Anal. Chem. 1986, 58, 20-25
20
Suppression of Analyte Signal by Various Concomitant Salts in Inductively Coupled Plasma Mass Spectrometry Jose A. Olivares' and R. S. Houk*
Ames Laboratory-USDOE and Department of Chemistry, Iowa State University, Ames, Iowa 50011
The Interference on the lonlzatlon of cobalt by five salts, NaCi, MgCiz, ",I, NH,Br, and NH,CI, in an Inductively coupled plasma (ICP)is flrst looked at theoretically, and subsequently the theoretical trends are established experimentally by mass spectrometry (ICP-MS). The Interference trends are found to be In the order of the most easily ionized element in the matrix salt, Le., Na > Mg > I > Br > Ci. Quantltatively the theoretlcal values for the amount of salt needed to produce a particular Interference are 1-2 orders of magnitude higher than the experlmentaily determlned values. The results reported here Indicate that ICP-MS Is somewhat more susceptible to lonlratlon suppression effects than ICP atomic emission spectrometry. I t Is also found that the most easily Ionized element In the salt domlnates the matrix Ion spectrum observed from the ICP In the order mentioned above. Total Ion current measurements by ICP-MS at solute levels above 1% are complicated by oriflce plugging and transport loss of the salt and analyte In the desolvatlon system for the ultrasonic nebullzer used.
sample containing approximately 5000 mg L-' dissolved solids. The use of high salt and acid concentrations in ion exchange and solvent extraction methodologies, for which ICP-MS may serve as a multielemental detector, prompted us to investigate the effect of high salt loadings on the observed analytical signal. In this paper we report the extent of ionization interference effects observed in ICP-MS and compare them with published results for ICP atomic emission spectrometry (AES) (5-10). Also, a simple theoretical model with similar ramifications as that shown by Mermet and co-workers (11, 12) is presented in order to describe the effect of the ionization of different concomitant salts on the analytical signal of a cobalt solution. The salts were chosen so as to contain elements of varying first ionization energies. The calculated trends are then compared to experimental observations using ICP-MS.
THEORY Consider ionization reactions for Ar, analyte (M), and a concomitant salt (formula A,B,C,) that have been atomized in the ICP
(1)
M+M++eThe use of inductively coupled plasma-mass spectrometry (ICP-MS) for elemental analysis is growing with the advent of commercial instruments. Yet, matrix interferences in ICP-MS have been little explored. Such interferences were found to be severe in the early instruments; as little as 100 mg L-l Na was found to cause 10% suppression in the analytical signal of 10 mg L-l cobalt and chromium solutions (1). The sampling orifice used in this early work consisted of a pinhole of 50-60 pm diameter. Ions were sampled from a thin aerodynamic and electrostatic boundary layer that formed in front of the sampling plate. In order to overcome the adverse effects of this type of sampling (orifice clogging, oxide formation, and short orifice lifetime), samplers of large orifice diameter (20.5 mm) are now used. The higher gas load is handled by an additional pumping stage containing a conical skimmer that skims a portion of the supersonic jet for introduction into the high-vacuum region of the mass spectrometer (2-4). This technique is called continuum sampling. Gray and Date (2) found that the continuum sampler tolerated up to 1000 mg L-l sodium concentrations with only 10% decrease in signal for 10 mg L-l bismuth and cobalt solutions. Douglas et al. (3) also found no interference from phosphate or aluminum concentrations of up to 1000 mg L-' on 2 mg L-l calcium solutions. These authors also reported that trace analysis of chromium, cobalt, copper, and manganese in a Iow-alloy steel (Standard Reference Material (SRM) 362) required only calibration with reference standard solutions, and no matrix matching was necessary. This was not the case in their determination of nickel, cadmium, and copper in National Bureau of Standards (NBS) water (SRM 1643a) where matrix-matched calibration was desirable. Furthermore, they only report a qualitative analysis of a marine sediment Present address: Battelle Pacific Northwest Laboratories, Richland, WA 99352.
