Element Selective Detectors in Gas Chromatography Since the Sobel Prize winning work of A. J. P. Martin in 1952, gas chromatography has grown into one of the most powerful molecular sorting and identification techniques available to the analytical chemist. Most commonly. qualitative identification of a n eluted species is based on its characteristic retention volume as indicated by an essentially nondiscriminating thermal conductivity or flame ionization detector. However, many ssmples. particularly those originating from environmental or biological investigations. contain so many constituent compounds that the resulting chromatogram is a complex maze of peaks. Often, the analyst is interested in only a few of these peaks and is faced with the problem of determining which they are and how he can eliminate interferences from nearby overlapping. or even obscuring. peaks. One method of peak discrimination is to employ a detector system which responds selectively or characteristically to some property of the eluted species such as its atomic or molecular spectral emission, its electrochemical activity in solution. its mass spectrum, its biological activity. or even its odor. Ideally. such a detector must be sensitive, quantitative, and have a rapid response time, but with these provisos the choice of selective principle may be largely u p to the ingenuity of the analyst. For example, male gypsy moths can be employed as selective (in this instance. specific) detectors to identify the gas chromatographic peak corresponding to the female gypsy moth sex attractant. Such a n arrangement clearly epitomizes the concept of selective detection. The most common selective detectors in use today respond to the presence of a characteristic element or group present in the eluted species. A 1184A
0-Rings Bulkhead Mirror Housing ,Exhaust Tube Windows-
-
-
Window Housing Radiator Filter Housing
--
-
igniter
=3
Collector
Burner
'r7-L
Filter
___-
-
I@"
-
COIumn Effluent -1 - l
I
Figure 1. S c h e m a t i c for Tracor FPD ( 5 )
much higher degree of specific molecular identification can, of course, be achieved with on-line mass spectrometry (GC-MS) or Fourier transform infrared spectrometry (GC-FTIR). but the necessary instrumentation is expensive and not widely available t o most gas chromatographers. Consequently, this article will deal only with element selective detectors, and the reader is referred elsewhere for discussions of GC-MS ( I ) and GCFTIR (2, 3 ) .
Flame Photometric Detectors (FPD) As its name implies, this detector is essentially a flame photometer. The eluted species passes into a flame (usually H 2 / 0 2 ) which supplies sufficient energy first to produce atoms and simple molecular species
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
and then to excite them to a higher electronic state. The excited atoms and molecules subsequently return to their ground states with emission of characteristic atomic line spectra or molecular band spectra. By monitoring a selected emission wavelength, a phototube signal reproduces the chromatographic peak of interest. Flame photometric detection of a gas chromatographic effluent has an intrinsic sensitivity advantage over conventional flame photometry of solution samples ( 4 ) . This is because all of the energy available in the flame can be utilized for the atomization and excitation processes since none is required for vaporization of the sample. Consequently, detection limits in the subnanogram range are often accessible. A schematic diagram of a typical
Report
David F. S. Natusch and Thomas M. Thorpe School of Chemical Sciences, University of Illinois, Urbana, 111. 61801
Selective gas chromatographic detectors accrue advantages not found with nondiscriminating thermal conductivity or flame ionization detectors, especially in analyzing complex environmental or biological samples
FPD is presented in Figure 1.Commercial FPD’s employ a narrow bandpass filter to isolate the appropriate analytical wavelength range (6).A mirror or lens focuses light from a large cross-sectional area of the flame onto the filter, thereby increasing sensitivity and reducing the influence of flame variations, but also appreciably enhancing the detection of flame background emission. Consequently, the mirror is eliminated from some recent detector designs. A variation on the detector construction illustrated in Figure 1 is employed in the Bendix FPD which utilizes fiber optics to isolate the phototube from the flame. Although this is generally considered to be an advantage. problems have been encountered owing to hightemperature breakdown of the cement retaining the fibers (7). Better cements are now being used, but the initial section of fiber optics can readily be replaced by a rigid glass rod (7). Stability of the FPD is usually limited by flame flicker ( 8 ) .and early systems were notorious for the ease with which the flame could be extinguished by large solvent injections or by high carrier gas velocities ( 4 , 9). T o obtain maximum reproducibility, therefore, the carrier and flame gas flow rates must be carefully controlled, and where large sample volumes are injected, it is often necessary either to reignite the flame after solvent passage or to vent the solvent past the detector. The most highly developed FPD’s are selective for phosphorus and for sulfur. These elements are detected by monitoring narrow band emissions from the simple molecular species H P O and Sz at 526 and 394 n m , respectively. Detector response to phosphorus compounds is linear; however, because of the presence of two sulfur
