Ultrahigh Resolution Chromatography - American Chemical Society

6 Use of a Small Volume Thermal Conductivity Detector for Capillary Gas Chromatography Practical Aspects RICHARD SIMON and GREGORY WELLS Downloaded by AUBURN UNIV on September 29, 2017 | http://pubs.acs.org Publication Date: April 26, 1984 | doi: 10.1021/bk-1984-0250.ch006

Walnut Creek Division, Varian Instrument Group, Walnut Creek, CA 94598

Capillary chromatography is attractive for analysis of complex samples, primarily because the capillary columns have very high efficiencies when compared with packed columns. In practice however, capillary columns are not operated at maximum efficiency, and a trade-off is made to reduce analysis time by s a c r i f i c i n g chromatographic resolution. The use of capillary columns is a relatively recent advance in chromatography when compared with the thermal conductivity detector (TCD). The ΤCD is well established and is among the most commonly used detectors in gas chromatography. Some of the advantages of the TCD Include the simplicity, stability, and universal nature of the detector. This paper addresses the operating characteristics and performance of a 30μlTCD when coupled with a high resolution capillary gas chromatography Differences in the peak shape, peak symmetry, electronic bandwidth, column flow rates, and column bleed invoke different responses for the TCD with a capillary column as opposed to a packed column. These differences raise several questions which will be addressed in this paper: (1) How does the detector volume of the TCD affect the observed chromatographic performance? (2) How is the best performance obtained in practice? (3) How do detection limits, linear dynamic range, and sensitivity change using a capillary chrornatograph as opposed to a packed chromatograph? 0097-6156/ 84/0250-0059S06.00/0 © 1984 American Chemical Society

Ahuja; Ultrahigh Resolution Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

ULTRAHIGH RESOLUTION CHROMATOGRAPHY

Downloaded by AUBURN UNIV on September 29, 2017 | http://pubs.acs.org Publication Date: April 26, 1984 | doi: 10.1021/bk-1984-0250.ch006

60

In depth d i s c u s s i o n s of the TCD can be found i n most texts on gas c h r o m a t o g r a p h y [1-4]· Lawson and M i l l e r [5] have w r i t t e n a r e v i e w o f t h e TCD, and g i v e m u l t i p l e r e f e r e n c e s t o d i s c u s s i o n s r e g a r d i n g t h e TCD* A c o m p a r i s o n o f the v a r i o u s modes o f operation has been made by Wells and Simon [6] which compares the constant voltage, constant current, and constant mean temperature modes of operation on both an e m p i r i c a l and a t h e o r e t i c a l basis* The TCD measures changes i n the thermal c o n d u c t i v i t y of the c a r r i e r g a s , p e r t u r b e d by t h e e l u t l n g a n a l y t e * Thermal c o n d u c t i v i t y detectors which r e l y on d i f f u s i o n by the analyte to the h o t f i l a m e n t g e n e r a l l y have t h e l o w e s t l i m i t o f d e t e c t i o n (due t o the v e r y low n o i s e o f the d e t e c t o r ) , y e t have v e r y l o n g time constants* Flow-through thermal c o n d u c t i v i t y detectors can r e t a i n peak symmetry without e x c e s s i v e l y l a r g e v o l u m e t r i c time constants* The s e n s i t i v i t y o f the TCD i s d e f i n e d as the s i g n a l o u t p u t per u n i t concentration [7], or as:

W

where

S i s the TCD s e n s i t i v i t y (mVmL/mg), A i s the Integrated peak area (mV min), F i s the gas flow r a t e a t the detector (mL/mln), and W i s the weight of the sample (mg)* S e n s i t i v i t y o f a TCD i s dependent upon the d e t e c t o r b l o c k t e m p e r a t u r e , t h e d e t e c t o r f i l a m e n t t e m p e r a t u r e , c a r r i e r gas, earnpie, d e t e c t o r volume, and d e t e c t o r c o n f i g u r a t i o n * These s h o u l d be l i s t e d when s p e c i f y i n g a s e n s i t i v i t y f o r a TCD, when comparing d i f f e r e n t Instruments, or d i f f e r e n t Instrumental conditions· Since the TCD i s a flow s e n s i t i v e (concentration) detector, the low flow r a t e s g e n e r a l l y associated with c a p i l l a r y systems s h o u l d enhance t h e o b s e r v e d r e s p o n s e o f a low volume (30 u£ ) TCD* The o b s e r v e d r e s p o n s e , i . e . peak h e i g h t , o r peak a r e a , i s I n v e r s e l y p r o p o r t i o n a l t o t h e gas f l o w r a t e a t t h e d e t e c t o r (assuming the same c a r r i e r gas flow r a t e through the column, with any a d d i t i o n a l gas being Introduced as a make-up flow j u s t p r i o r to the detector)* Optimum detector response should be obtained at those flow rates j u s t s u f f i c i e n t to e f f i c i e n t l y sweep-out the dead volumes a s s o c i a t e d with the detector and connecting tubing* S i n c e s h a r p , narrow peaks a r e g e n e r a t e d w i t h a c a p i l l a r y s y s t e m , the d e t e c t o r must be e f f i c i e n t l y purged by the c a r r i e r gas* Otherwise, the detector and connecting dead volumes operate as a m i x i n g o r an e x p o n e n t i a l d i l u t i o n chamber and t h e r e b y e e v e r l y d i s t o r t t h e a c t u a l c h r o m a t o g r a p h i c peak s h a p e * E l e c t r o n i c a l l y t h i s i s analogous to i n c r e a s i n g the e l e c t r o n i c c


6.

SIMON AND WELLS

time constant.

