- Email: [email protected]
Measurement of inorganic acids with rotating ball inlet mass spectrometry
Analytica Chimica Acta 390 (1999) 185±192
Measurement of inorganic acids with rotating ball inlet mass spectrometry Henrik érsnes, Thomas Graf, Hans Degn* Institute of Biochemistry, Physical Biochemistry Group, Odense University, Odense, Denmark Received 8 September 1998; received in revised form 13 January 1999; accepted 18 January 1999
Abstract Using a new acid resistant inlet we have evaluated the use of rotating ball inlet mass spectrometry (ROBIN-MS) as a tool for the measurement of inorganic acids in aqueous solution. EI-spectra of sulfuric, sulfurous, nitric, perchloric, bromic, iodic and boric acids were recorded and interpreted with respect to preionization decomposition and electron impact fragmentation. With the exception of sulfuric acid the detection limits for these acids were in the order of 1 mM and the response times are in the order of 1 s. Sulfuric acid had a signi®cantly higher detection limit and response time than the other acids. No mass spectrum of phosphoric acid could be detected. High concentrations of phosphoric acid reduced the signals of other acids. HCl formed by decomposition of perchloric acid caused a strong and slowly recovering decrease of the sensitivity of the mass spectrometer. Sodium sulfate or sodium nitrate in neutral solution did not yield any mass spectrum. The intensity of the sulfuric acid peak of acid solutions of sodium sulfate was measured at different molar ratios of sodium and sulfate. The results indicate that the evaporation of such samples leaves a remnant containing three molecules of sulfuric acid per sodium ion. Similar measurements on acid solutions of sodium nitrate indicate that the remnant contains one molecule of nitric acid per sodium ion. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Rotating ball inlet; Inorganic acids; Continuous mass spectrometry
1. Introduction In a previous paper we have described a new mechanical device, the rotating ball inlet (ROBIN), for continuous introduction of aqueous sample into a mass spectrometer [1]. ROBIN consists of a ball mounted on a shaft and connected to a gear motor. The ball is pressed against a polymer gasket placed around an aperture in the vacuum chamber. Sample situated in microscopic cavities in the surface of the *Corresponding author. Fax: +45-6593-2309; e-mail: [email protected]
ball is dragged past the gasket into the vacuum of the mass spectrometer by rotation of the ball. Water and volatile analytes evaporate from the surface of the ball when exposed to vacuum and migrate to the nearby ion source, where they are ionized by electron impact. We have shown that a wide range of volatile and semivolatile compounds can be measured continuously in aqueous solution with ROBIN-MS. In comparison with membrane inlet mass spectrometry (MIMS) [2,3], ROBIN-MS offers a wider range of compounds which can be measured. Except for compounds which are highly soluble in the membrane material the detection limits of ROBIN-MS are com-
0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 0 9 7 - 5
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parable to or lower than those of MIMS. The response time of ROBIN-MS is only determined by the transport of analyte inside the mass spectrometer. Therefore it is shorter than that of MIMS which has additional contributions from the diffusion through the unstirred layer of sample and through the membrane. In a recent work we have utilized the fast response of ROBIN-MS to build a stopped ¯ow mass spectrometer which was used to measure transients in the acetone concentration in the reaction of acetone with sul®te [4]. In our previous work, we found that the measurement of organic acids by ROBIN-MS requires acidi®cation of the samples because ions do not evaporate from the ball surface in vacuum [1]. When we acidi®ed samples with sulfuric acid we found that dilute sulfuric acid itself gives rise to a mass spectrum with ROBIN-MS. Other inorganic acids were also found to do so. The consequences of this ®nding are twofold. Firstly, the spectra of the acidifying agents may obscure the spectra of organic analytes. Secondly, ROBIN-MS may be used to measure concentrations of inorganic acids. We here present a study of mass spectra, detection limits and response times for some inorganic acids. The well-known tendency of many inorganic acids to decompose when water is removed by evaporation [5] is a complicating factor in the interpretation of the spectra. 2. Experimental Our prototype rotating ball inlet was made from stainless steel and it was not acid resistant [1]. We have now made a new version from acid resistant materials, PTFE and ruby (Robinx, Odense, Denmark). Fig. 1 shows a diagram of the new version of the rotating ball inlet with a sample cell. The steel ball of 20 mm diameter used in the prototype was replaced with a ruby ball of 10 mm diameter (Carl Zeiss, Oberkochen, Germany). Microscopic cavities were created in the surface of the ball by grinding it with silicium carbide powder in a ball mill before the shaft was mounted. The ball, ®tted with a shaft and attached to a gear motor (Maxon Motor, Interelectric AG, Sachseln, Switzerland), was pressed against a 25% graphite ®lled PTFE gasket situated at an aperture towards the electron impact ion source of the
Fig. 1. Diagram of rotating ball inlet made of acid resistant materials. A ruby ball of 10 mm diameter (1) is mounted on a shaft (2) and connceted to a gear motor (3) through a flexible joint (4). The ball is pressed against a PTFE gasket (5) by a PTFE plug (6) situated in the wall of a PTFE sample cell (7). The force from an adjustment screw (8) is transmitted to the plug (6) though the flexible wall of the sample cell (7). The assembly is mounted on a conflat flange (9) which has a 3 mm hole in extension of a 3 mm hole in the gasket (5).