where n represents the number density of the species identified by the subscript. Similar reactions with the appropriate ionization constants, Ki,are formulated for Ar, A, B, and C. The value of Ki for each species, i, is determined by the ionization temperature (Ti,,,,)(13,14),which is assumed to be (a) equal for all plasma species and (b) independent of salt concentration (15). The degree of ionization (ai) for each species is given by (13)
nit nit + ni
ai=-=-
Ki
Ki + ne
(3)
The total electron density, ne, represents the sum of the electrons contributed by the ionization of each species present. In the present work the analyte concentration is deliberately kept low enough so that the electrons formed from ionization of M are negligible compared to ne. Thus ionization of Ar, A, B, and C is the only source of electrons and
(4) ne = aArnAr + nABC(XaA + YaB + 2%) Here nABC ( ~ m - is~ the ) number density of hypothetical salt molecules that would be present if no atomization occurred. This quantity can be estimated from typical operating parameters for the ICP and ultrasonic nebulizer used in this study (16) ~ABCFAERT~~~ CABC= RNEBENEBEICPNO TROOM = 7.4 x 1 0 - 1 7 ~ ~ ~ (5) where Cmc is the concentration of salt in the analyte solution (mol L-l), RNEBis the solution uptake rate (2.5 mL min-l), ENEBis the nebulization efficiency (10%) (171, E1cp is the
0003-2700/86/0358-0020$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
overall transport efficiency of aerosol out of the nebulizer and into the axial channel (estimated to be 75%), FAER is the aerosol gas flow rate (0.5 L min-l), No is Avogadro's number, Tgasis the gas temperature of the axial channel (5000 K), and TRooM is the gas temperature of the room (298 K). Equations 3 and 4 can be combined for each species Ar, A, B, and C into the following quadratic forms:
Here P is pressure, 12 is Boltzmann's constant, T,,, is the gas temperature (5000 K), and the contribution of the electrons (ne 7 l O I 5 ~ m - is ~ )neglected relative to the total particle density ~rn-~). Solution of eq 6-10 at a given salt concentration yields the aivalues from which ne can be calculated (18). The assumption that Tionand KMremain constant as CAB, increases implies that analyte ionization, Le., reaction 2, shifts to the left, or neutral atom side, as Cmc and ne increase. The extent of this shift can be estimated by calculating the ionization suppression factor (y)
Le., y gives the ratio of analyte signal detected in the presence of concomitant ABC to that observed in the absence of ABC. Equations 3 and 11 can be used to derive the following form for y:
where (nJmCis the electron density when salt is present and (ne)ABC=O is the electron density when salt is absent. Thus y depends upon both the concentration of the concomitant salt and the ionization energies of the elements added. The theory presented in this section is only a first approximation to ionization conditions in the ICP because it neglects the contribution of electrons from ionized water and its fragments. This theoretical model also considers only processes characteristic of the ICP itself and assumes that the ion extraction, mass analysis, and detection processes yield a representative sample of ions from the plasma. No attempt is made to consider possible artifact effects from the ion extraction process, e.g., a decrease in Tgascaused by cooling of the extracted gas sample.