Small Volume Thermal Conductivity Detector

This v o l u m e t r i c time constant (τ ) i s given by:

τ = V

e f f

/F

(2)

c

where V i s the e f f e c t i v e volume of the TCD, and F i s the gas flow r a t e at the detector* S t e r n b e r g [8] has shown t h a t e l e c t r o n i c and v o l u m e t r i c time c o n s t a n t s c a n s e r i o u s l y d i s t o r t chromatographic peaks when τ (or RC) i s greater than one f i f t h the standard d e v i a t i o n σ of the chromatographic peak* The e f f e c t of a v o l u m e t r i c or e l e c t r o n i c time constant can be modeled as a c o n v o l u t i o n o f a G a u s s i a n peak g ( t ) w i t h a f i l t e r f u n c t i o n ( o r t r a n s f e r f u n c t i o n ) h(T). T h i s c o n v o l u t i o n generates the output s i g n a l f ( t ) defined by: e f f


61

Q

f(y)

- g(t)

* h(t)

(3)

For an e l e c t r o n i c or turbulent v o l u m e t r i c time constant i s given by: h(T) = (l/τ) exp (-Τ/τ) F o r p l u g f l o w the f i l t e r f u n c t i o n c o r r e s p o n d s averaging of the input f u n c t i o n and has the form:

h(t)

(4) to a

h(T) = 1/τ

Thus the output

τ,

simple

(5)

s i g n a l i s of the form:

f(t)

-jj"h(T)

g(t-T) dT

(6)

where a and b are the i n t e g r a t i o n l i m i t s ( f o r turbulent flow a 0, b - t , a n d f o r p l u g f l o w a » t - τ , b « t ) . B o t h t h e e x p o n e n t i a l f i l t e r and p l u g f i l t e r c a n be r i g o r o u s l y s o l v e d by the method o f L a p l a c e t r a n s f o r m s * However, t h e a n a l y t i c a l f u n c t i o n i s best c h a r a c t e r i z e d by the use of s t a t i s t i c a l moments* An a l t e r n a t i v e d e f i n i t i o n o f peak d i s t o r t i o n i s peak f i d e l i t y or the r a t i o o f the output peak height to the input peak height* Peak f i d e l i t y ( f ) can be approximated by: c 2 2 ^ 2, f = σ /(σ + τ ) lf


(7)

62


F o r a peak f i d e l i t y o f 98%, a σ/τ r a t i o o f 4*92 o r g r e a t e r i s required* I n o t h e r words, t h e s t a n d a r d d e v i a t i o n o f the peak must be greater than 4.92 V f f / * sharp, narrow peaks, the detector time constant can oe reduced by reducing the e f f e c t i v e detector volume, i n c r e a s i n g the make-up gas flow r a t e , or both* However, i n c r e a s i n g t h e make-up gas f l o w r a t e d e c r e a s e s the d e t e c t o r r e s p o n s e s , and r e s u l t s i n a l o s s I n d e t e c t i v i t y i f t h e noise remains constant* The t h e o r e t i c a l s e n s i t i v i t i e s w h i c h c a n be a t t a i n e d w i t h c o n c e n t r a t i o n d e t e c t o r s f o r b o t h packed and c a p i l l a r y s y s t e m s have been r e p o r t e d by Yang and Cram [9]· The c o n c e n t r a t i o n detectors w i l l have b e t t e r d e t e c t i v i t i e s when using a c a p i l l a r y column compared w i t h a packed column, defined by : F


e

F

o

r

c

V

η (8) D = ~ρ ΓΝ c" V Ν c L P J where D i s the d e t e c t i v i t y r a t i o of c a p i l l a r y to packed columns, V i s t h e r e t e n t i o n volume o f t h e packed column, V i s the r e t e n t i o n volume of the c a p i l l a r y column, and Ν , and N are the number of the t h e o r e t i c a l p l a t e s f o r packed and c a p i l l a r y columns respectfully* T h i s e x p r e s s i o n l m p l l c i t y assumes the same noise sources, m i x i n g e f f i c i e n c i e s w i t h i n the d e t e c t o r s , and c o m p a r a b l e e l e c t r o n i c time constants* T h i s expression can be r e w r i t t e n as: c

c

σ

F

D = ρ p σ F c c

(9)

Since ο c « σ ρ and F < F , superior performance i s expected f o r c a p i l l a r y systems as opposed to packed columns w i t h a TCD* E q u a t i o n (9) shows t h a t when a TCD i s used to d e t e c t t h e e l u e n t f r o m a c a p i l l a r y column, i t w i l l have a l o w e r d e t e c t i o n l i m i t than that f o r a packed column, or a higher s i g n a l to noise r a t i o f o r the same sample s i z e i n j e c t e d * Because the minimum d e t e c t a b l e q u a n t i t y f o r a TCD i s p r o p o r t i o n a l to the concentration of sample molecules at the peak maximum, C _ . . the Instantaneous concentration, C, f o r a Gaussian max d i s t r i b u t i o n In volume u n i t s i s : c

2

-z 2 / 2σ/ ν Z

C = C e max

The

(10)

t o t a l mass, M, can be w r i t t e n as: 2

C max

-z 2 e / σ dz ν z


(11)

6. SIMON AND WELLS


63

or

M - C

ν/2ϊΓ σ

v

max

ν

Here α ν corresponds to the gas volume a s s o c i a t e d w i t h the peak standard d e v i a t i o n or In the time domain.: σ ν

=

σ

* F c

(12)

The d e t e c t i o n l i m i t , or d e t e c t i v i t y d, i s o f t e n defined as :


d

=

2N/S

(13)

where Ν i s the n o i s e (mV) and S i s t h e d e t e c t o r s e n s i t i v i t y a s d e f i n e d i n e q u a t i o n 1. The d e t e c t i v i t y c o r r e s p o n d s t o t h e c o n c e n t r a t i o n o f sample m o l e c u l e s a t t h e peak maximum i n t h e detector volume. Thus the t o t a l mass M which must be accomodated by the column a t the d e t e c t i o n l i m i t i s given by: F

M = wy/ïJT °t c

(14)