quadrupole mass spectrometer. The sample cell was made entirely of PTFE. The force of an adjustment screw was transmitted to the ball through the ¯exible wall of the sample cell. We used a quadrupole mass spectrometer (QMG420, Balzers, Liechtenstein) with a home-made semiclosed electron impact ion source and a secondary electron multiplier detector. The entrance of the ion source was placed opposite the vacuum exposed ball surface at a distance of 6 mm. The sample cell was not thermostatted. The temperature in the cell was about 358C due to heating by the ®lament in the nearby ion source. The mass spectrometer was pumped with a 50 l/s turbomolecular pumping unit. The total pressure was measured with a Penning gauge (TPG300, Balzers, Liechtenstein) at a point remote from the ion source. The pressure in the ion source was unknown but it was higher than the pressure measured by the gauge. In order to obtain the highest possible signal, the mass spectrometer was
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Fig. 2. Mass spectrum of 0.7 M sulfuric acid recorded with rotating ball inlet. The observed peaks we assign as follows: m/z 98: H2SO4; m/z 81: HSO3; m/z 80: SO3; m/z 65: HSO2; m/z 64: SO2; m/z 48: SO; m/z 32: O2, S.
operated at a pressure of 4.510ÿ5 mbar which is about the highest pressure endured by the ®lament. At this pressure, electron impact ionization is prevalent but low intensity peaks due to chemical ionization are observed. 3. Results and discussion Fig. 2 shows the mass spectrum of dilute sulfuric acid, H2SO4, recorded with the rotating ball inlet. Since the natural distribution of sulfur isotopes is 95.8% 32 S and 4.2% 34 S each major peak in the spectrum is accompanied by a small peak at m/z plus 2. The peaks are easily assigned to different sulfur oxide species as indicated in the ®gure legend. As concentrated sulfuric acid is stable [5], we assume that H2SO4 evaporates as intact molecules from the surface of the ruby ball inside the mass spectrometer. Fragmentation is probably taking place entirely by electron impact in the ion source. In a previous work we have shown that water disintegrates or reacts with impurities at the hot ®lament in an electron impact ion source to form molecular oxygen, hydrogen and carbon dioxide [6]. Both m/z 32: O2 or S and m/z 44: CO2 are seen in the mass spectrum of H2SO4 shown in Fig. 2 and in all the other spectra shown. Since the contributions from different
sources to the oxygen peaks in these spectra are dif®cult to assess we do not discuss them further. Measurements on a neutral solution of sodium sulfate revealed no peaks due to the solute. This would be expected as evaporation of a neutral solution of sodium sulfate will leave the stable, non-volatile salt on the surface of the ball. Measurements were also done on mixtures of equimolar solutions of H2SO4 and Na2SO4 at different volume ratios. The dependency of the signal of H2SO4 on the molar ratio of sodium and sulfate is shown in Fig. 3. We had expected that each
Fig. 3. Intensity of m/z 98: H2SO4 peak in mixtures of equimolar solutions of H2SO4 and Na2SO4 as a function of relative sodium concentration.
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Fig. 4. Mass spectrum of 0.1 M nitric acid recorded with rotating ball inlet. The observed peaks we assign as follows: m/z 63: HNO3; m/z 46: NO2; m/z 44: N2O, CO2; m/z 32: O2; m/z 30: NO; m/z 28: N2, CO.
sodium ion would retain one sulfate as NaHSO4, and consequently, the slope of the plot would be ÿ1. Surprisingly, the slope of the experimental plot is about ÿ3. This result seems to indicate that one sodium ion immobilizes three sulfate ions at the surface of the ball. Fig. 4 shows the mass spectrum of dilute nitric acid, HNO3. Assignments of the peaks are indicated in the ®gure legend. Because anhydrous nitric acid is unstable, decomposing to form N2O, H2O and O2 [5], we assume that the observed fragmentation is partially due to decomposition during the evaporation from the surface of the ball and partially due to the electron impact ionization. The comparatively low molecular peak at m/z 63 may indicate that preionization decomposition is predominant. When measurements were done on mixtures of equimolar solutions of nitric acid and sodium nitrate at different volume ratios we found the relationship between the nitric acid signal and the relative sodium concentration to be consistent with the assumption of one nitrate ion per sodium ion being retained on the ball surface. Fig. 5 shows the mass spectrum of dilute perchloric acid, HClO4. Assignments of peaks are indicated in the ®gure legend. The doubling of peaks is due to the natural distribution of chlorine isotopes being 75% 35 Cl and 25% 37 Cl. Anhydrous perchloric acid is stable [5] and it is likely to evaporate as intact mole-
cules. Accordingly, the molecular ion is present in the mass spectrum. The fragmentation is probably entirely due to electron impact [6]. Whereas ClO3 is abundant in the spectrum, HClO3 is not seen. During the measurements on dilute perchloric acid we found the sensitivity of the mass spectrometer to decrease rapidly with time and it took several hours for the instrument to recover after the sample had been replaced by pure water. This is probably caused by contamination of the mass spectrometer with HCl formed by decomposition of perchloric acid. In previous work we have observed similar effects when we measured on dilute hydrochloric acid with MIMS (unpublished). Because of this problem we have not included HCl, HBr and HI in the present study. Negative ion electrospray mass spectrometry has previously been used as a detector of oxyhalide in connection with ion chromatography [7]. All the peaks of the mass spectra of chlorate and chlorite obtained with the negative ion electrospray technique are present in our positive ion EI-spectrum of perchloric acid but the peaks at m/z 38 : H37 Cl and m/z 36 : H35 Cl found in the EI-spectrum are absent in the electrospray spectrum. Fig. 6 shows the mass spectrum of dilute orthoboric acid, H3BO3. Assignments of the peaks are indicated in the ®gure legend. The doubling of peaks is due to the natural distribution of boron isotopes being 20%
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Fig. 5. Mass spectrum of 0.25 M perchloric acid. The observed peaks we assign as follows: m/z 102: H37 ClO4 ; m/z 100: H35 ClO4 ; m/z 85: 37 ClO3 ; m/z 83: 35 ClO3 ; m/z 69: 37 ClO2 ; m/z 67: 35 ClO2 ; m/z 53: 37 ClO; m/z 51: 35 ClO; m/z 38: H37 Cl; m/z 37: 37 Cl; m/z 36: H 35 Cl, m/z 35: 35 Cl.
