GCD_A

1

BOOK 4

Chrom-Ed Book Series Raymond P. W. Scott

GAS CHROMATOGRAPHY DETECTORS

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2

Chrom-Ed Book Series Book 1 Book 2 Book 3 Book 4 Book 5 Book 6

Principles and Practice of Chromatography Gas Chromatography Liquid Chromatography Chromatography Gas Chromatog Chromatography raphy Detectors Liquid Chromatography Chromatography Detectors Detectors The Plate Theory and Extensions Extensions for Chromatography Columns Book 7 The Thermodynamics Thermodynamics of Chromatography Chromatography Book 8 The Mechanism Mechanism of of Retention Retention Book 9 Dispersion in Chromatography Columns Book 10 Extra Column Dispersion Dispersion Book 11 Capillary Chromatography Chromatography Book 12 Preparative Chromatography Book 13 GC Tandem Systems Book 14 LC Tandem Systems Book 15 Quantitative Analysis Book 16 Ion Chromatography Book 17 Silica Gel and Its Uses in Chromatography Book 18 Thin Layer Chromatography Book 19 Chiral Chromatography Chromatography Book 20 Bonded Phases Book 21 Chromatography Applications

COPYRIGHT COPYRIGHT @2003 by LIBRARYFORSCIENCE, LLC ALL RIGHTS RESERVED

 Neither  Neither this book or any part may be reduced or transmitt transmitted ed in any form or by any means, electronic or mechanical , including photocopying, You may make electronic copies copies of this eBook for academic.You may also print out copies on paper for academic use in. You may not give, sell or distribute this document except as permitted by the EULA

2

Chrom-Ed Book Series Book 1 Book 2 Book 3 Book 4 Book 5 Book 6

Principles and Practice of Chromatography Gas Chromatography Liquid Chromatography Chromatography Gas Chromatog Chromatography raphy Detectors Liquid Chromatography Chromatography Detectors Detectors The Plate Theory and Extensions Extensions for Chromatography Columns Book 7 The Thermodynamics Thermodynamics of Chromatography Chromatography Book 8 The Mechanism Mechanism of of Retention Retention Book 9 Dispersion in Chromatography Columns Book 10 Extra Column Dispersion Dispersion Book 11 Capillary Chromatography Chromatography Book 12 Preparative Chromatography Book 13 GC Tandem Systems Book 14 LC Tandem Systems Book 15 Quantitative Analysis Book 16 Ion Chromatography Book 17 Silica Gel and Its Uses in Chromatography Book 18 Thin Layer Chromatography Book 19 Chiral Chromatography Chromatography Book 20 Bonded Phases Book 21 Chromatography Applications

COPYRIGHT COPYRIGHT @2003 by LIBRARYFORSCIENCE, LLC ALL RIGHTS RESERVED

 Neither  Neither this book or any part may be reduced or transmitt transmitted ed in any form or by any means, electronic or mechanical , including photocopying, You may make electronic copies copies of this eBook for academic.You may also print out copies on paper for academic use in. You may not give, sell or distribute this document except as permitted by the EULA

3 microfilming, and recording or by any information storage and retrieved system without permission in writing from the publisher except as  permitted  permitted by the in-user in-user license license agree agreement. ment.

