Chapter 9 The detection of inorganic and organometallic compounds by electron-capture gas chromatography

191

Chapter 9

The detection of inorganic and organometallic compounds by electroncapture gas chromatography COLIN F. POOLE and ALBERT ZLATKIS

CONTENTS 9.1. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. The determination of metals as their chelate derivatives. . . . . . . . . . . . . . . . . . . . . 9.3. The determination of organoarsenic compounds . . . . . . . . . . . . . . . . . . . . . . . . . 9.4. The determination of organomercury compounds . . . . . . . . . . . . . . . . . . . . . . . . 9.5. The determination of selenium as piazselenols . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6. The determination of inorganic anions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.7. The determination of miscellaneous inorganic compounds . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

..

191 191 193 194 195 199 200 202

9.1. INTRODUCTION The application of the electron-capture detector to the analysis of inorganic and organometallic compounds is limited primarily by the poor chromatographic properties of many of these substances. Most metals and their salts are either ionic or too involatile to be separated by gas chromatography. A further problem is the high chemical reactivity of many organometallic compounds which results in their loss at low levels due to irreversible interactions with the chromatographic system. However, when these problems can be overcome, excellent sensitivity can be obtained with the electron-capture detector for appropriately substituted inorganic compounds. 9.2. THE DETERMINATION OF METALS AS THEIR CHELATE DERIVATIVES The general area of the separation of metals as their neutral chelate derivatives by gas chromatography has been extensively reviewed [l-81. A limit to the success of this approach has been the paucity of suitable reagents which can confer the necessary volatility, thermal stability and chemical inertness (with respect to the components of the chromatographic system) on the metal ion. Numerous examples exist of metal chelate derivatives which can not be quantitatively eluted at low levels by gas chromatography. For trace metal analysis using the electron-capture detector, the most frequently employed complexing ligands are 1,I ,I -trifluoropentane-2,4-dione (HTFA) and 1,1,1,5,5,5-hexafluoropentane-2,4-dione (HHFA). Chromatographically stable complexes are formed with ions such as Be(II), Al(II1) and Cr(II1) which form chelate derivatives having a preferred coordination number that is twice the oxidation number of the metal.

192

INORGANIC AND ORGANOMETALLIC COMPOUNDS

By contrast, metal ions such as Ni(II), Co(II), Fe(II), and lanthanide(II1) which readily adduct additional ligands to assume a coordination number greater than twice their oxidation number are often non-quantitatively eluted from the gas chromatographic column. The HTFA chelate derivatives formed with trivalent hexacoordinate metal ions exist in two isomeric forms (cis and trans) which may be resolved into two peaks depending on the chromatographic conditions selected for the analysis. Only in the case of the HTFA derivatives of beryllium, aluminium and chromium have extensive environ-

TABLE 9.1 DETECTION LIMITS FOR SOME METAL CHELATES DETERMINED BY GAS CHROMATOGRAPHY WITH ELECTRONCAPTURE DETECTION Metal

Ligand*

Detector operating parameters

Detection limit of the metal (g)

Ref.

Be(I1)

HTFA HTFA HTFA HTFA HTFA AcAc HTFA AcAc HTFA HHFA HTFA HTFA HTFA HTFA HTFA AcAc HTFA HHFA HTFA HATFP H, bnTFA, HATFP H,enTFA, HTFA DEDTC DEDTC DEDTC DEDTC DEDTC HFOD

'H, d.c. )H, d.c., 180°C 'H, pulsed, 176°C 3H, d.c., 200°C 63Ni,d.c. 3H, d.c. 3H, d.c jH,d.c. 'H,d.c. 'H, d.c. 'H, 190°C 63Ni,pulsed, 200°C 63Ni, d.c. 245°C 63Ni,pulsed 3H, d.c. 'H, d.c. 'H, d.c. 3H, d.c. 3H, d.c. 63Ni,pulsed, 200°C 3H, d.c. 63Ni, pulsed 'H, d.c. 'H, d.c. 'j3Ni, d.c., 270°C 63Ni,d.c., 270°C 63Ni,d.c., 270°C 63Ni,d.c., 270°C 03Ni, d.c., 270°C -

