Chapter 5 Gas chromatographic and supercritical fluid chromatographic techniques for elemental speciation

Chapter 5

Gas chromatographic and supercritical fluid chromatographic techniques for elemental speciation Peter C. Uden

5.1 INTRODUCTION The chemical analysis of materials with the goal of determining individual free elements is rarely of great practical importance except in metallurgy and mineralogy. Elements seldom exist in an uncombined, native condition or as free ionic species but occur as compounds resulting from a series of chemical, geochemical and biochemical reactions which occur in gaseous, liquid or solid phases. Although methods to determine total elemental composition are important analytically, the critical understanding of chemical elemental speciation and reaction in the biosphere is a far more difficult endeavor and calls for great sophistication of experimental design and information gathering. Elemental bioavailability, toxicity, or essentiality is not attributed to an element itself but to compounds with distinct biological, physical, and chemical properties. The question of how to acquire this molecular information from a qualitative and quantitative elemental basis requires the field of elemental speciation analysis which invariably requires a separation and isolation step prior to detection. Typically the on-line coupling of a high-resolution multi-stage analytical separation with an element specific spectroscopic determination is utilized [1]. In gas chromatography, the development of detection has paralleled advances in column separation. Bulk property sensors such as the thermal conductivity detector, which respond to changes produced by Comprehensive Analytical Chemistry, Vol. XXIII J.A. Caruso, K.L. Sutton and K.L. Ackley (Eds.) © 2000 Elsevier Science B.V. All rights reserved

123

eluates in a characteristic of the mobile phase itself, approach universality in detection, but suffer from limited sensitivity and inability to characterize the eluate species. Solute property detectors, however, measure directly some physico-chemical property of the eluate and can show high sensitivity and also give eluate identification and characterization. Spectroscopic detectors, by virtue of the different spectral properties measured, may be 'element selective', 'structure or functionality selective', or 'property selective'. From the spectroscopic viewpoint, the 'chromatographic sample' involves analytes changing in nature and in time. In high resolution chromatographies, these changes occur very rapidly thus placing rigorous demands on the analytical interface and detection system. An effective analysis must optimize both the separation and the detection processes, as well as the interface between them. Element selective GC detectors include the alkali flame ionization nitrogen/phosphorus detector (NPD), the flame photometric detector (FPD), selective for sulfur and phosphorus, and the Hall electrolytic conductivity detector, for halogens, nitrogen and sulfur, but they are limited in elemental scope in comparison with a full spectral technique such as atomic emission. Atomic emission spectroscopy is a natural choice for such interfaced detection since it can monitor all elements. The wide employment of plasma excitation sources in atomic emission spectroscopy has focused the efforts of chromatographers to employ its capacities in on-line detection. In the context of elemental speciation, interfaced chromatography-atomic plasma emission spectroscopy (CAPES) has these goals: (i) to monitor eluates for their elemental composition with high elemental sensitivity; (ii) to determine the target element with high selectivity over co-eluting elements; (iii) to tolerate incomplete chromatographic resolution from complex matrices; and (iv) to detect a number of elements simultaneously for empirical and molecular formula determination and hence enhanced speciation analysis.

5.2 ATOMIC PLASMA EMISSION GAS CHROMATOGRAPHIC DETECTORS (AED) The most important plasma emission sources for gas chromatographic detection have been the microwave induced and sustained helium plasma (MIP), and the DC argon plasma (DCP). Alternating Current 124

(ACP), capacitively coupled (CCP) and radio frequency induced (RFP) plasmas have also been used, but the argon inductively coupled plasma (ICP) has found less use because of poor excitation efficiency for some of the non-metallic elements that are of primary gas chromatographic concern. 5.2.1 Microwave induced plasmas as gas chromatographic detectors Gas chromatography with atomic emission detection with an interfaced microwave induced plasma (MIP), was described by McCormack et al. in 1965 [2]. The detector responds only when a molecule containing that element is eluted, by monitoring emission wavelengths that are specific to the particular element. In a MIP, an argon or helium plasma is initiated and sustained in a microwave 'cavity' which focuses power from a magnetron source, typically at 2.45 GHz, into a discharge cell made of quartz or other refractory material [2-4]. Power levels for analytical microwave plasmas are typically 50-100 watts, but power densities are similar to those of the ICP or DCP due to the smaller size of the MIP. High electron temperatures are produced, notably in the helium plasma, giving intense spectral emission for many elements, including non-metals which have poor sensitivity in the argon ICP or DCP. Risby and Talmi compared microwave cavities in their general review of GC-MIP [5]. Until 1989, most GC-MIP studies were carried out with laboratory built systems, owing to a lack of commercial instrumentation. The original GC-MIP was modified and improved. Helium was favored as the plasma gas, as it provides a simpler background spectrum, higher excitation energy and improved linear response range than argon. The plasma source was also developed from a reduced pressure plasma (less than 1 atm) to the helium plasma sustained at atmospheric pressure using the TMon cavity introduced by Beenakker [6]. This resonant 0 cavity improved microwave transfer efficiency and allowed the axial viewing of the plasma, instead of transversely through the tube walls whose properties change with time. The use of atmospheric pressure plasmas simplified and improved the GC detection, however, problems such as memory effects, non-specificity and low sensitivities were still present [7]. The performance of the GC-MIP has been improved with a threaded tangential flow torch (TFT) [8,9], to give a self-centering plasma which provides enhanced emission and better stability. The 125