S

When a s p l i t i n j e c t i o n technique i s used, only a f r a c t i o n of the sample i s a c t u a l l y I n j e c t e d onto t h e column. This fraction c o r r e s p o n d s t o t h e s p l i t r a t i o o f t h e i n j e c t o r , χ . The s p l i t r a t i o i s d e f i n e d as t h e r a t i o o f c o l u m n f l o w r a t e t o t h e s p l i t point flow r a t e . Sample c a p a c i t y i s a measure o f t h e maximum sample mass w h i c h c a n be accomodated w i t h o u t o v e r l o a d i n g the column. O v e r l o a d i n g o c c u r s when t h e m o b i l e t o s t a t i o n a r y p h a s e d i s t r i b u t i o n Isotherm becomes non-linear, r e s u l t i n g i n d i s t o r t e d peak shapes and l o s s of chromatographic r e s o l u t i o n . The phase r a t i o , i . e . the r a t i o n o f the volumes o c c u p i e d by the gas phase and t h e s t a t i o n a r y phase, i s g i v e n t h e symbol 3 · For w a l l coated c a p i l l a r y columns, 3 i s approximated by: 3

=

r

/2d

(15)

f

where r i s t h e d i s t a n c e f r o m the c e n t e r o f t h e column t o t h e s u r f a c e o f t h e l i q u i d phase, and d c o r r e s p o n d s t o t h e f i l m ( s t a t i o n a r y phase) thickness. The range f o r 3 values t y p i c a l l y vary from 5 to 35 f o r packed columns to 50 to 1500 f o r c a p i l l a r y columns. The e f f e c t of i n c r e a s i n g 3 f o r a f i x e d column diameter and column l e n g t h i s t o d e c r e a s e the sample r e t e n t i o n t i m e and sample c a p a c i t y o f t h e column. I n c r e a s i n g the s t a t i o n a r y f i l m t h i c k n e s s r e s u l t s i n i n c r e a s e d sample c a p a c i t y , b u t a t t h e expense o f l o n g e r r e t e n t i o n t i m e s o r h i g h e r t e m p e r a t u r e s . I n a s i m i l a r manner, i n c r e a s i n g the column diameter, but maintaining Q

f



64


the same s t a t i o n a r y phase thickness, r e s u l t s i n lower r e t e n t i o n t i m e s but an i n c r e a s e i n column sample c a p a c i t y due to the increased s t a t i o n a r y phase present on the column. A c o n v e n t i o n a l c a p i l l a r y column, 0.200mm χ 12.5M w i t h a 3 of 250 has a sample c a p a c i t y of about 50 ng a t k' « 10· In comparison, a 0.480 mm χ 28M column with a β of 250 has a sample c a p a c i t y of about 540 ng a t a k' • 10· These v e r y low sample c a p a c i t i e s a f f e c t the TCD r e q u i r e m e n t s v e r y s e v e r e l y s i n c e the minimum detectable quantity, i.e. the d e t e c t i v i t y , approaches the maximum sample c a p a c i t y o f the column as the column d i a m e t e r decreases· Experimental The s m a l l volume thermal c o n d u c t i v i t y detector used i n t h i s study was a Y-type [ 10,11,12] gas flow p a t t e r n 30 μ£ Gow-Mac Model 10955 c e l l (Gow-Mac Instrument Company, Madison, New Jersey)* T h i s c e l l was compared with the standard V a r i a n TCD (Gow-Mac Model 10952) i n a V a r i a n s e r i e s 3700 gas c h r o m a t o g r a p h . The s t a n d a r d e l e c t r o n i c s were modified f o r o p e r a t i o n with the 30 μ£ TCD. All measurements were made i n the c o n s t a n t mean t e m p e r a t u r e mode. The c a r r i e r gas was He. The flow r a t e s were regulated by two 060 ml/min mass flow c o n t r o l l e r s (model 1000, Porter Instrument Company, H a t f i e l d , Pa.) or by a p r e s s u r e r e g u l a t o r (Model 8601, Brooks Instrument D i v i s i o n , Emerson E l e c t r i c Company, H a t f i e l d , Pa.). The c a p i l l a r y column was a t t a c h e d to a m o d i f i e d 1/16" to 1/16** Swagelok u n i o n w h i c h i n t u r n was c o n n e c t e d to the a p p r o p r i a t e TCD. A l l make-up f l o w s were r e g u l a t e d so t h a t the t o t a l flow through both the reference and the sample sides were matched* A 0*48 mm χ 28M 0V-101 column (SG&E, A u s t i n Texas) was used with a V a r i a n 1080 i n j e c t o r . A 0.20 mm χ 12.5M SE-54 column (J & W S c i e n t i f i c Inc.) was used w i t h a V a r i a n 1070 i n j e c t o r . D i s t i l l e d i n g l a s s 2,2,4-trlmethylpentane (Burdick & Johnson L a b o r a t o r i e s Inc., Muskegon, MI.) was used as solvent without any a d d i t i o n a l p u r i f i c a t i o n . S o l u t e s (n-pentadecane, n-hexadecane, and n - t e t r a d e c a n e ) were o b t a i n e d from Poly Science Corporation ( N l l e s , II.) and were used without a d d i t i o n a l p u r i f i c a t i o n . A l l c a l i b r a t i o n c u r v e s were made by the method of s e q u e n t i a l dilution. Data a c q u i s i t i o n was c a r r i e d out by e i t h e r a V a r i a n 401 data system, or by a HP-1000 computer system* Peak areas, r e t e n t i o n t i m e s , peak w i d t h s and peak h e i g h t s were o b t a i n e d u s i n g b o t h systems* A l l data were obtained with a 20 Hz sampling rate* F a s t peaks were g e n e r a t e d u s i n g a m o d i f i e d f a s t s a m p l i n g v a l v e ( L i n e a r Dynamics, E. P e p p e r e l l , Ma*), to i n j e c t butane (Matheeon, I n s t r u m e n t grade) onto an u n c o a t e d 0.060mm χ 2.0m f u s e d s i l i c a column* The 1070 I n j e c t o r cap was d r i l l e d out t o a l l o w a p i e c e of 1/16" s t e e l t u b i n g to s e a l the f a s t s a m p l i n g v a l v e w i t h the 1070 i n j e c t o r body* The s t a n d a r d gas i n l e t l i n e


