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Chapter 5 Misme-Ionization Detector
91
Chapter 5
FLAME- ION I Z A T ION D E E CTOR CONTENTS
............................ ........... ....... .............. ...............................
5.1. I n t r o d u c t i o n 5.2. Hydrogen atmosphere flame-ionization d e t e c t o r 5.3. Hydrogen atmosphere flame-ionization d e t e c t o r f o r s i l i c o n compounds 5.4. Flame-ionization d e t e c t o r w i t h hydrocarbon background 5.5. Selective d e t e c t i o n o f halogen compounds References 5.1.
91 92 103 105 106 106
INTRODUCTION
The basic r e a c t i o n s taking place i n the oxygen-hydrogen flame can be described by the f o l l o w i n g equations t :
H
t
O2
Z OH
t
0
(5.1)
0
t
H2
OH
t
H
(5.2)
H2 f H20
t
OH
t
H
(5.3)
The r a d i c a l s formed i n the r e a c t i o n zone o f the flame r e a c t exothermically as f o l l o w s (M represents a general flame-gas molecule): H OH
H
t
t
t
M + H2
H + M
+
t
'
M
H20 +
(5.4)
M
(5.5)
and, i f organic compounds are present, chemi-ionization occurs: CH +
o
-f
CHO+ + e-
(5.6)
Various s u b s t i t u e n t s on the i n d i v i d u a l carbon atoms i n t h e molecule o f a compound a f f e c t i o n i z a t i o n i n d i f f e r e n t ways (commonly reducing the i o n i z a t i o n compared w i t h hydrogen). This e f f e c t has l e d t o the determination o f t h e soc a l l e d number o f e f f e c t i v e carbon atoms i n a molecule o f a compound2, the i o n i z a t i o n o f an alkane carbon atom being taken as a reference, i.e.,
a value
o f u n i t y i s ascribed t o the response due t o an alkane carbon atom. For t h i s reason, the f l a m e - i o n i z a t i o n d e t e c t o r (FID) i s expected t o respond o n l y t o organic compounds containing an e f f e c t i v e ( i o n i z a b l e ) carbon. Inorganic gases, carbon oxides and carbon d i s u l p h i d e should y i e l d no response ( t h e e f f e c t i v e carbon number f o r C=O and C=S being zero). I t has been found, however, t h a t ,
a t increased hydrogen flow-rates
3
and w i t h an electrode geometry t h a t causes
the electrodes t o be heated by the flame4, carbon d i s u l p h i d e gives a response t h a t i s a thousandth t o a hundredth o f t h a t o f hydrocarbons. A t increased hydrogen flow-rates, the detector gives a response t o n i t r i c oxide5 t h a t i s commensurate w i t h the methane response. Under these conditions, the d e t e c t o r a l s o t o carbon dioxide, carbon monoxide, n i t r o u s
gives a lower
oxide, n i t r o g e n dioxide, helium, oxygen and hydrogen sulphide. None o f the above-mentioned responses has the character o f a s e l e c t i v e response, because the molar responses t o organic carbon compounds d i f f e r from each other only i n the number o f the e f f e c t i v e carbon atoms i n the s o l u t e molecules; the s l i g h t l y anomalous responses t o inorganic gases are small compared w i t h the hydrocarbon responses and they d i s p l a y the same p o l a r i t y ( t h e number o f ions i s increasing). The aim o f t h i s chapter i s t o describe m o d i f i c a t i o n s o f t h e f l a m e - i o n i z a t i o n detector t h a t g i v e a s e l e c t i v e response t o c e r t a i n types o f compounds w i t h regard t o a higher response and opposite p o l a r i t y . I n f a c t , t h i s chapter should also include the a l k a l i flame-ionization d e t e c t o r (AFID), because i t i s a m o d i f i c a t i o n o f the FID. However, the modifications o f the AFID cover a broader spectrum (e.g.,
a d d i t i o n a l electrode, the method o f heating the a l k a l i metal
source) and, e s s e n t i a l l y , the AFID i s so widely and manufactured d i s t r i b u t e d t h a t i t has been considered as a separate type o f detector. I n 1969, an FID was described
7 t h e electrodes o f which were made o f 0.051 mm
diameter h i g h - p u r i t y platinum wires. These microelectrodes were adjustable and i t was w i t h t h e i r varying distances from the d e t e c t o r j e t t h a t a s e l e c t i v e
response, i.e.
, different
responses f o r d i f f e r e n t compounds, was obtained. The
responses t o ketones and a1 i p h a t i c and aromatic hydrocarbons displayed maximum values a t a v e r t i c a l distance o f the electrodes from the j e t o f ca. 6 mm, whereas the response t o polychloroalkanes was maximal a t a distance o f ca. 2 mm. When the electrodes were placed i n the region o f the flame corresponding t o the optimum response f o r a l i p h a t i c o r aromatic hydrocarbons, t h e chloroalkane response was f o u r orders o f magnitude lower. 5.2. HYDROGEN ATMOSPHERE FLAME-IONIZATION DETECTOR Aue and H i l l 8 found i n 1972 t h a t , when the hydrogen and a i r flows were interchanged i n the flame-ionization detector, i.e.,
when a i r was introduced
i n t o t h e j e t w h i l e hydrogen was flowing around it, t h i s d e t e c t o r gave a s e l e c t i v e response t o organometallic compounds. I n t h i s instance the flame burns i n a hydrogen atmosphere and the hydrogen f l o w i s an order o f magnitude h i g h e r than the a i r f l o w (hence t h e name hydrogen atmosphere f l a m e - i o n i z a t i o n detector,
93
-90v
Collector electrode
from Gc
F i g . 5.1. C r o s s - s e c t i o n o f a s i l a n e - d o p e d HAFID. (From r e f . 10.) HAFID). F u r t h e r i n v e s t i g a t i o n s showed t h a t t h e i n t r o d u c t i o n o f s i l i c o n compounds i n t o t h e d e t e c t i o n system (e.g.,
by t h e c a r r i e r gas f r o m t h e s t a t i o n a r y phase,
t h e s i l i c o n septum, e t c . ) i s e s s e n t i a l f o r t h e HAFID response t o o r g a n o m e t a l l i c 9 compounds t o o c c u r
.
On t h e b a s i s o f t h e s e f i n d i n g s , a d e t e c t o r was designed” s i l a n e was s u p p l i e d ( F i g . 5.1).
t o which gaseous
The c a r r i e r gas f r o m t h e column and an oxygen-
n i t r o g e n m i x t u r e e n t e r t h e 1.2 mm q u a r t z j e t o f t h e d e t e c t o r , which i s 70 mm f r o m t h e base o f t h e d e t e c t o r body. The hydrogen and s i l a n e i n l e t i s s i t u a t e d a t t h e d e t e c t o r base n e a r t h e j e t . The c o l l e c t o r e l e c t r o d e i s o f t h e shape o f a 5-mm d i a m e t e r r i n g , 50 mm f r o m t h e j e t . Under t h e s e c o n d i t i o n s , t h e HAFID responds t o organornetall i c compounds .and t h i s response, depending on t h e presence o f a metal i n a m o l e c u l e o f t h e compound, i s s e v e r a l o r d e r s o f magnitude h i g h e r than t h e response t o hydrocarbons ( s e e T a b l e 5 . 1 ) . The d e t e c t o r i s most s e n s i t i v e t o manganese and aluminium compounds, t h e minimum d e t e c t a b l e mass r a t e f o r these compounds and f o r i r o n , t i n and chromium compounds b e i n g l o w e r t h a n o r commensurate w i t h t h e FID minimum d e t e c t a b i l i t y o f hydrocarbons (1.7 g/ 11 sec o f Mn f o r manganese 1. The background c u r r e n t o f t h e HAFID i s h i g h e r t h a n t h a t o f t h e FID12’13. T h e r e f o r e , t h e n o i s e o f t h e HAFID i s i n c r e a s e d by a p p r o x i m a t e l y an o r d e r o f magnitudeI3. The d e t e c t i o n l i m i t and s e l e c t i v i t y a r e a l s o e v i d e n t f r o m F i g s . 5.2 and 5.3. A comparison o f t h e g a s o l i n e chromatograms
ID
P
TABLE 5.1
R E S P O N S E S OF MODEL COMPOUNDS S e l e c t i v i t y a g a i n s t tetradecane, taken as t h e r a t i o of i n j e c t e d amounts o f tetradecane and t h e o r g a n o m e t a l l i c t h a t produce t h e Same peak area o f 1-10-11 C. A minus s i g n preceding a s e l e c t i v i t y v a l u e i n d i c a t e s a n e g a t i v e response. (From r e f . 10.) Compound
Column temperature (OC)
Selectivity
A1 u m i n i u d 111) hexafluoroacetylacetonate Ferrocene T e t r a v in y l t i n T e t r a p r o p y l t in Tetraethylt i n Chromium( 111) hexafluoroacetylacetonate Chromi um hexacarbonyl Chromium( 111) trifluoroacetylacetonate T e t r a e t h y l 1ead Tetrabutyl lead T r i phenylan timony Tungsten hexacarbonyl Molybdenum hexacarbonyl T e t r a b u t y l germane T r i - n - b u t y l phosphate Chl orobenzene Bromobenzene Triphenyl arsine T r i p h e n y l b i srnuth
50 145 110 140 90 60 30 170 100 190 250 50 30 140 190 70 80 250 250
3.2 1.9 2.6 1.9 1.6 1.6 1.5 1.1 1.1 6.0 1.2
8.6 5.0 3.5 -3.2 -3.2 -3.2 -3.2 1.3
. 105 .. 105 104 104 .. 104 104 -. 104 .. 104 104 103
. 102 103 lo2
-
102 102 102 102 102
lo2
D e t e c t i o n 1i m i t (9)
6.0 2.1 1.0 7.8 2.1 1.5 2.3 1.7 5.1 7.5 1.3 5.3 3.5 5.5 1.1 1.4 1.4 2.3 6.3
. 10-13 10-12
- 10-11 - 10-11 10-11
-- 10-11 10-10
. 10-9 10-10 . 10-9 10-9 10-9
-. 10-9 10-9
Piaselenole Iron( I I I ) t r i f l uoroacetylacetonate Di -n-butyl di sul phi de F1 uorobenzene Ni trobenzene Diethylrnercury Tetraethyl s i 1 ane Tetradecane
165 175 140 30 110 70 50 140
-9.2 8.0 -2.0 7.1 3.1 2.9 1.9 1.0
- 10; - 10101 10; 10 - 100 100 100
5.7 4.1 2.3 6.7 1.8 1.3 4.1 3.7
.. 10-9 10-9 .. 10-7 10-7 . 10-7 10-7
96 (Fig. 5.3) obtained by means o f an HAFID and a FID c l e a r l y shows the advantage of the HAFID i n d e t e c t i n g organometall i c compounds i n mixtures w i t h hydrocarbons.
