Anal. Chem. 1900, 60,1308-1313
1300
Sensitivity Enhancements Obtained at High Temperatures in Atmospheric Pressure Ionization Mass Spectrometry Jan Sunner,’ Michael G. Ikonomou, and Paul Kebarle*
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
The sensitlvlty of atmospheric pressure lonlzation mass spectrometry to a range of analytes In atmospherlc air was studled as a functlon of ion source temperature between 25 and ca. 400 OC wlth a SCIEX TAGA 6000E mass spectrometer. Many sulfur and carbon bases have extremely low sensitivities at room temperature. With the samples heated, the sensitlvitles of many of these analytes were found to increase by several orders of magnltude, aliowlng for subpartper-billlon detection of these compounds. I n contrast, oxygen bases with low sensltlvlty at room temperature show only smal sensftlvlty Increases with lncreaslng temperature. Thls was also the case for those analytes, mostly nitrogen bases, that have very hlgh sensltlvlty at room temperature. The dlfferent sensltlvlty changes with temperature for the varlous analytes can be predicted on the bask of the analyte gas-phase baslcltles (OB)and the energetlcs for the gasphase hydratlon of the protonated analytes.
Atmospheric pressure ionization mass spectrometry (API-MS) (1,2)is a very sensitive technique for the detection of trace compounds in air (3) with detection limits often in the parts-per-billion to parts-per-trillion range. However, many compounds, including some which are environmentally important, have very low sensitivities in API (4). For example, some organosulfur compounds cannot even be detected with API. This fact has slowed the acceptance of API for trace analysis. In air monitoring using API, ambient air is being ionized at atmospheric pressure, typically by a corona discharge. Due to the naturally present moisture, the ionization is followed by a sequence of ion/molecule reactions (5)which results in the rapid formation of hydronium ion-water clusters, H30+(HzO)h.An equilibrium cluster distribution is attained rapidly through successive, reversible clustering reactions
H3O+(H2OIh+ H 2 0 = H30+(H20)h+l
(1)
Under typical ion source conditions of 25 “C and 5 Torr partial water pressure (relative humidity 21 %) and at equilibrium, the majority of the hydronium ion water clusters in the ion source contains from four to seven water molecules in addition to the hydronium ion. The hydronium ion-water clusters are the main reagent ions in ambient air API. The clusters protonate analyte molecules according to the reaction H30+(HzO)h
+ €3 = BH+(H,O)b + ( h + 1 - b)HZO
(2)
The factors controlling the sensitivity of analytes (B) in air API were recently studied in this laboratory (4). The results are briefly reviewed here. In the preceding paper in this issue ( 4 ) it was found that the API sensitivities of analytes B are best discussed in terms Present address: Department of Chemistry, Montana State University, Bozeman, MT 59717.
of the gas-phase basicities of the analytes, GB(B). (A graph showing sensitivity vs gas-phase basicity, for a number of compounds at room temperature, is shown in Figure 3 of the Results and Discussion section of this paper.) The gas phase basicity values were taken from Lias (6). The analytes can be divided into three groups. Analytes with high-gas basicities, GB > ca. 200 kcal/mol, usually have high sensitivities which are determined by the fast kinetics for proton transfer from the reagent H30+(H20)hions to the analyte, reaction 2 (kinetic control). These compounds will be referred to here as “K”type compounds. The majority are nitrogen bases. Occasionally, the proton transfer reaction 2 for a particular analyte may be slow. The sensitivity is then low but still kinetically controlled. Most analytes with gas-phase basicities 200 kcal/mol; 0,thermodynamic control group T.; 0, low-sensitivity group L.