plasma loses relatively little energy to the walls, thus atom formation and excitation are enhanced by comparison with the straight capillary torch. A disadvantage, however, is the high volume (liters per minute) of helium flow gas required. In 1989, the first commercial automated multi-element atomic emission GC detector was introduced, thereby increasing the interest and the availability of atomic emission detection [10-12]. This instrument incorporates a plasma, which is generated in an atmospheric pressure flow of helium, comprised of GC capillary column flow and a makeup of 40-200 ml min- '. The plasma is sustained in a thin walled silica discharge tube within a water-cooled microwave 'reentrant' cavity, which is a circularly symmetric cavity having its central portion narrower than the periphery. Microwave power at around 50 watts is coupled through a waveguide, removing the need for frequent microwave tuning. Additional low concentrations of various reagent gases such as hydrogen and oxygen are added to optimize performance for different elements. A purge flow in the exit chamber of the cavity eliminates deposits on the viewing window of the spectrometer, which uses a movable photodiode array in a flat focal plane spectrometer. Figure 5.1 shows a view of this microwave cavity. The modifications in the Beenaker cavity overcame some limitations of earlier plasma emission detectors. The cooling system of the discharge tube reduced problems induced by high temperatures, such Make Up Purge Flow And Reagent Outlets Ge Inlet

~

i

Cooling Water Inlet And

Capillary Colunu I

FromGC and Transfe Line

Fig. 5.1. Schematic view of AED Cavity [11]. 126

as limited lifetime of discharge tubes, interactions between analytes and active sites of the tube and high oxygen and silicon background emissions. The system has sealed chromatographic interfaces and optical path that prevents air leakage into the plasma, allowing the improvement of detection of air constituents such as nitrogen, oxygen and carbon. There is also the option of helium flow in the reverse mode, through the discharge tube towards the GC. This keeps the column flow out of the discharge tube and the plasma, thereby avoiding extinguishing the plasma when a large amount of material, such as solvent, enters the cavity. The light emitted from the plasma is detected by a movable photodiode array detector. The system uses a fixed concave grating to focus emitted radiation on a flat focal plane containing 211 diode tranducers, such that each diode corresponds to a portion of the spectrum, making it possible to monitor different wavelengths simultaneously. Recently, a new version of the GC-AED was introduced with a spectrometer with higher optical resolution, 0.25 nm vs 0.4 nm in the original version [13]. The wavelength per diode in the new fixed diode-array spectrometer is 0.08 nm/pixel compared with 0.22 nm/pixel in the original. These characteristics improve detection and increase selectivity. The new version also allows higher hydrogen gas flows that help to improve detection of some elements, and it also facilitates the use of the cyanogen molecular emission band at 388 nm to detect nitrogen, improving selectivity for that element. 5.2.2 Other plasma systems for GC detection The 'Surfatron', atmospheric pressure plasma cavity, operates by surface microwave propagation along a plasma column which may be viewed axially or transversely, since it extends outside of the plasma structure [14]. A Fourier transform spectrometer was interfaced to a surfatron MIP to obtain multi-element determination [15]. The near-IR region of 680-1080 nm was selected for simultaneous detection of fluorine, oxygen, chlorine and carbon because of high halogen sensitivity, simplicity of background and absence of interferences. A 60 Hz alternating current helium plasma (ACP) acts as a stable, selfmaintaining emission source, needs no external initiation and does not extinguish under high solvent loads [16]. The ACP operates as a microarc at power line frequency and may be the simplest and least expensive atomic source to construct and operate. As a way to reduce 127

instrumentation costs, Pedersen-Bjergaard et al. [17] reported a 350 kHz helium plasma sustained inside the end of a 0.32 mm I.D. capillary column. As a result of the small volume of the detector cell, the energy within the miniaturized plasma was very high, giving improved detection limits, and reduction of gas consumption. Skelton et al., reported a radio frequency plasma detector (RPD) operating at 50-80 W at ca. 335 kHz for sulfur selective capillary GC of fossil fuels [18]. Sulfur selectivity greater than 103 at 921 nm was obtained for petroleum distillates and coal extracts and a detection limit of 0.5 pg/s with 4 decades linear range was reported. The direct-current argon plasma (DCP) discharge is sustained by a continuous DC arc and stabilized by flowing inert gas [19]. For GC detection, a cathode jet is placed above two symmetrically placed anode jets in an inverted 'Y' configuration [20]. A 'thermal pinch' gives an arc column of between 500 and 700 watts at an operating potential of 40-50 volts. Since, unlike the MIP, the DC plasma is not constrained in a discharge tube, additional make-up argon must be introduced annularly around the GC eluent stream to direct it into the excitation region.

