1192
Anal. Chem. 1984, 56, 1192-1194
Table 111. Determination of 20 pg of Si/mL in Aqueous Standard Solution Addition of Various Concentrations of Si Standard Solutionsa C,O added, pg/mL IP, nA CXofound, fig/mL 5.00 -2.45 f 0.03 18.9 10.0 -1.63 i 0.02 19.6 20.0 -0.1 b 20.6 40.0 3.34 t 0.06 20.0 a Transient intensities are averages of three measurements at Din” = 4.0 and Im = 3.33 i 0.01 nA. This measurement is an approximate value.
results for the determination of Si a t various dilutions of concentrated phosphoric acid (85%) are summarized in Table IV. The mean values obtained decrease with increasing sample viscosity. This phenomenon reflects the change in nebulization efficiency with sample viscosity which was not compensated for using standard addition method but should be minimized if an internal reference were measured simultaneously with the silicon.
ACKNOWLEDGMENT We appreciate the loan of the Fiatron instrument from Baird Corp., Spectrochemical Products Division, Bedford, MA. Registry No. Si, 7440-21-3; phosphoric acid, 7664-38-2.
Table IV. Determination of Si in 85% Phosphoric Acid Sample concn of sample in solution, g/L 57.802 100.01
289.01
concn of Si in solution, pg/mL 0.341 0.565 1.350
LITERATURE CITED initial concn of Si, in sample, pg/g 5.90 * 0.23 5.65 f 0.20 4.70 z 0.12
to support the above speculations for achieving higher accuracy. It may be concluded that accuracy is enhanced by the choice of high injection volumes, up to a value of Din”of ca. 4, and that a ratio C,O/C,O between 2 and 2.5 is adequate. The precision was examined by running within-day experiment of an aqueous solution containing 20 pg of Si/mL to which 40 pg of Si/mL standard solution was added at D = 3.3. The relative standard deviations for P and IP were 1.2 and 1.9% (two series, 17 measurements each). The mean values were then used to calculate the concentration of the analyte yielding results which deviated by 0.1% and -0.3% for series I and 11. As stated above the standard addition method is not expected to compensate spectral and chemical interferences, such as those that may be exhibited by practical samples. This work mainly examined the validity of eq 13 with standard solutions. However, the influence of viscosity on the results obtained by the present method was also investigated. The
(1) Ruzicka, J.; Hansen, E. H. “Flow Injection Analysis”; Wiiey: New York, 1981; pp 146-176, pp 15-17. (2) Betteridge, D. Anal. Chem. 1978, 50, 832A. (3) Tyson, J. F.; Idris, A. B. Analyst (London) 1981, 106, 1125. (4) Tyson, J. F. Anal. R o c . 1981, 78, 542. (5) Tyson, J. F.; Appleton, J. M. H.; Idris, A. B. Analyst (London) 1983, 108, 153. (6) Tyson, J. F.; Appleton, J. M. H.; Idris, A. B. Anal. Chim. Acta 1983, 145, 159. (7) Greenfield, S. Spectrochhn. Acta, Parts 1983, 388, 93. (8) Ruzlcka, J.; Hansen, E. H. Anal. Chlm. Acta 1978, 99, 37. (9) Vandersllce, J.; Stewart, K. K.; Rosenfeld, A. G. Talanta 1981, 28, 11. (IO) Mahanti, H. S.;Barnes, R. M. Anal. Chem. 1983, 55, 405. (11) Cave, M.; Barnes, R. M.; Denzer, P. 1982 Winter Conference on Phsma Spectroscopy, Orlando; ICP Information Newsletter: Amherst, MA, 1982; Abstract 23.
‘
On leave from I M I Institute for Research and Development, Inc., Haifa 31002, Israel.
Yecheskel Israel’ Ramon M. Barnes* Department of Chemistry GRC Towers University of Massachusetts Amherst, Massachusetts 01003-0035
RECEIVED for review October 14, 1983. Accepted February 15, 1984. Supported in part by Department of Energy Contract DE-AC02-77EV-0432.