dimethyl sulfide 2-ClEtEtS pyrrole ferrocene ani1ine pyridine nickelocene piperidine
G.B.(B)," S,,I(B)~'" 174.1 174.4 174.6 178.6 180.2 180.6 180.6 182.0 185.0 185.6 186.1 188.9 189.5 192.4 192.5 192.8 200.3 201.8 202.5 213.1 216.0 218.0
S,,i(BIb
28 "C
kcal/mol
1.7
TOpt/"C TOpt/"C 4.0
X
400
c
C
3.2 X 2.9 X 3.8 X 2.0 X c 5.0 X 2.9 X 6.8 X 4.7 X 5.0 X 1.6 X 8.4 X 5.0 X 1.0x 3.7 X 1.2 X 0.23 1.00 8.0 X 0.50
2.4 X 1.8 X 8.3 X 3.6 X
220 290 380 320
X
c
C
C
8.2 X low3 2.1 X lo4? 6.5 X 0.69 2.9 X 0.67 0.30 lo-* 0.11 10-5 0.15 0.26 lo+ 0.17 0.23 1.00 0.19 0.65
410 410 400 290 430 220 400 430 400 310 280 28 28 210 70
"Gas phase basicity of analyte B at 28 "C, from Lias ( 6 ) . *Sensitivityof analyte B relative to the high sensitivity K group analyte-pyridine at 28 "C and at a higher temperature Toptwhere the sensitivity of B was highest. 'Not detectable. dQuestion marks indicate signal is close to the background level. ~~~
10-8 I
160
I
1
1
I
I
1
I
170
180
190
200
210
220
230
G.B. (kcal/mol) Figure 4. Relative sensltiiity for analytes as a functlon of analyte gas phase basicity and temperature: open symbols, 25 O C ; full symbols, maximum sensitivity at high temperature: A, group K; 0,group T: 0, group L analytes.
results will be discussed and compared with model calculations. The sensitivities of a range of analytes B relative to that of pyridine observed a t room temperature and plotted versus the gas-phase basicity of B, GB(B), are shown in Figure 3. These are essentially the same data as used in the previous publication ( 4 ) except that a few more analytes have been added. As already discussed in the Introduction, the analytes can be divided into three groups (K, T, and L); see Figure 3. The temperature effect on the sensitivities is summarized in Table I and Figure 4. The table gives the sensitivities a t 25
"C and at "high" temperature. The "high or optimum temperature is the temperature a t which the observed sensitivity for the given compound was the highest. For several compounds the optimum corresponded to the maximum air temperature attained in the ion source (ca. 400 "C). The sensitivities from Table I are plotted in Figure 4 vs the gas-phase basicities (GB)of the respective analytes. I t can be seen that a number of the low-sensitivity "L" compounds show dramatic increases in sensitivity as the temperature of the ion source is increased. Thus, the sensitivities for the sulfur compounds dimethyl sulfide, thiophene, and 2chlorodiethyl sulfide, increase by at least factors of lo4 to lo6. Very large increases are also observed for furan, xylene, anisole, ethylene oxide, pyrrole, and ferrocene. Several of the L compounds in Figure 4 can not be detected at room temperature except at very high concentrations. Even a t these concentrations, the signals for furan, thiophene, and dimethyl sulfide a t room temperature were hidden in the background noise. Therefore, the room temperature sensitivities for these compounds indicated in Figure 4 are only upper limits. The sensitivities for the group T compounds show much less dramatic increases with increasing temperature in the ion source. For the majority of the compounds the increase is less than a factor of 10. The increase for benzaldehyde is somewhat larger whereas the sensitivities for methanol and acetonitrile are quite unaffected by temperature change. The compounds with high GB and high sensitivities at room temperature, i.e. group K, are represented in Figure 4 by pyridine and piperidine. The sensitivities change very little with temperature. A decrease in sensitivity with increasing temperature may indeed sometimes be observed. This is the case for piperidine above 185 "C. The L compounds included in Figure 4 are those that could be detected at a higher temperature. However, some compounds with GB below ca. 180 kcal/mol could not be detected either at room temperature or at higher temperatures. Thus, we did not observe protonated benzene, toluene, chloro-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 13, JULY 1, 1988
1311
These facts determine the formalism used in the subsequent equations. The analysis (4) uses the "equilibrium ratio" expression R,, which is defined by
be rewritten as shown
10-3 ,
,
Substituting eq 5 into eq 10 and rearranging, one obtains
Furan
,I 104
1
I
I
Equation 11 shows that when R,,(B) and the sum of the intensities of the hydronium ion hydrate peaks are known, the sensitivity for B, S,,(B), can be calculated. The sum CI,(H30+(H20),,) measured in our experiments was typically 2 X lo6 counts per second. At a water concentration of 7.75 Torr and a total pressure of 700 Torr, the water is present at 1.1x 10'O parts per trillion. Substituting the counts per second and the parts per trillion for water into eq 11, one obtains Str(B) =
/5500
(12)
where S,(B) is given in counts per second per parts per trillion.
