prompted the following classification of gas chromatography according to the state of the mobile phase: (1) GC at mobile phase pressures of 1-25 atm and (2) GC with the mobile phase at high pressures (100-500 atm), including the supercritical state. A schematic diagram of a gas chromatograph is shown in Fig. 3.4. The carrier gas is continuously supplied from the high-pressure cylinder I to the carrier gas preparation unit 2, where it is further purified and the required parameters of the mobile phase (pressure, velocity) are maintained throughout the experiment. The carrier gas then passes to the sample input system 3, which normally in laboratory chromatographs is an independently thermostated cylindrical flow chamber provided with a self-sealing thermostable rubber septum through which a sample (1-10 pl) is injected from, e.g., a syringe (9) into the carrier gas flow at high temperature. In the input system the liquid sample rapidly vaporises and the carrier gas flow transfers it to the chromatographic column 4 located in a thermostat. The separation is usually run at 20-400°C. Special thermostats allow the separation to be performed at temperatures as low as liquid nitrogen boiling point, which is mainly used in separating the isotopes of low-boiling gases. Analytical separations are usually performed on 0.5-5 m X 0.5-4 m m I.D. packed columns and 10-100 m x 0.2-0.5 mm I.D. open capillary columns. The columns are packed with sorbents consisting either of a non-volatile liquid (stationary phase) supported as a thin coating on a solid macroporous carrier with low specific surface area (0.5-2 m2/g) or of a solid with a developed surface (50-500 m2/g). The weight of the stationary phase is normally 2-20% of that of the solid support. The mean diameter of the sorbent particles is usually 0.1-0.4 mm, but fine fractions are used when filling the column. The separation of the components of a mixture occurs in a chromatographic column as a result of the solute bands moving at different velocities. The bands are passed by the carrier gas flow to a detector 6 whose signal, which is proportional to the analyte masses, is recorded
3.1. General considerations
19
continuously by a recorder 7. A microcomputer 8 serves for instrument control and data processing. Gas chromatography is a hybrid technique: separation of a mixture into components occurs in the chromatographic column, and quantitative analysis of the eluate is performed by the detector. Hence separation and quantitative (and, frequently, qualitative) analysis are separated in time and space. Therefore, the final result of the analysis is determined by the characteristics of both the column and the detector. The detector plays a very important role in gas chromatography. Now mostly differential detectors are used whose signal is proportional to the instantaneous value of the solute concentration or mass flow. The most useful types are the katharometer (thermal conductivity detector) and flame ionization, electron-capture and flame photometric detectors. The highly sensitive ionization and flame photometric detectors are employed in the analysis of trace impurities and very small samples and in capillary chromatography. The analytical potential of gas chromatography can be substantially expanded by employing several detectors. This makes it possible to determine quantitatively and qualitatively the composition of unresolved bands containing two or more compounds, provided that the selectivity of their detection by the detectors employed is different. The use of the mass spectrometer for detection has led to the development of highly e f i cient combined gas chromatography-mass spectrometry (GC-MS). A gas chromatograph with a packed or capillary column is connected to a mass spectrometer via a molecular separator serving to concentrate the separated bands and partially remove the carrier gas. The ion sources of a number of mass spectrometers equipped with powerful high-vacuum pumps sometimes allow the direct connection of a chromatographic column to a mass spectrometer without a molecular separator, with the solutes passing directly from the column to the ion source for ionization. The flow of charged particles is then supplied to a mass analyser where separation according to mass-to-charge ratio ( m l z ) occurs. Each substance is characterized by its specific mass spectrum, reflecting its structure. The analyte is identified by comparing its mass spectrum with standard spectra measured on pure samples or available in the literature. The