atoms in S z , the response t o compounds containing a single sulfur atom is proportional to the square of the compound concentration (6, 10). This characteristic provides a useful means of determining the number of sulfur atoms present in an eluted species, although caution should be exercised since the square relationship often does not hold precisely (6, 10). The FPD response to sulfur and phosphorus atoms is commonly on the order of 10,000 times that elicited by hydrocarbons (9).For example, a thousandfold excess of methylphenylacetate eluted simultaneously with triethyl phosphate produces no interference in the detection of the latter species ( 6 ) .Discrimination between sulfur and phosphorus is, however, less impressive. Thus, whereas phosphorus is detected about one-hundredfold less sensitively t h a n sulfur when the detector is operated in the S mode, sulfur is only detected fourfold less sensitively t h a n phosphorus for P mode w e r a t i o n (9, 11).This differential cross response, which arises because the band spectra of H P O and SZ effectively overlap by different amounts within the bandpass of the two filters, means that sulfur-containing species may act as interferents in the detection of phosphorus compounds. Such influences can. however, normally be overcome by suitable choice of column parameters. Selectivity can, of course, also be improved by using a monochromator for wavelength discrimination in place of filters. Sensitivities of the FPD for sulfur and phosphorus are essentialy comparable as indicated in Table I, and detection limits are normally about an order of magnitude lower t h a n can be achieved for these compounds with a flame ionization detector. Some rep-
resentative detection limits are listed in Table 11, although it must be recognized that at these low levels, detection is generally limited by column adsorption ( 1 4 ) . In addition to their utility as element selective detectors for sulfur and phosphorus, dual FPD’s can be employed to provide information about the relative numbers of sulfur and phosphorus atoms in a compound (15).In this application a single flame is viewed by two phototube heads, one selective for sulfur and the other for phosphorus, and the response ratios, R p / x determined for each peak. These response ratios range from 5.2 to 6.0 for compounds containing PS, from 2.8 to 3.3 for compounds containing PS2, and from, 1.7 to 2.3 for those containing PS3
x
(15).
The major field of application of GC-FPD systems has been in the determination of pesticides and pesticide residues containing sulfur and phosphorus. For such analyses, the high sensitivity and selectivity of the FPD (Table 11) give it superiority over flame ionization or electron capture detectors. The FPD has also been used to detect gaseous sulfur compounds in air (14, 16), and a number of committed instruments utilizing the detector have been designed for this purpose ( 1 6 ) .T h e performance of that designed by Stevens et al. ( 2 4 ) is particularly impressive for automated determination of H2S and SO2 in ambient air. Detection limits are 2 ppb for HzS and 4 ppb for SOz. Although not commercially available, flame photometric detectors have been designed to respond to a number of elements other than sulfur and phosphorus. They have been used to detect a variety of volatile metal salts and chelates ( 4 , 17,organics ( 8 ) ,and silylated species ( 1 8 )(Table
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1 4 , D E C E M B E R 1973
1185A
Table I. Flame! Photometric (Trimode), Alkali Flame Ionization, and Electron Capture Detector Comparison (7 2) P-mode
Parameters
-
Photomultiplier d a r k current, amps Background current; flame on, amps Base line noise; f l a m e on, amps Detectability, g/sec
S-mode
ECD (3H)
AFlD (CsBr)
Halogen-mode
4.10V9
4.10-5
4.10-8
...
6.10-8
2.10-8
3.10-7
3.10-5
5.10-10
2 ' 10-10
1.10-8
3.10-12
3.10-8 standing current 8.10-12
1.1.10-l2 methyl parathion
8.10-1Imethyl
1.1.10-10 aldrin
2.5.10W3 methyl parathion
1.10-13 g/ml aldrin
parathion
...
-5.10-14
1 . 3 . l O - l 3 atomic P 9.10-12atomic S None, calibration Linearity, slope of 105, slope 1.01 straight line only, log-log p l o t rosponse slope 1.1 vs. concentration Response to S-comSpecificity Response t o Ppounds -340 times com po u nd s -150 times t h a t for P-compounds. No ret h a t for S-comsponse for halopounds. No response for halogenated comgenated compounds pounds 10,000 (9) Specificity factor, 10,000 (9) with respect to hydrocarbons Gas flows, m l / n i i n N,, GC c o l u m n carrier 35 H? -220 Premixed, lower flame Air -220 Air -500 Upper flame only Flame extinguishing, >10 PI sample injection
6.10-11 atomic GI
3.10-14 atomic P
5.103, slope 1.01
--lo3
Response t o halo- Response t o P-corn- Response t o genated compounds -1000 all electronpounds (CI) times that for capturing -100 times the S-compounds, compounds interference N-compounds, signals from Sa n d halogenated and P-comcompounds pounds 15,000 parathion ( 9 ) IO7,aldrin ( 9 ) 10,000(73)
40 (He) 20 160 >10
I11 i . \\.a\.eiengt h isolation is norrriall>-
a> gmt3 a- t hi)>eot electron i.;ipt lire
;ichie\etl with ii monochromatiir \vhic,h provitie> hetter helectivity t h a n ii tiller h u t increa:;es the cost and
detectorh I 121.