EXPERIMENTAL SECTION The ICP-mass spectrometer used for this work is described elsewhere (4). The main ICP parameters were as follows: aerosol gas flow rate, 0.4 L mi&; outer gas flow rate, 17 L min-l; auxiliary gas flow rate, 0.4 L m i d ; ICP forward power, 1.1kW; and sampling position on center, 7 mm above the load coil. This sampling position is approximately 4 mm downstream from the tip of the initial radiation zone (19) during introduction of NaCl. The operating conditions were chosen to be consistent with other recent publications that describe the performance of this instrument. Also, the conditions represent those we would choose for mul-
21
tielement analysis, i.e., nearly optimum analyte ion count rates, reasonably uniform response for various elements, and low levels of oxide and doubly charged ions (4,20-22). The cobalt ion signal in each sample solution was determined with the mass spectrometer set up for selected ion monitoring at m / z 59. Sample solutions were introduced by flow injection to minimize salt deposition and orifice plugging ( 1 ) . Injection through a 0.5-mL injection loop with a solvent (H20)flow rate of 1.7 mL m i d to an ultrasonic nebulizer (9, 17) along with desolvation provided eluting peaks with heights that were approximately 90% of the signal observed under continuous flow of the analyte into the nebulizer. Stock solutions of 2.0 M and 0.20 M NaC1, MgCl,, NH41, NH,Br, and NH4Clwere prepared from reagent grade chemicals; MgCl, stock solutions were prepared from MgC12.6H20. Solutions of appropriate molar concentrations of the salts were prepared by diluting the stock solutions and then spiking with a 1000 mg L-' Co standard so as to contain a final Co concentration of 2.0 X M (1.2 mg L-l). The Co standard in distilled water was prepared at the same Co concentration. Each injection of salt solution was preceded and followed by one injection of Co standard in deionized H20. Peak heights, with background subtracted, were recorded for each injection. The ratio of the cobalt ion signal in the presence of a concomitant salt to the average of the cobalt ion signal for the two flanking injections of the cobalt standard was calculated for each salt solution. This corrects to, some extent for partial orifice plugging or some other change in sampling conditions between injections. At least two trials were performed for each salt. The averages of these ratios were then plotted. The points were connected by straight lines to serve as a visual aid and do not represent a statistical fit to the data. The mass spectra reported in this paper were recorded with the electron multiplier detector in an analog mode using a current-to-frequency converter (Model 151, Analog Technology Co., Pasadena, CA) with a signal averager (Model 1170, Nicolet Instrument Co., Madison, WI) to scan the mass analyzer and collect intensity data. This ensured observation of the major ions in the spectra without the signal losses that occur when ion counting procedures with high ion currents are used. A mass discrimination correction curve was prepared by recording the complete spectrum of four elements (Na, Co, Y, and Pr) at equimolar concentrations. Mass spectral scans were obtained repetitively during each sample introduction pulse for a total of 15 s and 128 sweeps, 40 s after injection of a 1.0-mL sample. It was critical to collect the mass spectra in the flattest portion of the top of the elution peak, thus assuring the recording of the ion composition sampled from the ICP under the full effect of the salt solution. The aerosol generation efficiency (i.e., solution nebulized/solution uptake) of the ultrasonic nebulizer was estimated by collecting the unnebulized "drain" solution at the nebulizer after uptake of 10 mL of solution to the nebulizer. The heating chamber of the desolvation apparatus was operated at the minimum temperature necessary for drying the aerosol particles to minimize possible loss of volatile species (8, 9).
RESULTS AND DISCUSSION Experimental Measurements. In Figure 1 the experimentally measured ratio (y) of cobalt ion signal in the presence of the concomitant salt to the cobalt ion signal from an aqueous standard is plotted vs. the salt concentration. The Go+ signal detected by ICP-MS either remains constant or is suppressed as the concentration of concomitant salt increases. Some authors have reported enhancements of ionic emission line intensities by concomitant salts, particularly at low observation heights (IO),but only suppression effects were observed in the present work. The extent of Co+ signal attenuation increases with salt concentrations above 0.01 M. The amount of salt that can be tolerated a t a particular y increases in the order NaCl < MgCl, C ",I < NH4Br < NH4C1. This trend follows the ionization energies of the most easily ionized element in the salt, Le., Na < Mg < I < Br < C1. The curve due to the presence of MgCl, has a more gradual descent than the others. This is due to partial clogging of the
22
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986 15
r---
'* t
1 I
\
'\C
I
-1
y 04 .
00
1
105
'&
11u-1-1
IO4 IO3 IO2 S A L T CONCENTRATION
4
,'
'i,
IO-'
A
IO'
100
(molesiL)
00 10.'
