a straight line for an initial Gaussian distribution, and any deviations from straight-line behavior would give information on the initial pulse shape.
CONCLUSIONS Plasma chromatography has been used largely as a simple detection or characterization tool, but the existence of a highly-developed kinetic theory of ion mobility and diffusion should make it capable of greater scope. Much useful information can be gained by the systematic variation of operating parameters much as temperature, gas pressure, and electric field, the use of various drift gases or gas mixtures, and the study of pulse widths and shapes. Further advances can be expected in the correlation of diffusion cross-sections with molecular structure.
ACKNOWLEDGMENT The authors thank T. H. Vu for his help with many of the calculations, and D. I. Carroll for his many helpful comments.
LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25)
F. W. Karasek, ReslDevel., 21 (3). 34 (1970). M. J. Cohen and F. W. Karasek, J. Chromatogr. Sci.. 8, 330 (1970). F. W. Karasek, Res.lDevel., 21 (12), 25 (1970). F. W. Karasek, M. J. Cohen, and D. I, Carroll, J. Chromatogr. Sci., 9, 390 (1971). F. W. Karasek, W. D. Kilpatrick, and M. J. Cohen, Anal. Chem., 43, 1441 (1971). F. W. Karasek, Anal. Chem., 43, 1982 (1971). F. W. Karasek and 0. S. Tatone, Anal. Chem., 44, 1758 (1972). F. W. Karasek and R. A. Kelier, J. Chromatogr. Sci., 10, 626 (1972). F. W. Karasek and D. M. Kane, J. Chromatogr. Sci., 10, 673 (1972). F. W. Karasek and D. M. Kane, Anal. Chem., 45, 576 (1973). F. W. Karasek, 0. S. Tatone, and D. M. Kane, Anal. Chem., 45, 1210 (1973). S. P. Cram and S. N. Chesler, J. Chromatogr. Sci., 11, 391 (1973). M. M. Metro and R. A. Keller, J. Chromatogr. Sci., 11, 520 (1973). S. Chapman and T. G. Cowling, "The Mathematical Theory of Non-Uniform Gases", 3rd ed., Cambridge University Press, London, 1970, Chap. 19. E. W McDaniel, "Collision Phenomena in Ionized Gases", John Wiley, New York, 1964, Chap. 9, 10, App. II. H. S. W. Massey, "Electronic and Ionic Impact Phenomena. Vol. 111. Slow Collisions of Heavy Particles", Oxford University Press, London, 197 1, Chap. 19. E. W. McDaniel and E. A. Mason, "The Mobility and Diffusion of Ions in Gases", John Wiley, New York, 1973. J. H. Whealton and E. A. Mason, Ann. Phys. ( N V , 84, 8 (1974). E. H. Kennard. "Kinetic Theory of Gases", McGraw-Hill, New York, 1938. Chap. 3. G. H. Wannier. BellSyst. Tech. J. 32, 170 (1953). H. Hahn and E. A. Mason, Phys. Rev. A, 6 , 1573 (1972). L. A. Viehland and E. A. Mason, Ann. Phys. ( N O , in press. J. 0. Hirschfelder, C . F. Curtiss, and R. B. Bird, "Molecular Theory of Gases and Liquids", John Wiley, New York, 1964, Chap. 8 . H. R. Hasse, Phil. Mag., 1, 139 (1926). H. R . Hasse. and W. R. Cook, Phil. Mag., 12, 554 (1931).
E. A. Mason and H. W. Schamp, Ann. Phys. ( N u , 4,233 (1958). E. A. Mason and J. T. Vanderslice, Phys. Rev., 114, 497 (1959). T. M. Miller, J. T. Moseley, D. W. Martin, and E. W. McDaniel. Phys. Rev., 173, 115 (1968). E . Graham IV, D. R. James, W . C. Keever, D. L. Albritton, and E . W. McDaniel, J. Chem. Phys., 59, 3477 (1973). K. Hoselitz, Proc. Roy. SOC.(London), Ser. A, 177, 200 (1941). E. A. Mason, H. O'Hara, and F. J. Smith, J. Phys. B: Atom. Molec. Phys., 5, 169 (1972). P. L. Patterson, J. Chem. Phys,, 53, 696 (1970). A. S. Dickinson, J. Phys. B: Atom. Molec. Phys., 1, 387 (1968). G. Heiche and E. A. Mason, J. Chem. Phys., 53,4687 (1970). 0. J. Orient, Can. J. Phys., 45, 3915 (1967). P. L. Patterson, Phys. Rev. A, 2, 1154(1970). S. Geitman, Phys. Rev., 90, 808 (1953). Ref. (10,Sec. 5-3-F. J. J. Thomson and G. P. Thomson. "Conduction of Electricity Through Gases", 3rd ed., Cambridge University Press, London, 1928, reprinted by Dover, New York, 1969, Vol. i, pp 187-189. G. M. Hidy and J. R. Brock, "The Dynamics of Aerocolloidal Systems", Pergamon Press, New York, 1970, Chap. 3. J. Gieniec. H. L. Cox, Jr.. D. Teer, and M. Dole, Proc. 20th Annual Conf. Mass Spectrometry, Dallas, TX, June 1972, p 276. B. K. Annis, A. P. Malinauskas, and E . A. Mason, J. AerosolSci., 3, 55 (1972). Ref. (23), pp 27-28. P. L. Patterson, J. Chem. Phys., 56, 3943 (1972). D. I. Carroll and E. A. Mason, Proc. 19th Annual Conf. Mass Spectrometry, Atlanta GA, May 1971, p 315. W. D. Kilpatrick, Proc. 19th Annual Conf. Mass Spectrometry, Atlanta, GA, May 1971, p 320. G. W. Griffin, I. Dzidic, D. I. Carroll, R. N. Stillwell, and E. C. Horning. Anal. Chem.. 45, 1204 (1973). L. B. Loeb, "Basic Processes of Gaseous Electronics", University of California Press, Berkeley and Los Angeles, 2nd ed. 1960, pp 129 ff. M. A. Biondi and L. M. Chanin, Phys. Rev., 122, 843 (1961). A. Blanc, J. Phys., 7, 825 (1908). S. I. Sandler and E. A. Mason, J. Chem. Phys.. 48, 2873 (1968). E. A. Mason and H. Hahn, Phys. Rev. A., 5, 438 (1972). H. 8.Milloy and R. E. Robson, J. Phys. B: Atom. Molec. Phys., 6, 1139 (1973). J. H. Whealton, E. A. Mason, and R. E. Robson, Phys. Rev. A, 9, 1017 (1974). B. K. Annis, A. P. Malinauskas, and E. A. Mason, J. Aerosol Sci., 4, 271 (1973). A. M. Tyndall, "The Mobility of Positive Ions in Gases", Cambridge University Press, London, 1938, Chap. 5. D. I. Carroll, Franklin GNO Corporation, West Palm Beach, FL 33402, oersonal communication, 1970. (58) A.E. Humphreys and A. Mills, Nature Phys. Sci., 238, 46 (1972). (59) T. Kihara. Adv. Chem. Phys., 5, 147 (1963). (60) T. Kihara, Prog. Theor. Phys. Suppl., 40, 177 (1967). (61) T. R. Galloway, Am. lnst. Chem. Eng. J., 18, 833 (1972). (62) S. N. Lin, G. W. Griffin, E. C. Horning, and W. E. Wentworth. J. Chem. Phys., 60, 4994 (1974). (63) S. I. Sandler and E. A. Mason, J. Chem. Phys., 47, 4653 (1967). (64) G. A. Lugg. Anal. Chem.. 40, 1072 (1968). (65) E. Graham IV. D. R. James, W. C. Keever, I. R. Gatland, D. L. Albritton, and E. W. McDaniel. J. Chem. Phys., 59, 4648 (1973). (66) I. R. Gatland, Case Stud. At. Phys. 4, 369 (1974). (67) J. Crank, "The Mathematics of Diffusion", Oxford University Press, London, 1956, Chap. 2.