10
B and 80% 11 B. Orthoboric acid is stable in anhydrous form and volatile with steam [5]. Accordingly there is a signi®cant molecular peak. The presence of a small peak at M1, corresponding to H4BO3, is probably due to chemical ionization by water. Measurements on a dilute solution of orthophosphoric acid, H3PO4, revealed no peaks which could
be assigned to the solute. This shows that phosphoric acid or its possible dehydration products, metaphosphoric acid and phosphoric acid anhydride, do not evaporate from the surface of the ball in vacuum. Phosphoric acid also impedes the evaporation of other acids. We have found that the addition of 2 M phosphoric acid reduces the peaks of other acids by about
Fig. 6. Mass spectrum of 0.05 M boric acid. The observed peaks we assign as follows: m/z 63: H4 11 BO3 ; m/z 62: H3 11 BO3 ; m/z 61: H3 10 BO3 ; m/z 45: H2 11 BO2 ; m/z 44: H2 10 BO2 ; H11 BO2 , CO2; m/z 43: H10 BO2 .
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Fig. 7. Mass spectrum of solution of a 0.1 M sodium sulfite acidified with phosphoric acid. The observed peaks, m/z 65: HSO2; m/z 64: SO2; m/z 48: SO; m/z 32: O2, S, were all present in the spectrum of dilute sulfuric acid shown in Fig. 2. The assignments are the same.
40%. We assume that phosphoric acid has this effect because it makes hydrogen bonds with other acids and thus impedes the evaporation. Since phosphoric acid does not give rise to any mass spectrometric signals this acid may be used as an acidifying agent to protonate anions of other acids which are measureable. The diminishing effect of phosphoric acid on the signals of other acids should be taken into account, but it is not strong enough to preclude the use of phosphoric acid as an acidifying agent in ROBIN-MS measurements. Fig. 7 shows the mass spectrum of sulfurous acid, H2SO3, measured in a solution of sodium sul®te acidi®ed with phosphoric acid. Assignments of the peaks are indicated in the ®gure legend. It is seen that the spectrum does not contain any peaks which are not present in the spectrum of sulfuric acid and it does not contain the molecular peak at m/z 82. Sulfurous acid disintegrates into sulfur dioxide and water at dehydration [5]. Because the molecular peak is absent we assume that sulfurous acid leaves the surface of the ball as sulfur dioxide which is then partially fragmented by electron impact. The peak at m/z 65: HSO2 may be explained by chemical ionization by water. The absence of a unique peak in the spectrum of sulfurous acid means that this acid cannot be easily determined in the presence of sulfuric acid.
Fig. 8 shows the mass spectrum of bromic acid, HBrO3, measured in a solution of potassium bromate acidi®ed with phosphoric acid. Assignments of the peaks are indicated in the ®gure legend. The doubling and tripling of peaks are due to the natural distribution of bromine isotopes being 50.5% 79 Br and 49.5% 81 Br. Bromic acid can only exist in aqueous solution. It is reported to decompose to Br2, O2 and H2O when water is removed by evaporation [5]. This explains why the molecular ion is absent and that of Br2 is present in the spectrum of bromic acid, but it does not account for the presence of different oxygen containing species. These species may be intermediates in the decomposition process and fragments of intermediates created during the ionization. A negative ion electrospray spectrum of bromate has been published [7]. Most of the peaks of the electrospray spectrum, m/z 113: 81 BrO2 ; m/z 111: 79 BrO2 ; m/z 97: 81 BrO; m/z 95: 79 BrO; m/z 81: 81 Br; m/z 79: 79 Br are also present in our EI-spectrum, but the electrospray spectrum has two additional peaks, m/z 129: 81 BrO3 ; m/z 127: 79 BrO3 , which are absent in the EI-spectrum. On the other hand our EIspectrum has peaks at m/z 162: 81 Br2 ; m/z 160: 79 Br81 Br; m/z 158: 79 Br2 ; m/z 98: H81 BrO, m/z 96: 79 H BrO; m/z 82: H81 Br; m/z 80: H79 Br which are absent in the electrospray spectrum.