mass spectrometer can be used first as a highly selective detector for taking chromatograms at a fixed mass-to-charge ratio and second for the rapid monitoring of a full mass spectrum during chromatographic analysis. Computers are widely used for data processing. The sensitivity of a mass spectrometer is l O - ” - l O - ” mg/s. GC-MS is currently one of the most universal and informative instruments and it is widely used in the analysis of air pollutants. Figure 3.5 demonstrates the relationship between the various parameters and the criteria of chromatographic separation for gas chromatography. It is made in the form of two quasicircumferences one of them includes criteria of chromatographic separation, another one units the parameters of chromatographic experiment. A similar approach was described by BERRIDGE for HPLC [20]. The inner circumference indicates the following major criteria: R,, peak resolution; t , , time of separation; AP, pressure difference on the column (the pressure drop); S, response factor (peak height) for a bulky (1) and a smaller sample (2). Intermediate parameters (dotted circumference) are k’, capacity factor; a,selectivity factor; N , column efficiency (number of theoretical plates); 7 , carrier gas viscosity; ps, volume fraction of the carrier gas component with displacing properties; d , , column diameter; T, column temperature; u , carrier gas linear velocity; d , , size of solid support (adsorbent) particles; L, column length; P,,content of SLP on the solid support; MCS, weight ratio between the mixed stationary phase components. The data in Fig. 3.5 indicate a complex relationship between the various parameters and the criteria of the chromatographic method. It is recommended that the relationships are used in the compilation of the programme for the optimization of a chromatographic separation.
20
3. Gas chromatography in the analysis of air pollutants
Fig. 3.5. Relationship between the various characteristics of chromatographic separation
Gas chromatography allows one to carry out qualitative and quantitative analyses of organic and inorganic compounds whose vapour pressures at the temperature of the column exceed 0.001-1 mm Hg and which are thermally stable at the decomposition temperature. This method is widely used for determination of compounds present in samples in trace amounts (10-4-10-8%).
3.2.
Peculiarities of the gas chromatographic analysis of impurities
The difficulties associated with the gas chromatographic determination of impurities are mainly due to the fact that when in very low concentrations the latter show a behaviour different to that exerted by macroscopic amounts of the same substances. A detailed discussion of this problem has been given elsewhere [21-231. Organic pollutants are often preconcentrated at reduced temperatures in order to analyse them in air (see, e.g., ref.24). An interesting example of determining Arctic hydrocarbon air pollutants at ppt levels was described by Norwegian researchers [25]. The detection limit was 1-5 ppt. The Arctic air was shown to contain various gases of this type (C2-C4). The 'main problem to be faced in the determination of trace impurities (10-4-10-8%)in air is potential analyte losses that may accompany any step of the gas chromatographic analysis. The disappearance of trace impurities can occur when taking a sample (sorption by container walls, incomplete consumption and chemical reactions in the concentrator, etc.), during the desorption of analytes from the trap (inefficient desorption), when injecting the samples into the chromatograph (e.g., sample decomposition in the vaporizer at high temperature) and owing to sorption on or chemical reactions with the sorbents and the inner surfaces of the
3.2. Peculiarities of the gas chromatographic analysis of impurities
21
chromatographic equipment. In the latter instance we are dealing with phenomena similar to the adsorption of traces of isotopes in radiochemistry. The concentration of the active sites in the sorbent becomes comparable to that of the analytes and therefore adsorption, which is insignificant at normal concentrations, is important at micro concentrations. Overestimated results may arise from analyte vapour adsorption on the walls of the sampling valve. There are various ways of preventing such losses, but the most useful involves continuous conditioning of the column and the whole system with analytes in order to saturate active sorbent sites and the equipment surfaces. Such conditioning may take a long time (several