ION MOBILITY SPECTROMETRY Herbert H. Hill, Jr., William F. Siems, and Robert H. St. Louis Department of Chemistry Washington State University Pullman, WA 99164-4630
Dennis G. McMinn Department of Chemistry Gonzaga University Spokane, WA 99258-0001
When a gaseous ion a t atmospheric pressure is placed in a constant electric field, it accelerates down t h e field until it collides with a neutral molecule, accelerates again until it has another collision, and so forth. T h i s chaotic sequence of accelerations and collisions at the molecular level translates into a constant ion velocity over macroscopic distances. T h e ratio of t h e ion velocity to the magnitude of the electric field is called the ion mobility, and separation of ions on the basis of mobility differences is called ion mobility spectrometry (IMS). Because it can measure ion currents below 10~ 12 A, I M S is highly sensitive as a detection technique. T h e combination of moderate separating power with trace-level sensitivity has led to applications of t h e ion mobility spectrometer as a stand-alone monitor a n d a chromatographic detector. When it emerged as an analytical 0003-2700/90/A362-1201 /$02.50/0 © 1990 American Chemical Society
technique in t h e early 1970s, I M S was also known by two other names: gaseous electrophoresis and plasma chromatography. T h e term gaseous electrophoresis emphasized the strong analogy between t h e mobility experim e n t a n d traditional liquid-phase electrophoresis (many of the key equations are identical), whereas the t e r m plasma chromatography referred to t h e chromatogram-like separations of ions t h a t can be achieved with I M S . These terms were confusing, however, because both electrophoresis and chromatography m u s t be stretched beyond
REPORT the meanings they generally have to apply to separation of ions on the basis of mobility. On the other hand, t h e t e r m mass spectrometry is a wellaccepted use of t h e word spectrometry, denoting a method for gas-phase separation of ions. T h u s ion mobility spectrometry seems to better represent the standing of the method in the hierarchy of separation techniques. I M S was discussed by F. W. Karasek in a 1974 REPORT (1) t h a t reviewed the development of mobility theory and t h e emerging analytical applications of I M S for detection of trace quantities of
gaseous organic compounds at atmospheric pressure. T h e advantages of IMS are the same today as they were then—this technique offers high sensitivity, i n s t r u m e n t a l simplicity, low cost, analytical flexibility, and realtime monitoring capability. IMS researchers in t h e 1970s gathered fundamental d a t a about the technique and its potential applications. Response ions were identified (2), mobility constants were measured (3), t e m p e r a t u r e effects were investigated (4), and ion mass-to-mobility correlations were made (5). Explosives, chemical warfare agents, drugs of abuse, and atmospheric and workplace pollutants were found t o produce strong I M S responses. It was soon realized t h a t a variety of operating modes were available. For example, by monitoring positive background reactant ions, one can gain essentially the same information from I M S as t h a t obtained from a flame ionization detector (6), whereas monitoring electron current with nitrogen gas in the spectrometer yields a response similar to t h a t of t h e electron capture detector (7). In addition, the instrum e n t can be t u n e d to perform selective mobility monitoring (8). Investigators in the 1980s pioneered new techniques for ionization, for adding trace dopants to the spectrometer
ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990 · 1201 A
REPORT
Controller Temperature control
Heated enclosure Drift gas exit
/
Sample inlet
Drift voltage supply
|
I Ι Γ' Gate driver
ih ^k^L
s
ιS
I
ι Computer interface
|
1— 63Ni foil
Plp^
" — - p ^ Guard rings
J
il ^ I
i
s
\\
SI
IS
ί
/
\ \> Gates
/
Y \ |ί
j
Aperture grid
Drift gas entrance Collector SSQ
Timer
A/D ι
Personal computer
ι
Ο
Drift gas flow control
Amplifier
Figure 1. Ion mobility spectrometer with an atmospheric pressure ion drift tube, 63Ni ionization, and capillary column sample introduction. Shaded areas are ceramic materials. Guard rings are fashioned of thin stainless steel and fit snugly against the inner wall of the ceramic tube; an electrical lead from each guard ring passes through the ceramic wall. The holder for the 63Ni foil is of stainless steel and is electrically connected to the first guard ring. Both collector and foil holder are perforated to allow passage of drift gas. A data system with analog-to-digital (A/D) conversion and timing functions, as well as a scanning square-wave generator (SSQ) for FT mode operation, occupy I/O slots in a personal computer.