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Contents Introductio Introduction n ...................... .......... ...................... ...................... ....................... ...................... ...................... ....................... .............. .. 1 Classificatio Classification n of Detectors Detectors...................... ........... ...................... ...................... ...................... ...................... .............. ... 1 Detector Specifications ........................ ...................................... .......................... ......................... .................. .....2 The Form of Detector Response ........................ ...................................... .......................... ............... ... 3 The Dynamic Range of the Detector .......................... ....................................... .................... .......4 Detector Linearity ......................... ...................................... .......................... ......................... ........................ ............5 Determination of Response Index .......................... ....................................... ........................ ...........7 The Incremental Method of Linearity Measurement ....................... 7 The Logarithmic Dilution Method of Linearity Measurement ........ ....... . 8 Alternative Method for f or Specifying Detector D etector Linearity........ .... ........ ........ ...... .. 10 Detector Response ......................... ...................................... .......................... ......................... ...................... .......... 10 Detector Noise......................... ....................................... .......................... .......................... .......................... ............... ... 11 Measurement Measurement of Detector Detector Noise....................... ........... ....................... ...................... ................. ...... 13 Detector Sensitivity or Minimum Detectable Concentration ...... 13 System Dispersion and Sensor Dimensions ...................... ........... ...................... ............. 14 Peak Dispersion from the Overall O verall Detector Time Constant. ........ .... ...... 14 Pressure Sensitivity ......................... ...................................... .......................... .......................... .................... ....... 16 Flow Sensitivity ......................... ...................................... ......................... ......................... ........................... ..............16 Temperature Sensitivity ........................ ...................................... ........................... ......................... .............. 17 Summary Summary of Detector Detector Criteria Criteria....................... ........... ...................... ...................... ....................... ................. ...... 17 Early Gas Chromatograp Chromatography hy Detectors Detectors ...................... .......... ....................... ...................... ................. ...... 19 The General General Properties Properties of GC Detectors Detectors ........................... ................ ...................... ................... ........ 27 The Katharomet Katharometer er Detector Detector....................... ........... ....................... ...................... ...................... ..................... .......... 29 The Simple Gas Density Balance ....................... ............ ...................... ...................... ....................... ............ 32 The Flame Ionization Ionization Detector Detector...................... .......... ...................... ...................... ....................... ................. ...... 33 The Design of the FID ......................... ....................................... ........................... .......................... ............... .. 34 Electrode Electrode Configurations Configurations ............................... .................... ...................... ...................... ................... ........ 35 The High Impedance Amplifier Amplifier .............................. ................... ...................... ..................... .......... 36

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4 The Response Mechanism of the FID .......................................... 37 The Operation of the FID............................................................. 41 The Nitrogen Phosphorus Detector (NPD)........................................... 43 The Emissivity or Photometric Detector .............................................. 47 Ionization Detectors............................................................................. 50 The Simple or Macro Argon Detector Sensor ............................. 51 The Micro Argon Detector ...........................................................55 The Thermal Argon Detector ....................................................... 57 The Helium Detector ..................................................................... 60 The Pulsed Helium Discharge Detector ........................................... 63 The Electron Capture Detector......................................................... 66 The Pulsed Discharge Electron Capture Detector ...................... 72 The Radioactivity Detector .................................................................. 74 Some Less Common GC Detectors...................................................... 76 The Thermionic Ionization Detector ............................................ 77 The Discharge Detector ................................................................ 78 The Spark Discharge Detector ..................................................... 80 The Radio Frequency Discharge Detector ...................................81 The Ultrasound Whistle Detector .................................................82 The Piezoelectric Adsorption Detector ......................................... 85 The Surface Potential Detector ..................................................... 88 References........................................................................................... 90

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1

Introduction

 A chromatography detector is a device that locates in the dimensions of space and time, the positions of the components of a mixture that  has been subjected to a chromatographic process and, thus, permits the senses to appreciate the nature of the separation.

The definition, by necessity, must be broad, as it needs to encompass all types of detecting systems ranging from elaborate electronic devices to the human eye or even the sense of smell. Tswett in his pioneering chromatographic separation of some plant pigments used the human eye to determine the nature of the separation and, even today, as one of the more common separation techniques is thin layer chromatography, the human eye is still one of the more frequently used detectors. Similarly, essential oil chemists smell the eluent from a gas chromatography (GC) column in organoleptic assessment. The detector, as well as being an essential supporting device for the gas chromatograph has also played a critical role in the development of the technique as a whole. There has been a synergistic interaction between column development and detector development. The need to develop higher column efficiencies has demanded higher detector sensitivities which has provoked the development of more sensitive detectors. In turn, the more sensitive detectors have encouraged the improvement of  column performance. In fact, the rapid development of GC in the 1950s was possible because or the swift introduction of high sensitivity linear  detectors. Classification of Detectors

Detectors can be classified into two types, bulk property detectors and  solute property detectors. The bulk property detector measures some  bulk physical property of the eluent (such as dielectric constant or  refractive index) and the solute property detector, measures some

