532
Anal. Chem. 1983, 55,532-535
(8) Schueler, B.; Krueger, F. R. Org. Mass Spectrom. 1979, 14, 439. (9) Vanderborgh, N. E.; Fletcher, M. A,; Jones, C. E. R. J . Appl. Anal. fyrol. 1979, 1 . 177. (IO) Jones, C. E. R.; Vanderborgh, N. E. J . Chromafogr. 1979, 186, 831. ( 1 1 ) Vanderborgh, N. E.; Jones, C. E. R.; Verzino, W. J.; Haverkamp, J. J . Appl. Anal. fyrol., in press. (12) Verzino, W. J.; Roofer-DePoorter, C. K.; Hermes, R. E, Catalytlc Coal Conversion Support, LA-9269-PR, Los Alamos National Laboratory: Los Alamos, NM, March 1982. (13) Raymond, R., Jr.; Gooley, R. "Analytical Methods for Coal and Coal Products"; Karr, C. J., Jr., Ed.; Academic Press: New York, 1979; Vol. 111, Chapter 48, p 337. (14) Sharkey, A. G.; McCartney, J. T. "Chemistry of Coal Utilization"; Elliott, M. A., Ed.; Wiley-lnterscience: New York, 1981; Chapter 4, especlally pp 204-214. (15) Meuzelaar, H. L. C.; McClennen, W. H.; Metcalf, G. S.; Hill, G. H. 29th Annual Conference on Mass Spectroscopy and Allled Topics, Minneapolis, MN, May 1981; 673.
(16) Helnen, H. J.; Meier, S.; Vogt, H.; Wechsung, R. "Advances in Mass Spectrometry"; Quayle, A., Ed.; Heyden: London, 1980; Vol. 8A, p 942. (17) Vandegrift, G. F.; Winans, R. E.; Scott, R. G.; Horwitz, E. P. Fuel 1980, 59, 627. (18) Hanson, R. L.; Vanderborgh, N. E. "Analytical Methods for Coal and Coal Products"; Karr. C. J., Jr., Ed.; Academic Press: New York, 1979; Vol. 111, Chapters 40 and 73. (19) Silbernagel, B. G.; Ebert, L. B.; Schlosberg, R . H.;R. B. "Long Coal Structure"; Gorbaty, M. L., Ouchi, K., Eds.: American Chemical Society: Washington, DC, 1981, Adv Chem. Ser. No. 192, p 23.
RECEIVED for review January 19, 1982. Resubmitted September 27,1982. Accepted November 18, 1982. This work was supported by the U S . Department of Energy under Contract No. DE-AC21-79MC11530.
Automated Multicomponent Analysis with Corrections for Interferences and Matrix Effects J. H. Kalivas and B. R. Kowalskl" Laboratory for Chemometrics, Department of Chemistry B E IO, University of Washington, Seattle, Washington 98 195
The generalized standard additlon method (GSAM), a multianalyte generalizationof the method of standard addltlons, can simultaneously correct for matrix effects and spectral interferences in a multicomponent analysis. Slnce the GSAM requires the standard addltions of all analytes be made, a completely automated instrument was deslgned to Implement the GSAM under computer control. Additionally, the GSAM has been adapted to a new experimental deslgn of maklng standard additlons by weight rather than the usual procedure of additions by volume. This new design is combined with the usual advantages of automation to yield a step toward Intelligent analytical instrumentatlon.
Many recently designed analytical instruments include microprocessor computers. These microprocessors have primarily been used for data acquisition, transformation, storage, and retrieval. Lately, new ways in which microprocessors can be used in chemical analysis are being investigated by analytical chemists ( I ) , namely, utilization of the "intelligence" of computers so they can be used to optimize analyses and detect and correct for problems that may render an analysis invalid. One example is a computer-controlled photon counting spectrometer that makes decisions to reduce wasteful time scanning spectral regions containing no information (2). A truly intelligent instrument would identify sample constituents, select optimal operating parameters, correct for all types of interferences (chemical,physical, spectral) and matrix effects, and accurately estimate the concentrations of all analytes. In the present study, the generalized standard addition method (GSAM) (3) is used in an automated mode, integrated with a microcomputer-controlled analytical instrument, representing a step toward a fully automated intelligent instrument. The GSAM is an experimental design and a calculation procedure for multicomponent analysis based on the method of standard additions ( 3 , 4 ) . The GSAM represents instrument responses as functions of the chemical species present in the sample undergoing analysis and enables one to detect 0003-2700/83/0355-0532$01.50/0
and correct for matrix effects and all forms of interferences.