to tailor its response to specific classes of compounds, and for sampling from vapor and condensed phases. New sig nal-processing methods, including a Fourier transform mode, were devel oped, and the importance of avoiding overload in the detector was realized. Engineering developments provided IMS-based detectors and alarms in both hand-held and stand-alone con figurations. Expanded uses in chro matographic detection included the use of supercritical fluids and liquids as well as vapor mobile phases. Franklin GNO Corporation, the original devel oper and supplier of IMS equipment, was reorganized as PCP, Inc., and to day a handful of new IMS manufactur ers are targeting the broad, but as yet largely untapped, range of monitoring
applications. The growing interest in IMS as an analytical method warrants an update on the technique and its po tential for the 1990s. Basic principles The ion mobility spectrometer. A typical IMS system is shown in Figure 1. The heart of the instrument is the drift tube, which provides a region of constant electric field where ions are created and allowed to migrate. In the design shown, the tube is built from a stack of metal guard rings separated by thin insulators; each ring is connected to a node in a string of electrical resis tors. This arrangement provides a smooth progression of voltages from one ring to the next when a supply volt age is connected across the whole
1202 A · ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990
string. A steady flow of ambientpressure drift gas, usually N2 or air, sweeps through the drift tube and min imizes the buildup of impurities that could otherwise react with ions and dis tort mobility spectra. Gates, fabricated from thin parallel wires, are used to block or pass ions traveling in the drift field. The ion paths terminate at the collector, a simple metal screen or plate. Many ion mobility spectrom eters contain an aperture grid close to the collector to capacitively decouple the collector from approaching ions. A number of additional components are needed to provide drift field high voltage, control the drift tube tempera ture and drift gas flow rate, generate timing signals for the gates, isolate gate timing signals from the high voltage of
the drift field, amplify the ion signal as it arrives at the collector, and provide signal averaging or other signal proc essing for the amplifier output. The ion mobility spectrum. The output spectrum is generated by first forming ions from the sample and then measuring ion migration in the electric field. The selection of ionization meth od is important because the ions formed and the ionization efficiency (and hence the quantitative measure of an analyte) depend on this choice. Once formed, each ion migrates at its own characteristic velocity down the constant electric field in the drift tube. The qualitative information from the experiment is usually reported as the spectrum of ion arrival times at the col lector. Figure 2 shows a typical signalaveraged ion mobility spectrum of a small peptide, L-leu-L-met-L-phe, in troduced in liquid methanol/water us ing an electrified spray to produce ions. The gate is pulsed open for 0.3 ms, ad mitting a mixture of ions to the drift tube, and the collector current is moni tored. The y-axis shows the ion current from the collector, and the x-axis re cords the arrival times of ions. Under normal IMS conditions, ions migrate at velocities between 1 and 10 m/s, pro ducing arrival time peaks in the 3 30-ms range. Thus, after the collector current is monitored for ~30 ms, the gate can be opened to admit another ion pulse and the experiment can be repeated. Repetitions are commonly averaged to increase the signal-to-noise ratio (S/N); 512 repetitions were aver aged to obtain the spectrum in Figure 2. Peaks with arrival times of 4-9 ms correspond to background ions pro duced from the solvents used for sam ple introduction, whereas peaks in the 10-16-ms range correspond to product ions derived from the peptide.
nr~
Reactant l i o n s UU
Product ions
Ο
30 Time (ms)
Figure 2. Ion mobility spectrum of a small peptide. Sample was introduced in methanol/water using an electrified spray nebulization/ionization source.
IMS peaks are typically rather broad compared with the range of possible drift times, and the amount of qualita tive information available from IMS is less than that from MS and IR spec trometry but more than that from flame ionization and electron capture detection. Ion formation. Although positive and negative gas-phase ions for IMS have been produced by a variety of methods, including photoionization (9), laser multiphoton ionization (10, 11), thermionic emission (12), and coronaspray (13), the most common ionization source used is still radioac tive 63 Ni foil. Using such a source, one can produce background ions from ni trogen gas:
N2 + β' — Nj + β'~ + e"
example, CI" reactant ions have been used to increase sensitivity to explo sives (15-17), NH^ reactant ions have been found to enhance selectivity and simplify IMS response to amines (18), and (acetone) 2 H + reactant ions have been used by Spangler and co-workers to ionize organophosphonates with specificity in hand-held detectors (19). Ion migration. After formation, ions are accelerated in the direction of the field between collisions with drift gas molecules. The energy gained from the electric field is randomized by these collisions, and the combination of acceleration and collision results in a constant average ion velocity (vj) that is directly proportional to the electric field (E). vd = KE
