Anal. Chem. 2007, 79, 2537-2540
Demonstration of Selected Ion Flow Tube MS Detection in the Parts per Trillion Range Daniel B. Milligan,†,‡ Gregory J. Francis,†,‡ Barry J. Prince,†,‡ and Murray J. McEwan*,†,‡
Syft Technologies Ltd., P.O. Box 28149, Christchurch, New Zealand, and Department of Chemistry, University of Canterbury, P.B. 4800 Christchurch, New Zealand
The rate coefficients of the ion-molecule reactions between H3O+, NO+, O2+, and phosphine were determined using a selected ion flow tube. Using these data, the selected ion flow tube mass spectrometry (SIFT-MS) method was applied to the real-time measurement of phosphine in nitrogen without sample preparation down to concentrations in the mid parts per trillion range. This is the first reported measurement using SIFT-MS in the parts per trillion range. Linear dependencies on concentration were found from 190 ppt to the ppm range, and the limit of detection for a 10-s scan was 190 ppt (0.27 pg/mL). Selected ion flow tube mass spectrometry (SIFT-MS) is an analytical technique based on the ion-molecule chemistry taking place in a flow tube reactor and was first introduced by Smith and Spanel.1 It is now an established technique that has advantages over many other analytical approaches in that it provides a quantitative measure of analytes in air mixtures in real time at sensitivities in the low parts per billion level for most analytes.2 SIFT-MS is based on the chemical ionization of analyte molecules in a sample mixture by mass-selected reagent ions. The reagent ions generally chosen for the technique are generated from a microwave or radio frequency discharge in air at low pressure and are H3O+, NO+, and O2+. In the SIFT-MS method, these reagent ions are each mass-selected in turn by a quadrupole mass spectrometer so only mass-selected ions are introduced into the flow tube reactor. Once the kinetic parameters of the ionmolecule reactions of the reagent ion with the analyte molecule are known along with the flow of gas into the flow tube, the ratio of the product ion signal to the reagent ion signal gives a quantitative measure of the amount of analyte in the air mixture.2 The technique has numerous applications in medicine such as examining individual breath profiles,3 monitoring solvents in blood,4 examining nitrogen-containing volatiles from soil after * To whom correspondence should be addressed. E-mail: murray.mcewan@ canterbury.ac.nz. † Syft Technologies Ltd. ‡ University of Canterbury. (1) Spanel, P.; Smith, D. Med. Biol. Eng. Comput. 1996, 34, 409-419. (2) Smith, D.; Spanel, P. Mass Spectrom. Rev. 2005, 24, 661-700. (3) Spanel, P.; Davies, S.; Smith, D. Rapid Comm. Mass Spectrom. 1999, 13, 1733-1738. (4) Wilson, P. F.; Freeman, C. G.; McEwan, M. J.; Allardyce, R. A.; Shaw, G. M. Appl. Occup. Environ. Hyg. 2003, 18, 759-763. 10.1021/ac0622678 CCC: $37.00 Published on Web 02/16/2007
© 2007 American Chemical Society
fertilizer application,5 and detection of peroxide-based explosives favored by terrorists.6 In a SIFT-MS experiment, there are no sample preconcentration steps; the gas sample containing the analyte is simply drawn into the lower pressure flow tube reactor via a capillary inlet. The sensitivity of SIFT-MS depends on the analyte being measured, but generally lies in the range 1-10 counts s-1 ppb-1. We show here that, for phosphine, a sensitivity of 3 counts s-1 ppb-1 is sufficient to detect phosphine at a level of 190 ppt in a 10-s scan. Typical reported sensitivities for previous SIFT-MS experiments were in the low-ppb range.2 Proton-transfer reaction mass spectrometry, another flow tube-based analytical technique can, in some circumstances, achieve lower sensitivities (10-100 ppt), but it does not have the range of chemical ionization agents available with SIFT-MS.7 Phosphine is a gas that is commonly used as an insecticide fumigant in the shipping container industry. It poses a significant health and safety risk to those personnel responsible for unpacking those containers. The accepted safety exposure levels of phosphine vary widely between countries and can be as low as 10 ppb.8 SIFT-MS provides a means of making fast, high-accuracy, quantitative measurements of phosphine at these levels, as it also does with the many other fumigation compounds used in the container