6. SIMON AND WELLS


65


to the 1070 was capped with a Swagelok plug and the gas l i n e was c o n n e c t e d t o t h e f a s t s a m p l i n g v a l v e (see F i g u r e 1). The gas f l o w r a t e was h e l d a t 300 ml/min. The s p l i t p o i n t f o r t h e I n j e c t o r was r a i s e d to w i t h i n 1 cm of the f a s t sampling valve's o u t l e t p o r t and was c o n s t a n t l y swept by 300 ml/min o f h e l i u m , the v a l v e was opened f o r 50 ms f o r each d a t a run. The d a t a was c o l l e c t e d on a N i c o l e t 1170 s i g n a l a v e r a g e r , 10 ms p e r p o i n t , 1024 points per run, synchronized with the f a s t sampling valve. D i f f e r e n t peak w i d t h s were g e n e r a t e d by c h a n g i n g t h e column t e m p e r a t u r e (e.g. 90 C f o r 0.33 second peak w i d t h , 230 C f o r 1.07 second peak width, and 340 C f o r 1.65 second peak width). Results And D i s c u s s i o n The standard thermal c o n d u c t i v i t y detector employed by Varian i s a Y-geometry 140 μ £ c e l l c o n s i s t i n g o f f o u r matched f i l a m e n t s arranged i n a Wheatstone bridge c o n f i g u r a t i o n and operated i n the constant mean temperture mode. The c e l l was replaced by a 30ul TCD and the e l e c t r o n i c s were modified to compensate f o r change i n filament resistance. The f i l a m e n t r e s i s t a n c e of the 30 μ £ TCD was determined as a f u n c t i o n of temperature by heating and c o o l i n g the 30 μ ϋ TCD i n an oven ( r e g u l a t e d 0.01°C), a l l o w i n g t i m e f o r t h e TCD t o t h e r m a l l y e q u i l i b r a t e p r i o r to measuring the f i l a m e n t resistances. P l o t t i n g the average bridge r e s i s t a n c e versus the oven t e m p e r a t u r e p e r m i t t e d f i t t i n g the 30 μ ϋ TCD f i l a m e n t r e s i s t a n c e as a f u n c t i o n of temperature to the form given by: R(t) = R

(1 + * T)

where Τ i s t h e f i l a m e n t t e m p e r a t u r e , " i s the r e s i s t i v i t y c o e f f i c i e n t , Ro i s t h e z e r o - p o i n t r e s i s t a n c e , and R(T) i s t h e r e s i s t a n c e a t f i l a m e n t t e m p e r a t u r e T. F o r the 140 μ & TCD, Ro 29.6 ohms, - 0.00322/°C. F o r the 30 \xi TCD, Ro - 13.1 ohms, cc - 0.00788/°C. U s i n g h e l i u m as c a r r i e r gas w i t h two matched mass f l o w c o n t r o l l e r s , (30 ml/min He through both s i d e s ) , and operating i n the constant mean temperature mode (no columns, empty s t a i n l e s s s t e e l tubing only), current versus Δ Τ (where Δ Τ i s the d i f f e r e n c e between t h e f i l a m e n t and w a l l t e m p e r a t u r e s ) was measured f o r t h e 30 μ * TCD (see F i g u r e 2). Use o f a 1/8" χ 20" 5% 0V-101 packed column ( V a r i a n , Walnut Creek, CA) a l l o w e d determination of s e n s i t i v i t y versus current as shown i n Figure 3. These c u r v e s r e v e a l t h a t a c o m p a r i s o n o f t h e 30 μ £ TCD and t h e 140 μ & TCD c a n not be made on the b a s i s o f c u r r e n t , b u t r a t h e r must be based on equal ΔΤ. This may seem to imply that the 140 μ ^ TCD may be p r e f e r r e d due t o g r e a t e r s e n s i t i v i t y , b u t t h i s s i m p l i f i c a t i o n t o t a l l y n e g l e c t s t h e e f f e c t s of d e t e c t o r volumetric time constants on peak shapes. Œ



ENERGIZED DEENERGIZED


SAMPLING 1

INJECTOR I

SEPTUM I

INJECTOR I

COLUMN OVEN

CAPILLARY—COLUMN

Figure 1* Fast sampling valve i n l e t system: I n j e c t o r and modified L i n e a r dynamics valve*

Varian 1070



6. SIMON AND WELLS


100

150 200 250 300 i (mA)

F i g u r e 2. C u r r e n t v e r s u s Δ Τ f o r 30 ]xi and 140 \ii t h e r m a l c o n d u c t i v i t y detectors* Reference and sample flow rates 30 ml/min of helium* Detector temperature 170°C*

5000r £2000 > Ε

140 μ I

900

Ζ % 500 * 400 300 200 100 10

100 ΔΤ (°C)

F i g u r e 3. S e n s i t i v i t y v e r s u s Δ Τ f o r 30 \xi and 140μ£ thermal c o n d u c t i v i t y detectors* Reference and sample f l o w r a t e s 30 ml/min. h e l i u m * D e t e c t o r t e m p e r a t u r e 140°C* I n j e c t o r temperature 180°C.