Band Broadening in Solid-Phase Derivatization Reactions for Irreversible First-Order Reactions Sir: The use of solid-phase reactors in flow-through analytical systems has been recently discussed (1). In liquid chromatography, this type of reactor is used to improve the sensitivity and/or selectivity of detection systems. Due to the low concentration of analytes detected, many derivatization reactions obey first-order kinetics with rate constant k,
For a skewed chromatographic peak, an exponentially modified Gaussian (EMG) defined mathematically as a convolution of Gaussian with exponential decay represents a suitable model (4-7). The response curve is given by eq 2
k,
A-D
If a narrow pulse of analyte, A, is introduced into the reactor, separation of derivative, D, from the pulse of A may occur. Therefore, the solid-phase reactor behaves as a so-called chromatographic reactor (2). The resulting peak of D is skewed and fused with the peak of A. This phenomenon, which restricts the use of solid-phase reactor in HPLC and FIA, has been called “reaction band broadening” and treated on the basis of a simple mathematical model (I). Usual band broadening mechanisms are not taken into account in this model derived for first-order kinetics. 0003-2700/84/0356-1192$01.50/0
where the essential parameters are as follows: A , peak area; 7,time constant of the exponential decay; t ~center , of the gravity of Gaussian; and u, standard deviation of Gaussian. The quantity t ’ is a dummy variable of integration. Also the reaction chromatogram of a first-order irreversible reaction is supposed to be a result of two independent processes: Gaussian band broadening and exponential decay. This fact led to the idea of treating the reaction chromatogiam as an EMG. The aim of this paper is to supplement our previous paper (1)and demonstrate that a reaction chromatogram of a.fast 0 1984 Amerlcan Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
?IJNI
1193
L
where A A is the amount of analyte A introduced and t is a dummy variable of integration. Dimensionless parameters p, cp, and r are given as p = k$A
r=
= 1- ( t A / t D )
t/tA
(4)
where t A and tD are retention times, k, is the rate constant, and N is the number of plates. Constraints a and /3 are defined as
9‘ 6
4
p
+
a = r ( l - ( P ) ( I V / ~P
/ ~ N-/N1j2 ~)
(5)
p = r(N1l2+ p / p W I 2 ) - N112
i’ 8
4
I
t
A family of curves were calculated according to eq 3-5 for various values of parameters p, p, and r , as shown in Figure 1. The curves have been subsequently evaluated as EMG; the fraction of A converted to D at the reactor output x = 1- exp(-p) (6)
i l i\,
exceeds 99.99% in all cases. Comparing the initial values of parameters p and cp with the estimates of time constant T given in Table I, one can deduce that
YEtnIiVE RElENilOll TIME
(7)
-p/P = T / t D
Figure 1. Derivatlve peaks calculated by means of a plate model (eq 2-4). Run numbers and essentlal parameters are identical with those
and therefore
glven In Table I.
and irreversible first-order reaction can be approximated by use of the exponentially modified Gaussian (EMG).
Thus, the values of tD, u, and r are obtained With a reasonable accuracy as it is evident from Table I. Relationship 8 is almost identical with the equation derived for the “reaction band broadening” in the previous paper (1). For an immediate conversion of A to D (K, = m ) or the lack of separation (tA = tD), the reaction-broadening phenomenon does not occur ( T = 0). The described approach makes it possible to separate the Gaussian component of band broadening from the reaction broadening characterized by the time constant 7. Consequently, the performance of HPLC or FIA derivatization reactors packed with a solid catalyst or reagent can be discussed more exactly, e.g., in terms of relative plate loss or relative system efficiency (4).