shipping industry. EXPERIMENTAL SECTION SIFT-MS Instrument. All the measurements reported here were made using a Syft Technologies Ltd. Voice100 instrument shown schematically in Figure 1. The principle of the technique has been described in many previous publications,1,2 and only the operations pertinent to this study will be stated here. In brief, ions are generated from a microwave or radio frequency discharge in the ion source region. These ions are then mass selected by the quadrupole in the upstream chamber and enter the flow tube via a Venturi orifice. The ions are carried along the flow tube in a stream of helium gas, which enters through an inner annulus in the Venturi nozzle, and argon gas, which enters through an outer annulus. The purpose of the carrier gas is to transport the ions (5) Milligan, D. B.; Wilson, P. F.; Mautner, M. N.; Freeman, C. G.; McEwan, M. J.; Clough, T. J.; Sherlock, R. R. J. Environ. Qual. 2002, 31, 515-524. (6) Wilson, P. F.; Prince, B. J.; McEwan, M. J. Anal. Chem. 2006, 78, 575579. (7) de Gouw, J.; Warneke, C.; Karl, T.; Eerdekens, G.; van der Veen, C; Fall, R. Int. J. Mass Spectrom. 2003, 223-224, 365-382. (8) Technical Rule for Hazardous Substances-TRGS 512. German Federal Labor Gazette. Issue 6, 2004.
Analytical Chemistry, Vol. 79, No. 6, March 15, 2007 2537
and diluted in nitrogen as phosphine undergoes reaction with oxygen. First, a parent sample of phosphine in nitrogen was prepared containing ∼5 ppm phosphine. Successive dilutions were then made by extracting measured amounts of the parent sample and injecting it into a new Tedlar gas sampling bag (SKC Inc.) filled with nitrogen. The extractions from the parent sample were measured using a 1-mL gas syringe. The sample bags were each filled with dry nitrogen using a timed, constant flow measured with a mass-flow controller and filled to a volume of 2.0 L. Samples with the highest concentrations were measured through both the conventional inlet (2 Torr L s-1) and the high-flow inlet and gave, after correction for the different flows, the same concentrations of phosphine within the experimental uncertainty of the measurement. The lower concentration samples were all monitored using the high-flow inlet. RESULTS AND DISCUSSION Kinetics. The kinetic parameters measured for phosphine are
H3O+ + PH3 f PH4+ + H2O O2+ + PH3 f PH3+ + O2
k ) 1.7 × 10-9 cm3 s-1 (1) k ) 1.5 × 10-9 cm3 s-1 (2)
Figure 1. Syft Technologies Ltd. Voice 100 SIFT-MS instrument used in this study.9
NO+ + PH3 + M f NO+‚PH3 + M
along the flow tube. The function of the argon is to reduce the loss of ions by diffusion to the walls. The ratio of helium to argon is typically 2:3, and the total pressure is ∼0.5 Torr. The sample is introduced into the flow tube via a heated capillary inlet at the point shown in Figure 1. After ∼4 ms of reaction time for chemical ionization of the sample, the remaining reagent ions and resulting product ions exit the flow tube and are focused by a lens array into a quadrupole mass filter where they can be mass selected and counted by a continuous dynode electron multiplier. For the experiments described here, a high-flow sample inlet was designed (4 Torr L s-1), which resulted in an approximate doubling of the instrument sensitivity. Phosphine Preparation. Phosphine was prepared by adding water to calcium phosphide and, after drying with P2O5, purified by successive freeze-pump-thaw operations.10 Analytical Method. The determination of phosphine was made using nitrogen-filled Tedlar bags and a static-dilution method. Phosphine samples were prepared from pure phosphine
Reaction 3 was very slow, having a pseudo biomolecular order rate coefficient of k ∼ 3 × 10-12 cm3 s-1. These parameters were determined in the usual way for flowtube kinetics: from the slope of the semilogarithmic plot of the reagent ion signal with respect to the flow of phosphine.11 The error associated with these measurements is estimated at (20%. The reaction chosen in this experiment to monitor the phosphine concentration was the proton-transfer reaction from H3O+. Consideration must also be made for the reaction of the water cluster ions H3O+‚H2O and H3O+‚(H2O)2 with PH3. These cluster ions are always present with H3O+ due to its reaction with trace amounts of moisture present in the carrier gases.