(1)
where β~ is the beta particle emitted from the 63 Ni source and β'~ is the beta particle after some of its energy has been used in ionization of the nitrogen molecule. The primary N j ion is too short-lived to appear in the mobility spectrum, but it begins a series of ionmolecule reactions with trace amounts of H 2 0 , NH 3 , NO, or sample (if in suffi ciently high concentration) in the drift gas. In the absence of sample, the re sulting stable secondary ion clusters have been identified as (H 2 0)„NH^, ( H 2 0 ) „ N O + , and (H 2 0) n H+. These background ions are normally called "reactant ions" in IMS because they undergo further ion-molecule reac tions with neutral gas-phase analytes to produce analyte "product ions." Similarly, thermalized electrons pro duced during the primary ionization process can undergo capture reactions with electronegative analytes to form negative product ions. When 0 2 is present in the drift gas, a negative reac tant ion cluster, (H20)„02~ or (H 2 0)„ (C0 2 ) m 0 2 ~, is formed and negative product ions are produced by ionmolecule reactions. For IMS cell tem peratures > 100 °C, ( H 2 0 ) n O - , (Η 2 0)„00 3 -, CN-, CI", and N0 2 " are also present (14). The ion-molecule re actions that occur in chemical ioniza tion MS can be used to help predict responses in IMS. Detailed investiga tions of ion-molecule reactions under the ambient pressure and electric field conditions common in IMS are scant, however. An important variation of IMS ion ization that can be used to enhance the sensitivity or selectivity of the tech nique for particular classes of com pounds or to simplify the response for certain analytes involves the modifica tion of reactant ion populations by adding carefully controlled concentra tions of dopants to the drift gas. For
(2)
The proportionality constant Κ is called the ion mobility and is usually computed in units of cm 2 V" 1 s _ 1
Κ = υά/Ε = L2/Vtd
(3)
where L is the ion drift distance in cen timeters, Vis the voltage drop across L, and td is the time it takes the ion to traverse L. In a review of ion mobility theory, Revercomb and Mason (20) have given the fundamental relationship between ion mobility and collision processes at the molecular level: Κ =
(3q/l6N)(2ir/kT)m X(m + Μ/τηΛί)1/2(1/Ω)
(4)
where q is the charge on the ion, Ν is the number density of the drift gas, k is Boltzmann's constant, Τ is absolute temperature, m is the mass of the ion, M is the mass of the drift gas, and Ω is the collision cross section of the ion in the drift gas. When the instrument op erating conditions are held constant (constant Τ and P, and thus constant N), the mobility depends only on ion charge, reduced mass, and collision cross section: Κας/μυ2η (5) where μ = mM/(m + M). For ions much more massive than the drift gas mole cules, μ is nearly equal to the drift gas mass M, and Κ varies only with q and Ω. Collision cross section is determined by ionic size, shape, and polarizability. Reduced ion mobility constant. Because analytical IMS is generally performed at ambient pressure and at a variety of temperatures, comparisons are facilitated by reporting a quantity as nearly independent of Τ and Ρ as possible. Because the number density, N, of Equation 4 increases with in creasing Ρ and with decreasing T, mo-
ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990 ·
1203 A
REPORT bility values are normally adjusted to standard conditions, 273 °C and 760 Torr. The adjusted value is called the reduced mobility constant (Ko) and is calculated from: K0 = K(273/T)(P/760)
(6)
Although KQ is expected to be a tem perature-dependent quantity (arising from the T~112 term and the tempera ture dependence of Ω in Equation 4), minimal change has been found in Ko over the range 85-220 °C for a variety of product ions (2, 21). Inherent in the definition of reduced mobility is the idea that the ratio of drift times for any two ions is indepen dent of temperature and pressure. Re duced mobilities and drift time ratios can also vary if the ions contributing to a mobility peak are a composite of clus ter ions whose proportions change with temperature and pressure (22). Instrumentation and operation Drift tube. Good performance can be obtained with a wide range of guard ring diameters, thicknesses, and sepa rations as long as the electric field ex perienced by the ions remains constant within a few percent (23). The stacked design has numerous parts and can be tedious to build. A one-piece construc tion based on a uniform resistive coat ing on a nonconductive substrate has been used successfully (24). The operating conditions most fre quently adjusted are temperature, drift voltage, and drift gas flow rate. Al though IMS resolving power decreases with increasing temperature, higher temperatures reduce ion cluster forma tion and interferences from contamina tion. Resolution loss from elevated temperatures may be countered by in creasing drift voltage, as long as the initial pulse width is not the dominant contributor to peak width. At very high drift fields, considerably above the 200-350 V/cm typical of present sys tems, Equations 2 and 4 are no longer valid (20). Adequate drift gas flows, on the or der of hundreds of milliliters per min
Table 1.