RECEIVEDfor review October 31, 1974. Accepted February 12, 1975. This work was supported in part by National Science Foundation Grant GP-37549X.
The Gas Density Balance and the Mass Chromatograph Erdogan Kiran' and John K. Gillham Department of Chemical Engineering, Princeton University, and Textile Research Institufe, Princeton, NJ
A theoretical development is presented to show the hydrodynamics of operation of the gas density balance (a gas chromatographic detector that is currently being used in the mass chromatograph). Theoretical aspects of the mass chromatograph and a formal derivation of its operational relationships are also presented and discussed. I P r e s e n t address, SEKA, C e n t r a l Research L a b o r a t o r y , I z m i t , Turkey.
The gas density balance was first introduced in 1956 ( I ) . Its design was motivated by a desire t o develop a gas chromatographic detector where response would be independent of the chemical structure of the substance being analyzed. The detector can be used for determination of molecular weights (2, 3 ) and in a modified form ( 4 ) , which has become known as the Gow-Mac gas density balance ( 5 ) ,it is currently being used in the mass chromatograph (6). ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
983
GAS DENSITY DETECTOR FILAMENT DIRECTION OF GRAVITY
R E FG ER A SE N Ck EA { I
1
&{I
k-
VENT
GAS F R O M CHROMATOGRAPHIC COLUMN
Figure 1.
Schematic diagram of the Gow-Mac type gas density bal-
ance The mass chromatograph utilizes two gas chromatographic systems that are coupled through a common injection port. The instrument gives two simultaneous sets of gas chromatographic peaks (one from each detector), and the ratio of a peak in one set to its counterpart in the other is related to the molecular weight of the constituent which is responsible for the particular peak. In providing the molecular weights as well as the gas chromatographic retention times, this analytical instrument combines the functions of a gas chromatograph and, in a sense, a mass spectrometer in one unit. In this laboratory, the instrument has been coupled with a programmable pyrolyzer and has been used effectively to study the decomposition products of polymeric materials under various heating conditions (79).
The purpose of the present report is to present a complete and formal analysis of the hydrodynamics of the gas density detector and of the determination of molecular weight using the mass chromatograph so as to achieve a better understanding for effective use of the mass chromatograph ( 7 ) .
GAS DENSITY BALANCE A Formal Analysis of the Hydrodynamics of Operation. A schematic diagram of the Gow-Mac type detector is shown in Figure 1. The reference gas, which is the same as the chromatographic carrier gas, is split at A and flows in
A 6.X.Y
0
5-PORT
0
0 h
Figure 2. 984
VALVE5
F-OW CONTROLLEP FLC#
ME7EP
P P E S S U F E PEGL-A-GR GPlEii
UEATEO -TAhlSFER h L E S
Pyrolyzer-thermal conductivity probe-mass chromatograph
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
parallel over the lower and upper filaments which are part of a Wheatstone Bridge. So long as no solute is carried by the carrier gas from the column, the flow of the reference gas over the filaments is not disturbed. However, when a solute with a density greater than that of the carrier gas enters the cell at C, the density of the gas in the vertical conduit BD becomes greater than that of the pure carrier gas, and, consequently, the pressure at D increases. As will be shown below, this causes the flow of the reference gas to decrease over the lower filament but to increase over the upper filament. The converse phenomenon applies for a solute which has a lower density than that of the carrier gas. The change in the flow rate of the reference gas over the filaments leads to a change in the temperature and in turn to a change in the resistance between the upper and lower filaments. This unbalance is recorded as a chromatographic peak (as in Figure 2). Despite the fact that the gas density balance was one of the first gas chromatographic detectors designed, search of the literature (1-4, 10-21) showed that a formal treatment of its hydrodynamics of operation was lacking. Such an analysis was recently developed (7) and is now presented below. To ascertain the kind of flow patterns that prevail, three dimensionless quantities will be considered. These are the Knudsen number ( N K ~which ) is the ratio of the mean free path ( A ) of the molecules of the gas to a characteristic length (1) of the conduit in the flow system, Le., A/1; the Mach number (NM,)which is the ratio of some characteristic velocity ( u ) of the fluid to the velocity of sound ( c ) in ) is the fluid, i.e., u l c ; and the Reynolds number ( N R ~which the ratio of the inertial forces to the viscous forces (in the equation of motion) Le.,
(- P;d)
where p and p are the density and viscosity of the fluid, and u and d represent a characteristic velocity and a characteristic length (diameter), respectively (22-24). A fluid can be treated as continuum only if A > ~ l),then the so-called Knudsen flow, that is free (collisonless) molecule flow prevails. The condition X = 1 (NK”= 1) is referred to as “slip flow” and
represents the transition between continuum and free molecule flow. The Mach number is of importance since it gives a measure from which one can decide if the fluid can be treated as incompressible. Compressibility is a measure of the changes of volume (or density) due to external forces such as pressure variations. It can be shown (22) that the relative change in density ( A p l p o ) due to a pressure increase A p is related to the Mach number through
and, clearly, when Rs.