taken [ i f atomic line spectra where they occur. Thu5. the inten-ity 01 t h e lead line a t 405.6 n m enable> high1)- selecr ive detection 01 lead alkyl- at the parts per hillion level as injcc~tetii2Oi. X \.ari,ition on the iia3ic tlame phoioiiiet ric Iirinciple ha- A o been p x 1 ) l o i t e d tor detection I J ~ ' halogen ccin~ i o u i i d I~n. thi- .-ybteni a pelier oi S i i 2 S O . $ ( 2 1I [ir indiun: metal ( 12, ;;]I i s ~ i o ~ i t i i ~at ~ ithe e t l tlame tip as in t hi. ,ilknli t'lame iiinizatioii detectur i l e>c r i t i e I: 1le1IJ w. C iini 1x)un& contain in:: ('I. H r . anti I niodii:\- sodium or intlium ;ironiic line >pectral emission3 \vhic,h are monitored. rr>pectively.at .-M;ind :I60nm.T h e intrinhic senrii i v i t y i'or h a oge11:~in this mode i b i~cinil)araiileto that tor conientional (let ect ion of buli'ur a n d p h o z p h o r u s I j?J. hut practical detection limits -omen hat higher iiwing to high t ' l a m e hackprouncl ('ralile I I . T h e d e tec,tiir ha3 the advantage that it can h e uwd in a dual mode for detection osphc sruz. and halogenpevie,,. although its haloxen tletwtion characteristic- are not 11, lie
1186A
Microwave Plasma Detectors (MPD) I n ~pectrci.cc)picprinciple. the microwa\-e plasrna detector is eswntially similar t o the Ilame photometric deTable li. Representative FPD Detection Limits for Sulfur and Phosphorus-Containing Compounds Compound
Species deDetection tected limit
1.1x
Methyl parathion
Parathion DEPP (diethyl phenyl phosphate) DEPPT (diethyl phenyl phosphothionate) Ro-Neet Malathion
so, HzS (CHdzS CHISH
P P
10-12
Ref
12
glsec 8 x 10-11 12 glsec 1 x 10-11 6 glsec 2.3 x 10-13 7 g P/sec
1.6 x 10-18
7
g P/sec
S
g/sec a Id ri n =3.10-14 atomic CI -5.102
5 x 10-11
7
g S/sec 0.25 n g 0.06 n g 0.03 n g 0.06 n g 0.28 n g
6 14 14 14 14
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14. DECEMBER 1973
40
... ...
PI
tectcir since hoi h monitor a characteristic high-temperature line or band emission. In the MPD. however. t h e relatively low energy t'lame of the FPD is replaced h y much higher energ>-micron.a\-e plasma excitation. T h e plasma is e s t a b l i ~ h e dhj- irradiating a n inert cdrrier ga?. such as argon or helium. flowing through a tuned microwave c a v i t y . Free electrons in this plasma ac,quire hufi'icient kinetic energy trom the elect romagnetic, field in the ravity t o cauhe tragmentation of molecular species eluted into the plasma and to excite these fragments. The, obser\.ed spectra arise principally from i,ither diatomic molecule> or free atoms. although emission also result> from home recomhination reactions in the pla?ma I 2 2 ) . A lilock diagram ot a C;('-l\IPD i:, s h i w n in Figure 2 , 'I'he GC'column is ciinnected to a quartz discharge tube in the microwave cavity Lvhich is powered by a microwave generator operating. most ccimmonly. at 2450 LIHz. T h e micro~vavedischarge can he sustained either at atmospheric pressure (with argon as t h e carrier gaa I or at reduced preshure ( l v i th helium, ( 2 2 .2.9i. T h e main limitation of the l I P D is clue t o widely distrihuted hpectral int ert e re n c ea from e fi'i c ie nt 1y excited species >uch a > C S . ( ' 2 . and CH.
