For the main functional groups (carboxyl, hydroxyl, etc.) several methods of obtain- ing stable derivatives for subsequent GC analysis have been developed. In elaborating
REAGENTS FOR SlLY LATlON
No. Reagent Formula B.P. ("C/mmHg) Examples (ref.)
1 N , 0-Bis (trime t h y lsily 1) ace tarnide 71-73/35 124, 125
CH,-C=NSi(CH,),
2 N,O-Bis( trimethylsilyl) trifluoroacetamide (BSTFA) Si(CH , ) , 401 12
I 122
3 N-T r imeth y lsily la ce ta m ide
0 I
CF,-C=N-SKH,),
CH,CONHSi(C'H,), 52-54 126,127
4 N-Methyl-N-trimethylsilyltrifluoroacetamide (MSTFA) CF ,CONCH , Si (CH , ) , 132 128,129
5 N-Trimethylsilyliniidazole (TSIM) 6 N-Trimethylsilyldiethylamine (TMSDEA)
(CH,),SiNCH=NCH= CH 99.5/14 (CH ,) ,SiN(C,H ), 1271738
130-133
n .
134-137
7 H exame t h y ldisilazane (HM DS) (CH,) ,SiNHSi(CH ,) , 126 126 75
7 5 5 3
$
2 5
8 Trimethylchlorosilane (TMCS) (CH, ),Sic1 5 7 126. 127, io
138-140 P?
141,142
9 Dimethylchlorosilane (DMCS) H(CH,),SiCI 36
10 1,3-Bis(chloromethyl)-l,l,3,3-tetramethyldisilazane (CH,Cl)(CH,),Si,NH 141,142
11 Chloromethyldimet hylchlorosilane (CH,CI) (CH ,) ,Sic1 143
r W
(li
METHODS USED TO OBTAIN DERIVATIVES FOR GC
the CFD method for a particular system the choice of a certain chemical method is often determined not only by the properties of the method but also by characteristics such as the composition of the sample and the chromatographic zones of unreacted components that can be superimposed on the chromatographic zones of derivatives.
The availability of certain derivatives also affects the choice of a CFD method. Hence it is expedient to consider the main methods used to obtain derivatives for GC analysis.
1.3.1. Silyl derivatives
Derivatives of this type are widely used in gas chromatography to obtain volatile derivatives of involatile compounds. Silyl derivatives, e.g., for the case of the TMS group donor, can be obtained in accordance with the following scheme:
O-Sl(CH,), C O O - S I ( C H ~ ) ~
---) S-SI(CH3& , S I (CH31-3
( 1 ) NH-SI(CH3)3 ( 2 ) -N, S I ( C H ~ ) ~
= N- SI (CH3 )3
: : : o +
* -NH2
-NH
When silyl derivatives are obtained, as a rule all of the functional groups of a molecule form the corresponding derivatives in one stage. Methods used to obtain silyl derivatives are discussed in detail in a book by Pierce [122] and in a review by Drozd [57], and those used for trimethylsilyl derivatives in a review by Miller and Pacakova [123].
Various reagents, some of which are listed in Table 1.4, are used t o obtain silyl derivatives.
Quantitative analysis in derivatization is discussed in refs. 45, 122, 144-147 and 457.
1.3.2. Esters
For the protection of the carboxyl group methods are used that result in the forma- tion of esters. Those used most frequently are the following.
1.3.2.1. The diazomethane method [148-1501
I
The ester is formed in accordance with the following reaction:
RCCOH + CH2N2- RCOOCH3 + N2
The reaction is conducted in anhydrous medium because diazomethane reacts with water. Diazomethane usually gives high yields of esters, but it is unstable, may explode and it takes a relatively long time to obtain the derivatives.
1.3.2.2. The methanol method
The reaction proceeds according to the equation RCOOH + CH30H-RCOOCH3 + HzO
Several compounds can be used as the catalyst for the esterification reaction. The use of boron trifluoride [151-1561 makes it possible, for many systems, to accomplish the
reaction within a few minutes. The optimum conditions for the reaction were determined [157]. In some instances it is expedient to use boron trichloride-2-chloroethanol instead of boron trifluoride-methanol; esters prepared this way can be analysed by GC with electron-capture detection. In other instances it is expedient to use boron tetrachloride instead of boron trifluoride [159, 1601. Hydrochloric [161, 1621 and sulphuric acid
[163, 1641 and ion-exchange resin I1651 are also used as catalysts.