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2  physical or chemical property that is unique to the solute (such as heat of  combustion or fluorescence). Detectors can also be classified as concentration sensitive devices such as the katharometer or  mass  sensitive devices such as the flame ionization detector (FID). Another  method of classification is to define detectors as  specific or non-specific. An example of a specific detector would be the nitrogen phosphorous detector (NPD), which as its name implies detects only those substances that contain nitrogen or phosphorous. A non-specific detector would be the katharometer detector that senses all vapors that have specific heats or thermal conductivities different from those of the carrier gas. In general (though not always), non-specific detectors have lower  sensitivities than the specific detectors, the reasons for which will be discussed in due course. In this treatment of GC detectors the classification of bulk property detector and solute property detectors will  be used. The ideal GC detector should have a sensitivity of about 10 -12 - 10-11 g/ml, and a linear dynamic range of about five orders of magnitude. It should have a catholic response, but be independent of the characteristics of the mobile phase. It should also be insensitive to changes in mobile phase flow rate through the sensor and also changes in temperature and pressure. No existing detector fulfills all these specifications but the flame ionization detector (FID) comes close to this ideal performance. Detector Specifications

The subject of detector specifications has been touched upon in Principles and Practice and Gas Chromatography but will be treated here in detail. In order to evaluate a detector for use in GC, accurate  performance criteria or specifications must be available to assess the  pertinence of a particular detector for a given application. Such information is also necessary to permit a rational comparison with other  detectors or detectors supplied by competitive manufacturers. The  principle characteristics of a GC detector that will satisfy these requirements are given below. It will be seen that in specifying the above properties of the detector it is important to employ the correct You may make electronic copies of this eBook for academic.You may also print out copies on paper for academic use in. You may not give, sell or distribute this document except as permitted by the EULA

3 units that will depend on the mechanism of detection. for example, the katharometer detector responds to the concentration of solute in the gas flowing through it so its sensitivity (minimum detectable concentration) would be defined in terms of g/ml. The FID, on the other hand, responds to the mass of solute flowing though it per unit time and thus, for this detector, the sensitivity would be defined in units of g/sec. Principle Characteristics of a GC detector

1. Dynamic Range 2. Response Index or Linearity 3. Linear Dynamic range 4. Detector Response 5. Detector Sensitivity or Minimum Detectable Concentration 6. System Dispersion 7. Sensor Dimensions 8. Detector Time Constant 9 Pressure Sensitivity 10. Flow Sensitivity 11. Operating Temperature Range The Form of Detector Response

There are three different forms of detector response, namely,  proportional, differential and integral. A proportional response is one that is directly related to the concentration of solute in the mobile phase  passing through it. All detectors with a proportional response are designed to give as near a linear response as possible. In many detectors, the actual sensor does not give a proportional response. Thus, suitable electronic circuitry must be employed to modify the signal from the sensor so that the actual detector output is proportional to the solute concentration in the mobile phase passing through it. For example, a sensor with a logarithmic response would be modified by an exponential amplifier to give an output linearly related to the solute concentration. The different types of detector response are shown in figure 1.

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4 A linear detector will provide a normal  response and follow the Gaussian concentration profile of the eluted peak as shown in figure 1. If the normal signal is electronically integrated with respect to time then an integral output is obtained. Similarly if the normal output is differentiated then the differential of the Gaussian curve is produced.

Figure 1. Different Types of Detector Response

In a normal response the area of the peak is proportional to the total mass eluted whereas with the integral response the step height of the integral curve is proportional to the total mass eluted. The differential curve is often used to identify the retention time that is the point where the signal crosses from positive value through zero to a negative value. The Dynamic Range of the Detector

A detector has two response ranges, the dynamic rang e and the linear  dynamic rang e and the two ranges are not synonymous. The dynamic range of a detector is that concentration range over which a concentration dependent output is produced. The minimum of the range will be the concentration at which the output is equivalent to twice the You may make electronic copies of this eBook for academic.You may also print out copies on paper for academic use in. You may not give, sell or distribute this document except as permitted by the EULA

5 noise level and the maximum that concentration where the detector no longer responds to any concentration increase. The dynamic range is usually given as a concentration ratio and is thus, dimensionless. Detector Linearity