4- lo-'' 8*10-14 4. 10-14 1. lo-', 10-11 g/g 6.5.10-14 2.0.10-'6 8.8. lo-' 9.10-I' 3.3610-" 5 ng/ml 3. 2.5.10-13 1.1043 2.1045 5.1.10-' 2.7.10-9 4.8. lo-'' 1.7. lo-'' 1-10-8 1.1043 8-10-" 7.10-13 2.2.10-'2 6. 3.10-6 2.10-6 3*10-6 4~10-~ 4.4- 10-11

9 10 11 12 16 27 21 28 28 28 13 14 15 17 21 28 28 28 27 29 29 29 29 30 31 31 31 31 31 32

Cr (111)

Al(II1)

Cu(I1) Ni(1I) Rh(I1) Pb (11) Zn(I1) Ni (11) Hg(11) Cd(I1) co

*HTFA = l,l,l-tnfluoroacetylacetonate, AcAc = acetylacetonate, HHFA = 1,1,1,5,5,5-hexafluoroacetylacetonate, HATFP = 4amino-1 ,l,l-trifluoropent-3-en-2-one,H,bnTFA, = N,"-butylenebis(trifluoroacetylacetoneimine),[bis-(trifluoroacetylacetone)butylenediimine], H,enTFA, = N,N'ethylenebis-(trifluoroacetylacetonehnine), [bis
ORGANOARSENIC COMPOUNDS

193

mental and biomedical applications of the chelate derivatization technique with electroncapture detection been employed [9-261. There is 110consensus in the literature concerning the optimum detector operating conditions to obtain maximum sensitivity for the chelate derivatives. The quored detection limits for the metal chelate derivatives by electron-capture de tectiori are summarized in Table 9.1. Most measurements have been made with the tritium electron-capture detector which has precluded a study of the influence of detector temperature on detector response due co the limited upper operating temperature of the tritium source. In spite of this, the quoted detection limits illustrate the excellent detector sensitivity of the electron-capture detector for the HTFA derivatives of chromium arid beryllium. It is believed that the detector response to these derivatives is due to both the metal ion and the trifluoroacetylacetonate ligand. The evidence offered for this is the general observation that the fluorine-containing ligands have a much greater detector response than their hydrocarbon anatogues.

9.3. THE DETERMINATION OF ORGANOARSENIC COMPOUNDS Cacodylic acid (hydroxydimethylarsine oxide) reacts rapidly and quantitatively with hydriodic acid (provided that sample water is limited to less than 20% v/v of the reagent HI) to form dimethylarsine iodide which can be detected down to the 0.05 ppm level with a 3H electron-capture detector at 200°C [33].

0

II

CH3-As-OH

HI

CH3-AS-I I

I

CH3 Cdcodylic acid

CH3 Dimethylarsine iodide

Inorganic and methylated arsenic compounds can be determined as their diethyldithiocarbamate complexes (Table 9.2) [34]. For these compounds, the observed trend in detector response is in the opposite direction to that predicted based on the number of dethyldithiocarbamate moieties introduced. This either reflects a genuine unexpected TABLE 9.2 ELECTRONCAPTURE DETECTOR CARBAMATE COMPLEXES Compound*

RESPONSE TO SOME ARSENIC DIETHY LDITHIO-

Electron-capture detection limit (ng/ml)**

~~

As(DEDTC), CH,As(DEDTC), (CH )*As (DEDTC)

13 40 15

*DEDTC = diethyldithiocarbamate complex. **Detector temperature 109°C.

INORGANIC AND ORGANOMETALLIC COMPOUNDS

194

trend in the electron-capturing ability of these compounds or represents increasing adsorption and/or decomposition of the diethyldithiocarbamate derivatives on the gas chromatographic column.