67



68

Thermal c o n d u c t i v i t y detectors g e n e r a l l y have been used with packed columns* Connections between the column and the detector are made by using GLT ( s t a i n l e s s s t e e l g l a s s l i n e d tubing)* The 6LT s e r v e s t o r e d u c e v o i d volumes a t t h e d e t e c t o r - c o l u m n i n t e r f a c e , provides a completely f i x e d geometry w h i c h i s reproducible, and s e l f - a l i g n i n g whenever the column i s changed, and minimizes the s o l u t e residence time and peak mixing e f f e c t s i n c o n n e c t i n g t u b i n g * The GLT was used w i t h c a p i l l a r y columns because the inner diameter of the GLT i s 0.30mm (comparable w i t h the wide bore 0.48mm g l a s s column, and compatible with the 0.20mm f u s e d s i l i c a column), and i s swept by not o n l y the column f l o w , but a l s o any make-up f l o w w h i c h i s b e i n g used as a purge f l o w , thus m i n i m i z i n g s o l u t e r e s i d e n c e t i m e , band b r o a d e n i n g , and t a i l i n g due to connecting tubing* As s t a t e d e a r l i e r , t h e TCD i s a c o n c e n t r a t i o n dependent d e t e c t o r and t h e r e f o r e has a d e t e c t o r volume t h a t must be e f f i c i e n t l y swept by e i t h e r c a r r i e r gas, o r make-up gas, t o prevent any s i g n i f i c a n t peak d i s t o r t i o n * Comparison o f a 140 μ£ TCD and a 3 0 T C D , r e q u i r i n g e q u a l v o l u m e t r i c t i m e c o n s t a n t s and equivalent f i d e l i t y of a Gaussian input peak shape, r e q u i r e s the detector f l o w r a t e f o r the 140 μ£ TCD to be 4*6 times l a r g e r t h a n t h a t f o r the 30 y £ T C D . T h i s d i f f e r e n c e i n t o t a l d e t e c t o r flow r a t e s a l l o w s the 30 μ £ TCD to give a greater response than the 140y£TCD by simply decreasing the flow r a t e thru the 30 u£ TCD. E x a m i n a t i o n o f the r e s p o n s e o f the 30 μλ TCD as a f u n c t i o n of flow r a t e (see F i g u r e 4) r e v e a l s apparent l i n e a r response f o r s e v e r a l d i f f e r e n t f l o w r a t e s , and a d e c r e a s e i n t h e minimum detectable quantity p r o p o r t i o n a l w i t h the decrease i n t o t a l flow t h r o u g h the d e t e c t o r . T h i s s t r o n g l y s u p p o r t s the c o n c e p t t h a t the TCD operates with g r e a t e s t s e n s i t i v i t y and minimum d i s t o r t i o n a t f l o w g r e a t e r t h a n 4.92 V * / σ (e.g. f o r 30 μ£ TCD and σ -1 second, the t o t a l flow thru the detector must be greater than 8*9 ml/min, and f o r the 140μ£ TCD the flow must be greater than 41.3 ml/min f o r minimum d i s t o r t i o n ) . Sternberg [8] has shown that the observed peak variance i s the sum o f t h e i n d i v i d u a l v a r i a n c e s o f a l l p r o c e s s e s that c o n t r i b u t e to band broadening, o r : e

°total

=

°inj.

+ < J

f

col.

+

"det.

+

°ct

where σ a r e t h e v a r i a n c e a s s o c i a t e d w i t h t h e i n j e c t o r , t h e column, the detector, and the connecting tubing. I n t h i s method, v a r i a n c e s a r e c a l c u l a t e d i n the t i m e domain (as opposed t o t h e column-length domain) and correspond to the second moment of the c h r o m a t o g r a p h i c peak, i . e . σ - 2 o * i o n i z a t i o n d e t e c t o r (FID), σ d e t i s a p p r o x i m a t e l y z e r o , t h e 7

M

/ M

1

8

1

F

o

r

a

f


l

a

m

e


SIMON A N D WELLS


F i g u r e 4. Response o f t h e 30 μ& TCD v e r s u s f l o w r a t e . D e t e c t o r t e m p e r a t u r e 170°C, i n j e c t o r t e m p e r a t u r e 180°C, column temperature 100°C, f i l a m e n t temperature 250°C, 170 i n j e c t o r , s p l i t r a t i o 1:65. 12.5 m χ 0.20 mm SE-5 column, column f l o w 0.79 ml/min. 9.45 ug hexadecane i n j e c t e d .



70

l i n e a r dynamic range i s o v e r 6 o r d e r s o f m a g n i t u d e , and t h e FID i s flow independent. One c a n a p p r o x i m a t e the s y s t e m v a r i a n c e ( σ s y s ) to be: 2

2

2

2

2

σ sys. = σ FID = σ i n j . + σ c o l . + ο et

S u b t r a c t i o n of (17) from the TCD, o r : 2

(16) y i e l d s the variance a s s o c i a t e d w i t h

2

σ TCD = σ t o t a l


(17)

2

(18)

- σ FID

D e t e r m i n a t i o n o f t h e TCD v o l u m e t r i c t i m e c o n s t a n t by t h e method of s t a t i s t i c a l moments r e q u i r e s a minimum s i g n a l to noise r a t i o o f 100, o p e r a t i o n w i t h i n t h e l i n e a r dynamic range o f t h e TCD and FID being used, and s u f f i c i e n t l y r a p i d data a c q u i s i t i o n so as t o p r e s e r v e t h e i n t e g r i t y o f t h e peak shape i n the h i g h e r moments [13]. E x p e r i m e n t a l l y the noise present f o r the TCD was t o o g r e a t t o a l l o w t h i s method t o be used w i t h a hexadecane standard. I n s t e a d f a s t peaks were g e n e r a t e d u s i n g a f a s t s a m p l i n g v a l v e ( a m o d i f i e d L i n e a r Dynamics S e r i e s 11, 2-way normally c l o s e d valve) to i n j e c t butane onto an uncoated 0.060mm χ 2m column. A p l o t of σ det versus flow r e v e a l s that the s m a l l volume TCD has an apparent volume of 48.8 μ£· T h i s e x t r a column v o l u m e i s due t o o t h e r than solely turbulent mixing considerations. Sternberg [8] has d e s c r i b e d the p o s s i b i l i t y of s y m m e t r i c band b r o a d e n i n g , i . e . new s p r e a d i n g . T h i s new s p r e a d i n g c a u s e s b o t h s y m m e t r i c s p r e a d i n g as w e l l as l a r g e i n c r e a s e s i n t h e o b s e r v e d second moment. New spreading occurs when a sample p u l s e e n t e r s a wide d i a m e t e r tube from a s m a l l e r d i a m e t e r tube. Here t h e sample p u l s e w i l l tend t o r e t a i n t h e i n t e g r i t y of the peak shape on a column l e n g t h b a s i s w i t h i n the detector; however, l a t e r a l d i f f u s i o n leads to i n c r e a s e s of the time based and volume based second moments (Figure 5 ) . A t low f l o w r a t e s m i x i n g i s v e r y n e a r l y c o m p l e t e , and t h e detector operates as an exponential d i l u t i o n chamber due both t o d i f f u s i o n and t u r b u l e n t mixing. As the flow r a t e i s Increased, the d e t e c t o r v o l u m e t r i c t i m e c o n s t a n t d e c r e a s e s and t h e peak becomes more symmetric (see F i g u r e 6). However, i n c r e a s i n g the make-up flow r a t e any f u r t h e r , i.e. F >3 V / ° j p T n > r e s u l t s i n a s y m m e t r i c b r o a d e n i n g o f t h e peak. Thus the TCD o p e r a t e s w i t h both exponential mixing and new spreading c o n v o l u t i n g the input peak shape. I n c r e a s i n g the diameter of the connecting tubing to that of the TCD c a v i t y d i a m e t e r would e l i m i n a t e t h i s p r o b l e m , a t t h e expense o f a v a s t l y I n c r e a s e d d e t e c t o r volume o r l a m i n a r f l o w spreading. W h i l e new s p r e a d i n g cannot be e l i m i n a t e d , i t s chromatographic e f f e c t s can be minimized by running the detector det


