Gas Chromatograph/Mass Spectrometer Interface: Analysis of the Molecular Effusion Separator W l l l m Fock CSIRO, Division of Chemical Physics, P.O. Box 160, Clayton, Victoria, Australia 3 768
Since the first description of molecular effusion separators (Watson and Biemann (1, 2), they have been used as interfaces between gas chromatographs and mass spectrometers for more than a decade. The enrichment of a separator is defined as the ratio of sample-gas flow to carriergas flow a t the entrance to the mass spectrometer divided by that a t the entrance to the separator. The yield is defined as the sample-gas flow into the mass spectrometer expressed as a percentage of that into the separator. An expression for the enrichment obtainable has been given by BrunnBe, Bueltemann, and Kappus (3), who suggested that by using multistage separators the enrichment could be made large a t the expense of reducing the yield. T o obtain this expression for the enrichment, one has to assume that there are no concentration gradients within the separator, and, hence, that the rate of effusion through the wall is everywhere the same and proportional to the concentration a t the separator exit. If there is a concentration gradient, then the concentration near the entrance is less than that near the exit and the average rate of effusion of sample gas through the wall will be less, thus giving a higher value of enrichment. In the next section, a simple theory is developed which allows for a concentration gradient in the separator and, in subsequent sections, experiments to test the theory are described.
THEORETICAL Separator Enrichment. The enrichment obtainable from a molecular effusion separator has been expressed as ff P(a!
- 1)
+
1
where N is the enrichment, /3 is the ratio of the carrier-gas flow out of the separator to that into it, and a is the square root of the ratio of the molecular weights of the sample and carrier gases. To obtain this expression, one has to assume that there are no concentration gradients in the separator and that the flow into the mass spectrometer is viscous. If, however, it is assumed that there is a concentration gradient down the length of the separator, but that the back diffusion due to this gradient is negligibly small, then the expression N = Io"' ( 2) can be obtained quite readily, where y = (a - l)/a.Equation 2 may be obtained also by considering an infinitely large number n of separators of the type described by Equation 1, each with Pn = (1 - x ) , where x is vanishingly small, and where /3 = (1 - x)". Equation 1 can then be rewritten as
log N = +log
(1 -
YX)) M
where Q is the gas flow 'in pressure volume units per unit time, c is the mole fraction of the sample, A is the crosssectional area of the separator, 6 ) is the diffusion coefficient of the mixture, p is the pressure in the separator, z is the position on the axis and subscripts refers to the sample gas. Because of the effusion through the wall,
If the substitution C#I = Q J Q is made, Equations 3 and 4 become, respectively:
and
If the wall of the separator is uniform, and if the pressure in the separator is essentially constant, then
Q
= Q o (1 -
):
where Qo is the input carrier-gas flow and L is the length of the separator divided by (1- 0). Equations 5 , 6 , and 7 can be used to obtain
(3)
N =
values of N between the extremes given by Equations 1and 2. If the diameter of the separator is small so that the concentration a t the wall is essentially the same as that on the axis, then the following differential equation can be set up:
where E = A m Q o / L ;and on making the substitution x = Q/E1/2
The solution of this differential equation can be expressed as: n
To evaluate R , the conditions a t the exit of the separator are used. If there is viscous flow a t this point, then @e = ce, and
+nyx
but log 0 = n log (1 - x) x -nx so that Equation 2 follows immediately. I t is evident that a more realistic expression must have
xe
+
= 0
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
2447
/-
- log,o B Flgure 1. Theoretical enrichment factors
and from Equations 9 and 10, R can be evaluated:
I t is evident from Figure 1 that to obtain the most enrichment the diffusion parameter D must be as small as possible, as must be the flow into the mass spectrometer; the latter, however, must be viscous. Calculations of N assuming molecular flow a t the exit give a series of curves whose limiting values as p approaches zero are those of Figure 1reduced by the factor l/a. It should be noted that the pore size does not appear in any of the expressions of this section; this is because the only requirement is that the product of pore diameter and pressure is sufficiently small for molecular flow to occur in the pores ( 4 ) ,thus validating Equation 4. Flow in Exit Tube. The exit tube not only has to transport gas from the separator to the mass spectrometer but also provide a suitable pressure drop. The pressure drop is usually achieved by having a constriction somewhere along the line, and invariably there is molecular flow a t this point. One of the assumptions of the previous section was that at the inlet of this tube 4 = c. Consider a tube of cross-sectional area a and length 1 , with a constriction at the exit where the flow is molecular. Let the pressure in the tube be sufficiently high so that the flow is viscous in the tube, and let the pressure drop across the tube be small compared to that across the constriction. Equation 5 can be used to describe the variation of concentration down the tube, where Q and d, are now constants. The solution is:
At the exit, ce = a4, and hence Y
z 0
+
I"I (2r + r) (%(Z?z
+
l)!
