+
the gas mixture, (vl u t ) , Equation 7 may be written as follows:
If detectors of the first category are run wit'h scavenging gas, any advantages are achieved a t the expense of sensitivity, since sensitivity decreases by a factor, (vl uz)/v3. Valuable qualitative effects can be obt,ained with relatively small v3 values, compared with (vl 02). In this case, therefore, the loss in sensitivity is tolerable. In quantitative analysis, 03 >> (ul v2), as was shown previously. The advantages for quantitative determinations, however, cannot be fully utilized for the time being. The category-one detectors a t present available-e.g., katharometers-do not, compensate for t'he high losses in sensitivity caused by the addition of a scavenging gas. No loss in sensitivity is experienced as a consequence of operat,ion with a scavenger if the time equivalent of a peak width is smaller than t2. I n order to verify this conclusion for detector volumes 1,' in the order of 0.1 ml., peak widths of some 0.1 second would be required, as may be evidenced by pertinent calculations. Performance wit'hout loss in sensitivity by operation with small peak width and in presence of a scavenger cannot be realized unless future technical development improves the detectors of category one so that'
+
In this case, the signal, SI,depends on the flow rate of the sample v1 (Equation 8). The peak area A is given by the following expression:
A
r
= J .
fvl
.dt
(9)
03
The peak area is now independent of the flow rate of sample plus carrier gas (vl v2). This means that a detector of the first family behaves as if it were a member of family 11. Nevertheless, the detector signal is proportional to the sample concentration in the detector. The latter, however, is a function exclusively of vl, since u3 >> (vl u2) = constant. The use of a scavenger in detectors of the second group may also provide beneficial qualitative effects in the form of improved resolution through degeneration of any downstream dead volumes which may be present. However, the matrix and the amount of scavenger added may alio influence the response value of the detector.
+
+
+
+
they also conform to the above mentioned condition tl < t2. NOMENCLATURE
A
=
c
=
m
=
rl, r2
= =
SI SP td
= =
tl
=
tz
=
2'1
=
Z'n
=
L'3
=
z'd
Wl
= =
W2
=
W3
=
peak area concentration of the sample in the carrier gas quantity of one sample component response factors or response signal of a detector of category 1 signal of a detector of category 2 resiionse time of a detector response time of a senqing element fluqh time of the effective dead volume masb flow rate of the sample mass flow rate of the carrier gas mass flow rate of the scavenging gaq effective dead volume effective volume flow rate of the sample effective volume flow rate of the carrier gas effective volume flow rate of the scavenging gas
RECEIVEDfor review Llarch 26, 1964. Accepted April 1, 1964. Presented at the Second International Symposium on Advances in Gas Chromatography, Vniversity of Houston, Texas, March 23-26, 1964.
Performance of Argon Detectors in the Fie1d-I nte nsif ied Current Region JULIUS Z. KNAPP Research laborafories, Schering Corp., Bloomfield,
N. J .
ANDRE S. MEYER Deparfmenf o f Biology, Western Reserve Universify, Cleveland, Ohio
b A general equation describing the current variation of an argon detector in the field-intensified region as function of applied potential and solute concentration has been devised. This new understanding should spur improved detector operation and design. The extended linear dynamic range noted by Lovelock is shown to b e an inherent feature of cell operation in the field-intensified current region; the sensitivity is found to b e proportional to the indirect ionization coefficient. With a suitable restriction, a simplified derivation for the plateau region is obtained which is identical with the earlier expression by Platzman as employed by Jesse and Sadaus1430
ANALYTICAL CHEMISTRY
kis. The effects of space charge, load resistance, and secondary emission are briefly discussed.