'r ht,se c(In t ri bu t e signi f'ican t 1y to the
with respect to hexane is not as good as that o f t h e FPD because o f t h e greater number of interfering spectra emissions, These background emissions also limit the excellent intrinsic sensitivity of the MPD.although detection limits are commonly in the nanogram or picogram ranges. Thus: while its utility and potential as a multielemental detector are established. it must be stressed that the SIPD requires considerable optimiza tion for detection of individual analyte species. Rapid switching among range of elements cannot. therefore, be achieved without sacrificing perfor m an ce . A s with the FPD.most analytical studies using the SIPD have been directed to analysis for pesticides and drugs ( 2 4 ) .However. recent concern about contamination of living organisms with methylmercury and dimethylmercury has seen applica-
spectral background and necessitate t h e use ( ] f a monochromator for navelength srlection in place ot'the simpler narrow bandpass filters e m plo:.the selectivity ratio
Microwave Cavity \ Microwave Generator
Quartz Discharge ,,Tube
Quartz /Lens
H= 7- - -
-'
-11-
Monochromator
7
Amplifier
Inlet
Recorder
Injection ' Port
Quartz
' 6
Micrwave Cavity
Figure 2. Schematic of GC-MPD system ( 2 2 )
Table 111. FPD Detection Limits (19) Sample
Band or line, A
TiClp ASCI, ZrCli Rh(hfa)id Cr(hfa),d Cde
5449 AsO, 5000 5640 Rh, 3692, 3701 Cr, 4254, 4275, 4290 CH, 4314 CH,4314 CH, 4314 5200 5200 A10, 4866
cor
CH iCOCH 3 MoF, WFe Al(tfa),e
Limit of detec- Limit of detecLimit of datec- tion with spection nontion with Du,a g tronic 20,b g selecttve,e g
4
x 10-11
z x 10-9 2 x 10-8
x 10-12 1 X 10-1O 1 x 10-7 7 x 10-7
1
...
...
...
...
...
... ...
7 x 10-11
...
... ... 3 x 10-'0
1 x 10-6
2 X 10-10 2 x 10-8
2 x 10-6 3 x 10-10 2 X 10-10 7 x 10-9
3x 2x 6x 2X
6X
...
...
10-8
lo-" 10-4 10-3
Results o b t a i n e d with B e c k m a n DU f l a m e photometer. b Results o b t a i n e d w i t h m o d i f i e d Spectronic 20. c Results o b t a i n e d wrth a UV-absorbing filter a n d p h o t o m u l t i p l i e r t u b e 5.hexafluoro-2,4-pentanedione. e tfa = l , l , l - t r i f l u o r o 2,4-pentanedione. d h f a = 1,1,1,5,5, a
CIRCLE 138
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1187A
Table IV. Microwave Plasma Detector Performance Data ( 7 9 ) Band o r line, A
Compound introduced
C?,5165 CN, 3883
Hexane Hexane CsFs CbFs
Phosphorus
SI,2516 5611 CCI, 2788 2985 I , 2062 P, 2555
Sulfur
CS, 2576
cs2
Element
Carbon Fluorine Chlorine Bromine
Iodine
(A
CHCI? CHBr, CHdl
(CzH ,O)r PO
Limit of detection, gisec
x x
lo-'' 10-18
3x 8x 2x
10-10 10-12 10-10 10-7
3
2 5
x
Selectivity ratio vs. n-hexa nea
1
1 20 10 20 10
7 x lo-'* 1 x 10-11
104-1 0' 100
1 x 10-9
100
Selectivity r a t i o cIf hexane IS 1 0 with respect t o itself
tion ot GC-SIPD instrumentation to these compounds (25, 2 6 ) .The high volatility of mercury limits detection to the high parts per billion level as injected. but selectivity ratios with respect to hydrocarbons lie in the range -1O(i-40.000 ( 2 6 ) In . some cases. spectral emissions only 0.1 n m from the analytical mercury line (253.65 nm 1 have been successfully rejected by careful uavelength selection ( 2 6 ) . At the present time. there is no commercially a\-ailable GC -SlPD instrument. although detector components are supplied b y Scintillonics and Kayethon. Since the majority of applications for selective detectors can currently he satisfied ti>-the FPD. the lack of availabilith-. higher cobt. and greater complexit\- of construction and operation of the SIPD have minimized its use in analytical gas chromatograph!-. The detector doe*. ho~vever.appear t o have a bright future f'or the selective determination ot metal chelates and organic derivatives of toxic metal species in environmental samples:.