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Fig. 8. Mass spectrum of solution of 0.1 M potassium bromate acidified with phosphoric acid. The observed peaks we assign as follows: m/z 162: 81 Br2 ; m/z 160: 79 Br81 Br; m/z 158: 79 Br2 ; m/z 113: 81 BrO2 ; m/z 111: 79 BrO2 ; m/z 98: H81 BrO; m/z 97: 81 BrO; m/z 96: H79 BrO; m/z 95: 79 BrO; m/z 82: H81 Br; m/z 81: 81 Br; m/z 80: H79 Br; m/z 79: 79 Br; m/z 32: O2.
We also recorded the spectrum (not shown) of the analogous acid HIO3 and found only two peaks that we assign as follows: m/z 254: I2; m/z 127: I. The
negative ion electrospray spectrum of iodate [7] has three additional peaks, m/z 143: IO; m/z 159: IO2; m/z 175: IO3, but the m/z 254: I2 peak is absent. The
Fig. 9. Multiple ion monitoring of transients in intensities of major peaks of H2SO4/ H2SO3 after rapid replacement of pure water with sample solution or vice versa. Samples of 0.1 M NaHSO3 were twice introduced and removed after 20 s. A sample of 1 M H2SO4 was introduced and removed after 50 s. A sample of 0.5 M H2SO4 was introduced and not removed within the frame shown. The peaks at m/z 65 and m/z 50 were monitored at 10 times higher gain than the peaks at m/z 64 and m/z 48.
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absence of oxygen containing ions in our spectrum of iodate suggests that iodic acid decomposes completely at the ball surface during the evaporation. The response times of sulfurous acid and sulfuric acid are illustrated in Fig. 9. Initially the cell contained pure water and the monitoring of the four major peaks of the sulfurous acid spectrum was started. After a few minutes, the water was removed from the sample cell by aspiration and a solution of NaHSO3 was injected. The replacement of the water by sample took about 3 s. It is observed that all four traces become constant within a few seconds. After replacement of the sample with pure water by the same procedure the four traces fall to zero within a few seconds. The half time for sulfurous acid is too short to be precisely determined by manual replacement of the sample. We estimate that it is not more than 1 s. In a similar experiment with H2SO4 the traces are seen to respond with a half time of about 150 s. The large difference between the half times of the two compounds may be explained in view of the conclusions that H2SO3 decomposes on the surface of the ball and leaves it as SO2, whereas sulfuric acid is stable and leaves the surface of the ball as H2SO4. The latter molecule adsorbs much more strongly to the surfaces in the vacuum chamber than the former. The stronger binding molecule spends longer time bound to surfaces in the vacuum chamber and arrives later at the ion source. The lack of symmetry of the increasing and decreasing transients in the signal of sulfuric acid is a well understood characteristic of compounds which bind to the surfaces [8]. In similar experiments where the molecular ion of H2SO4 was also monitored, we observed that the transient of the molecular ion is identical to the transients of the fragments. This suggests that all the ions are created from H2SO4 in the ion source. The data presented in this paper were obtained with the use of a semiclosed ion source. With the exception of sulfuric acid the detection limits (signal/noise3) for inorganic acids were found to be in the order of 1 mM and the response times (t1/2) in the order of 1 s. Sulfuric acid has a higher detection limit and a longer response time. We have repeated some of the mea-
surements with an open ion source and found that the detection limits were higher and the response times, as far as they could be determined, were shorter than with the semiclosed source. It follows that the detection limits may be improved at the expense of response times by the use of a more tightly closed ion source. Since the response times generally are very short, some sacri®ce of response time for sensitivity is affordable. We have done a few measurements at pressures in 10ÿ4 mbar range. At such high pressures where the ®lament lasted for only a few hours, we found chemical ionization by water to be prevalent and higher ionization ef®ciencies could be obtained than with electron impact ionization. In order to pursue this ®nding we are presently redesigning the ion source and vacuum chamber. The detection limits of ROBIN-MS for inorganic acids found in this study are not particularly low. We believe that the technique at its present stage is far from optimal and considerably lower detection limits may possibly be achieved. We hope to be able to develop the technique to such an extend that it can be used as a detector in combination with liquid chromatography. The technique as it is may be used to measure most inorganic acids at moderately low concentrations. In combination with a preconcentration procedure, the technique is potentially useful for the measurement of inorganic acids at low concentrations. References [1] H. érsnes, T. Graf, S. Bohatka, H. Degn, Rapid Commun. Mass Spectros. 12 (1998) 11. [2] H. Degn, J. Microbiol. Meth. 15 (1992) 185. [3] F.R. Lauritsen, T. Kotiaho, Rev. Anal. Chem. 25 (1996) 237. [4] H. érsnes, T. Graf, H. Degn, Anal. Chem. 70 (1998) 4751. [5] J.C. Bailar, H.J. EmeleÂus, R. Nyholm, A.F. TrotmanDickenson (Eds.), Comprehensive Inorganic Chemistry, vol. 2, Pergamon Press, Oxford, 1973. [6] H. érsnes, S. Bohatka, H. Degn, Rapid Commun. Mass Spectros. 11 (1997) 1736. [7] L. Charles, D. Pepin, Anal. Chem. 70 (1998) 353. [8] F.R. Lauritsen, Int. J. Mass Spectrom. Ion Processes 95 (1990) 1.