5.3 PLASMA INTERFACING WITH CAPILLARY GC Since eluent from GC columns is at atmospheric pressure, interfacing is simpler for atmospheric pressure plasmas than for reduced pressure plasmas. For the latter, the interface involves evacuating a sample chamber within the MIP cavity to a pressure of ca. one torr. For packed columns, little peak broadening occurs, but the volume of the chamber degrades capillary peak efficiency. The atmospheric pressure cavities such as the TMoo are very simple to interface with capillary GC columns since these can be brought to within a few millimeters of the plasma, giving minimal 'transfer volume' [21]. Helium make-up gas or other reactant gases can be introduced within the transfer line to optimize plasma performance.

5.4 ANALYTICAL INFORMATION AND SPECIATION BY GC-AED The detection limit and sensitivity for an element depends on its emission intensity at the measured wavelength and the signal to background level. Each element has many emission wavelengths and 128

the best must be chosen for analytical purposes. Sensitivity, defined by the slope of the response curve, is less often used in GC-AED than 'detection limits', given as an absolute value of element mass (in a resolved peak) or in mass flow rate units. Interelement selectivity depends on emission properties, on possible interferences, and on the resolution of the spectroscopic system. A definition is the peak area response per mole of analyte element divided by the peak area response of the 'background' element, per mole of that element. Dynamic Measurement Range of response typically extends from the upper linear analyte capacity, ca. 100 ng, to the detection limit of the target element typically in the pg range. Carbon selective detection may be thought of as a 'universal' mode of detection for organic compounds. This AED mode response is analogous to flame ionization but is more completely independent of carbon atom environment and exhibits as great or greater sensitivity. Table 5.1 [22] summarizes elemental detection limits, selectivities and linear dynamic ranges for atmospheric pressure microwave induced helium plasma capillary GC detectors. Element specific chromatographic detection alone does not provide element speciation of the analyte. However, a combination of knowledge of chemical and chromatographic behavior accompanied by accurate elemental microanalysis obtained by AED quantitation can approach reliable identification and speciation. The ideal detector for gas chromatography responds to any compound independent of the molecular structure of the analyte. This would permit the use of just one or few standards to quantify any sample, using what is called compound independent calibration (CIC), which has very important advantages for analytical chemistry. CIC reduces costs of analysis, since many of the required standards are expensive, toxic or extremely difficult to obtain, and also reduces analysis time, since preparation of multicomponent standard mixtures is not necessary. In concept, the helium plasma has enough energy to atomize and excite all the elements of the compounds eluting from the column. Consequently, the response should be related only to the element concentration in the plasma and not be influenced by the molecular structure of the compounds. Thus, CIC could be used with standards that contain the target elements. The GC-AED also has several other advantages including that compounds reaching the detector have high 129

TABLE 5.1 GC Detection with helium microwave induced plasmas (MIP) Element

Wavelength (nm)

Detection Limit pg/s (Pg)

Selectivity vs. C

LDR

Carbon (a) Carbon (b) Hydrogen (a)

247.9

2.7 (12) 2.6

1 1 160

>1,000

7.4 (20)

variable 194 9,300 25,000 11,400

6,000 500 500

Hydrogen (b) Deuterium (a) Boron (a)

193.1 656.3 486.1 656.1 249.8

7.5 (22) 2.2

21,000 500

Chlorine (b) Bromine (b)

479.5 470.5

3.6 (27) 39 10

Fluorine (b) Sulfur (b) Phosphorus (b)

685.6 180.7

40 1.7

30,000 150,000

177.5

5,000

20,000 1,000

Silicon (b) Oxygen (b)

251.6 777.2 174.2

1 7.0

90,000

40,000

25,000

4,000

6,000 >10,000

43,000 >1,000

19,000

>1,000 >500

Nitrogen (b) Aluminum (b) Antimony Gallium (b) Germanium (a) Tin (a) Tin (b) Arsenic (b) Selenium (b) chromium (b) Iron (b) Lead (a) Mercury (b) Vanadium (b) Titanium (b) Nickel (b) Palladium Manganese (b)

75 7.0

>1,000 2,000

396.2 217.6

5.0 5.0

294.3 265.1 284.0

ca. 200 1.3 (3.9) 1.6 (6.1)

>10,000

303.1 189.0

30,000

196.1

(0.5) 3.0 4.0

267.7

7.5

47,000 50,000 108,000

302.1 283.3 253.7

0.05 0.17 (0.71) 0.1

3,500,000 25,000 3,000,000

>1,000

292.4 338.4

4.0

36,000

>1,000

1.0 1.0 5.0 1.6(7.7)

50,000 200,000 >10,000

>1,000 >1,000 >1,000

110,000

>1,000

301.2 340.4 257.6

7,600 36,000

Detection limit: 3 times the signal to noise ratio. LDR: Linear Dynamic Range. (a) Conventional TM 01 0 MIP, University of Massachusetts. (b) Hewlett Packard 5921A, Hewlett Packard or University of Massachusetts. From Ref. [22]. 130