The molar response o f the compounds seems t o be given by the number o f heteroatoms i n the molecule o f the compound13 (Table 5.2).
S i m i l a r l y t o t h e FID, t h e
l i n e a r dependence o f the molar response on the number o f e f f e c t i v e carbon atoms 12 i n the molecule i s r e t a i n e d
.
The wide gap between the j e t t i p and the base o f t h e d e t e c t o r body i s o f great importance w i t h regard t o the s e l e c t i v i t y o f t h e HAFID response13. Table 5.3 compares the response s e l e c t i v i t i e s and t h e d e t e c t i o n 1 i m i t s f o r a common massive j e t and a j e t t i p modified w i t h q u a r t z o r s t a i n l e s s - s t e e l t u b i n g (10 mm), w i t h the same distance o f the c o l l e c t o r electrode from the j e t t i p . Compared w i t h a normal j e t , the response o f a d e t e c t o r w i t h a j e t t i p i s s u b s t a n t i a l l y lower f o r hydrocarbons and, therefore, the s e l e c t i v i t y o f t h e response t o hydrocarbons i s higher. The reduction i n the response t o hydrocarbons i n t h e
I1
HAFID compared w i t h the FID i s believed t o be due t o an o x i d a t i o n step t h a t occurs i n the oxygen-rich precombustion zone o f t h e HAFID12’13, i n a d d i t i o n t o
a
C
1
-b
Fig, 5.2. Chromatograms of model compounds: ( a ) 5 pg o f aluminium(II1) hexafluoroacetylacetonate, 50OC; ( b ) 10 pg o f ferrocene, 145OC; ( c ) 50 pg o f chromium( 111) trifluoroacetylacetonate, 170%. Glass column ( 1 m) packed w i t h 6% OV-17 on Chromasorb W, temperature as i n d i c a t e d , Flow-rates: hydrogen 1600 ml/min plus 7 pl/mi.n s i l a n e , oxygen 125 ml/min, n i t r o g e n 100 ml/min, n i t r o g e n from a GC column 40 nl/min. (From r e f . 10).
97 FID
HAFID
I
4
6
30
50 90120
HAFID
'C
30
50 90120 'C
Fig. 5.3. HAFIO and FID analyses o f a l k y l l e a d compounds i n g a s o l i n e . L e f t , r e g u l a r gasoline; r i g h t , premium gasoline. Glass column (6 ft. x 2 nun I.D.) packed w i t h 80-100 mesh Ultrabond 20 M. Temperature, programmed from 30OC ( 5 min) t o 120Oc ( 5 min) a t 2OoC/min. HAFID flow-rates: hydrogen 1600 ml/min, s i l a n e 34 ppm, oxygen 150 ml/min, a i r 120 ml/min. (From r e f . 11 w i t h permission.) the effect o f the p o s i t i o n and p o t e n t i a l o f t h e c o l l e c t o r electrode. The massive j e t d i s s i p a t e s the heat from the precombustion zone, decreasing the o x i d a t i o n 13 therein. The j e t t i p then reduces the c o o l i n g e f f e c t and enhances o x i d a t i o n
.
The l i n e a r i t y o f response o f the d e t e c t o r i n which oxygen enters t h e detect i o n space a t the d e t e c t o r base covers about t h r e e orders o f magnitude f o r aluminium, chromium, t i n , lead and i r o n compounds".
I f hydrogen i s supplied t o
the d e t e c t o r above t h e j e t t i p only, the response i s n o t a l i n e a r f u n c t i o n o f the amount o f compound 11
.
TABLE 5.2 EVALUATION OF RESPONSE (From r e f . 13 w i t h permission.) Compound
Dodecane Diallyldibutyl t i n Tetraethyl t i n Tetra buty 1ti n Tetra-n-propyl t i n Hexabutyl d i t i n
*
Due t o the column.
Detection 1 i m i t (pg)
S e n s i t i v i t y (C/mol)
Selectivity
FID
HAF ID
FID
HAFID
FID
HAF I D
300 800 2000 250 1400 760
48,000 800 38 70 22 13
1.4 0.38 0.52 1.8 1.4 2.1
0.034 12' 29 31 38 55
1 .oo 0.27 0.37 1.3 1 .o 1.5
3.5 8.5 9.1 1.1 1.6
1 .oo
102
lo2
102 103
lo3
TABLE 5.3 COMPARISON OF JET T I P S
(From r e f . 13 w i t h permission.) Compound
Tetraethyl t i n Dodecane
Mass ive j e t
Stainless-steel j e t t i p
Quartz j e t t i p Detection 1i m i t
Sel ec t iv i t y
10 P9 0.5 ug
5.9 1
D e t e c t ion 1i m i t
Selectivity
Detection limit
Sel ec t i v i t y
38 P9 48 ng
110 1
15 P9 0.2 119
1.2 1
.
104
-
104
W W
F i g . 5.4. Background c u r r e n t and responses o f model compounds a t d i f f e r e n t c o l l e c t o r e l e c t r o d e p o t e n t i a l s . 1, Background c u r r e n t ; 2 , t e t r a d e c a n e ( 2 p g ) ; 3, t e t r a b u t y l l e a d ( 1 0 n g ) ; 4, t e t r a b u t y l t i n (10 n g ) ; 5, f e r r o c e n e ( 1 n g ) . h = Peak h e i g h t ( a r b i t r a r y u n i t s , l o g a r i t h m i c s c a l e ) . (From r e f . 10.) F i g . 5.4 shows t h e r e l a t i o n s h i p s between response and c o l l e c t o r e l e c t r o d e p o t e n t i a l f o r t i n , l e a d and i r o n compounds and f o r a hydrocarbon. The n e g a t i v e l y p o l a r i z e d e l e c t r o d e i s c l e a r l y more advantageous; compared w i t h t h e p o s i t i v e c o l l e c t o r e l e c t r o d e , t h e response t o o r g a n o m e t a l l i c compounds i n h i g h e r and t h e hydrocarbon response and background c u r r e n t a r e l o w e r . The responses o f a l l o f t h e i n v e s t i g a t e d compounds and t h e background i o n i z a t i o n c u r r e n t i n c r e a s e t o maximum values as t h e p o t e n t i a l i n c r e a s e s . The optimum e l e c t r o d e p o t e n t i a l w i t h r e g a r d t o t h e s e l e c t i v i t y and t h e minimum background c u r r e n t i s -90 V. F o r a c o n s t a n t c o n c e n t r a t i o n o f a s i l i c o n compound e n t e r i n g t h e d e t e c t o r , t h e response o f t h e HAFID depends on .the d i s t a n c e o f t h e c o l l e c t o r e l e c t r o d e from t h e
The response t o o r g a n o m e t a l l i c compounds i n c r e a s e s t o maximum
values w i t h i n c r e a s i n g d i s t a n c e o f t h e e l e c t r o d e . T h i s response enhancement i s 1.5 o r d e r s o f magnitude f o r a d i s t a n c e r a n g i n g f r o m 10 t o 50 mm. On t h e o t h e r hand, t h e hydrocarbon response decreases by a p p r o x i m a t e l y an o r d e r o f magnitude w i t h i n t h i s range. The background c u r r e n t a l s o decreases w i t h i n c r e a s i n g distance o f t h e c o l l e c t o r electrode from the j e t . The d i s t a n c e o f t h e c o l l e c t o r e l e c t r o d e f r o m t h e j e t a l s o a f f e c t s t h e amount o f s i l a n e t h a t has t o be s u p p l i e d t o t h e d e t e c t o r . As mentioned above, t h e HAFID g i v e s a s e l e c t i v e response t o o r g a n o m e t a l l i c compounds o n l y i n t h e presence o f a s i l i c o n compound. The HAFID response and t h e background c u r r e n t i n c r e a s e t o
101
ELECTRODE HEIGHT
OX
0.3 m
0
-i
0.2
L
I
2 w
I 0.1 Y
4
[L
0.0 V
\;
i
3
3
5
4
6
7
8
9
1
1011
Fig. 5.5. HAFID response as a f u n c t i o n o f s i l a n e c o n c e n t r a t i o n f o r v a r i o u s e l e c t r o d e heights. Flow-rates: hydrogen 1500 ml/min, oxygen 120 ml/min, a i r 165 ml/min. (From r e f . 14.) maximum values w i t h i n c r e a s i n g c o n c e n t r a t i o n o f s i l a n e i n t h e d e t e c t o r . The response t o hydrocarbons and t h e background c u r r e n t a t t a i n t h i s maximum v a l u e 10 a t h i g h e r s i l a n e c o n c e n t r a t i o n s than does t h e response t o o r g a n o m e t a l l i c s
.