ute with most tube designs, are needed to continuously sweep contaminants and analytes from the spectrometer and minimize peak broadening and sensitivity loss resulting from ion-mol ecule reactions in the drift space. When spectrometers are not gas tight, the drift flow also helps to reduce unwant ed diffusion of ambient vapors into the spectrometer. Ion gates. Gates are used to block or pass ions traveling in the drift field. The most common design is a planar array of thin parallel wires oriented perpendicular to the ion drift. When the gate is closed, voltage on nearestneighbor wires is (Vr — VJ2) and (Vr + Vc/2), where Vr is the voltage of the drift field in the plane of the gate and Vc is the gate closure voltage. When Vc is large enough, essentially all the ions are collected on the gate wires. To open the gate, the voltage on all the gate wires is pulsed to Vr. An alternative design uses two close ly positioned planar arrays (15). The "downstream" array (closest to the col lector) is kept at a constant voltage, Vn and the voltage on the upstream array is changed to open and close the gate. For positive ions, a voltage somewhat above Vr allows ions to pass; if the volt age is below Vr, ions are collected on the upstream array and the gate is closed. If Vc is too small, ions can leak through the gate. On the other hand, Vc can become so large that the region im mediately upstream of the gate be comes significantly depleted of ions, a condition known as the gate depletion effect (25,26). A plot of sensitivity ver sus Vc has a rather broad maximum where the closure electric field is 1-10 times the drift electric field. Many ion mobility spectrometers contain an aperture grid. This grid has a parallel wire construction similar to a gate, although all the wires are main tained at the same potential (typically 5-25 V above ground for positive ions). The aperture grid is positioned as closely as possible to the collector (typically 0.5 mm) and reduces the ca
pacitive response of the collector to ap proaching ions. Peaks are significantly broadened if the aperture grid is omit ted. Sample introduction. An ion mo bility spectrometer is relatively easy to overload with analyte, and sample size must be controlled with care. Symp toms of overloading include disappear ance of reactant ion peaks, appearance of multiple analyte peaks from forma tion of dimer product ions and other cluster ions, and peaks that persist for long times. An overloaded instrument must be given time to be swept clear by the drift gas flow (sometimes days in the case of gross overloading). For many analytes, appropriate sample sizes are in the range of a few picograms to a few nanograms. From the stand point of sample size, capillary GC has proven to be a particularly good match for IMS. Instruments intended for va por-monitoring applications often in corporate membrane inlet systems (27) to moderate both the class and amount of sample introduced into the detector. Although vapor-phase samples are most common, condensed phases can also be introduced in the spectrometer using nebulization/ionization tech niques (13). Effluents from supercriti cal fluid chromatography (SFC) (28), LC (29), and capillary electrophoresis (30) have been introduced to the ion mobility spectrometer in this way. For these applications, however, the reac tant ions described above are probably no longer present but are modified by the reagents contained in the con densed mobile phase. Operational problems not yet entirely solved in clude incomplete breakup of solvent clusters and long-term stability of the nebulization/ionization process. Operating modes. The two basic types of data from IMS instrumenta tion are generation of complete mobil ity spectra of the ion population of the instrument and selective monitoring for the appearance of ions having drift times within some specified range. There are also three basic modes of op eration: signal averaging (or one-gate),
Modes and typical electronic operating conditions for IMS Mode
Gates
Sample pulse
Signal averaging Hardware monitoring
One + aperture Two
Fourier transform
One + aperture or two gates
0.2-0.5 ms 0.5 ms (gate 1) 0.5-20 ms (gate 2) Square-wave chirp
Conditions Cycle time Amplifier gain (V/A) 5-50 ms 5-50 ms
109- •101° 1011--1012
0.25-10 s
109--1011
1204 A · ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990
Amplifier rise time 0.1 ms 2s 10 ms (two gates) 0.1 ms (one gate)
-τ
70
1—
Temperature (°C)
230
φ w φ
η 70
τ 230 Temperature (°C)
Figure 3. Gas chromatogram with ion mobility detection of Aroclor 1248 spiked with 4,4'-dibromobiphenyl. (a) Mobility monitoring of drift times between 2.2 and 10.0 ms. Arrow denotes the 4,4'-dibromobiphenyl peak, (b) Bromide-selective monitoring of drift times. (Adapted with permission from St. Louis, R. H.; Hill, H. H„ Jr. J. High Résolut. Chromatogr. 1990, 13, 628-32).
hardware monitoring (or two-gate), and Fourier transform (25). Each mode can perform spectral or monitoring experiments, and each combination has different signal generation, gate configuration, amplification, and data processing requirements (Table I). Signal averaging, the fastest way to acquire ion mobility spectra, involves periodically pulsing the gate and recording the current arriving at the collector in real time. (If the drift tube contains two gates, as in Figure 1, the gate closer to the collector is kept permanently open.) The resulting ion mobility spectrum can be monitored with either an oscilloscope or a data acquisition system (if enhancement of S/N or a permanent record is desired). If signal averaging is applied, a few tens to a few hundreds of spectra are typically averaged. When IMS is used to monitor the output of another analytical technique (e.g., chromatography), integration of a portion of the stored spectrum may be desirable. In the hardware-monitoring mode, as in the signal-averaging mode, the entrance gate is opened to admit a pulse
show peak broadening because of ionmolecule reactions downstream of the entrance gate, the F T mode does not show this effect. However, it requires additional hardware and software as well as longer signal-processing times.