20,000

>1,000 >1,000 >1,000 500 >1,000 >1,000 >1,000 >1,000

purity and the elements in the compound can be detected with high selectivity and some of them simultaneously, thereby reducing the amount of sample required for analysis. Because of those characteristics, GC-AED has been widely advocated for the determination of empirical molecular formulas. Although accurate and reliable microanalytical methods are available for such determination, they typically require from 1 to 10 mg of pure material for each element determined and thus cannot be made for analytes that are components in mixtures, which are available in limited quantity or cannot be purified. However, the use of compound independent calibration and determination of empirical formulas has shown some conflicting results, largely due to the elemental response dependence on the molecular structure. Dingjan and deJong [23] found good agreement with theoretical empirical formulas of halogen and sulfur compounds, but precision and accuracy tended to become worse with molecular size. Yie-ru et al. [24] using a polychromator based reduced pressure MIP, concluded that the structure of the reference compound used for the empirical formula determination has a significant effect. Hydrocarbons, polynuclear aromatic, chlorinated hydrocarbons and brominated hydrocarbons were studied and the accuracy of carbon to hydrogen ratios was observed to be poor when halogen atoms were present in the compound. Sullivan and Quimby using the commercial GC-AED system [25] found response factors that varied within 2% and 7%, depending on the element determined, C, H, N or 0. They also recognize that the AED can not be used indiscriminately to determine molecular formulas, even when in some cases accurate formulas were obtained. Valente and Uden [26] evaluated the GC-AED for empirical formulas of chlorinated hydrocarbons. Significant errors in elemental molecular coefficients were found for hydrogen, probably due to concentration dependencies. Jelink and Venema [27] found that the molecular structure of the analyte influences the element response of hydrocarbons, oxygen and nitrogen containing compounds. They conclude that calibration of the system and empirical formulas determinations have to be performed with an identical molecular structure or with the analyte itself, to obtain accurate and precise results. A software algorithm was developed by Wylie et al. [28] to calibrate and measure elemental ratios for all peaks. When accuracy and precision limits the exact determination of empirical formulas, the algorithm provides a list of possible empirical formulas. Examples of 131

the application were reported for samples such as phenol mixture, pesticides, pollutants and petroleum products. Nonlinearity for hydrogen response was again observed. Kovacic and Ramus [29] reported important compound dependence of response factors for some chlorine compounds and most fluorine compounds. Nitrogen response factors did not show similar compound dependent behavior, which could have been concealed by the high background noise for this element. Reagent gases in the plasma were found to have a significant effect on fluorine response factors. PedersenBjergaard et al. [30] evaluated the C:H and C:N ratios of several compounds, concluding that the ratios are affected by both the molecular structure and the concentration of the reference standard used. However, similarity in structure did not always improve the C:N ratio determination. They also found important molecular structure dependence of elemental responses of C, H, Cl and S [31]. CIC was used to determine concentration of a reference compound, but errors ranging from 10% to 30% were observed. Accuracy was improved by using a reference standard with a similar molecular structure. Albro et al. [32] concluded that CIC for sulfur compounds is accurate and precise enough for the analysis of petroleum samples. Comparing calibration curves for different pesticides, Slowick [33] found important compound dependencies when Cl, P and N were analyzed by GC-AED. Similar dependencies were not found for S. Jandk et al. [34] reported that the AED response was affected by the molecular structure, the concentration of the compound and the elution temperature. Equations for the quantitative determination of overlapping peaks were derived and applied to a known mixture. Webster and Cooke [35] concluded that elemental responses in phenols are independent of molecular structure for chlorine substituents, but the opposite was found for carbon and oxygen. Becker et al. [36] evaluated the elemental response ratios for a series of alkylated and arylated phosphates for carbon, hydrogen, chlorine, phosphorus and oxygen. Accurate empirical formulas were obtained when the calibrating compound was closely related by structure, elemental composition, molecular weight and amount, to the analyte. Analyte concentrations larger than 30 ng were needed to obtain an efficient calibration. Herbicides were analyzed by Olson et al. [37], using GC-AED, compound independent response being seen for C1, N and S In 1995, a new version of a commercial system, the HP G2350A, was introduced and was evaluated by Szelewski for empirical formula 132

determinations [38]. For compounds of similar structure, elemental response factors were determined within 10% accuracy. 5.4.1 GC-AED detection of non-metallic elements Although many of the more interesting applications of GC-AED are for inorganic and organometallic speciation, a system must provide good analytical performance for non-metals, carbon, hydrogen, the halogens, sulfur, nitrogen and oxygen. The helium MIP has proved effective for such detection, in contrast with argon metastable energy carriers which have insufficient collisional energy transfer for adequate excitation. McLean et al. [4] used scavenger gas to prevent carbon deposits within the plasma tube and obtained limits between 0.03 and 0.09 ng/s for C, H, D, F, C1, Br, I and S with selectivities against carbon between 400 and 2300. Detection limits for O and N were around 3 ng/s. As and Sb were determined by derivatization to form stable triphenylarsine and triphenylstibine, determined by reduced pressure GC-MIP with detection limits of 20 and 50 pg [39]. Hagen et al. [40] used a multi-channel polychromator with a low pressure Evenson cavity and employed chlorofluoroacetic anhydride derivatization to 'tag' acylated amines, enabling determination through the F and Cl substituent atoms. Hooker and DeZwann reported simultaneous atomic emission and mass spectrometric detection, effluent being split within the GC oven [41,42]. Olsen et al. [43] compared reduced pressure and atmospheric pressure MIP systems for Hg, Se and As speciation in shale oil matrices, and found the latter to be superior. Efficient microwave power transfer to the plasma in cavities such as the Beenakker TMolo, allows them to be maintained at atmospheric pressure and flexible fused silica capillary columns can be interfaced to within a few mm of the plasma. These direct interfaces have been widely used, although there are advantages in incorporating a gas switching device to introduce additive gases or purge segments of the chromatographic eluates. For halogen detection, the MIP has advantages over other selective GC detectors, the electron capture detector (ECD) and the Hall Electrolytic Conductivity detector (HECD). Although it has poorer sensitivity than the ECD for polyhalogenated compounds, it has the advantage of almost uniform elemental response for each halogen, irrespective of analyte molecular structure. A major advantage over the HECD is element specificity for different halogens. 133