+
< 1 and R4 < R1.
4) Minimize
-
interesting to note the dimensions reported for a gas density balance in Ref. 4: R4 (2 mm) Rs ( 2 mm), R4 ( 2 mm) < R2 (4 mm) < R1 (15 mm), 1‘ (10 mm) < l ( 1 5 mm) < h (150 mm). This is most interesting in that, except for one condition, the dimensions satisfy the requirements suggested from the foregoing detailed analysis and, therefore, much credit is due to the intuition of the designer. The experimental results obtained using a gas density balance with these inner dimensions indicate a simple direct proportionality between change in flow rate and change in density ( 4 ) . I t is now demonstrated that this is indeed predicted from the nonlinear general expression derived herein, (i.e., Equations 40 or 37) when the detector parameters are chosen so as to conform with the requirements of Equation 41. Imposition of the requirements of Equation 41 leads to the following approximations:
+
and R4
-
< Rs.
[ ( 3 ( 3 4 (341 +
Furthermore, if the flow rate of the reference gas (of density p and viscosity f i ) is chosen to be sufficiently greater than the flow rate of the carrier gas, then p ” and fi” (the density and viscosity of the gases exiting from the detector) can be approximated as p and f i , that is: p”
+
jl”
x p x p
(43)
Substitution of Equation 43 into Equation 40 results in: that is, choose I’ < h, RP < R4, R2 < Rs, R2 < R1, 1’ < I , h < 1, and R4 < R1. These conditions cannot be all satisfied simultaneously. However, they suggest choosing R4 < R1, R P < R1, R4 < Rs, and I’ < 1 without any ambiguity. The selection of h > 1 is essential in order to maximize the premultiplying factor. This selection however will make [B], I C ] , and [D] terms larger unless R4 I , one has to choose h > 1’; this helps minimize [D] but has an adverse effect in [A]. Similarly choosing R2 > R4 and R2 > Rs maximizes [A] but has an undesirable consequence in [D]. However, it is more important to maximize [A] than to minimize [D] since [A] directly multiplies and thus amplifies the change in density. These considerations suggest that in designing a gas density balance, the dimensions should be chosen so as to satisfy the following conditions: R, 0.07 R, N R ~must , be satisfied (23). Here L,, R,, and N R ~represent , the length, radius, and the Reynolds number in each (vertical and horizontal) conduit of the detector. I t must be further realized that the greater the lengthhadius ratio in each conduit, the smaller will be the relative magnitude of pressure losses a t the bends and T-joints.] In view of the conditions suggested in Equation 41, it is 988
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
In view of Equation 42 and the requirements R P > R4 and R2 > R3 of Equation 41, so long as [ ( f i / f i ’ ) ( p ’ / p ) ] is not much less than 1 (which will be true for low solute contents), the denominator of Equation 44 can be approximated as (jd fi’)(p’/p)[A],and thus Equation 44 can be reduced to a simple form: (Q?
-
Q*) = 77gHR44 81.11
- p)
(45)
or, if the radius is expressed in terms of diameter, and the change in flow rate and density are expressed as A Q and AP, AQ =
?rgHD4
128111
(461
is obtained; [This linear relationship is identical to the expression which has been reported to fit the experimental data for methane, propane, and butane (as solutes) in a gas density balance using nitrogen as the reference and carrier gases (4).] The effect of density change on the change in flow in the vertical conduit YS (Figure 3), i.e. (v4 - u s ) is also of interest. Even though a general relationship can be obtained by simultaneously solving Equations 23 and 33 [as was done to derive Equation 37 for ( u 7 - us)], the following simplified analysis is found to be more instructive. I t was mentioned earlier in connection with Equations 7 through 1 2 that, in the vertical conduits of the detector, there are two contributions to the pressure drop, Le., a static contribution and a dynamic contribution. An analysis of the relative importance of the two contributions leads to interesting observations.
0
N
M
which are the simplified forms of Equations 18 and 25, respectively. If the pressure losses a t the junctions (i.e., Z A P M and Z A&') are neglected, simple rearrangement of Equation 51 yields
L
or or rgHD4 128~1
AQ = C
I f /
1
D
F
G
Figure 3. Schematic diagram of the gas density balance detector
(see text for restrictions on dimensions)
which is incidentally identical to Equation 46 derived earlier. Substitution of Equation 53 into Equation 52, after rearrangement, provides an explicit expression for the difference ( u 4 - u 3 ) :
Consider the ratio (y) of the absolute value of the static contribution ( - p g h ) to the dynamic contribution [-(8&/ R12) u1] in the pressure drop expression (Equation 7) applicable for conduit PR:
($)
A(+') (P' P"
(47)
or, from Q 4 = i ~ R 2 ~ uand 4
Q 3
- P")}
(54)
= rRz2v3, it follows that
The velocity u1 can be related to the total reference gas flow rate ( Q ) entering the detector by Q = 2~17rR1~. (The approximate sign is needed since the upward and downward flows in conduit P B are not necessarily equal.) Thus, "pg
(e&)
(48) 4P It is clear that depending on the detector and fluid parameters, y can assume values anywhere from much less than 1, to much greater than 1. The implication is that under certain conditions, such as y >> 1, the dynamic contribution to the pressure drop in the vertical conduits may be negligible when compared to the static contribution, and vice versa. Equation 48, when evaluated using data for carbon dioxide [Le., p = 1.97 X lo-" g/cm3 and p = 1.48 X g/cmsec, (25)] becomes N
(%) x 104
When a solute with density greater than that of the carrier (and consequently the reference) gas enters the detector, p' is greater than both p and p" and Equation 55 predicts that the downward flow in the conduit YS will be greater than the upward flow. The converse is predicted if the solute is of lower density. If the reference gas flow rate is greater than the carrier flow rate (which is in fact the case in practice), then the approximations p" = p and y" p are permissible, and Equation 55 becomes
(49)
(where R1 and Q must be expressed in cm and cm3/sec, respectively). For a typical reference gas flow rate of 100 ml/ min, and a radius of 1.5 cm for conduit B P ( 4 ) , the value of y becomes = 3 X lo4 >> 1; and for a typical column flow rate of 10 ml/min and a radius of 0.4 cm for conduit Y S ( 4 ) , the value of y is = 2.4 X lo7which is again much greater than 1. Thus the dynamic contributions to the pressure drops in the vertical conduits of a gas density balance are negligibly small when compared to the static contributions. Consequently, Equations 7-12 can be simplified by neglecting the second terms. As an example, Equation 7 can be written as:
(PP- pR)
-pgh (50) Substitution of the simplified forms of Equations 7-12 into Equations 17 and 24 results in
I t is important to note the implication of this equation in terms of the linearity of the relationship being limited to low solute contents. (For linearity, plp' = 1 is needed.) I t is the difference (Q7 - Q s ) which is sensed by the detector. Whenever there is a change in the flow rates over the filaments, there occurs a change in the temperature and, consequently, in the resistance of the filaments, and since the filaments are part of a Wheatstone Bridge, an electrical signal is generated. There is experimental evidence ( 4 ) that this electrical response of the detector (which we will designate by the symbol E ) is linearly related to the change in flow rate, that is, (57) E 0: (Qi. - Qg) However, as has been shown above, (Q; - Q s ) is in turn related to the change in density by an equation such as Equation 45. Thus,
or
and
8p"l' R , ($)'(uB
- 2.7)
CAP,'
= O
(52)
(59) E = k(p' - p ) where h is an overall proportionality constant. Since the densities can be related to the molecular weight, the gas density balance finds its unique use in determination of the molecular weights of solutes. ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
989
MASS CHROMATOGRAPH Unified Treatment for Determination of Molecular Weight. A sample, when injected into the mass chromatograph (see Figure 2), is automatically split into two approximately equal fractions and carried (by helium) onto two external traps. Then valves A and B are turned to their backflush positions and the traps are simultaneously heated; this operation permits the carrier gases 1 and 2 to flow through the traps and carry the sample (which is released by heating) onto the matched chromatographic columns. As they elute from the columns, the constituents are detected by the two gas density detectors. The recorder output displays two sets of peaks corresponding to the responses from the two gas-chromatographic systems for the same constituents of the mixture. The ratio of the responses are then related to the molecular weights of the constituents as explained below. T o a first approximation, the density of a gas is given by the ideal gas expression, and a t a given pressure and temperature, it is proportional to its molecular weight: P RT (Under standard conditions, p = (1/22.4)M, where p is e r pressed as gramdliter and M as gramdmole.) The density of the pure carrier (or reference) gas can therefore be expressed as P p =-Mc (61) RT where M c is the molecular weight of the carrier gas. When a solute of molecular weight Mx is introduced to the carrier gas stream, the density and molecular weight become p’ and M‘, respectively. The instantaneous average molecular weight of the carrier gas M’can be expressed as a linear sum of the molecular weights of the solute and the pure carrier gas, Le., M’ = (1 - Y)’VC+ YM, (62) where Y represents the mole fraction of the solute. Consequently P p’ = F ( ( 1 - Y)rWc + YAWX} (63) p =-M
Substitution of Equations 61 and 63 into Equation 59 results in
which represents the instantaneous electrical response generated by the detector when the effluent from the chromatographic column containing nX moles of a solute of molecular weight M x and nc moles of the carrier gas of molecular weight Mc enters the detector. The total electrical response during the elution of the solute can be obtained by integrating Equation 64 with respect to time: Et = L - E d t
or Et = k
(g)mx
- Mc1
(65)
Joy
nx
+” nc ) d t
(66)
(where it has been assumed that in the time interval for the elution of the solute, the pressure and the temperature of the effluent remain unchanged so that the ( P I T ) term can be taken outside the integral sign). For small values of n x , the following approximation is permitted:
990
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
Furthermore, the integral (So”rtxdt) is simply equal to the total number of moles of the solute that has eluted, Le., N x , and therefore,
Substitution of Equation 68 into Equation 66 results in:
Since the total electrical response is directly proportional t o the chromatographic peak area A x , A x = k’(P/RT)Nx[Mx - M c ] (70)
or Ax
k’(P/RT)Wx[l
- Mc/Mx]
(71)
where k’ is a new proportionality constant that also includes the klnc term, and W X ( =N x M x ) is the total mass of the solute that has eluted. Equations 70 and 71 involve all measurable quantities and form the operational relationship of a gas density detector. I t is clear that at a given temperature and pressure, the factor k’(P/RT) becomes a cell constant for a given carrier gas. I t can be evaluated from a measurement of the peak area due to a known number of moles, N x , (or known mass, WX) of a solute of known molecular weight M x . Once the cell constant is determined, the molecular weights of unknowns can be calculated by measuring the peak area due to a known mass (or number of moles) of the unknown. The h priori knowledge of the cell constant is not required if a standard of molecular weight M s is purposefully added to the unknown before its chromatogram is obtained. The following equations can be written for the detector response for the standard (Le., A s ) and for the unknown (Le., A x ) :
where m and n are the weight fractions of the standard and the unknown, respectively, in the total amount of sample ( W )injected, and Mcl is the molecular weight of the carrier gas used. When these equations are divided, the cell constants cancel out and:
This shows that if the ratio of the weight fractions of the standard and unknown, Le., m / n , is known (which in practice usually necessitates knowing the weight fraction of each) then M X can be calculated from the ratio of the areas of the chromatographic peaks since the molecular weights of the standard and the carrier gas are known. The need to know m / n can be removed however, if another sample of the same mixture is chromatographed using this time a different carrier gas (of molecular weight Mc,). A new expression, analogous to Equation 74, involving Mc2 can be derived:
Division of Equation 74 by Equation 75 leads to a new relationship, that is,