G
is a t present the most powerful tool for the analysis of relatively heat stable substances. Any approach toward ultimate sensitivity has to contend with the resolving power of the chromatographic column, the sensitivity of the detector, and absorptive interferences within the chromatographic system. The currently preferred hypersensitive differential detectors-the flame ionization and the argon detectors-are both based on gas ionization phenomena. AS CHROMATOGRAPHS
;ilthough ionizat'ion chambers were not widely employed in general gas analytical work ( 7 . S I ) , their potentialities with respect to gas chromatography were soon recognized. Indeed, four years after the successful launching of gas liquiti chromatography by James and Martin in 1952, the first radiation detectors for measuring chromatographic eluates were described by I3oer and by Deal et al. (reviewed in 21). In these detectors:, a radioactive source provided the init,ial ionization. Such detectors were subsequently refined to the fairly sensitive 80-pl. (26) and more recently to the 6-pl. micro cross-section detectors. Through the introduction of noble
gases as carrier (most conveniently argon) and adaptation of operating conditions, Lovelock vastly extended the sensitivky of such devices. He event,ually developed a whole family of so-called argon det'ectors-the simple, the small, and the triode-based on the effective utilization of metast,able carrier gas energy (24). The triode, though reportedly some 20 times more sensitive t,han the small detector, has not found the wide acceptance which could have been expected, presumably because of adjustment difficulties. Incidentally, although this triode is mentioned here, it may, with greater justification, be classified among the subsidiary discharge detectors. ;in excellent review of ionization detectors can be found in Litt,lewood's treatise (21) and later developments have been summarized adequately (6). An interesting variant has been recently described by Shahin and Lipsky (20, 33). Their coaxial microdetector would seem to be a complex combination of cross-section and argon detectors, the lower sensitivity, extended linear dynamic range, and greater catholicity of the former supplementing the high sensitivity of bhe latter. The popularity of the small argon detector made a reinvestigation of it's response charact'eristics desirable. I t is not obvious in which field region this cell should be operated, and indeed the extremely critical role of the anode position ( 2 4 ) in generating a part'icular field cannot be emphasized too strongly. The present study was undert'aken to establish operat'ing conditions, in which the overall response of the cell would fall in exclusively one region, the fieldintensified current region. There, the current increases with solute concentrations as an exponential to the first power, thus realizing increased sensitivity. Such a current output can then be correlated with t'he established knowledge of gaseous conduction under conditions of field-intensified growth ( I , 9 , 26, 36). Detector performance mas evaluated by a steady-state calibrat,ion, a procedure usually applied to transducers. X kinetic analysis of the variation of output current with addition of solute traces to noble gases is presented for the plateau region. ,\ssuming analogous mechanisms in the field-intensified current region, a general equation has been developed which conforms well with the experimental results. This derivation is, with an appropriate restriction, identical with the plateau expression. Particular attention has been paid to the effect of field-int,ensification on the linear concentration range of the cell. 13y putting detectors which utilize met,nstahle po1)ulations on firmer theoretical grounds, it, is hoped to make
possible the design and operation of more sensitive detectors. MECHANISTIC ASPECTS
The energetic part,icles of the radioactive source of the detector can raise the argon of the carrier gas to various excited states as listed in Table I. ;1 number of t,hese states can transfer t'heir energy to solute molecules introduced in the cell, thereby generating an increase in ionization current. The two listed metastable states, because of their longer lifetimes, are of greatest importance for solut,e ionization. In addit,ion, the carrier gas can be ionized by collision with sufficiently energetic source particles, resulting in a quiescent current. The rates of these reactions are dependent on cell and source geometry. ;Is will be demonstrated below, for a given configurat'ion the production of ion pairs and metastable species is proportional to the emission rate of the source and it,s energy. The concentration of metastable argon is thus determined by source, geometry, and electric field strength and acts as a n energy reservoir. dddition of traces of solute cannot increase this metastable population, but merely governs the pathways of its energy conversion. The argon gas density determines whether the excited states are prevalently present as atomic or molecular species. .it the near atmospheric pressures of the conventional argon detectors, three-body collisions of an excited argon atom with t'wo neutral argon atoms have to be taken into account. This has been shown to be the case for the argon ion (11, 12, 2 7 ) and later for the metastable state of argon (2, 28, 29). As a matter of fact, Colli (2) in her experiments with excited argon a t pressure ranges beyond 150 mm. Hg observed only the presence of molecular metastable argon with a lifetime inversely proportional to the square of the gas pressure [see also Phelps (29)]. Moreover, a nonpressure dependent process produced photons of about 10 ev.; this energy is inadequate for ionization of most organic compounds, but can on occasion contribute to secondary emission by cathode bombardment. Because of the small energy difference of the two metastable levels (Table I), no distinction was made. The existence of combinational forms of excited states has been discussed by Weissler ( 3 7 ) and must, for instance, play a role in the low levels of ionizat,ion achieved by methane in argon ( 3 9 ) . This and other subsidiary processes are, however, of lesser import for the ionization of solute molecules with an ionization potential below the metastahle energy levels of argon. Disregarding these reactions permits a simple kinetic analysis of the events within the cell.
Table I.
Excited
Excitation energy, e.v.
Term 'So 3p*
11 11
3P, 3p0
11
1>3P
14
'P, 11 1,3S,P,D 12 ',3P,D,F 14 15
0 545 620 720 825 7-13 3 0-14 9 1-14 3 76
Levels of
Argon
Remarks Fundamental state Metastable level Resonance level hfetastable level Resonance level
Ionization level
After Colli and Facchini ( 4 ) . Ionization level from Reed ( 3 2 ) .
