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Anal. Chem. 1983, 55, 1821-1823
log (Ix/clml) log E2 - log (Idc2m2) log El (3) log E2 - log El
Table 11. Detection Limits for Trace Elements in Urine by XRF after Preconcentration with a Chelating Filter detection limit,a ng/mL
element Cr3 Mnat
A and B are the slope and the offset of the linear line, re-
RSD,~ %
3
+
B=-
18
Fe3 coat Ni2+ cu2+
2 30 2 0.2 3
zn=+
1
7.2 8.2 17 11 10
9.9
a Detection limit is determined as the concentration corresponding to twice the RSD. RSD is determined from four measurements.
Table 111. Comparison ;If Analytical Results for Trace Elements in Urine (ng/nnL) Obtained by Ordinary Calibration Curve Method and the Present Internal Standardization Method sa m-
Mn
Ni
ple ccma isrnb ccm
A
1.7 1.5 0.7 0.6 c 1.9 1.8 D 1.0 0.9 E 2.0 1.6 F 1.8 1.6 1.5 G 4.7 2.1 1.9 a Calibration curve method. standardization methodl. B
5.0 5.3 4.7 2.3 3.5 2.0 4.9
cu
ism
4.4 5.0 4.4 2.2 3.3
ccm
isin
14.7 1 7 . 1 12.7 16.3 20.9 24.1 10.5 18.2 17.8 23.0 17.0 22.2 12.7 1 4 . 1
Zn ccm ism 319 349 185 443 574 204 762
294 322 170 436 553 212 741
Present internal
observed in the filter sample. Therefore, we attempted to calculate the concentrations of Mn, Ni, Cu, and Zn in real urine by the internal ;standardization method using Cr and Ga as internal standards. The concentration, ci, of each analyte is calculated according to eq 1.
spectively, and the subscripts 1,2, and i refer to the internal standards Cr, Ca, and the analyte element, respectively. I is the fluorescent X-ray intensity, c is the concentration, E is the energy of the characteristic X-ray, and m is the recovery of each element for the chelating filter. The results obtained by the present method are compared with those obtained by the conventional calibration method and are shown in Table 111, which shows satisfactory agreement. As a result of the experiment, the present internal standardization method proves to work satisfactorily for trace elements collected on a chelating filter. The method does not require a calibration curve for each element and accordingly the procedure for preparation of the standard solution is much simpler compared with the conventional methods. Although, for powder samples, three internal standard elements were required to obtain satisfactory results ( I ) ,the addition of only two internal standard elements is sufficient for the present samples. This would mean that a chelating filter is a more suitable sample for the present internal standardization method. Moreover, since chelating filters are almost uniform in their shape and surface structure and the X-ray intensities of the internal standard elements do not differ from one filter to another, the addition of the internal standard elements is required for only one sample. Other samples can be analyzed by using the X-ray intensities of the internal standard elements added to that fiiter. Therefore, with respect to the point that only one standard sample added with two internal standards is required for calibration, the present method would be as simple as the so-called absolute method and can be used as a simple and rapid calibration method in energydispersive XRF. Registry No. Cr, 7440-47-3;Ga, 7440-55-3; Mn, 7439-96-5;Ni, 7440-02-0; Cu, 7440-50-8; Zn, 7440-66-6; Fe, 7439-89-6; Co, 7440-48-4; Ti, 7440-32-6.
LITERATURE CITED (1) Matsumoto, K.; Fuwa, K. Anal. Chem. 1979, 57, 2355-2358. (2) Van Grieken, R. Colloq. Spectrosc. Int., 22nd 1981, 1L71,224 (3) Christian, G. D. Anal. Chem. 1969, 4 1 , 24A-40A.
RECEIVED for review April 12,1983. Accepted May 27, 1983.