The linear dynamic range of a detector is that concentration range over  which the detector output is linearly related to solute concentration. Thus,

y= Ac

where (y) is the detector output (c) is the concentration of solute in the mobile phase, and (A) is a constant. In practice, no detector has a truly linear response (despite manufacturers claims) but most detectors will have a response approaching that of linear. It is difficult to apply a standard to detector linearity, but the  Response  Index (1) does help comparisons to be made between one detector with that of another. Providing the response of the detector approaches linearity then its response can be described by the following simple equation, y=Acr

where (r) is the response index and the other symbols have the meaning previously ascribed to them. For a truly linear detector, r=1, and the extent to which (r) deviates from unity would be a measure of its non linearity. Curves relating the detector output to different solute concentrations  passing though it for different response values are show in figure 2.

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6 All the curves shown in figure 2 appear to closely approximate to a straight line (i.e. are linear). However, if linearity is assumed considerable errors can result as shown in table 2.

Figure 2 Curves Relating Detector Output to Solute Concentration for Different Response Indices. Table 1 The Analysis of a Two-Component -Mixture Using Detectors Having Different Response Indices Solute

r=0.94

r=0.97

r=1.00

r=1.03

r=1.05

1

11.25%

10.6%

10.0%

.42%

9.05%

2

88.75%

89.4%

90.0%

90.58%

(0.95%

It is seen that the results for the lower level component (10 % w/w) can  be as great as 12.5 % (1.25 % absolute) when (r) is a low as 0.94 and

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7 9.5 % (0.95 % absolute) if (r) is as high as 1.05. It follows, that to  provide adequate accuracy for concentration ranges of 10 or more then (r) should lie between 0.98 and 1.02. Determination of Response Index

There are two basic methods for measuring the response index of a detector and they are the incremental method  and the logarithmic dilution method . The incremental method requires no special apparatus other than the chromatograph itself but the latter does require special equipment but the apparatus is very simple to construct. The Incremental Method of Linearity Measurement

This procedure provides a curve relating detector output to solute concentration over the concentration range of interest employing the associated chromatograph. Replicate samples are placed on the column and the eluted peaks monitored. The sample solution is made up in an appropriate volatile solvent at the maximum concentration of interest and duplicate samples placed sequentially on the column and the eluted  peaks monitored. The sample is then diluted by three and the analyses repeated. The sample is again diluted and the process repeated until the  peak heights are about five times the noise level (see later for a definition of noise). The concentration at the peak maximum for each ample injected is calculated as follows,

c

ms wQ

where, (c) is the concentration of solute in the mobile phase at the  peak maximum in g/ml, (m) is the mass of solute injected, (w) is the peak width at 0.6065 of the peak height. (s) is the chart speed in cm/min. and (Q) is the mobile phase flow rate in ml/min. The logarithm of the peak height (y) is plotted against the logarithm of  the solute concentration at the peak maximum (c), (cf. equation 1)

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8

log (y) = Log (A) + rLog c) The slope of the curve will give the response index (r) that will be /4 or  unity for a perfectly linear detector. The Logarithmic Dilution Method of Linearity Measurement

This method of linearity measurement was introduced by Lovelock (2). The procedure requires some special apparatus that is diagramatically represented in figure 3.

Figure 3 The Logarithmic Dilution Apparatus.

A known mass of solute is introduced into a well – stirred vessel through which passes a flow of gas. The exit gas is arranged to pass directly into the detector. As a consequence, the mixture is continuously diluted and the concentration of the solute in the exit flow continuously monitored  by the detector. Let a volume (dv) of pure solvent enter the vessel (volume V) and displace a similar volume (dv) from the vessel. The mass of solute removed (dm) is given by

dm

C t dv

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9 where (Ct) is the concentration of solute in the vessel after time (t). The mass change (dm) will result in a change in concentration (dC t) in the vessel, thus,

VdC t 

C t dv

0

and

dC t  C t 

dv V 

Integrating

logC t 

v V 

k 

Qt  V 

k 

where (Q) is the flow rate, (v) is the volume flow of mobile phase through the system after time (t) and (k) is the integration constant.  Now when t=0, Ct = Co, where Co is the initial concentration of solute.

