Limits to Sensitivity in Laser Enhanced Ionization J. C. Travis National Bureau of Standards. Washington, DC 20234 Laser enhanced ionization (LEI) occurs when a tunable dye laser is used to excite a specific atomic population in a flame ( 1 4 ) . The thermal ionization rate of the laser-perturbed atomic population is greatly enhanced over that of the thermally distributed population. Resonant (R), non-resonant (N), stepwise (S), and non-resonant stepwise (NS) laser excitation schemes for LEI are illustrated in Figure 1,which also indicates the role of collisional (thermal) processes in LEI. In all cases. the final ionization sten is collisional. The ions produced hg I.El are decected by impressing a high voltaee across the flamf.. The current in this high-voltace circug is a measure of the volume ionization rate (7) (ions created per cm3 per second), and, hence, changes in this current induced by the laser represent the LEI signal. Under appropriately controlled conditions, the LEI signal varies linearly with the concentration of the resonantly excited element in the flame, yielding the basis for a new method of chemical analysis. Flames have long been used to render free atoms for spectroscopic analysis f& elements (usually metals) in solution (8). For this purpose, a fine mist of solution drawn from a sample heaker is mixed with the fuel and oxidant in the chamber of -~~~ a pre-mix burner (the fuel and oxidant are mixed before combustion). Upon passing into the flame, the tiny droplets rapidly evaporate, leaving a fraction-often near unity-of the desired element in the free atomic state. For a given set of burner adjustments, the ratio of free atom number density in the flame to element concentration in the original solution is constant, but not accurately known. Typically, a 1-ppm solution yields 1010-10" atoms of a n a l y t e l c d i n the flame ~~
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solutions of known concentration to calibrate the instrument. Flame atomic emission spectroscopy (FAES), flame atomic absorption spectroscopy (FAAS), flame atomic fluorescence spectroscopy (FAFS), and LEI share a common technology u p to the point of detecting free atoms in the flame. Each method has unique advantages, and all have a lasting place in atomic spectrometry. A particular advantage of LEI is its extremely high sensitivity (2,lO). Limits of detection (LODs) range from ng/mL (parts-per-billion) to pg/mL (parts-per-trillion), as shown in
~
(9).
Free atoms in the flame may be identified by their unique absorptions andlor emissions and quantifird by the strength, or intrnsitv, oi these spectral fttnturrs. Accurate measurrments of solution concentrations require the use of "standard"
TI Co Sn In Figure 1. Excitation schemes for LEI in a 2500 K flame ( k T = 0.22 eV): R = resonant; N = nonresonant; S = stepwise; NS = nonresonant stepwise.
Volume 59 Number l i
November 1982
909
LEI Detection Limits (nglmL) a
Element
A.d
XI5
LODC
CHART
RECOROER
TRIG.
AVERAGER
Figure 2. Block diagram of an LEI spectrometer.
+ h.
'From relerence (41 for A, only and (10)lor A, h, = lower transition wavelength and A( = upper, in run.
'See
text for definition.
dNonresonance: lowest level 01 scheme is not the ground state.
the table. This range corresponds to number densities of 1@5-10s cm-"n the flame. For a typical 1 cm3 laserlflame interaction volume. and a 10 Hz laser. the minimum numher of atoms detected is -106 in a 1-s signal-averaging time. The intent of this paper is to explore the origin of this high sensitivity, and to identify possible avenues to even higher sensitivity. The approach to this goal is to describe the instrument and experimental characteristics in more detail; discuss how ions are formed and detected: use this information to derive ultimate, ideal detection limits; and extend these results to more realistic cases. ~
~
Instrumentation A simple block diagram of an LEI spectrometer is shown in Figure 2. The flame is a commercially available FAAS burner supporting CzHzIair, Hnlair, or C2HnIN20 flames. Of these, acetylenelair is the most common for LEI as well as is used to conventional methods: nitrous oxide/acetvlene , maximize the ircr atom yield for retractor). (oxide-forming) elements; and hydrogen air is especially attractive for I.Kl brcause of it5 low natural ion and clrcrron density hut isa lrss effective s a m ~ l atomizer e than the hottor flames 161. Many elecirodc. systems have heen used to d e t e c t ~sig~l nnli ( 1 . I I , 12). Some actually employ the hurner head as the anode, with one eathode in the tlame, or two 5traddling the flame. The parallel plrite split-cathode scheme ihuwn in Figure 2 includes the hurner head as anude and the flame reaction zone in thi, acti5.r circuit and is less amenable to theoretical treatment than the case of uppositely biased parallel plate elwtrod~.~. Electrode ctmfigurarion has negligible hearing un ultimate limits-t~f-~lttt~.ction tor ultra-rrece cltments in distilled water. However, electrode configurations do play an i m ~ o r t a nrole t for solutions containine laree amounts of easilv ionized elements, as will he discussed briefly in the final section. A pulsed laser, as shown in Figure 2, is presently the most practical type of laser for analvtical LEI. Pulsed dve lasers. kith associ~n~ted nonlinear n&g optics, can reach ail hut ih; very shortest 6 2 1 7 nm) wa\,eleo~thsof interest in atomic flake spectrometry. Continuous wave (cw) lasers are quite capable of performing.LEI (1.13.14). but lack this wavelenrth coverage. ~
