1.4.1. Alcohols and phenols
For the analysis of hydroxyl-containing compounds they are converted into esters [227-2291, including those compounds which contain fluoro, chloro and nitro groups [230-2331, or ethers [186, 191, 228, 234-2361. Of particular importance are the derivatives containing phosphorus, obtained with the aim of subsequent selective and highly sensitive detection with an alkali FID or a flame-photometric detector that selec- tively detects phosphorus-containing compounds. TMS derivatives are the most popular, and are widely used to protect the hydroxyl group [237-2551. These methods are discussed in detail in the books by Pierce [122] and Blau and King [45] and in compre- hensive reviews [57, 1231. Methods of obtaining derivatives for the separation of optically active alcohols have been described [199,256-2581.
1.4.2. Aldehydes and ketones
To obtain derivatives of aldehydes and ketones for GC analysis the most widely used reaction is that of condensation with the corresponding amines, e.g., N-aminopiperidine, pentafluorophenylhydrazine or phenylhydrazine [259] . The most popular reagent is 2,4-dinitrophenylhydrazine, with the help of which carbonyl compounds can be
N
TABLE 1.5
LIMITS O F DL:TI:CTION I N TfIE GAS CHROMATOGRAPHIC ANALYSIS OF ELEMENTS IN THE FORM O F VOLATILE COMPLEXES AND COMPOUNDS
No. Elcinen t Ligand (radical) Detection liniit Reference
(1: ' 10'")
1 Aluminium Trifluoroacetylacetonc (TFA) 0.09 212
Pivaloyltrifluoroacetone ( l T A ) 0.04 212
2 Arsenic Phenyl 4 21 3
3 Beryllium w A 0.0004 214
4 Cobalt 1,1,1.2,2.3,3-Heptafluoro-7,7-dimethyl-4.6-octanedione (HDO) 0.4 215
5 Chromium TI-A 0.0025 2 16
Acetylacetone (AA) 0.03 217
6 Copper TFA 0.08 21 7
Tetradcntate p-ketoiniine (TDKI) 0.001 218
7 Iron TFA 0.13 217
8 Gallium TFA 0.027 217
9 Nickcl Trifluorornonothioacetylacetone (TMTA) 0.5 219
TDKI 0.007 218
10 Palladiu in TMTA 2000 2 20
1 1 Pla t i n u i n TMTA 3000 2 20
12 Scandium AA 0.021 217 V
TFA 0.03 217 7
13 Thorium Hexafluoroacetylacetone (HFA) and 1
3-n-butylphosphineoxide (TBPO) 8000 221 m
14 Thalliuni Cyclopentadiene 10,000 222 4
5
15 Uranium HFA + TBPO 12,000 22 1
16 Rare earths l,l, 1,2,2,6,6,7,7,7-Decafluoro-3,5-heptandione and 2
dibutyl sulphoxide 200-300 223
0.1 224 $
17 Vanadium HDO
18 zinc Bis(diethyldithiop1iosphinate) (DETP) 7000 47 >
19 Cadmium DETP 7000 47
20 Lead DETP 10,000 47
P P
>
4
cn 1 e
!- -
cn
53 separated. The initial compounds are then regenerated from the condensation products formed by heating them, e.g., with a-ketoglutaric acid or by using some other reagents [242, 260-2681. Relatively light carbonyl compounds can be analysed by GC in the form of their 2,4-dinitrophenylhydrazones [269-2781. In some instances other derivatives are used, e.g., oximes to be separated from hydrocarbons extracted with pentane [279] or thiosemicarazones to determine selectively the carbonyl compounds with a flame-photometric detector [280] . Methods for oxidizing aldehydes into corre- sponding acids [281] and the formation of corresponding acetals [282] have also been described.
1.4.3. Amines
As amines are active compounds that tend to adsorb and form complex compounds with some common metals, preliminary protection of the amino group is often conducted by acylation, silylation and formation of Schiff bases by condensation reactions. The acylation method was used for the first time to form derivatives of amines directly in the column by Anders and Mannering [ 1961 ; the possibility of obtaining sufficiently reliable results in analysing impurities was shown by Marmion et al. [283]. It is expedient to conduct acylation using halogen derivatives of acylating agents, which makes it possible to increase considerably the sensitivity of detection of the derivatives using an ECD and, in a number of instances, to increase their volatility [284]. Acetic anhydride and fluoro derivatives are used for this purpose [285-2991. The method is widely used in the analysis of biological amines [300-3041.
To obtain non-polar derivatives silylation is also used, but it should be noted that amino groups are relatively difficult to silylate [131, 305-3141. Nanogram amounts of amines are determined via their dinitrophenyl derivatives formed with 1 -fluoro-2,4- dinitrobenzene [191, 315-3181 or 2,4-nitrobenzenesulphonic acid [319] . A sensitive method for determining amines using an ECD was proposed by Moffat and Horning [320], who used the condensation of pentafluorobenzaldehyde with amines to obtain derivatives .
In the reaction of amines with carbon sulphide, isocyanates are formed, small amounts of which can be recorded on an ECD [321, 3221. Other reactions can also be used to obtain amine derivatives for subsequent GC analysis [323-3341.