Oxygen-selective detection over carbon of 102 with 1000 linear dynamic range was reported by Bradley and Carnahan [44] with a TMo0 cavity. Background oxygen interference from plasma gas impurities, leaks or back-diffusion into the plasma was minimized to give oxygen detection limits between 2 and 500 ppm in petroleum distillates. Further examples of petroleum analysis by GC-AED are noted by Kosman [45] showing compound independent calibration wherein methane is used to calibrate the detector simultaneously for carbon and hydrogen. The importance of element selective detection profiles in simulated distillation applications, and pattern recognition using element ratioing to type crudes and oil spills is stressed [46]. Selective measurement of sulfur and nitrogen compounds in refinery liquids represents a challenge which the present GC-AED instrumentation can address through optimization of element wavelength, detector gas flows and GC conditions. Sulfur detection is optimized for three emission lines in the 181-183 nm region while nitrogen detection using cyanogen molecular bands in the 388 nm region provides greatly enhanced selectivity over carbon by comparison with the previously favored 174 nm atomic emission lines [47]. Instrumental advances particularly in gas flow control have also greatly enhanced the precision of retention time and peak area measurements. Figure 5.2 shows an expanded portion of the sulfur chromatogram from a gasoline standard chromatographed 15 times repetitively over a one week period, wherein precision of peak area was 1.8% RSD. Light cycle oil is an important petroleum stream and contains high sulfur and nitrogen concentrations. Figure 5.3 shows C, S and N specific chromatograms of such an oil [47]. GC-AED has been compared directly with other element selective detectors, the ECD, ELCD (Hall detector), NPD, and FPD for the determination of pesticide residues [48]. Twelve agricultural products were fortified with 10 commonly used pesticides which were then extracted. High selectivity for C, P, Cl, F, N and S allowed analysis in all extracts but interferences restricted the ECD use to five samples, the ELCD to eight, and the FPD and NPD to nine. For the samples with the greatest interferences for other detectors, onions, strawberries and alfalfa, AED provided the most selectivity and sensitivity. The enhanced performance and repeatability of the new generation of GC-AED [13] has enabled a standard method for pesticide analysis to be recommended by the US Environmental Protection Agency (EPA) as method 8085 [49]. Low and sub-ppb levels of pesticides are detected in 134

%rsd total S area= 1.8 %

Counts

60 50 40 30 6.5

6.25

7.5

6.75

20

10 on

1.,

1

5

2.5

0

LLLiII-,

i

7.5

'''

'