9.4. THE DETERMINATION OF ORGANOMERCURY COMPOUNDS The recognition of organomercury compounds as the causative agent of the outbreak of the tragic neurological diseases inflicted upon the inhabitants of Minamata and Niigata in Japan and the discovery of toxic levels of these compounds in the inland waters of Scandinavia and the Northern United States of America has prompted numerous studies TABLE 9.3 REAGENTS FOR THE FORMATION OF VOLATILE DERIVATIVES O F INORGANIC MERCURY Reagent

Reaction products

Tetramethyltin

CH,HgCI + (CH,),SnCI yield 85-90%, reaction time 5.0 min CUB1 RHgCl CH,HgBr CzHsHgBr C,H,HgBr

Tetraalkyltin

RHg' + R,SnCl R = CH,, C,H,, C,H, yield 10-20%

[Co(III) (CN),RIJ-

RHg R = CH,, C,H,, C,H,

[CICH,Cr(III) (H,O,]

'+

R,TI (IlI)CO, R'

Detection limit (Hg)

Electron-capture detectar operating conditions

Ref. 46

1.36 ppb 1.30 ppb 1.12 ppb

'H, d.c, 150°C

150°C 185°C 41

10 nglml

'H, d.c., 210°C

41

CICH,Hg' derivative is thermally unstable RHg'

47

R = CH,, C,H,

R' = CH,, C,H, non-quantitative reaction Tetraphenylborate

C6 H sHg+ reagent unstable in acid solution

C, H, SO, H

C,H,HgCl + SO, + HCI

Sodium 2,l'-dimethyl2-silapentane-5-sulfonate C,F,SO,H (C6Hs14 HgBr, + KBr

KI

CH,HgCI + CH,HgI (C6Fs)zHg

-

2Hg(C,H,),

+

3HCI + NaCl+ B(OH),

RHgBr R,Hg conversion of dialkylmercurials R = CH, R = C,H, R = C3H7 R = C,H, R = C,H,

41

2.10-'0 g

63Ni, pulsed, 280°C

48

<2.5 ppb

63Ni,pulsed, 250-275°C

49

20 ppb

'H, dx., 210°C

50

45 pg

'H

51

10 Pg 25 pg 50 Pg 25 pg 200 Pg

'H, d.c., 165°C

52

SELENIUM

195

on the distribution and levels of these compounds in the environment. The organomercuric halides and organomercurials have favorable gas chromatographic properties and this coupled with their high response to the electron-capture detector makes gas chromatography with electron-capture detection the method of choice for their analysis at trace levels in biological samples. The gas chromatographic properties of organomercurial compounds have been reviewed by Fishbein [35] and Mushak [36]. Of note with respect to the electron-capture detection of organomercurials in environmental samples are the early studies of Sumino et al. [37,38], Yamaguchi and Matsumoto [39] and Nishi and Horimoto [40,41] carried out in Japan and the subsequent development of improved extraction and sample cleanup techniques developed by Westoo [42,43], Uthe et al. [44] and Von Burg et al. [45] in particular. Although there is no consensus of opinion in the available literature as to the optimum operating parameters for the electron-capture detector, detection limits for alkylmercurial compounds at the low picogram level and at the tens of picogram level for arylmercurial compounds have been obtained. The use of the electron-capture detector in the study of the concentration, distribution, speciation, impact, and fate of organomercurials in the environment is now too extensive to be reviewed in this section. Gas chromatography with electron-capture detection remains the principal analytical tool for the study of organomercurial compounds in the environment. The facile alkylation of mercury in the environment coupled with the high sensitivity of alkylmercurial compounds to electron-capture detection has prompted studies on the use of alkylating reagents as a means of derivatizing inorganic mercury for gas chromatographic analysis with electron-capture detection. The alkylating reagents evaluated for the derivatization reaction are summarized in Table 9.3. Among the more common reagents are the arylsulfinates (Peters’ reaction) [48,50], [CO(III)(CN),CH~]~ ; [47], sodium tetraphenylborate in basic or weakly acidic solution [47,51] and tetramethyltin [46,47]. Several of the reactions given in Table 9.3 are rapid, quantitative and can be used directly with sample digests. The electron-capture detector responds to parts per billion concentrations of the derivatives being competitive in sensitivity terms with other instrumental techniques for the determination of inorganic mercury. Thus gas chromatography with electron-capture detection can be used to determine the total mercury content of a sample as well as to study the speciation of the organomercurials in that sample. 9.5. THE DETERMINATION OF SELENIUM AS PIAZSELENOLS Selenium(1V) reacts with substituted 1,2-diaminobenzene or 2,3-diaminonaphthalene at low pH in aqueous solution to form a stable cyclic piazselenol derivative which can be extracted into an organic solvent. When the 1,2-diaminobenzene reagent is used in large excess, the reaction is quantitative and the piazselenol derivative can be determined at trace levels by gas chromatography with electron-capture detection. The structure and nomenclature for naming the piazselenol derivatives is illustrated below.