It requires that when there are r analyte concentrations to be determined, the responses from p sensors (electrodes, wavelengths, etc.) be recorded (p 2 r ) before and after n standard additions are made ( n 2 r). The linear model is T
m=l,
..., n
1 = 1 , ..., p
where rm,lis the response of the lth sensor after the mth addition of analyte s, cm,sis the total concentration of the sth component after addition m, and ks,l is the linear response constant for the 2th sensor to the sth component. In matrix notation
R = CK where R is the n X p matrix of measured responses, C is the n X r concentration matrix, and K is the r X p matrix of linear response constants. For further details on solving for C and K in the above equation, the reader is referred to ref 3 and 4. Recently, the model for the GSAM has been extended to include instrument responses as functions of nonchemical parameters, such as time (5). Expressing responses as a function of time allows for the detection and correction of drift that may be occurring during an analysis. The GSAM has also been extended to the case where the number of sensors is greater than the number of analytes ( p > r ) , allowing for the detection and correction of potential interferents (5). Using p > r also makes possible the combination of several relatively unsensitive sensors for a given analyte to form, in effect, a more sensitive sensor. The model for the GSAM with the inclusion of time is 111
rm,l =
C cm,&L + 2 tikti,L
s=l
(3)
i=l
where w represents the polynomial order of the drift process model. Further details are given in ref 5. The GSAM has been applied to inductively coupled plasma atomic emission spectrometry (6),anodic stripping voltametry
a 1983 American Chemical Society
ANALYTICAL CHEiMISTRY, VOL. 55, NO. 3, MARCH 1983
1)
533
Table I. Total Standard Additions with Total Weights of Solution after .Each Addition
AN,mmol
v P D P 11/05
1
SAMPLII
[-g
I I I i
1
Flgure 1. Block diagram of the components for the automated design. The balance holds three standard solutions A, B, and C.
(a,
ion-selective electrode potentiometry (8), and spectrophotometry ( 4 ) . It has been shown t o possess the ability to characterize multicomponent instruments with minimal effort (6) thereby providing a tool for analytical method development. In all of the above applications, the GSAM experimental design was performed manually. When r becomes substantially large, manual ojperation of the GSAM can represent considerable time and effort for the analyst especially when the number of standard additions for each component is greater than one. Therefore, automating the GSAM promises to reduce the workload, analysis time, and even experimental error. In addition, automating the GSAM with time additions, should make possible the ability for automatic drift detection and correction and the ability t o flag a drifting sensor as potentially defective. This paper deals with the applicability of using the GSAM with visible light spectrophotometry under computer control t o perform self-correcting multicomponent analysis in the presence of spectral interferences and matrix effects. The instrument constructed for this purpose is shown in Figure 1. Sample is pumped in a continuous manner through the spectrophotometer and the standard addition system. In this study an important new feature has been added to the GSAM, performing standard additions by weight rather than by volume as done in previous applications. Handling additions by weight replaces the larger absolute errors in a volume-based system with lower absolute errors from weight measurements. Utilization of the weight of solutions rather than volumes in a automated fashion was first realized by Malmstadt and co-workers (9). Details of the automation are found in the following section. EXPERIMENTAL S E C T I O N Reagents. Three aqueous solutions of 1.50 M NiC12.6Hz0,1.30 M CoCl2-6H20,and 0.695 M CuCl2.2Hz0(ACS Reagent Grade) served as standards. Two test samples (“unknown”mixtures) were also prepared. One consisted of 40.0 mL total volume containing 3.28 mmol of Ni, 3.08 mmol of Co, and 0.677 mmol of Cu. The other test sample consisted of the same quantities of Ni, Co, and Cu as above in 20.0 mL plus 20.0 mL of a 1% starch solution to add a matrix effect arising from the light scattering properties of starch in water. Procedure. A total of nine additions, three for each analyte, were made to the two 40.0-mL test samples. Table I contains the order and millimoles of the standard additions made along with the total weights of the first test sample after the standards had been added. A similar experimental design was devised for the test sample with starch. Wavelengths used were XI = 394.5 nm, X2 = 511.6 nm, and X3 = 820.0 nm. Computer and Interfaces. The host computer system used was a PDP 11/05 with a general-purpose interface (IO). The interface is equipped with switched AC used for manipulating
Ni 0.575 1.15 1.73 1.73 1.73 1.73 1.73 1.73 1.73
