924
Anal. Chem. 1991, 63,924-925
Influence of Coexisting Analytes in Atmospheric Pressure Ionization Mass Spectrometry S. N. Ketkar,* S. M. Penn, and W. L. Fite Entre1 Corporation,575 Epsilon Drive, Pittsburgh, Pennsylvania 15238
The sensltivtty of atmospheric pressure lonlratlon mass spectrometry to an analyte, In the presence of another analyte wtth higher proton afflntty, was studied by using an EXTREL API-MS/MS system. A simple rate equation analysis was used to model the Ion-molecule chemistry occurrlng In the lonizatlon source. The experimental resuits obtained for the detectlon of dimethyl methyl phosphonate, In the presence of varying concentrations of dllsopropyl methyl phosphonate, agree with the rate equatlon analysis. The analysis also lndlcates that a hlgher denslty plasma Is better sutted for the detection of trace quantities of analytes, In the presence of hlgher concentrations of other analytes.
molecules, with proton affinity (PA) greater than that of the water cluster ions, through the following proton-transfer reaction:
(H20),.H+ + X
EXPERIMENTAL SECTION An EXTREL API-MS/MS instrument was used for these experiments. This instrument has been described elsewhere in detail (6),so only a brief overview will be presented. The atmospheric pressure ionization source uses a point to plane corona discharge as a source of primary ionization. Air at rates of up to 5 L/min flows through this source. A low-pressure declustering region is used to alleviate the problems associated with the water-clustered ions, which are always present in an atmospheric pressure discharge. Solutions of DMMP and DIMP in hexane were used, together with two syringe pumps, to generate the desired fractional concentrations. The first syringe pump was used at a constant flow rate to generate 320 ppt DMMP in air. The flow rate of the second syringe pump was varied to generate concentrations of DIMP in the range 2-600 ppb. The transitions m / z = 125 (DMMPH+) m/z = 93 was used to monitor DMMP.
-.
RESULTS AND DISCUSSIONS In a corona discharge operating at atmospheric pressures, a rapid series of ion-molecule reactions occur, to form a series of protonated water molecular cluster ions of the type (H20),H+ with n 2 1. These protonated water cluster ions serve as the primary reagent ions for the ionization of the trace 0003-2700/9 1/0363-0924$02.50/0
X.(H20),.H+
+ ( n - m)(H20)
If there are two trace components, X and Y, present, the following reactions will take place:
(H20),.H+ + X (H20),.H+
+Y
+
X.(H20),.H+ Y.(H20),.H+
+ ( n - m)(H20) + ( n - m)(H20)
If PA(Y) > PA(X) then the following reaction will also take place: X4H20),-H+
Atmospheric pressure ionization (API) is an extremely sensitive technique for the ionization of trace constituents in air. Much progress has been made in the use of API as an ion source for mass spectrometric analysis, since its introduction by Horning et al. (I). API-MS finds widespread use in air pollution studies, where it is important to detect trace levels (sub-ppb) of contaminants in air (2-5). API-MS/MS has been reported to detect select contaminants, in air, in the 10 ppt range (6). Attention has been mainly focused on the sensitivity of API and the factors influencing it (7,8).However in the presence of more than one analyte, as often is the case, the sensitivity of API depends on the relative concentrations of the analyte as well as their proton affinities. We report here a rate equation analysis to calculate the loss in sensitivity of API to an analyte, in the presence of another analyte with higher proton affinity. Experiments performed by using an API-MS/MS system using dimethyl methyl phosphonate (DMMP) and diisopropyl methyl phosphonate (DIMP) agree with our rate equation analysis.
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+ Y .-,Y.(H20),.H+ + ( n - m)(H20) + X
Let nl+= sum of densities of (H20),H+ type clusters, nl = density of HzO molecules, n2 = density of the neutral trace component X, n2+= sum of densities of X.(HzO),H+ type clusters, n3 = density of the neutral trace component Y, n3+ = sum of densities of Y-(H20),H+ type clusters, n+ = total ion density = nl++ n2++ n3+,n- = electron density = n+,T = residence time, a = recombination rate coefficient, and K = forward rate constant for all relevant reactions. For steady state, we have
n1+ -d -h + ) - Qnl - anl+n+- - ~ n ~ ++(n3) n ~= 0
(1) dt 7 where Q is the source term for the primary ionizing events that produce the protonated water ions and its clusters, the second term is the loss term due to recombination, the third term is the loss term due to diffusion, and the fourth term is the loss term due to the formation of protonated water cluster ions of the trace components X and Y. Similarly, we have
-d(n2+) - -~ dt
n2+ = n ~ -+~ nn ~~ -+anp+n+ n ~ -0 7
(2)
Since X and Y are present in trace quantities, the source term can be ignored in eqs 2 and 3. The primary ionizing events will produce negligible amounts of X+ and Y+. On solving eqs 1-3, we obtain
+
nl+ = n+[n+ P]/[n++ p
p= =
+ y(n2+ n3)]
1/a7 K / a
Similarly, we have
n2+/n2= yn+[n++ Bl/[(n++ P
+ yn&+ + P + ~ ( n +2 n3))l
n3+/n3= yn+/(n++ /3
+ yn3)
In the above discussion, it is implicitly assumed that the 0 1991 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 63, NO. 9, MAY 1, 1991
925
x L
9
L
0
Y CONC (PPB)
Conc Of DIMP(PPB)
Flgure 1. Sensitivity for X as a function of concentration of Y for different plasma densities: (El) 5 X lo8 cm? (+) 8 X 10' cm-? (0) 1 X lo9 ~ m -(4) ~ ; 2 X loe ~ m - (D) ~ ; 5 X lo9 ~ m - ~ .