Measurement of inorganic acids with rotating ball inlet mass spectrometry Henrik érsnes, Thomas Graf, Hans Degn* Institute of Biochemistry, Physical Biochemistry Group, Odense University, Odense, Denmark Received 8 September 1998; received in revised form 13 January 1999; accepted 18 January 1999
Abstract Using a new acid resistant inlet we have evaluated the use of rotating ball inlet mass spectrometry (ROBIN-MS) as a tool for the measurement of inorganic acids in aqueous solution. EI-spectra of sulfuric, sulfurous, nitric, perchloric, bromic, iodic and boric acids were recorded and interpreted with respect to preionization decomposition and electron impact fragmentation. With the exception of sulfuric acid the detection limits for these acids were in the order of 1 mM and the response times are in the order of 1 s. Sulfuric acid had a signi®cantly higher detection limit and response time than the other acids. No mass spectrum of phosphoric acid could be detected. High concentrations of phosphoric acid reduced the signals of other acids. HCl formed by decomposition of perchloric acid caused a strong and slowly recovering decrease of the sensitivity of the mass spectrometer. Sodium sulfate or sodium nitrate in neutral solution did not yield any mass spectrum. The intensity of the sulfuric acid peak of acid solutions of sodium sulfate was measured at different molar ratios of sodium and sulfate. The results indicate that the evaporation of such samples leaves a remnant containing three molecules of sulfuric acid per sodium ion. Similar measurements on acid solutions of sodium nitrate indicate that the remnant contains one molecule of nitric acid per sodium ion. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Rotating ball inlet; Inorganic acids; Continuous mass spectrometry
1. Introduction In a previous paper we have described a new mechanical device, the rotating ball inlet (ROBIN), for continuous introduction of aqueous sample into a mass spectrometer [1]. ROBIN consists of a ball mounted on a shaft and connected to a gear motor. The ball is pressed against a polymer gasket placed around an aperture in the vacuum chamber. Sample situated in microscopic cavities in the surface of the *Corresponding author. Fax: +45-6593-2309; e-mail: [email protected]
ball is dragged past the gasket into the vacuum of the mass spectrometer by rotation of the ball. Water and volatile analytes evaporate from the surface of the ball when exposed to vacuum and migrate to the nearby ion source, where they are ionized by electron impact. We have shown that a wide range of volatile and semivolatile compounds can be measured continuously in aqueous solution with ROBIN-MS. In comparison with membrane inlet mass spectrometry (MIMS) [2,3], ROBIN-MS offers a wider range of compounds which can be measured. Except for compounds which are highly soluble in the membrane material the detection limits of ROBIN-MS are com-
0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 0 9 7 - 5
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parable to or lower than those of MIMS. The response time of ROBIN-MS is only determined by the transport of analyte inside the mass spectrometer. Therefore it is shorter than that of MIMS which has additional contributions from the diffusion through the unstirred layer of sample and through the membrane. In a recent work we have utilized the fast response of ROBIN-MS to build a stopped ¯ow mass spectrometer which was used to measure transients in the acetone concentration in the reaction of acetone with sul®te [4]. In our previous work, we found that the measurement of organic acids by ROBIN-MS requires acidi®cation of the samples because ions do not evaporate from the ball surface in vacuum [1]. When we acidi®ed samples with sulfuric acid we found that dilute sulfuric acid itself gives rise to a mass spectrum with ROBIN-MS. Other inorganic acids were also found to do so. The consequences of this ®nding are twofold. Firstly, the spectra of the acidifying agents may obscure the spectra of organic analytes. Secondly, ROBIN-MS may be used to measure concentrations of inorganic acids. We here present a study of mass spectra, detection limits and response times for some inorganic acids. The well-known tendency of many inorganic acids to decompose when water is removed by evaporation [5] is a complicating factor in the interpretation of the spectra. 2. Experimental Our prototype rotating ball inlet was made from stainless steel and it was not acid resistant [1]. We have now made a new version from acid resistant materials, PTFE and ruby (Robinx, Odense, Denmark). Fig. 1 shows a diagram of the new version of the rotating ball inlet with a sample cell. The steel ball of 20 mm diameter used in the prototype was replaced with a ruby ball of 10 mm diameter (Carl Zeiss, Oberkochen, Germany). Microscopic cavities were created in the surface of the ball by grinding it with silicium carbide powder in a ball mill before the shaft was mounted. The ball, ®tted with a shaft and attached to a gear motor (Maxon Motor, Interelectric AG, Sachseln, Switzerland), was pressed against a 25% graphite ®lled PTFE gasket situated at an aperture towards the electron impact ion source of the
Fig. 1. Diagram of rotating ball inlet made of acid resistant materials. A ruby ball of 10 mm diameter (1) is mounted on a shaft (2) and connceted to a gear motor (3) through a flexible joint (4). The ball is pressed against a PTFE gasket (5) by a PTFE plug (6) situated in the wall of a PTFE sample cell (7). The force from an adjustment screw (8) is transmitted to the plug (6) though the flexible wall of the sample cell (7). The assembly is mounted on a conflat flange (9) which has a 3 mm hole in extension of a 3 mm hole in the gasket (5).