Thus d, can be evaluated at Q = Q, and at Q = Q e = PQ,, i.e., at x = x o = 1/D and at x = x , = P/D, where D = (Asp/ Q&)l12 and is the diffusion parameter. The enrichment is given by
c o exp
=
aDp
+ e - 11
therefore, if exp(Ql/aap) >> ( a - l),then 4 = co and hence the boundary condition used in the previous section is satisfied. One of the requirements is 12-
'L7 = d,/q
All the sums in Equations 9 and 11 tend to zero as x becomes large, but the individual terms become unmanageably large before the sums converge. To overcome this problem, the asymptotic representation of 4 can be used, viz: @ =
(x) 6 [exp(s) @P
Q
(log(b,
- 1) + 4.6)
where the constant 4.6 has been chosen to give a maximum difference of 1%between 4 and c,. The other requirement, to ensure viscous flow in the tube, is ( 4 ) :
p
>
957
($)i ' c q / d
where p is in Torr, d is the diameter in centimeters, and 17 is the viscosity in poise. For helium a t 150 "C, this condition becomes
P >
+
+
where k l 1 and k2 1 are the values of n which give the smallest terms. (The solutions of Equation 8a can be obtained by making the substitution z = - x 2 / 2 . The differential equation then takes the form of Kummer's equation, the solutions of which are known (6). By resubstituting into these solutions, Equations 9 and 12 are obtained quite easily.) For values of x o S 8, values of 4 obtained from Equations 8a and 11 were used to evaluate N. For x o > 8, values of 4 were obtained for x = x,, 4 and 6 from Equations 8a and 11, and d, values a t x = 4, 6, and x , were obtained from Equation 12. X and Y were obtained by matching values of d, a t x = 4 and 6 and, hence, 4o was evaluated. The results of these calculations for (Y = 6 are shown in Figure 1, with 1/D as parameter. The curves for D = m and 0 were obtained from Equations 1 and 2. 2448
2.3/d
(15)
An estimate for D p can be obtained from the expression given for the diffusion coefficient by Hirschfelder, Curtis, and Bird ( 5 ) . Some practical values of the quantities involved in Equations 14 and 15 are: d = 0.05 cm, a = 0.00196 c m 2 , a p = 128 Torr cm2 sec-l, Q = 0.3 Torr cm3 sec-l (0.016 ml min-'), and a = 6. With these values, the requirements for pseudo viscous flow become: 1 > 5.2 cm and p > 46 torr. The only practical difficulty that may arise is that the pressure in the separator may not be high enough when low flow rates are used.
EXPERIMENTAL Apparatus. The apparatus used for measuring the enrichment factor of separators is shown schematically in Figure 2. The effluent from a gas chromatograph passed into the separator through 1 m of stainless-steel capillary tube and a flow control valve. The major part of the flow was pumped away by a rotary
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
,SEPARATOR,
MASS SPECTROMETER
2.0
ROmRY PUMP
Figure 2.
Schematic of apparatus
Table I. Comparison of Theoretical and Experimental Enrichment Factors Separator
I I
Q o m l min-'
11.9 11.9
I1
4.0
I1
16.1 16.1
I1
l/D
8
8 16 32 32
-IOgj$
2.57 2.87 2.40 2.70 3.00
lOg,d\i,xp
1.10 1.10
(1.58) 1.76 1.89
log,,yVth
1.40 (1.44) 1.58
1.83 (1.88)
pump. Flow into the mass-spectrometer ion-source reglon was through 7 cm of 0.05-cm internal diameter glass-lined tubing and another flow control valve. The ion source region was connected to the analyzer region by conductances totaling about 15 1. sec-', and the analyzer section was pumped a t 150 1. sec-'. The pressure in the ion-source region was measured by a nude Bayard Alpert ionization gauge. The mass spectrometer was constructed in these laboratories and consisted of a monopole mass analyzer driven by a variable-frequency power source and coupled to a Nier-type ion source. The gas chromatograph column, a packed one of 3-mm diameter stainless steel, was run at 100 "C with a helium flow rate of 30 ml min-'. The separator, control valves, and connecting tubing were all heated to about 150 "C. As reference for estimating enrichment factors, a glass T-piece and throttle valve replaced the separator under test, the throttle valve being adjusted to reproduce the operating conditions that existed when the separator was used. The T-piece was constructed from 1.5-mm bore capillary tubing. In order to make allowance for variation of the ion-source sensitivity with helium pressure, some zinc granules were introduced into the ion-source region to provide a reference mass peak a t mle 64. Flow through the separator was measured at the exit of the rotary pump by means of a soap-film flow meter. Flow into the mass spectrometer was estimated from the measured ion-source pressure and the calculated conductance between the ion-source region and the analyzer region of the mass spectrometer. Procedure. To obtain enrichment factors, the following procedure was adopted: the flow control valves (and the throttle valve if the T-piece was in use) were adjusted to give the required flow rates; a 6-pg sample of nonan-2-one (M = 142) was injected into the gas chromatograph inlet, and a mass scan taken when the flame detector signal was a maximum; the ratio of the heights of the peaks a t masses 71 and 64 noted; the enrichment was taken to be the ratio of this quantity obtained with the separator to that obtained with the T-piece replacing the separator. By using this technique, the enrichment of the separator only is obtained as allowance is made for any effects due to the connecting tubes and the ion source. Separators. The performance of two separators was investigated, these being designated I and 11. I. This separator was made from commercially available sintered-glass tubing of length 6 cm, outside diameter 0.75 cm, and inside diameter 0.45 cm. 11. This separator was made from a sintered-glass tube of length 3.5 cm, outside diameter 0.3 cm, and inside diameter 0.1 cm, which was fabricated in our workshops from Pyrex glass powder with grain size in the range 5-35 pm. The grain size used in I is not known, but is considerably smaller than the average grain size of particles in the powder used to manufacture 11.
