Determination of Ammonia by Mercury-Sensitized Luminescence Willlam Ho* and A. 6. Harker Science Center, Rockwell International, Thousand Oaks, Calif. 9 1360
A method uslng the prlnclple of mercury-sensltlred luminescence was developed and demonstrated for the detectlon and quantitatlve analysis of ammonla. The method Is based on observing the emlsslon from the decay of a bound state of the mercury-ammonia exclplex formed by reactlng ammonia wlth metastable 03P0mercury atoms. The emission is speclflc In frequency for ammonia. For samples contalnlng low concentrations of interfering gases such as 02,real-the measurements were shown feasible at sensltlvltles well below the plcogram level. For ambient measurements, gas chromatographic separation technlques were used to isolate the ammonia samples and a sensltlvlty of 3 p$ wlth a slgnal-to-noise ratio of 5 was achieved. In addltlon, the linearity of the emission signal was established for ammonia samples between 3 and 5000 pg.
Evidence is accumulating that ammonia, though present in ambient air a t low concentrations (0.1to 100 ppb), plays a' significant role in the formation and stabilization of ambient pollution aerosols, and that such aerosol particles can potentially cause adverse health effects in humans. In response to the need for a sensitive method of continuously monitoring gaseous NH3 at ambient levels, a detection method has been developed based on the observation of the emission intensity from the decay of the excited mercury-ammonia complex (exciplex). The exciplex is produced by the reaction of NH3 with Hg atoms in the excited metastable 63P0 state which can be formed by irradiating Hg vapor with 253.7-nm light in the presence of Nz. The emitted light from the exciplex is in a continuum b'and centered at 345 nm with a half-width of approximately 20 nm. An extensive examination of the physics and chemistry of the process has been carried out and the rate constant of the exciplex formation reaction as well as the lifetimes of the exciplex and Hg(63Po) atoms have been measured (1,2). In this previous study, it was found that the exciplex emission is linear with concentration and for NH3 in Nz, measurements can be made to well below the part-per-billion by volume level. For ambient measurements, it was found that specific gases, primarily molecular oxygen, lowered the sensitivity of the method through the quenching of the excited Hg atoms. Therefore, gas chromatographic techniques capable of separating NH3 at ppb concentrations were developed and used in conjunction with the exciplex emission detection method for the purpose of measuring NH3 in ambient air. Since other compounds also form emitting mercury exciplexes, this technique can potentially also be employed to detect such species as amines, alcohols, hydrazine, and water.
PRINCIPLE OF DETECTION Several investigators ( 3 , 4 )have established that the metastable mercury (63P0)state forms an excited complex (exciplex) with ammonia in the gas phase which primarily decays 1780
via the emission of light in a continuum centered at 345.0 nm. This emission band is specific for the ammonia exciplex and does not overlap any other known mercury exciplex emissions such as those for water vapor, alcohols, or other amines. Mercury (63P0) metastable atoms are created by the quenching (usually by Nz) of mercury in the 63P1 state which can be produced by irradiating mercury vapor with 253.7-nm light. The proposed mechanism for the formation and decay of the exciplex are given by the reactions: Hg
+ hv(253.7 nm)
-
Hg(63P1)
(la)
where M denotes any gas species present other than nitrogen. The intensity of the emission produced in reaction l e is given by,
-
Emission (345.0 nm) = h,[(Hg NH3)*]
(2)
where the steady-state exciplex concentration for the mechanism given above can be expressed as,
At milliTorr Hg concentrations, the absorptivity is sufficiently large such that in a few cm pathlength, virtually all 253.7-nm radiation is absorbed. Hence, the Hg(63P1) production rate is proportional only to the incident 253.7-nm light. This allows the spatial and time averaged steady-state Hg(63Po)concentration in a flow reaction cell to be written as, [Hg(63Po)Iss
-
kb[aI] [Nzl (kh[N2] + hi[MI + ["3I(hc + hd[Nzl)I(kb[N~I hj[MII (4)
where LY is a constant of proportionality. For low concentrations of "3, hh[Nz] + hi[M] >> ( h , + hd[N2])[NH3].In addition, at high Nz concentrations and low impurity levels hb[Nz] >> hj[M]. Therefore, the expression for the emission intensity of the exciplex can be approximated by,
