New Methods for Programmed Heating of Electrically Heated Nonflame Atomic Vapor Cells Akbar Montaser' and S. R. Crouch2 Depafiment of Chemistry, Michigan State University, East Lansing, Mich. 48824
Four methods for the programmed heating of nonflame atomizers have been investigated and are described. The instrumentation for current, power, radiation, and direct temperature programming is presented along with the advantages and disadvantages of each programming technique. Three of these programming techniques have been applied to a platinum wire loop atomizer and a graphite braid atomizer. The method of programmed heating is shown to influence the atomizer lifetime, the time required for the atomizer to reach a steady state temperature, the long and short term reproducibility of atomization temperatures, and the separation and optimization of the parameters which influence the atomization process.
Nonflame atomizers for atomic absorption (AA) and atomic fluorescence (AF) spectrometry have been shown to be of considerable value for the detection and analysis of trace amounts of metals in a variety of matrices ( 1 - 3 ) . T o reduce matrix effects and to allow some control over the atomization process, a two- or three-stage programmed heating process is utilized for heating the atomization element. These steps are employed to dry, ash, and atomize the sample. There are essentially 5 different methods of programmed heating of nonflame atomizers, depending upon the electrical or physical parameters which are controlled during the heating steps. Most nonflame atomizers utilize a two- or three-stage electrical current program for heating the atomization elements. However, programmed heating can also be accomplished by controlling the voltage across the atomizer, the power dissipated in the atomizer, the radiation emitted by the atomizer ( 4 ) , and the actual atomizer temperature. In this paper, the fundamental principles upon which each heating technique is based are described along with the instrumentation for implementing current, power, radiation, and direct temperature programming. All of the methods presented are applicable to any electrically heated nonflame atomic vapor cell. The application of current, power, and radiation programming to two filament-type nonflame atomizers, a platinum wire loop atomizer ( 5 - 7 ) and a graphite braid atomizer (GBA) (8, 9 ) , is presented to illustrate the influence of the method of programmed heating on the atomizer lifetime, the time required for the atomizer to reach its maximum temperature, and the reproducibility of the atomizer temperature. T h e radiation programming method is shown to aid in the separation and optimization of the many parameters which influence the atomizer temperature.
FUNDAMENTAL PRINCIPLES In this section, the basic equation which describes the temperature of a nonflame atomizer heated by direct cur-
rent is derived and then simplified to illustrate the assumptions implicit in current, voltage, and power programming. The temperature of an electrically heated nonflame vapor cell is a function of a variety of factors, such 3s the heating technique, time, the input power, and the heat losses by convection, conduction, and radiation. For the following treatment, it is assumed that the atomizer is heated by a constant current i and that the physiochemical properties of the atomizer are not influenced from one experiment to another. Furthermore, it is also assumed t h a t the work done on the system by pressure-volume variations is negligible compared to the magnitude of the internal energy and various heat losses and gains. In general, we have 0' = Q, where Q is the net heat transfer and U is the internal energy, or qinternal energ).
radiation
%lectrical
-
'-
conduction
qTbomson heating 'convection
-
'sample
(1
For a filament-type atomizer of circumference P, diameter D, and surface area s, Equation 1 can be expanded into the following equation
P ( T - T , ) h ( T ) - PWZSH where p and C, are the density and specific heat of the atomizer, respertively. R 0 is the atomizer electrical resistance a t room temperature, N is the temperature coefficient of resistance. D is the Stefan-Boltzmann constant, t is the total emissivity of the atomizer, 7'0 is the sheath gas temperature away from the atomizer, h o is the atomizer thermal conductivity, 3 is the coefficient of thermal conductivity, x is the distance from the center of the atomizer, Ii is the combined convective heat transfer coefficient due to forced and natural convection, m is the amount of analyte deposited on the atomizer, AH is the amount of energy required to atomize the compound, and f is the Thomson coefficient, which may be of either sign. The heat due to the Thomson effect disappears if ac heating is used. It can be seen that the atomizer temperature is a complicated function of time and various parameters. For programmed heating involving the control of an electrical quantity (current, voitage, or power), several of the terms in Equation 2 must be neglected if the atomizer temperature is to bear a direct relationship to the controlled variable. If only ohmic heating is assumed and the heat losses due to radiation, conduction, convection, and the sample are negligible, Equation 2 reduces to
'
Present address, Ames Laboratory-USAEC and Department of Chemistry, Iowa State University, Ames, Iowa 50010. Author t o whom requests for reprints should be addressed.