The optimum s i l a n e c o n c e n t r a t i o n , however, i s d i f f e r e n t f o r each d i s t a n c e between t h e c o l l e c t o r e l e c t r o d e and t h e j e t 1 4 ( F i g . 5.5). The g r e a t e r t h e e l e c t r o d e distance, t h e lower i s t h e optimum s i l a n e c o n c e n t r a t i o n . With optimum s i l a n e c o n c e n t r a t i o n s t h e HAFIO response t o o r g a n o m e t a l l i c s increases w i t h i n c r e a s i n g d i s t a n c e o f t h e e l e c t r o d e . Greater e l e c t r o d e d i s t a n c e s ( 7 0 mm) a r e advantageous f o r t h i s reason, because b o t h minimum d e t e c t a b i l i t y f o r organom e t a l l i c s (improved s e n s i t i v i t y o f response, l o w e r background c u r r e n t and n o i s e ) and s e l e c t i v i t y ( h i g h e r s e n s i t i v i t y o f response t o o r g a n o m e t a l l i c s and reduced s e n s i t i v i t y t o hydrocarbons) a r e increased. As can be seen from F i g . 5.5,
t h e response o f an HAFIO o f t h e t y p e used i s
n e g a t i v e a t a d i s t a n c e o f t h e c o l l e c t o r e l e c t r o d e from t h e j e t o f 10 mm14. T h i s n e g a t i v e response i s r e l a t e d t o t h e shape and p o s i t i o n o f t h e c o l l e c t o r e l e c trode. The e l e c t r o d e was pin-shaped and s i t u a t e d o u t o f t h e j e t c e n t r e i n t h i s instance. When u s i n g a ring-shaped e l e c t r o d e s i t u a t e d above t h e j e t , t h e response i s a l s o p o s i t i v e f o r a d i s t a n c e o f 10 mm".
The optimum d i s t a n c e f o r
n e g a t i v e response depends on t h e e n t i r e d e t e c t o r design15. Negative responses a l s o occur a t low p o s i t i v e p o t e n t i a l s o f t h e c o l l e c t o r e l e c t r o d e (see F i g . 5.4) and a t h i g h s i l a n e concentrations".
The d e t e c t i o n l i m i t s f o r t e t r a e t h y l t i n and
t e t r a e t h y l l e a d i n t h e negative mode are lower by about one o r d e r o f magnitude than those i n t h e p o s i t i v e model5. However, parameter s e t t i n g s f o r t h e n e g a t i v e response must be c a r e f u l l y c o n t r o l l e d and a r e l a t i v e l y h i g h s i l a n e concentrat i o n may prove t o cause long-term degradation o f response due t o s i l i c o n d i o x i d e
102 deposits on the electrode. Thus f o r r o u t i n e analyses t h e p o s i t i v e rode seems t o 15 be more s u i t a b l e
.
The HAFID r e q u i r e s s u b s t a n t i a l l y higher hydrogen flow-rates than t h e FID.
14
The HAFID response t o organometallics increases t o a maximum a t about 3 l/min
w i t h increasing hydrogen f l o w - r a t e (4 ppm o f s i l a n e , 70 mm e l e c t r o d e distance), b u t t h e hydrocarbon response d r a s t i c a l l y increases w i t h flow-rates below 650 ml/min9. The means o f supplying hydrogen t o t h e detector a f f e c t s t h e amount o f s i l a n e needed t o achieve maximum response, both i n p o s i t i v e ”
.
and negative mode 15
I n the p o s i t i v e mode, i f hydrogen enters the detector 21 mn above t h e j e t t i p t h i s concentration i s 34 ppm, and i f hydrogen i s supplied t o the detector base the optimum s i l a n e concentration i s below 10 ppm (see Fig. 5.5). The dependence o f t h e response o f an organometallic compound on t h e percentage o f n i t r o g e n i n the air-oxygen m i x t u r e supplied a l s o shows a maximum ( a t a 1 : l n i t r o g e n t o oxygen r a t i o ) . Therefore, f o r a t o t a l f l o w - r a t e through the j e t o f 305 ml/min and a n i t r o g e n c a r r i e r gas f l o w - r a t e o f 20 ml/min, the optimum a i r and oxygen flow-rates are 165 and 120 ml/min, r e ~ p e c t i v e l y ’ ~With . higher n i t r o g e n t o oxygen r a t i o s the response t o organometall i c s decreases, the hydrocarbon response increases and, consequently, t h e d e t e c t i o n s e l e c t i v i t y i s diminished 10
.
5.3. HYDROGEN ATMOSPHERE FLAME-IONIZATION DETECTOR FOR SILICON COMPOUNDS I f a s i l i c o n compound i s introduced i n t o the HAFID, i t gives a s e l e c t i v e
response t o m e t a l l i c compounds. If an i n v e r t e d system i s used, i.e.,
i f an
organometallic compound i s supplied t o the detector, the HAFID responds t o organic s i l i c o n compounds. Hence t h i s i s again an HAFID, b u t an i r o n compound, 16
commonly ferrocene, i s supplied i n t h e hydrogen f l o w i n t h i s instance
.
The detector geometry plays an important r o l e i n peak t a i l i n g I 7 . Pronounced t a i l i n g occurs w i t h l a r g e r i n n e r diameters o f the d e t e c t o r (above 11 mm). Hydrogen i s introduced i n t o the detector space below the j e t l e v e l and oxygen i s supplied t o the detector j e t together w i t h the c a r r i e r gas from t h e column. The c o l l e c t o r electrode (-90 V ) i s 110 mm from the j e t , The amount o f i r o n compound supplied t o the d e t e c t o r fundamentally changes the response 16s18-21 , (Fig. 5.6). With increasing amounts o f an i r o n compound the response t o s i l i c o n compounds increases t o a maximum (about 5 ppm i n t h i s instance) and then decreases, passing t o a negative response ( a t about 12 ppm). The s p e c i f i c course o f t h i s dependence i s a f f e c t e d by the distance o f the p o i n t o f the i n l e t o f the i r o n compound from the detector flame2’ ( t h e i n v e r s i o n from a p o s i t i v e t o a negative response i s s h i f t e d t o higher amounts o f the i r o n 18,20 compound a t l a r g e r distances) and by the type o f i r o n compound supplied The maximum negative response (about 35 ppm o f ferrocene) i s three times greater
.
than the maximum p o s i t i v e response. The detector noise increases a t the same
103 ? 20
?Q
Y
2 c
15 10
; 5
E -0 I v)
Amount of ferrocene (pprn) '
"
I
\
"
I
'
I
'
I
I
'
,
'
I
J3 -5
LL
4
10
-7 5
-20
J
Fig. 5.6. E f f e c t o f amount o f ferrocene on t h e HAFID-Si response. (From r e f . 18 w i t h permission.) TABLE 5.4 DETECTION LIMITS FOR THE HAFID-Si (From r e f . 21.) ;,lode o f HAFID o p e r a t i on
Coumpounds containing S i 1icon
Phosphorus
Iron
Chlorine
Non-doped P o s i t i v e mode Negative mode
4 ng ( r e f . 16) 50 pg ( r e f . 17) 1 ng ( r e f . 18)
7 ng 0.5 ng 1 n9
0.6 ng 1 ng 14 ng
50 ng 2.5 ng 9 ng
time, so t h a t the d e t e c t i o n l i m i t i s smaller by a f a c t o r o f about 20 i n t h e negative mode (see Table 5.4).
L i n e a r i t y o f d e t e c t i o n covers a range o f about
three orders o f magnitude i n both modes
21
.