k = log Co

Thus,

or 

and

C t 

C oe

logC t 

v V 

logC o

Qt  V 

Consequently, if the logarithm of the detector output is plotted against time, then, for a truly linear detector, a straight line will be produced having a slope (Q/V). If the detector has a response index of (r) and the slope of the line is ( ), then

Qr  or r  V 

V  Q

The accuracy of the measurement will depend on the flow rate remaining constant throughout the calibration, and, consequently, a  precision flow controller must be employed. Manufacturers do not This eBook is protected by Copyright law and international treaties. All rights are reserved. This book is covered by a multi-user academic End User Licensee Agreement (EULA). The full EULA may be seen at http://www.library 4 science.com/eula.html.

10 usually provide the response indices for their detectors and so it is left to the analysts to measure it for themselves. Thus, "the linear dynamic range of a detector can be defined as that range of  concentration of a solute over which the response index lies between 0.98 and 1.02."  Alternative Method for Specifying Detector Linearity

The E19 committee suggested an alternative procedure for defining linearity (3). They defined the linear dynamic range as follows, "the linear dynamic range of a detector is that range of concentration of a test substance over which the response of the detector is linear to within 5%, determined form a linearity curve" . The range should be expressed as a ratio of the highest concentration to the minimum detectable concentration. Although defining linearity by this method ensures a minimum linear performance and, consequently, a reasonable quantitative accuracy, the definition is not sufficiently explicit. Conversely, if the response index is employed, any slight nonlinearity can be taken into account by correcting the peak height (or the  peak area) using the numerical value of the response index. Thus, in effect, the useful linear dynamic range of a detector for quantitative  purposes can be significantly extended by employing correction  procedures when using the response index. It is clear that the logarithmic dilution method should not be used if the linearity is to be measured by the method recommended by the E19 committee of the ASTM. Detector Response

There are two ways of defining detector response, either as detector  output (usually in mv) per unit change in solute concentration or as the detector output per unit change in the units of detector measurement (e.g. the sensitivity of a electrical conductivity detector would be defined You may make electronic copies of this eBook for academic.You may also print out copies on paper for academic use in. You may not give, sell or distribute this document except as permitted by the EULA

11 in terms of detector output per unit change in electrical conductivity). The detector response (R D) is determined by injecting a known mass (m) onto the column and measuring the peak height (h) in (mv), then,

R D

h wQ ms

where the symbols have the meanings previously ascribed to them. In general the response is defined in terms of solute concentration but the solute must be defined. Detector Noise

Detector noise was discussed in some detail in Principles and Practice  but will be briefly reiterated here. Examples of the different types of  detector noise are given in figure 4.

Figure 4 Different Types of Detector Noise

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12 Detector noise is any perturbation on the detector output that is not related to an eluted solute. It is a fundamental property of the detecting system and determines the ultimate sensitivity or minimum detectable concentration. Detector noise has been divided into three types,  ' shorterm noise', 'lon- term noise' and 'drift' all three of which are depicted in figure 4. Short-term noise results from baseline perturbations that have frequencies significantly higher than those of an eluted peak. Short-term noise is not a serious problem as it is easily removed by appropriate noise filters without significantly affecting the profiles of the peaks. Its source usually originates from either the detector sensor system or the amplifier. Long-term noise results from baseline perturbations that have frequencies similar to those of an eluted peak. This type of noise is the most damaging as it cannot be differentiated from very small peaks. Long-term noise cannot  be removed by electronic filtering without affecting the profiles of the eluted peaks. It is clear from figure 4 that the  peak profile can easily be discerned above the high frequency noise but is lost in the long-term noise. Long-term noise usually arises from temperature, pressure or flow rate changes in the sensing cell. and is largely controlled by detector cell design. It is this noise that ultimately limits the detector  sensitivity or the minimum detectable concentration. Drift results from baseline perturbations that have frequencies that are significantly larger than those of the eluted peak. Drift is almost always due to either changes in ambient temperature, changes in mobile flow rate, detector cell pressure or column bleed in GC. As a consequence, certain detectors have very significant baseline drift at high column temperatures. Drift is easily constrained by choosing operating  parameters that are within detector and column specifications. A combination of all three sources of noise is shown by the trace at the  bottom of figure 4. The sensitivity of the detector should never be set above the level where the combined noise exceeds 2% of the F.S.D. (full scale deflection) of the recorder (if one is used), or appears as more than 2% F.S.D. of the computer print-out simulation of the chromatogram. You may make electronic copies of this eBook for academic.You may also print out copies on paper for academic use in. You may not give, sell or distribute this document except as permitted by the EULA