Equation 41, Equation 40 led to Equation 44 and, for low solute contents, to Equation 45 (which is the linear relationship between the change in flow rate and the change in density). Assumption of a linear relationship between the detector signal and the change in the flow rates over the filaments of the detector (substantiated by experimental evidence) led to Equation 59 and in turn, under the restrictions of low solute content and ideal gas behavior, to Equation 69. The assumption of direct proportionality between the electrical signal and the chromatographic peak area resulted in Equation 71 (the key expression involving measurable quantities). And finally, under the restrictions that the split ratio be fixed and the temperature and pressure conditions in the detectors be identical, the operational expression, Equation 80, for the mass chromatograph was obtained. Thus, whether or not K is indeed an invariant of the instrument depends on whether these assumptions and restrictions are indeed obeyed. [The incorporation of the nonlinear forms (such as Equations 40 or 44) for the difference ( Q ; - Q s ) in Equation 57, and the consequences that will follow in developing the operational relationships for the mass chromatograph are presently being studied and will form the subject matter of a future communication (261.1 I t is clear from Equation 71 that the response of a gas density detector is positive when M x > M c , negative when M x < M c , and zero when M x = M e . The conditions of M x and >> M c or Mx > M c , then M c l M x 1, and A x 01 W x ( M c l M x ) 0: where AI and A2 are the chromatographic peak areas for Nx; that is, the response becomes proportional to and more unknown X, kl’ and k2’ are detector constants, and M c , sensitive to the number of moles of the sample. Thus, a and M e 2 are the molecular weights of the carrier gases 1 given carrier gas can be expected to display optimal funcand 2. tionality only in a certain range of molecular weights. Some Division of Equation 77 by Equation 78 leads to experimental studies on the response of the gas density - Mc balance as a function of the nature of the carrier gas have (79) been reported. In particular, hydrogen ( 1 6 ) , helium (16, 1 7 ) , nitrogen (12, 13, 16, 17), argon (12, 13, 1 7 ) , carbon where K = /3k2’/aklf.(Provided that both columns and dedioxide (12, 13, 16), chloromethane ( 1 6 ) , difluoroethane tectors are operated under identical temperature and pres( 1 3 ) , dichlorofluoromethane (13, 1 6 ) , sulfur hexafluoride sure conditions, PIRT terms will cancel). (14, 16, 171, bromotrifluoromethane ( 1 6 ) , and octafluorocyT h e knowledge of the actual value of the split ratio (PICY) clobutane (13) have been investigated. In one study (171, is not required so long as it does not vary from one experithe optimum molecular weight ranges of operation were exment to the next. If this condition is satisfied, K becomes plicitly reported to be 10-50 for helium, 40-125 for nitroan instrument constant that can be determined by using a gen, and 50-130 and 160-300 for sulfur hexafluoride. compound of known molecular weight and measuring the In the mass chromatograph, whereas the use of two carresponse ratio A J A 2 from the chromatographic outputs. rier gases makes it possible to obtain Equations 77 and 78 Once K is established, the molecular weight of any unand consequently derive Equations 79 and 80, the selection known can be determined by measuring the response ratios of a high- and a low-molecular weight carrier gas provides a from the chromatographic outputs and using the equation wider range of operation for calculations of molecular weights. The selection of the low molecular weight carrier gas for use in the mass chromatograph requires certain considerwhich is obtained by rearranging Equation 79. ations. Very light gases such as helium or hydrogen are of limited use. This is because the detector response is proDISCUSSION portional to [ ( M x - M c ) / M x ] (see Equation 71 for examEquations 71,77, 78, 79, and 80 constitute the operationple) which very rapidly tends to a constant as Mx: inal relationships of the mass chromatograph and, a t this creases, thus putting limitations on the functionality of the stage, it is instructive to summarize the assumptions and detector. Furthermore, back-diffusion in the conduits of restrictions involved in their derivations. Assumptions of the detector may become appreciable with light gases. In continuum, incompressible, and laminar flow conditions in this laboratory, carbon dioxide and chloropentafluoroethane ClC2Fj (Freon-115) are being used as the low- and the conduits of the gas density balance led to Equation 37 high-molecular weight carrier gases, respectively. which, when the pressure losses a t the bends and T-joints I t is clear from Equation 80 that the accuracy of molecuwere considered negligible, reduced to Equation 40. Under lar weight determinations depends strictly upon the invarthe restrictions that the flow rate of the reference gas be iance of the instrument constant, K , and the accuracy and greater than that of the carrier gas, and the dimensions of the gas density balance conform with the restrictions of precision with which the response ratios are measured. I t is
which no longer involves terms related to the masses of the standard or the unknown. The use of such methods in calculating molecular weights of unknowns has been reported in the literature (2, 3, 13). In particular, in view of Equation 76, it has been shown that by using several carrier gases with molecular weights greater and less than the molecular weight of the unknown, the molecular weight of the unknown can accurately be bracketed and interpolated ( 1 3 ) . This concept of using more than one carrier gas also forms the basis of molecular weight determination in the mass chromatograph. However, in the mass chromatograph, instead of carrying out the chromatographic analyses of two samples of an unknown mixture containing a standard, the first with one carrier gas and the second with another, a sample (which does not require inclusion of a standard once the instrument has been calibrated, as will be shown below) is injected directly and split automatically into two fractions (CYW and /3W where CY /3 = 1.0, and W = amount injected), and each fraction is simultaneously analyzed by the respective chromatographic system. Therefore, for a given solute X, two simultaneous responses, describable by two equations of the form of Equation 71 are obtained:
-+
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
991
Table I. Influence of Error Made in Evaluation of K(A1IAz) on the Calculation of Molecular Weightsa ?4 envr
% error
MX
4 10 20 30 40
3.76 4.25 5.60 8.88 28.54
4.53 10.42 20.28 30.15 40.04
13.25 4.20 1.40 0.50 0.10
6.54 12.11 21.37 30.74 40.19
50 60 70 80 90 100 110 120 130 140 150
17.44 5.91 3.25 2.07 1.40 0.97 0.67 0.45 0.28 0.15 0.04
49.94 59.85 69.80 79.76 89.84 99.70 109.70 119.79 129.83 139.85 149.96
0.12 0.25 0.28 0.30 0.18 0.30 0.27 0.17 0.13 0.11 0.02