Hence, the s e l e c t i v i t y o f response o f t h i s d e t e c t o r , ' c a l ? e d the HAFID-Si,
is
conditioned by the response l e v e l and response p o l a r i t y ( F i g . 5.7). With smaller amounts o f doping i r o n compounds i n the p o s i t i v e mode, the HAFID response t o s i l i c o n compounds i s higher than t h a t o f a d e t e c t o r i n t h e absence o f a doping agent and approximately commensurate w i t h the response o f a common FID, whereas the response t o hydrocarbons i s reduced. The s e l e c t i v i t y o f response o f S i t o C i s 18'*'
4
about 10 : I . With higher concentrations o f i r o n compounds i n the nega-
t i v e mode, the response t o s i l i c o n compounds i s negative whereas t h a t t o hydrocarbons remains p o s i t i v e . The change from a p o s i t i v e t o a negative mode i s easy t o accomplish, by increasing t h e f l o w - r a t e o f the gas containing the
104 12 L
FIO
b
HAFIO ( S i 1 positive mode
1
1
70
8L
1
1
1
1
98 112 126 9:
I
,
,
70
8L
98
,
.
112 126 T
C
2
8
HAFID ISi 1 negative mode
5
Fig. 5.7. Chromatograms o f equal amounts o f compounds using d i f f e r e n t d e t e c t i o n methods. Chromatograms obtained w i t h a standard FID (a); a HAFID doped w i t h 5 ppm o f ferrocene (b); a HAFID doped w i t h 30 ppm o f ferrocene ( c ) . Peaks: 1 = Hexane ( s o l v e n t ) ; 2 = chlorobenzene; 3 = decane; 4 = octanol; 5 = t r i e t h y l phosphate; 6 = dodecane; 7 = decanol; 8 = ferrocene; 9 = tetradecane. Methylsilicone-coated f u s e d - s i l i c a c a p i l l a r y column (10 m x 0.2 nun I.D.). Temperature, programmed from 70 t o 150OC a t 8oC/min. Helium f l o w - r a t e : 1.2 ml/min, HAFID gas flow-rates: hydrogen 1600 ml/min, oxygen 130 ml/min. FID gas flow-rates: hydrogen 30 ml/min, a i r 240 ml/min. (From r e f . 21.) doping i r o n compound, A s e l e c t i v e enhancement o f the response t o s i l i c o n compounds compared w i t h hydrocarbons was a l s o observed18 when other metals were used as doping agents, such as aluminium, n i c k e l , chromium, molybdenum, brass, platinum, copper and magnesium. The l i f e t i m e s o f the sources o f these metals are short, however.
105 The HAFID-Si gives a response approximately commensurate w i t h t h a t o f s i l i c o n compounds a l s o t o phosphorus, i r o n and c h l o r i n e compounds 18y21 (see Table 5.4). The response t o t h e l a t t e r compounds a l s o depends on t h e amount o f doping agent applied. The course o f t h i s dependence f o r phosphorus and i r o n compounds i s s i m i l a r t o t h a t f o r s i l i c o n compounds, i.e.,
t h e response t o these compounds
i s negative f o r l a r g e r amounts o f dopants supplied t o the d e t e c t o r (Fig. 5.7). Alcohols, ketones, ethers and n i t r o g e n and f l u o r i n e compounds g i v e a low posit i v e response 20
.
The HAFID responds t o organic s i l i c o n compounds 16,'7 and t o phosphorus, i r o n and c h l o r i n e compounds*' even i f no i r o n compound i s supplied t o t h e d e t e c t o r , I n t h i s instance, t h e s e n s i t i v i t y o f d e t e c t i o n i s lower than i n the presence o f ferrocene i n the p o s i t i v e mode (see Table 5.41, being approximately two t o three orders o f magnitude lower than t h a t o f t h e FID. The d e t e c t o r does n o t d i f f e r i n design from t h a t w i t h ferrocene17, the S i t o C s e l e c t i v i t y i s about 3.5 orders o f magnitude and the d e t e c t i o n l i m i t i s 4 ng t e t r a e t h y l s i l a n e . 5.4. FLAME-IONIZATION DETECTOR WITH HYDROCARBON BACKGROUND F r i t z e t a1 .22 found w i t h an FID t h a t the response t o s i l i c o n compounds showed
an i n v e r s i o n s t a r t i n g from a c e r t a i n mass f l o w - r a t e o f the s i l i c o n compound. They ascribed t h i s e f f e c t t o two c o n t r a d i c t o r y phenomena: t h e production and the l o s s o f charged p a r t i c l e s i n the system. Based on these f i n d i n g s , they created conditions causing the i o n l o s s t o p r e v a i l i n the detector. They used a hydrogen-hydrocarbon flame (by supplying acetylene t o the d e t e c t o r burner) and found t h a t the detector response t o organic s i l i c o n compounds was negative when t h e hydrocarbon concentration i n the d e t e c t o r exceeded a c e r t a i n value. On modifying t h i s d e t e c t i o n system by i n t r o d u c i n g methane i n t o the hydrogen f l o w z 3 ( t e n t h s o f a m i l l i l i t r e per minute o f methane) t o t h e FID (oxygen-hydrogen flame), the negative response o f t h i s d e t e c t o r a t t a i n e d a value o f 0.2 C/g o f s i l i c o n . This value considerably exceeds the response o f t h e common FID as expressed i n coulombs per gram o f carbon. The FID t o which a hydrocarbon i s supplied gives, a t c e r t a i n hydrogen f l o w rates, a negative response t o inorganic gases t o which the common FID mostly i s n o t s e n s i t i v e (see section 5.1). On supplying methane t o the detector, e i t h e r i n t h e hydrogen o r i n the c a r r i e r gas i n amounts causing t h e background c u r r e n t due t o hydrocarbon i o n i z a t i o n t o a t t a i n values o f ca. 3
lo-''
A, the response
o f t h i s detectorz4 t o hydrogen sulphide, sulphur dioxide and carbon d i s u l p h i d e mole), i s about 2 10- 4 C/mole (which represents a d e t e c t i o n l i m i t o f 1
the response t o oxygen and n i t r o u s oxide i s about 4 response t o carbon oxide and carbon d i o x i d e i s about 3
C/molez5 and t h e C/mole6. When
perfluoromethane was used as t h e c a r r i e r gas, the response t o nitrogen, helium C/moleZG.
and carbon dioxide was about 5
5.5. SELECTIVE DETECTION OF HALOGEN COMPOUNDS Under c e r t a i n conditions (higher hydrogen f l o w - r a t e , the e l e c t r o d e positioned near the flame)
, the
s e n s i t i v i t y o f t h e flame-ionization d e t e c t o r f o r halogen
compounds may be higher than i t i s under common conditions4. When hydrogen i s employed as the c a r r i e r gas and oxygen instead o f a i r , the s e n s i t i v i t y o f t h e FID towards halogen compounds i s approximately two orders o f magnitude h i g h e r than t h a t o f the common d e t e c t o r
27 The response i s i n d i r e c t p r o p o r t i o n t o the I
number o f c h l o r i n e atoms i n t h e molecule. For instance, w i t h flow-rates f o r hydrogen o f 100 ml/min and oxygen o f 50 ml/min, and an a p p l i e d p o t e n t i a l o f
+170 V , the molar responses o f mono-, d i - and trichlorobenzene were 23, 45 and 69 C, respectively, i.e., i n t h e r a t i o 1:2:3. However, t h e background c u r r e n t and the detector noise are a l s o several orders o f magnitude h i g h e r than they are w i t h the standard FIDZ8 ( 1
10"
and 5
10-l' A, r e s p e c t i v e l y ) .
REFERENCES
1 T.M. Sugden, Ann Rev. Phys. Chem., 13 (1963) 369. 2 J.C. Sternberg, W.S. Gallaway and D.T.L. Jones, i n N. Brenner, J.E. Callen and M.D. Weiss ( E d i t o r s ) , Gas Chromatography, Academic Press, New York, 1962, p. 231. 3 B.L. Walker, J . Gas Chromatogr., 4 (1966) 384. 4 M. Dressler, J . Chromatogr., 42 (1969) 408. 5 P. Russev, T.A. Gough and C.J. Woolam, J . Chromatogr., 119 (1976) 461. 6 B.A. Schaefer and D.M. Douglas, J . Chromatogr. Sei., 9 (1971) 612. 7 R.W. McCoy and S.P. Cram, J. Chromatogr. Sci., 7 (1969) 17. 8 W.A. Aue and H.H. H i l l , Jr., J . Chromatogr., 74 (1972) 319. 9 H.H. H i l l , Jr. and W.A. Aue, J. Chromatogr. S c i . , 12 (1974) 541. 10 H.H. H i l l , Jr. and W.A. Aue, J . Chromatogr., 122 (1976) 515. 11 M.D. DuPuis and H.H. H i l l , Jr., AnaZ. Chem., 51 (1979) 292. 12 J.H. Wagner, C.H. L i l l i e , M.D. DuPuis and H.H. H i l l , Jr., AnaZ. Chem., 52 (1980) 1614. 13 D.R. Hansen, T.J. G i l f o i l and H.H. H i l l , Jr., AnaZ. Chem., 53 (1981) 857. 14 J.E. Roberts and H.H. H i l l , Jr., J . Chromatogr., 176 (1979) 1. 15 D.R. Hansen and H.H. H i l l , Jr., J . Chromatogr., 303 (1984) 331. 16 H.H. H i l l , Jr. and W.A. Aue, J. Chromatogr., 140 (1977) 1 . 17 M.A. Osman, H.H. H i l l , Jr., M.W. Holdren and H.H. Westberg, AnaZ. Chem., 51 (1979) 1286. 18 M.A. Osman and H.H. H i l l , Jr., J . Chromatogr., 213 (1981) 397. 19 M.A. Osman and H.H. H i l l , Jr., J . Chromatogr., 232 (1982) 430. 20 M.A. Osman and H.H. H i l l , Jr., Anal. Chem., 54 (1982) 1425. 21 M.A. Osman and H.H. H i l l , Jr., J . Chromatogr., 264 (1983) 149. 22 0. F r i t z , G. Garzb, T. Szekely and F. T i l l , Acta Chim. Hung., 45 (1965) 301. 23 G. Garzd and D. F r i t z , i n A.B. L i t t l e w o o d ( E d i t o r ) , Gas Chromatography 1966, I n s t i t u t e o f Petroleum, London, 1967, p . 150.