Figure 2. Intensity for DMMP as a function of concentration of DIMP. Solid line is the result of rate equation analysis for a plasma density of 5 x io9 cm3.
proton-transfer reaction rate constant, K, is the same for all proton-transfer reactions. This assumption is valid for most compounds with PA greater than that of water, and a value of K = cm3/s can be used. Similarly, a generic value of a = lo4 cm3/s can be used for the recombination rate coefficient. In corona discharge API sources, the approximate residence time of the ions in the interaction region is 7 = lo4 s. Using these values of K, a,and 7,one obtains 0= 1Olo and y = Figure 1shows a plot of n2+/n2 as a function of n3for plasma densities of 5 X log,2 X log,1X l@,8 X 108,and 5 X 108 ~ m - ~ , respectively. As can be seen from this figure, nz+/n2 decreases as n3 increases. As the plasma density increases, the curves shift to increasing values of n3. Figure 2 shows the plot of the intensity a t transition m / z = 125 93 (corresponding to DMMP) in the presence of varying concentrations of DIMP. Also plotted is the result from the rate equation analysis for the case of a plasma density of 5 X lo9 ~ m - There ~. is an excellent agreement between the calculations and the experimental observation. Moreover, a plasma density of 5 X lo9 seems to be in the right range for plasma densities associated with corona discharges at atmospheric pressure. Strictly speaking, a corona discharge source cannot be considered as a true plasma source. In a true plasma source, all the ion-molecule reactions take place in an electrically neutral plasma, i.e. in the plasma n+ = n-. In a corona discharge, the field from the corona point drops off rather quickly. The interaction region for the ions is a field-free region governed by space charge, and this region is dominated by positive charges. Recently, we have tried to model a corona discharge source. Initial results are qualitatively similar to the results obtained for a plasma. As shown in Figure 1,as the plasma density increases the curves shift outwards. In low-pressure plasmas, like glow discharges, the plasma densities are of the order of 10l2cmS (9). Moreover, in the case of microwave-induced discharges, even higher plasma densities can be obtained. It seems
conceivable that the use of higher density plasma sources with mass spectrometers will make trace detection using such devices immune from the presence of high concentrations of other analyte. API-MS is also being used to detect trace impurities in semiconductor grade gases like N2 (IO). In such applications charge-exchangereactions are used to ionize trace constituents, rather than the proton-transfer reactions as described above. A similar rate equation analysis can be applied to this problem by using the charge-exchange reaction rate constants rather than the proton-transfer reaction rate constants. For the case of detection of trace impurities in semiconductor grade gases like Ar, Nz, H2, etc., there is a wealth of experimental information on the ion-molecule reaction rate constants as well as recombination coefficients (11). We have performed rate equation calculations for the case of impurities in N2. Experiments are in progress to verify our calculations. Registry No. DMMP, 756-79-6;DIMP, 1445-75-6.
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LITERATURE CITED (1) Horning, E. C.; Hornlng, M. G.; Caroll, D. I.;Dzidic. I.; Stillwell, R. N. Anal. Chem. 1973, 45, 936. (2) McKeown, M. C.; Siegel, M. W. Am. Leb. 1975, Nov 91. (3) S i l l , M. W.; Fite. W. L. J . Phys. Chem. 1978, 80, 2871. (4) Kambara. H.; Kanomata, I . Anal. Chem. 1977, 49, 270. (5) Mitchum, R. K.; Korfmacher, W. A.; Freeman, J. P. Anal. Instrum. 1988, 15, 37. (6) Ketkar, S. N.; Dulak, J. 0.;Fite, W. L.; Buchner, J. D.; Dheandhanoo, Seksan. Anal. Chem. 1989, 6 1 , 260. (7) Sunner, J.; Nlcol, G.; Kebarle, P. Anal. Chem. 1988, 60, 1300. (8) Sunner, J.; Ikonomou, M.; Kebarle, P. Anal. Chem. 1988, 60, 1308. (9) Vertes, A.; Gijbels, R.; Adams, F. Mass Spectrom. Rev. 1990. 0 . 71. (10) Yabumoto, N.; Mineglshl, K.; Sako, K.; Harada, H. Ukfacleen Techno/. 1990, 1 . 13. (11) Albritton, D. L. At. Data Nucl. Data Tables 1978, 22, 8.
RECEIVEDfor review November 16,1990. Accepted February 11,1991. This work was supported by the U.S. Army Program Manager for Chemical Demilitarization, Aberdeen Proving Grounds, Edgewood, MD, under Contract No. DAAA15-86C-0107.