Alkali Flame Ionization Detectors (AFID) This detector. ivhich is also called a thermionic detector ( T I D i . is essent iall!. a conventional flame ionization detector ivith a pellet of an alkali metal salt pobitioned at the flame tip. However. kvhen cornpounds containing nitrogen. phosphorus. sulfur. or halogens enter the H ? 0 2 flame. the resulting ion current i a much greater than occur? in a conventional flame ionization detector. Three possible mechanihms ha\-e been suggested for thih phenomenon ( ? T i : the analyte and alkali salt vapors react in the flame to enhance the equilibrium concentration of alkali metal ions: active intermediates in the analyte breakdou.n procesh react Lvith the alkali salt t o gi\-e \-(]latileproducts Lvhich are ionized in the flame: and compound3 containing phosphorus alter the flame color and produce photc\ei-ai)oraticlnot the alkali salt 1188A
e
which is then ionized. Each of these theories accounts for portions ofthe detector's behavior. hut none completely explains the observed responses. Figure :3 is a s h e m a t i c diagram ot a commercially available AFID sholving the position ot the alkali metal salt (RC1. KbCI. Kb2S04.NazS04. CsC1. or CsBr) 129).However. although the detector components and their arrangement appear simple. the selectivity and sensitivity of the XFID strongly depend on such parameters as the electrode geometry. the electrode polarization and potential. the detector temperature. the nature and purity of the alkali metal salt. the carrier gas flow rate and flow configuration through the detector. and the
flow rates of the fuel gases i 9.27). For example. 0.05% change in H1 velocity can alter the ion current by 17~. Optimization of these parameters leads to selective detection ofparticular elements. Thus. phosphorus selectivity depends \-ery sensitively on the fuel gas ratio. and alteration of detector geometry permits selective detection of compounds containing nitrogen and halogens at the expense of those containing phosphorus ( 2 7 ) . Optimization can. however. be time consuming. and frequent replacement of the salt tip is required if base line stability. sensitivity. and quantitative response are to be maintained. Even with salt replacement. calibrations should be performed ivith each analysis. and large solvent injections I which produce prolonged disturbances in detector performance) must be avoided ( 9 ) . The AFID is most selective for phosphorus compounds which can he detected about one-thousandfold more sensitively than compounds containing nitrogen. halogens. or sulfur (Table I ) (9. 27). Selectivity for phosphorus compounds with respect to hydrocarbons is impressive (Figure 4 ) . though somewhat less than that of the FPD. and detection limits are commonly at least a n order ot magnitude lower than f'or the FPD. Discrimination between compounds containing nitrogen. sulfur. or halogens is relatively poor (Table I ) . The high sensitiL-ity and selectivity
W Top View
1. Collector (anode) 2 . Igniter (cathode)
3. Salt Tip 4. Vent port
View Figure 3 . Varian Aerograph A F I D ( 2 8 )
A N A L Y T I C A L CHEMISTRY. VOL. 4 5 . N O . 14. DECEMBER 1973
Anything you can do we can do better: Our pollution is better. Because when you’re measuring pollutants from stack gases or automobiles, you want better pol Iution than yours. All our calibration gases come with a guaranteed analysis, accurate t o ppm levels. And we have all kinds: carbon monoxide, nitrogen oxides, sulfur dioxide, carbon dioxide, propane, hydrocarbons, hydrogen sulfide and others. We’ll even mixa new one if you need it. Details? Phone or write Hank Grieco (201)464-8100. Airco Industrial Gases,575 Mountain Avenue, Murray Hill, N.J. 07974.
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o f t h e AFID for phosphorus com w cost have estabpounds and its l lished its main area of use in pesticide analysis ( 2 7 ) .It should be emphasized. however. that the susceptibility of the AFID perf'ormance t o t h e numerous influences mentioned above recommend its use mainly t o experienced gas chromatographers.
R
FID
Microcoulometric Detectors (MCD) In t h e JICD the analyte reacts with a n electrochemically generated species. and the current passed t o the coulometric generator to reestablish equilibrium is t h e source ot the chromatogram (.'301.Quantitative analysis is achieved through t h e proportionalit) between the total number ot'coulombs passed a n d the chromatographic peak area. Sulfur. nitrogen. and halogen-containing species can be determined in this manner after preliminary conversion to a suitably reactive chemical form such as S 0 3 2 . S z - . S H 4 . or halogen ions ( 3 1 ) . Some typical preliminary a n d coulometric reactions are given in Table V. T h e electrochemical section of a microcoulometric detector contains a potentiometric sensing circuit in addition to a reagent generating circuit. T h e former produces a n error signal which, after amplif'ication. control> t h e rate of reagent generation via a servo loop t o keep it in step with the . addirate of arrival ofanalyte ( 3 0 ) In tion. a n appropriate preliminary reactor system must be interposed between t h e chromatograph's exit port and the coulometric cell. This requires heaters for eluent transfer lines. reaction chambers. reactant gas supplies. or pyrolysis equipment. depending on t h e element t o be detected. Both the coulometric a n d preliminary reactor systems require careful optimization to obtain accurate a n d precise quantitative analyses. a n d se-
4
8
12
0
Time (min) Figure 4. A F I D selectivity hydrocarbons (27)
. .,
lectivity depends entirely on the specificity of the preliminary a n d coulometric reactions. Thus. in addition t o SO2. any species (such as halide ions and oxides of nitrogen) capable of reducing I3 will elicit a detector response ( .'11). However. t h e selectivity of the J I C D Ivith respect to hydrocarbons is good Lvhen preliminary reaction conditions are carefully chosen i.Y.5). Detection limit3 are most commonly reported t o lie in the microgram range (;MI. although t h i s high level i> prot)abl>-due t o column sorption losses since as little as 4 n g of H2Shas been detected ivhen liresented directly t o a Dohrmann Brr JICD ( U I , Applications of'the SIC11 include the determination of sulfur-contain-
compounds in Kraft process effluents I .lli a n d natural gases (:j7, 4 2 ) . LTseof the XICI) ha5. however. been limited by it5 poor detection limits in many analyses, although thi3 characteristic is not intrinsic to the detector.