Oxygen Ashing Attachment for a Furnace Atomizer Power Supply David K. Eaton andl James A. Holcombe* Department of Chem,istry, University (of Texas at Austin, Austin, Texas 78712 There are vast quantities of literature dealing with the utilization of graphite furnace atomic absorption (GFAA) for ultratrace and microtrace analysis of metals in a variety of sample types. There is an equally complementary volume of literature outlining procedures for de,aling with complex samples. Often, the use of some chemiical additive or pretreatment of the sample to minimize the interferences produced by the presence of'the matrix is necessary. The general objective, in many instances, is to prevent gas phase coincidence of the matrix component and analyte and/or to remove the impact of the matrix on the graphite surface/analyte interaction. In general, the highest possible ash or char temperature is recommended to remove the maximum amount of matrix prior 0003-2700/83/0355-1821$01S O / O
to vaporization of the analyte. However, when analyzing for volatile elements in biological or organic based matrices, an ash temperatwe that would be required to remove a significant fraction of the matrix cannot be used because a loss of the analyte would result. A high temperature ash in an inert sheath gas (e.g., argon or nitrogen) accomplishes only a pyrolysis of the matrix and leaves a carbonaceous residue as a result. This residue, which contains a significant amount of carbon, often cannot be removed even with a high temperature cleanout cycle. Beaty et al. ( I , 2 ) were among the first to show that the addition of oxygen during the ash cycle removed a significant fraction of the organic matrix without significantly affecting the analytical signal for a large number of the volatile ele0 1983 American Chemical Society
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ANALYTICAL CHEMBTRY, VOL. 55, NO. 11, SEPTEMBER 1983 115
Table I. CRA-90 Board Modifications Timer PWB 1. cut trace from pin 38 to pin 5 of RLC2 and connect pin 38 to pin 1 2. cut trace from pin 41 to pin 1 of RLD/2
and connect pin 41 to pin 1 0 of RLDl 3. cut trace from pin 45 to pin 4 of RLE/2 and connect pin 45 to pin 10 of RLEl 4. add 1 0 p F polyester film capacitor (63 V) in parallel with C43 Control PWB no modifications needed Power PWB no modifications needed Front Panel Controls Figure 1. Schematic of oxygen ashing supplementary circuitry: IC1 and IC3,CD4066 bilateral analog switch; IC2, 7555 CMOS tlmer; IC4 CD4001 Quad NOR gates; P1, 25K potentiometer; P2, 2 MO potentiometer; C1, 33 pF electrolytic capacitor; C2 and C3, 0.1 pF capacitor; R1 thru R6, 1K I/, W reslstors; R7 and R8, O.OOK W resistors; R9 and R10, 5K W resistors; SW1 and SW2, SPST toggle switches; SW3, SPDT toggle switch; V1 and V2,26 Vac normally closed solenoid gas valves; TR1 and TR2, ECG5600 Trhcs; T1,2N2222 npn transistor; LED1, PR5534S red light emitting diode: D1. 1N21B silicon diode: TF1 24 Vac, 2 A transformer. ments. Salmon et al. (3) pointed out the possible mechanistic impact that chemisorbed oxygen resulting from the oxygen ash would have on the analysis of the volatile elements. They also demonstrated that vaporization temperatures can be increased by as much as 500 "C for these elements by the utilization of oxygen during the ash cycle. These results suggest that a combustion rather than a pyrolysis of the biological matrix can be realized through the use of oxygen and a much higher ash temperature can be used during the analysis. Both these factors should prove advantageous when dealing with biological or organic matrices such as blood, plasma, oils, etc. Recently a fairly comprehensive study utilizing oxygen ashing for the analysis of lead in whole blood demonstrated the increased simplicity of the analysis if an oxygen ash cycle is included during the heating program. Eaton and Holcombe (4)make note of the fact that no measurable decrease in furnace lifetime should be expected if the atomizer is properly flushed with inert gas after the oxygen ashing step and before the atomization cycle, and as long as the ashing temperature is kept at or below 1000 "C. In their study, for example, lead was ashed in oxygen a t 900-950 "C without any measurable loss of the lead and minimal sample pretreatment prior to analysis. Besides elevating the appearance temperature of volatile metals such as lead, cadmium, zinc, and silver, the main function of using oxygen during the ashing cycle is to combust the organic materials. Consequently, oxygen has little effect on reducing interferences caused by inorganic salts unless the ability to ash a t the higher temperature is beneficial to the analysis. I t has been shown that use of oxygen ashing with certain elements such as chromium or tin may actually decrease the analytical sensitivities (1). The cause for such a decrease is not known a t this time. The ability to oxygen ash can provide distinct advantages from many types of analysis if the instrument being used has the capability for introducing oxygen during the ash cycle and shutting off the flow of oxygen prior to atomization. This paper discusses the additional circuitry and modification of a CRA 90 power supply (Varian Techtron Pty. Ltd., Springvale, Australia) to permit the use of oxygen ashing.