hours), especially when analysing reactive inorganic gases or unstable and readily hydrolysed substances (nitrogen and sulphur oxides, ozone, halogens, mixed halogen compounds, hydrides, etc.) even if all the parts of the chromatograph are made of materials resistant to corrosion (nickel, PTFE, glass, etc.), and inert sorbents (PTFE, graphite, polymers based on polytrifluoromonochloroethylene) are utilized for separation [26]. Frequently, the preliminary sorbent treatment takes about 20-30 h (e.g., for nitrogen dioxide) and must be repeated (1-2 h) before each set of analyses. It should always be borne in mind that the absence of peaks of the impurities being analysed (especially when analysing reactive, unstable and readily hydrolysed substances) by no means indicates their absence in the sample or an undetectable concentration. This may be due to irreversible sorption or chemical reactions undergone by the impurities (hydrolysis, pyrolysis, reactions with sorbents, etc.). Such interactions can be minimized by taking advantage of cryogenic gas chromatography. By analysing corrosive and reactive compounds (ozone, nitrogen, halogen and sulphur oxides, fluorides, etc.) at low temperature (below - 20°C) or with a programmed increase from - 100°C until the elution of one of the components (e.g., sulphur or nitrogen dioxide) one can successfully separate the impurities of corrosive substances in the presence of moisture and low-boiling compounds (CO, C 0 2 , N2, 0 2 , etc.) without losses. One of the problems attending the analysis of impurities is the drying of air dehumidification, which is particularly important when dealing with large samples. As the amount of moisture accumulated in traps is several orders of magnitude greater than that of the concentrated impurities, analyte losses often result which requires the solutions being analysed to be strongly diluted. A widely used means of drying air consists in passing it through a dehydration plug filled with absorbents or chemicals that selectively absorb water. One of the best absorbents of this type are molecular sieves 3A, which absorb water but allow organic and inorganic impurities except ammonia and methanol to pass through [27]. The interaction of analytes with the chromatographic equipment and sorbents results in insufficient resolution when analysing trace amounts of flavours and many other compounds of natural and biological origin. One of the means of increasing the sensitivity of such analyses is to utilize packed or capillary columns and vaporizers made of fused silica, which decreases the possibility of irreversible adsorption. Another helpful approach consists in injecting a sample of thermally unstable compounds or large volume of diluted solution directly into the chromatographic column. The purity of the solvent used for the extraction of impurities form the sorbent following sample concentration is of great importance. The solvent must not contain impurities at concentration comparable to and eluted within a short time interval of the analyte substances. It is desirable that these sorbents (e.g., chlorobenzene) should be purified of impurities by ordinary distillation. Of no less importance is the purity of the carrier and diluent gases employed for the preparation of standard mixtures and the calibration of detectors. The thorough purification of cylinder nitrogen used for sorbent vaporization and trace compound analysis by GC and GC-MS involves five traps connected in series [28]. Two of them are filled with a mixture of Carbopak, activated carbon and Chromosorb W, the latter being coated with Apiezon. The
22
3. Gas chromatography in the analysis of a h pollutants
third, fourth and fifth traps are filled with Zeolyte 3A, silica gel impregnated with 20% sulphuric acid and Carbosieve S , respectively. Neglect of the above requirements may cause appreciable errors and make the results of the analysis useless. One of the significant sources of errors can be the glass-wool used as an insert or plug in chromatographic columns and splitters. The treatment of used glass-wool with n-pentane, dichloromethane, gaseous hydrogen chloride, and carbon disulphide followed by gas chromatography recovered traces of previously analysed hydrocarbons, phthalates and organic acid salts at levels of 0.3-0.005 mg/g. It is advisable for the glass-wool to be pretreated with gaseous hydrogen chloride and then extracted for 24 h with dichloromethane in a Soxhlet apparatus. It is also recommended that its surface be silylated. Another frequently