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
Emission (345.0 nm)
Table I. Quenching Rate Constants of Various Gases Reacting with the Excited States of Mercury Bimolecular Rate Constants
Alternatively, Equation 5 can be expressed in the form,
-
he(kc
+ kd[N21)[NH31[~~I~[Hg(63P~)I~(Hg * "3)* (6)
where ~ [ H g ( 6 ~ P ois)the ] lifetime of the metastable Hg atom in the system, and r(Hg NH3)* is the lifetime of the exciplex. These two expressions show that the exciplex emission intensity is directly proportional to the ammonia concentration, with its overall sensitivity controlled by magnitude of the rate constants, incident excitation light intensity, and the level of impuritiei3, or M gases, existing in the reaction cell. Therefore, for a given set of operating conditions and for a constant level of trace contaminants, the emission intensity a t 345.0 nm provides a simple measure of the ammonia concentration present. The presence of impurities in the system influences the observed ammonia exciplex emission primarily via the quenching reactions l j and l i which reduce the steady-state concentration of the Hg(63P1)and Hg(63P~) atoms necessary for the formation of the ammonia exciplex. From the data given in Table I, the relative importance of these two processes can be estimated. For the first process, if it is assumed that less than 10% of the Hg(6c1P1)state produced is quenched t D the ground state, i.e., k,[M]/hb[Nz] 5 0.1, then the maximum allowable concentrations for some common gases listed are: 0 2 < 1.4%;C02 < 7.9%; I-12 < 3.3%; NO < 0.8%. The estimates for the second effect, whereby the mercury atoms in the 3P0state are quenched to the ground state, require a slightly different approach. Since the Hg(3Po) state is metastable and the transition to the ground state is doubly forbidden, the major mechanisms controlling the lifetime of the Hg(3Po) state in a given system are quenching by trace contaminants from outgassing and by wall reactions. What one wishes to compare, therefore, is not the quenching rate constant of various gases relative to NH3 and Ne, but rather to compare them to the effective quenching rate in a clean system which can be determined by measuring the lifetime of the Hg(3Po) state. The observed lifetime of the Hg(3Po)state for a typical reaction cell containing 1 atm of pure N2 was found to be of the order of 100 ks corresponding to a first-order rate constant of lo4 sV1 for the quenching of Hg(3Po)to the ground state. Using the data iin Table I, the required concentrations for the various gases in order to give a rate constant equal to or less than IO4 s-l are given by: 0 2 6 2 ppm; H2 < 6.5 ppm; NO < 1.5 ppm; CO < 34 ppm; C2Hs 6 3000 ppm; C2H4 < 0.8 ppm. These values correspond to the impurity level for a given contaminant gas such that the lifetime of the Hg(3Po) state, and therefore the emission intensity of the ammonia exciplex, is reduced by a factor of 2. For other atmospheric trace gases such as SO2 and NO2, it is expected their quenching rate constant would be comparable to that for 0 2 . Consequently, they can also be tolerated at the parts-per-million level. As can be seen, the conditions imposed by the quenching of the Hg(3Po)state by the impurity gases via reaction l i are much more critical than those for the quenching of the Hg(3P1) state. For ambient monitoring, molecular oxygen removal is by far the most critical problem, owing to its abundance and relatively large quenching rate constant. Its concentration in a sample obtained from ambient air must be reduced by approximately a factor of 105 in order to maintain the same detection sensitivity as that achieved in measurements made
Hg 6(3P,) at 2 9 5 K
Hg 6(3P1)at 2 9 8 K (From Cvetanovit ( 5 ) ) k(cm3 molecule- s-l)
Emission (345.0 nm)
Procedure Ia 3.02 X 2.08 X lo-'' 3.36 X lo-'' 3.85 X lo-'' 6.48 X lo-" 5.91 X lo-" 1.94 X lo-" 3.25 X lo-" 1.66 X lo-'' 1.3 X lo-'' 4.2 X lo-'' 1.7 X lo-''
-
(From Callear and McGurk $6))
Procedure IIa 4.31 X lo-'' 2.98 x lo-'' 4 . 8 1 x lo-'" 5.45 x 10-10 9.27 X lo-" 8.37 x lo-" 2.77 x lo-'' 4.64 x 2.37 x lo-" 1.8 X lo-" 6.0 X lo-'' 2.5 X lo-''
k(cm molecule-'^-^) Very small 1.8 x lo-'' 5.37 x 10-11 2.51 X lo-''' 1 . 0 3 5 X lo-'' 3.1 x 10-13 Very small 4.39 x 1 0 - 1 3 1.12 x 5.9 x 10-15 4.2 X lo-'' 8.88 X
Hg/NH,/N, system (1)
3.1 x
2.2
i
0.2
x 10-31
1.7
i
0.1 ( p s )
80-120 ( W )
a The t w o procedures reviewed b y Cvetanovif give absolute bimolecular quenching rate constants which differ by about a factor of 1 . 4 3 . T h e relative values are in good agreement.