of ions. After a measured interval, the exit gate opens for a short period (selective monitoring, as in Figure 3b) or a long period (nonselective monitoring, as in Figures 3a and 4). The chromatogram-like output of the amplifier indicates the quantity of ions having drift times within the selected range, and the amplifier output can be passed directly to an integrator or chart recorder. Spectra can be generated by sweeping the delay time between entrance and exit gates. Although hardware monitoring is quite slow for gathering spectra, its instrumentation is the simplest of the IMS modes because a slow amplifier is used and no processing of collector signal is required. Because the S/N can always be increased by averaging for longer periods of time or, equivalently, by filtering with longer time constants, one must be sure to compare equal time periods for data collection when evaluating S/N characteristics of different methods. Thus, for a given duration of data acquisition, the one-gate and two-gate modes produce comparable S/N. FT-IMS uses a square-wave generator capable of a linear frequency ramp, or chirp, to drive entrance and exit gates simultaneously. The amplifier output must then be digitized and Fourier transformed. For a given data acquisition time, the F T mode produces a modest S/N advantage of X2-X10 compared with the signal-averaging mode. Although signal-averaging and hardware-monitoring spectra often
Analytical characteristics
Separation efficiency. For IMS, like other separation methods, the ratio of the spatial variance of the sample zone at the end of the separation to the length of the separator provides a convenient measure of separation efficiency. This measure is the theoretical plate height H. H = c2/L = (*/ + a/)/L
(7)
The total spatial variance is divided into contributions from diffusion (σ 0.01 can be achieved for caffeine. Thus with e, in the range 1.0-0.00001, detection efficiency ranges from about 10 _1 down to 10~6. In a typical signal-averaging experiment of 1 s duration, Nrms is on the order of 1.5 Χ 10~13 and
of 200 V/cm, Γ of 150 °C, and 1 atm pressure, η is approximately 2.7. The maximum number of theoretical plates possible for IMS can be deter mined from N m a x = L/Hm{n = qV/2vkT
(9)
and is a function of the charge on the ion, the voltage across the tube, the temperature, and the Townsend factor. Note t h a t separation efficiency in creases as drift field energy (qV) in creases relative to random thermal en ergy (kT). With typical values of V = 3500 V and Τ = 150 °C, iVmax is on the order of 18 000 for singly charged ions. However, because ag in Equation 7 is usually about the same magnitude as ad, the actual number of theoretical plates is most often considerably below JVnax. Spangler and co-workers have found that Ν goes through a maximum of 1700 as the drift field is varied around 100 V/cm in a miniature ion mobility spectrometer cell (33), al though the value of about 3800 exhibit ed by the product ion peaks in Figure 2 is more typical of larger tubes. Resolution. Following the chro matographic definition, IMS resolu tion, R, is defined as the ratio of the separation of two ion peaks in a drift time spectrum to the average width of the peaks, R = (td2 ~ tdl)/2{atl
+ at2)
(10)
where each at is the temporal counter part of the spatial σ in Equation 7 (σ = vdat). From the expression for the num ber of theoretical plates (\N = L/σ = tjat), it is easy to derive a useful rela tion for resolution (33) R = (|AtJ/i d )( v 'iV/4) =
(\AK\/K)(^N/4)
(11) where the barred quantities refer to av erages. Using an experimentally deter mined value of Ν for a particular in strument and set of operating condi tions, in conjunction with a table of reduced mobility values (3), one can predict how well two analytes might be resolved by the instrument. For a resolution of 1.00 and the iVmax of 18 000 value calculated above, it is apparent that KQ values must differ by at least 3% to be fully resolved by IMS; in actual instruments, 10% differences are typical. Minimum detectability. Defined as the mass flow rate required to produce an analytical signal three times the root mean square (rms) noise of the system, the minimum detectability of IMS is DmiB = SNTmsUF
(12)
where Dmm is the minimum detectabil ity in moles per second, NTms is the root
£ m i n = 3 X 10" 17 to 3 X 10" 12 mol/s (14) Minimum detectability may be fur ther reduced by additional signal aver aging and by multiplexing. Also, multi ply charged ions have increased detect ability by a factor equal to their number of charges. In practice, 3 X 10~17 mol/s has not been achieved but the method routinely operates below 3 X 1 0 " u mol/s (36).
J
IMS applications
I n the 1980s, work in IMS moved from fundamental studies toward applications to specific analytical problems. mean square noise of the system in am peres, F is Faraday's constant (96 500 coulombs per mole), and e is the overall detection efficiency measured as the ratio of the flow rate of ions detected (moles per second) to the flow rate of molecules in the sampled stream (moles per second). Detection efficien cy (e) can be divided into several com ponents: f
= Un)(fi)(ft)(fd)
(13)
where en is the fraction of neutrals transferred from the sampling stream to the spectrometer, e, is the ionization efficiency of the source, et is the effi ciency of ion transport through the drift tube, and ed accounts for intensity loss from diffusion, gate depletion, and electronic time constants. Approxi mate efficiencies for signal-averaging IMS are en = 1.0, tt = 0.5, and ed = 0.3. Ionization efficiency depends on the
1206 A · ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990
In the 1980s, work in IMS moved from fundamental studies toward applica tions to specific analytical problems, and the ion mobility spectrometer was used both as a stand-alone spectrome ter and as a chromatographic detector. Motivated by the adverse effects of surface contamination in semiconduc tor manufacturing processes, Carr used thermal desorption and a carrier gas to transport trace contaminants to the ion mobility spectrometer for detection (37). Lawrence and co-workers of the Canadian National Research Council have applied IMS to the detection of explosives (16) and drug particulates on the hands of emergency room pa tients suspected of drug overdose (38). They have also used the technique as a screening procedure for the identifica tion of wood products (39). Kolaitis and Lubman studied the ionization of various purine and pyrimidine bases and their ribose sugars, amino acids, vitamins, antidepressant drugs, and catecholamines using laser desorption with IMS (40,41). The real-time monitoring capabili ties and simple hardware needs of IMS have motivated Environmental Tech nologies Group (formerly Bendix Envi ronmental Systems Division) and Graseby Dynamics of England to de velop portable stand-alone alarms and field monitors. To improve the porta bility of these systems, a miniaturized ion mobility spectrometer was devel oped (33, 42) using continuously recir culating carrier and drift gases. A dimethylsilicone membrane inlet (27) re-