107
24 25 26 27
B.A. B.A. W.C. A.E. 158 28 C.F.
Schaefer, Anat. C k m . , 42 (1970) 448. Schaefer, J . Chromatogr. Sci., 10 (1972) 110. Askew, Anat. Chem., 44 (1972) 633. Karagozler, C.F. Simpson, T.A. Gough and M.A. Pringuer, J . Chrornatogr., (1978) 139. Simpson and T.A. Gough, J . Chromatogr. S c i . , 19 (1981) 275.
Chapter 5
FLAME- ION I Z A T ION D E E CTOR CONTENTS
............................ ........... ....... .............. ...............................
5.1. I n t r o d u c t i o n 5.2. Hydrogen atmosphere flame-ionization d e t e c t o r 5.3. Hydrogen atmosphere flame-ionization d e t e c t o r f o r s i l i c o n compounds 5.4. Flame-ionization d e t e c t o r w i t h hydrocarbon background 5.5. Selective d e t e c t i o n o f halogen compounds References 5.1.
91 92 103 105 106 106
INTRODUCTION
The basic r e a c t i o n s taking place i n the oxygen-hydrogen flame can be described by the f o l l o w i n g equations t :
H
t
O2
Z OH
t
0
(5.1)
0
t
H2
OH
t
H
(5.2)
H2 f H20
t
OH
t
H
(5.3)
The r a d i c a l s formed i n the r e a c t i o n zone o f the flame r e a c t exothermically as f o l l o w s (M represents a general flame-gas molecule): H OH
H
t
t
t
M + H2
H + M
+
t
'
M
H20 +
(5.4)
M
(5.5)
and, i f organic compounds are present, chemi-ionization occurs: CH +
o
-f
CHO+ + e-
(5.6)
Various s u b s t i t u e n t s on the i n d i v i d u a l carbon atoms i n t h e molecule o f a compound a f f e c t i o n i z a t i o n i n d i f f e r e n t ways (commonly reducing the i o n i z a t i o n compared w i t h hydrogen). This e f f e c t has l e d t o the determination o f t h e soc a l l e d number o f e f f e c t i v e carbon atoms i n a molecule o f a compound2, the i o n i z a t i o n o f an alkane carbon atom being taken as a reference, i.e.,
a value
o f u n i t y i s ascribed t o the response due t o an alkane carbon atom. For t h i s reason, the f l a m e - i o n i z a t i o n d e t e c t o r (FID) i s expected t o respond o n l y t o organic compounds containing an e f f e c t i v e ( i o n i z a b l e ) carbon. Inorganic gases, carbon oxides and carbon d i s u l p h i d e should y i e l d no response ( t h e e f f e c t i v e carbon number f o r C=O and C=S being zero). I t has been found, however, t h a t ,
a t increased hydrogen flow-rates
3
and w i t h an electrode geometry t h a t causes
the electrodes t o be heated by the flame4, carbon d i s u l p h i d e gives a response t h a t i s a thousandth t o a hundredth o f t h a t o f hydrocarbons. A t increased hydrogen flow-rates, the detector gives a response t o n i t r i c oxide5 t h a t i s commensurate w i t h the methane response. Under these conditions, the d e t e c t o r a l s o t o carbon dioxide, carbon monoxide, n i t r o u s
gives a lower
oxide, n i t r o g e n dioxide, helium, oxygen and hydrogen sulphide. None o f the above-mentioned responses has the character o f a s e l e c t i v e response, because the molar responses t o organic carbon compounds d i f f e r from each other only i n the number o f the e f f e c t i v e carbon atoms i n the s o l u t e molecules; the s l i g h t l y anomalous responses t o inorganic gases are small compared w i t h the hydrocarbon responses and they d i s p l a y the same p o l a r i t y ( t h e number o f ions i s increasing). The aim o f t h i s chapter i s t o describe m o d i f i c a t i o n s o f t h e f l a m e - i o n i z a t i o n detector t h a t g i v e a s e l e c t i v e response t o c e r t a i n types o f compounds w i t h regard t o a higher response and opposite p o l a r i t y . I n f a c t , t h i s chapter should also include the a l k a l i flame-ionization d e t e c t o r (AFID), because i t i s a m o d i f i c a t i o n o f the FID. However, the modifications o f the AFID cover a broader spectrum (e.g.,
a d d i t i o n a l electrode, the method o f heating the a l k a l i metal
source) and, e s s e n t i a l l y , the AFID i s so widely and manufactured d i s t r i b u t e d t h a t i t has been considered as a separate type o f detector. I n 1969, an FID was described
7 t h e electrodes o f which were made o f 0.051 mm
diameter h i g h - p u r i t y platinum wires. These microelectrodes were adjustable and i t was w i t h t h e i r varying distances from the d e t e c t o r j e t t h a t a s e l e c t i v e
response, i.e.
, different
responses f o r d i f f e r e n t compounds, was obtained. The
responses t o ketones and a1 i p h a t i c and aromatic hydrocarbons displayed maximum values a t a v e r t i c a l distance o f the electrodes from the j e t o f ca. 6 mm, whereas the response t o polychloroalkanes was maximal a t a distance o f ca. 2 mm. When the electrodes were placed i n the region o f the flame corresponding t o the optimum response f o r a l i p h a t i c o r aromatic hydrocarbons, t h e chloroalkane response was f o u r orders o f magnitude lower. 5.2. HYDROGEN ATMOSPHERE FLAME-IONIZATION DETECTOR Aue and H i l l 8 found i n 1972 t h a t , when the hydrogen and a i r flows were interchanged i n the flame-ionization detector, i.e.,
when a i r was introduced
i n t o t h e j e t w h i l e hydrogen was flowing around it, t h i s d e t e c t o r gave a s e l e c t i v e response t o organometallic compounds. I n t h i s instance the flame burns i n a hydrogen atmosphere and the hydrogen f l o w i s an order o f magnitude h i g h e r than the a i r f l o w (hence t h e name hydrogen atmosphere f l a m e - i o n i z a t i o n detector,
93
-90v
Collector electrode
from Gc
F i g . 5.1. C r o s s - s e c t i o n o f a s i l a n e - d o p e d HAFID. (From r e f . 10.) HAFID). F u r t h e r i n v e s t i g a t i o n s showed t h a t t h e i n t r o d u c t i o n o f s i l i c o n compounds i n t o t h e d e t e c t i o n system (e.g.,
by t h e c a r r i e r gas f r o m t h e s t a t i o n a r y phase,
t h e s i l i c o n septum, e t c . ) i s e s s e n t i a l f o r t h e HAFID response t o o r g a n o m e t a l l i c 9 compounds t o o c c u r
.