"
38
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
pcp,s-d T := i2R dl
Integration of Equation 3 gives:
7
I
where k I = ( C , p ) - l and P , is the power dissipated in the atomizer. Under the conditions implied by the assumptions leading to Equation 4,the atomizer temperature is proportional to the current through the atomizer, the voltage across the atomizer, or to the power dissipated in the atomization element. In addition to the assumptions upon which Equation 4 is based, several other disadvantages of controlling electrical quantities in heating nonflame devices are apparent from Equation 4 itself. First, for the atomizer temperature to be a reproducible function of current or voltage from one run to the next, the atomizer resistance R and its surface area s must remain constant. Second, control of the electrical power dissipated in the atomizer, while compensating for resistance changes, does not compensate for changes in the atomizer surface area, which may occur because of evaporation losses as the atomizer is heated. If, on the other hand, a parameter more directly related t o the atomizer temperature (such as the radiation emitted by the atomizer) is monitored and used in a feedback control system, compensation may be achieved for changes in the atomizer properties as well as for the various heat losses which occur in a real nonflame system.
MINICOMPUTER
3
COMPJTER INTERFACE BUFFER
CURRENT REGJLATED SU'PLY
5 OCTAL 3 3
P-,
DECODER CARD
I O
PATCH CARD
D A
CONVERTER
Figure 1. Computer interface for electrical heating of nonflame atomizers
POWER SUPPLY
POWER TRANSISTORS
INSTRUMENTATION General Principles. In this section, the instrumentation for current, power, radiation, and temperature controlled heating of nonflame devices is described. T h e instrumentation for voltage control is trivial and is not presented here. T h e components of t h e nonflame AA/AF spectrometer have been previously described ( 7 , 8 ). T h e instrumentation employed for each heating method was similar in t h a t all techniques utilized one or more sensing devices, a d c power supply, a feedback system for heating regulation, and a sequencing system for providing a variable reference signal and t h e time for each heating stage. I n all programming operations presented here, a dc rather t h a n an ac power supply was utilized for heating t h e atomizer. T h e ac evaporation rate of t h e atomizer material has been shown ( I O ) t o be greater than t h e dc evaporation rate under t h e constraint of either equal effective current or voltage. For all of t h e operations reported here a PDP L a b 8/3 (Digital Equipment Corp., Maynard, Mass.) computer was utilized as t h e sequencing system as well as for d a t a acquisition and treatment. T h e computer provides a n operator-selected reference analog signal which is proportional t o the electrical or physical parameter being controlled. T h e parameters which control the heating of t h e atomizer and t h e times for each heating stage are stored in t h e computer through an initial dialog between t h e operator and the teletype. Figure 1 shows t h a t t h e stored digital information relating to t h e electrical or physical parameter for each heating step is transferred t o t h e Computer Interface Buffer (Model EU-801-19, Heath Co., Benton Harbor, Mich.) under computer control. T h e I/O Patch card (Model EU-801-21, H e a t h Co., Benton Harbor, Mich.) directs t h e digital signal t o a 10-bit digital-to-analog converter (DAC) card (EU-ROO-GC, Heath Co., Benton Harbor, Mich.), which produces a corresponding analog voltage between 0 t o -10 \.' when it is properly addressed. T h e DAC output is sent through a unity gain inverter t o provide appropriate polarity for t h e regulator circuit. I n order to be addressed unambiguously, t h e DAC also requires a Strobe and a Device Select signal from t h e computer. T h e 1/0 patch card provides three different Input-Output signals ( I O P I , 2, and 4), and IOPl was used as t h e Strobe signal. T h e Octal Decoder card (EU-800-SA,Heath Co., Benton Harbor, Mich.) was also e m ployed to provide t h e Device Select Code 11. Current Programming. T h e current regulator shown in Figure 2 consists of a raw power supply, power transistors, and a n operational amplifier (OA) feedback system for current regulation. During operation. the raw supply is on a t all times. However, t h e
Figure 2. Current-regulated power supply
-
/
I
Figure 3. Circuit diagram of the instrumentation for power programmed heating of nonflame atomizers
power transistors prevent current from reaching t h e atomizer until a command is received from t h e computer. T h e resistor R,, senses t h e current through t h e atomizer a n d a corresponding voltage iR is is produced a t t h e OA input. T h e OA compares this voltage with t h e output of t h e DAC and allows a current, i, through the atomizer, such t h a t iR,, equals t h e DAC output voltage. A raw supply capable of providing 30 A at 25 V was utilized. With the specific raw supply and pass network used. t h e current applied to t h e atomizer could be varied u p t o 30 A a t 15 V. Power Programming. In both current and voltage programmed heating, only one electrical variable. either current or voltage, is controlled during t h e three heating stages. Since it is practically impossible t o prevent t h e physiochemical transformation of t h e atomizer, it is desirable to monitor both the current and voltage simultaneously. T h e product of t h e two parameters is, of course, t h e electrical power dissipated in the atomizer. T h e instrumentation for power programming, shown in Figure 3 , consists of two sensing devices for monitoring t h e current and t h e
ANALYTICAL CHEMISTRY. VOL. 47, NO. 1, JANUARY 1975
39
INSTRUMENTATION
1 KO
1 K11
AMPLIFIER
1OK11
1OK11
POWER
SUPPLY
TRANSISTERS
I
i
I
I
I
ANALOG SWITCH
i
r ANALOG MULTIPLIER I
ANALOG
OPE R AT ION A L
swircH
AMPLIFIER
LOGIC
&.