Figure 3. Measured enrichment factors of ( 0 )Ion source pressure = 2 X 1O+ Torr
separators I and I1 lov5 Torr. (El)Ion source pressure =
4 X
The flow of air through the separator walls was measured as a function of internal pressure a t 20 "C; in both cases, this flow was found to be a linear function of pressure for flows of 10 ml min-' or less, indicating that, in this range, the pressure was low enough for the flow through the walls to be molecular. Therefore, from the relations given in reference (4) for flow through tubes and for the transition pressure, it may be inferred that for helium a t 150 "C the flow through the walls is molecular for ail flows in the range 0 to 90 ml min-'.
RESULTS Logarithms of the experimentally determined enrichment factors are shown in Figure 3 plotted vs. the logarithms of the input helium flow. Straight lines have been drawn, even though the theory predicts lines with a slight curvature, as the precision of the results does not justify a more elaborate curve fit. Inspection of Figure 3 shows that an increase in the enrichment by a factor of 8 has been obtained by reducing the internal diameter of the separator from 0.45 to 0.1 cm. Values obtained from this figure at the points indicated by arrows have been used to make a comparison with those obtained from the theoretical treatment described above; these two sets of values are listed in Table I. The figures in parentheses were obtained by extrapolation. DISCUSSION The yield of a separator (y = NP) has been used as an indicator of the performance of separators ( 3 ) ;in this paper only the enrichment has been used as it is thought to be the better indicator. The output of the mass spectrometer is proportional to N p where p is the ion-source pressure, and this pressure in turn is proportional to PQoIS where S is the pumping speed a t the ion source. It is desirable to operate the ion source a t its maximum pressure, so that under these conditions the mass spectrometer output is directly proportional to N . If Q o is reduced and fi is increased in proportion to keep p constant, then Y is increased and N reduced, so that the mass spectrometer output is decreased in spite of the increased yield. Comparison of the enrichment factors in Table I shows that separator I1 behaves as expected from the theoretical treatment, but that the enrichment factor of separator I is only half that predicted by the theory. One of the assumptions made in the theoretical section was that concentration gradients in the radial direction could be ignored. However, with large-bore separators, the concentration at the wall could be considerably greater than that on axis, thus giving an enrichment less than that predicted.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
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It appears then that the bore of the separator should be as small as possible consistent with the requirement of the inequality Equation 15, that the porosity of the wall should be small to allow the length of the separator to be large, and that the pumping speed of the ion-source region of the mass spectrometer should be small to allow the use of a small exit flow and a large ion-source pressure. Unfortunately, there are limits to the gains achievable by carrying out these procedures: for example, a very small exit flow will cause hold-up in the connecting tube and small pumping speed causes a large background spectrum to appear.
CONCLUSIONS A considerable improvement in the separation efficiency of molecular effusion separators can be made by decreasing the cross-sectional area and increasing the length. With the use of low porosity frits, useful enrichments should be obtainable with flows as low as 1ml min-' provided the massspectrometer gas flow is kept low. It should be advanta-
geous therefore to use small-bore separators with capillary columns. ACKNOWLEDGMENT I thank K. I. Grosz for making the separators and frits and V. W. Maslen for suggesting the use of an asymptotic solution of Equation 8a. LITERATURE CITED (1) J. T. Watson and K. Biemann, Anal. Chem., 36, 1135 (1964). (2) J. T. Watson and K. Biemann, Anal. Chem., 37, 844 (1965). (3) C. Brunee, H. J. Bueltemann, and G. Kappus, presented at the 17th Annual Conference on Mass Spectroscopy and Allied Topics, Dallas, Texas, 1969. (4) C. M. van Atta, "Vacuum Science and Engineering", McGraw-Hill, New York, 1965, p 39, 46. (5) J. 0. Hirschfelder, C. F. Curtis, and R. B. Bird, "Molecular Theory Of Gases and Liquids", John Wiley & Sons: New York, 1954, p 14. (6) M. Abramowltz and I. A. Stegun, Ed., Handbook of Mathematical Functions", Dover Publications, New York, 1965, Chapter 13.