co 0 0 0 1.03
2.05 3.04 3.04 3.04 3.04
cu 0 0 0 0 0 0
0.252 0.51 3 0.771
wt, g 40.820 41.269 41.727 42.627 43.385 44.385 44.777 45.183 45.585
-
the standard addition valves. The interface also supplies Logic levels providing +3.5 V output when logic “1” is loaded into the bit corresponding to that level and 0.0 V output for a logic 0. These logic levels were used to control the pump as discussed below. The interface is equipped with a series of relays which were used to switch computer control between the spectrophotometer and the balance via a RS 232C interface contained in the PDP 11general interface. A series of assembly language routines, callable from FORTRAN or BASIC, control the experiment. Spectrophotometer. A Kontron UVIKON 820 spectrophotometer equipped with a RS 232C interface was used. All UVIKON 820 keyboard functions are callable by sending the appropriate ASCII character from the host computer to the spectrometer. Balance. A digital Sartorius 1265 MP balance was used to measure the amount of standards added by weight. Data were transferred from the balance to the PDP-11/05 through the relays mentioned above. Pump. A Fluid Metering, Inc., Lab Pump RP-B was connected to a power supply voltage controller which was interfaced to the PDP 11 through the logic level feature of the general purpose interface. Using two logic level ports allows for four pump rates. That is, high, medium, low, and off are obtained through the appropriate combination of the logic levels which would then dictate the voltage Level for the pump. Addition Valves. Altex automatic slider valves using pneumatic actuation were used. Valves operated in a three-way configuration and clould be controlled by the interface. Software. The main program was written in FORTRAN IV and called assembly language subroutines to drive the pump, activate selenoids,read the balance, and control the spectrometer. The GSAM (FORTRAN IV program, available from Infometrix, Inc., Seattle, WA) was used t o analyze the data from the experiments. Operation Sequence. The program begins by asking the operator for the number of analytes, concentrations of standards, number of standard additions to be performed for each analyte, number of moles to be added on each addition, wavelengths at which absorptions are to be measured, and the initial weight (not volume) of sample. The design of the system dictates that the system loop be dry before pumping the weighed sample through the system. The weight of the sample is the last entry needed by the program. At this point the computer assumes complete control. A general flow chart of the program is shown in Figure 2. After entering the above parameters, the sample is pumped through the system loop. The computer tares the balance and pumps the desired rnoles of standard t o the sample. A rough calibration of the Teflon tubes located between the standard reservoirs and the valves for their respective flow rate in unlits of seconds per gram is helpful. This allows the PDP 11to pump for the appropriate time to deliver the required amount of standard. Weight of the standard solution added is then measwed and converted to millimoles of standard for entry into the GSAM data analysis program. The sample is then mixed with a magnetic stirrer and pumped through the system loop for 75 s. The pump is stopped and an absorbance reading is obtained at the characteristic wavelength for the analyte corresponding to the added standard. The solution is again mixed for 30 s and another measurement is acquired. If there is a significant difference between the two absorbance measurements (>0.002 absorbance
534
ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983
Table 11. Final Results GSAM
Ni Co
cu
true analyte quantities, mmol
calcd analyte quantities, mmol
3.28 3.088 0.677
3.23 2.97 0.699
3.28 3.08 0.677
3.38 3.07 0.655
std dev
re1 error,a %
Without Starch 0.11 0.16 0.038
SAM calcd analyte quantities, mmol re1 error,a %
1.5 3.6 3.2
3.40 3.32 0.887
3.7 7.8 31.0
3.0 0.32 3.2
3.63 3.10 0.820
10.7 0.65 21.1
With Starch Ni co
cu a
0.13 0.11 0.035
relative error ( W ) = (true - calcd)/true x 100. Table 111. Calculated Response Constants for for Test Samples Input’ initial parameters
A2
d l o r e balance
J perform standard addition and obtain weight mix solution
J o b t a i n measurements
m a d e f o r 011 onalytes ?
Figure 2. General flow chart of the automation program.
units, the precision of the UVIKON spectrophotometer) the solution is mixed further repeating this process until a stable reading is attained. The 75-s and 30-s mixing intervals were found to be quite sufficient. After homogeneity is attained, absorbance is measured at all wavelengths entered during the intialization of the program. Following the duplication of this addition and measurement process for the desired analytes, the GSAM program prints out estimates of K , the linear response constants, and the initial analyte quantities in units of millimoles.