Biz
Ref
Reaction
+
S-contg compd 01 SO? S03'I HA0 * SO*231SO* Br2 H 2 0 -+ S O P 2HBr 52Br2 2 H - 4 So 2 H B r
+ + + + + +
-+
$-
Br?
+
+ +
+ 2H-
32 32 33 34
Nitrogen SI-XpO
Preliminary reaction Coulometric reaction Chlorine Preliminary reaction Coulometric reaction 1190A
... H ,O'
.. . Ag'
0
6 VCI-13 C 701Cl,PS ( 1 7 X ' 0 9 g ! 7 Malathion i 2 8 X 10 a g ) 8 Parathion ( 4 3 X 10 g1 9 SuponaC,zH ,O,Cl:P 13 0 X 1 0 'g1 10 Ethion 3 0 X 10 ' g S = Solvent
Coulometric reagent
11-
4
for phosphorus-containing pesticides with respect to
Table V, Coulometric and Preliminary Reactions for Microcoulometric Detectors
Sulfur Preliminary reaction Coulometric reaction
8
Time (min)
I n j e c ~ e dmixture contalned 1 n-Dodecane ( 1 5 X 10 ' g l 2 n-Tetradecane i l 5 X 10 ' g ! 3 n-Hexadecane 12 3 X 10 "'91 4 n-Octadecane 13 8 X 10 ' 9) 5 n-Nonadecane ( 3 6 X 10 g)
*
Element detected
12
+
c a d ) -t
N-contg cornpd H? -----i"3 NH H?O NHI' OHOHH $0-? 2H?O
35 35 35
Combustion of CI-contg cornpd CIAg -+ AgCl
36 36
+
+
+
+
ANALYTICAL CHEMISTRY VOL. 4 5 , NO, 14. DECEMBER 1973
Coulson Conductivity Detector (CCD) This detector operates by converting the analyte t o an ionic hpecies ivhohe conductance is monitored in a dc. conductivity cell t'rom nhich the analyte ions are cont inuouJy removed SO that the detector has a ditferential. rather than a n integral. response. Selectii,ity depends on the +peciiicity ( ~ the f chemical reactions producing the conducting specie3 1 Ti. Four modes ot operation enable detection ot compounds containing nitropcn. chlorine. and sulf'ur ! 2. Figure 5 is a schematic diagram of the CCD which consists of a heated transfer line from the chromatograph. a valve for admitting reactant gases. a vent for remo\-ing the solvent front. a quartz reaction tube. a furnace. and a dc conductivity cell. This last element ha> a solution circulating system which enables removal ot the analyte ions in an ion-exchange column. T h e operating conditions employed in the reaction htage e are listed in Table L-I. T h e sensitivity a n d selectivity of the C C D are influenced 11y the furnace temperature. the reactant and carrier gas flow rates. and the flow rate of water through t h e conductivity cell j 7 1T . h e best sensitivity and selectivit h - are obrained for nitrogen compounds. Both the chlorine and sult'ur
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We doubled the evaporation area to double the evaporation speed. THOMAS MAGNE-FLASH' EVAPORATOR. Ordinary rotary evaporators increase surface area by a factor of about 4 The Thomas Magne-Flash Evaporator increases it by about 8 Twice the area-twice the speed-twice the efficiency Simpler, too No spinning flasks A large magnetic stirring bar drives the sample surface up the wall of the flask and also produces Innumereble spray aroplets each with its own spherical evaporating surface We eliminated troublesome rotary seals and swiveling connectors using just a simple jointed assembly We made it safer too With a stable cabinet-style base recessed stainless steel heating and cooling baths and a silicone-sheathed heating pad Loading and unloading is easy Circle 201 o n Reader Service Card for details.