10.~ IO-' IO-' loo S A L T C O N C E N T R A T ION (molesiL)
10'
1 o2
Figure 1. Experimentally determined suppression of cobalt ion signal vs. concomitant salt concentration. The matrix salts used are NaCl ( O ) , MgCI, (X), NH,I (+), NH,Br (A),and NH,CI (0).
Figure 2. Theoretically determined suppression of cobalt ion signal vs. concomitant salt concentration at Tbn= 7500 K. The matrix salts used are NH,CI (a), NH,Br (b), NH,I (c), MgCI, (d), and NaCl (e).
Table I. NaCl Levels ( % NaCl by Weight) Necessary To Suppress Analyte Ion Signal by 10%
operating conditions for analytical applications and by no means list all the published studies of the effects of concomitant elements in ICP-AES. The concentrations are listed in percentages for this table because these units are commonly employed by atomic spectrometrists in sample preparation and dilution. The following trends are evident from Table I. First, ICP-MS with ultrasonic nebulization is more susceptible to analtye signal suppression than ICP-MS with pneumatic nebulization. This is expected because the ultrasonic nebulizer introduces 3-10 times more solute into the plasma than a pneumatic nebulizer (9, 17). Second, ICP-MS with either nebulizer would seem to be more susceptible to analyte signal suppression by concomitant elements than ICP-AES. The practical consequence of this difference is that elemental analysis by ICP-MS may require a more dilute sample solution (by a factor of 3-10) than ICP-AES. Such dilution, when feasible, will also attenuate solute deposition on the walls of the sampling cone. The detection limits obtainable by ICPMS (10-100 ng L-l) are in general superior to those obtainable by AES even if the above disparity in tolerable solute levels is considered. Correlation with Theoretical Calculations. Solution of the equations presented in the theoretical section gives the relationship shown in Figure 2 for the five salts under study and cobalt as the analyte, using a typical Tionof 7500 K (25). The overall shape of the theoretical curves and the predicted trends in the extent of signal suppression for the various salts agree with the experimental observations given in Figure 1. The theory predicts greater tolerance to dissolved salts than actually observed. In the case of NaC1, the ICP-MS data (Figure 1) show signal suppression at NaCl concentrations approximately 1order of magnitude lower than predicted by the theory. The reason(s) for this discrepancy is not known at this time, but a change in Tionwith salt concentration or collisional or gas dynamic effects during the ion extraction process could be contributing factors. Our choice of Tion= 7500 K for the calculations given in Figure 2 is based on the fact that elements with high first ionization energies are extensively ionized in the ICP. For example iodine has been determined to be 40% ionized (26) in the plasma, which at electron number densities of 5 X 1014cm-3 (15) corresponds to a Tionof approximately 7400 K. The chosen TI,is arbitrary and represents a reasonable value for the ICP. The relationship between predicted amount of interference with change in NaCl concentration a t various values of Ti, is shown in Figure 3. The NaCl concentration predicted to cause ionization suppression can vary over 1order of magnitude for a change from 7000 to 8000 K in Tion.Thus the extent of
measurement technique MS MS MS AESd AESd
nebulizer
analyte
ultrasonic Cot pneumaticb pneumaticb Cot, Bi+ pneumaticb Cr+ Cat ultrasonic Ca+
-
% NaCly 0.90
0.06 0.15c 0.1 0.7 1.3 0.15
ref a
24 2 7 9
"Present work. Cross flow geometry. cRecommended total Ionic emission lines observed.
solute level.