quadrupole mass spectrometer. The sample cell was made entirely of PTFE. The force of an adjustment screw was transmitted to the ball through the ¯exible wall of the sample cell. We used a quadrupole mass spectrometer (QMG420, Balzers, Liechtenstein) with a home-made semiclosed electron impact ion source and a secondary electron multiplier detector. The entrance of the ion source was placed opposite the vacuum exposed ball surface at a distance of 6 mm. The sample cell was not thermostatted. The temperature in the cell was about 358C due to heating by the ®lament in the nearby ion source. The mass spectrometer was pumped with a 50 l/s turbomolecular pumping unit. The total pressure was measured with a Penning gauge (TPG300, Balzers, Liechtenstein) at a point remote from the ion source. The pressure in the ion source was unknown but it was higher than the pressure measured by the gauge. In order to obtain the highest possible signal, the mass spectrometer was
H. érsnes et al. / Analytica Chimica Acta 390 (1999) 185±192
187
Fig. 2. Mass spectrum of 0.7 M sulfuric acid recorded with rotating ball inlet. The observed peaks we assign as follows: m/z 98: H2SO4; m/z 81: HSO3; m/z 80: SO3; m/z 65: HSO2; m/z 64: SO2; m/z 48: SO; m/z 32: O2, S.
operated at a pressure of 4.510ÿ5 mbar which is about the highest pressure endured by the ®lament. At this pressure, electron impact ionization is prevalent but low intensity peaks due to chemical ionization are observed. 3. Results and discussion Fig. 2 shows the mass spectrum of dilute sulfuric acid, H2SO4, recorded with the rotating ball inlet. Since the natural distribution of sulfur isotopes is 95.8% 32 S and 4.2% 34 S each major peak in the spectrum is accompanied by a small peak at m/z plus 2. The peaks are easily assigned to different sulfur oxide species as indicated in the ®gure legend. As concentrated sulfuric acid is stable [5], we assume that H2SO4 evaporates as intact molecules from the surface of the ruby ball inside the mass spectrometer. Fragmentation is probably taking place entirely by electron impact in the ion source. In a previous work we have shown that water disintegrates or reacts with impurities at the hot ®lament in an electron impact ion source to form molecular oxygen, hydrogen and carbon dioxide [6]. Both m/z 32: O2 or S and m/z 44: CO2 are seen in the mass spectrum of H2SO4 shown in Fig. 2 and in all the other spectra shown. Since the contributions from different
sources to the oxygen peaks in these spectra are dif®cult to assess we do not discuss them further. Measurements on a neutral solution of sodium sulfate revealed no peaks due to the solute. This would be expected as evaporation of a neutral solution of sodium sulfate will leave the stable, non-volatile salt on the surface of the ball. Measurements were also done on mixtures of equimolar solutions of H2SO4 and Na2SO4 at different volume ratios. The dependency of the signal of H2SO4 on the molar ratio of sodium and sulfate is shown in Fig. 3. We had expected that each
Fig. 3. Intensity of m/z 98: H2SO4 peak in mixtures of equimolar solutions of H2SO4 and Na2SO4 as a function of relative sodium concentration.
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Fig. 4. Mass spectrum of 0.1 M nitric acid recorded with rotating ball inlet. The observed peaks we assign as follows: m/z 63: HNO3; m/z 46: NO2; m/z 44: N2O, CO2; m/z 32: O2; m/z 30: NO; m/z 28: N2, CO.
sodium ion would retain one sulfate as NaHSO4, and consequently, the slope of the plot would be ÿ1. Surprisingly, the slope of the experimental plot is about ÿ3. This result seems to indicate that one sodium ion immobilizes three sulfate ions at the surface of the ball. Fig. 4 shows the mass spectrum of dilute nitric acid, HNO3. Assignments of the peaks are indicated in the ®gure legend. Because anhydrous nitric acid is unstable, decomposing to form N2O, H2O and O2 [5], we assume that the observed fragmentation is partially due to decomposition during the evaporation from the surface of the ball and partially due to the electron impact ionization. The comparatively low molecular peak at m/z 63 may indicate that preionization decomposition is predominant. When measurements were done on mixtures of equimolar solutions of nitric acid and sodium nitrate at different volume ratios we found the relationship between the nitric acid signal and the relative sodium concentration to be consistent with the assumption of one nitrate ion per sodium ion being retained on the ball surface. Fig. 5 shows the mass spectrum of dilute perchloric acid, HClO4. Assignments of peaks are indicated in the ®gure legend. The doubling of peaks is due to the natural distribution of chlorine isotopes being 75% 35 Cl and 25% 37 Cl. Anhydrous perchloric acid is stable [5] and it is likely to evaporate as intact mole-
cules. Accordingly, the molecular ion is present in the mass spectrum. The fragmentation is probably entirely due to electron impact [6]. Whereas ClO3 is abundant in the spectrum, HClO3 is not seen. During the measurements on dilute perchloric acid we found the sensitivity of the mass spectrometer to decrease rapidly with time and it took several hours for the instrument to recover after the sample had been replaced by pure water. This is probably caused by contamination of the mass spectrometer with HCl formed by decomposition of perchloric acid. In previous work we have observed similar effects when we measured on dilute hydrochloric acid with MIMS (unpublished). Because of this problem we have not included HCl, HBr and HI in the present study. Negative ion electrospray mass spectrometry has previously been used as a detector of oxyhalide in connection with ion chromatography [7]. All the peaks of the mass spectra of chlorate and chlorite obtained with the negative ion electrospray technique are present in our positive ion EI-spectrum of perchloric acid but the peaks at m/z 38 : H37 Cl and m/z 36 : H35 Cl found in the EI-spectrum are absent in the electrospray spectrum. Fig. 6 shows the mass spectrum of dilute orthoboric acid, H3BO3. Assignments of the peaks are indicated in the ®gure legend. The doubling of peaks is due to the natural distribution of boron isotopes being 20%
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Fig. 5. Mass spectrum of 0.25 M perchloric acid. The observed peaks we assign as follows: m/z 102: H37 ClO4 ; m/z 100: H35 ClO4 ; m/z 85: 37 ClO3 ; m/z 83: 35 ClO3 ; m/z 69: 37 ClO2 ; m/z 67: 35 ClO2 ; m/z 53: 37 ClO; m/z 51: 35 ClO; m/z 38: H37 Cl; m/z 37: 37 Cl; m/z 36: H 35 Cl, m/z 35: 35 Cl.