1.3.2.3. The pyrolysis method
Pyrolysis of tetramethylammonium salts at 360-400°C makes it possible to obtain methyl esters of fatty acids [166-1681 :
RCOOH + (CH3)4NOH - RCOOCH3 + H2O + (CH3)3N
The acids are usually titrated with a methanol solution of tetramethylammonium hydrox- ide with phenolphthalein as the indicator. The solution of the salts formed is introduced directly into the device for sample insertion heated at 360-400°C.
Methyl esters are also formed by reaction with 2,2-dimethoxypropane in the presence of dimethyl sulphoxide [I691 and by other methods [170-1721.
Vorbeck et al. [ 1731 compared the yields of the esterification reaction of organic acids with diazomethane and two reaction mixtures: methanol-hydrochloric acid and methanol-boron trifluoride. The best results were achieved when the diazomethane method was used.
Methods have been developed for obtaining other non-methyl esters. In addition to simpler techniques these methods were aimed at increasing the sensitivity and volatility and, sometimes, decreasing the volatility (for lower acids with high volatility) and improv- ing the separation and selectivity of determination. Other workers [ 174-1761 have described the application of diazoethane, diazopropane, diazobutane and diazotoluene to obtain esters. Solutions of higher diazoalkanes are more stable and less explosive than diazomethane solutions.
The use of propanol or butanol solutions with boron trifluoride has been described [177-1793. To separate optical antipodes it was proposed t o use L-menthol [180].
Obtaining esters by means of the reaction of the acid with methanol and higher alcohols in the presence 01' pyridine (catalyst) and a large excess of N,N '-dicyclohexylcarbo- diimine was described by Felder et ai. [ 1811 :
H ' + R,COO- R,OH
RN=C=NR- RN=C-NHR----CRN=C-NHR-R~COOR~ + RNHCONHR
I
0-CORI
The use of 8,"-dimethylformamide acetals makes it possible to obtain esters of organic acids :
P--:c,'jk +
p - - 3
' C ~ ~ ~ , C H ~ : ? - Q C O C - + R'-o-cH--I;(cH~)~ - R C O O R ' + H C O N ( C H ~ ) ,
.. /, n - - :
Using the alcoholysis of imidazolides of acids obtained from the reaction of the acid with N,N'-carbonyldiimidazole, one can obtain the corresponding esters [ 1821 :
N=CH
I: Y="
R-C-N, I + R ' O H -- R C O O R ' + I )NH
H,C--CH, H C = C H
49
Esters can be also obtained in the reaction of an alkyl iodide with tetramethyl- ammonium salts of the corresponding acids [ 1831 :
(CH ,),N+OH-
RCOOH *[RCOO] [(CH&N] RCOOR'
In the opinion of Drozd [57], special consideration should be given to isopropyl esters of acids. These esters are formed in the reaction of 2-bromopropane with sodium hydride [184]. With the help of this method isopropyl derivatives can be obtained for other classes of organic compounds:
- COOCH (CHs )2 -NHCH(CHJ)I
- SCH(CH312 -COOH
- SH - N H 2
-OH 3 p - E ( C H 3I2C H 0 r /-- - OCH(CH3)p
In connection with the necessity to increase the sensitivity and selectivity of the determination of acids, halogen derivatives are used to obtain esters. Such methods have been described for trichloroethyl [185], pentafluorobenzyl [186, 1871, penta- fluoropropionic [188] and heptafluorobutanol esters [189].
1.3.3. Ethers
This method is applied to protect the hydroxyl group:
ROH + CH,I-ROCHj + HI
The reaction proceeds in the presence of silver oxide in dimethylformamide or in an ethereal solution of tert.-butanol-potassium alcoholate [ 1901 .
To determine trace amounts of hydroxyl-containing compounds, nitro [186] and fluoro [ 1911 derivatives are used, e.g.,
F F F F
/"=" FC%
'; - [' c=c
R O O , + O r C H Z C 'CF - R O O - C H 2 - - C \ IC F
F F F F
The reaction is catalysed by potassium carbonate. To carry out reactions of this type extractive alkylation has recently been applied, in which tetraalkylammonium is used as the counterion and methylene chloride as the solvent [192]. Extractive alkylation usually takes 15-25 min.
1.3.4. Acyl derivatives
The formation of acyl derivatives is used to protect hydroxyl, amine and thiol groups:
R - 0 - C O R '
:I:z23 (R'CO),O
R-SH R-S-COR'