The selected major reactions under the condition of complete collection of all generated ion pairs are listed in Table 11. These reactions will be considered for equilibrium conditions. Therefore the rate of concentration change with respect to time is in each case identically zero-Le., the rate of species production is equal to its destruction [compare (Sb)]. I n addition, equilibrium conditions presuppose a charge balance within the cell which requires an equal production rate of positive and negative charges. Furthermore it is assumed that source particle range lies within the cell volume and that space charge effects can be neglected. The quiescent or standing current is the result of the charge collection of reactions 3, 4, and 5 and multiplication by the Faraday constant. Ib =
+
F(k3[Air+] kq[Ar2+]
+ k5[e-l)
(1)
Applying t,he charge balance relationship, Equation 1 can be siniplified to I b
=
2F(k3[.4r+]
+ k4[Arz+])
(la)
The concentrations of these ions are computed as follows:
Substituting the thus determined concentrations of the atomic and molecular argon ions in Equation la, yields the following expression for the quieycent current:
VOL. 36, NO. 8, JULY 1964
1431
PLATEAU REGION
On simplification this becomes
Twice the electrochemical equivalent of the production rate of the atomic metastable shall be defined as l . ~i.e. ,
T h e equation indicates that the quiescent current is directly proportional to the production rate of the atomic argon ion which is dependent on the concentration and mean energy of the source particles and on the argon carrier density. These conclusions of the kinetic analysis are readily susceptible to experimental verification. Addition of solute molecules ( M ) generate a current increase according to
I,
k s [Ar~m] [MI =
=
(5a)
ks[Ar][P] = k7[Arml[.4rl2
The above equation is more familiar in its inverse representation:
Equation 13 is identical in form with the expressions of Platzman (SO) and Littlewood (21) obtained from earlier kinetic analyses. These were, however, derived from a model in which solute molecules were assumed to be ionized by atomic metastable argon. Since these analyses were based only on the constant pressure, static data of Jesse and Sadauskis ( I S ) , the existence of the molecular metastable species could not be recognized. Under the experimental conditions, however, the evaluation of the constants yields identical results regardless of the model chosen.
(8)
Substituting Equation 8a in Equation 7a vields
Equation 5a can now be rewritten
Table II.
KO.’
Simplified Reaction Scheme
2
3 4 5
Quiescent current eqs.
Description Ion generation:
1
(11)
k , [~M + ] (6)
2Fks [Arzm][ M ]
The concentrations of the metastable species are
Reaction
2Fk6[Ar][P]
Thus in similar fashion to the quiescent current I , is seen to be proportional to the carrier gas density and the source strength. These conclusions of the kinetic analysis are again readily susceptible to experimental verification. If moreover the solute concentration [ X ] / [Ar] is symbolized by C, the current increase equation now reads
Since at equilibrium
AI
=
Atomic Molecular Charge collection: Atomic ion Molecular ion Electron
+ ++eAr+ Ar+ + Cathode .4r Ar2+ + Cathode 2 Ar e - + Anode -0 Ar Radiation + A r + Ar+ 2Ar + Ar2+ --t
-c
or Rate Constant This equation describes the proportional region asymptote. On the other hand when C >>
ki
k2
ki k, ks
JCd/k,
AI
Solute current eqs
-6 i
8 9 10 11
Metastable generation: Atomic Molecular Metastable destruction: Thfough collision with carrier gas Through colliaion with ROlute (solute ionization) Charge eollwtion: Solute ion Electron
++ Arzm + Ar
Ar Radiation Arm 2 Ar
+ :If
ArZm
Cathode ++.-\node
-c -+
-+
-+
Arm
+ Ar 3 .4r + hv 2 Ar + M + + e Arp
M+
+
M
e-
-c
0
k6
k7
k,
= kd
ko
=
k,
kio k,,
r o t e : The radioactive source particles of specified mean energy will be denoted by 8.
1432
ANALYTICAL CHEMISTRY
The discussion of current response in the plateau region will proceed with a more detailed description of the earlier experiments of Jesse & Sadauskis (13) [Compare also inter. al. (as)].These authors studied the ionization of helium and other noble gas carriers generated by single alpha particles in a cylindrical ionization chamber with an eccentric anode. The chamber was operated under the normal plateau conditions which exhibit no appreciable fieldintensification. The observed increase of this ionization which followed trace additions of gaseous solute molecules was examined. I n concordance with Equation 13, the measurements yielded a linear Platzman plot of I/AZ os. l/C, indicating that indirect solute ionization was the only mechanism operative a t the given conditions. The convenience of this straightline plot must, however, be balanced against the scanting of the graphical weight of high concentration points. Jesse and Sadauskis have already commented on this difficulty with regard t o this plot. Improved accuracy may be obtained when the observed current increase is graphed in a logarithmic plane according to Equation 12. As an example, in Figure 1 the data of Jesse and Sadauskis for argon in a helium carrier are so plotted according to the latter expression. The curve clearly reveals the saturation characteristic of the cell processes. To obtain the asymptotes to this curve, the detector response for extremely low and extremely high concentrations will be considered. I n the former case (C