One challenge facing researchers in the 1990s
capillary GC, SFC, and LC. Ion mobility detectors have been designed specifical ly to meet the stringent demands of de tection after capillary GC (13, 47). By using a unidirectional gas flow, a re duced ionization cell volume, and direct capillary sample introduction, it has been possible to minimize peak broad ening in the detector. The ion mobility detector has been used for the determi nation of naphthalenes in gasoline (13, 48); hydrocarbons in petroleum ether (47); fatty acid methyl esters in cabbage extract (48); and opiates, barbiturates, benzodiazepinones, and tricyclic anti depressants in urine (49). As a chromatographic detector, the positive ion mode provides a response similar to that of the flame ionization detector but with greater sensitivity (36). The nonselective negative ion mode response is similar to that pro vided by the electron capture detector. Tunable selectivity, an important ca pability of ion mobility detection (IMD), is possible in either mode by monitoring a specific mobility window. Figure 3a shows a gas chromatogram of Aroclor 1248 (a mixture of polychlorinated biphenyls) spiked with 4,4'-dibromobiphenyl for which response is
comparable to that of an electron cap ture detector chromatographic tracing. Figure 3b shows the same chromato gram when the detector is set to moni tor the mobility window specific to bro mide ion. The unidirectional flow ion mobility detector has also been interfaced suc cessfully to a supercritical fluid chromatograph (28). Advantages of IMD after SFC include sensitive detection of compounds that do not contain chromophores (50), compatibility with a wide range of mobile phases (51), and an electron capture-like response un der pressure-programmed conditions (52, 53). Figure 4 shows a supercritical fluid chromatogram of polydimethylsilicone oligomers in which individual oligomers of > 3000 amu were detected using the ion mobility detector. A more complete review of IMS as a detection method for SFC is available (54). IMS in the 1990s
One challenge facing researchers in the 1990s is the use of the ion mobility spectrometer as a chromatographic de tector for the determination of com pounds separated by LC or other liq uid-phase methods such as capillary
is the use of IMS for chromatographic detection.
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duces clustering complications from ambient water and ammonia. Continuous IMS monitoring of toxic chemicals in the industrial environ ment has been studied by Dam (43) and by Watson and Kohler (44). In work directed toward monitoring N 0 2 , SO2, HC1, and H 2 S in stack gases, Eiceman and co-workers have encountered difficulties in quantifying analytes in complex and dynamic mixtures (45). Difficulties of this type are typical of IMS under conditions of changing background matrix or sample overload (46) and must always be kept in mind as a possible problem with stand-alone applications. Matrix and overload problems gener ally are not experienced when highresolution chromatography is used for sample introduction. Microcolumn sep arations and IMS detection are an ex cellent match from the standpoint of sample size and simplicity as well as de tector sensitivity. Work in our own labo ratories has been directed primarily to ward developing the ion mobility spec trometer as a detector following
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electrophoresis or ion chromatography. A significant step toward this goal was the recent development of a coronaspray ionization source for IMS (13). By introducing the effluent from a liq uid stream into the spectrometer via an electrically charged capillary, ion mo bility spectra from nonvolatile neutral and ionic compounds dissolved in liq uids have been obtained (29, 30). Al though simple liquid chromatographic separations have been reported (29), some difficult operational problems re main. Reactant ions and analyte re sponse factors vary considerably with small changes in geometry of the coronaspray source. Also, the high tempera tures and high drift gas flow rates need ed to break up solvent clusters have led to problems with sample decomposi tion, corona instability, and detector noise. The considerable practical potential of IMS for monitoring applications re mains largely untapped, in spite of its high sensitivity, low cost, small size, and versatility. This lack of develop ment is largely a result of initial dis couragement caused by overload and matrix problems. It must be recognized that IMS functions best with trace lev els of analytes that make only minor perturbations of drift tube conditions. Each application presents challenges such as analyte sensitivity and volatili ty as well as matrix interferences, and must be considered individually. The powerful techniques of preseparation by membranes, selective adsorption, and chromatographic inlets coupled with reactant ion modifications are helpful in many cases. Some fundamental problems remain in developing IMS. The resolving pow er of existing instruments seems to be less than expected on the basis of diffu sion and initial pulse width (55), and the reason for this is not understood. The extra broadening may be attribut able to ion-molecule reactions in the drift space, inhomogeneity of the elec tric field, or even coulombic repulsion. The FT mode should be able to probe reactions in the drift space, but it has not yet been used for this purpose. Oth er detection modes, such as an organophosphorus selective configuration that would simulate the nitrogen/phos phorus detector (56), have been pro posed but not yet implemented. Final ly, recent articles on peak shape analy sis (55) and a second-derivative algo rithm for separation of overlapping peaks (57) are reminders that several powerful signal-processing techniques are not fully used in IMS. Although IMS is considered an old technology, in many ways it is new technology waiting to be discovered. Its
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low detection limits and simplicity of design assure IMS a strategic position in analytical chemistry between uni versal nonspecific detectors, such as flame ionization and electron capture, and the more powerful qualitative techniques, such as MS. IMS research in the authors' laboratory has been sponsored in part by a grant from the Public Health Service (GM29523). The authors are most grateful to Glenn E. Spangler for his careful critical reading of this manuscript and many helpful suggestions.