REFERENCE VOLTAGE
Figure 4. Circuit diagram of the instrumentation for the combined power and radiation programmed heating of nonflame atomizers
voltage. an analog multiplier (MC 1594, Schweber Electronics, \Vestbury, N.Y.), a control operational amplifier, a power supply, and power transistors. T h e current through the atomizer is sensed by t h e resistor K,,, and a corresponding voltage is produced a t one multiplier input. T h e voltage across t h e atomizer is monitored by an instrumentation amplifier (EU-900-DA, Heath Co., Renton Harbor. Mich.). and its output is connected t o t h e other multiplier input. ‘I’he 1/10 voltage divider network provides the proper voltage levels for the instrumentation amplifier. T h e control OA compares the multiplier output voltage with t h e reference voltage supplied by the DAC (proportional t o power), and allows a current through the atomizer such t h a t the multiplier output equals the reference voltage (power). As t h e atomizer resistance is increased, because of‘the influence of factors such as vaporization, t h e voltage drop across the atomizer increases, and the feedback loop allows less current through t h e atomizer. Consequently, t h e product. power, remains constant despite t h e fact t h a t current and voltage vary from one experiment t o another. R a d i a t i o n P r o g r a m m i n g . According to Stefan’s law, the total rate of radiation emitted by a body increases with t h e 4th power of the Kelvin temperature. T h e radiation emitted by any electrically heated atomizer can be utilized for programmed heating. A radiation transducer such as a phototransistor can function as a radiation sensing device. Since the nonflame atomizers emit visible radiation only during the ashing and atomization periods, other control methods can be utilized for heating the atomizers during the desolvation period a h e r e there is no visible radiation. Alternatively, an infrared sensitive photodetector can he used for radiation control during all three heating stages ( 4). An instrumentation system employing a phototransistor sensitive t o visible radiation is shown in Figure 4. T h e power control technique is used for t h e desolvation period. and the radiation control method is employed during the ashing and atomization periods. A two-channel analog switch ( D G 152 AP, Siliconix Incorporated, Santa Clara, Calif.) selects t h e appropriate control method during the three heating stages. One analog channel is active a t all times. As soon as t h e desolvation period is complete. the computer supplies a +5-Y signal to the analog switch logic input for the entire length of the ashing and atomization stages. T h e analog switch disconnects the power sensing cir40
A N A L Y T I C A L C H E M I S T R Y , VOL. 47,
NO. 1.
cuit from the control OA and simultaneously connects the radiation sensing circuit. T h e phototransistor (Type T I L 64, Texas Instrument. Inc. Dallas, Texas) monitors the atomizer radiation and produces a corresponding voltage, which is amplified and applied to the control operational amplifier input. T h e OA compares this voltage with the reference voltage from the DAC (proportional t o radiation) and allows a current through the atomizer such t h a t the radiation circuit output voltage equals the reference output voltage (radiation). T o test the radiation method with filament atomizers, such as the GBA and hot wire loops, the phototransistor was mounted in a lens holder. T h e X, Y, and Z positions of the phototransistor with respect tu the atomizer could be changed by means of suitable a d justments. Because of the intense atomizer emission, light filters were also employed to provide the proper emission intensity for the phototransistor. T e m p e r a t u r e P r o g r a m m i n g . In all the heating methods discussed so far. an electrical or a physical parameter proportional t o the atomizer temperature was monitored in the programmed heating. Direct temperature regulation can be performed if a thermocouple is employed as a sensing device. T h e instrumentation required for direct temperature control is similar to t h a t employed in the current technique except t h a t the thermocouple output voltage is amplified and applied t o t h e control operational amplifier instead of t h e voltage drop across t h e sensing resistor Kc,. However, in temperature measurements, the parameter of importance is the thermal time constant, Tth, of the measuring device. For a very fine thermocouple, which is immersed in an environment of high thermal conductivity, 7th ranges from msec to seconds. For nonflame atomizers, the temperature continuously changes with time and it is necessary to measure t h e temperature instantaneously. However, when a thermocouple is subjected to a temperature step, its temperature approaches t h e steady state value after several time constants. T h e temperature growth, to a first approximation, is exponential for t h e thermocouple. T h e output voltage of the thermocouple V ( t) as a function of time r is given by Equation 5:
J A N U A R Y 1975
(5)
where V Ois the thermocouple output voltage corresponding to the steady state temperature. Differentiating Equation 5 with respect to time, gives Equation 6:
which when substituted in Equation 5 provides the following equation:
In principle, by combining the temperature at any instant V ( t ) with the rate of change of temperature, dV(t jldt, the ultimate temperature Vo, can be measured instantaneously. LVhen a nonflame atomizer is heated, it can be assumed that the atomizer is subjected to an infinite number of temperature steps before t h e final steady state temperature is achieved. The ultimate output of the thermocouple for each step Vol is given by an equation similar to Equation 7. If the resulting ultimate thermocouple output for each step can be evaluated instantaneously, a stair case waveform pattern, which is proportional to the heat input to the atomizer. can be generated. This stair case function
i: i= 0
10
A T O M I Z E R T E M P E R A T U R E , OC
I'Oi
can be utilized as the controlling variable for the input to the control operational amplifier. This also prevents temperature over-shoot which might occur because of the slow response of the ther-
mocouple. Evaluation of Vol can be performed either by software or by analog circuitry. When a minicomputer is available, the thermocouple output voltage V(tj is amplified and applied to the analog-todigital converter (ADCj of t h e computer, where ti-to-D conversions are made at a specified rate. .4 real-time calculation routine can be employed for evaluation of VO,,and Z V", can subsequently be applied to the control OA. Since the computer should function as a digital integrator f o r the A.4 or AF signals. it may be advantageous that evaluation of Voi, be executed by an analog circuit. An analog circuit for the determination of VO has recently been described ( I J 1. The limiting factor in the evaluation of V Ois the electronic time constant of the circuit. Calcuation of the ultimate thermocouple output for each temperature step can be executed in 0.1 msec if the thermocouple time constant T,), is = 100 msec.
RESULTS AND DISCUSSION For those heating methods involving t h e control of a n electrical quantity, Equation 4 demonstrates t h a t such a t omizer properties as resistance and surface area must remain constant from r u n t o run in order for reproducible temperatures to be achieved for t h e same value of t h e controlled variable. Because of vaporization of t h e atomizer material during heating, these atomizer properties change from one experiment to t h e next. A simple calculation was made t o determine t h e evaporation rate for a carbon atomizer. T h e calculation was based on a modification of Equation 8, which has been derived for t h e rate of evaporation R F, in a vacuum ( 1 2 ) .
I t , = trP,
M \ I 2 nR T
(8 )
where P,is t h e saturated vapor pressure at the temperature T , A4 is t h e molecular weight, and a is t h e condensation coefficient. I t was assumed t h a t carbon is monatomic in t h e vapor phase and t h a t t h e condensation coefficient was unity. Under these conditions Equation 8 reduces to:
Figure 5. Influence of the atomizer temperature on the atomizer evaooration rate
T h e evaporation rate for a carbon atomizer was calculated and plotted as a function of atomizer temperature as shown in Figure 5 . I t can be seen t h a t when t h e atomizer temperature is varied from 2100 to 3200 "C, t h e evaporation rate changes by five orders of magnitude. If t h e sheath gas or t h e impurities in the sheath gas react with t h e atomizer, t h e evaporation rate is still higher. T h e evaporation rate is also higher in a dynamic system compared t o a static system. As the flow rate is increased, t h e vaporized material is swept out of t h e atomization chamber, and the atomizer consequently produces more vapor. These processes result in a change in atomizer properties. T h e temperature variation which results is a function of parameters such as t h e type of atomizer, t h e number of experiments performed, and t h e method of programmed heating. In t h e following sections, t h e influences of t h e above effects are presented along with t h e effect of sheath gas flow rate and heating method on t h e atomizer temperature. C u r r e n t a n d V o l t a g e P r o g r a m m i n g . In current-programmed heating, the power supply provides the atomizer with t h e preselected amount of current during the various heating stages. In voltage programming, t h e voltage across t h e atomizer is preselected a n d kept constant during each heating stage. T h e magnitude of t h e voltage, therefore, would be different for t h e 3 heating stages. Both current and voltage control of programmed heating can result in a t omization temperature drift If current programming is employed as the heating method, gradual vaporization results in higher electrical resistance and smaller cross-sectional area for the atomizer. Thus, t h e atomization temperature for the same current will increase from run to run. For voltage-controlled heating, the increase in the resistance allows less current through the atomizer and t h e temperature decreases. T h e rate of atomizer evaporation, however, would be smaller than with current programming. These variationh can be multiplied if the atomizer or its holder do not have t h e same temperature a t t h e start of successive experiments. Since t h e atomizer resistance is a function of temperature ( R = R o ( 1 tuT)),different effects may be observed depending on the sign and magnitude of a . t h e coefficient of resistance. If t h e atomizer and its holder are made of the same material, a positive value of N result5 in the intensification of temperature variations, while a negative value may operate in the opposite direction. If the atomizer and
+
(9) in g cm-2 sec-' where P,is in Torr. At atmospheric pressure, it can be assumed (13, 1 4 ) t h a t the evaporation rate is about one-sixtieth of t h e rate in vacuum. X i,tm = R ~:/60.