Figure 5. C o u l s o n conductivity detector ( 7 )
modes experience interferences a n d decreased sensitivities. and the pyrolytic mode offers virtually no discrimination between the elements (Table VI). In addition, the sulfur and p ~ r o lytic modes experience response variations with changes in analyte structure (7). T h e principal advantage of the CCD is its ability to detect nitrogen compounds almost specifically and with high sensitivity. Only the MCD and AFID are capable of nitrogen detection. but both are less sensitive than the CCD. In addition. the sulfur mode of the CCD, although subject to some interferences, provides better sensitivity than that of the F P D (7, 43).T h e CCD is capable of reliable pesticide analyses ( 4 4 ) and is most frequently used in this area. It is less widely used than the F P D and AFID. owing in part to the simplicity and high performance of these detectors. The CCD is, however, commercially available (Tracor Inc.) and offers considerable potential as a nitrogen selective detector.
Electron Capture Detector (ECD) Although it is not normally classified as such, no discussion of element selective detectors would be complete without mention of the electron capture detector. T h e E C D consists of a radioactive source (63Ni or 3H) which ionizes t h e carrier gas (K2or Ar + 10% CH4) to produce a standing current. When a n electronegative species is introduced into t h e detector, it reduces t h e standing current and is thus detected. T h e extent of current decrease depends both on the number of electron-capturing species present and on their electronegativity, so t h a t the E C D is strictly selective only for highly electronegative species (e.g., compounds containing halogens, oxygen, and unsaturated groupings) (9,
45). Typical ECD performance d a t a are presented in Table I which illustrates the excellent exclusion characteristics for saturated hydrocarbons and t h e extremely high sensitivity of t h e detector. However, the ECD can only be
considered to be element selective for halogen or oxygen-containing compounds in the absence of other species of comparably high electronegativity.
Conclusion Comparison of element selective detectors in terms of their sensitivity and selectivity shows t h a t no single detector is unequivocally superior in performance. Strictly, such comparisons should be made for the same compound, and in this regard, the chlorine, phosphorus, sulfur. and nitrogen-containing pesticide Dursban has been suggested (7) as a suitable reference. Some relative response ratios for different detectors to Dursban are presented in Table VII. Even for a given element, however. the ideal choice of detector will depend upon the nature of the other species present with the analyte of interest and on the detection limits required.
References (1)G. A. .Junk, Int. J . M a s s Spectrom. lonPh\,s., 8, l ( 1 9 7 2 ) . (21 D. LVelti, “Infrared Vapor spectra,"^ 45.Hevden and Son Ltd.. London. E n gland.“1970. (3) M ,.J. D. Low. Anal. Chem., 1 1 ( 6 ) . 97A (1969). (1)R. S. Juvet and R. P. Durbin. ibid., 38,565 ( 19661. (5) “Operation and Service hlanual: Flame Photometric Detector,” l‘racor Inc., Austin. Tex.. 1968. ( 6 ) S. S. Brody and .J. E. Chaney. J . Gas C‘hromalogr.. 1 , 4 2 (19661. ( 7 ) FV, P. Cochrane and R. G. Greenhalgh. “Evaluation and Comparison of Specific GC Detectors for the Analysis of Pesticide Residues.” 3rd Annual Symposium on Recent Advances in the Analytical Chemistry of Pollutants. Athens. Ga.. l l a y 1973. ( 8 ) R. S. Braman. Ana!. Chem.. 38,734 i1966j . ( 9 ) C. H. Hartmann. ibid.. 13 (21. 113A i1972). (10) D. G . Greer and T. .J. Bydalek. Environ. Sei. Techno/., 7, 153 i1973). (11) M. C. Bowman and hT.Beroza. J . Ass. Off. Ana/. Chem.. 51,1086 (19711.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
1193A
Everything Necessary
For
Simplifiec Testing
12) B. Versino and G Rossi, Chromntographio, 4.331 (1971). 13) M. C. Bowman, M. Beroza. and G. Nickless, J Chromotogr Sci., 9,44 ,ln.-l/
,'a",.
14) R. K. Stevens. J. D. Mulik, A. E. O'Keefe, and K. J . Krost, Anal. Chem., 43,827 (1971). 151 M. C. Bowman and M. Beroza. ibid.. 40,1448 (1968). :E) C. D. Hollowell, G. Y. Gee, andR. D. MeLaughlin, ibid., 45 (1). 63A (1973). ,171 F. M. ZadoandR. S. Juvet. ibid.. 3R.