1. disconnect center tap of ASH potentiometer from pin 1 9 control PWB and connect to pin D of 0,
ashing supplementary board Table 11. Pin Assignments Oxygen Ash Supplementary Boarda function connection A B C D E F G
+ 1 5 V regulated ground oxygen ash trigger ash I1 (inert gas) temperature potentiometer ash temperature control operate switch disable dry cycle indicator
pin 2 timer PWB pin 5 timer PWB pin 45 timer PWB front panel of CRA-90 pin 19 control PWB pin 38 timer PWB pin 41 timer PWB
a Note: All letters refer to pin assigments of the interconnection cable between the CRA-90 power supply and the 0, ash supplementary board.
POWER SUPPLY MODIFICATIONS AND OPERATION The circuit diagram is shown in Figure 1. The object circuitry incorporates CMOS integrated circuits to reduce the load on the CRA 90 power supply. Basic design features include the use of CMOS bilateral analog switches to decrease switching time and minimize equipment costs, a built-in fail-safe system to prevent oxygen introduction during the atomization cycle, an inert gas flush prior to atomization to reduce furnace degradation, the ability to operate in a nonoxygen ashing mode by simply throwing a switch, and minimal amount of modifications to the existing CRA 90 circuitry. The modifications that need to be made to the CRA 90 power supply circuitry are shown in Table I and the pin assignments corresponding to the oxygen ashing supplementary board are shown in Table 11. As is noted in Table I, a timing capacitor has been added to the CRA 90 timing board to double the ash time accessible from the front panel controls. The CRA 90 ASH potentiometer now governs the total ashing time, which can extend up to 120 s. The 2 MQ potentiometer (P2) incorporated with IC 2 controls the time for the oxygen ash. Temperature for the oxygen ash is controlled by the 25K variable resistor (Pl) connected to IC 1. Thus, independent temperature and timing controls for the oxygen and inert gas cycles are available. Outputs (pins 4 and 10) of IC 4 are used to activate two triacs, which activates either the inert gas or oxygen solenoid valves, respectively. The solenoid valves within the power supply are bypassed in order that the auxiliary valves may control the sheath gases. Inlet pressures were approximately 1 atm (14-15 psi), and two rotameters were used to control flow rates. Although an auxiliary 26 Vac power supply is used, it is possible to drive the solenoid valves di-
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Anal. Chem. 1983, 55, 1823-1826
rectly from the CRA 90 26 Vac supply, if caution is used not to overload the power rsupply. The operation of the “modified” CRA-90 furnace power supply is relatively normal with the exception of the setting of the oxygen ashing time and temperature. Oxygen ashing is selected by closing SW2. This enables the various logic and timer chips, and lights lLED1. The O2 i3sh time is set with ash time potentiometer (P2). The total ash time (0, ash + inert gas ash) is set with the front panel ASH control, which has been doubled by the addition of the extra timing capacitor. Thus a reading of 30 011 the scale will eqlual a 60 s ash. The length of the inert gas flush before atomization is the total ash time minus the O2 ash time. This shlould be a minimum of 20-30 s to allow complete flushing of the gas lines and furnace of any residual 02. The inert gas ash cycle temperature is set by depressing the ASH set button and adjusting the temperature potentiometer to the desired 3etting. The O2 agh cycle temperature is set by turning O2 ash set switch (SW1) to the on (SET) position. Press the ASH set button andl adjust the temperature with the O2 ash cycle temperature potentiometer (P2) to the desired setting. SW1 also disable13the operate button and turns on the O2 solenoid so that the O2 flow rates can be adjusted. SW1 must be opened (OPERATE) before the power supply can be activated.