with NHB in pure N2. It should be noted, however, that the presence of 0 2 only decreases the sensitivity and does not change the linearity of the emission intensity with NH3 concentration so long as the 0 2 concentration remains constant. Therefore, the degree to which it can be tolerated is largely dependent on the required sensitivity for a given application. A feasible method for separating NH3 from other gases in ambient samples is by the use of chromatographic columns. The described detection method can then be used as a highly sensitive and specific detector in a gas chromatograph for the measurement of NH3 as well as other species such as amines and alcohols. It is in this context that the use of the proposed technique for the detection of NH3 was most thoroughly investigated in a series of experiments.
EXPERIMENTAL The experimental apparatus used for the measurements consisted of a standard gas chromatograph column and injection system, and an emission cell with its associated optical system. A schematic of the arrangements of the components is shown in Figure 1. The chromatographic system consists of a dual-coil microvalve suitable for oncolumn injection. The column best suited to the present purpose was experimentally determined to be a 1.5-meter, 0.32-cm 0.d. Teflon column packed with 60-80 mesh Chromosorb 104 which was treated with a 10%by weight solution of tetrahydroxyethylethylenediamine (THEED). The injection valve and column were enclosed in a constant temperature oven maintained a t 80 "C. Ultrapure grade N1 was used as the carrier gas and the flow rate through the column was set between 80 to 100 cm3/min. The elution from the column was then mixed with a 500 cm3/min flow of Nz which was passed through a Hg bubble saturator thermostated at 22 & 1 O C . The column effluent and saturator flow were then passed into a quartz (Suprasil) reaction cell whose dimensions were 1.25 cm in diameter and 5 cm long. The cell was concentrically irradiated with three (Ultra-Violet Products) low-pressure mercury pen-ray lamps. It was observed in preliminary tests that during prolonged operation of the system, the walls of the cell become coated with products (primarily HgO) formed from reactions taking place within the cell involving trace contaminants. However, it was experimentally discovered that by introducing a very small concentration of methanol
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
1781
,,q,i II .