13

Measurement of Detector Noise

Detector noise is defined as the maximum amplitude of the combined short –  and long-term noise measured over a period of 10 minutes (the E19 committee recommends a period of 15 minutes). The detector is connected to a column and mobile phase must be passed through it during measurement. The detector noise is obtained by constructing  parallel lines embracing the maximum excursions of the recorder (or   printer) trace over the defined time period (see figure 5). The distance  between the parallel lines measured in millivolts is taken as the measured noise (vn), and the noise level (N D) is calculated in the following manner.  N  D

vn A

vn where (A) is the attenuation factor   B

and (B) is the amplification factor.

Figure 5. The Measurement of Detector Noise

It should be noted that at the high sensitivity range settings of some commercial detectors, filter circuits are automatically introduced to reduce the noise. Under such circumstances the noise level should be determined at the lowest attenuation (or highest amplification) that does not include noise-filtering devices (or at best the lowest attenuation with the fastest response time) and then corrected to an attenuation of unity. Detector Sensitivity or Minimum Detectable Concentration

Detector sensitivity or minimum detectable concentration (MDC) is defined as the minimum concentration of solute passing through the detector that can be unambiguously discriminated from noise. The size of the signal that will make it distinguishable from the noise (the  signal   –  This eBook is protected by Copyright law and international treaties. All rights are reserved. This book is covered by a multi-user academic End User Licensee Agreement (EULA). The full EULA may be seen at http://www.library 4 science.com/eula.html.

14 to – noise ratio) is an arbitrary choice. It is generally accepted that with electronic measuring instruments discrimination is possible when the signal to noise ratio is two and this criteria has been adopted for  chromatography detectors. Thus for a concentration sensitive detector, the detector sensitivity (X D) is given by

 X  D

2 N  D ( g / ml )  Rc

(R c) and (ND) being determined in the manner previously described. System Dispersion and Sensor Dimensions

One problem common to all detectors is the peak dispersion that takes  place in the mobile phase conduits and sensor volumes of the detector. Dispersion of this type is particularly serious in LC where solute diffusivities are 4 to 5 orders of magnitude smaller than those in gasses. In GC however, due to the much higher diffusion rates detector  dispersion is minimal and does not significantly affect chromatographic  performance. Consequently detector dispersion in GC detectors will not  be discussed in this book, but dispersion in LC detectors will be considered in detail in book 5, LC Detectors. Peak Dispersion from the Overall Detector Time Constant.

Peak dispersion resulting from the time constant of the sensor and its associated electronics can be significant in both GC and LC, particularly when filter circuits are introduced to remove inherent detector noise. The effect of the detector time constant can be theoretically examined (see  book 14 Extra-Column Dispersion) and calculated and the results from such calculations are shown in figure 6. The undistorted peak, that would be monitored by a detector with a zero time constant, is about 4 seconds wide. Thus, for a GC packed column operating at 20 ml/min. this would represent a peak having a volume of about 1.3 ml. It is important to note

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15 that the dispersion is only apparent . The term apparent  is used as the solute concentration profile, itself, is not actually changed, only the  profile as presented on the recorder or computer printer. Modern sensors and electronic systems employ fast solid state sensors and solid state electronic components. Thus, the majority of detector systems commercially available are sufficiently fast for the vast majority of  chromatography applications. In general, the overall time constant of  the detecting system should be less than 50 milliseconds.