20

15

10 Time (min)

Fig. 5.2. Chromatograms from standard gasoline, run 15 times over a one week period. Traces are offset to zero, overlaid, percent rsd of total sulfur response area = 1.8% [47].

~~~

isl-~~~~~~~~~~~~~~~~ i~~~~j-c-

I'A2, i

-

=t

Carbon

6130 ppm

total Sulfur Sulfur 634 ppm

total Nitrogen

Nitrogen q

5

10

-~~* 20

15

" 25

30

Time (min)

Fig. 5.3. Carbon, sulfur, nitrogen chromatograms of Light Cycle Oil [47].

135

solid and water matrices. A four-laboratory round robin study was run on two spiked extract samples, the matrix extract consisting of a soil extract that was diluted to approximate the background levels expected in a surface water extract. Eighteen common pesticides were spiked at the 0.4-10 ppb level and recoveries ranged from 88-104% with less than 20% RSD. A powerful analytical procedure for screening and identifying 567 pesticides and suspected endocrine disrupters has been developed using GC-AED and retention time locking (RTL); precise retention data and elemental content of the target analytes is followed by elemental ratio calculation and GC-MS confirmation [50]. Figure 5.4 shows a sample GC-AED chromatogram with nitrogen, phosphorus and chlorine detection, nine pesticides being determined in green onion extract using retention time locked conditions. Sandra has shown the utility of GC-AED for environmental air pollution applications [51] determining highly toxic carbonyl sulfide in the presence of high levels of hydrocarbons. Vapor samples were injected onto porous polymer-coated capillary columns for sulfur determination at the ppb level. 1. Dichlorvos 2. 2,4,5.Trichlorophenol 3. Propoxur

3

4. Trichlaronaphthalene 5. Chlorothalonil 6. Chlorpyrifos-methyl

7. Folpet 8. Mirex 9. Prochloraz

N 174

1

I 1

I

I

2

a,

I

4

, P178

I

§

I

_

_

I

8

I

_,

E

10

I I

12

I

I

14

Time (min)

Fig. 5.4. Nitrogen, phosphorus, chlorine chromatograms of green onion extract under retention time locked conditions [501. 136

9

m 11

A A-

AA

0 25,000

0

20

10

30

time (minutes)

Fig. 5.5. Organoselenium compounds in elephant garlic determined by Headspace GCAED. Selenium peak identities: 4, dimethylselenide; 5, methanesulfenoselenoic acid methyl ester; 6, dimethyldiselenide; 7, bis(methylthio)selenide; 8, allylmethylselenide; 9, 2-propylsulfenoselenoic acid methyl ester; 10, 1-propylsulfenoselenoic acid methyl ester; 11, (allylthio)-(methylthio)selenide. Structures confirmed by mass spectrometry [52].

Selenium speciation by GC-AED has afforded broad insights into natural abundance and enhanced level organoselenium volatiles present in and generated by genus Allium plants, notably garlic and onion. Headspace-GC techniques with carbon, selenium and sulfur detection have shown the presence of methyl and allyl selenides, diselenides, thioselenides and related simple molecules whose presence has been related to biosynthetic pathways involving selenoaminoacids with substantive anticarcinogenic character [52]. Figure 5.5 shows a typical headspace GC-AED chromatogram of elephant garlic. The use of GC-AED for characterization of complex pyrolyzates of organic geochemical samples was demonstrated by Seeley et al. Element specific pyrograms were obtained for a Monterey kerogen [53], Fig. 5.6 showing a portion of an arsenic selective chromatogram, in which high element selectivity leaves no doubt as to the authenticity of the organoarsenic compounds. 137

->

SI t

(b) .

60

40

20

0

5

10 Time/min

15

20

Fig. 5.6. Carbon (a, 193 nm), arsenic (b, 189 nm) selective chromatograms from Monterey kerogen pyrolyzed at 800°C [53].

The ability of GC-AED to differentiate and quantify isotopes of elements such as hydrogen, carbon, nitrogen and oxygen has potential for stable isotope studies. Deruaz et al. [54] monitored H, D, S and I in a study of the purity of deuteroiodomethane used for derivatization of immunoenhancer drugs. Quimby et al. [55] and Sullivan [56] reported the ability of GC-AED to determine 13C selectively over 12C, utilizing the molecular emission from intense CO bands in the vacuum ultraviolet region. Figure 5.7 shows 13C and 12C selective chromatograms 12

c

A,

, 4

. 6

a

.

.1...

10 Time (min)

... 12

14

16

Fig. 5.7. Carbon-13 determination by molecular emission detection of 30 pg of C-13 nitrobenzene, spiked into urine, extracted [55]. 138

where [ 3C] nitrobenzene was spiked into urine at 3 ng ml- '. Thirty pg of the 3C compound is readily detected. The instrumental recipe is set to measure selectivity over 2C taking into account the natural abundance (ca. 1% ) of 13C reported as >2000.

5.4.2 GC-AED detection of metallic elements GC-AED is valuable in confirming elution of metallic compounds and in acquiring analytical data, since metal elemental detection is often more sensitive and selective than for non-metals. Many volatile binary metal compounds, organometallics and metal chelates can be gas chromatographed [57], although in some cases the absence of confirmatory elution data based on elemental content of eluates has cast doubt on GC authenticity. GC-AED may serve to provide such. 5.4.2.1 GC-AED of main group metallic compounds. As shown in Table 5.1, GC-AED data is available for many main and transition group metals. Among the former, tin, lead, and mercury have been the most studied as they are determinable with TMo10 cavities with sub-pg/second detection limits. Trialkyllead chlorides extracted from an industrial plant effluent were reacted with butyl Grignard reagent to form trialkylbutyllead derivatives [58]. The high interference from carbon compounds prevented any determination of the trialkyl lead compounds by GC-FID, GC-ECD or GC-MS without extensive clean-up and loss of analyte. Lobinski and Adams [59,60] developed speciation of organolead compounds in water at the sub-ng dm 3 level. Sample preparation involved extraction of ionic organolead species as dithiocarbamate complexes, followed by their propylation by Grignard reaction. Sample injection to the capillary column was by concentration on a Tenax trap. Figure 5.8 shows a mixture of propylated lead species present in tap water samples and polar snow samples at the ng/l level. These authors published a widely cited study in which the lead content of wine vintages bottled over a period of more than 30 years was followed by derivatization GC-AED and shown to bear close correspondence with the industrial and commercial usage of organolead compounds during that period as shown in Fig. 5.9 [61]. Emteborg et al. [62] developed a method for simultaneous determination of methyl and ethyl mercury and inorganic mercury by dithio139