occurring source of errors is gases evolved from the septa and vaporizers of chromatographs, which give rise to ghost peaks that distorts the results. The composition and concentration of these gases have been established to depend on the septum material, temperature and the period during which the septum was used in the chromatograph. The best properties are exhibited by septa made of perfluorinated elastomers and some silicone rubbers. At any step in the analysis of impurities one should take into account the possibility of thermal and exclusive desorption of the components previously adsorbed on the column or their exclusion from subsequent samples by more polar compounds ( e g , water and acids). Competitive sorption is also observed when concentrating impurities from air, the possible exclusion of one compound by others creating conditions that are unfavourable for sorption of the former, as occurs with non-polar impurities in the presence of polar substances. When the concentration of impurities in a trap containing a sorbent increases by a factor of 102-105and the molecules are within the range of the action of the sorbent, the possibility of chemical (catalytic, heterogeneous, etc.) transformation of the sample increases sharply. Sample heating during thermal desorption (150-250°C) further increases this possibility owing to potential pyrolysis and other interactions of the concentrated compounds. This substantially distorts the results of analysis and frequently makes them useless under these conditions. Catalytic reactions lead to losses of styrene when it is being concentrated on charcoal, and the analysis of atmospheric air containing alkenes and halogens (chlorine and bromine) performed in the presence of ozone results in the formation of halogenated hydrocarbons on the column sorbent. Thus, the presence of butenes and chlorine gives rise to 2,3-dichlorobutene and such reactions can occur on both porous polymers and carbon-containing sorbents. To avoid false peaks of halogenated hydrocarbons, glass-fibre impregnated with 10% sodium thiosulphate solution is installed before the sorbent. The false peaks of halogenated hydrocarbons change with the ozone and nitrogen oxide concentrations in the atmosphere. When concentrating toxic impurities from flue gas on Tenax, the sorbent undergoes partial transformation into 2,6-biphenyl-p-quinone under the action of sulphur dioxide and nitrogen oxide. The analysis of sulphur and nitrogen oxides contained in industrial emuents is accompanied by the sulphonation and nitration of porous polymeric sorbents (Tenax and XAD 21, which affects their capacity and selectivity (especially with polar sorbents). Gas chromatography has shown ihat extremely toxic N-nitrosodimethylamines may be formed in a Tenax concentration trap during sorption from air through the interaction of dimethylamine present in the air with nitrogen oxides and ozone in the presence of traces of moisture [29]. This is a serious obstacle to the concentration and quantitation of N-nitroso compounds. The false peaks of the latter are also formed when impurities are concentrated on other sorbents (charcoal, silica gel, alumina, Florisil and the sorbents treated with aqueous KOH solution, ascorbate or phosphate-nitrate buffer). The formation of “false” N-nitrosamines is particularly intensive with activated carbon and silica gel.
References
23
Account should also be taken of the direct interaction of the sorbent in the trap with the impurities being analysed, which leads to losses of the latter. This can be exemplified by the absorption of nitrogen dioxide impurities by Porapak Q (accompanied by sorbent nitration) or irreversible interaction between sulphur-containing compounds and activated carbon..The concentration of ozone and chlorine dioxide impurities is not safe because when concentrated these compounds are explosive [26]. When employing the thermal desorption of impurities from sorbent-containing traps it is essential that the temperature applied is not too high (150°Cis the optimal value), as certain compounds undergo isomerization at 250°C to yield “false” compounds of higher molecular mass. Thus, C1-C3 hydrocarbons give cis-2-butene and cis-2-pentene under these conditions 1301. It should be noted that the adsorptive activity of the inner walls of capillary columns, including the quartz type, may cause irreversible adsorption of polar analytes [31, 321. The above peculiarities in the determination of impurities should be taken into account when analysing air pollutants.