EXPERIMENTAL SECTION A Model 3600 Data Station (Perkin-Elmer,Norwalk, CT) was used as a microcomputer programmed in BASIC. Response curves of the derivative formed in a first-order reaction were simulated by means of eq 3-5; the integral of error function in eq 3 was calculated by means of a polynomial approximation (3). All curves were evaluated according to a procedure proposed by Foley and Dorsey (4). The coordinates of curve maxima and the values of peak asymmetry (B/A) at 10% of the peak height were estimated numerically with precision of about f0.5% rel. RESULTS AND DISCUSSION
A plate model of the chromatographic reactor derived by Kallen and Heilbronner (8)has been used for the simulation of reaction chromatograms. The model, valid for first-order kinetics, takes into consideration chemical reaction, separation, and Gaussian band broadening. The response curve of D is given by
ACKNOWLEDGMENT The assistance of StBptin KrupiEka in computer programming is greatly appreciated.
Table I. Essential Parameters of Derivative Peak peak asymmetry
input valuesa run no. 1 2 3 4 5 6 7 8 tA ..
= 1s;
PI@
tD,s
201-3 301-3 201-2 301-2 20/- 1 101-1 301-1 1O l - I l 3
0.250 0.250 0.333 0.333 0.500 0.500 0.500 0.750
= 1x
io3.
1 0 4 ~ ,s
0.791 0.791 1.054 1.054 1.581 1.581 1.581 2.371
EMG estimates
103?, s
BIA
tD, s
3.00 2.50 3.33 2.22 2.50 5.00 1.67 2.50
3.32 2.86 2.86 2.12 1.74 2.86 1.38 1.38
0.251 0.250 0.332 0.333 0.500 0.499 0.500 0.749
s
103~,
0.797 0.799 1.066 1.067 1.610 1.599 1.630 2.404
2.89 2.49 3.32 2.24 2.51 4.99 1.63 2.51
1 0 4 ~ ,
1194
Anal. Chem. 1984, 56, 1194-1196
LITERATURE CITED (1) Nondek, L.; Brinkman, U. A. Th.; Frei, R. W. Anal. Chem. 1983, 55,
1466. (2) Langer, S. H.; Patton, J. E. "New Develooments in G a s Chromatography"; Wiley: New York, 1973;pp 294-367. (3) Abramowitz, M., Stegun, I . A,, Eds. "Handbook of Mathematical Functions"; National Bureau of Standards: Washinaton. DC. 1964: Applied Mathematics No. 55, p 932. (4) Foley, J. P.; Dorsey, J. G. Anal. Chem. 1983, 55, 730. (5) Yau, W. W. Anal. Chem. 1977, 4 9 , 395. (6) Grushka, E. Anal. Chem. 1972, 4 4 , 1733. (7) Pauls, R. E.; Rogers, L. B. Anal. Chem. 1977, 4 9 , 625.
(8) Kallen, J.; Heilbronner, E. Helv. Chim. Acta 1960, 4 3 , 489.
LuboiS Nondek Institute Of Chemical Process Fundamentals Czechoslovak Academy of Sciences 165 O2 Prague 6, Suchdol, Czechoslovakia
RECEIVED for review November 14,1983. Accepted February 16, 1984.