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Journal of Chemical Education
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Among pulsed lasers, flashlamp dye lasers are desirable for single wavelength LEI (2,4) (i.e., Fig. 1, "R"and " N ) because of their relatively long -1 ps pulse duration. In general, however, Nn- or Nd:YAG-pumped dye lasers have the advantage. Although the short, 5 ns pulse of these lasers is a disadvantage, as discussed in the next section, the high peak powers improve the efficiency of non-linear uv generation, and each " D U ~ D " laser can he used to excite more than one dve head f i r twb-color LEI (10,15) (as illustrated in Figs. 1S and INS). For all oulsed dve lasers. radiofreauencv . " interference (RFI) emitted by the iaser during gas discharges andlor Qsw~t~.hing operatluns can he a major rxprr~mrntdproblem for LEI ( 2 ) .TI! date, the Nd:SAG-pumped iyittLmsiwns to have caused the least prohlrm in t h ~ regard. s With \rhagesof -1(1lXI\'tu -2000Vappl1ed to the parallel created by I.El are plate spl~t-cathodein Figure 2. ~~lecrrons detected within a microiecond, as discussed l a t e r . ~ h u sthe simple R-C network of Figure 2 passes these current pulses T the signal processing r q m & - n t , t h r ~ h the ~ h C H ~ H C ~ I Ointo while n~utingthe DC hackground current to ground ihrough the resistor. The hieh-eain orenmo rvi~lelvused fur analvtical LEI (16) yields -ipspul;es regkdlesiof the actual pulse width, because of its own bandwidth limitations. For a 10-Hz laser, the preamp output thus yields > T R , or In fact, this condition of "ionization saturation," or nearly complete conversion of a11 atoms in the laser beam into ions, is not difficult to achieve. Modern, high-powered pulsed lasers typically "saturate" optical transitions, driving R to its maximum value. This saturated value of R(R,) depends on the statistical weights of the quantum.states, but taking R, = 0.5 (its value for two states of equal statistical weight) is a reasonable estimate. Since h* can approach the collision rate of >10Ys-1, even 5-ns pulses may satisfy the condition in eqn. (15) for excited states very near the ionization potential. Figure 5 is a hydrodynamic analogy to the ionization process discussed above (26). The lower pump (laser) maintains a constant proportion of fluid (neutral population) in the lower two tubs. The upper pump (collisional ionization) thus depletes both lower tubs, even though drawing directlv. onlv " from rhr middle one (excited stare). Since loss &urn the upper tuh is jm;ill (rerornhination), the prurrss will continur until most of the fluid is in the upper tub (ions), if the lower pump remains on. The discussion to this point has actually assumed laser excitation from a ground state, as in Figure 1, "R" and "S." For nonresonant (not originating from the ground state) LEI to he efficient for ion production, a criterion virtually identical to eqn. (15) is applied to the energy gap between the ground state and the lowest level, except ki* is replaced by he, the thermal excitation rate from the ground state. Thus, if thermal excitation "mixes" the ground state and excited state more rapidly than TL-l, then ground state atoms have an opportunity to be pulled into the LEI process. For 10-6 s pulse lengths, 10,000 cm-I (or -1.2 eV) has proven to he a reasonable upper limit for this energy gap (4). Detecting Ions
One of the greatest assets of LEI is that no transducer is required to convrrt a physical responrr into an elerrricnl signal. Thr only question we nrrd to address here is the efficiency
Height (mm)
Figure 6. "images" of LEI ions and electrons. obtained by taking the LEI signal from a thin rod translated across the frontof the normal collecting plate at the indicated high voltages. The experimental apparatus is shown in the inset: (1) high voltage repelling plate; (2) laser excitation volume; (3)flame reaction zone; (4) burner head: (5)low voltage electrode plate; (6)verticelly moveable signal pick-off wire.
with which an external circuit can reeister the nroduction of ions and electrons in a flame. Since charged species in the flame can induce charges (and currents) in the external circuit without actually impacting the electrodes (27), collection of ions and electrons is not absolutelv necessary to the ~ r o d u c tion of an LEI signal. However, the-maximum response of the external circuit for a given number of LEI ions and electrons is obtained when theipecies are actually neutralized a t their respective electrodes. Thus, we need to consider the processes whkh transport andlor destroy (neutralize) ions and electrons. In a buffer gns, charged species typically move at constant n t with the proportionality constant velocity in H c ~ ~ n s t af~eld, fur a singly charged spectes be~ngcalled the molnlity. K:
where u,,