OXYGEN ASHXNG CIRCUIT OPERATION At the start of dry cycle, RLDl (timer PWB) closes and pin 10 goes high (+15 V). The signal is input to pin 1of IC3. If the O2 ash enable switch is closed, pin 13 is pulled high and the CMOS analog switch is closed. This routes the dry signal to the input (pins 2 and 9) of IC4. This forces pin 10 of IC4 low, turning off the inert gas solenoid (Vl), and the inverted output turns the O2sollenoid (V2) on. If SW2 is open, the CMOS analog switch if3 held open and the inert gas solenoid remains on throughout the cycle. At the start of the ash cycle, RLEl (timer PWB) closes and pin 10 goes high. The signal is capacitively coupled to the base of the switching transistor (Tl). Thle transistor is briefly turned on, and pin 2 of IC2 is pulled low. This triggers the start of the O2ash timing cycle. The output of IC2 is input into IC1, the CMOS analog switch, which switches the ash cycle temperature control to P2. This controls the ash tem-
perature during the O2 ash. At the end of IC2 timing cycle, the output goes low, pin 13 of IC1 opens the connection between P2 and the ash temperature control. However, the inverted output is input to pin 12, which connects the ash temperature control to the front panel mounted ASH potentiometer. This now controls the ash temperature for the remainder of the ash cycle, which is utilized as an inert gas flush. If SW2 is off, the reset line of IC2 is pulled low by R3, and triggering i s prevented. The sheath gas control system consists of two triacs (TR1 and TR2) and two normally closed, 26 Vac solenoid gas valves (V1 and V2). When SW2 is off the inputs to the logic control (IC4) are held low, disabling the O2 triac (TR2), and the inert gas solenoid is held on through out the timing cycle. For O2 ashing, the logic is as follows. If the dry or O2 ash cycle is on, the O2solenoid is on. If any other cycle is on, the inert gas solenoid is activated. The outputs of IC4 are input into a divider network of resistors (R5-R8) to lower the voltage and current to the triac gate specifications. The logic is designed so that at no time can both gas solenoids be on the same time. It is recommended when using oxygen ashing that the atomizer temperature not exceed 1000 “ C when oxygen is present within the furnace to prevent any detrimental effect on the furnace lifetime. Circuit provision allows for the changeover from oxygen to an inert gas ash prior to atomization. The cycle is provided to ensure that there is no oxygen on the atomizer under high temperature conditions. It is also recommended that the inert sheath gas ash cycle temperature be held at a relatively low value to prevent the desorption of oxygen which ‘has been attached to the sample and surface during the oxygen ash cycle. Typical operation is such that a 900 “C oxygen ash for 20-30 s is followed by a 200-300 “C nitrogen ash for 20-30 s. Registry No. 02,1182-44-7.
LITERATURE CITED (1) (2) (3) (4)
Beaty, R. D ; Cooksey, M. M. At. Abs. News/. 1978, 17, 53-58 Beaty, R. D ; Barnett, W. A t . Spectrosc. 1980, 1 , 72-77. Salmon, S. G.;Holcombe, J. A. Anal. Chem. 1978, 50, 1714-1716. Eaton, D. K : Hoicombe, J. A. Anal. Chem. 1983, 55, 946-950.
RECEIVED for review May 5, 1983. Accepted June 3, 1983. This work is supported in part by a grant from the National Science Foundation, CHEW-07632.
Extraction and Spectrophotometric Determination of Molybdenum with Thiocyanate and Amides Khageshwar Singh €’atel,*Hiralal Khatri, and Rajendra Kumar Mishru’ Department of Chemistry, K. Gout. Arts & Science College, Raigarh 496-001, Madhya Pradesh, India Amidine dimer (1) was recently reported for the spectrophotometric determination of Mo in complex materials. Similarly, the commonly available amides (HAL) such as N-phenylacetamide l(PAA), N-(methylpheny1)acetamide (MPAA), N-(dimethylpheny1)acetamicle (DMPAA), N-(diethylpheny1)acetamide (DEPAA), N-phenylpropionamide (PPA), N-phenylbutramide (PBA), and benzamide (BA) are described as very coiivenient extraction reagents for the spectrophotometric dletermination of IMo(V). The organic solution of HAL derived from aliphatiic acid and aromatic Present address: Department of Chemieitry, Ravishankar University, Raipur 492 010, Edadhya Pradesh, India.
amine or aromatic acid and ammonia is capable of extracting Mo(V) from SCN- solutions. Mo(V1) was reduced into Mo(V) with mild reducing agent like ascorbic acid and allowed to react with S C N in strongly acidic media and then the species formed was extracted with benzene solution of amide dimers (H2A2L2)as a mixed-ligand complex. HAL may dimerize through intermolecular hydrogen bonding by association of enolic and keto tautomers (2) in the fashion similar to carboxylic acid (3) to act as a N,O-dentate chelating agent in the formation of six-membered Mo(V) complex as does the amidine dimer (1). The special advantage of the method is that the HAL has a good chemical stability with a very simple method of preparation from readily available chemicals.
0003-2700/83/0355-1823$01.50/00 1983 American Chemical Society