Figure 6. Peak Profiles Demonstrating Distortion Resulting from Detector Time Constant

For special applications involving very fast separations, this value may need to be reduced to around 15 milliseconds. Sensors and electronics, with very small time constants, unfortunately, will also readily respond to high frequency noise. Consequently, the chromatographic system must be carefully designed to reduce short-term noise, which, as already stated, is not normally a problem in general chromatographic analysis. This may involve careful magnetic screening to reduce the effect of  stray, low-frequency electromagnetic fields from nearby power supplies and any high energy consuming laboratory equipment.

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16 Pressure Sensitivity

The pressure sensitivity of a detector can be very important as it is one  parameter that determines both the long term noise and the drift. As it influences long term noise, it will also have a direct impact on detector  sensitivity or minimum detectable concentration. Some detectors are more sensitive to pressure changes than others. The katharometer  detector, which is used frequently for the detection of permanent gases in GC, can be very pressure sensitive. Careful design can minimize the effect of pressure. It should be noted that all bulk property detectors will tend to be pressure sensitive. The pressure sensitivity (D P) should be given as the output in millivolts for unit pressure change in the detector  (e.g . as mV/psi or mV/kg/m 2). The pressure sensitivity can be used to calculate the pressure change (N P) that would provide a signal equivalent to the detector noise (N D), i.e.

 N  P 

 N  D  D P 

Thus, a knowledge of (N P) can be used in detector design when a  particular sensitivity is the objective. Flow Sensitivity

Flow sensitivity is another detector property that can have a significant effect on long term noise and, consequently, also on the detector MDC. Again it is the bulk property detectors that are the most likely to exhibit high flow sensitivities (e.g., the katharometer). To reduce its flow sensitivity, the katharometer is usually fitted with a reference cell through which a flow of mobile phase also passes. The two sensors for  the column flow and the reference flow are placed in the arms of a Wheatstone bridge so that any changes in flow rate are to a large extent compensated. The flow sensitivity (D Q) is defined in a similar manner to  pressure sensitivity (i.e. mV/ml/min). The flow sensitivity can be used to calculate the flow change (NQ) that would provide a signal equivalent to the detector noise (N D),

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17 i.e.

 N Q

 N  D  DQ

A knowledge of (NQ) can also be utilized in detector sensor design to minimize long term noise. Temperature Sensitivity

Detector temperature sensitivity varies greatly from one detector to another. The FID used in GC is virtually insensitive to temperature changes but this may not necessarily be true for the associated electronics. In contrast the katharometer detector is extremely sensitive to temperature changes (the reason for this will be clear when the katharometer detector is discussed) and must be thermostatted in a separate oven. Temperature changes together with changes in flow rate are the two main sources of drift in GC detectors. The overall temperature sensitivity of the detector system (D T) is defined as the change in output in millivolts for one degree change in temperature (˚C). Some detectors have a limited temperature range over  which they can operate satisfactorily and thus the maximum and minimum operating temperatures should also be available. The temperature sensitivity can be used to calculate the temperature change (NT) that would provide a signal equivalent to the detector noise (ND),  N T 

 N  D  DT 

It is clear that a knowledge of (N T) can be used in the same way as (N P) and (NQ) in detector design . Summary of Detector Criteria

1. Dynamic Range  –  (R D)  –  The dynamic range of a detector is that concentration range over which it will give a concentration dependent output. The units are dimensionless.

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18 2. The Response Index  –  (r)  –  The response index of detector is a measure of detector linearity and would be unity for a truly linear  detector. In practice the value of (r) should lie between 0.98 and 1.02. If  (r) is known, quantitative results can be corrected for any non linearity. 3. Linear Dynamic Range  –  (DL)  –  The linear dynamic range of a detector is that concentration range over which the detector response is linear within defined response index limits. It is also dimensionless and is important when the components of a mixture cover a wide concentration range. 4. Detector Response – (R c) – The detector response can be defined as the detector output per unit change in concentration (e.g. volts/g/ml) or, as the detector output per unit change of physical property being measured (e.g. for the FID, volts/gram of carbon/sec). In conjunction with the detector noise level it allows the sensitivity or minimum detectable concentration to be measured. 5. Noise Level  –  (ND)  –  The noise level of a detector is taken as the maximum amplitude of the combined short and long term noise taken over a period of 10 minutes; it is usually measured in volts.  It must be emphasized that detectors cannot be compared on the basis of their noise or response. They can only be compared on the basis of  their sensitivity or signal-to-noise ratio at a specific solute concentration. 6. Detector Sensitivity  –  Minimum Detectable Concentration (MDC) –  (XD)-Detector sensitivity can also be defined as that concentration that will produce a signal equivalent to twice the noise or, as that change in the physical property being measured that will provide a signal equivalent to twice the noise. The sensitivity defined in concentration units is, in general, more useful to the analyst.