10
B and 80% 11 B. Orthoboric acid is stable in anhydrous form and volatile with steam [5]. Accordingly there is a signi®cant molecular peak. The presence of a small peak at M1, corresponding to H4BO3, is probably due to chemical ionization by water. Measurements on a dilute solution of orthophosphoric acid, H3PO4, revealed no peaks which could
be assigned to the solute. This shows that phosphoric acid or its possible dehydration products, metaphosphoric acid and phosphoric acid anhydride, do not evaporate from the surface of the ball in vacuum. Phosphoric acid also impedes the evaporation of other acids. We have found that the addition of 2 M phosphoric acid reduces the peaks of other acids by about
Fig. 6. Mass spectrum of 0.05 M boric acid. The observed peaks we assign as follows: m/z 63: H4 11 BO3 ; m/z 62: H3 11 BO3 ; m/z 61: H3 10 BO3 ; m/z 45: H2 11 BO2 ; m/z 44: H2 10 BO2 ; H11 BO2 , CO2; m/z 43: H10 BO2 .
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Fig. 7. Mass spectrum of solution of a 0.1 M sodium sulfite acidified with phosphoric acid. The observed peaks, m/z 65: HSO2; m/z 64: SO2; m/z 48: SO; m/z 32: O2, S, were all present in the spectrum of dilute sulfuric acid shown in Fig. 2. The assignments are the same.
40%. We assume that phosphoric acid has this effect because it makes hydrogen bonds with other acids and thus impedes the evaporation. Since phosphoric acid does not give rise to any mass spectrometric signals this acid may be used as an acidifying agent to protonate anions of other acids which are measureable. The diminishing effect of phosphoric acid on the signals of other acids should be taken into account, but it is not strong enough to preclude the use of phosphoric acid as an acidifying agent in ROBIN-MS measurements. Fig. 7 shows the mass spectrum of sulfurous acid, H2SO3, measured in a solution of sodium sul®te acidi®ed with phosphoric acid. Assignments of the peaks are indicated in the ®gure legend. It is seen that the spectrum does not contain any peaks which are not present in the spectrum of sulfuric acid and it does not contain the molecular peak at m/z 82. Sulfurous acid disintegrates into sulfur dioxide and water at dehydration [5]. Because the molecular peak is absent we assume that sulfurous acid leaves the surface of the ball as sulfur dioxide which is then partially fragmented by electron impact. The peak at m/z 65: HSO2 may be explained by chemical ionization by water. The absence of a unique peak in the spectrum of sulfurous acid means that this acid cannot be easily determined in the presence of sulfuric acid.
Fig. 8 shows the mass spectrum of bromic acid, HBrO3, measured in a solution of potassium bromate acidi®ed with phosphoric acid. Assignments of the peaks are indicated in the ®gure legend. The doubling and tripling of peaks are due to the natural distribution of bromine isotopes being 50.5% 79 Br and 49.5% 81 Br. Bromic acid can only exist in aqueous solution. It is reported to decompose to Br2, O2 and H2O when water is removed by evaporation [5]. This explains why the molecular ion is absent and that of Br2 is present in the spectrum of bromic acid, but it does not account for the presence of different oxygen containing species. These species may be intermediates in the decomposition process and fragments of intermediates created during the ionization. A negative ion electrospray spectrum of bromate has been published [7]. Most of the peaks of the electrospray spectrum, m/z 113: 81 BrO2 ; m/z 111: 79 BrO2 ; m/z 97: 81 BrO; m/z 95: 79 BrO; m/z 81: 81 Br; m/z 79: 79 Br are also present in our EI-spectrum, but the electrospray spectrum has two additional peaks, m/z 129: 81 BrO3 ; m/z 127: 79 BrO3 , which are absent in the EI-spectrum. On the other hand our EIspectrum has peaks at m/z 162: 81 Br2 ; m/z 160: 79 Br81 Br; m/z 158: 79 Br2 ; m/z 98: H81 BrO, m/z 96: 79 H BrO; m/z 82: H81 Br; m/z 80: H79 Br which are absent in the electrospray spectrum.