ANALYTICAL CHEMISTRY. VOL. 47, NO. 1 , JANUARY 1975
41
ISC,
(a)
>
60
!=
V,
a+
t
50-
v)
5
20
60
40
80
100
120
140
160
IBQ
I
50
100
150
200
250
300
350
400
NUMBER OF EXPERIMENTS 40
80
1iO
I60
200
?LO
280
NUMBER OF EXPERIMENTS
Figure 7. Variation of GBA emission intensity with number of twostage heating experiments for power-controlled heating ( a ) Atomization power = 185 W (initial current = 15 A); ( b ) atomization power = 130 W (initial current = 12 A)
Figure 6. Variation of GBA emission intensity with number of twostage heating experiments for current-controlled heating ( a ) Atomization current = 15 A ; ( b )atomization current = 12 A
700
t h e holder are made of materials with different a values, as is normally the case, other temperature variations can be expected. T o illustrate these effects, the graphite braid atomizer (GBA) temperature was monitored as a function of the number of experiments performed by measuring its emission intensity for atomization currents of 9 t o 21 A. Each experiment corresponded t o a two-step current-controlled heating. All experimental conditions, summarized in Table I, were kept constant, and no operator adjustments were made during the set of experiments. Figure 6 shows the variation in the GBA emission intensity from experiment to experiment for two different atomization currents. Higher values of emission intensity indicate higher temperatures. I t can be seen t h a t the atomizer temperature gradually increases from experiment to experiment even though the electrical current is controlled. T h e final sharp rise in temperature, observed in Figure 6, occurs just prior to braid deterioration. When braid deterioration occurs, electrical conduction ceases, and t h e atomizer must be replaced. Hence, the total number of experiments prior to braid deterioration represents t h e lifetime of t h e atomizer in terms of analytical runs. Assuming t h a t the atom population is observed in a n area where the atomizer background emission variation does not influence t h e measurement process, the temperature variation can alter t h e optimized parameters in t h e system. The distribution of free atoms and the sheath gas flow pattern are influenced by temperature variations. T h e atom residence time would be shorter, which may require a faster data acquisition system. If the analyte is a volatile sample, gradual vaporization may occur during desolvation and/or ashing with a consequent decrease in the AA or AF signal. Power Programming. As can be seen from Equation 4, the atomizer temperature is also proportional t o t h e power dissipated in the atomizer. Because the power control technique compensates for atomizer resistance changes from run to run, power-programmed heating should yield superior results in terms of reproducibility of atomization temperatures, atomizer lifetimes, and optimization of experimental parameters compared to either voltage or current regulation techniques. T h e variation of the GBA temperature as a function of the number of two-stage experiments, shown in Figure 7, was monitored by measuring the emission intensity of t h e GBA as described previously for current-programmed
!!
-
600
5
g
533-
W
0. w X ACC
~
9
\ ' m'
U
0 E
?GO
-
200
-
W
m
5Z
8170 -
I ATOMIZATION 9 A T 3 M l Z A T l O V 95 ATOM ZATION 1485
I I2 130 1650
IS
15
185 2'00
250 POWER W 1 2 5 0 - E * I P E R A - U R E 'C
CJRRENT A
Figure 8. Lifetime of GBA (number of experiments) and maximum temperature variation during lifetime (emission intensity ratio) vs. atomization parameters for current- and power-controlled heating Power programming; 0 current programming; - number of experiments vs. atomization parameters: - - - emission intensity ratio vs. atomization parameters
heating and under the same experimental conditions. A comparison of Figures 6 and 7 indicates t h a t the total number of experiments before atomizer deterioration (lifetime) has improved substantially with power programming. Furthermore, the rate of temperature increase from run to run is much lower than when the atomizer heating is currentprogrammed. A more comprehensive comparison of the two techniques in terms of atomizer lifetime and temperature variations is shown in Figure 8. The X axis is linear in current through the atomizer. T h e corresponding atomization power and temperature are also shown. T h e unbroken lines show the total number of experiments which could be performed before braid deterioration (left hand Y axis) as a function of atomization current for current-controlled heating and power-controlled heating. T h e dashed lines show the ratio of the GBA emission intensity for the last experiment before deterioration to t h a t of the first experiment as a function of atomization current (right hand Y axis) and, therefore, indicate the maximum deviation from t h e initial temperature. Each data point is the average of three sets of experiments, and all data have been obtained under the experimental conditions shown in Table I. It can be seen that with power programming a n average of 678 determinations can be performed a t about 1485 "C before the braid deter-
42 * ANALYTICAL CHEMISTRY, VOL. 47, NO. 1 . JANUARY 1975
I
Table I . Experimental Conditions for the Investigation of Various Heating Techniques with t h e GBA Inner sheath g a s flow rate, l./min argon 2 Outer sheath gas flow rat'e, l./inin argon 2 Desolvation current, A 1.5 Desolvation, time, sec. 11 Atomization time, sec. 1.6 Cooling period, sec. 10
