Nonlinear laser wave mixing in a common graphite furnace atomizer is presented as a zeptomole-level, sub-Doppler, high-resolution atomic absorption spectrophotometric method. A nonplanar three-dimensional wave-mixing optical setup is used to generate the signal beam in its own space. Signal collection is efficient and convenient using a template-based optical alignment. The graphite furnace atomizer offers advantages including fast and convenient introduction of solid, liquid, or gas analytes, clean atomization environment, and minimum background noise. Taking advantage of the unique features of the wave-mixing optical method and those of the graphite furnace atomizer, one can obtain both excellent spectral resolution and detection sensitivity. A preliminary concentration detection limit of 0.07 parts-per-quadrillion and a preliminary mass detection limit of 0.7 ag or 8 zmol are determined for rubidium using a compact laser diode as the excitation source.

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Sensitive absorption-based detection of anthracycline antibiotics, daunorubicin and doxorubicin is demonstrated using a capillary electrophoresis system interfaced to a nonlinear wave-mixing detection system. Unlike conventional absorption methods, this nonlinear absorption method can detect very thin analytes (50 μm) efficiently. At the same peak height, the wave-mixing CE peak is narrower than a conventional CE peak, and hence, compared to other laser-based or non-laserbased CE on-column detection methods, our wave-mixing detection method offers intrinsically enhanced separation resolution even when using identical CE separation conditions. In this unusually sensitive "absorbance" detection method, two input laser beams interact to produce a thermally induced grating from which coherent laser-like wave-mixing signal beams are created. Using our sensitive "absorbance" on-column CE detector, we report a preliminary concentration detection limit of 9.9 × 10 −10 M using a 50 μm i.d. capillary column. The corresponding "injected" mass detection limit is 9.1 × 10 −18 mol using an injection volume of 9.2 nL. The corresponding preliminary "detected" mass detection limit inside the 12-pL detector probe volume is 1.2 × 10 −20 M.

Laser wave-mixing spectroscopy is presented as a simple, sensitive, on-column detection method for capillary electrophoresis. The use of a single focusing lens to focus and mix two input beams significantly simplifies the optical alignment requirement of this nonlinear laser method. High signal collection efficiency allows excellent detection sensitivity for both fluorescing and non-fluorescing analytes, since the signal is a sharp coherent beam. This laser detection method can be conveniently interfaced to capillary-based separation systems since it offers small detector probe volumes, efficient use of short excitation or absorption path lengths, efficient use of low laser power levels available from small, compact, inexpensive laser sources, and inherently narrower peak widths (i.e., squared Gaussian). Potential applications include sensitive detection of biomolecules, either labeled or in their native fonns, using appropriate excitation wavelengths. Furthermore, laser wave-mixing detection can be used also in the indirect detection mode where the solvent or the buffer system yields a baseline signal, and nonabsorbing analytes can be measured indirectly in the form of negative peaks.

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