(9) www.syft.com. (10) Melville, H.; Gowenlock, B. G. Experimental Methods in Gas Reactions; MacMillan: London. 1964. (11) McEwan, M. J. In Advances in Gas Phase Ion Chemistry; Adams, N. G., Babcock, L. M., Eds.; JAI Press Inc.: Greenwich, CT, 1992; Vol 1, pp 1-42. (12) Meot-Ner (Mautner), M. M.; Lias, S. G. Binding Energies Between Ions and Molecules, and The Thermochemistry of Cluster Ions. In NIST Chemistry WebBook; Linstrom, P. J., Mallard, W. G., Eds.; NIST Standard Reference Database 69; National Institute of Standards and Technology: Gaithersburg MD, June 2005 (http://webbook.nist.gov). (13) Meot-Ner (Mautner), M. Int. J. Mass Spectrom. 2003, 227 (3), 525-554. (14) Spanel, P.; Smith, D. Rapid Commun. Mass Spectrom. 2000, 14, 18981906.
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Analytical Chemistry, Vol. 79, No. 6, March 15, 2007
(3)
H3O+ + H2O + M f H3O+‚H2O + M
(4)
H3O+‚H2O + H2O + M f H3O+‚(H2O)2 + M
(5)
In the present case, only the ion at m/z ) 37 (H3O+‚H2O) in addition to H3O+ exhibited a collision rate, proton-transfer reaction with PH3 and this was included in the analysis. In the present study, the phosphine sample was present in dry nitrogen and higher order water clusters were not significant. Proton transfer from m/z ) 37 to phosphine is endothermic; however the contribution from the entropy component drives the reaction to be exoergonic with respect to the Gibbs free energy.12,13 The reactions of m/z ) 55 (H3O+‚2H2O) and m/z ) 73 (H3O+‚3H2O) were observed to occur via a ligand switching reaction at an appreciable rate with phosphine to yield a clustered product.
Figure 2. Measurement of a sample of phosphine in nitrogen contained in a Tedlar bag. The concentration of PH3 is determined by the ratio of PH4+ product ion to the reagent H3O+ ion counts. The counts corresponding to PH4+ have been averaged over 4 s and those for H3O+ over 100 ms.
background was obtained from a measurement of a Tedlar bag containing nitrogen but no phosphine. It was found to be stable and was subtracted from each measurement. The lowest concentration of phosphine determined in the present series of measurements was 190 ppt. An example of an individual measurement for one sample is shown in Figure 2. The linearity of the measurements down to the lower level of 190 ppt is shown in Figure 3 along with the calculated dilution line. The agreement is very good, and any variation can be attributed as much to the dilution technique as to any other factor. The quantitation of phosphine using SIFT-MS in this way was also compared with and agreed with the levels of phosphine produced from a G-Cal type (room temperature) permeation tube containing phosphine provided by Vici Metronics,15 yielding phosphine at a concentration of 1.4 ppm with an appropriate flow of nitrogen. Limits of Detection (LOD) and Quantitation. The LOD is usually defined as the minimum concentration or weight of analyte that can be detected at a known confidence level.16 It is governed by the minimum signal detectable above the background noise, Bm:
Bm ) Bµ + 3Bσ
(9)
where Bµ is the mean background signal and Bσ is the standard deviation of the background signal. The LOD is then given by
LOD ) (Bm - Bµ)/s
Figure 3. Successive dilutions of phosphine measured using the SIFT-MS technique on a commercial instrument. The error bars shown in the graph represent the instrument background at the phosphine product mass.