SIMON AND WELLS



1.251

.10

.20 .30 FLOW -1 (min/ml)

F i g u r e 5. σ versus flow Fast sampling valve (50 ms) I n j e c t e d Butane onto an u n c o a t e d 0*060 mm χ 2 M column* D e t e c t o r t e m p e r a t u r e 120°C, f i l a m e n t t e m p e r a t u r e 180°C* S p l i t r a t i o 1:300. d

e

t

W-j/2 = .33 Seconds

0

1

2 3 TIME (SECONDS)

4

5

Figure 6· Peak shape as a f u n c t i o n of detector flow r a t e . Conditions same as f i g u r e 5·



72


a t i t s optimum f l o w r a t e f o r the e x p e c t e d peak w i d t h , or by c o m p r o m i s i n g peak shape ( i . e . f i d e l i t y ) to g a i n i n b o t h s e n s i t i v i t y and to e l i m i n a t e any e x c e s s i v e new spreading c o n t r i b u t i o n s to peak shape. For packed columns, new spreading i s a minor p e r t u r b a t i o n of the peak variance. Here the detector e f f e c t i v e l y operates as a plug f i l t e r on the Gaussian input f u n c t i o n . Since Ο » V ^ ^ / F , the e f f e c t s of new spreading i s hidden i n the u n c e r t a i n t y of the peak variance. U s i n g 0.20 mm χ 12.5M SE-5 column, 1070 i n j e c t o r , column f l o w 0.78 m l / m i n , TCD t e m p e r a t u r e 170 ° C , TCD filament temperature 250°C, the S/N v a r i e d from 50 at 2.90 ml/min to 5 at 33.5 m l / m i n f o r 2.50 ug hexadecane i n j e c t e d a t a s p l i t r a t i o o f 1:300. I n c r e a s i n g the sample i n j e c t e d to t r y and i n c r e a s e the S/N r e s u l t e d i n s e v e r e o v e r l o a d i n g of the 0.20 mm χ 12.5M SE-54 column. T h i s can be o b s e r v e d by e x a m i n i n g the peak shape by expanding the time a x i s , and observing severe f r o n t a l d i s t o r t i o n of the Gaussian peak shape. As sample concentration i s increased the peak h e i g h t r e m a i n s c o n s t a n t and the peak w i d t h b e g i n s to dramatically increase. T h i s s e r v e s to g r e a t l y d e c r e a s e the r e s o l u t i o n a v a i l a b l e using a c a p i l l a r y column. In e f f e c t , t h i s negates some of the advantages that are Inherent using c a p i l l a r y columns over packed columns regarding r e s o l u t i o n , or d e t e c t i v i t y . As the column sample c a p a c i t y i s exceeded, the peak width begins to increase d r a m a t i c a l l y (Figure 7). Recently wide bore fused s i l i c a g l a s s c a p i l l a r y columns have been g e n e r a t e d w h i c h have v e r y t h i c k bonded phases ( i . e . 5 ym) which are s t a b l e (14). These preparative columns have increased sample c a p a c i t y ( g r e a t e r than 10 pg) w h i l e simultaneously m a i n t a i n i n g h i g h s e p a r a t i o n e f f i c i e n c i e s . However, they a l s o have the d i s a d v a n t a g e of s u b s t a n t i a l l y i n c r e a s i n g r e t e n t i o n times. The use of one of these columns together with a 30 y£ TCD would I n c r e a s e the dynamic range of the s y s t e m a f a c t o r of 200. Thus f o r a 1 second peak h a l f width, and a t o t a l detector flow of 9 ml/min, the dynamic range of the 30 \xl TCD would be 4000. The c h r o m a t o g r a p h i c s y s t e m would s t i l l be l i m i t e d a t h i g h sample concentrations, but r e a l q u a n t i t a t i v e work could be done. Α 5μ£ t h e r m a l c o n d u c t i v i t y c e l l has been d i s c u s s e d by C r a v e n and C l o u s e r (15,16) w h i c h i s modulated a t 10 c y c l e s per second. Here the a n a l y t i c a l and r e f e r e n c e gas f l o w s a r e a l t e r n a t e l y switched to flow through a s i n g l e d e t e c t i o n c e l l . An a d v a n t a g e of the modulated TCD i s t h a t the c e l l r e m a i n s n e a r l y constant f o r s e q u e n t i a l pulses, and thus the modulated d e t e c t i o n removes the low f r e q u e n c y components of the TCD s i g n a l . This e f f e c t i v e l y e l i m i n a t e s any p r o b l e m s t h a t a r e a s s o c i a t e d w i t h d r i f t i n g baselines. The modulated TCD r e t a i n s the l i m i t a t i o n s imposed on c o n v e n t i o n a l t h e r m a l c o n d u c t i v i t y d e t e c t o r s w i t h r e s p e c t to s t a b i l i t y of the r e f e r e n c e and a n a l y t i c a l flows, detector w a l l t e m p e r a t u r e s t a b i l i t y , and make-up f l o w s b e i n g r e q u i r e d to