iorates. This decreases to 162 determinations a t 2250 "C. When current programming is utilized under the same conditions, the number of determinations decreases to 490 and 86, respectively. Furthermore, the emission intensity ratio, or the temperature variation, is higher with current-controlled heating in all cases. With power programming, the ratio of the emission intensity does not change significantly from unity a t higher temperatures. In practice, the GHA is replaced with a new braid as soon as braid deterioration is noted by the observation of a hot spot,. T h e hot spot usually forms a t a point which is located either between the inner and the outer sheath gas streams or a t the contact points of the atomizer and its holder. R a d i a t i o n P r o g r a m m i n g . In contrast to the current and power methods, where the atomizer temperature is a function of its dimensions, as shown by Equation 4, the radiation technique does not significantly suffer from this disadvantage as long as the radiation transducer monitors a fraction of the total emitted radiation. Furthermore, the variation of sheath gas flow rate and sheath gas type should not influence the atomizer temperat,ure in the radiation technique. This is a n important step in the separation of parameters in nonflame atomization. When the current-programming technique is employed for the carbon rod atomizer or a graphite tube atomizer, it may be shown that the rate of cooling water flow in the atomizer holder influences the AA or AF signal. Increasing the water flow may improve the sensitivity. This enhancement a t higher water flow rates can be attributed to an improvement, in the electrical contact between the atomizer and its holder. When radiation programming is employed, these physical changes should not affect either the atomizer temperature or the AA and AF signals. T h e influence of radiation-programmed heating on the reproducibility of' atomization temperature and the lifetime of the GBA was investigated using a procedure similar to that described previously for the other heating techniques. The ratio of the final to the initial emission intensity was about 1.3 a t 1485 "C.but the ratio was unity a t higher temperatures. Compared to the other programmed heating methods, radiation control pave the best temperature reproducibility over a wide temperature range. With the radiation control method, the GBA lifetime was shorter than when its heating was current or power controlled. T h e ratio of the average number of determinations before deterioration using power control to the average number of determinations using radiation control varied from 2 t o 4. T h e shorter GBA lifetime when the radiation control method is employed for heating can be explained in terms of the larger fraction of time that the atomizer spends a t the final steady state temperature. Figure 9 demonstrates the atomizer emission intensity-time profiles for the three heating techniques during the atomization stage. T h e atomization period was 1.6 seconds in each case, and the final GHA temperature was about 1650 "C. I t can be noted that the steady st,ate period is considerably longer in the radia-
I
U
D G S
10
I 5
Time. sec
Figure 9. Intensity of GBA emission (temperature) vs. time for three programmed heating techniques (a) Power-controlled heating; ( b ) current-controlled heating; ( c ) radiationcontrolled heating; maximum temperature = 1650 OC in all cases
Figure 10. Duration of steady state temperature period vs. atomization parameters for three programmed heating methods 0 Radiation-controlled heating; trolled heating
current-controlled heating; A power-con-
tion method. I t is also interesting to examine the regulation of radiation (temperature) in the radiation technique. T h e duration of the *steady state atomization period, t,, as a function of atomization current is shown in Figure 10 for the three heating methods. It can be seen t h a t t , increases with increasing temperature in the current and power techniques, while for the radiation method the steady state period decreases with increasing temperature. I t can be noted from Figure 10 t h a t as far as the vaporization of the atomizer is concerned, the current and the power techniques should cause nearly the same amount of vaporization in a single experiment. When radiation programming is u%ilized,however, the atomizer spends a considerably longer time a t its final temperature and a greater amount of vrlporization should be expected. The atomizer lifetime will, therefore, be shorter compared to the other techniques.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
43
of the atomizer is a complicated function of power applied to the atomizer, the sheath gas flow rate, and several other parameters as shown by Equation 2. 'The sheath gas can similarly influence t h e atomization efficiency and the residence time of the atomic vapor in front of the observation window. In the case of atomic fluorescence, it can also influence quenching of the excited atoms. I t is desirable, therefore, to isolate the interdependence of experimental parameters. The capability of the various heating techniques in the separation of experimental parameters is dis-
.-
I
,
'
I
.