I

(a)

I , c

I To

(d)

Ic) E6

Mu

4 4

5

3 3

2

4 2

2

2

2

0

T 6

7

8

9

6

7

a

9

Retention time/min

Fig. 5.8. Chromatograms of propylated organolead compounds: (a) blank from derivatization of pure hexane with propylmagnesium chloride; (b) blank from complete analytical procedure, (c) chromatogram from tap water, (d) chromatogram from polar snow sample. Peaks: 1, trimethyllead; 2, dimethyllead; 3, triethyllead; diethyllead [59].

carbamate resin concentration in a flow injection system, followed by butylation. Using water sample volumes of 500 ml, a detection limit for methylmercury of 0.05 ng/l was obtained. Figure 5.10 shows response from a 20 ml water sample from the 100 m level in the Gulf of Bothnia, showing 0.35 ng/l-l of methyl mercury. Lobinski et al. also reported parallel derivatization procedures for organotin compounds allowing detection at about 0.05 pg [63]. Tributyltin halides have been determined by GC-AED using solid phase extraction (SPE) concentration from sea water, followed by on-line conversion to the corresponding hydride using solid sodium borohydride reagent [64]. Tin interferences in the chromatogram were found to be produced by volatile residual organotin compounds in the SPE cartridge; these could be removed by solvent treatment. 140

DUU

_Lead phased out

c 400

_

300

r

Tetra

introd gasol

200

a 100

_ I

I

i

i

1950

o

Year

Fig. 5.9. Lead content of French wines as a function of vintage year, measured as trimethyllead by GC-AED [61].

200

o 150

E E 100 E o

Li

50 so

t

I

a

rz

0 o

Time/min

Fig. 5.10. Methylmercury measured as ethylated derivative (methylethylmercury) at 0.35 ng/l at 100 m water depth in Gulf of Bothnia [62]. 141

5.4.2.2 GC-AED of transitionmetal compounds Much GC emphasis has been placed upon chelating ligands of 2,4pentanedione (acetylacetone) and its analogs [65,66]. Black and Sievers [67] used GC-AED to determine chromium as its trifluoroacetylacetonate (TFA) in blood plasma with good quantitation and precision. GC-AED is used to study ligand redistribution and reaction kinetics of gallium, indium and aluminum chelates [68]. Tetradentate Schiff base ketoimine chelates of divalent transition metals show excellent GC behavior and quantitative elution. GC-AED selective chromatograms for copper, nickel and palladium and vanadium complexes of N,N'-propylene-bis(trifluoroacetylacetoimine) are shown in Fig. 5.11 [69]. The distorted profile of the vanadium specific chromatogram is due to the presence of stereoisomers of the oxovanadium chelate for which interconversion occurs on the GC column. A characterization study of platinum chelated was recently completed [70]. Vanadium specific GC-AED has also been of value in fingerprinting of metalloporphyrins in crude oils [71]. Figure 5.12 illustrates the selectivity of vanadium detection against a carbon matrix for a fraction extracted from Boscan crude oil. Detection using two independent wavelengths at 269 nm and 292 nm confirms the identity of the vanadium profiles in the two spectral regions. Many pi-bonded organometallics such as metallocenes elute well in capillary GC and provide good model compounds for evaluation of 1200 1000

Ij

Cu 325 nm

.

00 Pd 340 nm

0

· 400 V 292 nm 200 0

Ni 301 nm 2

6

4

8

Timelmin

Fig. 5.11. Copper, palladium, vanadium, nickel selective chromatograms of N,N'-propylenebis(trifluoroacetylacetoneimine) complexes [69]. 142

UJ

Fig. 5.12. Carbon, vanadium selective chromatograms of extract of Boscan crude oil [71] .

detection for their constituent elements. Sensitive and selective detection of iron, cobalt, nickel, chromium, manganese and vanadium in a series of cyclopentadienyl carbonyl/nitrosyl compounds was obtained, verifying elution of some previously unchromatographed compounds [72]. The manganese organometallics used as gasoline additives have been determined by GC-AED using a DC Plasma system [73] and GC-MIP may also be used if solvent removal is carried out following sample injection. The desirable analytical characteristics of metal specific detection are very clear, and in general, are limited far more by any shortcomings of GC column behavior than by spectral detection limitations.

5.5 PLASMA DETECTORS FOR SUPERCRITICAL FLUID CHROMATOGRAPHY Although analytical SFC was introduced in the early 1960s, the availability of high resolution packed and capillary SFC columns has led to renewed interest in the technique. SFC along with supercritical extraction (SFE) can facilitate when neither GC nor HPLC may be possible. Detector development for SFC has been in two main directions; for methodology derived from GC, the flame ionization detector has been favored, while for development from HPLC, the UV/ visible spectrophotometric detector is preferred. Plasma detection is a natural development because of its use in both GC and HPLC. An initial report described an ICP interface with close to 100% atomization 143

efficiency [74]. Shen et al. [75] coupled SFC with ICP-MS for the determination of tetraalkyltin compounds. Detection limits were 0.034 pg for tetrabutyltin and 0.047 pg for tetraphenyltin. A surfatron helium MIP was described for SFC detection, giving sulfur-specific detection at 921.3 nm with a 25 pg/s limit for thiophene [76]. Spectral characterization was carried out for two common SFC mobile phases, carbon dioxide and nitrous oxide [77]. Skelton et al. used a radio frequency plasma for detection of S and Cl in the near infrared region, with 50-300 pg/s detection limits [78]. Webster and Carnahan coupled SFC with a 500 watt helium MIP, the near infrared region being used for Cl and S and the UV-visible region for detection for C and H. Detection limits of 0.8 ng/s were obtained for S using carbon dioxide [79]. The authors compared 500 W and 60-150 W helium MIP for packed column SFC, peak areas remaining unchanged between 150 and 250 atmospheres pressure [80]. Interference in plasma excitation by SFC solvents may be less troublesome than for typical organic HPLC solvents and element specific detection and speciation by atomic plasma emission affords a viable option.