Re, values can be obtained from the equilibrium cluster distributions of H30+(H20)hand BH+(H20)b,expected to be present in the ion source for the given experimental conditions. The equilibrium constants for the successive clustering reactions
+ + H2O = BH+(H2O)b+l
H30+(Hz0)h H20 = H30+(H20)h+l BH'(H2O)b
(13a) (13b)
are given by
Thus, the ratios between the concentrations of the successive clusters are determined by the water concentration and by the equilibrium constants. Standard thermodynamic formulas relate the equilibrium constants, eq 14, to the thermodynamic functions for reaction 13a, AHoh,h+l and A S o h , h + l (and correspondingly for reaction 13b). These thermodynamic data can be obtained with pulsed highpressure mass spectrometry (PHPMS) (7), and extensive compilations are available (8, 9). Evaluations of eq 14 for the hydronium ion and for the protonated analyte ion (BH+) at given temperatures and water pressures lead to the respective cluster distributions. The two distributions are reactively coupled by reactions like eq 2. For thermodynamically controlled analytes, these reactions are in equilibrium by the time the ions reach the interface gas ( 4 ) . For the purpose of numerical calculations, it is easiest to use the reaction H30+
+ B = BH+ + H 2 0
(15)
1312
ANALYTICAL CHEMISTRY, VOL. 60, NO. 13, JULY 1, 1988
-G E =ZOO kcal/mol
I
I
,
I
200
400
600
800
I
0
1000
Temperature ( " C )
IlTf P
I
10-2t fi i i 10-3i
P
' i 200 400 g 0 1
-lo-' 1
I
1
'
600
800
1000
1200
110-8
Temperature ("C) Flgure 6. Calculated R, (seeeq 10) and sensitivities for nonhydrating analytes BH+. The ordinate on the right side gives sensitMty of analyte 8 relative to the high sensitivity kinetically Controlled pyridine (seeFigure 5). For R e , Ilo5 protonation is kinetically limited.
to calculate the relative intensities of the bare ions. The equilibrium constant for reaction 15 is
The proton transfer equilibrium constant (KP,Jis calculated from the gas-phase basicities of water and of the analyte (B), respectively. Since the ratio [H30+]/[BH+]is determined by eq 16 and the ratios of the H30+(HzO)hclusters are fixed by eq 14a and those for BH+(H,O)b by eq 14b, combination of the results from eq 16, 14a, and 14b allows the evaluation of the relative concentrations of the H30f(H20)hand BH+(H@)b clusters. From these also Re, can be evaluated via eq 9. The complete analytical expression for Req, obtained in this manner, is found to be independent of [B], but a complicated function of [HzO]. Therefore, the Re, expression (eq 10) is useful for comparisons involving different [B] at constant [H,OI. Compounds in group L have low sensitivities because their conjugated acids (BH+) form very weak clusters with water. For example, whereas the BH+-HzO dissociation enthalpy for B = methanol is 27.6 kcal/mol, the corresponding value for B = furan is only 10.3 kcal/mol (IO). This means that at a water pressure of a few Torr, protonated furan looses its last water already around 70 "C whereas protonated methanol keeps its last water up to ca. 440 "C. The "wont case" analyte in API is one for which the corresponding BH+ does not cluster at all with water. In order for clustering to be negligible at room temperature and above, the BH+-H20 dissociation enthalpy must be less than ca. 6 kcal/mol. Many compounds like polyaromatic hydrocarbons can be expected to have such low BH+ to HzO bond energies. It is of interest to calculate the API sensitivities of such "worst-case" analytes. Figure 6 shows the calculated R, for the worst case, i.e. for nonclustering analytes with GB's from 160 to 210 kcal/mol. It is seen that Re, increase very rapidly with temperature. Nonclustering analytes with GB = 210 kcal/mol reach Re, = lo5 already at 25 "C. This value, when substituted into eq 12 leads to a sensitivity of ca. 20 (counts/s)/pptr which is typical for the high sensitivity kinetically controlled analytes; see value for pyridine in Figure 5. This means that at this temperature the kinetic control limit has been reached and
Flgure 7. Calculated R, and sensitivities for analytes furan ( O ) , acetonitrile (AN) (A),and methanol (0),see dashed curves. Ordinate on right side gives sensitivity of analyte relative to the high sensitivity pyridine. The experimentally measured sensitivities from Figure 6 are shown as full curves and indicated with X.