piazselenol

196

INORGANIC AND ORGANOMETALLIC COMPOUNDS

Tile optimum pH for piazselenol formation in aqueous solution is midway between the acid dissociation constant for the diprotonated diamine and the first acid dissociation constant of selenous acid, the reactive species in the derivative formation reaction being identified as the monoprotonated diamine and the undissociated selenous acid. Shimoishi [53] has compared the properties of thlrteen substituted 1,Zdiaminobenzene reagents for the determination of selenium by formation of piazselenol derivatives. The reagent 1,2-diamin0-3,5-dibromobenzene was selected as the reagent of choice based on its gas chromatographic properties, electron-capture sensitivTABLE 9.4 ANALYTICAL PROPERTIES O F SOME PIAZSELENOL DERIVATIVES Shimadzu Model GC-SA, 63Ni electron-capture detector, and detector temperature 280°C. Piazselenol

Retention index*

Distribution ratio**

Relative electron-capture detector sensitivity

Piazselenol 5-Methylpiazselenol

1307 1421 1515 1541 1611 1454 1547 1653 1632 1812 1864 1908 1800

172 379 4010 297 1661 1405 2576 250 N.B. N.B. N.B. 946 N.B.

1.0 1.4 1.2 1.7 1.6 17 30 128 102 363 172 25 5 25

5-Ethy lpiazselenol 5-Methoxy piazselenol 5-Ethox ypiazselenol SChloropiazselenol 5-Bromopiazselenol 5-Nitropiazselenol 5,6-Dichloropiazselenol 4,6-Dibromopiazselenol 4,7-Dibromopiazselenol 4-Bromc-6-nitropiazselenc)1 Naphth ylpiazselenol

*1 m X 3 mm I.D. column of 15% SE-30 on Chromosorb W (60-80 mesh), 180°C. **Calculated from the amount of piazselenol backextracted from toluene by 1.0 M hydrochloric acid. N.B. = not back-extracted. TABLE 9.5 OPTIMUM CONDITIONS FOR THE DETERMINATION OF PIAZSELENOLS Pye Unicamb GCV, 63Ni constant-current detector, detector current 10 nA, detector temperature 350°C. Piazselenol

Column temperature PC)

Gas chromatography* retention time (min)

Electron-capture detector response Se (x lo-'' g)

Piazseienol Naphth ylpiazselenol 5,6-Dichloropiazselenol 5-Nitropiazselenol

180 230 220 220

1.59 4.69 1.78 3.94

90 20 1.0 1.0

*lS-ft. column of 7% OV-225 on GasChrom Q (100-120 mesh), nitrogen flow-rate 35 ml/min.

SELENIUM

10.0

197

I 1.0

1.5

2.0

1, lo3

T

Fig. 9.1. Plots of lnAT3'Zvs. 1/T for (A) piuselenol, (B) naphthylpiazselenol, (C) 5-nitropiazselenol, and (D) 5,6-dichloropiazselenol. The negative slope of the lines indicates a dissociative mechanism of electron capture.