References (1) Karasek, F. W. Anal. Chem. 1974, 46, 710 A-717 A. (2) Carroll, D. I.; Dzidic, I.; Stillwell, R. N.; Horning, E. C. Anal. Chem. 1975, 47, 1956-59. (3) Shumate, C; St. Louis, R. H.; Hill, H. H., Jr. J. Chromatogr. 1986, 373, 14173. (4) Karasek, F. W.; Kane, D. M. J. Chroma togr. Sci. 1972,10, 673-77. (5) Griffin, G. W.; Dzidic, I.; Carroll, D. I.; Stillwell, R. N.; Horning, E. C. Anal. Chem. 1973,45,1204-09. (6) Karasek, F. W.; Hill, Η. Η., Jr.; Kim, S. H.; Rokushika, S. J. Chromatogr. 1977, 135 329-39. (7) Karasek, F. W.; Spangler, G. E. In Elec tron Capture: Theory and Practice in Chromatography; Zlatis, Α.; Poole, C. F., Eds.; Elsevier: Amsterdam, 1981; Chapter 15, pp. 377-406. (8) Bairn, Μ. Α.; Hill, Η. Η., Jr. Anal. Chem. 1982,54, 38-43. (9) Bairn, Μ. Α.; Eatherton, R. L.; Hill, Η. Η., Jr. Anal. Chem. 1983, 55, 1761-66. (10) Lubman, D. M; Kronick, M. N. Anal. Chem. 1982,54,1546-51. (11) Kolaitis, L.; Lubman, D. M. Anal. Chem. 1986, 58, 1993-2001. (12) Spangler, G. E.; Kim, S. H.; Epstein, J.; Campbell, D. N.; Carrico, J. P., Jr. Pro ceedings of the 1988 CRDEC Scientific Conference on Chemical Defense Re search; CRDEC: Aberdeen Proving Ground, MD, 1988; U.S. Patent 4 839 143, June 13,1989; U.S. Patent 4 928 033, May 22 1990. (13) 'Shumate, C. B.; Hill, Η. Η., Jr. Anal. Chem. 1989,61, 601-06. (14) Carr, T. W. Anal. Chem. 1979,51, 70511. (15) Proctor, C. J.; Todd, J. F. Anal. Chem. 1984,56,1794-97. (16) Lawrence, A. H.; Neudorfl, P. Anal. Chem. 1988,60,104-09. (17) Spangler, G. E.; Carrico, J. P.; Camp bell, D. N. J. Test. Eval. 1985,13(3), 23440. (18) Kim, S. H.; Karasek, F. W. Anal. Chem. 1977,50,152-55. (19) Spangler, G. E.; Campbell, D. N.; Car rico, J. P. Presented at the 1983 Pitts burgh Conference and Exposition on An alytical Chemistry, Atlantic City, NJ, March 1983; U.S. Patent 4 551 624, No vember 5,1985. (20) Revercomb, H. E.; Mason, E. A. Anal. Chem. 1975,47, 970-83. (21) Lubman, D. M. Anal. Chem. 1984, 56, 1298-1302. (22) Preston, J. M.; Rajadhyax, L. Anal. Chem. 1988,60, 31-34. (23) Spangler, G. E.; Cohen, M. J. In Plas ma Chromatography; Carr, T. W., Ed.; Plenum Press: New York, 1984; p. 6. (24) Carrico, T. F.; Sickenberger, D. W.; Spangler, G. E.; Vora, K. N. J. Phys. E.