above this temperature these analytes will exhibit the high and approximately temperature-independent sensitivities of the kinetically controlled group K. The sensitivity scale shown on the right ordinate of Figure 6 is relative to pyridine and is set equal to 1 at the point where Re, = lo5. It should be noted that due to the extreme steepness of the GB > 200 curves, even a very moderate cooling of the ion source leads to an almost total loss of ion signal for these analytes. Comparison of the results in Figures 5 and 6 shows that furan, anthracene, and pyrene behave like nonclustering analytes. The higher gas-phase basicity anthracene (GB = 200 kcal/mol) and pyrene (GB = 200 kcal/mol) have very steeply rising sensitivities while the furan sensitivity (GB 185 kcal/mol) rises more gradually (see Figure 5) and these are the trends predicted in Figure 6. Nonclustering analytes with lower GB down to 182 kcal/mol reach the kinetic limit at increasingly higher temperatures; see Figure 6. When the GB is lower than 182 kcaljmol, the kinetic limit is not reached at any temperature. A t high temperatures above ca. 650 "C not only BH+ but also most of the H,O+ ions are "bare", i.e. do not cluster with water. The analyte sensitivity is then determined by the equilibrium constant for the proton transfer reaction 15. This equilibrium constant decreases with temperature as the exothermic enthalpy change of the reaction becomes less important, i.e. as the exponential A S term in K,, = exp(-AH/RT) exp(AS/R) becomes dominant. This explains the decrease in sensitivity at these very high temperatures; see curves for GB < 180 kcal/mol in Figure 6. All real analytes are expected to have a sensitivity equal to or higher than that of a nonclustering analyte with the same GB. We may conclude, therefore, from the results in Figure 6 that at room temperature all real analytes with GB I210 kcal/mol should be kinetically controlled and (unless the proton transfer reaction is slow) the sensitivity should be high. Likewise, real analytes with GB 1 185 kcal/mol should have high sensitivities at the elevated temperatures that were reached in the present investigation (unless the analytes decompose or are otherwise reactive). Sensitivities for real analytes that are expected to hydrate can be calculated by including in the calculations hydrate clustering data also for BH+. Figure 7 shows results for furan, acetonitrile, and methanol at 7.75 Torr water and temperature between 0 and 1000 "C. For comparison purposes, Figure 7 also shows the corresponding experimental sensitivities from Figure 5. It is seen that there are differences of up to a factor of around 50 between the experiments and the calculations. However, the gross features of the experimentally observed
-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 13, JULY 1, 1988
-104 torr Kinetic Limit
105
U
cr"
10'
? 10-4
0
200
400
600
800
1000
Figure 8. Calculated R , and sensitivity for methanol at different temperatures and water partial pressures in 700 Torr air.