6. SIMON AND WELLS


1.8 ml/min

0

.2

.4

.6 .8 1.0 Time (minutes)

1.2

1.4

Figure 7· Peak shape f o r hexadecane. 12.5 m χ 0*20 mm SE5 column, 1070 i n j e c t o r , d e t e c t o r t e m p e r a t u r e 170°C, f i l a m e n t temperature 250°C, column flow 0.78 ml/min, s p l i t r a t i o 1:300, 46 ug hexadecane i n j e c t e d .


73


74


m i n i m i z e band b r o a d e n i n g a t the column to d e t e c t o r union* In a d d i t i o n to the make-up f l o w s b e i n g r e q u i r e d to m i n i m i z e peak broadening at the detector union, a d d i t i o n a l gas flow i s required to e f f i c i e n t l y sweep-out the d e t e c t o r volume and to e f f e c t the s w i t c h i n g a c t i o n * These a d d i t i o n a l f l o w s t h r o u g h the c e l l a r e r e f l e c t e d as a d e c r e a s e i n the a n a l y t e c o n c e n t r a t i o n a t the d e t e c t o r and thus r e d u c e s e n s i t i v i t y * The m o d u l a t i o n scheme i t s e l f r e s u l t s i n a f a c t o r of 2 ( a t b e s t ) a t t e n u a t i o n of the a n a l y t i c a l signal* Craven and Clouser have stated almost instantaneous b a s e l i n e and s e n s i t i v i t y s t a b i l i z a t i o n * Although the b a s e l i n e w i l l almost i n s t a n t a n e o u s l y s t a b i l i z e , the s e n s i t i v i t y can not s t a b i l i z e r a p i d l y s i m p l y because the d e t e c t o r w a l l t e m p e r a t u r e w i l l increase when the f i l a m e n t i s turned on, and time i s required f o r the w a l l to r e a c h an e q u i l i b r i u m t e m p e r a t u r e * Indeed, most b a s e l i n e d r i f t f o r c o n v e n t i o n a l TCD's i s due p r i m a r i l y to t e m p e r a t u r e c y c l i n g of the d e t e c t o r w a l l and o x i d a t i o n of the filaments. In the modulated mode, t h i s d r i f t i s masked, but the s e n s i t i v i t y of the c e l l i s always changing, as the detector w a l l temperature v a r i e s * The modulation frequency of 10 Hz i m p l i e s that the c e l l i s being f i l l e d with the a n a l y t i c a l flow f o r 1/20 second, and then d u r i n g the next 1/20 second b e i n g purged by the r e f e r e n c e f l o w * F o r a 5μ£ TCD volume ( a s s u m i n g no d i f f u s i o n i n t o the r e f e r e n c e f l o w l i n e s of the a n a l y t e , no a d s o r p t i o n of the a n a l y t e on the c e l l w a l l s , and t o t a l l y r e v e r s i b l e l a m i n a r f l o w t h r o u g h the c e l l ) , the minimum f l o w r a t e i s 6*0 ml/min to t o t a l l y f i l l or purge the c e l l * Any a d d i t i o n a l f l o w o n l y s e r v e s to d i l u t e the sample; however, any lower flow serves to not completely purge the a n a l y t e , r e s u l t i n g i n l o w e r s e n s i t i v i t y t h a n c a n be t h e o r e t i c a l l y attained* One should be aware that a s i g n i f i c a n t component of the a n a l y t i c a l and reference gas flows can c o n s i s t of the m o d u l a t o r flow* The recommended m o d u l a t o r f l o w i s 12*5 ml/min; however, i n p r a c t i c e , even h i g h e r m o d u l a t o r f l o w s a r e necessary f o r proper s i g n a l modulation* Thus simple measurement of the make-up f l o w and the column f l o w i s not s u f f i c i e n t t o describe the t o t a l gas flow across the filament* The conventional TCD i s configured w i t h the f i l a m e n t s being c o n n e c t e d to f o r m a Wheatstone b r i d g e * A p r o p e r t y of the Wheat s tone bridge i s common mode r e j e c t i o n of the noise which i s p r i m a r i l y due to the e l e c t r o n i c s (i.e. power supply s t a b i l i t y and the a m p l i f i e r c i r c u i t ) * The TCD noise spectrum resembles white ( s h o t ) n o i s e r a t h e r than the 1/f ( f l i c k e r ) n o i s e of i o n i z a t i o n detectors* Modulation t e c h n i q u e s f o r n o i s e r e j e c t i o n of w h i t e noise i s no b e t t e r than a simple Wheatstone bridge* I n c o m p a r i s o n , b o t h t h e 30 μ£ and t h e 5 y£ thermal c o n d u c t i v i t y d e t e c t o r s r e q u i r e good s t a b i l i t y of the d e t e c t o r w a l l t e m p e r a t u r e , and matched a n a l y t i c a l and r e f e r e n c e f l o w s . A l t h o u g h b a s e l i n e d r i f t i s a r e a l p r o b l e m f o r the 30μ£ TCD systems which have a i r leaks, extremely poorly matched f i l a m e n t s ,