5.6 ATOMIC PLASMA MASS SPECTRAL GAS CHROMATOGRAPHIC DETECTORS While atomic plasma emission spectral detection has been widely adopted for gas chromatography, the capability to use the mass spectral ion profiles of such plasma for element selective detection and elemental speciation is a substantive one and is discussed in this section. The most widely adopted atomic plasma mass spectral analytical technique is ICP-MS [81-83] in which an argon ICP, comprising ionized argon at 8000-9000°K, acts as a mass spectral ion source. For a dissolved analyte, after aerosol formation in a nebulizer and spray chamber, it is injected into the plasma where it is desolvated, vaporized, atomized and ionized. Ions are sampled through a watercooled metal cone, separated from most of the argon and directed, through a low pressure interface (10-5 torr) into the mass spectrometer. ICP-MS combines features of ICP-AES such as multielement analysis, wide dynamic range and speed, with mass spectral acquisition, enhanced detection limits and isotopic analysis. Elemental speciation 144

of chemical form is not directly accessible but introduction techniques incorporating prior separation and/or concentration provide a major analytical advantage. Flow injection, analytical, liquid and gas chromatographic interfaces and applications for ICP-MS were comprehensively reviewed [84] and there is now an extensive literature on HPLC-ICP-MS. Interfacing of GC with ICP-MS is inherently simpler than for HPLC since no solvent removal with concomitant analyte losses is required. However, development has been limited until recently for various reasons, the wide adoption of GC-MIP, the limited use of GC-ICP itself and the relative insensitivity to non-metallic elements all contributing. However, coupling packed column GC with ICP-MS was early reported with detection limits from 0.001 to 400 ng/s for nonmetals including halogens, silicon, phosphorus and sulfur [85]. Trace metal speciation analysis at low pg/s levels has now become more widespread and Fig. 5.13 depicts a 12 0Sn ion selective chromatogram of tetraalkyltin compounds at low pg levels from a harbor sediment [86]. A low-pressure ICP can be used for both molecular and atomic mass spectrometry in the interfaced chromatographic mode [87], speciation being obtained for lead, tin, iron, chlorine, bromine and iodine compounds in the range from 13 pg (Pb) to 500 pg (Cl). Lower power and plasma gas flow enabled a helium plasma to be sustained using only carrier gas from the gas chromatograph, and mass spectra were obtained for halogenated compounds similar to those obtained for electron impact MS. Adjustment of the plasma gas flow and forward power influenced the fragmentation of the organic species. c

300

D IA

Q. ._

100-

(I t Bil 2

4 6 8 Retention

1 ii2 Time

4 6 18 (mins.)

0 Fig. 5.13. 12 Sn ion selective GC-ICPMS of tetraalkyl tin compounds from a harbor sediment. Peaks: A = SnEt4 ; B = BuSnEt 3; C = Bu 2 SnEt 2; D = Bu 3 SnEt.

145

As improved transfer line interfacing has accompanied general improvement of performance characteristics of the ICPMS, sensitivities for metal speciation have been substantially improved and now exceed those obtained by GC-MIP for a number of elements. Analyte condensation during transfer must be avoided as metallic transfer lines must be, an aerosol carrier make-up gas must be used to achieve adequate flow to transport the analytes to the central plasma channel. Also simple coupling and decoupling with the GC is important. A number of such devices have been described. Bayon et al. [88] used a simple demountable T-piece interface applied to a quadrupole ICPMS instrument and obtained detection limits between 50 and 100 fg for organotin compounds. De Smaele et al. [89-90] developed a similar practical system involving a concentric tube interface and applied it successfully to aqueous in-situ derivatization of organomercury, lead and tin compounds. Wasik et al. [91] developed an automated accessory based on a constant temperature multicapillary GC with cryofocussing to provide rapid-pulse sampling and achieved detection in the 150 fg range for organomercury compounds. Cryofocussing at -175°C was also successfully used for organoarsenic, indium and gallium compounds to the 100 fg level. Armstrong et al. compared GC-ICPMS and GC-AFS (atomic fluorescence spectroscopy) for methylmercury measurement in standard reference materials [92]. Packed capillary columns interfaced with a micro-concentric nebulizer provided another viable interfacing method [93]. The considerable advantages in sensitivity and selectivity of a double focusing sector field ICPMS for GC interfacing were reported by Prohaska et al. [94] with sensitive application to organoarsenic speciation. Argon plasmas have not solely been employed for GC-MS interfacing; Waggoner et al. [95] described a low power reduced-pressure helium ICPMS source and obtained organotin detection to the 100 fg range. Shen et al. [96] evaluated ICPMS for SFC detection for organotin detection and showed feasibility, obtaining detection limits for tin in the 50 fg range with three orders of magnitude linear range and better than 5% RSD. The general detection limits seen for tin, lead, mercury and arsenic by GC-ICPMS appear to be an order of magnitude better than those obtained optimally by GC-MIPAED at its present state of development. This factor should ensure the continued development of this technique 146

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