Set. Instrum. 1983,16,1058-62. (25) Knorr, F. J.; Eatherton, R. L.; Siems, W. F.; Hill, H. H., Jr. Anal. Chem. 1985, 57, 402-06. (26) Aaronson, E. A. Technical Report No. SAND87-0072 UC-32, 1987; Sandia Cor poration, Albuquerque. (27) Spangler, G. E.; Carrico, J. P. Int. J. Mass Spectrom. Ion Phys. 1983, 52, 26787. (28) Eatherton, R. L.; Morrissey, Μ. Α.; Siems, W. F.; Hill, H. H., Jr. J. High Résolut. Chromatogr. Chromatogr. Commun. 1986,9,154-60. (29) McMinn, D. G.; Kinzer, J.; Shumate, C. B.; Siems, W. F.; Hill, H. H., Jr. J. Microcolumn Sep. 1990, 2,188-92. (30) Hallen, R. W.; Shumate, C. B.; Siems, W. F.; Tsuda, T.; Hill, H. H., Jr. J. Chromatogr. 1989,480, 233-45. (31) Spangler, G. E.; Collins, C.I. Anal. Chem. 1975,47, 403-07. (32) McDaniel, E. W. Collision Phenomena in Ionized Gases; Wiley: New York, 1964; Chapter 10. (33) Spangler G. E.; Vora, Κ. Ν.; Carrico, J. P. J. Phys. E. Sci. Instrum. 1986, 19, 191-98. (34) Smith, R. D.; Loo, J. Α.; Edmonds, C. G.; Barinaga, C. J.; Udseth, H. R. Anal. Chem. 1990,62, 882-99. (35) Ikonomou, M. G.; Blades, A. T.; Kebarle, P. Anal. Chem. 1990, 62, 957-67. (36) St. Louis, R. H.; Siems, W. F.; Hill, H. H., Jr. J. Microcolumn Sep. 1990, 2, 138-45. (37) Carr, T. W. Thin Solid Films 1977,45, 115-22. (38) Nanji, Α. Α.; Lawrence, A. H.; Mikhael, N. Z. J. Toxicol. Clin. Toxicol. 1987, 25(6), 501-15. (39) Lawrence, A. H.; Elias, L. Presented at the 3rd Chemical Congress of North America, Toronto, June 1988.
(40) Kolaitis, L.; Lubman, D. M. Anal. Chem. 1986, 58, 2137-42. (41) Lubman, D. M. Rev. Sci. Instrum. 1988 59 557—61 (42) Carrico, J. P.; Davis, A. W.; Campbell, D. N.; Roehl, J. E.; Sims, G. R.; Spangler, G. E.; Vora, K. N.; White, R. J. Am. Lab. Feb. 1986,152,155-57; 159-63. (43) Dam, R. J. In Plasma Chromatogra phy; Carr, T. W., Ed.; Plenum Press: New York, 1984; pp. 174-214. (44) Watson, W. M.; Kohler, C. F. Environ. Sci. Technol. 1979, 23(10), 1241-43. (45) Eiceman, G. Α.; Leasure, C. S.; Vandiver, V. J. Anal. Chem. 1986, 58, 76-80. (46) Metro, M. M.; Keller, R. A. J. Chroma togr. Sci. 1973,77,520-24. (47) St. Louis, R. H.; Siems, W. F.; Hill, H. H., Jr. J. Chromatogr. 1989, 479, 22131. (48) St. Louis, R. H.; Siems, W. F.; Hill, H. H., Jr. LC/GC 1988,9, 810-14. (49) Eatherton, R. L. Ph.D. dissertation, Washington State University, 1987. (50) Rokushika, S.; Hatano, H.; Hill, Η. Η., Jr. Anal. Chem. 1987,59, 8-12. (51) Morrissey, M. A. Ph.D. Dissertation, Washington State University, 1988. (52) Tarver, E.; Hill, Η. Η., Jr. Presented at the Western ACS Convention, Los Ange les, 1988. (53) St. Louis, R. H.; Hill, H. H , Jr. CRC Crit. Rev. Anal. Chem. 1990, 21(b), 32155. (54) Hill, H. H., Jr.; Morrissey, M. A. In Modern Supercritical Fluid Chromatog raphy; White, C. M., Ed.; Huthig: Heidel berg, 1988; p. 95. (55) Glasser, M. L. J. Appl. Phys. 1988, 63(10), 4823-31. (56) Wohltjen, H. U.S. Statutory Invention Registration H406, Jan 5, 1988. (57) Roehl, J. E.Opt.Eng. 1985,24,985-90.
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H. H. Hill, Jr. (left), professor of analytical chemistry at WSU, received his B.S. degree (1970) from Rhodes College (Memphis), his M.S. degree (1973) from the University of Missouri, Columbia, and his Ph.D. (1975) from Dalhousie Universi ty. Before joining WSU in 1976, he studied IMS in F. W. Karasek's laboratory at the University of Waterloo. His research interests include analytical instrumen tation for trace organic analysis using IMS, SFC, flame ionization detection, simultaneous derivatization and extraction, and MS. William F. Siems (second from left) received his Sc.B. degree (1966) from Brown University and his Ph.D. in physical chemistry (1974) from WSU, where he is a research associate and lecturer. His research interests include data acquisition, signal processing, and instrument development. Robert H. St. Louis (third from left), currently a research chemist in the Physical and Analytical Division of Tennessee Eastman Chemicals, received his Ph.D. in chemistry from WSU in 1990. His research interests include the use of IMS as a detector following capillary GC and SFC, chromatographic interfacing technology, and applications and fundamental studies of subambient LC. Dennis G. McMinn (right), professor and chair of the Department of Chemistry at Gonzaga University, received his B.Sc. degree from the University of Alberta in 1966 and his Ph.D. in organic chemistry from the University of Minnesota in 1970. He taught undergraduate chemistry at Carleton College for four years before joining Gonzaga University in 1974. His research interests include chromato graphic detectors and the application of IMS to liquid systems.
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