AIDS FOR ANALYTICAL CHEMISTS Determination of Airborne Free Chlorine in the Presence of Ammonia by Capillary Column Gas Chromatography John M. Cheplen,' Craig Barrow,* and Earl L. White2 Analytical Services Unit, Department of General and Biochemical Toxicology, Chemical Industry Institute of Toxicology, P.O. Box 12137, Research Triangle Park, North Carolina 27709 Chlorine ((21,) is a potent respiratory tract irritant. Acute exposure of mice and rats to high concentrations of Clz (1000 ppm) caused death in 28 and 53 min, respectively (1). Rats exposed to 9 ppm C12 (6 h/day, 5 days/week, for 6 weeks) developed severe inflammatory reactions in the upper and lower respiratory tract (2). Interpretation of toxicity studies of C12is complicated because microbial degradation of urine and feces of animals generates ammonia (NH,) that is released into the air of inhalation chambers (3). NH3 and C12react rapidly to produce monochloramine (NH,Cl) ( 4 ) which can alter the concentration of free C12. Due to the formation of chloramines, determination of C1, in the presence of NH3 can yield falsely high values for free C12 using methodology which does not differentiate between free and combined Cl,. Numerous methods for the analysis of C12in air have been reported. Many of these involve trapping the C12in a solution containing a chromophore that is reactive with Cl,, resulting in a change in the spectral properties of the solution (5). These methods lack the sensitivity needed for sub-part-per-million determinations of Cl, without resorting to very long sampling times. Another complication is the possibility of the further formation of chloramines from the presence of NH3 during one or more of the steps of some analytical procedures. Gas chromatographic procedures that use thermal conductivity detectors are available for determining free C1, in air directly, but they also lack the sensitivity necessary for sub-partper-million determinations (6). Other methods available determine total Cl,, both free and combined, by reaction with potassium iodide to produce iodine proportional to the Cl2 species. The relative amounts of iodide and iodine are then measured with ion-selective electrodes (7). Recently, a technique has been published which discriminates between free and combined C12 in water based upon the chlorination Present address: Union Chemicals Division, Union Oil Company of California, Solvent Technical Service Center, 8901 Research Drive, P.O. Box 26009, Charlotte, NC 28213. Present address: Department of Chemistry,University of North Carolina, Chapel Hill, NC 27514.
of the para position of 2,6-dimethylphenol by free C12 (8). The present paper describes a modification of this technique for specifically determining gaseous C12 at low concentrations in the presence of NH3.
EXPERIMENTAL SECTION Reagents. Cylinders of chlorine (-105 ppm) and ammonia (-105 ppm), both diluted with nitrogen, were obtained from Matheson Gas Co. (Morrow, GA). The 2,6-dimethylphenol were (2,6-DMP)and 4-bromo-2,6-dimethylphenol(4-Br-2,6-DMP) obtained from Aldrich Chemical Co. (Milwaukee, WI), and 4chloro-2,6-dimethylphenol(4-C1-2,6-DMP) from Crescent Chemical Co. (Hauppauge, NY). n-Hexane (Burdick and Jackson, Muskegon, MI), distilled in glass grade, was used without further purification and C12-freewater was obtained by distillation from KOH/KMn04. The 2,6-DMP stock solution was prepared by dissolving (sonicated for 30 min, then stirred for 1.5 h) 300 mg of 2,6-DMP in 1 L of C12-freewater. The stock solution of 4Br-2,6-DMPwas prepared by dissolving 180 mg of 4-Br-2,6-DMP in 1 L of water as described for 2,6-DMP. NH&l was prepared as described by Corbett et al. (9). Apparatus. Mass spectral data were collected on a Finnigan Model 4023GC/MS (San Jose, CA) system equipped with an Incos data system. A Hewlett-Packard Model 5880A (Palo Alto, CA) gas chromatograph equipped with capillary injection and a flame ionization detector was used for the 4-C1-2,6-DMPdeterminations. The capillary column (25 m X 0.3 mm i.d.) was fused silica (Hewlett-Packard, Avondale, PA) coated with OV-1. Helium was used as the carrier and makeup gas. Carrier gas pressure at the head of the column was 10 psi resulting in an average linear velocity of 35 cm/s. Splitless injections were used with a split vent delay of 45 s. Injector and detector temperatures were set at 250 OC and 300 "C, respectively. Column temperature was held at 40 O C for 2 min and then programmed to 200 "C at 10 OC/min for a 20-min sample analysis time. The packed column was a 1.8 m, 2 mm i.d. glass column packed with 5% SP 2250 on lOO/l20 mesh Supelcoport (Supelco, Inc., Belleforte, PA). Injector and detector temperatures were the same as for capillary with a helium flow rate of 35 mL/min. The column oven was held constant at 130 "C for a lO-min sample analysis time. Generation of Test Atmospheres. Clz or C12-NH3 test atmospheres were generated under conditions of dynamic air flow. Clz or NH3 was metered from cylinders through calibrated
0003-2700/84/0356-1194$01.50/00 1984 American Chemical Society