(19,-J. D: Wikfordner and T. H. Glenn,
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R. Hill, J. Chromatogr. Sci., 9,162 (1971). ( 2 2 ) A. .J, McCormackLS. C. Tong, and W. D. Cooke. Anal. Chem., 57,147U 11965). (23) c. A. Bache and D. J. Lisk, ibid., 39, 786i1967). (24) C. A. Baeheand D. J. Lisk, J. Ass. Off. Anal. Chem., 50.1246 (1967). (25) C.A.BacheandD. J.Lisk,Anal. Chem., 43,950 (1971). (26) W. E. L. Grossman, J. Eng,and Y. C. Tong, Anal. Chim. Acta, 60,447 (1972). (27) V. V. Brazhnikov, M. V. Gur'ev, and K. I. Sakodynsky, Chromatogr. Reo., 12.
Y
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l(1970).
(28) "Alkali Flame Ionization Detector In-
struction Manual," Varian Aerograph, Walnut Creek, Calif., 1970. 29) N. F. lves and L. Giuffrida, J. Ass. Off. Anal. Chem., SR,l(ISfiT). 30) J. A. Stamm, "Recent Advances in Applications of Micracoulometric Titrating Systems," in "Lectures on Gas Chromatography, 1966,"L. R. Mattick and H. A. Szymanski, Eds., p55, Plenum Press, New York, N.Y., 1967. 31) D. A. Leathard and B. C. Shurlock, "Identification Techniques in Gas Chromatography," p 179, Wiley-Interscience, NewYork, N.Y., 1970. 321, H. V. Drushel. Ana!. Chem.. 41.569 11969). 33) P. d. Klaas, ibid., 33,1851 (1961). 134) R. Koppe and D. Adams, Environ. Sei. Techno!., 1,479 (1967). '35) R. L. Martin. Anal. Chem.. 38,1209
* ANALYTICAL
I
-
(1966).
:36) D. M. Coulson and L. A. Cavanagh, ibid., 32,1245 (1960). :37) E. M. Fredricks and G. A. Harlow, ibid., 36, 263 (1964). 138) F. W. Williams and M. E. Umstead, ibid., 40,2232 11968). 139) H. L. Pease and J . J. Kirkland, J. Agr. Food Chem., 16,554 11968). (40) F. W . Williams, Anal. Chem., 44, 1317 (1972). (41) D.F.AdamsandR.K.Koppe, J.Air Pollut. Contr. Ass., 17,161 (1967). (42) H. Sehulz and M. Muniv, Eidoel Kohle, Erdgos, Petroehem., 25,14 (1972); CA, 76:88082. (43) W. P. Cochrane and R. Greenhalgh. Int. J. A d Anal. Chem.. inmess
..
(1973). (44) D. M. Coulson, J. Gas Chromatogr., 3,134 (1965). (45)J. Q.Walker, M. T. Jackson, and J. B. Maynard, "Chromatographic Systems: Maintenance and 'I'roubleshoating," pp 163-67, Academic Press, New York, N.Y., 1972. T. M. T. expresses his appreciation to the Perkin-Elmer Corp. for sponsoring the ACS analyti.
CIRCLE 93 ON READER SERVICE CARD
1194A
D a v i d F. S. N a t u s c h reccived BS a n d MS(hons) degrees in chemistry from the University of Canterbury, New Zealand, in 1961 a n d 1963. H e t h e n attended Oxford University, E n gland, as a Rhodes Scholar a n d ohtained a P h D under t h e supervision of R. E. Richards, F.R.S., in 1966. After years as .a research . scientist . "" with . four . -. the New Zealand u e p a r t m e n t Of 5c1entific and Industrial Research, he came t o t h e University of Illinois as a Fulbright Fellow in 1970 a n d has been a n assistant professor of chemistry at Illinois since 1971. Dr. Natusch is a member of the ACS, Society for Applied Spectroscopy, a n d t h e Air Pollution Control Association. His research interests involve development and application of analytical techniques to problems of environmental chemistry, with particular reference t o studies involving trace metals in t h e environment a n d inorganic gases and aerosols in t h e atmosphere.
CHEMISTRY, VOL. 45. NO. 14, DECEMBER 1973
T h o m a s >I. Thorp? received a B L degree in chemistry frrim Lafayel e College in 1971. He earned a n MS at t h e University of Illinois in 1973 a n d is currently working toward a P h D at t h a t institution under the direction of D. F. S. Natusch. MI. Thorpe is t h e recipient of a 1973 American Chemical Society Fellowship in analytical chemistry sponsored by t h e PerkinElmer Corp. H e is a member of t h e Society for Applied Spectroscopy a n d P h i L a m b d a Upsilon. His research interests include t h e development of analytical methods employing elem e n t selective gas chromatographic detectors and their application t o the determination of volatile metallo-or-