RESULTS AND DISCUSSION Table I1 contains the results of analyzing the two samples described in the Experimental Section. These samples were analyzed several times with the same results. Also listed are estimated standard deviations obtained by using a Monte Carlo pertubation (11). For the Monte Carlo estimations a normal distribution of measurement error was assumed with a standard deviation equal to 1 % of the relative standard deviation on the absorbance readings, the estimated precision limit of our spectrophotometer. Fifty Monte Carlo perturbations were performed t o compute 50 sets of random responses. The 50 sets of random responses were then used in the GSAM program to estimate 50 respective sets of the
A3
Without Starch Ni Co Cu
2.18 i O.lga 0.125 i 0.14 0.0254 i 0.40
Ni Co
2.32 I 0.018 0.161 i: 0.11 0.141 0.48
-0.0734 i 0.16 2.34 i 0.13 0.0142 i 0.61
0.294 i 0.14 0.148 i 0.081 7.38 i: 0.38
With Starch Cu a
-0.0206 i 0.13 2.36 i 0.11 0.0734 ?r 0.48
0.340 i 0.12 0,139 i 0.071 7.59 I 0.44
95% confidence limits,
corresponding K matrices and initial analyte quantities. These 50 sets of results were used to estimate standard deviations for initial analyte quantities and K. From Table I1 it is clear that, using the GSAM in an automated mode, the recovery of initial quantities of test analytes in the presence of spectral interferences and matrix effects (sample with starch) is excellent. Most important, in comparing the results of the GSAM with those estimated by the normal standard addition method (SAM), also given in Table 11, the relative errors in the test analytes are seen to be higher for the SAM. This is due to the fact that SAM cannot correct for spectral overlap, which is occurring in our test sample. In Table 111, the K matrices for the two test samples are shown to reveal the interferences present. The 95% confidence limits for each k are given in Table I11 and disclose many insignificant k’s. Namely, kcu,xl,kcu,x2,and k N i , h , (krow,co~umn) are found to be not statistically different from zero. hc,,x, has questionable significance, while kNi,h3and kco,x3show sizable spectral interferences of Ni and Co on As. Because of these interferences, the SAM cannot estimate the initial quantity of Cu correctly. The SAM results for Ni and Co are not statistically different from the GSAM results. This can be verified a t the 95% confidence level as expected since the interferences on the respective X’s, as seen from the K matrices, are essentially zero. It was observed that when starch was added to the test sample, the absorption readings were enhanced. This is seen as increases in the k values for the K matrix presented in Table 111. From the results in Table I1 it is seen that the GSAM is also able to correct for the starch matrix effect as is the case with the SAM. Thus with the GSAM, corrections for matrix effects and all types of interferences can be made simultaneously while the SAM only allows for corrections of matrix effects. Automating the GSAM has drastically reduced analysis time. The usual time needed for an analysis is dependent on the lengths of standard addition tubes and the system loop tube. The latter affects the mixing time needed to obtain a
Anal. Chem. 1983, 55,
homogenous mixture. No attempt was made in this first feasibility study to minimize analysis time.
ACKNOWLEDGMENT The authors are gr'ateful to c' Jochum and de Koven for their helpful discussions and to Kontron Analytical for use of their UVIKON spectrophotometer.
535-539
535
(6) Kalivas, J. H.; Kowalski, B. R . Anal. Chem. 1981, 5 3 , 2207-2212. (7) Gerlach, R.; Kowalski, B. R. Anal. Chim. Acta 1982, 134, 119. (8) Moran, M.; Kowalski, B. R., Laboratory for Chemometrics, DepaiZment of Chemistry, University of Washington, unpublished work, June 1981. (9) Renoe, B. W.; O'Keefe, K. R.; Malmstadt, H. V. Anal. Chern. 1976, 4 8 , 661-666. (10) Danielson, J. D. S.; Brown, S. D.; Appellof. C. J.; Kowalski, B. R. Chem., Biomed. Envlron. Instrum. 1979, 9 , 29-47. (1 1) Naylor, T. H.; Balintfy, J. L.; Burldick, D. S.;Chu, K. "Computer Sirnulation Techniques"; Wiley: New York, 1966; Chapter 4.
LITERATURE CITED Enke, C. G. Science 1982, 215, 785-791. Niemczyk, 'r. M.; Ettinger, D. G. Appl. Spectrosc. 1978, 32, 450-453. Saxberg, Bo W. H.; Kowalski, B. R. Anal. Chem. 1979, 51, 1031- 1038. Jochum, c.; JoChUWl, p.; Kowalski, 6. R . Ana/. Chem. 1981, 53, 85-9 2. Kaiivas, J. H.; Kowalski, B. R . Anal. Chem. 1982, 5 4 , 560-565.
RECEIVED for review August 17, 1982. Accepted November 12, 1982. This work was supported in part by the Office of Naval Research. The authors also gratefully acknowledge supportfrom the , ~ ~ science t i ~ ~~ ~~ l ~under~~~~~t d NO. CHE-8004220.