,
I
2
3
4
5
b
7
6
10
9
FLOW R A T E . I/niin
Figure 11. Influence of sheath gas flow rate on atomization temperature for three programmed heating methods
T
Radiation-controlled heating; power-controlled heating; 9 current-controlled heating: ~ - Graphite braidatomizer: - - - platinum ICOD atomizer T
I t should be noted that the length of the pre-steady state period is a function of the maximum amount of current t h a t the power supply can deliver. T h e !arger t,he current, the shorter the pre-steady state period. Furthermore, since in the radiation method, the atomizer achieves its final temperature a t a higher rate, atomization of the analyte should be complete in a shorter time. The lifetime of the atomizer can therefore be improved if the atomization pcriod is terminated shortly after the sample is completely aiomized. Temperature Programming. Thermocouples lend themselves convenientl!r to the measurement of temperature profiles in flame and nonflame atomizers, t i r i t empirical correction factors must be utilized t o allow for radiation and conductjon losses, arnd in any case they can be user! only for low temperature systems. Catalytic heating on the wire surface may also cause errors in temperature measurements. For nonflame atomizers, where a violent reaction zone does not exist. a tungsten-rhenium thermocouple system may be used for temperatures from 1600 to 3001, "C. 'I'his thermocouple system ha:; not heen used in either flame or nonflame atomizers thus f?r, but it has been evaluated for use in aerospace and nuclear industries (2.5 1. The stability of a 0.51-mm diameter wire thermocouple in an induction-heated vacuum furnace has been shown t.o be *2?6 for 15 hours a t 2600 "C ( 1 5 ) . When a nonflame atomizer is employed, even if a n inert gas is used as a sheath gas, oxidation of the thermocouple can occur, and the Thermocouple lifetime is expected to be shorter. Furthermore, attachment of a n y thermocouple to the atomizer presents a practical problem. Therefore, direct temperature measurement and regulation, despite its fundamental advantages, suffers from major practical disadvantages a t the present state of the art. Influences of Flow Rate and Heating Methods on Atomizer Temperatures. There are many interdependent parameters which can influence the free atom concentration in nonflame atomizers. For example, the temperature 44
Figure 11 demonstrates the influence of three heating methods and the sheath gas flow rate on the atomization temperatures of a graphite braid and a Pt loop atomizer. The vertical markings around each experimental point indicate the standard deviation of 4 successive measurements. A ten-second cooling period between measurements was used in all cases. The atomizer emission intensiiy was monitored as a measure of atomizer temperature in all cases. The GBA and P t loop temperatures for an inner and outer sheath gas flow rate of 1 l./min were about 1650 and 1200 "C, respectively. When the radiation control method is utilized, the atomizer temperature for both the GR.4 and the Pi, loop does not change with the flow rate. Furthermore, t h e atomizer temperature is very reproducible. With both the power and the current techniques, the Pt loop temperature decreases with gas flow at nearly the same rate. Although the power technique compensates for resistance changes caused by the flow rate variation, it cannot correct for convective arid conductive heat losses in the systeni. For the P t loop atomizer, the current control technique seems to provide higher reproducihility of atomization temperature compared to the power control technique. No explanation of this variation in precisicin can be provided a t this point. When the current, and power techniques are applied to the GBA, the atomizer temperature appears to pass through a minimum, but increases when the flow rate becomes greater than 6 Urnin. The decline in temperature a t lower flow rates can be explained in terms of convective and conductive losses by t h e GBA. The gas flow also cahses a decrease in the electrical resistance of the atomizer holder. At higher flow rates. the holder resistance is further decreased a n d this allows more current through the atomizer u ith a subsequent increase in temperature. This should aiso explain the higher standard deviation a t higher flow rates. S o t e that the precision of the atomizer temperature, provided by the power control technique. is superior to the current control method. In hoth cases, the precision at higher flow rates should be improved if the holder is kept a t constant temperature either b y water cooling or by allowing cooling periods of greater t,han 10 seconds.
CONCLUSIONS Five different methods of programmed heating of nonflame atomizers were presented along wit,h the required instrumentation for four techniques. The method of programmed heating was shown to influence the atomizer lifetime, the time required for the atomizer to reach a steady state temperature, and the separation and optimization of atomization parameters. T h e power-programmed heating method provides the largest improvement in terms of atomizer lifetime. The radiation method results in the best separation of atomization parameters. Since the radiation method can provide the fastest achievement of a desired temperature, the atomization of a given sample is accom-
A N A L Y T I C A L CHEMISTRY, VOL. 4 7 . N O . 1, J A N U A R Y 1975
plished in a shorter period. Because of this, radiation programming should be beneficial t o those systems which use peak height measurements of t h e AA or A F signals rather than integration techniques. I n those cases t h a t t h e atomization process goes through a n oxide formation step a n d volatilization of t h e oxide occurs a t a temperature much lower than t h e atomization temperature ( 1 6 ) , it is essential, to prevent sample loss due to oxide formation, that t h e atomizer achieve f he desired final temperature instantaneously. I t is in these situations t h a t t h e radiation method of' programmed heating can provide better powers of detection for certain elements compared t o other programming techniques. Also, recent work by Lundgren et a1 ( 4 ), who used radiation controlled heating with a graphite furnace, has demonstrated t h a t radiation programming can facilit a t e analyses which are impossible with conventional heating methods. These authors demonstrated that cadmium can be determined in a NaCl matrix with no nonspecific absorption interference from t h e salt, when t h e furnace heating is radiation controlled.