On t h e b a s i s o f t h e s e f i n d i n g s , a d e t e c t o r was designed” s i l a n e was s u p p l i e d ( F i g . 5.1).
t o which gaseous
The c a r r i e r gas f r o m t h e column and an oxygen-
n i t r o g e n m i x t u r e e n t e r t h e 1.2 mm q u a r t z j e t o f t h e d e t e c t o r , which i s 70 mm f r o m t h e base o f t h e d e t e c t o r body. The hydrogen and s i l a n e i n l e t i s s i t u a t e d a t t h e d e t e c t o r base n e a r t h e j e t . The c o l l e c t o r e l e c t r o d e i s o f t h e shape o f a 5-mm d i a m e t e r r i n g , 50 mm f r o m t h e j e t . Under t h e s e c o n d i t i o n s , t h e HAFID responds t o organornetall i c compounds .and t h i s response, depending on t h e presence o f a metal i n a m o l e c u l e o f t h e compound, i s s e v e r a l o r d e r s o f magnitude h i g h e r than t h e response t o hydrocarbons ( s e e T a b l e 5 . 1 ) . The d e t e c t o r i s most s e n s i t i v e t o manganese and aluminium compounds, t h e minimum d e t e c t a b l e mass r a t e f o r these compounds and f o r i r o n , t i n and chromium compounds b e i n g l o w e r t h a n o r commensurate w i t h t h e FID minimum d e t e c t a b i l i t y o f hydrocarbons (1.7 g/ 11 sec o f Mn f o r manganese 1. The background c u r r e n t o f t h e HAFID i s h i g h e r t h a n t h a t o f t h e FID12’13. T h e r e f o r e , t h e n o i s e o f t h e HAFID i s i n c r e a s e d by a p p r o x i m a t e l y an o r d e r o f magnitudeI3. The d e t e c t i o n l i m i t and s e l e c t i v i t y a r e a l s o e v i d e n t f r o m F i g s . 5.2 and 5.3. A comparison o f t h e g a s o l i n e chromatograms
ID
P
TABLE 5.1
R E S P O N S E S OF MODEL COMPOUNDS S e l e c t i v i t y a g a i n s t tetradecane, taken as t h e r a t i o of i n j e c t e d amounts o f tetradecane and t h e o r g a n o m e t a l l i c t h a t produce t h e Same peak area o f 1-10-11 C. A minus s i g n preceding a s e l e c t i v i t y v a l u e i n d i c a t e s a n e g a t i v e response. (From r e f . 10.) Compound
Column temperature (OC)
Selectivity
A1 u m i n i u d 111) hexafluoroacetylacetonate Ferrocene T e t r a v in y l t i n T e t r a p r o p y l t in Tetraethylt i n Chromium( 111) hexafluoroacetylacetonate Chromi um hexacarbonyl Chromium( 111) trifluoroacetylacetonate T e t r a e t h y l 1ead Tetrabutyl lead T r i phenylan timony Tungsten hexacarbonyl Molybdenum hexacarbonyl T e t r a b u t y l germane T r i - n - b u t y l phosphate Chl orobenzene Bromobenzene Triphenyl arsine T r i p h e n y l b i srnuth
50 145 110 140 90 60 30 170 100 190 250 50 30 140 190 70 80 250 250
3.2 1.9 2.6 1.9 1.6 1.6 1.5 1.1 1.1 6.0 1.2
8.6 5.0 3.5 -3.2 -3.2 -3.2 -3.2 1.3
. 105 .. 105 104 104 .. 104 104 -. 104 .. 104 104 103
. 102 103 lo2
-
102 102 102 102 102
lo2
D e t e c t i o n 1i m i t (9)
6.0 2.1 1.0 7.8 2.1 1.5 2.3 1.7 5.1 7.5 1.3 5.3 3.5 5.5 1.1 1.4 1.4 2.3 6.3
. 10-13 10-12
- 10-11 - 10-11 10-11
-- 10-11 10-10
. 10-9 10-10 . 10-9 10-9 10-9
-. 10-9 10-9
Piaselenole Iron( I I I ) t r i f l uoroacetylacetonate Di -n-butyl di sul phi de F1 uorobenzene Ni trobenzene Diethylrnercury Tetraethyl s i 1 ane Tetradecane
165 175 140 30 110 70 50 140
-9.2 8.0 -2.0 7.1 3.1 2.9 1.9 1.0
- 10; - 10101 10; 10 - 100 100 100
5.7 4.1 2.3 6.7 1.8 1.3 4.1 3.7
.. 10-9 10-9 .. 10-7 10-7 . 10-7 10-7
96 (Fig. 5.3) obtained by means o f an HAFID and a FID c l e a r l y shows the advantage of the HAFID i n d e t e c t i n g organometall i c compounds i n mixtures w i t h hydrocarbons.
The molar response o f the compounds seems t o be given by the number o f heteroatoms i n the molecule o f the compound13 (Table 5.2).
S i m i l a r l y t o t h e FID, t h e
l i n e a r dependence o f the molar response on the number o f e f f e c t i v e carbon atoms 12 i n the molecule i s r e t a i n e d
.
The wide gap between the j e t t i p and the base o f t h e d e t e c t o r body i s o f great importance w i t h regard t o the s e l e c t i v i t y o f t h e HAFID response13. Table 5.3 compares the response s e l e c t i v i t i e s and t h e d e t e c t i o n 1 i m i t s f o r a common massive j e t and a j e t t i p modified w i t h q u a r t z o r s t a i n l e s s - s t e e l t u b i n g (10 mm), w i t h the same distance o f the c o l l e c t o r electrode from the j e t t i p . Compared w i t h a normal j e t , the response o f a d e t e c t o r w i t h a j e t t i p i s s u b s t a n t i a l l y lower f o r hydrocarbons and, therefore, the s e l e c t i v i t y o f t h e response t o hydrocarbons i s higher. The reduction i n the response t o hydrocarbons i n t h e
I1
HAFID compared w i t h the FID i s believed t o be due t o an o x i d a t i o n step t h a t occurs i n the oxygen-rich precombustion zone o f t h e HAFID12’13, i n a d d i t i o n t o
a
C
1
-b
Fig, 5.2. Chromatograms of model compounds: ( a ) 5 pg o f aluminium(II1) hexafluoroacetylacetonate, 50OC; ( b ) 10 pg o f ferrocene, 145OC; ( c ) 50 pg o f chromium( 111) trifluoroacetylacetonate, 170%. Glass column ( 1 m) packed w i t h 6% OV-17 on Chromasorb W, temperature as i n d i c a t e d , Flow-rates: hydrogen 1600 ml/min plus 7 pl/mi.n s i l a n e , oxygen 125 ml/min, n i t r o g e n 100 ml/min, n i t r o g e n from a GC column 40 nl/min. (From r e f . 10).
97 FID
HAFID
I
4
6
30
50 90120
HAFID
'C
30
50 90120 'C
Fig. 5.3. HAFIO and FID analyses o f a l k y l l e a d compounds i n g a s o l i n e . L e f t , r e g u l a r gasoline; r i g h t , premium gasoline. Glass column (6 ft. x 2 nun I.D.) packed w i t h 80-100 mesh Ultrabond 20 M. Temperature, programmed from 30OC ( 5 min) t o 120Oc ( 5 min) a t 2OoC/min. HAFID flow-rates: hydrogen 1600 ml/min, s i l a n e 34 ppm, oxygen 150 ml/min, a i r 120 ml/min. (From r e f . 11 w i t h permission.) the effect o f the p o s i t i o n and p o t e n t i a l o f t h e c o l l e c t o r electrode. The massive j e t d i s s i p a t e s the heat from the precombustion zone, decreasing the o x i d a t i o n 13 therein. The j e t t i p then reduces the c o o l i n g e f f e c t and enhances o x i d a t i o n
.
The l i n e a r i t y o f response o f the d e t e c t o r i n which oxygen enters t h e detect i o n space a t the d e t e c t o r base covers about t h r e e orders o f magnitude f o r aluminium, chromium, t i n , lead and i r o n compounds".
I f hydrogen i s supplied t o
the d e t e c t o r above t h e j e t t i p only, the response i s n o t a l i n e a r f u n c t i o n o f the amount o f compound 11
.
TABLE 5.2 EVALUATION OF RESPONSE (From r e f . 13 w i t h permission.) Compound
Dodecane Diallyldibutyl t i n Tetraethyl t i n Tetra buty 1ti n Tetra-n-propyl t i n Hexabutyl d i t i n
*
Due t o the column.
Detection 1 i m i t (pg)
S e n s i t i v i t y (C/mol)
Selectivity
FID
HAF ID
FID
HAFID
FID
HAF I D
300 800 2000 250 1400 760
48,000 800 38 70 22 13
1.4 0.38 0.52 1.8 1.4 2.1
0.034 12' 29 31 38 55
1 .oo 0.27 0.37 1.3 1 .o 1.5
3.5 8.5 9.1 1.1 1.6
1 .oo
102
lo2
102 103
lo3
TABLE 5.3 COMPARISON OF JET T I P S
(From r e f . 13 w i t h permission.) Compound
Tetraethyl t i n Dodecane
Mass ive j e t
Stainless-steel j e t t i p
Quartz j e t t i p Detection 1i m i t
Sel ec t iv i t y
10 P9 0.5 ug
5.9 1
D e t e c t ion 1i m i t
Selectivity
Detection limit
Sel ec t i v i t y
38 P9 48 ng
110 1
15 P9 0.2 119
1.2 1
.