Determination of Ammonium, Nitrate, and Urea Nitrogen in Molecular Absorption Spectrometry Fertilizer by Gas-Phase Vincent C. Anigbogu, Mark
L. Dietz, and Augusta Syty"
Department of Chemistry, Indiana University of Pennsylvania, Indiana, Pennsylvania
Ammonium nltrogen In fertilizer is determlned by Injecting an aliquot of fertilizer solution Into strong base and measuring the translent absorbance exhibited by the evolved ammonia gas. Urea is converted to iimmonlum wlth the aid of urease and then determined as above. Nltrate Is reduced to nitrite by metallic cadmium and determlned by lnjectlng an allquot Into strong acid and measuring the absorbance of the evolved nitrogenous gases. Thie burner head of an atomic absorptlon spectrophotometer Is replaced wlth a flow-through absorptlon cell. A glass reactlon vessel is used to evolve the absorbing specles into tho gas phase. Ammonlum and urea can be determlned down to O.ID05 % and nitrate down to 0.015 % by weight In fertllirer, based on a 103 sample diluted to 500 mL. The reported methods are considerably faster than the corresponding AOAC methods, as they involve no dlstlllatlons. Data for flve commerciial fertllizers are given and the results agree with those obtallned by standard methods.
Nitrogen, phosphorus, and potassium are the main active ingredients of chemical fertilizers. Nitrogen is usually present as inorganic salts of aimmonia or nitrate, or as urea. Government regulation and quality control processes require the availability of rapid, convenient, and accurate methods for the analysis of fertilizers. The basis for the majority of N determination methods is the conversion of each form of N to ammonia. The determination of total N by the Kjeldahl method, or some modification thereof, involves digestion, distillation, and titration. Although the application of atomic absorption/emission spectrophotometry to the determination of fertilizer components such as K, as well as that of micronutrients such as IFe, Zn, Cu, and others, is quite common, the flame spectrophotometer cannot offer straightforward evaluation of nitrogen. However, the atomic absorption instrument can be modified very simply to allow molecular absorption measurements in the gas phase without the flame. This has made possible the determination of I-, Br- ( I ) , S032-(2-4), S2-(5),NO; (6),CN(7), and NH4+ (8) in a variety of materials. The objective of this report is to describe the use of gasphase molecular absorption spectrometry for the rapid de0003-2700/83/0355-0535$0 1.50/0
15705
termination of ammonium, nitrate, and urea nitrogein in commercial water-soluble fertilizers.
EXPERIMENTAL SECTION Approach. After the fertilizer is dissolved in water, the ammonium ion concentration is determined by injecting aliquots of the solution into sodium hydroxide and measuring the absorbance of the evolved NH3 gas. For the determination of urea, the fertilizer solution is first treated with urease to convert urca to NH4+. The total lVH4+ is then measured as above, and the concentration of urea is deduced by subtraction. For the determination of nitrate, the fertilizer solution is passed through a Cd column to reduce NO3- to NOz- and the concentration of nitrite is then determined by injecting aliquots of the reduced solution into hydrochloric acid and measuring the absorbance of the evolved nitrogeneous gases which consist predominantly of nitrosyl chloride. Apparatus. All absorbance measurements were made on a Perkin-Elmer Model 460 atomic absorption spectrophotometer and recorded on a 10-mV strip chart recorder. The instrument was modified for molecular absorption measurements in the gas phase by removing the burner head from the nebulizer/burner and replacing it with a 15 cm long flow-through cell with quartz windows. The deuterium arc lamp, whose normal function in the instrument is to provide for background correction, served as the source of exciting radiation. For the measurement of evolved ammonia, the wavelength was set at 194 nm with a slit corresponding to a bandwidth of 2 inm, as recommended by Muroski and Syty (8). For the measuremlent of the gases evolved from nitrite (believed to be nitrosyl chloride (9) and oxides of nitirogen),absorbance was measured at 195 nm with a bandwidth of 0.7 nm, as suggested by Syty and Simmons (6). A specially designed glass reaction vessel was used for evolution of the absorbing species into the gas phase. The vessel is illlustrated in Figure 1 of ref 5. The cylindrical glass vessel has an internal volume of about 60 mL. A 6-mL aliquot of 10 N NaOH (in the case of ammonium) or of 8 N HC1 (in the case of nitrite) is introduced into the reaction vessel from a 25-ml buret attached to a side arm via a small sleeve of Tygon tubing. Ammoniumor nitrite-containing samples are injected into the basic or acidic solution through the rubber septum covering the injection port by means of a 1-mL Hamilton syringe. The evolved ammonia or nitrogenous gases are swept by a continuously flowing stream of nitrogen carrier gas from the reaction vessel to the flow-through absorption cell and then vented into the hood. The carrier ,gas 0 1963 American Chemical Society
~
t