RECEIVEDfor review April 14, 1975. Accepted August 18, 1975.
Pesticide Residue Analysis by Mass Fragmentography B. A. Karlhuber, W. D. Hormann, and K. A. Ramsteiner Ciba-Geigy Ltd., Agrochemicals Division, & d e . Switzerland
Up to now, the technique of mass fragmentography has not found very widespread use in trace analysis. Skinner et al. ( I ) showed for the first time the application of mass fragmentography in residue analysis, namely, the determination of PCBs along with DDE in extracts from a sewage effluent and in sturgeon ovary extracts. In a review article, Oswald et al. (2) recently mentioned the use of mass fragmentography for the quantitative analysis of various environmental agents. A comprehensive summary of coupling gas chromatography-mass spectrometry, including mass fragmentography, was recently given by Fenselau (3). In this paper, 70 references can be found. The analysis of pesticide residues depends on specific and sensitive detectors. Using multiple ion monitoring, confirmation of a compound is achieved by the gas chromatographic retention time, by the presence of one or more characteristic ion fragments of the compound, and by the ratio of the intensities of the fragments. This high degree of specificity cannot be achieved by any other known instrumental technique. Compounds not entirely separated by gas chromatography can even be determined by mass fragmentographic detection by comparing the peak heights or areas of one or more ion fragments with the corresponding peak heights or areas of known amounts of standards. To ensure efficient use of this technique for pesticide residue determinations, the following conditions have to be met: To achieve the required sensitivity, it must be possible to inject solvent volumes of up to 10 111, without impairing the proper function of the mass spectrometer. To avoid contamination of the ion source, column bleeding must be less than tolerated for other gas chromatographic detectors. The response of the mass spectrometer used as a gas chromatographic detector must be reproducible and linear in the nanogram range. 2450
The aim of the present study is to show the application and utility of gas chromatography-mass fragmentography to some typical problems in pesticide residue analysis. Some hints are given to improve the technique for trace analysis.
INSTRUMENTATION G a s C h r o m a t o g r a p h - M a s s S p e c t r o m e t e r . The instrument used was a Finnigan Model 9500 gas chromatograph coupled with a Finnigan Model 3000 D quadrupole mass spectrometer. T h e system was equipped with a glass jet separator and a two-channel programmable multiple ion monitor (Promim, manufactured by Finnigan Corporation, Sunnyvale, Calif.). Between the GLC column and separator, a T-piece connected by a solenoid valve to a vacuum line was installed to vent large volumes of solvent vapors without impairing the function of the mass spectrometer. The construction of this automatic venting valve assembly was described by Karlhuber et al. ( 4 ) . A similar system was recently described also by Kuehl et al. (5). The experimental parameters for the mass spectrometer detector used were as follows: electron energy, 70 eV; beam current, 0.4-0.6 mA; electron multiplier voltage, 1.0-1.2 kV; preamplifier, A/V; interface oven temperature, 220 O C ; transfer line temperature, 220 " C ; manifold temperature, 120 OC; and helium flow rate, 25 ml/min. The signals were recorded on a two channel W W recorder model 1200 (W W Electronic AG, Basle, Switzerland). Gas C h r o m a t o g r a p h i c D e t e c t o r s . For comparative studies, the following two GLC-detectors were used: Electron capture detector (Tswett, OKBA, Dzershinsk Gorkowskoj, Oblasti, USSR) attached to a Tswett 5-68 gas chromatograph and Coulson electrolytic conductivity detector (Tracor Instruments, Austin, Texas) attached to a Varian 1700 gas chromatograph (Varian Aerograph, Walnut Creek, Calif.). The experimental parameters for these detectors were as follows. Electron capture detector: detector, 239Pu foil; carrier gas, Nitrogen, 60 ml/min; injection port temperature, 240 O C ; and detector temperature, 270 O C . For the Coulson electrolytic conductivity detector: reactant gas, hydrogen, 60 ml/min; carrier gas, helium, 70 ml/min; transfer line temperature, 240 OC; furnace temperature, 780 " C ; catalyst, Ni; and scrubber, Sr(OH)2.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
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+