LITERATURE CITED (1) (2) (3) (4)
G. F. Kirkbright, Analyst (London), 96, 609 (1971). J. D. Winefordner and T. J. Vickers. Anal. Chern., 44 (5),150R (1972). J. D. Winefordner and T. J. Vickers, 46 (5),192R (1974). G. Lundgren, L. Lundrnark, and G. Johansson, Anal. Chern., 46, 1028 (1974). (5) M. P. Bratzel. R. M. Dagnall, and J. D. Winefordner, Anal. Chim. Acta., 48, 197 (1969). (6) M. P. Bratzel, R. M. Dagnall, and J. D. Winefordner, Aoof. . . Soectrosc., . 24, 518 (1970). (7) S. R. Goode, Akbar Montaser, and S. R. Crouch, Appl. Spectrosc., 27, 355 (19731. (8) Akbar Montaser. S. R. Goode. and S. R. Crouch, Anal. Chern., 46, 599 (1974). (9) Akbar Montaser and S.R. Crouch, Anal. Chern., 46, 1817 (1974). (10) A. D. Wilson, Appl. Opt., 2, 1055 (1963). (11) S. H. Praul and L. V. Hrnureik, Rev. Sci. lnstrurn.. 44, 1363 (1973). (12) I. Langrnuir, Phys. Rev., 2, 329 (1913). (13) G. R. Fonda, Phys. Rev., 21, 343 (1923). (14) G. R. Fonda, Phys. Res., 31, 260(1928). (15) R. S. Asarnoto and P. E. Novak, Rev. Sci. lnstrurn., 38, 1047 (1967). (16) D. J. Johnson, T. S. West, and R. M. Dagnall. Anal. Chim. Acta.. 67, 79 (1973). ~
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RECEIVEDfor review April 22, 1974. Accepted September 24,1974.
Automated Identification of Mass Spectra by the Reverse Search Fred P. Abramson Division of Laboratory Medicine, Department of Pathology and Department of Pharmacology, George Washington University School of Medicine, Washington. D.C. 2003;
A new method for the automatic identification of mass spectra which used the library spectrum as the basis of the comparison is described. This process, called reverse search, is contrasted with other methods for mass spectral library searches where the unknown spectrum itself becomes the basis. The reverse search is shown to be fully automated, requiring no operator judgment to output qualitative and quantitative data. The other significant feature of a reverse search is its inherent rejection of interference. A specific compound obscured by other compounds may still be identified by this method. A number of areas of routine analysis are suggested where this system could have significant application.
This paper presents a situation of d a t a interpretation where t h e order of comparison between known and unknown d a t a is of great significance. An arbitrary distinction can be made between the two possible mechanisms for searching a library: forward and reverse. A forward search method compares a n unknown to a library entry, while a reverse search compares a library entry to a n unknown. Although these two cases seem similar, t h e significant advantages o f a reverse search will be described. Automated identification processes are especially valuahle when operating a gas chromatograph/mass spectrometer system. owing to t h e large number of' unknown peaks frequently encountered. My experience with library searches of mass spectral d a t a has been unsatisfactory. T h e principal difficulty is t h a t conventional searches provide equivocal answers regarding t h e composition of t h e spectrum in
question. This is especially bothersome when analyzing materials of biological interest, because few such compounds are included in commercial lihraries. Furthermore, biological samples are often complex and, even following gas chromatography, multiple compounds may be present in a n unknown mass spectrum causing inaccuracies. F e a t u r e s of Forward Search Techniques. T h e numerous methods for computerized searches of mass spectral d a t a u p to 1970 have been reviewed ( 1 ) . Several additional papers have appeared subsequently (2-51. All of these search methods are in t h e forward sense; t h a t is, they process a n unknown spectrum for comparison to their library. As a consequence, they suffer from interferences, d of automation, and an inflexibility of their compound identification algorithms as will be described. T h e presence of significant levels of interference may artificially suppress t h e relative intensity of relevant masses and produce a bad fit. Even more import,antly. when data are compressed (e.g., saving only the two largest peaks in a 14-amu region), interferences of any nature may cause relevant masses to be excluded. T o eliminate interferences, t h e operator must first detect such admixed spectra, then identify some other spectrum t o subtract from t h e first to remove this interference, and, finally, determine how much of this second spectrum t o subtract from the first. In addition, t h e operator must decide which, if any, of t h e multiple suggestions reported by most forward search methods is the correct answer. These human interventions make the automation of t h e identification process difficult. T h e algorithm generating t h e similarity index in a forward search is fixed. Whatever t h e method (if any) for reducing spectra, whatever t h e method for increasing t h e im-
ANALYTICAL CHEMISTRY, VOL. 47. NO. 1, JANUARY 1975
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