ity and distribution ratio between aqueous solution and organic solvents. The results are summarized in Table 9.4. The response of the electron-capture detector to the different substituents in the piazselenol structure was H < CH3 or C2H5< CH30 or C z H 5 0< C1 < Br < NOz. The highest detector response was observed for 4,6-dibr~~nopiazselenol. The response to the 4,6-dibromopiazselenol derivative was about three times greater than the response observed for 5-nitropiazselenol. Poole e t al. [54] have determined the response of the electron-capture detector to four piazselenol derivatives under optimum operating conditions (Table 9.5). The mechanism of electron-capture for the four piazselenols was shown to be dissociative (Fig. 9.1) and the maximum detector response was obtained at high detector temperatures. Without some knowledge of the products generated by capture of the thermal electrons, it was not possible to propose a molecular basis for the electron-capture process for these compounds. Chronologically, Nakashima and Toei [55] were the first to describe the use of the electron-capture detector to determine selenium as its 5-chloropiazselenol derivative. Using a 3H detector at 200°C, the detection limit for selenium was found to be 0.04 pg. To improve the detection limit of the selenium derivative, Shimoishi and Toei prepared the 5-nitropiazselenol derivative [56]. The detection limit under optimum conditions for selenium as the 5-nitropiazselenol derivative was found to be l.O.lO-'z g [54]. The 5-nitropiazselenol derivative has been used to determine selenium in pure sulfuric acid [56], pure tellurium [57], sea water [58], plant material [59], copper and its salts [60,61], milk, milk products and albumin [62], arsenic and arsenic oxide [63] and in human blood, hair, urine and placenta, bovine liver and orchard leaves [54]. Young and Christian [64] used the naphthylpiazselenol derivative to determine selenium in human blood and urine. The detection limit for selenium was found to be 0.5 ng in this study and 0.2 ng by Poole e t al. [54]. The 5,6-dicNoropiazselenol derivative was used to determine selenium in food products such as vegetables, fruits, mushrooms, soil and whole egg powder [ 6 5 ] . Using a 1 .O-g sample and a final solution volume of 20 ml, a detection limit

INORGANIC AND ORGANOMETALLIC COMPOUNDS

198

of 0.01 ppm of selenium was found. Under optimum conditions the detection limit for selenium as its 5,6-dichloropiazselenol derivative is 1.O kg [54 I. The 4,6-dibromopiazselenol derivative was used to determine selenium in bovine liver [53] and in human blood [66]. The detection limit for selenium as its 4,6-dibromopiazselenol was stated to be 2.5 times better than obtained with the 5-nitropiazselenol derivative. The reaction between selenous acid and the monoprotonated form of the diamine reagent is very selective and virtually free from interferences. None of the cations Ag', Cuz+, SnZ+, PbZ+, Hgz+, CdZ+, MgZ+, Mnz+, NiZ+, ZnZ+, Ga3+, In3+, As3+, Te3+, Sb3+, Bi3+, A13+,Cr3+, Fe3+, Ge4+, V5+,Mo6+present in over a 1000-fold excess concentration compared to selenium interfere in the formation of the piazselenol derivative [63]. The reaction is selective for Se(1V) and Se(0) and Se(V1) are not detected by this method without prior conversion to Se(1V). Biological samples are usually digested under strong oxidizing conditions prior to piazselenol formation and this results in the oxidation of Se(0) to Se(N). Shimoishi and Toei [56] used a bromine-bromide redox buffer to oxidize Se(0) to Se(IV) in samples of pure concentrated sulfuric acid. Se(V1) can be easily reduced to Se(1V) by heating on a water bath with concentrated hydrochloric acid for a few minutes [54,62,66]. By analyzing a sample with or without hydrochloric acid reduction enables a value for total selenium and Se(IV) to be determined and by difference Se(V1) to be calculated. As far as biological samples are concerned, the concentration of Se(0) is considered to be vanishingly small. Although chemical interferences in the formation of the piazselenol derivative are not a problem, chromatographic interference by co-extracted electron-capturing compounds from biological samples can be. To some extent this can be eliminated by careful choice of the gas chromatographic column used for the separation. In a comparative study of three sample digestion techniques for the analysis of trace selenium levels in human placenta, it was found that very clean chromatograms were obtained when a nitric acidmagnesium nitrate digestion technique was employed (Fig. 9.2) [54]. The sample is A

t

5e L

&,

-

i

Fig. 9.2. Determination of selenium as its 5-nitropiazselenol derivative (NP) in 0.1 g of standard reference placental material after digestion with (A) nitric acid, (B) nitric acid-perchloric acidsulfuric acid-hydrogen peroxide, and (C) nitric acid-magnesium nitrate.