6. SIMON AND WELLS


75


and p o o r l y matched a n a l y t i c a l and r e f e r e n c e f l o w s , a p r o p e r l y set-up c a p i l l a r y system tend to minimize these e f f e c t s and show e s s e n t i a l l y no b a s e l i n e d r i f t f o r the 30 μ£ TCD. The advantage of the 30U* TCD i s that i f b a s e l i n e d r i f t i s present, then there i s an i n d i c a t i o n of a problem, i.e. poor chromatographic technique. A steady b a s e l i n e ( w i t h the Wheatstone b r i d g e ) , r e v e a l s c o n s i s t e n t s e n s i t i v i t y t h r o u g h o u t t h e a n a l y s i s , and f o r s e q u e n t i a l a n a l y s e s . T h i s may o r may n o t be t h e case w i t h t h e modulated 5 u£ TCD, since the modulation serves to remove slow d r i f t components from the a n a l y t i c a l s i g n a l , constant s e n s i t i v i t y f r o m r u n t o r u n (and thus r e p r o d u c i b l e q u a n t i t a t i o n ) i s n o t n e c e s s a r i l y ensured. Conclusions (1)

(2)

(3)

(4)

A s m a l l volume TCD s t i l l r e q u i r e s a r e f e r e n c e f l o w t o be w e l l matched with the sample flow. For most wide bore WCOT columns, make-up gas i s r e q u i r e d t o o b t a i n the a v a i l a b l e r e s o l u t i o n o f t h e s y s t e m . F o r SCOT columns t h i s c a n be neglected without serious loss i n e f f i c i e n c y . Regulation and b a l a n c i n g o f t h e r e f e r e n c e and sample f l o w s a r e r e l a t i v e l y easy when only i s o t h e r m a l operation i s employed, but become very d i f f i c u l t when temperature programming i s necessary. L i n e a r dynamic range o f the 30μ& TCD i s good when used w i t h a packed column. When the 30μ£ TCD i s c o u p l e d w i t h WCOT columns, the l i m i t e d sample c a p a c i t y of the columns s e v e r e l y limits i t s analytical u t i l i t y . While the dynamic range can be somewhat a d j u s t e d by j u d i c i o u s s e l e c t i o n o f t h e s p l i t r a t i o o f a c a p i l l a r y i n j e c t o r , major and t r a c e components can not be both simultaneously determined. The upper end of l i n e a r range i s always l i m i t e d by column capacity. Increasing the s e n s i t i v i t y of the detector, i.e. i n c r e a s i n g Δ Τ o f t h e TCD, g i v e s a m o d e r a t e ( f a c t o r o f 2 t o 5) improvement i n l i n e a r dynamic range. The l i m i t i n g f a c t o r s are the increased noise, and the degradation of the f i l a m e n t lifetime. High f i l a m e n t temperatures degrade the f i l a m e n t and a l l o w r a p i d o x i d a t i o n of the f i l a m e n t s i f any a i r leaks a r e present. The 30 μ& TCD i n u s e w i t h packed columns i s o f d u b i o u s usage. The 140μ£ TCD has s u p e r i o r s e n s i t i v i t y a t most packed column f l o w r a t e s . A l t h o u g h t h e 30 μ£ TCD has marginal usage with wide bore (0.480mm) columns, n e i t h e r the 30]\l o r the 140μ£ TCD have any p r a c t i c a l usage w i t h h i g h r e s o l u t i o n narrow bore columns ( i . e . d< .25mm) columns. The 30 μλ TCD may have some u t i l i t y w i t h SCOT columns with high s t a t i o n a r y phase loading, but these were not i n v e s t i g a t e d i n t h i s work. The 30μ£ TCD combined w i t h t h e p r e p a r a t i v e



76

(5)

t h i c k f i l m (5 μ m) bonded phase c a p i l l a r y columns would a l l o w d e t e c t i o n o f up t o 10 m i c r o g r a m s o f a s i n g l e component i n the sample* While a 5u& modulated TCD i s a v a i l a b l e f o r use i n c a p i l l a r y chromatography, the 10 Hz m o d u l a t i o n f r e q u e n c y l i m i t s t h e minimum peak w i d t h t o be 1 s e c o n d , and t h e m o d u l a t i o n t e c h n i q u e r e q u i r e s i n l a r g e e x c e s s of 6 ml/min o f f l o w through the detector c e l l * These f a c t o r s s e v e r e l y l i m i t the u t i l i t y o f the modulated c e l l f o r f a s t , h i g h r e s o l u t i o n c a p i l l a r y chromatography*


Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

D.J. David, Gas Chromatographic Detectors, Chapter 3. Wiley, New York (1974). A.B. Littlewood, Gas Chromatagraphy, Chapter 9, Academic Press, New York (1970). H. Purnell, Gas Chromatagraphy, Chapter 12, John Wiley & Sons, Inc. New York (1967). S.D. Nogare, R.S. Juvet, Jr., Gas Liquid Chromatography, Chapter 10, Interscience Published, New York (1962). A.E. Lawson, J.M. Miller, J. of Gas Chromatogr., 4 (1966) 273. G. Wells, R. Simon, J. High Resol. Chromatogr. and Chromatogr Commun., 256 (1983) 1. ASTM standards on Chromatography, Philadephia, Pa., ASTM E516-74 (1981) 661. J.C. Sternberg, Adv. in Chromatogr., J.C. Giddings and R.A. Keller, Eds., Marcel Dekker, Inc., New York, Vol. 2 (1966) 205. F.J. Yang, S.P. Cram, J. High Resol. Chromatogr. and Chromatogr. Commun., 2 (1979) 487. C.H. Lochmuller, B.M. Gordon, J. Chromatogr. Sci., 15 (1977) 285. C.H. Lochmuller, B.M. Gordon, J. Chromatogr. Sci., 16 (1978) 141. C.H. Lochmuller, B.M. Gordon, J. Chromatogr. Sci., 16 (1978) 523. S.N. Chester, S.P. Cram, Anal. Chem., 43 (1971) 1922. K. Grob, G. Grob, J. of HRC & CC, 6, (1983), 133. D.E. Clouser, J.S. Craven, U.S. Appl. 949312, 06 Oct. 1978. D.E. Clouser, J.S. Craven, Analusis, 2, (1980), 3.

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Ultrahigh Resolution Chromatography - American Chemical Society

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