1.4.4. Carboxylic acids
Before conducting the quantitative analysis of carboxylic acids, especially of their bi- functional derivatives, as well as high-boiling acids, they are converted into more stable and less adsorptionally active derivatives. Methyl esters are frequently used as derivatives of acids in GC analysis. To obtain methyl esters of carboxylic acids, diazomethane [101, 148, 150, 3351, a methanolic solution of hydrochloric acid [164, 3361, a methanolic solution of boron trifluoride- [r51, 152, 3371 , pyrolysis of tetramethyl- ammonium salts [166, 338, 3391 and other methods [181, 340-3441 have been used.
In a number of instances [345-3471 higher esters are used. The sensitivity of detection with an ECD is increased when halogen-containing reagents are used to obtain esters
[348-3521. Of particular interest is the use of esters of a-oxyphosphonic acid for the analysis of trace amounts of carboxylic acids using a selective phosphorus detector [353]. TMS esters are also used in chromatographic analysis [254,354-3661. Interesting methods of analysing keto-acids have been described [367-3721 and the separation of optical isomers has been studied [199,373,374].
Methods of analysing involatile acids [375] and those previously separated on ion- exchange resins [376] were developed by Drawert and co-workers. Much attention has been paid in recent years to the analysis of biological samples. The determination of derivatives of bile acids [377], derivatives of ascorbic acid [378, 3791 and of organic acids in biological fluids [380] have been described. Methods of obtaining derivatives of acids and their chromatographic analysis were also described in reviews [381,382] .
In a number of instances it is also important to solve another problem, viz., the deter- mination of TMS groups in TMS esters of carboxylic acids and N-silyl compounds. The method that has been elaborated is based on desilylation with phenol followed by the GC analysis of the trimethylphenoxysilane formed [383]. The analysis of amino acids was described in a book 191 and a review [ 10) .
Questions related to the GC analysis of the individual classes of organic compounds and groups of industrially important organic products have been considered in a number of' reviews and books. Thus, the analysis of antibiotics was examined by Wagman and Weinstein [384], of steroids by Heftmann [54], of the chemistry of wood by Kholkin [385] and that of environment pollutants was described in the book edited by Grob [386] and in Fishbein's three-volume book [387-3891.
1.4.5. Analysis of inorganic acids and metals
GC can be applied successfully in the analysis of inorganic acids and cations. It is especially promising in the analysis of trace amounts of inorganic compounds. As inorgapic compounds are, as a rule, involatile, volatile derivatives are first obtained, which are then analysed by GC. The success of analysis is usually determined by the first stage when stable volatile derivatives are obtained. General problems related to this method of analysis have been examined [45, 21 1 1 . The analysis of phosphates, glycero- phosphates and their derivatives [226, 255, 390-3991, that of sulpho-acids and their derivatives [359, 400-4041 and the determination of trace amounts of selenium in sulphuric acid [405] have been considered. Chlcrides, bromides and iodides can be determined using the reaction with ethylene oxide, which results in the formation of a corresponding 2-halogenoethanol [406]. The formation of alkyl halogenides as a result of the reaction with tetraalkylammonium has been described 14071.
A vivid example of the extensive possibilities of gas chromatography in the analysis of inorganic substances is the study of trimethylsilylated silicate anions. T h s method was proposed by Lentz [408] and Gotz and co-workers [409, 4101 for investigating the structure of silicates. In this method silicate minerals that react with acids and form silicic acids can react in acidic medium with monofunctional organosilicon compounds, forming stable organosilicon derivatives. If, eg., trimethylsilanol formed from trimethyl- chlorosilane in aqueous medium reacts with silicic acid the reaction proceeds according to the following equation:
=SiOH + (CH3)3SiOH- 3i-O-Si(CH3)3 + H20
The trimethylsilylated silicic acids formed in this instance are soluble in conventional organic solvents, and their volatility is sufficiently high for them to be analysed by gas chromatography. Carzo and Hoebbel [411] carried out a comprehensive study of the chromatographic retention of various trimethylsilylated silicic acids on different station- ary phases: Apiezon L and silicone OV-1 and OV-17. The analysis of metals in the form of volatile complexes continues t o attract attention, and have been described for analys- ing sodium [412], potassium [412], radium [413], caesium [413], barium [414], calcium [414], strontium [415], beryllium [416, 4171, magnesium [418], zinc [419, 4201, nickel [419], mercury [421], copper [422, 4231, silver [424, 4251, cadmium [421], indium 1426, 4271, gallium [428], scandium [217], cobalt [421], thallium [426] , hafnium [429, 4301 , lead [431, 4321, titanium [430], vanadium [433], chro- mium [434-4361, manganese 14261, iron [437], yttrium [438], platinum [439,440] , palladium 1439, 441, 4421, zirconium [430], molybdenum [443], ruthenium [444], rhodium [445] , rare earths [446-4491, thorium [221, 450, 45 11 and uranium [221, 4521 . The literature on GC analysis of metal chelates was reviewed by Sokolov [458].
The cited examples are indicative of the progress of reaction GC in what at first sight might be regarded as a ‘non-chromatographic’ field.