INORGANIC ANIONS

199

TABLE 9.6 COMPARISON OF THE RESPONSE OF THE ELECTRONCAPTURE DETECTOR TO SOME ALKY LSELENIUM COMPOUNDS Compound Dimethyl selenide Diethyl selenide Dipropyl selenide Dimethyl diselenide Diethyl diselenide Dipropyl diselenide Ethyl selenocyanate

Detector response ratio ECD/FID 0.01 0.002 0.001

130 135 150 320

initially dissolved completely at a low temperature (< lOO"C), excess acid removed and the nitrates decomposed at a higher temperature. Finally, the sample digestion is completed by heating in a muffle furnace at 500°C for 0.5 h. Under these conditions the organic matrix is essentially eliminated entirely from the chromatogram. This can be seen by comparing the chromatograms A, B, and C in Fig. 9.2. The electron-capture detector has also been used to determine some alkylselenium compounds [67]. The operating conditions for the electron-capture detector were not given, but the response of the detector compared to the flame ionization detector indicated that the alkyl diselenides and selenocyanates had good electron-capture properties while the alkyl selenides showed only a poor detector response (Table 9.6). 9.6. THE DETERMINATION OF INORGANIC ANIONS The cyanide anion reacts with chloramine-T (sodium p-toluene sulfonchloramide) at ice-bath temperatures to form cyanogen chloride [68].

CN- t chloramine-T

+

ClCN

Reaction times are 5-6 min and the detection limit for the cyanogen chloride derivative was 0.025 pg/ml using an electron-capture detector. This detection limit corresponds to 12.5 pg of cyanide anion as cyanogen chloride injected on-column. The cyanide and thiocyanate anions in aqueous solution can also be determined as cyanogen bromide after reaction with bromine [69].

+ Br2 + BrCN + Br+ BrCN + SOA2-+ 7Br- + 8H' SCN- + 4Br2 + 4 H 2 0

CN-

The thiocyanate anion can be quantitatively determined in the presence of cyanide by adding an excess of formaldehyde solution to the sample which converts the cyanide ion to the unreactive cyanohydrin. The detection limit for the cyanide and thiocyanate anions was less than 0.01 ppm using a 3H detector at 150°C.

200

INORGANIC AND ORGANOMETALLIC COMPOUNDS

An organic solution of tetraheptylammonium carbonate can be used for the extraction of halide anions from aqueous solution. Injection of the organic phase into the gas chromatograph results in a thermal decomposition of the tetraheptylammonium salts to form the 1-haloheptanes of the chloride, bromide, and iodide anions [70]. The fluoride salt pyrolyzes to form heptene and hydrogen fluoride.

AH [R,N]+ X- --+ R3N + RX,where R = C,HI5 and X = C1, Br, I

A 3H electron-capture detector was used to detect the haloheptanes, but the method was rendered unusable in practice due to the large amount of involatile triheptylamine formed in the pyrolysis reaction which condensed in the detector and completely masked its response. The fluoride anion reacts specifically and quantitatively with triethylsilanol and other alkylsilanol compounds to form alkylfluorosilane derivatives which are stable to gas chromatography [71]. (C,H,)$iOH