References (Chapter 3) BEREZKIN, V. G. ; GAVRICHEV, V. S.; KOLOMNETS, L. N.; KOROLEV, A. A,; LIPAVSKII, V. N.; NIKITINA, N. S.; TATARINSKII, V. S.: Gazovaya Khromatografiya v Neftekhimii (Gas Chromatography in Petrochemistry), Moscow: Nauka 1975, p. 17. GROB,R. L. (Ed.): Modem Practice of Gas Chromatography. New York: John Wiley 1985. KATZ,E. (Ed.): Quantitative Analysis Using Chromatographic Techniques. Chichester: John Wiley 1987. SUPINA,W. R.: The Packed Column in Gas Chromatography. Bellefonte, Pennsylvania: Supelco 1974. POOLE,C. F.; SCHUETI’E, S. A,: Contemporary Practice of Chromatography. Amsterdam: Elsevier 1984. LEIBNITZ, E.; STRUPPE, H. G. (Ed.): Handbuch der Gaschromatographie. Leipzig: Akademische Verlagsgesellschaft Geest & Portig K.-G. 1984. KISELEV,A. V.; YASHIN,Y. I.: Gas-Adsorption Chromatography. New York: Plenum Press 1969. BEREZKIN, V. G.: J. Chromatogr. 98 (1974) 477. BEREZKIN,V. G. In: GIDDINGS, J. C.; GRUSHKA, E.; BROWN,P. R. (Eds.): Advances in Chromatography, vol. 27. New York: M. Dekker 1987, pp. 1-35. BEREZKIN,V. G.; RETUNSKY, V.N.: J. Chromatography 330 (1985) 71. JENNINGS, W.: Gas Chromatography with Glass Capillary Columns. New York: Academic Press 1980. JENNINGS, W.: Comparisons of Fused Silica and Other Glass Columns in Gas Chromatography. Heidelberg: Huthig 1981. LEE,M.; YANG,F.; BARTLE, K.: Open Tubular Column Gas Chromatography. New York: Wiley-Interscience 1984. GROB,K.; GROB,G.: J.Chromatogr. Sci. 7 (1969) 584. GROB,K.; GROB,G.: J. Chromatogr. Sci. 7 (1969) 587. BEREZKIN, V. G.: Gas-Liquid-Solid Chromatography (in Russian). Moscow: Khimiya 1985. MASHALY, M.; SANDRA, P. In: VIIIth Intern. Symp. on Capillary Chromatography. Vol. 1. Riva del Garda, May 19-21, 1987. SANDRA, 0. (Ed.): Heidelberg: Huthig 1987, p. 476. MATSUSHITA, H.; IIDA,Y.: J. HRC and CC. 9 (1986) 710. BEREZKIN, V. G.: J. Chromatogr. 159 (1978) 359. BERRIDGE, J. C.: Techniques for the Aromated Optimization of HPLC Separations. Chichester: John Wiley 1986. BEREZKIN, V. G.; TATARINSKII, V. S.: Gas-Chromatographic Analysis of Trace Impurities. New York: Consultants Bureau, 1973.
24
3. Gas chromatography in the analysis of air pollutants
Yu.S.; BEREZKIN, V. G.: Gas Chromatographic Analysis of Pollution Air. (in Russian). DRUGOV, Moscow: Khirniya 1981. JENNINGS, W.G.; UP, A,: Sample Preparation for Gas Chromatographic Analysis. Heidelberg: HUthig 1983. BRETIZLL, T.A.; GROB,R. L.: Intern. Lab., Apr. 1985, p. 30. SCHIDBAUER, N. S.; OEHME,M.: Journal HRC and CC. 8 (1985) 404. ANVAER,B. I.; DRUGOV,Yu.S.: Gazovaya Khromatografiya Neorganicheskikh Veshchestv (Gas chromatography of inorganic compounds). Moscow: Khimiya 1976. BEREZKIN, V. G.; DRUGOV,Yu.S.; GORYACHEV, N. S.: Zhurnal analyticheskoi khirnii. 37 (1981) 319. WESTRICK, T. J.; LAMPARSKI, L. L.: Anal. Chem. 53 (1981) 22. BERKLEY, R.E.; PELLIZZARI, E.D.: Anal. Lett. A l l (1978) 327. CRIP, S.: Ann. Occup. Hyg. 23 (1980) 47. PURCELL, J . E.: Chrornatographia 15 (1982) 546. BLOMBERG, L.G.: Journal HRC and CC 7 (1984) 232.