bI2 C A R R I E R GAS
I
l
MERCURY SATURATOR
I1
Y, C A R R I E R GAS
I I! tll SAMPLE I N
Ill
BANOPASS F I L T E R 1
EXHAUST
I InETECTnR '-i
-1-1
S T R I P CHART RECORDER
I
I
I
I
340
345
350
355
1
S I G N A L PROCESSER & DISPLAY
Figure 1. Schematic diagram of experimental apparatus vapor into the main N:! flow, this product buildup can be virtually eliminated. Presumably, methanol acts as a gas phase scavanger of reaction products which can combine to form condensable materials. Since the exciplex emission from the (HgCH30H)* exciplex occurs at 295 nm, it is isolated from the (HgNH3)* emission and does not Therefore, a small interfere with the measurements for the "3. concentration of methanol vapor was added to the system with a saturator which permitted very stable long term operation of the system with no detectable degradation of the signal intensity. The spectral emission from the reaction cell was measured with a $-meter Jarrell-Ash grating spectrometer whose exit slit was modified in the standard manner to interface with a PAR model 1205A optical multichannel analyzer (OMA). This 500-element multichannel analyzer was set up to have a 20-nm display width with a resolution of 0.04 nm per channel element. This system was chosen because it is capable of performing simultaneous real-time, as well as integrated timeaverage, sampling of the emission over the entire spectral region of interest while maintaining sensitivity approaching that obtainable by direct photon-counting techniques for each optical element. To reduce the scattered light background from the Hg emission lines in the photolysis lamp (see Figure 2), the emission radiation from the reaction cell was passed through an interference band-pass filter centered at 348 nm, with a 5-nm half-width. The resulting emission signal detected for this experimental arrangement is the convolution of the exciplex emission bandshape with the transmission characteristics of the interference filter. Gas samples containing ammonia at various concentrations were prepared by passing nitrogen or air over permeation tubes of various lengths located in a constant temperature bath. Concentrations ranging from 20 ppm to 100 ppb were generated by adjusting the temperature and the flow rate accordingly. For the low concentrations in the range of a few ppb, the flow from the permeation tube enclosure was further diluted by mixing with additional clean gas. It was found that after an initial equilibration period of about 1h, the concentration of ammonia generated remained stable to within a few percent. For experiments performed with samples of ammonia in nitrogen, Matheson ultrapure grade nitrogen gas was used. Further purification of the gas as delivered from the manufacturer, performed by passing the flow from the cylinder through a cryogenic trap cooled to liquid argon temperature, produced negligible change in the results. For the case of ammonia in air samples, both room air and zero gas were used. The described experimental apparatus were utilized to make three different types of measurements. The injection system was bypassed and gas samples of ammonia in nitrogen generated by the permeation tube were flowed continuouslythrough the reaction cell. The resulting real-time signal displayed by the OMA were used to study the spectral 1782
335
W A V E L E N G T H ("1)
Figure 2. Measured (Hg-NHs)"emission spectra for a continuousflow of N 2 containing 0.1 and 0.05 ppm NH3 through the reaction cell at an integration time of 1 s.The 348-nm filter was not used content and intensity of the exciplex emission, as well as scattered light background interferences. The signals was integrated for various time intervals to establish the achievable sensitivity and the signalto-noise of the system. Known volumes of sample containing ammonia at various concentrations were also injected directly into the cell to study the time dependence of the emission as a function of carrier flow rate and reaction cell configuration. The data obtained were used to verify the degradation in sensitivity due to trace gas contaminants. Finally, using samples of ammonia in nitrogen and ammonia in air, and injecting known volumes into the cell through the column, studies were carried out to test the feasibility of the system for ambient ammonia concentration measurements, as well as the degradation of the signal due to column effects such as incomplete separation, and loss of material due to irreversible wall absorption.
RESULTS Some typical data obtained with continuous flow of Nz containing various concentrations of NH3 through the reaction cell are shown in Figure 2. The upper curve represents the measured emission spectrum for 0.1 ppm by volume NH3 in NZwhile the middle curve is that for 0.05 ppm. The bottom curve is the background emission and scattered light signal without the presence of NH3 and shows the presence of scattered light emission from the photolysis lamp. The structure on top of the emission spectra is not noise but, rather, emission lines from the mercury atoms in the cell due to its interaction with the ammonia exciplex. Since this emission signal is also proportional to ammonia concentration, it represents additional signal which enhances the sensitivity of the detection scheme. The real signal-to-noise ratio per channel of the optical multichannel analyzer was measured and found to be 70 to 1at 0.1 ppm, or 10 to 1at 14 ppb. By placing the previously mentioned 348-nm interference filter in the optical path between the cell and the monochromator, the background scattered light interferences were greatly reduced, and a further improvement in the signal-to-noise ratio of an order of magnitude or better was achieved. The results obtained by injecting known volumes of samples containing NH3 at various concentrations are shown in Figure 3. Figure 3a represents the real-time signal for a single channel of the OMA corresponding to the peak frequency of the
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
R E A L - T I M E S I G N A L FOR NH3 I N N2 AND I N A I R
l
a
NH3 I N N2
k 6 n SEC* 2 . 3 PICOGRAMS
6 . 9 PICOGRAMS
R . 7 PICOGRAMS
NH3 I N A I R 460 P I C O G R A W
94 PICOGRAMS
23 PICOGRAMS
12 PICOGRGMS
NH3 I N A I R
'I I
!