t F-

+

(C2H5&3iF

However, the detection limit for the triethylfluorosilane derivative was much higher by electron-capture detection than that measured with the flame ionization detector. In order to improve the detection limit for the fluoride anion determination and to increase the selectivity of the derivatization reaction the use of the pentafluorophenyldimethylchlorosilane (flophemesyl chloride) reagent has been suggested [72]. Iodine in acid solution reacts with acetone to form monoiodoacetone which can be detected at high sensitivity with an electron-capture detector [73-751. Strong acid conditions are required to inhibit the formation of the diiodoacetone derivative. The reaction is specific for iodine and iodide is determined after oxidation with potassium iodate. The other halide anions are not oxidized by iodate to the reactive molecular form and do not interfere in the derivatization reaction. Using a @Ni pulsed (500 ps) electron-capture detector at 1 50°C, the detection limit for total iodine (iodine t iodide) in milk was established as 0.004 ppm [75]. In two reactions, the concentration of free iodine and after iodate oxidation the concentration of iodine t iodide can be determined for the same sample. The nitrate anion can be determined in aqueous solution after conversion to nitrobenzene by reaction with benzene in the presence of sulfuric acid [76,77]. The yield of nitrobenzene was 90 f 8% for nitrate anion concentrations in the range 0.12-62 ppm. The derivative was prepared by vigorously shaking the unhomogeneous mixture of benzene and acidified aqueous sample at 75'C for approximately 5 min. The nitrite anion can be determined after oxidation to nitrate with potassium permanganate. The detection limit for the nitrate anion was found to be less than 0.1 ppm using a 63Ni pulsed (1 50 ps) electron-capture detector at 275OC. 9.7. THE DETERMINATION OF MISCELLANEOUS INORGANIC COMPOUNDS The electron-capture detector has been used to determine trace quantities of nitrogen dioxide in the atmosphere [78,79]. Using a 3H detector at 2OO0C a detection limit of less than 3 ppm was established. Chlorine trifluoride can be detected at less than 0.1 ppm in

MISCELLANEOUS INORGANIC COMPOUNDS

201

TABLE 9.7 ELECTRONCAPTURE DETECTOR RESPONSE TO SOME SULFUR GASES Compound

Detector temperature TC)

Has

225 250

Detection limit (ppb, v/v)* -

cos

CH,SH (CH,),S CSa

250 25 0 250 250

350 (CH,),Sa

250

C4H4S

350 250

9

12

0.5** 5

50 1 0.2 12

3 500

*Pye 104, “)Ni 10-mCi, 15C-r~~ pulse. **Highest response found at a pulse frequency of 500 MS.

the atmosphere by electron-capture detection [80]. Phosgene was detected in air at less than 1 ppb with a d.c. ’H electron-capture detector [81]. The electron-capture detector has been used to detect sulfur gases in carbon dioxide at the ppb level (Table9.7) [82] . The detector response to dimethyldisulfide was about five times greater at a detector temperature of 350°C compared to that at 25OoC, in keeping with a dissociative mechanism of electron-capture. For the other sulfur gases, the highest response occurred at a detector temperature between 225 and 275°C. By comparison with the sulfur specific flame photometric detector, it was found that the electron-capture detector was as sensitive to disulfides, less sensitive to mercaptans and much less sensitive to thioethers than the flame photometric detector. The inorganic fluorine compounds F,, MoF,, UF6, SbF,, and SbF5 can be separated by gas chromatography and detected with an electron-capture detector [83,84]. No detection limits for the fluorides were given. The fluorides are very corrosive and special precautions were required in the handling and separation of these compounds. Lead tetraalkyls used as anti-knock additives in gasoline can be determined with an electron-capture detector [ 8 5 ] . Detection limits were not quoted in this study but the relative electron absorption coefficient of tetraethyllead was found to be 115 compared to a value of 1.O for chlorobenzene. Triphenyltin, diphenyltin and phenyltin complexes extracted from aqueous solution as their chlorides are readily reduced to the equivalent hydride with lithium aluminium hydride [86]. The hydrides could be detected down to the 0.2 ng level with a 3H electron-capture detector at 210°C. The organotin hydrides are strong reducing agents and any contact with oxidizable substances has to be avoided. Elemental sulfur when gas chromatographed produced three identifiable peaks [87]. The largest peak d.c. accounts for 98.8% of the total molar mass and could be determined with a electron-capture detector down to the 0.1-0.4 ng level.

202

INORGANIC AND ORGANOMETALLIC COMPOUNDS

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