sampling orifice, presumably due to MgO formed adjacent to the sampling cone. Magnesium oxide has a reported boiling point of 3600 "C (23),which is higher than the estimated gas temperature (-3000 K) in the sampling orifice (3) and certainly higher than the temperature of the sampling cone walls. This affects the results by causing the signal for a subsequent injection of pure cobalt standard to decrease from its previous value. Even though flow injection and normalization with the results from subsequent injections of Go in deionized H20 corrects to some extent from orifice plugging and sampling fluctuations, when the problem becomes severe the results become biased at high salt concentrations. This is the case for MgO where the pressure in the first stage dropped from 1.5 to 0.8 torr after one complete run, Le., 10 injections of salt solution with increasing concentrations. Some of the analyte signal suppression observed in Figure 1may be attributable to loss of material in the spray chamber and/or desolvation system. There have been conflicting reports of the extent of these losses when desolvation is employed with ultrasonic nebulization (8,9).Our experience with this sample introduction device and that of other workers in this laboratory ( 5 , 9 , 1 7 )indicates that aerosol transport losses are probably not a factor for MgCI, and NaCl concentrations such that 1 > y k 0.6. This phenomenon could be significant for higher concentrations of NaCl and MgC1, and for the other, more volatile, salts particularly because more concentrateed solutions of the latter are required to induce a given level of analyte suppression. From an analytical perspective it is useful to compare the extent of ionization suppression observed in the present work to that seen in both ICP-MS and ICP-AES by other investigators. For this purpose the NaCl concentrations necessary to suppress the analyte signal by approximately 10% are listed in Table I for the above techniques. The AES data chosen for Table I are meant to represent a typical instrument and
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986 1
I
I
I
23
!
a
AAr'
C
Flgure 3. Theoretically determined suppression of cobalt ion signal vs. concomitant NaCl concentration at different T,. (K) values: 7000 (a), 7250 (b), 7500 (c), 7800 (d), and 8000 (e). a 10"
c L v
-
10'6 =
I
1
10
c
I
I
20
I
50
40
30
MASS (rnlz)
c
v
1015
Flgure 5. Mass spectra of three solutions of varying NaCl concentration: (a) blank, (b) 0.020 M, (c) 0.20 M.
a
(Ar'
Flgure 4. Theoretically determined total electron number densities vs. concomitant salt concentration at T, = 7500 K. The matrix salts used are NaCl (a), MgCI, (b), NH41 (c), NH,Br (d), and NH,CI (e).
ionization suppression predicted by the model is dependent on the value chosen for Tion. Mass Spectra for Concentrated Salt Solutions. The above theory predicts that will increase at high salt concentrations (Figure 4). Such an increase should be noticeable by ICP-MS even at salt contents of several tenths molar. The total ion count rate in the mass spectrum should also increase if the total electron density in the plasma increases because the plasma will remain electrically neutral. An attempt to measure the total ion concentration at several different high salt concentrations (0.02,0.2, 2 M) was made. The results were erratic, and no particular trends in the data could be established. This could be due to sampling problems mentioned in the previous section. However, it was also observed that the tuning capacitance changed when salt concentrations above 0.2 M were introduced into the ICP, suggesting a change in the electrical properties of the ICP. The capacitance returned to the normal setting when the salt solution was spent. At high salt concentrations, e.g., above 0.2 M, aerosol particles also tended to coalesce in the desolvation apparatus, causing loss of transport of analyte and matrix to the plasma. Future experiments in the measurement of total ion currents will be performed by using lower salt concentrations (0-0.02 M) in order to avoid the adverse effecta of very high salt loadings. Despite these experimental difficulties there are some interesting qualitative features in the detected mass spectra at high salt concentrations (Figures 5-7). First compare the ion signal from the most easily ionized element in the matrix salt (e.g., Naf from NaCl) to the total ion signal detected at all
C
A
I 25
50
75
L 100
125
M A S S (m/z)
Flgure 6. Mass spectra of (a) blank, (b) 0.02 M ",I, and (c) 2.0 M ",I.
mlz values. When sufficient salt has been added to suppress analyte ionization, e.g., Figures 5b, 6b, and 7c, the primary matrix ion does indeed comprise a significant fraction of the total ions observed. It is thus reasonable to attribute the attenuation of the analyte signal to the addition of "extra" electrons from the ionization of the concomitant, which forces reaction 1back to the neutral atom side. It is also interesting
24
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
Table 111. Aerosol Generation Efficiency for Various Solutions soln HZO
2.0 M 2.0 M 2.0 M
NaCl NH4Br NHdI
wt 70 salt
efficiency, %"
0
10.0-10.1 9.5-10.0 9.0-10.0 10.0-10.1
12 19 29
"Range of three determinations.
b
1 I
I
20
Flgure 7. NH,Br.