Ligand switch reactions are a common occurrence for H3O+‚nH2O clusters where proton transfer is endergonic.14
H3O+‚H2O + PH3 f PH4+ + 2H2O k ) 1.5 × 10-9 cm3 s-1 (6) H3O+‚2H2O + PH3 f PH3‚H3O+ + 2H2O k ) 6 × 10-10 cm3 s-1 (7) H3O+‚3H2O + PH3 f PH3‚H3O+ + 3H2O k ) 6 × 10-10 cm3 s-1 (8) The O2+ reagent ion was not used as its PH3+ charge-transfer product at m/z ) 34 coincided with the 18O16O+ ion, which is present at much larger ion densities when O2+ is the selected ion. The instrument background, due in part to electronic interference and in part to chemical interference coincident with the PH4+product ion at m/z ) 35, corresponded to 120 ppt. This (15) Velco Instruments Co Inc; www.vici.com. (16) Skoog, D. A. Principles of Instrumental Analysis, 3rd ed.; Saunders College Pub.: Philadelphia, PA, 1985.
(10)
where s is the sensitivity of the measurement. The concentration of analyte in a SIFT-MS measurement depends on the sample flow rate into the flow tube, the flow tube pressure and temperature, and the reagent ion and product ion signals.1 However, at the LOD, the only significant noise contribution in the measurement is from the product ion signal. Furthermore, we have observed that, for analyte concentrations less than 10 ppm, the product ion signal obeys the Poisson distribution and, therefore, has the property that the mean of the product ion signal is equal to the square of the standard deviation. We can therefore write
Bσ ) xBµ
(11)
where Bσ and Bµ refer to the total accumulated product ion counts in a given time period t, which are simply related to the standard deviation or mean of the count rate values bσ or bµ (in counts/s) by
bσ t ) Bσ
and
bµ t ) B µ
(12)
Thus, the LOD for a SIFT-MS measurement is obtained by substituting (9), (11), and (12) into (10):
LODppb )
3xbµ t st
(13)
Here bµ is the mean background count rate in count/s and the sensitivity s is in counts s-1 ppb-1. For phosphine, the background Analytical Chemistry, Vol. 79, No. 6, March 15, 2007
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of 120 ppt is equivalent to bµ ) 0.36 counts/s. Thus, with s ) 3 counts s-1 ppb-1, the limit of detection for a 10-s scan is ∼190 ppt. The limit of quantitation (LOQ) is commonly defined as the minimum concentration that can be detected with an acceptable precision. If we declare an acceptable precision to be a measurement having a relative standard deviation of (20%, then the LOQ will be achieved at the point where the following relation holds:
x(qµ + bµ)t ) 0.2 qµ t
(14)
In eq 14, qµ is the mean signal count rate (in count/s) due to the sample but not the background, so the denominator is equal to the total accumulated counts from the sample, and the numerator is equal to the standard deviation of the total measured signal. Making the substitution
qµ ) cppbs
(15)
and solving for t gives the measurement time required to achieve a given LOQ:
t ) 25
cppbs + bµ (cppbs)
2
(16)
Thus, for phosphine, a 10-s scan results in an LOQ of ∼940 ppt, or, using eq 16, to achieve a LOQ of 190 ppt requires a measurement time of ∼70 s. CONCLUSIONS SIFT-MS measurements have been demonstrated to be linear over a concentration range spanning 4 orders of magnitude down to 200 ppt. The limit of detection for SIFT-MS measurements is simply dependent on the measurement time, sensitivity, and instrumental background signal associated with the analyte being measured. For the detection of phosphine, a detection limit of 190 ppt is achieved in a 10-s measurement. In principle, detection limits of 20 ppt are achievable for a 10-min measurement. However, this was not established experimentally due to the technical difficulties associated with preparing standard concentrations at this level. A high-accuracy quantitative measurement at this concentration is achieved in ∼70 s. The speed and sensitivity of the technique, together with the lack of any sample preparation, the ease of traceability to NIST standards, and the inbuilt and independent calibration methods based on known reaction kinetics, make SIFT-MS a simple solution to monitor environmental levels of volatile compounds. ACKNOWLEDGMENT D.B.M. and B.J.P. thank FRST for the award of postdoctoral fellowships. G.J.F. thanks Technology New Zealand for the award of a Ph.D. scholarship.
Solving for cppb gives the LOQ for a given measurement time (note that only one root of this quadratic is positive).
LOQppb )
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25 + 5x25 + 4bµ t 2st
Analytical Chemistry, Vol. 79, No. 6, March 15, 2007
Received for review November 29, 2006. Accepted January 19, 2007.
(17)
AC0622678