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Fig. 8. Mass spectrum of solution of 0.1 M potassium bromate acidified with phosphoric acid. The observed peaks we assign as follows: m/z 162: 81 Br2 ; m/z 160: 79 Br81 Br; m/z 158: 79 Br2 ; m/z 113: 81 BrO2 ; m/z 111: 79 BrO2 ; m/z 98: H81 BrO; m/z 97: 81 BrO; m/z 96: H79 BrO; m/z 95: 79 BrO; m/z 82: H81 Br; m/z 81: 81 Br; m/z 80: H79 Br; m/z 79: 79 Br; m/z 32: O2.
We also recorded the spectrum (not shown) of the analogous acid HIO3 and found only two peaks that we assign as follows: m/z 254: I2; m/z 127: I. The
negative ion electrospray spectrum of iodate [7] has three additional peaks, m/z 143: IO; m/z 159: IO2; m/z 175: IO3, but the m/z 254: I2 peak is absent. The
Fig. 9. Multiple ion monitoring of transients in intensities of major peaks of H2SO4/ H2SO3 after rapid replacement of pure water with sample solution or vice versa. Samples of 0.1 M NaHSO3 were twice introduced and removed after 20 s. A sample of 1 M H2SO4 was introduced and removed after 50 s. A sample of 0.5 M H2SO4 was introduced and not removed within the frame shown. The peaks at m/z 65 and m/z 50 were monitored at 10 times higher gain than the peaks at m/z 64 and m/z 48.
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absence of oxygen containing ions in our spectrum of iodate suggests that iodic acid decomposes completely at the ball surface during the evaporation. The response times of sulfurous acid and sulfuric acid are illustrated in Fig. 9. Initially the cell contained pure water and the monitoring of the four major peaks of the sulfurous acid spectrum was started. After a few minutes, the water was removed from the sample cell by aspiration and a solution of NaHSO3 was injected. The replacement of the water by sample took about 3 s. It is observed that all four traces become constant within a few seconds. After replacement of the sample with pure water by the same procedure the four traces fall to zero within a few seconds. The half time for sulfurous acid is too short to be precisely determined by manual replacement of the sample. We estimate that it is not more than 1 s. In a similar experiment with H2SO4 the traces are seen to respond with a half time of about 150 s. The large difference between the half times of the two compounds may be explained in view of the conclusions that H2SO3 decomposes on the surface of the ball and leaves it as SO2, whereas sulfuric acid is stable and leaves the surface of the ball as H2SO4. The latter molecule adsorbs much more strongly to the surfaces in the vacuum chamber than the former. The stronger binding molecule spends longer time bound to surfaces in the vacuum chamber and arrives later at the ion source. The lack of symmetry of the increasing and decreasing transients in the signal of sulfuric acid is a well understood characteristic of compounds which bind to the surfaces [8]. In similar experiments where the molecular ion of H2SO4 was also monitored, we observed that the transient of the molecular ion is identical to the transients of the fragments. This suggests that all the ions are created from H2SO4 in the ion source. The data presented in this paper were obtained with the use of a semiclosed ion source. With the exception of sulfuric acid the detection limits (signal/noise3) for inorganic acids were found to be in the order of 1 mM and the response times (t1/2) in the order of 1 s. Sulfuric acid has a higher detection limit and a longer response time. We have repeated some of the mea-
surements with an open ion source and found that the detection limits were higher and the response times, as far as they could be determined, were shorter than with the semiclosed source. It follows that the detection limits may be improved at the expense of response times by the use of a more tightly closed ion source. Since the response times generally are very short, some sacri®ce of response time for sensitivity is affordable. We have done a few measurements at pressures in 10ÿ4 mbar range. At such high pressures where the ®lament lasted for only a few hours, we found chemical ionization by water to be prevalent and higher ionization ef®ciencies could be obtained than with electron impact ionization. In order to pursue this ®nding we are presently redesigning the ion source and vacuum chamber. The detection limits of ROBIN-MS for inorganic acids found in this study are not particularly low. We believe that the technique at its present stage is far from optimal and considerably lower detection limits may possibly be achieved. We hope to be able to develop the technique to such an extend that it can be used as a detector in combination with liquid chromatography. The technique as it is may be used to measure most inorganic acids at moderately low concentrations. In combination with a preconcentration procedure, the technique is potentially useful for the measurement of inorganic acids at low concentrations. References [1] H. érsnes, T. Graf, S. Bohatka, H. Degn, Rapid Commun. Mass Spectros. 12 (1998) 11. [2] H. Degn, J. Microbiol. Meth. 15 (1992) 185. [3] F.R. Lauritsen, T. Kotiaho, Rev. Anal. Chem. 25 (1996) 237. [4] H. érsnes, T. Graf, H. Degn, Anal. Chem. 70 (1998) 4751. [5] J.C. Bailar, H.J. EmeleÂus, R. Nyholm, A.F. TrotmanDickenson (Eds.), Comprehensive Inorganic Chemistry, vol. 2, Pergamon Press, Oxford, 1973. [6] H. érsnes, S. Bohatka, H. Degn, Rapid Commun. Mass Spectros. 11 (1997) 1736. [7] L. Charles, D. Pepin, Anal. Chem. 70 (1998) 353. [8] F.R. Lauritsen, Int. J. Mass Spectrom. Ion Processes 95 (1990) 1.