I
1
I
I
40 60 M A S S (rn/z)
00
Mass spectra of (a) blank, (b) 0.02 M NH,Br, and (c) 2.0 M
Table 11. Experimentally Measured Ion Ratios for Five Salts Introduced into the ICP at High Solution Concentrations" salt blank NH4Cl NH4Br NHJ
soh
interfering
concn, M
ion (i+)
0 0.020 2.0 0.020 2.0 0.020
none 35*37C1
0.48 0.44 0.27
7g381Br
0.48
1271
0.27 0.45
2.0
MgC1,
NaCl
0.020 0.20 0.020 0.20
i+/(i++ Ar+/tot+ i+/totc
24-zeMg 23Na
0 0.0019
0.10 0.012 0.20 0.029
Ar+) 0 0.0043 0.27 0.025 0.42 0.059
0.27
0.40
0.24 0.012 0.14 0.011
0.25
0.60 0.51
0.44
0.97
0.39 0.69
0.74 0.98
" Data were corrected for mass discrimination. that the Ar+ signal tends to decrease as the concentration of injected salt increases. Table I1 summarizes this phenomenon for all five salts. In the last column, a value of 1.00 for the ratio indicates that the argon ion has been completely suppressed. Note that even NH4C1suppressess Ar+ if enough salt is added. The ArH+, O+,OH+,and HzO+peak heights remain similar in the spectra for NaCl (Figure 5 ) and NH4Br (Figure 7 ) , whereas addition of NH41 (Figure 6) suppresses all the background ions to a similar extent. If these spectra are representative of the ICP (which is hard to evaluate), then ions from the most easily ionized element ip the salt can dominate the ionic composition of the central channel of the ICP if sufficient salt is added. For example, Na+ rather than Ar+ is apparently the most abundant ion when 0.2 M NaCl is introduced (Figure 5c), and I+ represents a major fraction of the total ion signal when 2.0 M NH41 is nebulized (Figure 7c, Table 11). The treqds in Table I1 are consistent with the theory presented above, which predicts a decrease in the degree of ionization of any element, including argon, with increasing electron density. In fact, of all the ions in the ICP the theory would predict Ar+ to be the most susceptible to suppression by interferents because the high ionization energy of Ar (15.76 eV) implies KA, is a relatively
small number in eq 3. It is also desirable to determine quantitatively if suppression of Ar+ and other ambient ions (e.g., 0") corresponds to that predicted by the theory, which will obviously require extension of the theory to include dissociation and ionization of HzO. The ICP-mass spectrometric approach provides a potentially attractive means to study this phenomenon because it can directly monitor ionic species such as Ar+ that do not emit intense enough lines in the visible or ultraviolet regions for study by emission spectrometry. Finally, we note several other observations. As shown in Table 111, the aerosol generation efficiency did not change even at high solute concentrations, and therefore this did not cause the observed signal suppression. The determined values for aerosol generation efficiency agree very closely with the nebulization efficiency (i.e., solute into plasma/solute uptake) of 11% measured by Olson et al. for the same type of ultrasonic nebulizer and desolvation apparatus (17). Changing the sampling position to 5 , 7 , and 9 mm above the load coil caused no discernible change in the Co+ signal from a 0.2 M NH,Cl matrix. In contrast, our preliminary data and that of several other ICP-MS users indicate that the extent of ionization suppression induced by NaCl is dependent upon sampling position. The ICP-mass spectrometer has a very complex set of operating parameters, many of which will certainly affect the observed analytical signal (20, 27). Therefore, a more in-depth spatial profile of such measurements under different parameter settings is desirable in order to understand this system better.