49:73 59.33 69.04 78.81 88.73 98.67 108.68 118.85 129.02 139.34 149.79
0.54 1.11 1.37 1.48 1.41 1.01 1.20 0.95 0.75 0.47 0.14
160 170 180 190 200
0.04 0.12 0.19 0.24 0.29
160.04 170.09 180.36 190.30 200.67
0.02 0.05 0.20 0.15 0.33
160.27 170.81 181.62 192.25 204.00
0.17 0.47 0.90 1.18 2.00
210 220 230 240 250 260 270 280 290 300
0.33 0.37 0.40 0.43 0.46 0.49 0.51 0.53 0.55 0.57
210.60 220.73 231.21 241.38 251.85 261.86 272.20 282.56 292.77 303.09
0.28 0.33 0.53 0.57 0.74 0.72 0.81 0.91 0.95 1.03
214.12 225.36 236.53 247.71 259.39 270.52 282.30 294.23 306.04 317.67
1.96 2.43 2.84 3.21 3.75 4.04 4.39 5.08 5.53 5.89
320 340 360 380 400
0.59 0.62 0.65 0.67 0.69
323.63 344.96 365.08 386.70 407.33
1.13 1.46 1.41 1.76 1.83
341.63 367.34 391.88 417.91 443.31
6.76 8.04 8.85 9.97 10.82
63.50 21.10 6.85 2.47 0.47
450 500
0.73 460.80 2.40 512.81 13.95 0.75 512.01 2.40 582.39 16.47 M c , = 154.46 (molecular weight of Freon-115).Mc2 = 44.01 (molecular weight of carbon dioxide). = K ( A 1 / A 2 ) + 0.01 K ( A 1 / A 2 ) .C = K ( A 1 / A 2 ) + 0.05 K(AiIA2). Q
important to realize that errors in K and A1/A2 are not linearly related to the error they cause in the molecular weight. This is illustrated in Table I. In its preparation, the theoretical value of the factor K(A1IAZ) was calculated for a given molecular weight for the case in which the carrier gases are carbon dioxide and Freon-115. A defined error was then purposefully imposed on the value of K ( A J A " , and the molecular weight corresponding to this new value was back-calculated and compared with the initial value of the molecular weight. As examples, the errors induced in the molecular weights due to 1 and 5% errors in the ratio K ( A J A 2 ) have been tabulated. Table I indicates that error becomes magnified in the high molecular weight region. (The percent error is also large for extremely low molecular weights; but the error in terms of mass units is still low.) A graphical presentation of the dependence of the ratio K ( A , / An) on the molecular weight is shown in Figure 4. As can be seen from this figure, K ( A I / A Ptends ) to a limit as M x increases and, consequently, in this region, small errors in K ( A l I A 2 ) may lead to large errors in M x . 992
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
In regions of low molecular weight, there is a significant variation in A1/A2 as molecular weight changes and small errors in K are not magnified with respect to molecular weight. In particular, near the molecular weights of the carrier gases, A1/A2 approaches either infinity or zero (Figure 4), thereby masking any possible errors inherent in K . However, in high molecular weight regions, A1/A2 tends to a constant and K ( A J A 2 ) becomes very sensitive to errors in K . The implications are that a K value evaluated by using low molecular weight standards may, in practice, lead to large errors if used in estimating the molecular weights of unknowns in the high molecular weight range. (This is illustrated in the section on Instrument Calibration below.) On the other hand, a K value determined using high molecular weight standards is expected to be reliable in estimating the molecular weights of low molecular weight unknowns as well. This is because in the low molecular weight range, the ratio A1IAZ (rather than K ) is the governing factor. It should be apparent however that more accurate values
H~ -.r
,5
6
;*--
I
7
8
IO
7,
l,l
-- -
12
14
I6
18
20
2 2 2 4 26
28
I
I
1
I
I
MCI
I
:1 5 4
4 6 I M O L E C U L A R WEIGHT OF FREON-115 j
M c Z = 4401
( M O L E C U L A R WEIGHT
OF CARBON DIOXIDE )
Figure 5.
Mass chromatogram of a mixture of known saturated nor-
mal hydrocarbons (C, to CZ8)
I I
10-2
0
Figure 4.
50
I
I00 I50 200 250 300 3 5 0 400 M,, M O L E C U L A R WEIGHT OF UNKNOWN
450
500
Theoretical response of the mass chromatograph
of molecular weights of unknowns can be obtained by using more than one K value, each applicable to a certain range of molecular weights. I n s t r u m e n t Calibration. The process of calibration involves the determination of the constant K in Equation 79. In principle, K can be evaluated by injecting just one solute (of known molecular weight) into the mass chromatograph and measuring the response ratio AI/A2 from the chromatographic outputs. However, the accuracy of measuring the response ratio varies with molecular weight and, consequently, K should be evaluated by using more than one compound if it is to be applicable in a wide range of molecular weights. A synthetic mixture containing 15 known saturated normal hydrocarbons (C5 to C ~ S was ) used for calibration. A chromatogram is shown in Figure 5 where peaks have been identified with respect t o their carbon numbers. The top and bottom series of peaks correspond to the detector responses using Freon-115 and carbon dioxide gases, respectively. [In obtaining this chromatogram, columns (Ys-in. X 1 2 f t , 10% SE-30 on 60-80 mesh Chromosorb W-AW) were
program-heated (at 5 "C/min) from 30 to 300 "C. The peak attenuations were X8 except for Cs and Cs peaks in the Freon-115 channel for which the setting was X64. After elution of the C11 peak, the polarity of the Freon-115 detector response ?as reversed to give downward peaks for constituents with molecular weights greater than the molecular weight of Freon-115. Flow rates were 10 ml/min a t 100 psig for CO:! and Freon-115 carrier gases; 41 ml/min a t 56 psig for the Freon-115 reference gas and 122 ml/min a t 100 psig for the COa reference gas. Detector currents were 100 milliamperes. The detector oven temperature was 242 "C.] The K value was calculated using the response ratio (based on peak heights) for each hydrocarbon analyzed. The values are tabulated in Table 11. In the range (2.5to Cs and also C12 to (228, K is reasonably constant; the average values being 0.206 and 0.189 in the respective regions. However, in the vicinity of the molecular weight of Freon115,K fluctuates considerably. The molecular weight of each hydrocarbon can be backcalculated using the K values thus determined. Table I11 summarizes the calculations. This table shows that if a K value based on low molecular weight standards (such as c 5 - C ~ )is used to recalculate their molecular weights, the error is within 0.2%. However, if the same K value is used to back-calculate the molecular weights of higher molecular weight constituents, the calculated values are in large error. (The error increases rapidly as molecular weight increases,
Table 11. Analysis of a Synthetic Mixture of Normal Saturated Hydrocarbons Used for Calibration of the Mass Chromatograph o e
Hydrocarbon
Carbon No.
bP,
c
MX a
MX MX
-
-
%C(;K
MC2
A', "!