94 PICOGRAMS
b
INTEGRATED E M I S S I O N S I G N A L FOR YH3 I N A19 ,120 SEC INTEGRA'ION
A
X2 G A I N
i-10
0 . 7 6 PICOGRAMS
2.3 PICOGRAYS
nn+
2 . 7 PICOGSAMS
Figure 3. Real-time single channel (348 nm) emission signal and time-integrated intensity of the NH3 emission band for direct injections of samples of NH3 in N2 into the reaction cell
L
I
,(ASS 3i
NH3
TIPE
(PICOGRAVS!
Figure 4. Relative intensity of measured emission as a function of injected NH3 imass for direct injections of samples of NH3 in N2
emission spectra. Figure 3b represents the intensity of the emission band integrated over the residence time of the sample while flowing through the detector cell. The shape of these spectra is mainly dominated by the frequency response of the measurement system. As can be seen by comparing the areas under these curves, the frequency and time integration enhances ithe emission signal by about two orders of magnitude. Figure 4 shows the relative intensity of the signals detected as a function of NH3 concentration. The signal was found to be linear in the mass of NH3 gas injected over the
3 3 PICOGRAMS
6.0 PICOGRAMS
12 D I C 3 G R ' A S
Figure 5. Real-time and integrated signals from NH3 emissions for injections of NH3 in N2 and NH3 in ambient air through the chromatographic column
range of measurements, and a sensitivity of the order of a few tenths of a picogram was achieved. Extensive tests were carried out with injections of NH3 in N2 and in ambient air through the chromatographic column. Figure 5 shows some typical real-time and integrated signals for the two cases under similar conditions. As can be seen, the retention time of NH3 for the conditions used was about 1min, and the time necessary for complete elution of the NH3 peak was of the order of 2 min. The retention for 0 2 is about 10 s, which can be measured from the position of the strong negative signal caused by O2 quenching of the background Hg fluorescence from the cell. A comparison of the two results indicates that the emission signal is lowered by about a factor of 6 for air samples. This is most likely caused by residual 0 2 contamination of the system from the tailing of the air peak. It was, however, found that, by the use of methanol vapor as a gas phase scavenger, excellent repeatability can be obtained for measurements made over long periods of time. The integrated emission signals are also shown as a function of frequency in Figure 5 . Again, an enhancement of about a factor of two orders of magnitude in signal was achieved over the real-time single channel measurements. The integrated areas under these curves are shown in Figure 6 for both samples of NH3 in N2 and in air. The signal was shown to be linear for NH3 concentrations up to 5000 pg. The achieved sensitivity in the present system for samples of NH3 in air is about 3 pg (with a signal-to-noiseof 5 ) , which corresponds to a detection limit of the order of 4 parts per billion in a 1 cm3 sample. DISCUSSION The results of this study have shown the technique of mercury-sensitized luminescence to be capable of determining ambient NH3 in the 1- to 100-ppb range with linearity in concentration over three orders of magnitude. The ultimate sensitivity of the method is limited by the stringent requirement on the level of allowable interfering gases such as molecular oxygen. For specific applications where these gases are
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
1783
PASS
O F NH3 !N I Y J I C T E O SAMPLES ( P I C O G R A M % )
Figure 6. Relative intensity of the measured integrated emission signal as a function of injected NH3 mass for samples of NH3 in N2 and NH3 in ambient air through the chromatographic column
not present, the potential sensitivity of the described technique for detecting NH3 exceeds that for other competitive methods. For ambient air monitoring, the practical limitation on sensitivity is determined by the degree with which chromatographic techniques can be developed to reliably separate samples containing NH3 at concentrations below 1ppb. The detection principle itself has a number of fundamental advantages. The mercury-ammonia exciplex has a high efficiency for emission, with about 70% of the exciplexesdecaying through dissociation with photon emission ( 3 ) .Second, the exciplex emission occurs in a relatively narrow spectral range
in the near ultraviolet, allowing high-efficiency photomultiplier tubes to be used for the measurements. Third, the emission is specific in frequency and the spectral information may be used to distinguish the signal from emissions due to various other gases which form excited mercury complexes. Fourth, the exciplex emission reaction is proportional to the total pressure of the system so that measurements can be carried out a t atmospheric pressures or higher. Finally, the exciplex reaction mechanism does not remove NH3 from the system except via secondary product formation. Therefore, with proper choice of reaction conditions, each NHs molecule can, in principle, react and emit many times, thus enhancing the sensitivity of the method. For example, if the lifetime of the Hg(63Po) in a cell is 100 ps and the incident photolysis power density is 250 mW per cm3, then the NH3 exciplex emission rate is of the order of 100 photons-molecule-l s-l. Hence, for an average residence time in a reaction cell of 1s, a signal enhancement of 2 orders of magnitude can be realized over other emission measurements involving degenerative processes. Work is currently being continued to utilize the method as an analytical tool in laboratory research programs as well as to adapt the technique to the detections of other gas species.