7. Detector Dispersion  – (

2 ) – This d

is generally not significant in GC

detectors

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19 8. Detector Time Constant  –  (Dt)  –  The overall time constant of the sensor and electronics is given in milliseconds. It is of interest in high speed chromatography. 9. Pressure Sensitivity - (D P) – The pressure sensitivity of a detector is the output that results from unit change in pressure. It is usually specified in V/p.s.i. or V/kg/m 2 . It is important in detector design. 10. Flow Sensitivity  –  (DQ)  –  The flow sensitivity is the output that results from unit change in flow rate. It is specified in V/ml/min. It is important in detector design. 11. Temperature Sensitivity  –  (DT)  –  The temperature sensitivity is defined as the output that results from 1 oC change in temperature. It is given in V/oC. Early Gas Chromatography Detectors

The first GC detector was invented by James and Martin [4] in 1952, and used for the separation of some fatty acids. It consisted of a titration apparatus situated at the end of the column and the eluent gas was  bubbled through a suitable aqueous liquid to absorb the solutes. The solution contained an indicator and, as each solute was eluted, the solution was manually titrated. The titration process was eventually automated and an integral chromatogram was obtained by plotting the volume of base solution added against time. The integram consisted of a series of steps, one for each solute. This rather primitive arrangement validated the gas chromatographic concept but also indicated that a detector with greater sensitivity and a more catholic response was necessary for the effective use of the technique.

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20 The next detector, the first really practical detector to be developed, was also invented by James and Martin but, for some reason it seems, was never formally reported in the literature. Its description, however, did appear in a review by A. T. James [5] and a detailed explanation of the function described by Munday and Primavesi [6]. The gas density balance was a very complicated and ingenious device and, incidentally, the modern 'so – called' gas density bridge bears little or  no resemblance to the original design. A diagram of the gas density  balance is shown in figure 7.

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21

Figure 7. The Martin Gas Density Bridge

The detector consisted of a compact Wheatstone network of capillary tubes, drilled out of a high conductivity copper block. The reference flow of mobile phase and the eluent from the column entered at two opposing junctions of the bridge arms (the center of tube (C)) such that the eluent was contained in one vertical arm (C) and the pure mobile  phase in a parallel vertical arms (A) and (B). This eBook is protected by Copyright law and international treaties. All rights are reserved. This book is covered by a multi-user academic End User Licensee Agreement (EULA). The full EULA may be seen at http://www.library 4 science.com/eula.html.

22

The increase in pressure at the base of tube (C) due to the presence of  solute in (C) applied a pressure to the bottom of tube (A). This caused a flow of gas through the anemometer from tube (A) to tube (B) providing an output that was fed to a recording milliammeter. Subsequently all flows exited from the top and bottom of tube (C). The anemometer was  particularly unique. It consisted of a cylindrical chamber, 1.5 cm in diameter and 4 mm wide. A length of 0.001 in O.D. copper wire, containing 2 mm of 0.001 in Constantan wire arc welded to the copper wire in the center, passed through the conduit connecting the chamber to tubes (A) and (B). (The construction of these dual thermocouples with the equipment available in 1952 was a feat in itself). Beneath the copper Constantan junctions was situated a heater loop that raised the temperature of both junctions  by convection currents circulating round the cylindrical chamber as shown in the center of the anemometer diagram. When a flow of gas  passed through the anemometer as a result of solute vapor being present in tube (C), the convection currents above the heater loop were displaced so that one junction was cooled and the other heated as shown in the right – hand side of the anemometer diagram. The differential output from the two thermocouples was passed to an appropriate recording milliammeter. The detector was quite robust, but initially difficult to set up. The sensor  had a linear response (0.98