(3) a,
K
Pentane c5 36.07 72.15 -2.9250 -14.25 0.205 Hexane c6 68.95 86.18 -1.6 191 -7.81 0.207 ci 98.42 100.21 4.9653 4.68 0.206 Heptane Octane Ca 125.66 114.23 4.5729 -2.76 0.207 Decane c 10 174.10 142.29 -0.1238 -0.52 0.238 195.9 156.32 +0.0121 +0.16 0.076 Hendecane Cli c 12 216.3 170.34 +0.1257 +0.70 0.180 Dodecane c Id 253.7 198.40 10.2846 t1.46 0.195 Tetradecane Hexadecane 16 287 226.45 +0.3946 +2.04 0.193 Oc tadecane 18 316.1 254.51 +0.4752 +2.43 0.196 Eicosane c20 343 282.56 +0.5370 12.83 0.190 Docosane c 22 368.6 310.61 +0.5856 +3.12 0.188 Tetracosane c24 391.3 338.67 +0.6251 +3.35 0.187 Hexacosane cZS 412.2 366.72 +0.6577 +3.55 0.187 Octacosane c28 431.6 394.78 +0.6851 +3.63 0.187 "CRC Handbook of Physics and Chemistry". FreoniCOz response ratio, as measured from the chromatogram. Mc,l = 154.46 (Molecular Weight of Freon-115).Mcz= 44.01 (Molecular weight of Con). K = average value of K. Kc.,.[., = 0.206. KC1Z.cp8= 0.189. _______
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
993
Table 111. Back Calculation of the Molecular Weights of the Calibration Compoundsa Carbon NO.
c5
72.15 86.18 100.21 114.23 142.29 156.32 170.34 198.40 226.45 254.51 282.56 310.61 338.67 366.72 394.78
72.08 0.09 73.91 86.37 0.22 88.61 C? 100.24 0.04 102.62 c8 114.46 0.20 116.59 CiO 143.77 1.04 144.57 ci1 158.22 1.20 157.90 c 12 173.07 1.60 171.30 Cl, 201.96 1.79 196.55 c16 234.52 3.56 223.76 c18 265.17 4.19 248.27 czo 308.86 9.30 280.47 c22 353.15 13.69 312.43 c24 400.41 18.23 344.08 c26 455.06 24.08 378.67 C28 481.92 22.07 394.84 a W h e r e ,V,ca'cd values h a v e been c a l c u l a t e d f r o m M , = [K(Al/Az) 44.01 - 154.46]/[K(Al/Az) - 1.01 u s i n g t h e K parenthesis, a n d ( A 1 / A ~values ) measured f r o m t h e c h r o m a t o g r a m ( t a b u l a t e d in T a b l e I ) . c6
reaching about 20% by 400, i.e., an error of about 80 mass units.) On the other hand, a K value determined from higher molecular weight standards (such as C12-C28) estimates the molecular weights of these as well as those of low molecular weight constituents with reasonable reliability. Back-calculations of the molecular weights in the vicinity of 154.46 (molecular weight of Freon-115) is not affected appreciably by which K value is used, and the estimates are reasonable. The error is less, however, if the K value applicable for high molecular weight standards is used to estimate the molecular weights near but greater than 154.46 and the K value applicable for low molecular weight standards is used to estimate the molecular weights near but less than 154.46. These suggest the use of two K values, one applicable below 154.46 (Le., 0.206) and one applicable above (Le., 0.189). The observations in connection with Tables I1 and I11 are in accord with the expectations of theoretical considerations based on Figure 4. For example, near 154.46, A1 is small and cannot be accurately measured; consequently, measurement of A1/A2 is subject to error and leads to fluctuations in K if K is determined in this region. However, since A1IA2 is small, K ( A l / A z ) is not too sensitive to changes in K and thus the K values determined in other molecular weight regions turn out to be suitable for backcalculations in the vicinity of 154.46 also. The interesting conclusion is therefore that the variation in K in the immediate vicinity of the molecular weight of the high molecular weight carrier gas should not be of major concern. In fact, compounds with molecular weights in this region should not be considered as calibration compounds by themselves. This realization removes the need to use "effective" (27, 28) rather than actual molecular weight values for the carrier gases. [If, instead of their actual values, slightly different, Le., effective values are used for molecular weights of the carrier gases, the fluctuation in K that occurs near the molecular weight of the carrier gas of higher molecular weight (Le., 154.46) becomes somewhat smaller. This has no rigorous basis; it has been attributed to possible impurities in the gases that may be affecting the molecular weights of the gases (28).] The response ratios in this example have been calculated from the peak height measurements. In principle, however, calibration and molecular weight calculation must be based on peak area measurements as is evident from the develop994
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
2.4 2.8 2.4 2.0 1.6 1.0 0.56 0.36 1.20 2.4 0.74 0.59 1.6 3.2 0.01 v a l u e i n d i c a t e d in t h e
ment of Equation 80. The use of peak heights can be permissible only if the peaks are not skewed and the peak base widths are identical on both channels (which imposes a strict requirement for identical columns). However, area measurements are not without ambiguities with respect to the limits of integration. Peak area measurements are subject to greater error when the peak is not due to a single component. In the mass chromatograph, use of peak height ratios appears to be superior to peak area ratios determined using an on-line computer or electronic integrator (29). Caution must be exercised, however, since peak height is affected by column parameters; for example if the column is program-heated, peak height will depend on the rate of heating, whereas peak area would not be affected. One consequence of this is that if the analysis of an unknown mixture must be carried out under certain programmed heating conditions, then the instrument constant must be evaluated under similar conditions. Therefore, frequent recalibration may become necessary when using peak heights for calculations.