CONCLUSION It is clear from the results and discussion presented that the suppression of analyte signal in ICP-MS in the presence of high concentration matrix salts is due to several factors: (a) the ionization of the elements from the salt and possibly their total contribution to the electron densities in the plasma, (b) changing aerosol transport efficiency a t high salt concentrations, and (c) MS sampling conditions. For simplicity this work has evaluated the extent of analyte ion suppression caused by a single matrix element at a time, e.g., Na in NaC1. In analytical work with ICP-MS, situations will likely arise where the ionization of several matrix elements may add sufficient electrons to the ICP to suppress analyte ion signals. Because of the efficiency of ionization in the ICP even elements such as I that are not normally considered "easily ionized" may cause analyte signal suppression if present a t sufficiently high concentrations. The present work indicates the advisability of occasionally scanning mass spectra over the full mass range (e.g., Figures 5-7) during analytical use of ICP-MS. If the concomitant ions comprise a significant fraction of the total ions observed, then this scanning process has diagnosed the probable occurrence of ionization suppression. The analyst is thus warned of this possible interference and can take steps to evaluate its extent or compensate for it. Finally, the effects of ionization suppression in ICP-MS can be compensated for by stable isotope dilution or possibly by internal standardization. This latter calibration method will require experimental verification that ionization of the analytes and the internal standard element(s) are suppressed
25
Anal. Chem. 1986, 58,25-30
to a similar extent as the matrix composition changes.
ACKNOWLEDGMENT We thank Margo Palmieri and Jeffrey Crain for helpful suggestions and Kimberly LaFreniere for an early release of her manuscript. LITERATURE CITED (1) Houk, R . S.;Fassel, V. A,; Flesch, G. D.; Svec, H. J.; Gray, A. L.; Taylor, C. E. Anal. Chem. 1980, 52,2283-2289. (2) Date, A. R.; Gray, A. L. Analyst (London) 1983, 108, 1033-1050. (3) Dougtas, D. J.; Quan, E. S.K.; Smith, R. G. Spectrochlm. Acta, Part B 1983, 388, 39-48 (4) Olivares, J. A,; Houk, R. S.Anal. Chem. 1965, 57,2674-2679. (5) LaFreniere, K. E.; Rice, G. W.; Fassei, V. A. Spectrochim. Acta, Part B , in press. (6) Fassel, V. A. Science (Washlngton, D.C., 1883-) 1978, 202, 183-191. (7) Larson, G. F.; Fassel, V. A.; Scott, R. H.; Knisely, R. N. Anal. Chem. 1975, 47, 238-243. (8) Boumans, P. W. J. M.; DeBoer, F. J. Spectrochlm. Acta, Part B 1976, 318 355-375 --- - (9) Bear, B. R. M.S. Dissertation, Iowa State University, Ames, Iowa, 1983; submitted for publicatlon in Spectrochim. Acta, Part 6 ,1985. (IO) Blades, fd. W.; Horllck, G. Spectrochim. Acta, Part 6 1981, 368, 881-900. (11) Mermet, J.-M.; Robin, J. Anal. Chlm. Acta 1975, 7 5 , 271-279. (12) Jarosz, J.; Mermet, J.-M.; Robin, J. C . R. Acad. Scl. Ser. 8 1974, 278, 685-688. (13) Boumans, P. W. J. M. “Theory of Spectrochemical Excitation”; Hllger and Watts: London, 1966; Chapter 7. (14) de Galan, L.; Smith, R.; Wlnefordner, J. D. Spectrochlm. Acta, Part6 1968, 236,521-525.
-.