LITERATURE CITED (1) Alan B. Harker and C. S. Burton, J. Chem. Phys., 63, 885 (1975). (2) C. S. Burton, A. B. Harker, and W.W.Ho, Science Center Technical Report, Rockwell International Science Center, SCTR-74-3 (1974). (3) R. H. Newman, C. G. Freeman, M. J. McEwan, R. F. C. Claridge, and L. F. Phillips, Trans. Faraday Soc., 66, 2827 (1970). (4) A. B. Callear and J. H. Connor, Chem. Phys. Lett., 13, 245 (1972). (5) R. J. CvetanoviC, "Mercury Photosensitized Reactions", Progress in Reaction Kinetics, Vol. 2, G. Porter, Ed., The Macmiilan Co., New York, 1964, p 39. (6) A. 8. Callear and J. McGurk, "Flash Spectroscopy with Mercury Resonance Radiation", Part 5: Formation and relaxation of Hg 6(3P0), J. Chem. SOC. Faraday Trans. /I, No. 1, 97 (1973).
RECEIVEDfor review April 5, 1976. Accepted June 28, 1976.
Determination of p-Aminobenzoic Acid by Room Temperature Solid Surface Phosphorescence R. M. A von Wandruszka and R. J. Hurtubise" Department of Chemistry, University of Wyoming, Lararnie, Wyo. 8207 I
p-Aminobenzoic acid and several other molecules were found to phosphoresce at room temperature when adsorbed on sodium acetate. A unique phosphorimetric method is described for determining p-arninobenzolc acid without separatlon In multicomponent vitamin tablets. The reflected phosphorescence of p-aminobenzoic acid adsorbed on sodium acetate was measured in the quantitation step. The method is rapld, very selective, and sensitive for p-aminobenzoic acld. The limit of detection is about 0.5 ng per sample spot and the relative standard deviation is 2.4% at 5 mg per tablet. p-Hydroxybenzoic acid, folk acid, and the benzamide of p-aminobenzoic acid also exhibited room temperature phosphorescence when adsorbed on sodium acetate. Phosphorescence data on these molecules are reported.
Room temperature phosphorescence of adsorbed ionic organic molecules was first reported by Schulman and Walling 1784
(1,2) and later by Paynters et al. ( 3 )who put the phenomenon to its first analytical use. Also, Wellons et al. have reported on room temperature phosphorimetry of biologically-important compounds ( 4 ) .Some solid surfaces that have been used are filter paper, silica, alumina, and asbestos. In all instances, it was stressed that extensive drying of the surface was required. p -Aminobenzoic acid (PABA) was found to phosphoresce at room temperature when adsorbed on a surface of sodium acetate, even when the system was in contact with a solution of absolute ethanol. The new method of PABA determination discussed in this paper is insensitive to moisture and utilizes a thin-layer densitometer for rapid, sensitive determinations. The method is much more sensitive than the USP XVIII method (5)and it can be used for samples to which the USP method is not applicable. It compares favorably with a recently reported uv absorption method (6).
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