104
-
104
W W
F i g . 5.4. Background c u r r e n t and responses o f model compounds a t d i f f e r e n t c o l l e c t o r e l e c t r o d e p o t e n t i a l s . 1, Background c u r r e n t ; 2 , t e t r a d e c a n e ( 2 p g ) ; 3, t e t r a b u t y l l e a d ( 1 0 n g ) ; 4, t e t r a b u t y l t i n (10 n g ) ; 5, f e r r o c e n e ( 1 n g ) . h = Peak h e i g h t ( a r b i t r a r y u n i t s , l o g a r i t h m i c s c a l e ) . (From r e f . 10.) F i g . 5.4 shows t h e r e l a t i o n s h i p s between response and c o l l e c t o r e l e c t r o d e p o t e n t i a l f o r t i n , l e a d and i r o n compounds and f o r a hydrocarbon. The n e g a t i v e l y p o l a r i z e d e l e c t r o d e i s c l e a r l y more advantageous; compared w i t h t h e p o s i t i v e c o l l e c t o r e l e c t r o d e , t h e response t o o r g a n o m e t a l l i c compounds i n h i g h e r and t h e hydrocarbon response and background c u r r e n t a r e l o w e r . The responses o f a l l o f t h e i n v e s t i g a t e d compounds and t h e background i o n i z a t i o n c u r r e n t i n c r e a s e t o maximum values as t h e p o t e n t i a l i n c r e a s e s . The optimum e l e c t r o d e p o t e n t i a l w i t h r e g a r d t o t h e s e l e c t i v i t y and t h e minimum background c u r r e n t i s -90 V. F o r a c o n s t a n t c o n c e n t r a t i o n o f a s i l i c o n compound e n t e r i n g t h e d e t e c t o r , t h e response o f t h e HAFID depends on .the d i s t a n c e o f t h e c o l l e c t o r e l e c t r o d e from t h e
The response t o o r g a n o m e t a l l i c compounds i n c r e a s e s t o maximum
values w i t h i n c r e a s i n g d i s t a n c e o f t h e e l e c t r o d e . T h i s response enhancement i s 1.5 o r d e r s o f magnitude f o r a d i s t a n c e r a n g i n g f r o m 10 t o 50 mm. On t h e o t h e r hand, t h e hydrocarbon response decreases by a p p r o x i m a t e l y an o r d e r o f magnitude w i t h i n t h i s range. The background c u r r e n t a l s o decreases w i t h i n c r e a s i n g distance o f t h e c o l l e c t o r electrode from the j e t . The d i s t a n c e o f t h e c o l l e c t o r e l e c t r o d e f r o m t h e j e t a l s o a f f e c t s t h e amount o f s i l a n e t h a t has t o be s u p p l i e d t o t h e d e t e c t o r . As mentioned above, t h e HAFID g i v e s a s e l e c t i v e response t o o r g a n o m e t a l l i c compounds o n l y i n t h e presence o f a s i l i c o n compound. The HAFID response and t h e background c u r r e n t i n c r e a s e t o
101
ELECTRODE HEIGHT
OX
0.3 m
0
-i
0.2
L
I
2 w
I 0.1 Y
4
[L
0.0 V
\;
i
3
3
5
4
6
7
8
9
1
1011
Fig. 5.5. HAFID response as a f u n c t i o n o f s i l a n e c o n c e n t r a t i o n f o r v a r i o u s e l e c t r o d e heights. Flow-rates: hydrogen 1500 ml/min, oxygen 120 ml/min, a i r 165 ml/min. (From r e f . 14.) maximum values w i t h i n c r e a s i n g c o n c e n t r a t i o n o f s i l a n e i n t h e d e t e c t o r . The response t o hydrocarbons and t h e background c u r r e n t a t t a i n t h i s maximum v a l u e 10 a t h i g h e r s i l a n e c o n c e n t r a t i o n s than does t h e response t o o r g a n o m e t a l l i c s
.
The optimum s i l a n e c o n c e n t r a t i o n , however, i s d i f f e r e n t f o r each d i s t a n c e between t h e c o l l e c t o r e l e c t r o d e and t h e j e t 1 4 ( F i g . 5.5). The g r e a t e r t h e e l e c t r o d e distance, t h e lower i s t h e optimum s i l a n e c o n c e n t r a t i o n . With optimum s i l a n e c o n c e n t r a t i o n s t h e HAFIO response t o o r g a n o m e t a l l i c s increases w i t h i n c r e a s i n g d i s t a n c e o f t h e e l e c t r o d e . Greater e l e c t r o d e d i s t a n c e s ( 7 0 mm) a r e advantageous f o r t h i s reason, because b o t h minimum d e t e c t a b i l i t y f o r organom e t a l l i c s (improved s e n s i t i v i t y o f response, l o w e r background c u r r e n t and n o i s e ) and s e l e c t i v i t y ( h i g h e r s e n s i t i v i t y o f response t o o r g a n o m e t a l l i c s and reduced s e n s i t i v i t y t o hydrocarbons) a r e increased. As can be seen from F i g . 5.5,
t h e response o f an HAFIO o f t h e t y p e used i s
n e g a t i v e a t a d i s t a n c e o f t h e c o l l e c t o r e l e c t r o d e from t h e j e t o f 10 mm14. T h i s n e g a t i v e response i s r e l a t e d t o t h e shape and p o s i t i o n o f t h e c o l l e c t o r e l e c trode. The e l e c t r o d e was pin-shaped and s i t u a t e d o u t o f t h e j e t c e n t r e i n t h i s instance. When u s i n g a ring-shaped e l e c t r o d e s i t u a t e d above t h e j e t , t h e response i s a l s o p o s i t i v e f o r a d i s t a n c e o f 10 mm".
The optimum d i s t a n c e f o r
n e g a t i v e response depends on t h e e n t i r e d e t e c t o r design15. Negative responses a l s o occur a t low p o s i t i v e p o t e n t i a l s o f t h e c o l l e c t o r e l e c t r o d e (see F i g . 5.4) and a t h i g h s i l a n e concentrations".
The d e t e c t i o n l i m i t s f o r t e t r a e t h y l t i n and
t e t r a e t h y l l e a d i n t h e negative mode are lower by about one o r d e r o f magnitude than those i n t h e p o s i t i v e model5. However, parameter s e t t i n g s f o r t h e n e g a t i v e response must be c a r e f u l l y c o n t r o l l e d and a r e l a t i v e l y h i g h s i l a n e concentrat i o n may prove t o cause long-term degradation o f response due t o s i l i c o n d i o x i d e
102 deposits on the electrode. Thus f o r r o u t i n e analyses t h e p o s i t i v e rode seems t o 15 be more s u i t a b l e
.
The HAFID r e q u i r e s s u b s t a n t i a l l y higher hydrogen flow-rates than t h e FID.
14
The HAFID response t o organometallics increases t o a maximum a t about 3 l/min
w i t h increasing hydrogen f l o w - r a t e (4 ppm o f s i l a n e , 70 mm e l e c t r o d e distance), b u t t h e hydrocarbon response d r a s t i c a l l y increases w i t h flow-rates below 650 ml/min9. The means o f supplying hydrogen t o t h e detector a f f e c t s t h e amount o f s i l a n e needed t o achieve maximum response, both i n p o s i t i v e ”
.
and negative mode 15
I n the p o s i t i v e mode, i f hydrogen enters the detector 21 mn above t h e j e t t i p t h i s concentration i s 34 ppm, and i f hydrogen i s supplied t o the detector base the optimum s i l a n e concentration i s below 10 ppm (see Fig. 5.5). The dependence o f t h e response o f an organometallic compound on t h e percentage o f n i t r o g e n i n the air-oxygen m i x t u r e supplied a l s o shows a maximum ( a t a 1 : l n i t r o g e n t o oxygen r a t i o ) . Therefore, f o r a t o t a l f l o w - r a t e through the j e t o f 305 ml/min and a n i t r o g e n c a r r i e r gas f l o w - r a t e o f 20 ml/min, the optimum a i r and oxygen flow-rates are 165 and 120 ml/min, r e ~ p e c t i v e l y ’ ~With . higher n i t r o g e n t o oxygen r a t i o s the response t o organometall i c s decreases, the hydrocarbon response increases and, consequently, t h e d e t e c t i o n s e l e c t i v i t y i s diminished 10
.
5.3. HYDROGEN ATMOSPHERE FLAME-IONIZATION DETECTOR FOR SILICON COMPOUNDS I f a s i l i c o n compound i s introduced i n t o the HAFID, i t gives a s e l e c t i v e
response t o m e t a l l i c compounds. If an i n v e r t e d system i s used, i.e.,
i f an
organometallic compound i s supplied t o the detector, the HAFID responds t o organic s i l i c o n compounds. Hence t h i s i s again an HAFID, b u t an i r o n compound, 16
commonly ferrocene, i s supplied i n t h e hydrogen f l o w i n t h i s instance
.