behavior are reproduced in the calculations. Thus, the large increase in sensitivity for furan, the minimum around 200 "C for acetonitrile, and the relatively flat response observed for methanol between 50 and 400 "C are reproduced. The fact that large absolute differences in sensitivities between calculation and experiment are found is not surprising. This can be illustrated with acetonitrile (AN). At 25 "C and 7.75 Torr of water, the dominant clusters are ANH+(Hz0I5and H30+(H20)6,respectively. The calculated sensitivity then mainly depends on the free energy change for the reaction
+
to a sensitivity error by a factor of 160. Calculated sensitivities for methanol a t water vapor pressures between 10 and Torr are given in Figure 8 for the temperature range 0-lo00 "C. The curves for different water pressures are seen to converge at high temperatures, i.e. where the bare ions proton transfer, reaction 5 , dominates. The variable water pressure data demonstrate that a high sensitivity for methanol can be obtained only at very low water pressures. The calculated sensitivities of acetonitrile and methanol in Figures 7 and 8 show structure, i.e. maxima and minima. This structure is due to the discrete hydration numbers of given H30+(H20)b and BH+(H20)bthat are dominant in given ranges of temperature and pressure. An examination of the hydrate concentrations shows that maxima tend to appear where two successive H30+(H20)bclusters have the same concentrations. For example, the maximum at 680 "C in the methanol curve in Figure 7 occurs where [H30+] =
[H3O+(H@)I.
Temperature ( " C )
H30+(H20)6 AN = ANH+(H20),
1313
+ 2H20
(17)
The free energy change, AGO of eq 17 includes the difference in GB's between acetonitrile and water (21.6 kcal/mol) plus the difference between the sums of the free energy changes for the successive clustering reactions up to H30+(HzO)6(43.6 kcal/mol) and ANH+(Hz0)5(29.3 kcal/mol), respectively. The 11clustering equilibria were measured in different laboratories Meot-Ner (12)). Furby PHPMS (H30+,Lau (11);A"+, thermore, for the smallest clusters, the thermodynamic data are extrapolated beyond the range in which they were measured with PHPMS. It is therefore not surprising that a cumulative error of 3 kcal/mol may result. This corresponds
Registry No. PlP, 110-89-4;Pyr, 110-86-1;MeOH, 67-56-1; PhC1, 108-90-7;PhH, 71-43-2; MeCHO, 75-07-0;EtOH, 64-17-5; MeCN, 75-05-8; (CH2)20,75-21-8; PhMe, 108-88-3;PhN02,9895-3; 2-Me2CsH4,95-47-6; MeCOMe, 67-64-1; PhCHO, 100-52-7; PhOMe, 100-66-3;Me@, 75-18-3; 2-ClEtEtCl, 693-07-2; PhNH2, 62-53-3;furan, 110-00-9;thiophene, 110-02-1;pyrrole, 109-97-7; ferrocene, 102-54-5; nickelocene, 1271-28-9.
LITERATURE CITED Carroll, D. I.; Dzldic, I.; Hornlng, E. C.; Stillwell, R. N. Appl. Spectrosc. Rev. 1081, 17, 337. Proctor, C. J.; Todd, J. F. J. Org. Mass Spectrom. 1083, 18, 509. Reid, N. M.: Buckley, J. A.; French, J. B.; Poon, C. C. Adv. Mass Spectrom. 1070, 8 6 , 1843. Sunner, J.; Nicol, G.; Kebarle, P. Anal. Chem., preceding paper In this Issue. Good, A.; Durden, D. A.; Kebarle, P. J. Chem. Phys. 1970, 5 2 , 222. Llas, S. G.; Llebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data 1084, 13, 695. Kebarle, P. I n Techniques of Chemistry: Techniques for the Study of Gas-Phase Ion-Molecule Reactions ; Why-Interscience: New York, In press. Castleman, A. W., Jr.; Keese, R. G. Chem. Rev. 1088, 86, 589. Keese, R. G.; Castleman, A. W., Jr. J. Phys. Chem. Ref. Data 1088, 15, 1011. Nlcol, G.; Sunner, J.; Kebarle, P. Int. J. Mass Spectrom. Ion Proc., in press. Lau, Y. K.; Ikuta, S.; Kebarle, P. J. A m . Chem. SOC. 1082, 104, 1462. Meot-Ner, M. J. Am. Chem. SOC. 1984, 106, 1265.
RECEIVED for review October 8, 1987. Accepted February 8, 1988. This work was supported by a grant from the Canadian Natural Sciences and Engineering Research Council (NSERC).