LITERATURE CITED (1)A. J. P. Martin and A. T. James, Biochem. J., 63, 138 (1956). (2)A. Liberti, L. Conti, and V. Crescenzi, Nature, 178, 1067 (1956). (3)C. S.G. Phillips and P. L. Timms, J. Chromatogr., 5, 131 (1961). (4)A. G. Nerheim, Anal. Chem., 35, 1640 (1963). (5) Gow-Mac Instrument Co., 100 Kings Road, Madison, NJ. (6) C E. Bennett et al.. Am. Lab., 3 (5), 67 (1971). (7)E. Kiran, Ph.D. Thesis, Department of Chemical Engineering, Princeton University, Princeton, NJ, 1974. (8) E. Kiran and J. K. Gillham, J. Macromol. Sci. Chem., A8 (1).211 (1974). (9)E. Kiran and J. K . Giliham, SPE Tech. Pap., 19,502 (1973). (10)C. W. Munday and G. R. Primavesi, in "Vapor Phase Chromatography", D. H. Desly. Ed.. Butterworths. London, 1957,p 146. (11)E. A. Johnson, D.G. Chiids, and G. H. Beavan. J, Chromatogr., 4, 429 (1960). (12)C. L. Guillemin and F. Auricourt, J. Gas Chromatogr., 1 (lo),24 (1963). (13)J. S.Parsons, Anal. Chem., 36, 1849 (1964). (14)C. L. Guillemin and F. Auricourt, J. Gas Chromatogr., 2 (5).156 (1964). (15)I. A. Revel'skii. R. I. Borodulina. and T. M. Sovakova, Neftekhimiya, 4 (5).804 (1964):translated in Petrol. Chem. USSR, 4, 296 (1965). (16)C. L. Guillemin, F. Auricourt, and P. Blaise, J. Gas Chromatogr., 4 (9). 338 (1966). (17)J. T. Waish and D. M. Rosie, J. Gas Chromatogr., 5 (5),232 (1967). (18)E. C. Creitz, J. Res. Nat. Bur. Stand. Sect. C, Eng. Instrum., 72C (3), 187 (1968). (19)E. C. Creitz, J. Chromatogr. Sci., 7, 137 (1969). (20)A. Adam, F. Fock, and J. Harangozo, Meres Autom., 18 (3).81 (1970). (21) A. A. Datskevich et al., Vses. Nauchno-lssled. Geologo-Razvedochnyi Neftioni hstitut. Trudy, 64, 173 (1970). (22)H. Schlichting, "Boundary Layer Theory", 8th ed., McGraw-Hill Book Co., New York. 1968, Chap. 1. (23)R. B. Bird, W. E. Stewart, and E. N. Lightfoot, "Transport Phenomena", John-Wiley & Sons, Inc., New York, 1960.
H. Tennekes and J. L. Lumley,
"A First Course in Turbulence", MIT Press, Cambridge, Mass., 1972. "CRC Handbook of Chemistry and Physics", 52nd ed.. R . C. Weast, Ed., The Chemical Rubber Corporation, Cleveland, Ohio, 1971; a) p F170: b) p E41; c) p F43. E. Kiran and J. K. Gillham, work in progress. Chromalytics Corporation, Unionville, Pa. R. S.Swingle, Ind. Res., 14 (2). 40 (1972).
(29) A. C. Lanser et ai., Anal. Cbern.,45, 2344 (1973).
RECEIVEDfor review August 13, 1974. Accepted February 24, 1975. Financial support for this project was provided by the Chemistry Branch of the Office of Naval Research and The Camille and Henry Dreyfus Foundation.
Reference Compound to Calibrate Ion Abundance Measurements in Gas Chromatography-Mass Spectrometry Systems James W. Eichelberger, Lawrence E. Harris, and William
L. Budde'
Environmental Protection Agency, National Environmental Research Center, Methods Development and Quality Assurance Research Laboratory, Cincinnati, OH 45268
Gas chromatography-mass spectrometry (GCIMS) data from several surveys pointed out the need for a standard procedure for calibration of the ion abundance scale in GC/MS systems. In addition, there is a need for a standard test to evaluate the overall performance of these systems. A number of proposed reference compounds were evaluated with respect to a set of criteria for an ideal GC/MS reference compound. The compound decafluorotriphenylphosphine (DFTPP) was selected because its propertles best satisfied the crlterla. A set of standard relative abundance ranges for DFTPP were developed by examination of GC/MS data obtained on a variety of systems. Computerized data systems were considered an integral part of the GC/MS system for both ion abundance calibrations and performance evaluations.
Historically, the calibration of the mass/charge and ion abundance scales in mass spectrometry has been a user responsibility. Unlike some other forms of spectrometry, manufacturers of mass spectrometers (MS) could not and did not provide precalibrated chart paper. With the introduction of computerized systems, automatic calibration programs were developed for the m a d c h a r g e scale. These programs are based on the use of a reference material, usually perfluorotributylamine (PFTBA) or perfluoro kerosine (PFK), which have well defined ions at specific masses. With the computerized systems, ion abundance measurement calibrations remained a user responsibility. Several different types of mass spectromters, Le., magnetic, radio frequency (RF, quadrupole), and time-of-flight, are in widespread use as gas chromatography (GC) detectors, and variations in abundance measurements can be very large. There is a clear need for a standard calibration procedure to provide a reasonable basis for comparison of output from the large variety of equipment in use. In addition, a standard ion abundance calibration would support the increasingly heavy reliance on files of reference spectra to make empirical identifications of compounds in environmental, biomedical, and other types of samples. Clearly, correct identifications require some consistency between reference spectra and observed spectra, and better quality abundance data would improve the effectiveness of all empirical search systems. Author to whom correspondence should be addressed.
In addition to an ion abundance calibrant, there is a need for a reference compound to evaluate the overall performance of a computerized GC/MS system. We have observed spectra with acceptable ion abundances but, because of poor resolution adjustment, broad peaks that were interpreted by the data system as multiplets. A reference procedure would allow an operator to validate the performance of the GC column, the sample enrichment device, the ion source, the ion detection circuits, the analog-to-digita1 converter, the data reduction system, and the data ootput system. The application of this procedure would enhance the overall quality of results emerging from the systems in use. There is a special need to closely monitor the performance of the RF quadrupole mass spectrometer. Unlike the magnetic deflection spectrometer, the active ion separating device of the RF field spectrometer, the rods, is directly contaminated during operation. After prolonged operation, the rods are subject to severely degraded performance which usually affects the region above 300 amu first. Often this degraded performance is not detected because there is no generally accepted performance standard to form the basis for such judgments. The Environmental Protection Agency has developed and used experimentally a performance evaluation/abundance calibration procedure for the last several years. A set of chemical and physical properties criteria for a reference material was developed and a number of likely candidates, including PFK and PFTBA, were tested. The compound decafluorotriphenylphosphine (DFTPP) was selected as the one which met most of the criteria. This paper reports the criteria on which the compound was selected, its mass spectrum, some physical and chemical properties, and some performance data that were collected over the last few years. An RF field mass spectrometer, which has been tuned to give the suggested ion abundances in the reference compound spectrum will, in general, generate mass spectra of organic compounds which are very similar to spectra generated by other types of mass spectrometers. Thus RF field mass spectra become directly comparable to spectra of compounds in collections which have been obtained with other types of mass spectrometers.
EXPERIMENTAL Materials. All chemicals and solvents were obtained from commercial sources. Decafluorotriphenylphosphine was prepared ac-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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