(15) Kalnicky, D. J.; Fassei, V. A.; Kniseley, R. N. Appl. Spectrosc. 1977, 3 1 , 137-150. (16) Houk, R. S.;Fassel, V. A,; Svec, H. J. “Dynamic Mass Spectrometry”; Price, D., Todd, J. F. J., Eds.; Heyden: Philadelphla, PA, 1981; Vol. 6, Chapter 19. (17) Olson, K. W.; Haas, W. J., Jr.; Fassel, V. A. Anal. Chem. 1977, 49, 632-637. (18) United Kingdon Atomlc Energy Authority, HARWELL Subroutine Library, July 1983. (19) Kolrtyohann, S.R.; Jones, J. S.; Yates, D. A. Anal. Chem. 1980, 52, 1965-1966. (20) Olivares, J. A.; Houk, R. S.Appl. Spectrosc., in press. (21) Houk, R. S.Anal. Chem., in press. (22) Houk, R. S.,submitted for publlcation in Pure Appl. Chem., 1985. (23) “CRC Handbook of Chemlstry and Physics”. 58th ed.: CRC Press: Ohio, 1977-1978; p 8-128. (24) Douglas, D. J.; Houk, R. S. Prog. Anal. At. Spectrosc. 1985, 8 , 1-18. (25) Houk, R. S.;Svec, H. J.; Fassei, V. A. Appl. Spectrosc. 1981, 35, 380-384. (26) Houk, R. S.;Olivares, Jose A. Spectrochlm. Acta, Part B 1984, 398, 575-587. (27) Horlick, G.; Tan, S. H.; Vaughan, M. A.; Rose, C. A. Spectrochlm. Acta, Part B , in press.
RECEIVED for review June 27,1985. Accepted August 27,1985. J.A.O. gratefully acknowledges a research fellowship from Phillips Petroleum Company. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract W-7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Sciences.
Intermolecular Proton Transfer Reactions in the Laser Mass Spectrometry of Organic Acids Cass D. Parker and David
M.Hercules*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
+
-
Hlgh ylelds of both posltlve fM H)’ and negative (M H)quasi-molecular ions have been observed in the laser mass spectra (LMS) of carboxylic acids and amino acids. To date no studies have been reported concerning the posslble mechanlsm(s) of the protonation reaction involved In the formatlon of quasi-molecular ion in LMS. Two posslble protonation mechanisms are “Intermolecular proton transfer reaction” and/or “random protonation reactlons”. Based on deuterium labellng studies we are able to conclude that intermolecular proton transfer is the major mechanism contributlng to the production ofquasl-molecular lons, (M H)’ and (M H)-, in the LMS of amino acids.
-
+
Laser mass spectrometry (LMS) studies have been reported for a number of different organic compounds (1-3). Many of these compounds show intense protonated and deprotonated molecular ions, (M + H)+ and (M - H)-. Two processes can be envisioned for quasi-molecular ion production: acidbase reaction in the laser plasma and/or intermolecular proton transfer between acid-base pairs. Plasma acid-base reactions would involve acidic and basic species produced by plasma pyrolysis/volatilizationand subsequent reaction with neutral molecules volatilized in the plasma. The possible steps of this mechanism are
HA(s)
-
HA(s,g)
nhv
HA(g) volatilization of some species, HA
nhv
(1)
H+(g)
+ A-(g)
-
+ H+(g) RCOOH(g) + A-(g) RCOOH(g)
acid/base production from HA (2)
RCOOH2+(g) protonation
RCOO-(g)
(3)
+ HA(g)
proton abstraction (4) In eq 2-4, H+(g) and A-(g) have been written as general Bronsted acids and bases. An alternative to the scheme identified above involves simple proton transfer between two molecules of the same compound. Carboxylic acids and amino acids are known to exist in solids in head-to-head lattice structures (4),thus providing the possibility for intermolecular proton transfer induced by the laser. The overall process for such a proton transfer can be written as
nhv
BRCOOH(s,g) RCOOH2+(g) + RCOO-(g) (5) This reaction will be referred to as pair production because a pair of ions is produced by a single proton transfer between two neutral molecules. To distinguish between intermolecular proton transfer (eq 5) and random, gas-phase protonation (eq 1-4), we have
0003-2700/86/0358-0025$01.50/0 0 1985 American Chemical Society