The detector geometry plays an important r o l e i n peak t a i l i n g I 7 . Pronounced t a i l i n g occurs w i t h l a r g e r i n n e r diameters o f the d e t e c t o r (above 11 mm). Hydrogen i s introduced i n t o the detector space below the j e t l e v e l and oxygen i s supplied t o the detector j e t together w i t h the c a r r i e r gas from t h e column. The c o l l e c t o r electrode (-90 V ) i s 110 mm from the j e t , The amount o f i r o n compound supplied t o the d e t e c t o r fundamentally changes the response 16s18-21 , (Fig. 5.6). With increasing amounts o f an i r o n compound the response t o s i l i c o n compounds increases t o a maximum (about 5 ppm i n t h i s instance) and then decreases, passing t o a negative response ( a t about 12 ppm). The s p e c i f i c course o f t h i s dependence i s a f f e c t e d by the distance o f the p o i n t o f the i n l e t o f the i r o n compound from the detector flame2’ ( t h e i n v e r s i o n from a p o s i t i v e t o a negative response i s s h i f t e d t o higher amounts o f the i r o n 18,20 compound a t l a r g e r distances) and by the type o f i r o n compound supplied The maximum negative response (about 35 ppm o f ferrocene) i s three times greater
.
than the maximum p o s i t i v e response. The detector noise increases a t the same
103 ? 20
?Q
Y
2 c
15 10
; 5
E -0 I v)
Amount of ferrocene (pprn) '
"
I
\
"
I
'
I
'
I
I
'
,
'
I
J3 -5
LL
4
10
-7 5
-20
J
Fig. 5.6. E f f e c t o f amount o f ferrocene on t h e HAFID-Si response. (From r e f . 18 w i t h permission.) TABLE 5.4 DETECTION LIMITS FOR THE HAFID-Si (From r e f . 21.) ;,lode o f HAFID o p e r a t i on
Coumpounds containing S i 1icon
Phosphorus
Iron
Chlorine
Non-doped P o s i t i v e mode Negative mode
4 ng ( r e f . 16) 50 pg ( r e f . 17) 1 ng ( r e f . 18)
7 ng 0.5 ng 1 n9
0.6 ng 1 ng 14 ng
50 ng 2.5 ng 9 ng
time, so t h a t the d e t e c t i o n l i m i t i s smaller by a f a c t o r o f about 20 i n t h e negative mode (see Table 5.4).
L i n e a r i t y o f d e t e c t i o n covers a range o f about
three orders o f magnitude i n both modes
21
.
Hence, the s e l e c t i v i t y o f response o f t h i s d e t e c t o r , ' c a l ? e d the HAFID-Si,
is
conditioned by the response l e v e l and response p o l a r i t y ( F i g . 5.7). With smaller amounts o f doping i r o n compounds i n the p o s i t i v e mode, the HAFID response t o s i l i c o n compounds i s higher than t h a t o f a d e t e c t o r i n t h e absence o f a doping agent and approximately commensurate w i t h the response o f a common FID, whereas the response t o hydrocarbons i s reduced. The s e l e c t i v i t y o f response o f S i t o C i s 18'*'
4
about 10 : I . With higher concentrations o f i r o n compounds i n the nega-
t i v e mode, the response t o s i l i c o n compounds i s negative whereas t h a t t o hydrocarbons remains p o s i t i v e . The change from a p o s i t i v e t o a negative mode i s easy t o accomplish, by increasing t h e f l o w - r a t e o f the gas containing the
104 12 L
FIO
b
HAFIO ( S i 1 positive mode
1
1
70
8L
1
1
1
1
98 112 126 9:
I
,
,
70
8L
98
,
.
112 126 T
C
2
8
HAFID ISi 1 negative mode
5
Fig. 5.7. Chromatograms o f equal amounts o f compounds using d i f f e r e n t d e t e c t i o n methods. Chromatograms obtained w i t h a standard FID (a); a HAFID doped w i t h 5 ppm o f ferrocene (b); a HAFID doped w i t h 30 ppm o f ferrocene ( c ) . Peaks: 1 = Hexane ( s o l v e n t ) ; 2 = chlorobenzene; 3 = decane; 4 = octanol; 5 = t r i e t h y l phosphate; 6 = dodecane; 7 = decanol; 8 = ferrocene; 9 = tetradecane. Methylsilicone-coated f u s e d - s i l i c a c a p i l l a r y column (10 m x 0.2 nun I.D.). Temperature, programmed from 70 t o 150OC a t 8oC/min. Helium f l o w - r a t e : 1.2 ml/min, HAFID gas flow-rates: hydrogen 1600 ml/min, oxygen 130 ml/min. FID gas flow-rates: hydrogen 30 ml/min, a i r 240 ml/min. (From r e f . 21.) doping i r o n compound, A s e l e c t i v e enhancement o f the response t o s i l i c o n compounds compared w i t h hydrocarbons was a l s o observed18 when other metals were used as doping agents, such as aluminium, n i c k e l , chromium, molybdenum, brass, platinum, copper and magnesium. The l i f e t i m e s o f the sources o f these metals are short, however.
105 The HAFID-Si gives a response approximately commensurate w i t h t h a t o f s i l i c o n compounds a l s o t o phosphorus, i r o n and c h l o r i n e compounds 18y21 (see Table 5.4). The response t o t h e l a t t e r compounds a l s o depends on t h e amount o f doping agent applied. The course o f t h i s dependence f o r phosphorus and i r o n compounds i s s i m i l a r t o t h a t f o r s i l i c o n compounds, i.e.,
t h e response t o these compounds
i s negative f o r l a r g e r amounts o f dopants supplied t o the d e t e c t o r (Fig. 5.7). Alcohols, ketones, ethers and n i t r o g e n and f l u o r i n e compounds g i v e a low posit i v e response 20
.
The HAFID responds t o organic s i l i c o n compounds 16,'7 and t o phosphorus, i r o n and c h l o r i n e compounds*' even i f no i r o n compound i s supplied t o t h e d e t e c t o r , I n t h i s instance, t h e s e n s i t i v i t y o f d e t e c t i o n i s lower than i n the presence o f ferrocene i n the p o s i t i v e mode (see Table 5.41, being approximately two t o three orders o f magnitude lower than t h a t o f t h e FID. The d e t e c t o r does n o t d i f f e r i n design from t h a t w i t h ferrocene17, the S i t o C s e l e c t i v i t y i s about 3.5 orders o f magnitude and the d e t e c t i o n l i m i t i s 4 ng t e t r a e t h y l s i l a n e . 5.4. FLAME-IONIZATION DETECTOR WITH HYDROCARBON BACKGROUND F r i t z e t a1 .22 found w i t h an FID t h a t the response t o s i l i c o n compounds showed
an i n v e r s i o n s t a r t i n g from a c e r t a i n mass f l o w - r a t e o f the s i l i c o n compound. They ascribed t h i s e f f e c t t o two c o n t r a d i c t o r y phenomena: t h e production and the l o s s o f charged p a r t i c l e s i n the system. Based on these f i n d i n g s , they created conditions causing the i o n l o s s t o p r e v a i l i n the detector. They used a hydrogen-hydrocarbon flame (by supplying acetylene t o the d e t e c t o r burner) and found t h a t the detector response t o organic s i l i c o n compounds was negative when t h e hydrocarbon concentration i n the d e t e c t o r exceeded a c e r t a i n value. On modifying t h i s d e t e c t i o n system by i n t r o d u c i n g methane i n t o the hydrogen f l o w z 3 ( t e n t h s o f a m i l l i l i t r e per minute o f methane) t o t h e FID (oxygen-hydrogen flame), the negative response o f t h i s d e t e c t o r a t t a i n e d a value o f 0.2 C/g o f s i l i c o n . This value considerably exceeds the response o f t h e common FID as expressed i n coulombs per gram o f carbon. The FID t o which a hydrocarbon i s supplied gives, a t c e r t a i n hydrogen f l o w rates, a negative response t o inorganic gases t o which the common FID mostly i s n o t s e n s i t i v e (see section 5.1). On supplying methane t o the detector, e i t h e r i n t h e hydrogen o r i n the c a r r i e r gas i n amounts causing t h e background c u r r e n t due t o hydrocarbon i o n i z a t i o n t o a t t a i n values o f ca. 3
lo-''
A, the response
o f t h i s detectorz4 t o hydrogen sulphide, sulphur dioxide and carbon d i s u l p h i d e mole), i s about 2 10- 4 C/mole (which represents a d e t e c t i o n l i m i t o f 1
the response t o oxygen and n i t r o u s oxide i s about 4 response t o carbon oxide and carbon d i o x i d e i s about 3
C/molez5 and t h e C/mole6. When
perfluoromethane was used as t h e c a r r i e r gas, the response t o nitrogen, helium C/moleZG.
and carbon dioxide was about 5
5.5. SELECTIVE DETECTION OF HALOGEN COMPOUNDS Under c e r t a i n conditions (higher hydrogen f l o w - r a t e , the e l e c t r o d e positioned near the flame)
, the
s e n s i t i v i t y o f t h e flame-ionization d e t e c t o r f o r halogen
compounds may be higher than i t i s under common conditions4. When hydrogen i s employed as the c a r r i e r gas and oxygen instead o f a i r , the s e n s i t i v i t y o f t h e FID towards halogen compounds i s approximately two orders o f magnitude h i g h e r than t h a t o f the common d e t e c t o r
27 The response i s i n d i r e c t p r o p o r t i o n t o the I
number o f c h l o r i n e atoms i n t h e molecule. For instance, w i t h flow-rates f o r hydrogen o f 100 ml/min and oxygen o f 50 ml/min, and an a p p l i e d p o t e n t i a l o f
+170 V , the molar responses o f mono-, d i - and trichlorobenzene were 23, 45 and 69 C, respectively, i.e., i n t h e r a t i o 1:2:3. However, t h e background c u r r e n t and the detector noise are a l s o several orders o f magnitude h i g h e r than they are w i t h the standard FIDZ8 ( 1
10"
and 5
10-l' A, r e s p e c t i v e l y ) .
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